Well logging and interpretation techniques asin b000bhl7ou

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  • 1. FFF
  • 2. WELL LOGGING AND INTERPRETATION TECHNIQUES The Course For Home Study I. Log Interpretation Fundamentals: Open Hole II. Study Guide III. Induction Logs IV. Electrolog(st) V. Laterolog and Dual Laterolog VI. Flushed Zone Resistivity Devices VII. Spontaneous Potential Log VIII. Gamma Ray Log IX. Compensated Densilog" x. Acoustic Logs XI. Neutron Logs
  • 3. FOREWORD Lop, Interpretation Fundamentals: Open Hole is a home study course which covers the important basic elements of open hole log interpretation. The emphasis is on log interpretation, not on tool measurement theory. The first few lessons introduce relevant rock and fluid characteristics. Subse- quent lessons present progressively more complex log interpretation tech- niques. The number of interpretation techniques is kept to a minimum. This is a lesson-by-Iesson course. Participants should study each lesson and then answer the related questions. (A study guide has been provided). Supplementary reading is suggested throughout the text. The text, along with the supplementary reading, should provide a sound basis for basic open hole log interpretation. Comments or questions, regarding any of the course material, should be made to Dresser Atlas sales or log analysis personnel worldwide.
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  • 5. TABLE OF CONTENTS LESSON 1 Introduction to Rock Properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 LESSON 2 Basic Resistivity Concepts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 11 LESSON 3 Formation Fluid Properties 15 LESSON 4 How Subsurface Temperature Affects Formation Evaluation. . . . . . . . . . .. 22 LESSON 5 The Archie Equation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 32 LESSON 6 Induction Log 39 LESSON 7 uterolog . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 52 LESSON 8 Additional Resistivity Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 60 LESSON 9 Spontaneous Potential (SP) Log. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 69 LESSON 10 Gamma Ray Log . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 79 LESSON 11 Spectralog" ····· · · · . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 84 LESSON 12 Densilogs 88 LESSON 13 Acoustilog" 94 LESSON 14 Compensated Neutron Log 102 LESSON 15 Lithology 118 LESSON 16 Crossplotting Porosity Logs (Mixed Lithology) 123 LESSON 17 Tri-Porosity Interpretation .. · · · · · · 129 iii
  • 6. TABLE OF CONTENTS CONTINUED LESSON 18 Overview of Shale Content Evaluation 136 LESSON 19 Porosity and Clay Content Determination (Sand-Shale Sequences) 141 LESSON 20 Rwa Method: Fast Formation Evaluation 146 LESSON 21 Conductivity Derived Porosity (COP) 152 LESSON 22 The Formation Factor-Movable Oil Plot 161 LESSON 23 Hingle Crossplot 165 LESSON 24 Pickett Crossplot 175 LESSON 25 Water Saturation Determination (Sand-Shale) 179 LESSON 26 Predict Water Cut from Well Logs 188 LESSON 27 Computer Processed Interpretation 197 LESSON 28 Computer Generated Log Data 207 iv
  • 7. INTRODUCTION TO ROCK PROPERTIES 1INTRODUCIlON THE NATURE OF SEDIMENTARY ROCKS Throughout geologic time, global tectonic activity Sedimentary rocks may be classified as having beenaltered and continues to alter the earths crust. This formed primarily by mechanical weathering (e.g. sand-process has distilled out the lighter lower melting stones, conglomerates) or by chemical weatheringpoint materials which have accumulated on the sur- and/ or precipitation from solution. These altered parti-face forming the continents. Sedimentary rocks have cles may then be transported or dissolved in a fluid andevolved as a result of mechanical and chemical altera- deposited mechanically or precipitated chemically ortion of these rocks through exposure to the surface biogenetically under specific chemical and physical con-environment. Today, a thin veneer of sediments ditions. Sedimentary rocks are composed primarily ofalmost entirely covers the earths surface. The those minerals which are stable under normal conditionsgeneration of petroleum within this sedimentary rock of stress, temperature and pressure. Minerals normallyhas been occurring on the earth since life evolved. A associated with igneous and metamorphic rocks whichsmall percentage of the total organic remains has were formed under abnormal conditions of stress,escaped oxidation by burial beneath the surface in temperature or pressure may also occur with thesediments. These organic remains, when sufficiently sedimentary rock. Of the 2,900 naturally occurringconcentrated and subjected to moderate levels of minerals now known, less than 200 occur in sufficientgeothermal heat and overburden pressure in an ox- amounts to be classified among the common rock-ygen free environment, have evolved to form forming minerals. Of these, only two dozen or sopetroleum. The movement of pore fluids from source characterize the majority of sedimentary rocks.rocks into porous and permeable reservoir rocks Sedimentary rocks may be grouped conveniently where they were accumulated and trapped has into mechanically derived rocks, or clastics and resulted in the hydrocarbon bearing reservoirs found chemically precipitated rocks. Chemically precipi- today. tated rocks may be further subdivided into car- Most of the worlds petroleum occurs in sedimen- bonates and evaporites. (Fig. 1.1) tary rocks. The location of petroleum reserves re- quires an understanding of the nature of the rocks in Source Rock which these reserves occur, and well logs are one of the primary sources for such data. Well logs are par- I I I ticularly useful in the description and characteriza- Chemical Mechanical tion of sedimentary rocks and their pore fluids. Weathering Weathering Well logs are a fundamental method of formation I I analysis since they measure the physical properties of the rock matrix and pore fluids. They provide forma- Plant Extraction I Solution I New Minerals tion data not directly accessible by means other than coring. Well logs can be used to extend data obtained from core analysis to wells from which only logs are available. Utilizing log-derived measurements of such petro-physical properties makes it practical to Precipitation determine, for example, lithology, porosity, shale Biologic Extraction volume, water and hydrocarbon saturation and type, and Precipitation when oil and/or gas are present and to estimate permeability, to predict water cut, calculate residual Peat Shale oil saturation, and detect overpressured zones. Coal The primary purpose in the analysis of most well Evaporites Conglomerate logs is to describe the lithology of the reservoir rock (Some Limestone) Sandstone and fluid properties for the section cut by that par- Limestones Chert ticular well. A set of logs representative of an area Diatomaceous Shales Phosphates can be used as an exploration tool to describe local stratigraphy, structure, facies relationships and en- FIGURE 1.1 Simplified chart showing origins of sedimentary rocks. vironments of depostion.
  • 8. Sorting Grain Shape C1 °8° ~o~ Angular: having sharp corners and edges Very Well o Q and, therefore. showing little or no effects 000 of abrasion or wear. C68b 0 ~D{? Subangular: having edges and corners Well slightly rounded. so that wear is evident. 000 OPeO Subround: having most of the corners and Q~ l?~ Moderat~~y 00 °00 edges worn down to smooth curves, thus. showing extensive abrasion. 00°0 o 0 0 • 00 Round. having all edges and corners 0:)0 Poorly 1) 0°0 0 smoothed off to gentle curves by prolonged wear. 080FIGURE 1.2Texture of clastic rocks. Clastic rocks, principally sandstones and shales, comprise up to 40 alo or more of sands formed nearare those sediments composed of distinct grains granitic source areas in arid climates. A multitude ofwhich have been mechanically deposited. Their other minerals may occur in minor amounts (usuallycharacter is dependent upon the composition of the less than one percent of the rock). Some, such asgrains, their relative abundance, grain size and hematite, dolomite and siderite serve as cementingshape, orientation and packing of grains, cornposi- agents. Calcite is also quite common as a cement andlion and distribution of cement and nature of the a rock forming mineral. Textures can vary fromfluid content of the pore system. Typical clastic rock poorly sorted conglomerates as found in the granitenomenclature indicates the relative size of grains or wash to highly rounded, well sorted, fine blankettexture. For example, a siltstone is composed of silt- sands. A marl is usually 40 to 60070 carbonate withsized grains and a sandstone is composed of sand- the other 60 to 40 07 usually clay with silt or sand. 0sized grains. Greater than 60070 carbonate is considered a shaly The texture of clastic rocks is determined largely limestone or chalk. Less than 40 010 carbonate is oftenby grain shape and sorting. (Fig. 1.2) Grain shape or considered as a calcareous or dolomitic shale.sphericity is principally a function of the energy of Shale generally refers to a rock composed pri-the environment of transport reworking and deposi- marily of clay minerals, minor quartz silt with sometion. A grain which has been reworked through the feldspar, and from 0 to 20 070 organic matter. The car-sedimentary cycle several times generally has a higher bonate equivalent of shales are often called marls.sphericity. Younger grains near their source are They are composed primarily of calcite, clay, andusually very angular. Sandstones deposited in minor silica, and are occasionally dolornitized. Themoderate to high energy environments tend to be extensive "oil shale" deposits in Utah, Colorado andwell-sorted while those formed in low energy en- Wyoming are marls.vironments have poor sorting. Clay minerals are essentially hydrous aluminum The composition of sandstone is highly dependent silicates in which magnesium and/or iron mayupon the source, climate and environment of substitute for aluminum, and aluminum maytransport and deposition. Sandstones are usually substitute for silica within the structure. Claycomposed of quartz. Numerous sandstones contain minerals are formed by the weathering of silicateigneous or metamorphic rock fragments, some minerals from igneous and metamorphic rocks. In-feldspar and a few carbonate grains. Feldspar is the dividual particle size is of the order of 10 microns.most abundant mineral in igneous rock and may Shales comprise 50 to 60070 of the sedimentary 2
  • 9. record. The great majority of all shales are deposited tant producing horizons have been explored in in near shore marine environments along continental California. margins. The remainder are deposited as deep marine muds or in fluvial, and lacustrine environments. CHEMICAL ROCK CLASSIFICATION The color of gray to black shales is due primarily to organic content. Red or green shales have a high From the preceding brief discussion of the prin- iron content and relatively low organic content. cipal features of sedimentary rocks, it is evident that Yellow and brown shales have a low iron and low a simplified classification system for log interpreta- organic carbon content. tion is needed. Logging tools respond primarily to Shales have a low effective porosity and extremely the chemical nature of matrix and pore fluids. For low permeabilities. They have usually been bypassed this reason, a chemical rock classification has beenI.. by drillers even when good mud shows were en- countered. The Woodford shale in Oklahoma is a typical black organic-rich shale with beds of dark pyritic chert, siliceous shale and some siltstone. adopted. All rocks composed primarily of silica, in- cluding cherts, are called sandstones. All calcium car- bonate rocks are called limestones. All calcium magnesium carbonate rocks are called dolomites. Where relatively brittle zones have developed natural Rocks composed primarily of clay are called shale. fracture systems, good gas production and some oil Silt or siltstone is sometimes used as the calculated have been found. difference between shale volume and clay volume. Anhydrite, gypsum, halite and coal all have suffi- Carbonates comprise approximately 10010 of the stratigraphic section. Their importance as reservoir ciently unique log responses which are easily iden- rocks should not be underestimated. Approximately tified. SOOfo of the world hydrocarbon reservoirs are in car- RESERVOIR ROCK PROPERTIES-POROSITY bonate rocks. These consist primarily of calcite or dolomite with mica amounts of aragonite, ankerite, A thorough understanding of basic reservoir siderite, pyrite, chlorite. quartz and clays. parameters is a must for the analysis of any well log. The ultimate storage capacity of any reservoir is Evaporite sequences are deposited in marine basins defined as the percent of space not occupied by the having restricted circulation. Halite or salt beds ex- rock matrix. Total porosity, 0, is defined as: ceeding 3,000 ft thick are known in the geologic record. Sylvite (KCl) and other salts occur in lower f2S = pore volume/total volume (1) volumes, but are more important economically. Gyp- sum and anhydrite are the common sulfates. Gypsum or is altered to anhydrite by loss of water with increas- ing depth of burial and these deposits are frequently interlayered with shale, limestone, or dolomite. I2l l I-matrix volume ) Anhydrite may also crystallize directly under certain ~ total volume conditions. Primary porosity is the porosity that is built into the Phosphatic rocks also occur in complex mixed rock matrix during original deposition and includes lithologies. They include carbonate as well as clastic its reduction by subsequent cementation and com- mixtures. They frequently have high organic carbon paction. (Fig. 1.3) content and are thought to be the source rock for several major oil fields. Extensive deposits of chert have been formed in deep marine basins from the remains of siliceous micro-organisms. Chert also occurs as replacement deposits in shales and limestones. Cherts are general- ly not porous or permeable except where fractured. Depth An exception is the Mississippian "chert" zone in South Central Kansas and Northern Oklahoma which is a weathered chert with porosity of 30-40010 in places. The zone is also fractured which increases 1 the permeability. Diatomites and diatomaceous shales are accumula- tions of thin-walled unicellular siliceous micro- organisms with varying amounts of shale. These FIGURE 1.3 deposits have very high porosities (25-60010), but their Porosity vs. well depth. permeabilities are comparatively low. Several impor- 3
  • 10. In clastics, grain shape, size, sorting and energy physically and chemically with drilling and comple-level of the environment of deposition determine the tion fluids reducing permeability.packing arrangement. In general, fine-grained sand- Carbonate rocks usually show little evidence ofstones with poorly sorted angular grains will have physical compaction. Carbonate rocks are cementedlower porosity than sandstones composed of coarse, very quickly both during and soon after deposition.well-sorted grains. Angular grains tend to fit together Porosity reduction is primarily due to continuedand develop more intimate grain-to-grain contacts. growth of cement in the pore space.(Fig. t .4) In poorly sorted sediments, the smaller Secondary porosity is that porosity developed subsequent to original deposition, compaction and cementation. Secondary porosity includes fracture Porosity porosity, solution porosity and porosity caused by 47.6% dolomitization. 47.6% TRAPS The occurrence of accumulations of petroleum in nature requires the existence of an organically rich source rock, a porous and permeable reservoir rock 25.9% and a seal. Traps are classified as structural, stratigraphic or a combination. Structural traps include domes, faults and an- ticlines. Stratigraphic traps may be formed by lateral ~ <25.9% variations in lithology. The presence of fractured limestone stringers within impermeable shales is anFIGURE 1.4 example of a stratigraphic trap. (Fig. 1.5) In someGrain shape and size effects on porosity. cases, inclined bodies of rock have been eroded at the surface and overlain by impermeable bodies of rockgrains tend to fill the spaces in between the larger to form stratigraphic traps.grains. Packing is independent of absolute grain size.Rocks composed of grains of identical shape whichare equally well-sorted will theoretically have the PERMEABILITYsame porosity. However, sediment ranging from siltto very fine-grained sandstone frequently exhibit The interconnective porosity in a rock is called itslower porosities. Very small grains tend to have lower effective porosity. The degree to which this naturalsphericity and form smaller pores which are more plumbing system can conduct fluid is calledeasily cemented. The degree of sorting and average permeability. It is the key parameter in determininggrain size are directly related to the duration of the the rate of production.sedimentary process and the energy level present dur- Based on flow tests, Darcy determined that theing deposition. value of permeability, k, can be expressed by the After deposition, compaction and cementation equation:greatly reduce porosity. Shales exhibit the greatestdegree of compaction through the expulsion of in- k QJA/ A(AP/L) (2)terlayer pore fluids. This expulsion of fluids by com-paction at an increased temperature is the basic where:mechanism for primary migration of petroleum fromsource to reservoir rocks. Q flow per unit time Compaction effects in sandstones are less signifi-cant. The initial fabric of the grains is primarily JA = viscosity of flowing mediumdetermined at deposition. With increasing pressure, asand will compact no more than 10 to 15070 principal- A cross section of rockly due to grain rearrangement. At greatly increasedpressure and some increase in temperature, pressure L length of rocksolution occurs at stress points. Porosity reduction is primarily due to cementation ~P pressure differential (drop)and crystallization of certain minerals in the porespace. Clean sandstones and carbonates are relatively The unit of permeability is the darcy (D), equal tostable. Some clay minerals tend to react both 1,000 millidarcies (mD). By definition, a porous rock 4
  • 11. X X L- _-J b. Normal Fault X Limestone a.Dome Shale Sandstone 0 0 0 0 0 0 <0 CO 0 0 -- CO <0 XXl c -- - 8 A - -- B C J II Oil sand I I I I I J. . . . . . ..- ---r - - I / 1".--1---1 . . . . . . . 1, : ---f ,--,I I I ----------~~~~~------ -------------~---~-- d. Fractured limestone stringers within impermeable shales. c. Anticlinal TrapFIGURE 1.5Structural traps. 5
  • 12. exhibits a permeability of one darcy when a single Therefore, a fracture 0.01 in. wide has a permeability phase fluid of one centipoise viscosity (viscosity of of 5,440 darcys. water at 68 OF) which completely fills the entire pore In other words, solution channels, interconnectedspace will flow through it under viscous flow condi- vugs and fractures can significantly affect the pro- tions at the rate of one cm 3/sec per square centimeter duction behavior of potential reservoir rocks. of cross section area under a pressure gtadient of one Permeability can be determined from well resting atmosphere per centimeter. Potential hydrocarbon.. operations, wireline or DST, measurements on bearing rocks exhibit a wide range of perrneabilities. sidewall samples, core analysis using plug type or (Fig. 1.6) Frequently, permeability increases with whole (full) core measurements, or estimated through correlation to well log data. (Fig. 1.7) ffi High Permeability Area Rock ~ Low Permeability 0-)) )_ Pl , . - - L ~I P2 LengthFIGURE 1.6Range of permeability. Area of Core Permeability / . - - Pressure Drop porosity. However, even very low porosity rocks may "k x Q =. A-- be highly permeable. This is caused by natural frac.. L (P1 - P2) tures and/or solution channels. On the other hand, Flow / Fluid -, Core Length high porosity rocks, such as chalk, may have very Rate Viscosity low matrix permeability. A practical rule of thumb for classifying FIGURE 1.7 permeability is: poor to fair, k = < 1.0 to 15 mD; Determination of permeability. moderate k = 15 to 50mD; good k = 50 to 250 mD; very good k = 250 to 1,000 mD; and excellent k, in Reservoir permeability is a directional rock proper- excess of one darcy. ty. Cross bedding, ripple marks, bioturbation, cut Besides the typical matrix permeability, some and fill structures as well as variation in cementation, potential reservoir rocks, particularly low-porosity grain size, sorting and packing contribute to varia- carbonates, may have solution channels and vugs tion in permeability with a depositional unit. and/or natural fracture systems, which g~eatly Permeability in the direction of elongation of the enhance reservoir permeability. component grains is considerably greater than in any Permeability of solution channels, which exhibit other direction. Horizontal permeability (k h ) , circular or near circular openings, can be directly measured parallel to bedding, is the major con- related to and calculated from the size of the chan- tributor of fluid flow into the wellbore. nels: Vertical permeability (k-) is frequently lower than horizontal permeability. Bedding planes, the k(darcy) (3) presence of muscovite or other platy minerals and shale laminations act as barriers to vertical where: permeability. The ratio of kh/k v generally ranges from 1.5 to 3.0 and may exceed 10 for some reservoir d = diameter of channels rocks. Sometimes, however, unusually high vertical For example, the permeability of a solution channel permeability occurs in unconsolidated clean and having an opening of 0.001 in. is 20 darcys. coarse sandstones or due to fracturing or develop- ment of vertical jointing. Joints may occasionally be For typical fracture permeability, the above equa- filled with clay or other minerals which act as barriers tion becomes: to horizontal permeability. This condition (kh/k v ~ 1.0) greatly affects reservoir behavior due to bypass- k{darcy) (0.544)( 10 8) (,2) ing and coning effects in producing wells. where: w fracture width, in. 6
  • 13. ABSOLUTE, EFFECTIVE AND RELATIVE where:PERMEABILITY o and Swi are expressed in 010 Taking certain analytical precautions, a specificrock sample is characterized by a unique permeabilityvalue, regardless of whether gas or liquid has beenused in the measurement. (5) Provided only a single medium such as water, oilor gas flows through the rock (So or Sw or S = 1.0),the term absolute permeability is used. g 250 for oil However, since petroleum reservoirs contain gas C ={and/or oil and water, the effective permeability (k , 80 for gasko k w ) for a given medium in the presence of othe:smust be considered. Effective permeability is the where: 0, Swi are decimal fractions.permeability of the rock to a medium when anothermedium is present in the pore space. It is important For a graphical solution to the Timur equation andto realize that the sum of effective permeabilities will Morris and Biggs equation, refer to Figures 1.8 andalways be less than the absolute permeability. This is 1.9.due to mutual interference of simultaneous flows ofmore than one liquid. When dealing with flow of more than one fluidthrough a permeable reservoir rock, it is necessary toconsider relative permeability (krg~ k ro, k rw). Relative permeability is defined as the ratio of BIBLIOGRAPHYrelative permeability of one phase, during multiphasefluid flow, to the absolute permeability of that fluid Blatt, H., Middleton, G., and Murray, R. Origin ofduring single phase flow through the reservoir rock. Sedimentary Rocks, Second Edition. Prentice-Hall, Inc., 1980. Relative permeability = effective permeability/absolute permeability. Fertl, W.H. Knowing Basic Reservoir Parameters First Step in Log Analysis. Oil and Gas Journal, Porosity and water saturation are commonly used 1979.to predict production potential. It is apparent thatunless the relationship between porosity and Folk, R.L. Petrology of Sedimentary Rocks, Austin,permeability is known, as well as the relative Texas: Hemphills, 1968. (A "syllabus" periodicallypermeability, this practice entails considerable risk. revised, and used as a laboratory manual at the The importance of the presence of clay minerals as University of Texas).a determinant of permeability is often related notonly to their abundance, but also to their mineralogy Levorsen, A.I. Geology of Petroleum, Second Edi-and the composition of the pore fluid, principally its tion. W.H. Freeman & ce., 1967.salinity. In an undisturbed sandstone, the clayminerals are attached to or coat the grain surfaces Potter, O.E., Maynard, J.B., and Pryor, W.A.reducing the pore space. If the clays should be ex- Sedimentology of Shale. New York: Springer-Verlag,panded due to changes in the chemistry of the pore 1980.fluids, mud filtrate invasion or become dislodged tofloat through the pore channels and block porethroats, the permeability win be further reduced. There are several methods of determining reservoirpermeability from log data. Two commonly used em-pirical methods are the Timur equation and the Mor-ris and Biggs equation.Timur Equation: 0 4 .4 k (mD~ = 0.136 ~ (4) WI
  • 14. 80 r---------r----...-.......- -......- -.....- -.....- - - - - - - - - - - .....706050 I4030 1000 / 2000 /20 4000 /10 O~ ............ ~ ...... ._.._ ...._ __.l"__ ~ o 5 10 15 20 25 30 35 40 Porosity. +(%) FIGURE 1.8 Reservoir permeability estimate from log data (Timur equation). 8
  • 15. 80,..---------~---- ... ~--------------------------- ... 70 k = 0.1 60~ 50 i(/)C~~jCiS(J) 40~CG~~.0u j 30t)!~ 20 10 0 ... .... .... ..... ...- - - -.....- - -..... .... .... o 4 8 12 16 20 24 28 32 36 40 Porosity. +(0/0) FIGURE 1.9 Reservoir permeability estimate from log data (Morris and Biggs equation).
  • 16. QUESTIONS 1(1) Which of the following rock types arc usually clastics? a) limestone b) evaporites c) sandstones d) dolomites(2) Calculate the permeability of a fracture whose width is 0.2 in. a) 10 darcy b) 11 x 106 darey c) 2 x 106 darey d) 544 darcy(3) Sandstones are usually cornposed of: a) feldspar b) mica c) quartz d) calcite(4) Shales have (high, low) effective porosity and very (high, 10) permeability.(5) Relative permeability equals: a) effective permeability/absolute permeability b) absolute permeability/effective permeability c) absolute permeability x primary porosity d) primary porosity/effective permeability(6) Determine the reservoir permeability for a gas-bearing formation with 0 15ctJo and Swi 20% using both the Timur equation and Morris and Biggs equation. k (Timur) = k (Morris & Biggs) = 10
  • 17. BASIC RESISTIVITY CONCEPTS 2INTRODUCTION ticular case may violate the rule. Figure 2.1 il- lustrates the various symbols and terms used. Resistivity Concepts in Well Logging Resistance is the property of a substance that of-fers opposition to the electrical current flow. Ohms Aslaw describes the behavior of electrical current flow Mut1.----.......- - - Cakethrough a material. hmLI E (1) r = Flushed Zone s;: Undisturbed Formation Invaded Zone .--Ri---t-.... • 8, ~ r resistance, ohm E electromotive force, volts current, amperes R - ResistiVity xo - Flushed zone Resistivity is a measure of the resistance of a given S - Saturation me - Mud cake s - Shoulder bed h - Thicknessvolume of material. me - Mud cake w • Formation water A i-Invaded zone d - Diameter R = r - L (2) t - Non-invaded zone FIGURE 2.1 R resistivity, ohms m 2/m Formation parameters. r resistance, ohms The flushed zone, next to the borehole, is created A cross-sectional area, meters/ by the mud filtrate passing through it during the pro- cess of invasion. The hydrocarbon saturation in the L = length of material, meters flushed zones is at a minimum, and all the virgin for- mation water is replaced by mud filtrate. The invaded In practice, the resistance of a certain volume of zone is that portion of the formation which has beenthe formation is measured. The volume of formation penetrated by drilling fluids. (Fig. 2.2)measured is a function of the configuration of the in- Moving through the flushed zone next to the wellstrument, which is a constant, therefore, the bore, deeper into the transition zone, water satura-measurement is expressed in terms of resistivity. tion can vary. In a water zone there is no change in The resistivity of any formation is a function of the water saturation, only a change in water resistivity oramount of water in that formation and the resistivity salinity. In a hydrocarbon bearing lone, theof the water itself. Ion-bearing water is conductive; hydrocarbon saturation is reduced in the flushedthe rock grains and hydrocarbons are normally in- zone and increases in the transition zone until thesulators. original saturation in the undisturbed formation isFORMATION FLUIDS reached. These changes in water saturation" combin- ed with changes in the resistivity of the fluids filling Changes in resistivity that occur in porous and the pores, create resist ivity pro files.permeable beds between the wen bore and the virgin In fresh drilling muds, the mud resistivity is nor-zone influence most resistivity logs. The following mally higher than the formation water, In a water-discussion assumes that some invasion occurs during bearing zone, the formation resistivity is higher in thethe drilling process. The depth of invasion is a func- flushed zone due to R lll f > R w and decreases withtion of the mud and formation properties. In general, movement out into the undisturbed formation. In alow porosity formations invade more deeply than hydrocarbon-bearing zone . drilled with fresh mud, high porosity formations. Explanations as to why the resistivity behind the flushed zone may be higher are, at best, rationalizations. In general, any par- or lower depending on the water saturation and the 11
  • 18. resistivity of the formation water. In Figure 2.3 the invasion profiles are indicated with the relative posi- tion of the deep, medium and shallow resistivity curves. This assumes the shallow resistivity curve reads mostly the flushed lone, the medium reads some of the transition lone and the deep reads mostly the undisturbed formation. With a salt water based mud, the flushed zone nor- mally has a lower resistivity. With the undisturbed zone, resistivity is either the same or higher, if the formation contains equivalent or higher resistivity water . The virgin zone will have a higher resistivity, if there are hydrocarbons. Notice that the resistivity curve positions arc reversed because of the reversal of the resistivity profile. STEP PROFIL.: The step profile of invasion assumes the simplest geometry between the invading mud filtrate and undis- turbed formation. This invasion profile consists of a cylindrical interface moving laterally into a porous and QOAl _ - - [ ...... ., ... permeable homogeneous formation. The diameter of Distance from Borehole this cylindrical interface is d i- The formation within theFIGURE 2.2 interface is the flushed zone, with resistivity R xo •All resistivity/saturation profiles relate to this model. Beyond the interface lies the undisturbed zone, with a resistivity of Rt • Figure 2.4 shows a schematic diagram for this invasion profile. a: • Fresh Mud } cf....-. _Fresh Mud v Invaded System rl Formation A Undisturbed rf Formation 1 rl ~adius 9f _I Invaslo,;-J --Distance - -......Salt Mud System A FIGURE 2.4 Step profile of invasion.FIGURE 2.3• Schematic resistivity log response. The transition profile of invasion assumes that a D - Deep transition zone exists in a porous and permeable M-Medium homogeneous formation between the portion of the S -Shallow formation flushed by mud filtrate and the uncon- 12
  • 19. taminated formation. The conductivity in the transi- profile exists. This invasion pattern is usually refer-tion zone is assumed to vary linearly between the con- red to as a low resistivity zone or annulus profile, andductivity of the flushed zone and the undisturbed for- its existence is an indication of moveable hydrocar-mation. (Figure 2.5) bons. The width of this transition zone is dependent Due to the current patterns of the logging devices,upon the type of formation, rate of invasion by mud the induction devices are affected to a greater degreefiltrate and the length of time the formation has been by the low resistivity zone. The low resistivity zoneexposed to the invading fluid. has a more severe effect upon the medium induction device. Computations show that in the case of a severe annulus, existing in a shallow to moderately cS invaded formation, the medium induction may A record lower resistivity than the deep induction J Undistrubed device when the ratio of Rxo/R t is less than five. Formation When ratios of Rxo/R t are greater than five, or deeper invasion exists, the effect decreases and the 0 1 - Inner Boundary of medium curve will record resistivity approximately Transition Zone equal to or greater than the deep induction curve. ~ - Outer Boundary Calculations show that the deep induction curve is only slightly affected by an annulus, and the record- "Co Translnon ed resistivity is only about lOC1Jo low for Rxo/R t ratios ~~ Zone ,~ ~ E of three to five and S% low for Rxo/R. ratios greater v LL 0 than five. Figure 2.6 shows a low resistivity annulus V u.. 01 profile. Induction devices are covered in detail in J Chapter 6. D i s t a n c e - - - - - - - -.. 01 - Inner Boundary of cf Invaded V Transition Zone Formation 02 - Outer Boundary r! Rxo Undisturbed -05 Formation ~; U)«S ~ s 2E Low Resistivity Zone :; LLO LL iii Q) a: Transition Undisturbed Zone 0 1 O2 rf Formation O2 ~ 1.4 0, 1 rl 0, - - Distance - - - - - - - - . II---Distance----.. FIGURE 2.6 Low resistivity annulus profile of invasion.FIGURE 2.5Transition profile of invasion,ANNULUS PROFIL.: Since it is possible for mud filtrate invasion 0 f ahydrocarbon-bearing formation to create a zone offormation water which is displaced ahead of the inva-sion front by a process of miscible drive, considera-tion must be given to the response of the loggingdevices in the event this particular type of invasion 13
  • 20. QUESTIONS 2(1) The resistivity of the formation is a function only of the resistivity of the formation water. a) True b) False(2) A step profile of invasion assumes -- a) invading fluids are intermixed with the formation fluids, b) invading fluids are never mixed with the formation fluids. c) invasion seldom occurs and, as such, need not be considered in regard to resistivity devices(3) In a transitional profile the width of the transition Lone is assumed to vary only with the type of formation. a) True b) False(4) When an Annulus invasion profile exists and RXll/R. ratios are greater than five, a) deeper invasion exists b) the medium curve will record a resistivity approximately equal to or less than the deep induc- tion curve c) the medium curve will record a resistivity approximately equal to the deep induction curve d) the deep curve will record a resistivity approximately equal to the medium induction curve or less than the medium induction curve 14
  • 21. FORMATION FLUID PROPERTIES 3FORMATION WATERS the same water in a reservoir at a higher temperature. Subsurface waters represent a diversity of sources. Likewise, water-base drilling mud changes resistivitvThey are mixtures of newly formed waters, at- with depth, due to the change in temperature. Figur~mospheric waters, ocean waters and waters produced 3.1 shows how to correct the resistivity of mud, for..from diagenetic reactions. The history of any specific marion water, etc. for the effects of temperature.sample is exceedingly complex. Throughout geologic When the temperature of a solution is increased it istime, formation waters have undergone continuous assumed that the salinity stays constant, makingmodification by filtrations through clays, by ion ex- resistivity t he only other variable.change, by precipitation of minerals and by reactionwith rock matrix and other fluids. Example: Draw a straight line on Figure 3.1 from 139 0 and a resistivity at 0.06 Q-lll; the salinit v of this The initial fluid in most sedimentary rock was sea mixture is 62,000 ppm as shown on the center stringwater. Although it cannot be assumed that the com- of the nomograph. An increase in temperature toposition of sea water has remained constantthroughout geologic time, most data suggest that its 179 0 (the salinity stays constant so the line still goescomposition has not undergone significant change through 62,000 ppm) changes the resistivitv to 0.046over the past few billion years. Q-m. All aqueous solutions are handled in the same manner. The salinities shown are for sodium chloride The salinity of subsurface waters generally in- solutions, but the nomograph works well for mostcreases with depth. Reversals, however, are not un- practical well Jogging applications.common. Considerable variation within the same In many cases. brines are encountered with totalformation may occur within a basin. Studies ofcementation in sandstones indicate that significant solid concentrations composed of ions other than Nasalinity variation can occur over very short distances, and Ct. To accurately correct the Rw R In or Rmf" • • ,U~-both horizontally and vertically. Filtration through tng FIgure 3.1. the total ionic concentration must beclay membranes appears to be one of the key expressed as equivalent NaCl concentration. Fiauremechanisms capable of producing the gradients 3.2 is used to correct ionic concentrations toobserved. equivalent NaCI concentrations. The density of water depends upon its salt content, Example: If a brine with 50,000 ppm total solidstemperature and pressure. The specific gravity of a which includes 10,000 ppm Nat J6,000 ppm Cit 7,000 ppm Mg, 5~O(){) ppm Ca and 12,000 ppm S04 is en-substance is the ratio of its density to that of water at countered, Chart 3.2 can be used to determine aspecified temperatures. The density of water value for equivalent NaCI concentration to enter indecreases with increasing temperature, but increases Chart 3.1. From the chart read K -= 0.92 for Mg , atwith higher total solid concentration and pressure. 50,000 ppm, K ::: 0.18 for Ca at 50,000 ppm, and All porous rocks contain some water. Bv virtue of K = 0.36 for S04 at 50.000 ppm, noting thationized salts contained in solution, these -fonnation K = ).0 for the Na and C } at 50,000 ppm. Then add ..waters are electrically conductive, exhibiting (10,000 xl) + (16,000 xl) ,. (7,000 x 0.92) +resistivities ranging from 0.02 Q-m to several ohm- (5,000 x 0.78) + (12.000 x 0.36) -= 40,660 ppmmeters at formation temperature. The predominant equivalent NaCI. This value is used at the ppm NaCIsalt in these solutions is sodium chloride. Resistivity entry in Chart 3.1 when correcting R w R In or R mt. forof such an electrolyte normally decreases with in- the temperature.creasing salt concentration due to the higher amountsof ions which carry electric charges and highertemperature which affects the mobility of the ions.The resistivity of formation water may be determined OIL & GAS PROPERTIESby direct resistivity measurement on a sample, Specific gravity of oil is related to its API gravitychemical analysis, or an estimation of R or by the relationship:equivalent NaCI (in ppm) from well logs. The resistivity of an aqueous solution, which is water plus a salt such as sodium chloride. varies with 141.5 temperature . The resistivity of a water measured at p (1) °API + 13].5 surface conditions is different from the resistivity of 15
  • 22. T, (OF) 450 400 Aw.R m or Rmf (Q-m) NaCI 10 300 9 275 (ppm) 8 7 200 6 250 300 5 400 4 500 225 600 3 700 800 200 900 2 1000 190 180 2000 170 1 0.9 160 3000 150 4000 0.7 5000 140 6000 0.5 7000 130 8000 0.4 10.000 0.3 120 110 20.000 0.2 30.000 100 Equivalent NaGI in ppm 40.000 1.645 x ppm GI = ppm NaGI 50.000 0.1 90 60.000 0.09 80.000 0.07 100.000 80 0.06 150.000 0.05 200.000 0.04 70 300.000 003 60 0.02 Equations: 0.01 50 0.009 0.008 0.007 Salinity (ppm at 75°F) = 10x 3.562 - log (RW75 - 0.0123) x = ----~----- 0.955 3647.5 Rw = 0.0123+ --- 75 [NaCI (ppm)]0.955FIGURE 3.1Resistivity of saline solutions. 16
  • 23. +2.0 ...- ....- . . . - - - - -....----------------------------~- I - - - -.. - ..... Mg .....- . , I I .- --._. - J. I I I / .. I .. -- :/ I "Mg Ca •• -....... __ • . _.·A~ . ,/r- ... ,/ .. .. .. . . . .~ <, / . . . .,........ _... ----------.1 Na and CI~ +1.0 ~---~ , - .... -...... ..........~......~...----~~ ~..... -------..---------~-~...... - - - - - -......3::E -- ---- -- K -- ~ .. ~~ .. - ............ -. "~ ........ Ca ... ~ - ...".". "-" .. . . .---._----- .............. ---"- - .- ...... HCOa - .-- ....... - HC~ . ~ . " .-i..:. O... ..._ ...._ .........__.....lI-A. . . . . . . . . . . ._ - . . . _. . . . . . . .--A............~ ....._ . . . ._ . a . _......................_..._j~ . . . . 100 1000 10.000 100.000 200,1 Total Solids Concentration. ppm FIGURE 3.2 Equivalent NaCI concentrations from Ionic concentrations. where p is the specific gravity of oil at 60 OF. When 1.0 dissolved gas is present in oil, the specific gravity of the mixture decreases as the gas/oil ratio increases. M Figure 3.3 can be used for determining reservoir den- E 09 ~ sity of oil in g/cm for a known value of GOR. ~ Figure 3.4 shows the variation of specific gravity of <5 0 0.8 oils with temperature. Dry gas density as a function ?: of reservoir pressure and temperature is illustrated in iii c Q) Figure 3.5. C 0.7 ~ The viscosity of gas-free crude oil also decreases 0 e with temperature. (Fig. 3.6) From a knowledge of (J) en 0.6 (1) crude oil °API gravity and formation temperature, a: the viscosity of gas-free crude oil can be determined. The amount of gas dissolved in oil has an important 0.5 10 20 30 40 50 60 bearing on viscosity at reservoir conditons, figure Oil Gravity (OAPI) 3.7 shows how to correct the viscosity of dead oil, at reservoir temperature and atmospheric pressure, to Example: °API qravitv » 39 the viscosity of gas-saturated oil for the known GOR GOR = 1000 at reservoir conditions. Oil Density = 0.66 g/cm 3 FIGURE 3.3 Reservoir oil density as a function of oil gravity (OAPI) and gas·oil ratio (GOA). 17
  • 24. 400 100 200 300 400 500 600 700 800 900 1000 1 - Temperature In Degrees Fahrenheit Example: The specific gravIty of an oil at 60°F IS 0.85. The specific gravity at 10QoF = 0.83. --. ..... -- -- ~H6 = Ethane C:3Ha = Propane 20 30 40 50 60 C4 H 10 = Butane Crude Oil Gravity (OAPI) at 60°F and Atmospheric C4H 10 = Isobutane Pressure, C4H 10 = IsopentareTo find the weight densrty of a petroleum oil Ibs/ff~ at its FIGURE 3.6flowing temperature when the specific gravity at 60°F is ViSCOSity of gas-free (STO) crude oils.known. multiply the specific gravity of the oil at flowingtemperature (see chart above) by 62.4 lbs/It: the weightdensity of water at 60° F.FIGURE 3.4Specific gravity-temperature relationship for petroleum oils 100 .......- - - - , , - - - - - - - - - . . . . . - - - - - - - - - 80 60 Q) ~ 40 tJ) 0 C/) ~£ -c: 20 2000 ==0 o ~ 1O~---I__------_lfA__.........__,... ~OOg (j) 4000 ~~ ~ .>.. ~ ;j au ~ s 4 ~~ ,,~ C/) C/) Q) 6000 I ~ Ii: ~ ~ 2 ,.(o.... SOlution Gas/Oil 8000 C)Qi ,0".1 Ratio CU ft/bbl - a. -,,fOOo ~~ 1 t-----~~~ ......,........I«-" 00°;-.---------4 10,000 :~ .... 0.8 ",,~ 0 0.1 0.2 0.3 8 6 0.6 rz,r:fJ Density of Dry Gas (g/cm 3 ) 5 ~ 0.4 C/) ,,~$ ",~ C2 ~GOOFIGURE 3.5 ~O.2Density of dry gas as a fu nction of reservoir pressure andtemperature. Viscosity of Dead Oil (cP) (at Reservoir Temperature and Atmospheric Pressure) FIGURE 3.7 Viscosity of gas-saturated crude oils (J,l) at reservoir temperature and pressure. 18
  • 25. SUBSURFACE PRESSURE REGIMES PFG = 0.433 x p (3) Hydrostatic pressure is caused by the unit weight where p is the specific gravity of a representative col-and venical height of a fluid column. The size and umn of water.shape of this fluid column has no effect on the Overburden pressure originates from the combinedmagnitude of this pressure. Hydrostatic pressure, weight of the formation matrix (rock) and (he fluids (water, oil, gas) in the pore space overlying the for-P H• equals the mathematical product of the average yfluid density and its vertical heights, such as: mation of interest. Mathematically, the overburden pressure can be expressed as:P Hy = P X g x D (2) P = weight (rock matrix + fluid) (4) o area where: p average density g gravity value D height of the column Generally, it is assumed that overburden pressure increases uniformly with depth. For example, The hydrostatic pressure gradient is affected by the average Tertiary deposits on the U .5. Gulf Coast,concentration of dissolved solids (i.e., salts) and and elsewhere, exert an overburden pressure gradientgases in the fluid column and different or varying of 1.0 psi/ft of depth. This corresponds to a force ex-temperature gradients. In other words, an increase in erted by a formation with an average bulk density ofdissolved solids (i.e., higher salt concentration) tends 2.31 g/crrr. Experience also indicates that the pro-to increase the normal pressure gradient, whereas in- bable maximum overburden gradient in clastic rockscreasing amounts of gases in solution and higher may be as high as 1.35 psi/ft.temperature would decrease the normal hydrostatic Worldwide observations over the last few yearspressure gradient. For example, a pressure gradient have resulted in the concept of a varying overburdenof 0.465 psi/ft assumes a water salinity of 80,000 gradient used for fracture pressure gradient predic-ppm NaCI at a temperature of 77 of (25 O(~). tions in drilling and completion operations. Typical average hydrostatic gradients which may Formation pressure (P f) is the pressure acting uponbe encountered during drilling for oil and gas are the fluids (formation water, oil, gas), in the poreshown below: space of the formation. Normal formation pressures in any geologic setting will equal the hydrostatic head Equivalent Total (i.e., hydrostatic pressure) of water from the surfaceHydrostatic Mud Wt. Chlorides Basin to the subsurface formation. Abnormal formation Gradient ppg ppm Location pressures, by definition, are then characterized by any departure from the normal trend line. 0.433 8.33 Fresh Rocky Formation pressures exceeding hydrostatic pres- water Mountains sure (Pr > PH,,) in a specific geologic environment 0.442 8.5 20,000 Beaufort, are defined as abnormally high formation pressure Brunei, (surpressures) whereas formation pressures less than Malay, hydrostatic are called subnormal (subpressures). Sverdrup, These subsurface pressures and stress concepts are N. Slope in related. Alaska In normal pressure environments (PI = PH ) , the (most of . ~ matrix stress supports the overburden load due to worlds grain-to-grain contacts. Any reduction in this direct basins) grain-to-grain stress (0 -+ 0) will cause the pore fluid 0.452 8.7 40,000 North Sea, to support part of the overburden, the result being Delaware abnormal formation pressures (P f > PH,,). In other (older words, the overburden may effectively be "floated" portion- by high formation pressures. prePenn.) There are numerous factors that can cause abnor- 0.465 9.0 80,000 Gulf Coast mal formation pressures, such as surpressures and 0.478 9.2 95,000 Portions of subpressures, Frequently, a combination of several Gulf Coast causes superimposed prevails in a given basin and as such is related to the stratigraphic, tectonic and In general, the hydrostatic pressure gradient (in geochemical history of the area.psi/ft) can be defined as: In most cases, abnormal pressures are caused by 19
  • 26. early formation of impermeable barriers prior to BIBLIOGRAPHYcompaction and consolidation or other geologic anddiagenetic processes. Figure 3.8 shows subsurface Beal, C. The Viscosity of Air, Water, Natural Gas,pressure concepts. Crude Oil and Its Associated Gases at Oilfield Temperature and Pressure. A/ME, Vol. 165, 1946. Pressure (1000x psi) 2 4 6 O---.--......-----.-........- -.........----i---.---.---- Brown, K.E. The Technology of Artificial Lift Methods. Tulsa, Oklahoma: The Petroleum Publishing ce., 1977. 2 Dresser Atlas. Log Interpretation Charts. 1980. Fertl, W.H. Abnormal forma/ion Pressures. New £ 4 York-Amsterdam: Elsevier Scientific Publishing Co., o 8 1976. s: 0.. 6 Q) Katz, D.L., Cornell, D., Kobayashi, R., Poetman, c F.H., Vary, J.A., Elenbaas, J.R., and Weinaug, Surpressures C.F. Handbook of Natural Gas Engineering. (Abnormally 8 McGraw-Hill Book Co., 1959. High Pressure) Schowalter, T.T. Mechanics of Secondary Hydrocar- 10 bon Migration and Entrapment. AAPF Bulletin, May 1979.FIGURE 3.8 Standing, M.B. Volumetric and Phase Behavior ofSubsurface pressure concepts. Oil Field Hydrocarbon Systems. New York: Reinhold Publishing Corp., 1952.FLUID SATURATION DISTRIBUTION Fluid saturation is the percentage of the porosityof a rock occupied by a specific fluid. For example, awater saturation (Sw) of 50070 means that half thepore space is filled with water. In a water-oil system100070 - S" = So (oil saturation). The same logicwould apply to a water-gas system. Water saturation, a key parameter determinedfrom well logs, controls the reservoir productionbehavior and the amount of hydrocarbons in place.An oil reservoir at irreducible (non-movable) watersaturation produces water free, but with increasingwater saturation the percent of water production willincrease. There is no unique, single water saturationvalue to use as a cutoff for commercial or water-freeproduction. Each reservoir has its own uniquecharacteristics. 20
  • 27. QUESTIONS 3(1) What is the API gravity of an oil whose specific gravity is 0.85 g/cm 3? a) 65 b) 35 c) 40 d) 50(2) Viscosity of gas-free crude oil (increases, decreases) with increasing temperature.(3) Higher salt concentrations tend to (increase, decrease) the normal pressure gradient, whereas in- creasing amounts of gases in solution would (increase, decrease) the normal pressure gradient.(4) If Sw = 720/0 So = ---(5) If the total solid concentration of a Cae) water is 5,000 ppm, of which 2,500 ppm is Ca, and 2,500 ppm is CI, determine the R w at 160 OF. a) 0.06 b) 0.4 c) 0.6 d) 0.03 21
  • 28. HOW SUBSURFACE TEMPERATUREAFFECTS FORMATION EVALUATION 4INTRODUCTION 0-------------------..., Subsurface temperature normally increases with 2,000depth. The rate of increase with depth is called thegeothermal gradient: 4,000 GO = 100 (T f - T m)/D (1)where: GO = geothermal gradient, °F/lOO ft Tf = formation temperature, OF 16,000 Tm = mean surface temperature for a 18.000 given area, OF 20,000 22.000 D = depth of formation of interest, ft 24,000 Equation 1 can also be rewritten as: 26,000 E 80 ~~1 2 0...... ~~ 16~0~~~2~40~2~8~0~3~2~0~3~6~0---.~--~--- 440 60 100 140 180 220 260 300 340 420 Tf = T m + GG(D/l00) (2) Mean Surface Temperature. OF Temperaturewhich allows an estimate of formation temperature FIGURE 4.1provided the geothermal gradient and mean Estimating formation temperature.temperature for a given area are known. TABLE 4.1 Example: Using Figure 4.1, enter the bottom of the Heat Conductivity Valueschart with the bottom-hole temperature obtained Approximate Heat Conductivityfrom the log and proceed vertically to an intersection Material (meal cm- 1 see- 1 0c- 1) - - -with the value representing total depth. Use the scale Gas 0.1which most closely represents the proper mean sur- Oil 0.3face temperature. Water 1.4 From this point proceed diagonally, parallel to the Clay 2.4gradient lines, to the depth of the formation of in- Quartzite 6.8 - 18.9terest. Project this point vertically downward to readthe formation temperature. Furthermore, effects of local structures, salt domes, For example, assume total depth = 10,000 ft; bot- overpressure, etc., are important factors.tom hole temperature = 180 of; mean surface Formation temperature and heat conductivity is antemperature = 80 of and depth of formation of in- important parameter in formation evaluation, sinceterest = 6,000 ft. The temperature of the formation all resistivity data are temperature-dependent.at 6,000 ft is 140 of. Heat conductivity decreases hyperbolically with For older, well-drilled regions maps are available temperature. Thermal conductivity of water does notwhich show the geothermal gradient for the area. change appreciably with increasing salt concentra-However, since the geothermal gradient depends on tion, and the effect of pore fluids on gross conduc-the thermal conductivity of the rocks (Tab. 4.1) the tivity is relatively small for rocks of low to moderategradient is seldom constant. porosity. 22
  • 29. Abnormal Top abnormal Abnormal Fcrmauo« Pressure Formation Pressure Formation Pressure 8 3 -= 8 q ~T = 5.2 °F/1oo ft ~ 10 3 5 a CD a 12 ...... ~ ......_ ...._ ......._ _ 140 160 80 100 120 110 130 150 Temperature, of Temperature. 0 F Temperature, of A. South Texas B. North Sea C. South China SeaFIGURE 4.2Flowline temperature in normal. overpressured zones. However, with higher pore pressures (over- tion as to mud circulating time and time that the log-pressures), porosity is also higher which accounts for ging device was last at the bottom of the wellbore.a higher fluid content. This is similar to the observa- At times, the same BHT is recorded on differenttion that the thermal conductivity of clays (shales) logs run to the same depth on the same date in thevaries inversely with their water content. same well. This suggests that a BHT was measured In other words, geothermal gradients are larger in on one log run only. However, this will precludemassive shale zones overlying reservoirs and very determining formation temperature by the methodmuch reduced in the aquifer formations. Over- proposed.pressured, high-porosity shales represent a geother- The use of a maximum recording thermometer onmal gradient anomaly. This concept is the basis for each logging run should be the responsibility of theflowline temperature measurements at the rig site as a operator and logging service company if the operatorsupplementary overpressure indicator. wants this information. Changes in flowline temperature gradients of up to Since the rise in temperature is similar to a rise inlO°F/lOO ft have been observed prior to and/or pressure, it has been proposed that BHT data may bewhen entering overpressure intervals. (Fig. 4.2) analyzed in a manner similar to the Horner-typeHowever, since this pressure indicator is also affected pressure buildup technique. A detailed mathematicalby lithology, circulation and penetration rate, trip- study showed that under most practical field condi-ping the drill string for bit changes, length of the tions this method will give reliable estimates of true,riser, etc., certain precautions and refinements in use static formation temperature. Only the circulationand interpretation of such data must be considered. times for well conditioning in excess of a day or more would lead to static temperature estimates somewhatTEMPERATURE FROM LOGS lower than the actual temperature. The basic concept is the straight..l ine relationship Measured subsurface formation temperature on semilogarithmic paper of BHT in OF (from wellavailable from open hole well logs, or listed on the log heading) vs the ratio of ~t/(t + ~t), wherelog heading, is always lower than the true or static ~t = time in hours after circulation stopped;formation temperature. t = circulating time in hours for well conditioning. Due to the cooling of formations while drilling, or Then, extrapolation of this straight-line to a ratiocirculating drilling mud prior to logging, the record- of ~t/(t + .6t) = 1.0 will determine the true staticed bottom-hole temperature (BHT) may be 20 of to formation temperature.80 OF lower than the actual formation temperature. For example, after drilling a borehole to 10,000 ft Since true or static formation temperature is an im- the operator circulated six hours before pulling theportant parameter in exploration, drilling, logging, drill pipe. From the time the bit was pulled off bot-well completion and reservoir engineering, a method tom until the Induction-Electrolog" started out ofhas been developed which permits the determination the hole, another seven hours elapsed. The maximumof static formation temperature based on the max- recording thermometer showed 200 OF at that time.imum recording thermometer (BHn data recorded Similarly, the Compensated Neutron-Densilog"during all routine logging operations. combination was pulled off bottom after another The recommended technique requires the use of four and a half hours elapsed. The maximum record-BHT data on each logging run, including informa- ed temperature from this run was 225 OF. 23
  • 30. TABLE 4.2 Log- derived Depth, It Logging Runs Computations of Dimensionless Time BHT, of 10.000 I. IEL Llt I(t + ~t) = 7/(6 + 7) = 0.538 220 10.000 II. CNLog -COL 6t1(t + ~t) = (7 + 4.5)1(6 + 7 + 4.5) = 0.671 225 10.000 III. AL At/(t + At) = (7 + 4.5 + 8)1(6 + 7 + 4.5 + 8) = 0.765 228 After some discussion with the district office by . - - - - - - - - - - - - - - - - - - -...... 290radio, it was decided to run an Acoustilog" survey 284°F at ..which began when the instrument was tinally pulled off 12.548 ft »> bottom eight hours after the Compensated Neutron- Densilog" service combination left the same point in 2 ~---~at~/280 ~ 6,518ft /the borehole. The temperature recorded at the time was ~ I228°F. / 270 To determine the true static formation temperature / /from this formation,(Table 4.2), plot the BHT datavs the corresponding dimensionless time on Figure4.3. Note that the true static formation temperature Ie 260 LL o ! ::I ~is 235 OF at the depth of 10,000 ft. • CD 0. E CD t- CD 235°F 250 (3 , Static s: I E BHT at o ,. 10.000 ft t: , S ,, I 240 , I , I ,, ,I 230 • 230 u, 0 225 ~ ::I n1 0.1 0.5 1.0 Ci> a. Dimensionless time, ~U(t + LJ.t) E Q) t- FIGURE 4.4 220 Extrapolation technique. exploration concepts, is obvious. In the high temperature well (curve 1) four logs were run to 6,518 ft. The first log, four hours after mud circulation stopped, recorded 218 of. Straight-line extrapolation 0.2 0.5 to infinite time (log 1.0) indicates a bottom-hole 6t/(t + ~t) temperature of 281 of. FIGURE 4.3 Example calculation. In the deep Texas onshore well, (curve 2), three logs were run to 12,548 ft. The first recorded Figure 4.4. illustrates the extrapolation technique temperature was 272 OF, while actual stabilized BHTfor true static formation temperature in two wells is 284 OF.which have been drilled in different geothermal Figure 4.5 shows borehole temperature variationsregimes. Well No. 1 is a high temperature well with time and the true static formation temperatureslocated in the South China Sea; well No.2 is a deep in eight South China Sea wells. Note the drastically hole drilled onshore Texas. different temperature regime encountered. Such in- The drastic difference in the geothermal gradients, formation is of great importance in the search forin the two wells and the impact on completion and either oil or gas. 24
  • 31. South China Sea Area 320----------------~ 300 250 351---_ _"""-!-_ _.....&-- ..............--_~u, 1.0 12 1.4 1.6 1.8 2.0o Thermal Gradient. °F/l00 f1eli:;(ti FIGURE 4.6Qi Physical state of oil and gas.a.E~ in regions of known or suspected overpressures, since 200 recent experience indicates that steeper than normal temperature gradients do occur. Figure 4.7 shows such a field case from Offshore Louisiana, and 2 3 160"__ ... 4 0.2 03 0.4 0.5 06 0.7 0.8 0.9 1.0 5 Hours After } Stopping Time. ~t/(t + 6t) 6 CirculationAGURE 4.5 7 : 8 Mud Weight. .~Borehole temperature variations. 1b/9~ 1i __ 6 a 9 -13.5 ~ HYDROCARBON DISTRIBUTION ~ ~~ ~ 1°FI1 00 ft The wide range of geothermal gradients from area 12 -<---J <, -13.8 Top S~perto area over a continent makes the selection of a 13 -17.2 ...universal "normal" gradient impractical. ~~ -17.8 . +--{) I , , , , I , , 6 , In drilling for oil and gas, temperature gradient ex- 0.2 0.3 0.507 1 2.0100 200 300 Maximumtremes of 0.3 °F/IOO ft to 6 °F/lOO ft have been en- Formation Temperature. ofcountered. However, gradients of 1 °Flloo ft to 1.7 of1100 ft (3.1 °CI 100m) are found most frequently. FIGURE 4.7 For example, extremely deep wells (24,000 ft and Offshore Louisiana well.deeper) in Beckham County, Okla., have an averagegradient of 1.5 °F/IOO ft. A 28,500 ft well on the Figure 4.8 illustrates the temperature gradient changeRalph Lowe Ester, West Texas, has an average 1.4 in a deep Mid-Continent well.of/tOO ft temperature gradient. Temperature is the key factor for hydrocarbon Average gradients in the South China Sea area are transformation. Oil reservoirs can exist at greatas high as 2.5 °F/IOO ft to 3.5 °F/too ft. depths if the temperature gradient is low. In the study Landes (1967) proposed a universal relationship of of certain areas along the U.S. Gulf Coast, Timkooil and gas distribution with depth and temperature. and Fertl (1970) found essentially normal pressureLandes chart (Fig. 4 . 6) closely agrees with oil and and temperature gradients continue to great depths,gas production in several basins around the world. and concluded that such deep horizons are still highlyHowever, the author has reservations about Landes prospective for large oil accumulations.statement that most temperature gradients becomestraight lines below 3,000-4,000 ft and that this gra- HYDROCARBON ACCUMULATIONdient, once stabilized, can be extrapolated downwardduring drilling to forecast the content of prospective Together, formation temperature and pressure arereservoirs. the important factors which significantly affect the Cautious application of the chart is recommended physical state of hydrocarbons at depth. Both parameters can be determined from well logs. 25
  • 32. 10 60 Overpressured U.S. Gulf Coast Wells in Figure 4.9 14 -= ~ 18 ~ ~ Q. CD c 22 26 100 200 300 400 Temperature. ofFIGURE 4.8Midcontinent well. 16 Recently, an important pressure-temperature rela- 18tionship in about 60 U.S. Gulf Coast wells has beenstudied. Figure 4.9 shows the temperature-pressure 0.5 0.6 0.7 0.8 0.9 1.0relationship in these wells, with average geothermal Pore Pressure Gradient. psi/ft FIGURE 4.10 60 Overpressured U.S. Gulf Coast Wells o Pore pressures. 2 4 These aquifers usually contain rather fresh waters 6 with some gas in solution as well as zones with non- =:: 8 8 commercial oil and gas shows. ~ 10 £ 12 60 Overpressured U.S. Gulf Coast Wells in Figure 4.9,4.10 Q. ~ 14 16 -= ~ 1.3 a. 1.2 18 C Aquifers 20 CD 1.1 =0 (With Gas in Solution) (1 1.0~ 80 120 160 200 240 C5 ~ 0.9 .... CD Temperature. OF =:; CI) 0.8 ~-- 0FIGURE 4.9 CD ~Borehole temperatures. a.. 0.7 CD Mostly 0 0.6 a.. 0.5 Gasgradients ranging from t of/tOO ft to about 1.8 80 120 160 200 240 280 320 360 Temperature, of °F/l00 ft. Figure 4.10 shows the increase in abnor-mal formation pressures with depth for the same FIGURE 4.11wells. Pressure gradient, formation temperature. The most important findings, however, are shownin the crossplot of formation pressure gradient and BIBLIOGRAPHYtemperature. (Fig. 4.11) Note the bellshaped hydro-carbon envelope covering pressure gradients from 0.5 Dowdle, W.L., and Cobb, W.M. Static Formationpsi/ft to as high as 0.94 psi/ft and corresponding for- Temperature From Well Logs-An Empirical Method,mation temperatures between 90 and 365 OF. Journal of Petroleum Technology, November, 1975, Also, with further increase in temperature, the pp. 1326-1330.magnitude of overpressure gradients reverses itstrend and declines in potential reservoir rocks, coin- Dresser Atlas, Log Interpretation Charts. 1980.ciding with a simultaneous increase in the presence ofgas. Fertl, W.H. Abnormal Formation Pressures- At the same time, extremely high pressure gra- Implications to Exploration Drilling, and Productiondients are encountered in high temperature aquifers. of Oil and Gas Resources. Amsterdam-New York: 26
  • 33. Elsevier Scientific Publishing Co, 1976, pp. 325.Fertl, W.H. How Subsurface Temperature AffectsFormation Evaluation, Oil and Gas Journal, 1978.Fertl, W.H., Pilkington, P.E., and Reynolds, E.B.Evaluating Overpressured Formations in the SouthChina Sea. Petroleum Engineer, April, 1975, pp.40-47.Fertl, W.H., and Timko, D.J. How DownholeTemperature, Pressures Affect Drilling. Ten PartSeries, World Oil, June J972 - March 1973.Fertl, W.H., and Wichmann, P.A. Static FormationTemperature from Well Logs. Dresser Atlas,Technical Memorandum, April, 1976.Landes, K.K. Eometamorphism and Oil and Gas inTime and Space. AAPG Bulletin, 1967, Vol. 51, PP.828-841.Timko, D.J., and Fertl, W.H. Hydrocarbon Ac-cumulation and Geopressure Relationship andPrediction of Well Economics with Log-ealculatedGeopressures. SPE 2990, 45th A/ME Fall Meeting,1979. Also Journal of Petroleum Technology, 1971,Vol. 23, pp. 923-933.Wilson, G.J., and Bush, R.E. Pressure PredictionWith Flowline Temperature Gradient. Journal ofPetroleum Technology, 1973, Vo1. 25, pp. 135-142. 27
  • 34. QUESTIONS 4(1) A well located in the Gulf Coast has an 80 OF mean surface temperature. The BHT at 16,000 ft is 240 OF. What is the geothermal gradient? a) 0.89 b) 1.10 c) 1.0 d) 0.75(2) Using the data in question 1, what would the temperature be at 18,000 ft if the geothermal gra- dient remains constant? a) 30soF b) 260°F c) 220°F d) 244°F(3) If an area has a geothermal gradient of 1.45 and a mean surface temperature of 72 OF, what would the static formation temperature be at 8,000 ft?(4) You would expect the measured BHT during the logging operations to be than the true static formation temperature. a) higher b) lower c) the same(5) Given the following information, determine the static formation temperature. 8:30 a.m. - Circulated six hours before pulling pipe, TO = 12,000 ft 10:00 a.m. - Logger on bottom with 1st log run, BHT = 215° 2:30 p.m, - Logger on bottom with 2nd log run BHT = 223 0 8:00 p.m. - Logger on bottom with 3rd log run, BHT = 226 0 a) 2270 b) 220 0 c) 235 0 d) 230° 28
  • 35. t ! i f7 -t-1-.--;i C .... -r ~ : -.- =-rt-Tt--: L-:-: -v- +. ::T1-r-.:: ~ -, :1-:: r ~ !: ~ : ..... =...t.--. t i4-.... 1 :: n- ~ :~~; _= L i : : -. t : ~ -. - =. :-0 -0-, t : : : : : · ........ . + - i -or- -. . . • . , + •• ~ . . t - te -:+ i:: ! + ~ __• ~:£-.- -..- - ~ ;::;- ~_-=-:-: - -- - 1 i; 0 • .• t t ~ • • • to. , . -+:=1 I ~- ~-: + , -- t - : : + • 1 .. • ; : • l -: +-- - . . ; .. t ; . . t -+ - ... ~ : -: i .-- . . 0 : ; : , . .....!5~~---+------~-+------+----......-f------+-------+-----+------+-----~----+----~~----+-----+------4 --..- ....- .. ~+-.=;-: --0 . - .-:- • ..:-+: ~ -+ . -.-.- - 1 ... 1 _.-.--+ ----.- - . ~ - ~-.......--- , + : ~+- _ --- :t::r-: -L+.~ - ~ -1-::- · . - : ~ : :- t-: ; 1: i -: :: : ~ --+ t--+ -+-- 4 -+-4..-........-.----+-----+-----+----~~--- +- ~ - • --+ - -- t . . . . .- ----f-.. . . . .- --+--"""-----+-----1------+----T:_7 ~: -=- :--1 -?-- C = -: f ~ : : ! . - .- +- .... + • • =i -:--- -- :-. : ..=1---- -- ..- ~:-- .. - -:---:- i _ 0- +- 1 + , + - .. t •3t------+-----t-----t-~---__1~---_+----_+__----+_---__+----------+_----------+-------------4 + .- ... ~ .. ~ l + - . - t t- +- -- _.- ... ~ --~~ ~: 4: ·- : t .. t t-- + + • t - r +- 0- ., .. - t- -4 + +- • ~ --- .-. +-J~~:- 4--+-- - ---. -~ .. t ~ .. f !- -~ - .- :-1 ~ .-2 _4 __ +_... _. ~--: -: .-1 .: -: --. ~ r • + r.; .--: -=- , . t . - -: .. -+.- ~ + -4 ,---+ - t -. ~ 1 t i - ~ _ .. -+ ..... • -. + r- .-= ~. J - -+_ ~- • + I L I- T - - - f . -1 .- I J- : ~.! :- 6 •• f 1- i i:1-r- r! ------ Li l i , i-+- • T i ~ : ~ r~· :. -t- - ----4 - + -- • f-· + +- i ~ : ~--t .. -=-:-= ~ =- • =--. ~ .-: ~ .- - =- =--. - L .; _ --+ + I -+- .-+ -r-- ! .-. -~ ~ :-~ L:- ~ - .... - + - -0 +- .-! ..- - - .-... t - t-------+-~-- +--t --..--+-1- - --- ---. - ..... --.-..-.... .. t --~---- . ..-. -~ ~ +-- - -+-.,...- ~ ... + - - .~ r _. _ ._ t . . _ t _ ; ! 1 +2t----..--+-----+------+----~~----+---...,-----+-----~----+---- t . - f .-~ . .... j t .. ... t - 1 -0. i .. - I~ .-+ ~ t ----~---~~--~-+------~---~ ~ r : 0- :- t:::-:-:- .--~ {- . .. . , 1 t t - - -. - -. · -J : t t • ~ ,- , - +- -f - ; t .. t- . -.... it • - + +- +--_ -- ~ ,.- 0- ... _0-:_-=-[1 ~_ ~ t • .,I , t - + -+. 0 - -- --t -. -- - -t - ·- . --+ _:, t ~~l-:.- . - -- -- :-- t-: -- -. ~ t - -- t t -~-: . . ~- .. •• - +- +- --i • l .- ~ ..... ...- :- t • : - : -- : +- + + . - - .. ....-..- - -·-t .. • + rtf 1t - ; , t t t t t t , I .. • t .. • - ~ . 1 j -- : t I t ~ f , + t 29
  • 36. ... ~ 1 w lao (JI ca " CD rT--rir-Ti-"""""7""i-r.r-",,!,:ii---:~T. ""1----r.---"rriTil","",;·",,";irrl-rir. riiFiini"TI--r-rt,niT,;"""".Ti"IIT!~IT"jT1llrrllrrr"~llml~iT:I.JT1i1rnll"TIilmllmllirrrlllrT"i C) - N w • (JI ca ~ rl,--:-:Tj-j--jrT"1II"Tj"Tj"TI"TjII"Tj""TjiITI-rji,-rj"""Tj~jl""T"l..,..-rj-j"jl"T""iIrotIIMIMjnt--rI-· j!"TITjT"I"~IT"II""I""IIITIT"I;II;""·TTIITIlimllmll;tl"""1 j T ""1. CD G 0wo I I ,
  • 37. t I --L~ . . .- .. • ___._ -+. + ...... _ • :-r-~~ ~:- 1 ....- .- .... j ----- ---- :~ -_. ---- .- - . & 1• ~ j I t i I i . •-. t t---. : f -~ .-f--o i --. 1 ;-1 r : : : t-+:: .~ . i r t t7 H-Lt .--===.i- .- t-~:-t---! ::-7~ . -+---!..:,.~ ... 4-----....-. - - . .:.--:- . +- ,... :: • .~. _~ ! : . , . :--i • .. : ! .. 1 ... t+t .. ·r-::1~., --+- . • - .. ---1-. • ~ ~ :~:- - - . --~--. t --t .---: t :-.0--.,- :- - ET: :".-i : ; .-r- ! -: ~ r- t ~ : -= -: _-=-: ~ --+ . . . . ._ t·· : : : ; : : ~ - t - : -: -~ l ; : . . ~ +-- . . .. - • ~ 1 +U - r : ;~~:-~;-+!5 - • - . -+ . - - _. I ------ j ,-. - - . • • -- t .-.. • __. __ .. f-- , • - t ~ :~-:- +~L fI; -:- . ~~. :_-_:.=:.: ~ Of-i .: : ~ -:-.:-: ; - .: t : .; - .••• 1 ~-i ~:::_- ..! j T --= t 1--_- . - -i - ~ +- -- j • - f---- • -.- . - :4t-------+-+------t------+--~-___ir__- -=. : ~ =-1 : ~-= r - .- • 1 .. .......-__+_.....,..-----_+_--------__t----_+_-----+_------ - - -+- - - - - - - - - - -4 • j ____ t .. r : 1 :--1 · --- --. .- +- -~- ~ -.- : :-: ; t t l : .: ; : -.. .. .. - . . --.-+---- -- • t _-. .i.r.; _ -- t- ~ - ~ .t o......-+~- . ~ ----.- . .-_.-- I - :-: I~: .-~ i: t ..... 1-+3 - ,-. .. 1 L. .. ~_ .. ~-- .. - - t ~ .. . -+ . . -I j i ..... .. + I -I +- • - J 1 -;-. 31
  • 38. THE ARCHIE EQUATION 5INTRODUCTION R o F (2) Gustave E. Archie, an early pioneer in the develop- R wment of formation evaluation techniques, joinedShell Oil Company in 1934 and retired in 1967. In his and found to be essentially the same as the33 year career, Archie made a number of significantcontributions to quantitative log analysis. It is hewho is credited with coining the term F (3)"Petrophysics," which has since become a recogniz-ed specialty in the petroleum industry. His single,most significant contribution to log analysis techni- previously determined.ques was the development of the fundamental rela-tionship known throughout the industry as the "Ar- I f the formation resistivity factor is independent ofchie Formula." the resistivity of the fluid in the rocks pore volume, what rock property is it related to? In order to answerDEFINITION OF TERMS this question, the concept of formation porosity and the relationships between resistivity and the This fundamental set of equations establishes the resistance and physical dimensions of a conductingquantitative relationships among porosity, electrical medium must be considered.resistivity and hydrocarbon saturation of reservoir The term porosity, as it applies to formation rockrocks. Archies experiments show the resistivity of a properties, is the fraction or percentage of poreclean formation is proportional to the resistivity of volume per unit volume of rock. A sample of forma-the brine saturating the rock. The constant of pro- tion rock having a porosity = 0, simply means theportionality is termed formation resistivity factor, or total volume of fluid contained within the rock is 0simply the formation factor, F. times the bulk of the volume of the rock. For example, if the porosity of the sample = 20070, the total volume of fluid contained in the rocks pore volume = 20070 of the bulk volume of the rock. F (1) The resistivity, R w of the formation water con- tained in the rock sample may be determined by measuring the resistance of a volume of the water By definition, Ro is the resistivity of a rock with its having the same cross sectional area and length as thepore space 100070 saturated with water having a rock sample. The resistivity may then be determinedresistivity equal to R w • from the following equation: The value of F, for a given rock sample, remains rA Rw -- (4)essentially constant for a wide range of Rw values en- Lcountered in reservoir rocks. This means that the for-mation factor of a core may be determined from a r = the measured resistanceclean, water-bearing formation by measuring theresistance and physical dimensions of the core to A the cross sectional areadetermine its resistivity, Ro and measuring theresistivity of the water in the rock, Rw . The forma- L the length of the sample _ .tion factor could then be caluclated by using equa-tion (I). Taking 20070 of the volume of water just measured, it could be used to completely fill the pore volume in If all of the water could then be flushed from the the rock sample since the rock has 20070 porosity. core and replaced by water having a resistivity equal to Since formations conduct electrical currents by vir- R~, the resistance of the core could again be measured tue of the conductive fluids in the pore space, the and ~ determined. The formation factor could then resistance of the rock may be measured to determine be calculated from its resistivity. 32
  • 39. If the rocks porosity was considered to consist ofparallel capillaries equal in length to that of the rock F = (7)sample, L, then the ratio of the total cross sectionalarea of the capillaries to the total cross sectional area m = the cementation exponent.of the rock sample would be numerically equal to theporosity; the cross sectional area of the capillaries then As the term implies, "rn " is determined by the typewould be 200/0 of A. The measured resistance of the and degree of cementation holding the rock grainsrock sample would be five times the resistance of the together and may vary numerically from about 1.3 towater sample measured earlier since the resistance as high as 3.0. The values for "rn" most often ap-would be: plied to log interpretation problems range from 1.8 to 2.2. r (5) A more general form of Archies equation relating formation factor to porosity is: The physical dimensions of the rock sample are thesame as those of the water sample measured earlier so (8)its resistivity would be 5R w • Since the pore volume ofthe rock was completely filled with water having aresistivity = Rw the resistivity just determined is Ro . a = a constant that is determined empirically.The formation factor in this case would be: A value other than one is sometimes appropriate for "a" to compensate for variations in compaction, pore structure and grain size distribution in the rela- tionship between F and porosity. The numerical F = 5 (6) value for "a" generally falls between 0.6 and 1.0. The relationship between F and {Zj that usually pro- duces satisfactory results in carbonates and highly- However, the results of Archies work show that cemented sands is:the formation resistivity factor for a clean sand with20010 porosity is approximately 20. What this is say- 1 (9)ing, then, is that the value for R o in a clean sand hav- F=7ing 20070 porosity is approximately 20 x R w sinceRo = FR w • This is often referred to as the basic Archie formula. One of the most widely used relationships for The reason that the ratio Ro/R w is so much greater sands is the Humble formula:than the results of this exercise indicate, becomes ap-parent when the distribution of porosity in formation F = 0.62 (10)rocks is considered. The path that an electrical cur- rzJ2.15rent must take through a rock is much more tortuousthan the parallel capillaries in the sample in this exer- These two relationships are the bases for the watercise. The porosity is distributed through inter- saturation nomographs in Log Interpretation Charts.granular channels or capillaries that are effectively Another popular relationship for interpretinggreater in length than the length, L, of the rock sam- sands is the Tixier formula:ple. Also, the average cross sectional area of the porevolume is effectively somewhat less than the product (11 )of the porosity times the cross sectional area of therock sample. One way to visualize the distribution of porosity in This relationship is nearly identical to the Humblereservoir rock might be to consider that the rock is formula within the range of porosities normally con-much more analagous to a container filled with sand sidered (8070 to about 35070).than it is to parallel capillaries inbedded in rock Still other relationships for F vs 0 may be deter-matrix. The net result of these effects is to cause the mined from Formation Factor Chart in Log Inter-resistivity to be something considerably greater than pretation Charts. (Fig. 5.1) A specific F vs fZJ relation-the product of R w and 1/0 that this exercise in- ship obtained from this nomograph may prove to bedicated. useful in a certain field or area, once it has been The results of Archies experiments included an established by core analysis or production results.empirical relationship between formation factor and Otherwise, either the basic Archie or Humble rela-porosity. tionship should be use~ for log analysis. 33
  • 40. 40 rlIIIi. ~ "[, ~ ~ ~ :"lII.... Ill.. """IIil ...... """- :"... 30 - ~ .~ ~.- ..... ............. -- """"........ ---- -- -~~ l-- -~ ~~ r.....: ~ I""llIl -~- 1--1--- -- ~ -- - ~ I-- ...-- - ~ ~~- " >--.;..-. - - ,- - - ..., """ ~ ~ ~ ~~ 20 ...... "lIIIl~ t"l ...... f"lo.. .- -- - ~ -- -~ - - -~ .--. """- ... -- Ilo. Illill.. ....---..- >-- ..... I--- - ""IliIl " t". " ... ... " !lIlIo.. ~~ ~ ~ -- ~ ~ ~ ~-+-- " "~ ~ ~ ""~ ~ ~ ~.~ !l-. !lI. ~ ~ ~ ..... ~ -- ..... -~ -- - I- ~- - _ ....... ~. - ~ ~f-- T "" ~ N ~ • ~-< ~ >--< ~ " "- -..... ~~ " ~ ~ ""llll-. i~ 10 , ~ -- ~ IlliIo.. !Ill.. """ .~ ~ "z~ " ~ "" Iii...: " ",1IIIIIl... "" - ~ ""IIIlil ..... 11 ..... ..... ""lIl~ " ~ ~ """" ~ ~ ~~ ~ ""-.. I" """IIil -.~ " ~ lll "- ~ .... ~ " ..tn0 -. ..:~ ~ --.: ft 5 " """"Q. 4 ". " " " ""II1II ~ -. ...... ~ ..,.; -... rllIIo.: t ... ...... ....,; lO.:. """Ilii ~IV ~ ""lIIii " ~ """IIIIIl .... ~ """lIIii ..... ~ " ~ " ~ ... " r-. rIIlh f"IIIlI.. , ~ ,~ Iii..... ~ """IIil r""li-.... """Ilili " ""l1lI 3 " r.. ~ I. ~ "" ~ ~ ~ ""-. ~ ~ ...... ~ !. " ~ "" " ~ z JJ ~l" i"lllli ~ ~ l.. l 1B ~, ~~ 2 ~ "lIIIl~ _/~ ~ f ~ ~ ~~la ~~ ~ ~" lot "~ ~ ~~ 4ll~ m= 1 [4 ~ ~ 4ll~ ~~ ~~~ 1 ~ 2 5 10 20 50 100 200 500 1000 5000 10,OOC Formation Resistivity Factor, F Estimated m values Equations: Uncemented < 1.4 F:::: 1 0m Very slightly cemented, 1.4 to 1.6 Low +carbonates (Shell Oil) Slightly cemented, 1.6 to 1.8 m = 1.87 + 0.019 (21 Moderately cemented, 1.8 to 2.0 Humble Highly cemented sands, carbonates ~ 2.0 F = 0.62 0 2 . 15 RGURE 5.1 Formation resistivity factor, F, determination. A concept which should be helpful in understanding derived from porosity. other aspects of the interpretation problem is that of apparent formation resistivity factor, Fa. (The nota- s, 100070 tion F R is often used interchangeably with Fa denoting n, R1 that it is a formation factor derived from resistivity FR FfZ measurement as opposed to a porosity measurement.) 0w 0. o ta l In the case of a 100010 water-bearing interval, the R. value obtained from a deep-investigating resistivity However, if the porosity contains some hydrocarbons device is essentially the same as Ro so the apparent as well as water, the resistivity measurement is affected formation factor Fa (F R) equals the formation factor only by the water-filled porosity, since hydrocarbons 34
  • 41. arc non-conductors. In this case, the apparent forma- If both sides of this equation are squared,tion factor will be greater than the formation factorderived from porosity since the water-filled porosity 2 21 wmust be something less than the total porosity. s~. (16) s, < 100070 Ro < R. FR > R0 the appropriate terms from the basic Archie Equa- {ZJw < 0total tion relating porosity to R o and R w may be substituted. total fiuid volume Since 0 tota bulk volume Rw R 0; Rt and 0to ta1 w -- Ro (17) volume formation water and !25 w bulk volume so R.IRt R o Rthe relationship between the formation factor and S2 w x -- (18)water-filled porosity is, therefore: Rw/Ro R[ Rw (12) The term R w cancels out of the equation, leaving The apparent formation factor in terms ofresistivities then is as follows: (19) Rt = (13) Rw A more general form of this equation results from replacing the exponent "2" with "n", referred to as The concept of water saturation is important sincethat is one of the primary formation parameters the the saturation exponent:log analyst wants to obtain from well logs. Thesaturation of any fluid in a porous interval is theratio of the volume of that fluid to the total pore S~ (20)volume. volume formation water (14) Sw =: total pore volume The most genera] form of Archies saturation equation is as follows:Values of water saturation are normally calculatedthrough potential pay zones because the resistivity aRwmeasurements are determined by the amount of for- S" (21) wmation water in the interval and by its resistivity. Theremaining fluid in an interval that has less than 100070water saturation is then inferred to be hydrocarbons,but the resistivity measurements cannot normally (The saturation exponent, n t is normally equal todistinguish between oil and gas. two, but may vary slightly as local experience dic- Since the formations bulk volume is a common tates).denominator in both of the porosity definitions men-tioned previously t water saturation, Sw may be ex- pressed as follows: SAMPLE PROBLEMS Ow To best illustrate how Archies fundamental rela- (15) tionships are applied to actual practice, typical inter- 0total pretation problems need to be solved. 3S
  • 42. Example 1: "m" = 2.• The porosity log response through a sand interval • Rearranging the equation to determine porosity in a well gives an indicated porosity of 18010. gives the following:• The resistivity from an induction log indicated R, = 22 Q-m through the interval. 0 m = 1:: = ~5.• Rw is known to be 0.025 Q-m at formation temperature. The porosity then is• The analyst wants to know the calculated water {Zj =J 0.025 = O.16or16OJo. saturation for the interval. Using the saturation equation Still, another typical interpretation problem takes advantage of the basic Archie Equation in the estima- tion of R w from the porosity and resistivity response in a water-bearing interval. Example 3:local experience in the area dictates that a = 0.81 • A Compensated Densllogv-Gamma Ray survey in-and m and n = 2 for the sand evaluated. dicates 27ftJo porosity through a clean water sand. It is not necessary to solve for F as an intermediatestep in the solution since the analyst is not usually in- • The Induction Electrolog" survey through theterested in F. Therefore, the determination for water corresponding interval gives a value of R o = 0.35saturation is: Q-m. Since the interval is a sandstone, the Tixier relation- 0.81 x .025 ship between F and porosity is applied. Solving the sw = (0.18)2 x 22 equation for R w gives the following: Ro X 02 Sw = O. 17or 17010 0.81Another fairly typical interpretation problem is toestimate the porosity in an apparent water zone when 0.35 X (0.27)2only R w and the resistivity response from a deep- 0.81reading resistivity tool are available. When thiscalculation is made, it is assumed that the resistivity Rw = 0.03 Qrn.tool response is providing a value of Ro • Alsonecessary are the values for "a" and "rn." Knowing The example problem was a partial solution of thethese values, the Archie formula can then be solved Rwa method of log evaluation.relating F to porosity, R o and R w for porosity: Example 4: a F= rzJrn R w I f the water resistivity term in the saturation equation is replaced with mud filtrate resistivity, and the for-Example 2: mation resistivity term replaced with Rxo , the resistivity of the flushed zone, the fluid saturation• The induction log response in this instance is calculated would be the mud filtrate saturation in the 2Q-m. flushed zone, Sxo.• The formation water resistivity is 0.05 Q-m. S n• The interval is a highly-cemented sandstone so w local experience in this case dictates that the ap- propriate values to apply for "a" = 1 and 36
  • 43. Sxo n =When a micro-resistivity curve or shallow focusedcurve is recorded in a well to provide an Rxo log, solv-ing for Sxo is beneficial for determining residualhydrocarbon saturation. Sxo = (1 - RHS). The significance of knowing the value of Sxo through a porous interval as well as Sw is that it per- mits the determination of the degree of hydrocarbon flushing by the invading filtrate. As long as Sxo is numerically greater than Sw it can be inferred that t there are movable hydrocarbons present; if Sx_ o = Sw it can be inferred that there are no movable hydrocarbons present.BIBLIOGRAPHYArchie, G.E. The Electrical Resistivity Log as an Aidin Determining Some Reservoir Characteristics,Petroleum Technology, Vol. 5, 1942.Dresser Atlas. Log Interpretation Charts. 1980 Ed. 37
  • 44. QUESTIONS 5(1) The porosity in a water sand is 25010; R = 0.03 Q-m at T f and the resistivity from an Induction Log reads 0.4 Q-m. If a = 0.81 and m = 2, what is the apparent Formation Factor for the sand? _(2) If R mf = 1.8 Q-m at T f in question 1, what would be the value of the apparent Formation Factor in the sands flushed zone or R zone? __ __ __. xo(3) An lnduction-Electrologt survey and BHC-Acoustilog!> survey were recorded through a sand where an oil-water contact was observed. The Acoustilog" indicated the porosity to be constant through the interval; the Induction Log indicated 0.5 Q-m below the oil-water contact and 5.0 Q-m at the top of the interval. The formation has a water resistivity, R w = 0.02 Q-m. If m = 2 and a == 0.81 (Tixier), and n = 2, what porosity was indicated through the sand? _(4) What would the water saturation be in the top of the interval in the preceding question? _(5) A Micro-Laterolog on the well in the preceding problem indicated R xo in the top of the sand to be 16 Q-m and R mf = 0.5 Q-m @ formation temperature. What is the calculated value of Sxo? Are any movable hydrocarbons in- dicated? 38
  • 45. INDUCTION LOG 6INTRODUCTION ment is one of conductivity, errors that are insignifi- cant at high values can be a problem at low values Focused induction logs have proven to be the best (conductivities). For this reason the measurement ofmethod for obtaining formation resistivity in wells resistivities greater than 200 Q-m (or less than fivedrilled with fresh mud, air or oil base mud. The log mmhos/m) is not the best application for an inductioncurves recorded on the lnduction-Electrolog" survey device. The accuracy of the measurement is limitedare the SP, 16-in. short-normal, induction conductivi- above 200 Q-m.ty and its reciprocal, the resistivity curve. (Fig. 6.1) In Bed boundaries on the conductivity curve are half-boreholes containing non-conducting fluids, the Gam- way between the high and low reading that result fromrna Ray and Induction curves are recorded. changing from one bed to another. (Fig. 6.1) In thin Induction logging instruments are composed of beds, peak values, whether high or low need furthertransmitter-receiver coil pairs. The number of coils correction to read true resistivity. In thick beds whichand spacing of these coils determine the depth of vary in resistivity and are considered to be a singleinvestigation, borehole response and the resolution of unit, the curve is often averaged. In normal interpreta-the instrument. tion, the induction curve is assumed to be measuring An alternating current is applied to the transmitter true resistivity and is input directly in the Archie equa-coils which induces eddy currents by electromagnetic tion as R t •induction in the formation surrounding the coil Borehole effects on the induction curve are smallsystem. These currents have a magnetic field which except when highly conductive, salty fluids are presentinduces voltages in the receiver coils. These voltages in large boreholes. Normal borehole enlargementsare related to the conductivity of the formation. have little effect on the curve. The induction log may Conductivity is the reciprocal of resistivity. The be run in air, oil or foam filled holes as the inductionconventional units of conductivity used in well logging system does not require the transmission of electricityare mmhos/m which gives the relationship: through the drilling fluid. C = lOOO/R (I) SHORT NORMAI~where: The short normal resistivity measuring system has a C = conductivity in mmhos/m shallow depth of investigation, and is designed to measure the resistivity of what is normally the invaded R = resistivity in ohms-m-/m zone. When compared with deeper measured resis- tivity, such as the induction, the existence of invasion can be detected. Invasion indicates that the formation The measured value of conductivity is reciprocated is permeable.by the surface processing equipment as the log is run With its 16-in. spacing, the short normal recordsand is presented as a resistivity curve, in addition to good resistivity values in relatively thin beds, down tothe conductivity curve. four feet thick.The curve shape is symmetrical around The induction system works best where the undis- the center of the bed. In thick beds, an average valueturbed formation has lower resistivity than the in- may be used if a single value of resistivity for the bed isvaded zone. This is typical of logging in a fresh mud desired, otherwise, a thick bed may be divided intosystem. A low resistivity zone between the tool and the several thin zones. The short normal generallyundisturbed formation, as typified by invasion by salt measures the resistivity of the invaded portion of themud, creates problems if the invasion is deep. This is formation, which is partially saturated with muddue to the induction device being more influenced byhigh conductivity (low resistivity) zones than by low filtrate. I f the undisturbed formation contains oil,conductivity (high resistivity) zones. The induction log there is usually oil in the invaded zone, but to a lesserwin normally measure a good value of resistivity in extent. The short normal often reflects some of thezones of five feet or thicker. fluid properties in the undisturbed formation as well The induction device is better at measuring lower as those of the mud filtrate. The size of the borehole resistivities than higher resistivities. Since the measure- and the resistivity of the mud in the borehole influence 39
  • 46. GR SP lFPiH RESISTIVITY CONDUCTIVITY (Ohms rn-vm) (Mllhmhos/m) / ,1O, 16" NORMAL - i-I· INDUCTION CONDtJCl IVI Ty 40· SPACING O~ 10 I------ ~ - - ~--- 0 -- GAMMA RAY ~ BiJ ,APt 0, .. INDUCTION RESISTIVlii--- 8000 4000 ~o ___________ _3,E 40· SPACING 02 ~- ----- ------ - - -- 10 ; ~ , 1 ~ I: J i, ~ I r A I~" I I I !i IV ~~ 1 J ~~ ~-f r"-- I I I 1 t... ~ ~ I, HIGH t-- ~ [",/ BED BOUNDARY ~li t- ... """r---- ~~ I I ~N ~lOW D I "" I I ~ r., < Il I ! I " ~ I ! ~ , ~ , • ; I I ~ ~ ,t II ! JI .....-.II ~ V I , ~ --- I I~~ <..- , ~ I "".-- ~ ! 1 ! .. , " r ~ 0 ,t 1 ~ "- -(" I i ~ .., ~ ~ I ~ ... t- ~ SP i ,.> I " r"""-o ~~ " " . . . r---.~ ~~ Ie ~~ I I ! I j Ii,. t> ! I I v ~> ! I I I GAMMA RAY r )~ t-... i I, I ~ I ,i ,. 1 I 16" NORMAL I " ~ 1 Il ! j ! II I I I I tow-+-- I po , -- ( ~ ~~ ~ II ~~ r ~f.j I Ii HIGH, 1/IT ,.4 bX I I ! ~ ( I I~ Ii :1 1 Ii ~ I IIi I I I , I " lI! BED BOUNOARY .... "~~ It~ ~ ~ I INDUCTION. II I I I i I r--~ ~ / I . ! I I I , I 1" 1, (~ ~ :i! II I ~ ....... ~ ~ I q: I ,,,,,1-"- ~ ,~ I I ..- C t-- -.. ~, r---- ~rl "R.. I I r-- I I ~. ! ~ I ,,-.....- PI I - ,, ~ ............ 0 ! , IJ I I I I I ~r-, : " ,~ ~ ~ 0 t 1 ... ,. l ... ~ ---) , r~~ f CONDUC}VI+V ~ I I I I ~ ... ~, :1 j ! I I ! It I i I ( i , "-. 1 . 1 I I I I I ! ~ , • , : I 1 i I I 1 ~ I I I ~~ I t I ! I 11 j I I ~ ~ 1 I r I I 1", ) j I j ~ ~ , I , .... J I" : .1 j ! II, II I I 1 I " ! I I [I !FIGURE 6.1Bed boundary definition lAsing Induction Log conductivity. 40
  • 47. the short normal to some extent. The higher the mud GEOMETRIC THEORYresistivity the better the measurement as long as themud will conduct electricity. Salt muds are not the best To account for the effect of the borehole, invadedplace to use the short normal because the electrodes zone and adjacent beds upon the measured inductionwill easily short out. conductivity values, the geometric factor theory is Bed boundaries are indicated on the short normal as used. The geometric factor can be defined as; a dimen-inflection points on the curve. (Fig. 6.2) These inflec- sionless number for a horizontal loop of homogeneoustion points are actually one-half electrode spacing ground, having a circular shape with its center on theaway from the higher conductivity bed. If the distance axis of the hole, and whose cross section is a very smallbetween the two inflection points in a resistive bed is square of unit area. Thus, the formation surroundingnine feet, the bed is nine feet plus 16 inches or just the logging instrument can be considered to be madeover 10 feet thick. up of discrete, symmetrical elements which are in- tegrated to give the total formation response. Figure 6.4 represents a two-eoil induction logging Short Normal 16 in. coil system consisting of a single transmitter and - 2 = 8 in. receiver surrounded by a loop of homogeneous ground. ! Inflection PointFIGURE 6.2DUAL INDUCTION FOCUSED LOG This instrument was designed to provide the resis-tivity measurements necessary to estimate the effectsof invasion so more reliable values for the true forma- ....--r-I Tz -...._+-- l..tion resistivity may be obtained. The resistivity curvespresented are made up of a deep investigation induc-tion, a medium investigation induction and a shallowinvestigation guard-type focused device. These curvesare normally recorded on a four-cycle logarithmicscale covering a range from 0.2 to 2,000 Q-m. Thisscale has been selected so one scale is applicable for Tmost logging conditions. Visual observation of the Dual Induction FocusedLog can give valuable information regarding invasion,porosity and hydrocarbon content. The separationbetween curves recorded on a logarithmic scale is the FIGURE 6.4log of the ratio of the two curves, therefore the relative Two coil Induction system.position and separation of the curves can be used toestimate the extent of invasion and whether variations The voltage at the receiver from a unit ground loopin resistivity are due to a change in water saturation or of radius "r" and altitude "z" with respect to theporosity. The amount of separation between the center of the coil system is given bycurves depends on depth of invasion and the ratio ofthe resistivity of the mud filtrate to the resistivity of Vr = Kgo (2)the formation water, (Rn1f/Rw). In a porous andpermeable moderately invaded formation that is where K is a function of the area of the transmitterwater-bearing, the ratio of the focused curve to the and receiver coils, distance between thedeep induction curve (RFL/Ru.o) will be close to the coils, current in the transmitter, and fre-ratio of Rmr/Rw. The ratio decreases as hydrocarbon quency of the transmitter currentcontent increases. The ratio between the medium in-duction and deep induction curves (RILM/RILD) is an g is the geometric factor which depends on theindication of the depth of invasion. Figure 6.3 shows a geometric position of the unit loop asDual Induction Focused Log. related to the transm itter and receiver coils 41
  • 48. SP DEPTH RESISTIVITY (Millivolts) (Ohrns-mern) SHAllOW FOCUSED lOG 02 ~ 0 0 100 WOO l - -)20)--+ MEDIUM INDUCTiON LOG 02 ,0 - - - - - - - - - ..- - L- __- - - - _____ 10 00 1000 __•• J ••• _- _- ___ _ __•• _J _________________ ..L.___ DEEP INDUCTION LOG 02 10 10 iOO 1000 - - - - - - - L - - - _ ._____ L _________ L _________ L __ ) I . ~ ~ .., ,- .. ~~ ~ ~~. ... 1I MEDIUM -.;;; l" .4~ I r -t;;~ ;J ~. I~ ~ c-:::"~ SHALLOW FOCUSED l/ T 1 ~ro-I~ f •~ iI , ~ ~...::-Jr: ~ I ( r : 0 8 rr ... -- ~ ~ I ~-~ . r ~ "-- ~EEP 11 --::Q ~~ I I i I :I l ~ .,~ I I I V ~ I ~~ I II !~ V , ~~ ~b I II r .... - ~~~ I 11 l n I RLD = 0.33 1 I " . ~ = 0.6 1,L I I «~ I.... • II , , .~ i--" ~ ~ l RFL ~ RLM l = 3.0 I I I I II I l ~ I~~ (f!Cf (~ 1--" ..... ~ IRGURE6.3Dual Induction Focused Log. 42
  • 49. a is the conductivity of the ground loop factor, On plotted against the radius, r, for a transmitter-receiver spacing of 40 inches. The radial geometric factor considers the formation The signal measured by an induction logging tool as the combination of a large number of cylinders positioned opposite a thick formation usually reflects coaxial with the borehole. From the above definition the conductivity of that formation. However, in thin of "g", the integrated radial geometric factor, G r , is formations, the signal is affected by the conductivities the sum of all the "g" values for all the area of cross of the adjacent formations. The integrated vertical section within a cylinder of radius, r. This represents a geometric factor, Gv, becomes the sum of the discrete thick homogeneous formation invaded by mud filtrate geometrical factors for all of the cross sections above where conductivity changes radially. or below a horizontal plane at a distance, Z, from the Figure 6.5 represents a two-coil induction logging center of the tool. The integrated vertical geometric factor increases with vertical distance, Z, and must equal unity for all space. A transmitter-receiver coil pair in a conductive medium with an interface above the coil pair is shown in Figure 6.7. Geometric factors shown are applicable ............... ............... ............... ............... =-..----:.---=-=----------------.:Gs == 1 - Gv = 0.1:--,,--:- yr+----+------ll---....:.-------l~ .................... I, .•. ....•.•.•..•••• . ////////// ............... . , ~~~~~~~~~~ ~l~!!!!!m!~I!l!l!! .............. .............. .............. .............. .............. / / / / / / / / / / :::::::::::::::::::: .............. ....... . ///////~// ....... ::::.: ....... . .............. , . . .............. ................................................. . .............. .............. . . ......... .............. . . . FIGURE 6.5 Radial geometric factor reference to cylindrical boundaries. T h=40in. ... . .............. .................................................... .............. .............................................. - - - - .............................................. ........... : ........... ~ :~ .:~ ::::C~~te~ ot c~ii Spa~-: -:-::-.1:......:-::-:-: . : ........... . . ............ . 1 system in a thick homogeneous formation and shows the integrated radial geometric factor, G n applicable to cylindrical boundaries. The integrated radial geometric factor increases with the radial distance ......................................... . from the tool and must equal unity for all space. . Figure 6.6 shows the integrated radial geometric ...................................................... .................................................. ..... .......................................... .. .......................................... .. . .a 1.0.....--...--.....,..--...---.....---...--........- -...~ FIGURE 6.7~ Vertical geometric factor reference to an interface.L1. 0.8.gQ)E to the formation represented and are dependent upong 0.6C) the position of the interface with respect to the center(ij of the tool. Figure 6.8 shows the integrated vertical~ 0.4a: geometric factor, G v , plotted against the vertical~Q) distance from the interface, Z, for a 4O-in. transmitter-1U00.2 receiver coil spacing.s.£ The geometric factor theory, confirmed with experi- O~_--""" _ _........_ _........_ _....-.--.-j......._ ......._ _.... mental test results, is used to construct the environ- o 20 40 60 80 100 120 140 mental correction charts discussed in the next section. Radius, r (m.) A more detailed discussion of this theory is given in FIGURE 6.6 the Dresser Atlas publication, Induction Logs. Integrated radial geometric factor 40·in. transmitter-receiver Geometric factor tables are published in the Dresser coil spacing. Atlas Prolog manual. 43
  • 50. ,,1.0 conductivity, bed thickness effects and invasion .: effects. For the borehole correction, Figure 6.9 is used.2~ 0.8 for the deep induction and Figure 6.10 is used for the.Sl medium induction. Following the borehole correc-.:= C) tion, the apparent resistivity, Ra, is entered in the chartE 0.6oC) for the applicable adjacent bed resistivity, Rs, ina Figures 6.11 and 6.12. The corrected resistivity, Rcorr ,~ 0.4 is read at the intersection of the bed thickness line.t:~ If a Dual Induction-Focused Log is available, under~ 0.2 most reservoir conditions, reliable values of R, and~ diameter of invasion (di) and a good approximation ofC)S O......_...L.._.....A_-..a....-_.....&.--~~--"--~~~ Rxo may be obtained from the "tornado" Figures 6.13.E -80 -60 -40 o 80 and 6.14. These charts may be entered after the other Distance from Interface (in.) environmental corrections have been made. For the FIGURE 6.1 Integrated vertical geometric factor 40-in. transmitter-receiver case of Rt > R xo , Figure 6.14, R xo should be deter- coil spacing. mined from an auxiliary survey such as the Micro- Laterolog. ENVIRONMENTAL CORRECTIONS The log analyst will find, with experience, many situations in which any or all of the environmental For the most accurate values of R a, it is necessary to corrections may be safely ignored causing little signifi- make corrections for the contribution of borehole cant change to his interpretation. 44
  • 51. 15 ...- - -.....- -..........,.---....-~---.........-~..----~------~-~------ .....- - -...... - -..... -141312 f .. , .. - ... - - _.. -_. - - -- t .. , . .. ........ -- 7 . ---....----.- ..... - - ... .......--- ... . -,- _.... - - .. ~ .. - .: .: .: r:- 6 .....•. . i . 5 -.. ~ --. .. . . . . . - --- - .. .. .. .. .. .. . - . - . .. . .. .. - .. .. - - - . - " . - -.. .. ..- .. - .. . .. 4.-. ..... ---._ _....._ . A o - .... -..I ..r..- ...... ...- ~ ._. .....-1 4 5 1( Radial Geometric Factor 40 30 20 10 o -11 Hole Signal. mmho/m FIGURE 6.9 Deep induction log borehole correction. 4S
  • 52. , 15 ...- - - - - - - - - - - - - - - - - -.....- ....- ....- ...- .....- ..........- ..........-.-..........- - -...... - - - . - 2·.g h~.··: " . . . . : . . f . .: ::.: r. : : ~ t • • •• • •• • •••• T 14 • • • • • •• • -. • ••• - ~ •••••• - - • ~ • - ••• - •• - •••••• - •• • - • + •• • •• =: : . ·a.~ :,~: :.:.:::.: ~ ~ : ~ ::.: . . . : 1..~ h). _. . . . .. . ... . ... 13 -- -- -- .-_ - .- .. - - ..... ... . . -._. , ., 12 ............. . . . ... . ....... -- --.... . .. ~ .. -. .5 11...s~ftS 10CCD(5 9s:~ . - .........~ 8 ... - • • • - • -t - . ..• I. - • • - -...-- - ..•. • 7 6 , 5 4 - 1 4 5 10 Radial Geometric Factor 40 30 20 10 o -10 Hole Signal. rnrnho/rn FIGURE 8.10 Medium induction log borehole correction. 46
  • 53. 20 ....- - - - - - - - - - - - - - - - - - -.... 20 ....- -....- - - - - - - - - - - - - -... 10 10 - ..... ....:-..-4----._.- -0- --~.--:-.- ~L ~ - --of -•.. ~.~ -- -- :_~--- ------------1.-+----- . 5 ; . - ---r--;-::~ 4 -~~ ~- -- -------. _. . -- -.- . --:-1 -S!3 Rs. 1J 2 0.5 ~ ............. ..... _~ ...._........................_ _..... 0.5 ...................---.....--..... ~---..................- -.... 0.5 1.0 2 3 4 5 10 20 0.5 1.0 2 3 4 5 10 20 Rcorr (Q-m) Acorr (g·m) 25 . - - - - - - - - - - - - - - - - - -... 25 ....- - - - - - - - - - - - - - - - - . - . 18 ft and> 1ft 12 ft 20 20 5 O..-_ __.... ..... ~ _ __._I..._.__ ___I o ....__...... ..... ..... .....__ ~ o 5 10 15 20 25 o 5 10 1f5 20 25 Reorr (g-m) Roorr (Q-m) FIGURE 6.11 Bed thickness correction for induction log. 47 -
  • 54. 100 100 16 ft and> 80 80 fO ft 6ft 8ft- 60 - 60~ ~a ~J 40 As· 5 J 40 4ft 20 3ft 20 0...-.-_...... .....__.....__..............__.. o 20 40 60 80 100 20 40 60 80 100 Acorr (Q-m) Acorr (g-m) 100 100 ....- - - - - - - - - - - - - - - -.... 7 ft. 8 ft. 12 ft 10 ft 80 80- 60~ ---3ft~J 40 20 20 o O ...._ _..... --_ _--- .a..-_ _.... o 20 40 60 80 100 o 20 40 60 80 100 Acorr (g-m) Rcorr (g-m) FIGURE 8.12 Bed thickness correction for induction log. 48
  • 55. 20-------------~--------_, ~ (tri.) . 10 9 8 7 6 Thick Beds - ...---.... - . 8 in. Borehole Step Profile No Skin Effect 1.2 1.4 Rn.M AtLDFIGURE 6.13At from Dual Induction-Focused Log (At<Axo). 49
  • 56. 2O--+--...-+-......-+-......-+--+-~.............. -~~--+-O---.r-4...... ~........--+-~~ + - t -, - i---r-+-*+--+-~-+-..,. - -L-r- -+-# -+---+-...... #-+--"-"--+--~,,"-of---~~--""-+-#o-4-""""~~~ - -1- I 10 _ _ RtLD 5 ~#==-==-~~.J..--+--.--J~~..:4~Ue.....--+.-~--+......................... Axo .... + I I I 2 3 4FIGURE 6.14At from Dual Induction-Focused Log (Rt>Rxo). so
  • 57. QUESTIONS 6(1) If a zone has a conductivity of 1,500 mmhos what is the resistivity? a) 1.5 Q-m b) 0.66 Q-m c) 0.82 Q-m d) 0.15 Q-m(2) Determine the total bed thickness for zone A in Figure 6.1 using the short normal curve. a) 7 ft b) 17 ft c) 10 ft d) 15 ft(3) Determine di for the zone in Figure 6.3 at 10,143 ft to 10,148 ft. a) 0.67 inches b) 17 inches c) 67 inches d) not on the chart(4) What is R, for the zone in question three? a) 0.22 Q-m b) 0.67 Q-m c) 0.23 Q-m d) not on chart(5) What is R xo for the zone in question three? a) 4.03 Q-m b) 5.0 Q-m c) 12.73 Q-m d) not on the chart 51
  • 58. LATEROLOG 7INTRODUCTION The Laterolog instrument is a focused or guard elec-trode system. It is designed to produce reliableresistivity measurements in boreholes containing salinedrilling fluids. The instrument has a small current electrode posi-tioned between two long guard electrodes. A constantcurren t is applied to the center electrode. An auxiliarycurrent of the same polarity is applied to the guardelectrodes. The guard electrode current is automa..tically and continuously adjusted to maintain a zeropotential difference between the center electrode andguard electrodes. This forces the current emanatingfrom the current electrode to flow into the formation.A drop in potential is caused by the flow of the cur-rent through the surrounding formation to a remotecurrent return electrode. This potential difference isrelated to the resistivity of the formation. Invasion can be a major influence on the Laterolog.Experience shows that the use of salt base mudsgenerally results in only limited invasion which doesnot normally hamper the Laterolog in the determina-tion of true resistivity. If the muds are less saline (more resistive) than the formation water, the Laterologtends to be overly influenced by the invaded zone. This holds true unless the resistivity of the virgin zoneis considerably higher than the invaded zone. Boreholesize and bed thickness do affect the Laterolog FIGURE 7.1response, but normally the readings are accepted as Diagramatic representation of the magnetic face field induced by the tool transmitters. true resistivity and used directly in the Archie equa- tion. The Laterolog is normally used in high resis- tivity, low porosity formations in the presence of having the highest resistance. Note that in contrast, saline drilling fluids. the induction log, which measures conductivity, sees The Laterolog has good vertical resolution and will the different zones as constituting a parallel circuit of obtain good values of resistivity in beds thicker than resistors. 2 ft. Generally, the peak value of the curve represents The Laterolog is a superior device in high resistivity the resistivity of thinner beds while in a very thick bed, (>100 Q-m) formations. This is also true even in more an average in the zone is often used. resistive muds since the contribution of the mud The current pattern for the Laterolog is shown in diminishes in the series circuit compared with the con- Figure 7.1. tribution of the high resistivity uninvaded formation. The path taken by the measure current of the An exception occurs in a large diameter borehole where Laterolog constitutes a series circuit through the the contribution of the mud resistivity does become drilling mud, mud cake, flushed and invaded zones significant. and the undisturbed formation as illustrated in Figure The Laterolog is presented on a logarithmic scale. 7.3 Thus, from the analogy of electric circuits, the Figure 7.2. is a Laterolog recorded on a four-cycle greatest voltage drop will appear across those sections logarithmic scale from 0.. 2 to 2,000 Q-m. 52
  • 59. GAMMA RAY RESISTIVITY Radiation Intensity Increase8 DEPTH ,.. (API Units) 0 120 0.2 1.0 10 100 1000 -~ ~I- ~ _L-I-" If~ ~ I ! ...... ~~ i ---- .... ./11""" I -. : 5J> I I ""--. ~-~ ~ : I ill I i I - U~ 4lII~ ~ ........ ~ i I , I j I I ~~ ~ .:~ , ~ I +[ I i "< < I ~!, I I Id ,- ~ I 11 ~ I! L i J I I I K~ I I ~ I I I I ,.",. - I ! -- -- 1 1----- I/r I ;1 : ~~ I ..~61 i J T", R-..... ~ § 11: 1 I ,,- ,.... I I I ~~ t.. I } I I r~ ~~i,i I i I I; l/ ..--> I I : I I . [ 4- I-~ I l lie::::::: l> 1- J I I I ! ~ ... !I j I - I ~ ~ i I I .. - - _I J;j! II ~~ I j I I (~ ) i 1 Iii 1 ~~ L-~ I ! !t ]11 III I I I ~ I !II I I i11: 1 ~ ~ I I :~ 1 , I I ~- --~ ... I I I ~ I ~~ I ~ ! I l I § c .... :II, I T ~ i ~ I r: ; l---""l.--"" 1 ~ ~ t;> I ! I r I I If I ~ ~ ........... ~ - -~ - I III - ""--FIGURE 7.2Laterolog/Gamma Ray. 53
  • 60. Dual laterolog Simultaneous Shallow and t Salt Mud Deep Measurements I ,. ;~ Shallow- R ~ I " .... ........ _ ~ ,,::"-------~f ~:::::_---- Deep - RI ------:.--" WI 128 Hz . . ::::::::::::: ca 32 HI T ~--=-=:~~~-:f - --------- .... ========::::~-:.:. / 24-in 4------ . . ~ ,,----------. Beam Width ~-------E ~ -- ::===:::1 ---... ---- - ~_ ~-------- --- .... .... t#o ,.,_--_ ~.; .; ---- Mud Flushed Invaded Undisturbed FIGURE 7.4 Zone Zone Formation Dual Laterolog Current Paths, Ao~·~¥A •• ~,V¥w Am Ame s., AI At 8 3. The ratio of the borehole diameter and theFIGURE 7.3 diameter of the logging tool b small.Laterolog current path. DlJAL tATEROL()G When the above conditions are not present simultaneously, the values calculated will generally In highly resistive formations drilled with salt mud, depart to some degree from values that are actually two Laterologs with different depths of investigation recorded. The formula generally used is: are often recorded simultaneously in order to compute (I) true resistivity (R l ) . The shallow Laterolog primarily measures the resistivity of the invaded Lone, and the deep Laterolog measures the resistivity of a much This equation simply states that the total resistivity deeper part of the formation. The true resistivity (R ,) (R a )is the sum of the contributions of the borehole can thus be calculated more precisely by taking into resistivity (JmRn1) , the resistivity of the invaded zone account the effect of an invaded zone in the vicinity of (JiRi) and the resistivity of the undisturbed formation (JtR t). the borehole. The current pattern for the case of the Dual The pseudo geometrical factors for the mud (J rn ) . Laterolog is shown in Figure 7.4. The presentation of the invaded zone (Ji) and (he undisturbed formarlon the Dual Laterolog is usually on a logarithmic scale. (L) for the Laterolog depend upon the geometry and Figure 7.5 shows a Dual Laterolog./Gamrna Ray dimensions of the logging tool. borehole diameter (dh) and diameters of invasion (dj). recorded on a four-cycle logarithmic scale from 0.2 to 2,000 Q-m. The pseudo geometrical factors will satisfy the following classic expression: PSEUDO (~":OM":TRICAI~ .·ACTOR 1.0 (2) Mathematical calculations [0 correct apparent resistivity values can be made for the l.. aterolog based upon the pseudo geometrical factor concept. I f the The radial response curve for Dresser Atlas following conditions are complied with, the results Laterolog is shown in Figure 7.6 which gives the obtained from these calculations are within acceptable nurnerical values of the horizontal pseudo geometrical limits. factor in terms of diameter in inches. Also shown in the chart for the purpose of comparison is the radial 1. The formation of interest is greater in thickness response curve of the Dresser Atlas Induction l.ogs. than the disc of measure current. (>2 ft) The difference in the radial response curves for the Laterolog and the Induction Log illustrates that the 2. The resistivity contrast near the logging tool is not Laterolog pseudo geometrical factor for the invaded too great. zone (J j) is considerably greater than that for the 54
  • 61. GR DEPTH RESISTIVITY (Ohms m 2/m) GAMMA RAY (API Units) SHALLOW LATEROLOGo 120 02 1.0 10 100 1000 -----------~-----------------r-----------------r-----------------r--- DEEP LATEROLOG 02 1.0 10 100 1000 I r I. ! I <: I I :> I I S I I ( I i I I I( ~ SHALLOW LATEROLOG........ ~"~""1II ~ ~ I I f l ~~~ DEEP LATEROLOG II ~ I I o e I I I ("" I , r I c I I ~ 4~ f I GR ! / I I Ie , ~ I ! t I ~ IFIGURE 7.5Dual Laterolog/Gamma Ray Presentation Format. 55
  • 62. 0.8 From Dual Laterolog survey I I I I I ,.,.",.. ~ RLLS = 375fl 0.6 - Standard LaterologasLLg Ra = JmRm + JjR, + J,R t ~ ....... ~ ~ ~ ---- RLLD = 600Q) ~ ~ ~E 0.4 From surface measurementoQ) V ~C) ~ /o0~ 0.2 j" ~ ~ V Rm = 0.22 @ Tr ,/enQ."0 / RLLI)/R m = 2727~as V 800 Series Induction Log~ 0£ J -- ~ 1 = Gm + G 1 + Gt_ An I Am F Rt I I I - -0.2 Figure 7.7 gives: o 20 40 60 80 100 120 Diameter in Inches RLLDcorr/Rl.I.D = 0.89 FIGURE 7.6 Borenole Diameter V5. Geometric Factor. RLl.D = (0.89)(600) = 534 corrected for borehole Induction Log. However, where a low value of Ri/R. exists, the influence of the invaded zone (JjRi) on the RLLS = (1.01)(375) = 379 corrected for borehole apparent resistivity will be less for this device than for the Induction Log. This effect becomes more To enter Figure 7.8: pronounced with lower values of Rj/R, and increasing dj values. RLLD/RLLS -= (534) + (379) = 1.41 (534) -;- (15) = 35.6 ENVIRONMENTAL CORRECTIONS which gives: As with the Induction Log, it is often necessary to make corrections for borehole and invasion effects to obtain the most accurate value of RI . Rl/RLLD = 1.2; Rt/R xo = 43; dj = 22 in. For borehole effects, Figure 7.7 is entered with the ratio RLL/R 1 • The ratio Rl.Lcorr/RLL is read at the n R, = (1.02) (534) = 641 intersection with the borehole diameter line. Figure 7.8 corrects the RLLD value from the Dual R xo = (641) -;- (43) = 14.9 Laterolog for invasion if R.>R xo - Resistivities from the deep and shallow Laterolog along with the R xo measured by the Micro Laterolog or Proximity Log are necessary. The R xo devices may be run in combina- tion with the DLL. Example: Given: From Micro Laterolog survey Rxo 15 dbh 10 in. 56
  • 63. Borehole Correction for Laterolog Deep (LLD) 1.4 ,... ....;. ...;.. (Thick Beds) .. 1.3 1.2 dt. • 16 In. et: ~ 1.1 ---§ 0 ri 1.0 0.9 0.8 2 3 4 10 20 30 40 50 100 200 300 400 500 1000 5000 FLo/Rm Borehole Correction for Laterolog Shallow (llS) (Thick Beds) 1.5 r----------------------------.---------.. 1.4 1.0 0.9 . :"":J ..... " .. : . :. ~ . , ..... • .. ~ 4 .: ~ ~ 2 3 4 5 10 20 30 4050 200 300400 500 1000RGURE 7.7Borehole correction for Laterolog. 57
  • 64. 100 90 70 60 50 40 30 20 10 9 8 7 ~ 6 - .. --. --. a: C 5 .. .. cr.~ 4 3 ---. : 2 , 09 08 .-- . ... ----_ ..... -- . -------------~ ..-----... ....,.- ..r- -. . R, > A , xo 07 Thick Beds . . _ .._ 06 S In. Borehole Step Profile 05 04 . .: . -... : ..... . - -.,.._ . r- .: i ~.. , . : --: ..: "i i: 0.3 __ --_-- __ -~ _. ; ;..__ ~-i .. -+.. -:-~ .. ............ , . 0.2 o1 04 20 30 FIGURE 7.8 At from Dual Laterolog (for At > Axo )·. . ..-.. . iiiiiiiIiioooii~ _...... 58 .........
  • 65. QUESTIONS 7(1) Laterolog devices measure the drop in __created by the now of current through the formation to a current return electrode. a) induction b) resistivity c) current d} potential(2) If a Laierolog device is recorded in a 100070 saline water saturated formation, which i deeply invaded with fresh mud filtrate. the apparent resistivity will be a) too high b) too 10· c) the same d) not enough information(3) Determine the diameter of invasion for a Dual l.aterolog/Micro Laterolog combination given: RI. D = 17 Q-m. RI LS = 15 Q-nl. Rfli = 8 Q-m. a} 20 in. b) 40 in. c) 35 in. d) 30 in.(4) Determine R, for question three. a) 17 Q-m b) 19 Q-nl c) 14.1 Q-m 0) not enough information(5) In a 10 in. borehole with R n1 0.8 the I.aterolog deep reads 80 Q-In. What is the true resistivity? t a) 76.3 Q-m b) 81 Q-m c) 100.8 Q-m d) not enough information 59
  • 66. ADDmONAL RESISTIVITY CONCEPTS 8MiniI og® SP l"" RESISTIVITY t(~IT r-INTRODUCTION y. ,.. 1,., I, I " .. ~1 , I, The Minilog" service provides information useful ,. , -, I .... "in locating and evaluating porous and permeable ---_ ... -- . • -- -£formations penetrated by the drill bit. The closely f1, fC Ispaced electrodes, mounted on a fluid-filled pad in .-.--contact with the borehole wall, measure the resistivity I I j r ;;> tof a small volume of formation adjacent to the I r lt <;borehole wall. When formation conditions arefavorable, measurement of the resistivity of this por-tion of the formation provides values necessary to I ( I F-«. -- (. /,.- --- . . --- ~- .-+~ .. ~+~,~,determine the resistivity 0 f the formation flushed by -. ~ .-the invading mud filtrate. Knowledge of the resistivity jof the flushed formation is useful in estimating forma- I (Z -.tion resistivity factor and porosity. ;? ... .- Although formation conditions are not alwaysfavorable for estimating porosity, the log is useful "-- "l ~ ._-- _. ~ ~ ._. -qualitatively to detect porous and permeable intervals, )-to accurately determine the effective pay thickness and fto record borehole diameter variations. Mud resistivity ?may be measured in situ as the device is lowered into FIGURE 8.1the borehole with the pads in retracted position. Minilog· presentation formatOPERATION The Dresser Atlas Minilog is a hydraulicallyoperated pad-type logging device which records fourmeasurements simultaneously. The curves presentedare the spontaneous-potential or gamma ray curve forlithological correlation, a caliper for determiningvariations in borehole diameter and two shallow inves-tigation resistivity curves, the micro-inverse andmicro-normal. Figure 8.1 shows a typical Minilog. The electrode-pad and back-up-pad, located on op-posing arms of the device, are electro-hydraulicallyoperated by surface control and can be extended orretracted at any point in the borehole. The measuringelectrodes, mounted on the pad, are in contact withthe borehole wall during the logging operation. A photograph of the pad section of the Minilog instru- ment, with pads extended, is shown in Figure 8.2. The electrode arrangement consists of three small diameter electrodes in a vertical line spaced one-in. apart, embedded in the center of an insulated, fluid- filled rubber pad. Figure 8.3 shows the electrode arrangement. The intensity of the current is main- RGURE8.2 tained at a constant value at the current electrode Minilog pad section. "A." The electrode arrangement AMIAM2 is a 60
  • 67. cake. The curve is recorded on a scale which is calibrated in inches of borehole diameter on the left hand track of the log. THEORY Closely spaced electrodes on the face of an insulated pad maintained in close contact with the formation provide resistivity measurements of small volumes of formation in front of the pad. Because of the short electrode spacing, the borehole fluid tends to have a short-circuiting effect which must be eliminated. This is accomplished by hydraulic pressure which presses the pad tightly against the borehole wall so that the electrodes make direct contact either with the forma- tion or the mud cake. Since the effect of the mud is minimized, the Minilog" survey provides reliable data even when muds are saline. Radius of Invesdgatlon When a well is drilled with mud, a low permeability mud cake is deposited on the borehole wall as the drill bit penetrates a porous and permeable formation. The thickness of the deposited mud cake depends upon the mud filtrate loss to the formation and the sealingFIGURE 8.3 characteristics of this mud cake.Minilog" electrode arrangement. Readings obtained from the two micro curves are influenced by the resistivity of the mud cake, Rmce the zone completely flushed by mud filtrate, Rxo , thelateral-type measurement having an AO spacing of 1.5 invaded zone R, and in some cases the undisturbedinches. The AD spacing is the distance from the cur- formation, Rl . The magnitude of influence by each ofrent electrode "A ,. to the mid-point between elec- the above parameters is dependent upon the thicknesstrodes M) and M2. The potential difference between of the mud cake. depth of penetration of the invadingelectrodes M 1 and M2 is used to derive a resistivity drilling fluids, porosity and permeability of the forma-curve called the micro-inverse. Electrodes AM2 pro- tion.vide a two-inch micro-normal arrangement where the The micro-inverse curve, R 1 xl, a lateral typedifference in potential created by current electrode measurement, has a radius of investigation equal to"A" is measured between electrode M~ and a remote the AO electrode spacing of 1.5 in. Since this is a veryreference electrode. shallow investigation curve, its response is primarily The micro-inverse resistivity measurement, desig- influenced by the resistivity and thickness of the mudnated as R 1 xl, is presented on the log as a solid curve cake deposited on the borehole wall of porous andand the micro-normal measurement, R2 is presented permeable formations. The micro-normal curve, R2 isas a dashed curve. The measurements are recorded in a normal-type electrode measurement which has aQ-m. radius of investigation approximately two times the As the logging device descends into the borehole, electrode spacing of (0 inches. The four inch radiuswith the pads retracted, a measurement of the resis- of investigation, although substantially affected by thetivity of the mud is obtained and recorded from the mud cake, is primarily influenced by the formationmicro-in verse electrode arrangement. resistivity immediately behind the mud cake. When The pads arc extended at the bottom of the invasion of the formation is greater than six inches,borehole. A constant, firm pressure is exerted on the the formation beyond the invaded zone has very littlepads to maintain good contact with the borehole wall effect on either curve.as the pads slide upward over the rock surfaces and When a mud cake is deposited opposite a porousmud cake during the logging operation. and permeable formation, the two curves respond dif- The caliper provides a continuous measurement of ferently due to their differing radii of investigation.borehole variations from six to 16 inches, thereby in- The resistivity measured is directly related to thedicating borehole enlargement or the presence of mud resistivity of the mud cake and resistivity of the forma- 61
  • 68. tion flushed by mud filtrate. The resistivity of the and permeable formations providing permeability flushed formation is generally higher than the resistivi- allows invasion of an adequate volume of mud filtrate ty of the mud cake. The R2 being the deeper investiga- into the formation so that a mud cake is deposited. The tion curve) will give a higher resistivity than R I x J. The ability of the device to detect the presence of a mud cake difference between these two resistivity values is called is due to the differences in the depths of investigation of separation and the curves on the log may show the two curves. positive, negative or no separation. If the R2 curve has Opposite a porous and permeable formation, the a higher resistivity value than the R) x I curve, the ratio of the measured resistivity to the resistivity of the separation is referred to as positive. If the R2 curve has mud will generally be small with a maximum of 15 to a lower resistivity value than the R 1)( J curve, the 20. Although separation may exist above the max-separation is referred to as negative. imurn ratio, these formations are usually too dense for Since the mud cake represents a significant portion production. The log will generally have positiveof the small volume measured by each curve, the separation since the resistivity of the flushed forma-resistivity and thickness of this cake must be con- tion, R~o, is usually greater than the resistivity of thesidered in the evaluation of a formation. Although the mud cake, R tn l· • The amount of separation will becaliper presents evidence that a mud cake does or does dependent upon the contrast between the resistivity ofnot exist, its thickness cannot be determined accu- the mud cake and the flushed formation, Rmc/R xo,rately. Mud cakes filling borehole enlargements and the thickness of the mud cake, IUle. The largestopposite porous and permeable formations may be separation ",ill occur where mud cakes are thick andpartially or even totally unobserved by the caliper. the contrast is high. In the case of thin mud cakes and 10 resistive muds, separation may be negligible.APPLICATION Negative separation is observed opposite high porosity sands having good vertical permeability and The Minilog" service provides the resistivity curves containing very saline connate water, Vertical migra-necessary to detect mud cake formed on the borehole tion of the mud filtrate or very shallow invasionwall. Detection of the mud cake infers a porous and allows, R 2, to include in its measurement a portion ofpermeable formation. The ability of the logging device the formation beyond the invaded Lone which gives ato measure resistivity changes in great detail makes it lou value approaching the true resistivity of the for-an excellent log for accurate determination of effective mation while the R 1 x 1 reads the higher resistivity ofpay thickness. Under favorable conditions, Quantita- the flushed formation. This condition is often observ- tive evaluation provides a valuable approach towards ed in Gulf Coast water sands and might be expected whenever porosity and permeability are high and 10 determination of the resistivity of the flushed forma- tion which may be converted to formation resistivity water-loss muds are used ..An example of this effect is factor and porosity. shown by Figure 8.1. Posit ive separation exists in the The log also provides a means of measuring mud upper portion, and negative separation exists in the resistivity in situ when the instrument is run into the lower portion. Interpretation of the log would indicate borehole with the pads in retracted position, that hydrocarbons or deeper invasion occurs in the The caliper records borehole diameter variations upper portion of the sand having positive separation. which indicate borehole enlargement and presence of In this example, hydrocarbons are present where the mud cake. separation is positive, a transition zone occurs where the curves read the same resistivity and the zone is water-bearing where the separation is negative.INTERPRETATION Mud cakes are usually not formed through carbon- ates having Jugular or fracture porosity. These porousQualitative Use of the Minilog and permeable intervals are indicated by relatively low resistivity readings on both curves rather than by curve By inspection of the log, porous and permeable for- separation.mations can be defined. Since the separation between Mud cake tends to smooth out resistivity readings,the two curves is diagnostic, consideration must be and the curves do not show sharp variations evengiven to the appearance of the log opposite both when formation factor changes are appreciable withinpermeable and non-permeable formations for proper the formation.evaluation. Porous and Permeable Formations Impervious Formations Impervious formations are indicated by high Presence of mud cake on the borehole wall infers a resistivity values. When formations exhibit resistivities permeable formation. The Minilog can detect porous 62
  • 69. 20 times the resistivity of the mud, they are usually too Resistivitydense for production of hydrocarbons regardless ofthe separation. Rugose boreholes show numerous high Micro-Normaland low resistivity values and may have either positive Micro-Inverseor negative separation. This occurs because of distor-tion in the electrode current which causes the currentto now into the mud column when resistivity contrastsare high and the pad does not conform to the borehole Shalewall. I f the separation is positive, and the R I x I curvereads less than 10 times the resistivity of the mud, thepad is not conforming to the borehole wall. Tight ShaleShales Permeable It is expected that shales, generally considered to beimpervious formations, would show the same resis- Tight - ~ .. --,..~----_ .. Shaletivity for both curves. This is not always the case.When the borehole diameter is to gauge opposite a Permeable (POSS Hydrocarbon)shale formation, the pad may be separated from theformation by a thin mud film. The resistivity recorded Permeable (Water)for both curves will be low, and separation may indi-cate porosity and permeability. Correlation of the Permeableresistivity curves with the SP or gamma ray curves will (Water-no Invasion)distinguish shales from permeable sections. Thecaliper may be used to determine if positive separationis due to a thin porous streak, since a decrease in Shaleborehole size may indicate the presence of a mud cake,hence a permeable bed. Failure of the pad to conform to the wall of the holeor the existence of a washed-out section will cause positive separation. If the caving is deep, the resistivity reading for both curves will be close to the resistivity of the mud. A comparison of the Minilog" survey resistivity readings with the resistivity of the mud and 16-inch normal curve will indicate whether the readings are due to mud or formation. FIGURE 8.4 When the resistivity of the shale is approximately Typical MinilogJl: responses. equal to the resistivity of the mud cake, separation between the curves will be negligible, and when the resistivity of the mud is appreciably higher than the 4. Provides a record of borehole diameter variations resistivity of the shale, there will be substantial from the caliper which indicates mud cake and negative separation. This condition is often observed borehole enlargement. in the Gulf Coast Region. Typical responses of the Minilog are shown in MICRO I"ATEROLOG AND PROXIMITY LOG Figure 8.4. In summary, uses of the Minilog are: The pad-type focused logging devices, the Micro1. To determine effective pay thickness. Any forma- Laterolog and Proximity Log, provide shallow investi- tion that is sufficiently porous and permeable to gation resistivity measurements necessary to derive the admit mud filtrate and develop a mud cake, can resistivity of the formation flushed by mud filtrate. produce oil and gas. Knowledge of the resistivity of the flushed formation is useful, in evaluating the porosity and water satura-2. Porosity may be determined when formation tion of the formation. conditions are favorable. Leakage of current through the mud cake, which limits the use of the Minilog for quantitative resistivity3. Measures mud resistivity in situ .. as the instrument values in low porosity, high resistivity formations, is is lowered into the borehole with pads collapsed. alleviated by focusing the current through the mud 63
  • 70. cake into the formation. The current is allowed to caliper and a gamma ray or SP are recorded in the firstspread soon after entering the formation, keeping the track of the log. The second and third tracks are nor-resistivity measurement in the portion of the forma- maJJy a logarithmic scale of the respective resistivitytion which is near the borehole. measurement, covering a range of 0.2 to 2,000 Q..m. The Micro Laterolog is generally used where mud Figure 8.5 shows a Micro Laterolog run in combina-resistivities are low and mud cakes are not too thick. tion with a Dual Laterolog in a carbonate formation.The Proximity Log is affected less by mud cake and is The Proximity Log is very often run in combinationrecommended for the higher resistivity muds and with the Minilog" survey, with the Minilog curvesthicker mud cakes. Both devices successfully log low presented on a reverse scale in track one. Theporosity formations even when the ratio of the Minilog is also run simultaneously with the Microresistivity of the flushed formation to the resistivity of Laterolog, Densilog" service and Sidewall Neutronthe mud cake (R"o/R mc> exceeds 100. Logs. The Dresser Atlas Micro Laterolog and Proximity THf:ORY 0." M.:ASUREMENTLog are pad-type logging devices record threemeasurements simultaneously. The curves presented Micro Laterologon the log are the gamma ray or SP for lithologicalcorrelations, a caliper for determining variations in The pad of the Micro Laterolog comprises a beamborehole diameter and the Micro Laterolog or electrode in the center of the pad surrounded byProximity focused resistivity. focusing electrodes. These small electrodes are embed- The presentation for both logs is similar. The ded in an insulated fluid-filled rubber pad. GAMMA RAY ~{ r··" DUAL LATEROLOG CALIPER M1CRO LATEROLOG GAMMA RAY =E~ COMPENSATED DENSILOG hI V • CALIPER COMPENSATED NEUTRON • " ... " ....1,. . •• "III r[lIof,.·· PO~~lTV -, - - - u"t,lMA R"~ .... ..... " " .. ...... -. J - I f -- . -, ~- -- - , .... 1, •• .. "CLi-.Jn tt l,m~tne T.l,lt. PlIs:.- r IT .. r, -. f. ... - - - - r T T f I r :=zICr.~flI , I "I I·~ ~ r I i I 1 ~ ~J I I I I ..... _- - -~- -- IP1~ j i: , I. I i ~ I Ii ~J I.i ITt ~r-r I I I -~- : - ~~~ ! ~ I :1 ,II: --. ~ rill , III CALn ~~ I I I I .... ~ ~- ...... --[~;I I I I I I Btl .. II ! II I Irt I I i I ~~ I I I~ f:b l~ ~ I I ~ I ,I ] ,I I I I ~ I ! I ,f) 1 I <::: I ~ ~ , il I !f1 ~ I .~~ ~ I~I I I It::.... I lk I III I I~ i I ;~ I ---t=-a ~~i I~~ ~ ,. I ~ I I I I ! I d· I I iI ~ I ...~~ % ~~,J f--._ I .~ I i I 1( "~:lI I I ~ ~ i r~ p;~"L.1"~l1[1 ~ OLl ~ I l~ _~N I I~ " ~ I J -•• I < ~ l I ! I ~ c_ SLL1:· ;~ ./ ~ I ~ : I Ir""? I i I I [ ._1. ....... I 1(: , W--- I I ~ 11 __ ~~ 11 _.~! ~~ i I I p- I II I I I ~: I <:~~( I, I ~(~~~ ~~ < MLL1 ~ .~ ~ I r II ,1 i ~ } ~ !- I I I I ~! ~ 1 - <~~~; !~ ~J.illl ~ I I I ~~ I I I I ~ I I~ I I I I I I • I ; ~ I I ~ I ~ ! I 5;~H I t I t oC I II r- I II I I;~I I I f I I --t-- 4 -l > :> I i II I 1 !i~ I I il I ~ ~I I I I I I I I ~ ,[:? ~ I I I I «= nil II [, I Ii :i) . I I I -~~ 1ft ! i 11- , : II I: r ~ , I I .~FIGURE 8.5Typical Micro Laterolog/Dual Laterolog Combination. 64
  • 71. Measurement of a small volume of formation adja- Proximity Logcent to the borehole wall is achieved by applying acurrent of constant intensity to the beam electrode, The Proximity Log is a pad-type focused resistivitywhile a controlled supply of current is applied to the device which has additional focusing shields to reducefocusing electrode. The intensity of the current applied the effects of mud cake. This configuration is gen-to the focusing electrodes is automatically controlled erally referred to as a shielded-guard device. The beamand continuously regulated to maintain the potential electrode and focusing electrodes have larger cross sec-difference between the beam electrode and guard elec- tional areas than those of the Micro Laterolog. The in-trodes essentially at zero. By continuously maintaining crease in focusing area gives deeper radial investiga-this potential difference at zero. the current emanating tion characteristics.from the center electrode is forced to flow outward as The additional focusing shields, maintained at ana narrow beam perpendicular to the electrode and appropriate potential, tend to constrict the currentmud cake. The shape of this current beam is a func- beam emanating from the center electrode. Constric-tion of the distance and geometry of the focusing elec- ting the current beam through the mud cake, by pro-trodes in relation to the beam electrode. per design of the focusing shields in relation to the The current beam maintains a reasonably uniform guard and beam electrodes, reduces mud cake effects.shape through the mud cake, spreading as the distance Figure 8.7 shows the electrode configuration andfrom the borehole wall increases. A drop in potential shape of the current beam.is caused by the resistance of the formation to the flowof current to a remote current return electrode. Ameasurement of the potential difference between thecenter electrode and the return electrode in combina-tion with a calibrating constant gives the apparentresistivity of the formation. Figure 8.6 shows the elec-trode arrangement and shape of current beam. FIGURE 8.7 Proximity Log, electrode arrangement and shape of current beam. Radial Investlgation The pad-type focused resistivity logging devices are designed to in vestigate the formation near the borehole wall. In a porous and permeable formation invaded by drilling fluids, the measurements recordedFIGURE 8.6 by the instruments will be influenced by the resistivityMicro Laterolog electrode arrangement and shape of currentbeam. of the mud cake, thickness of the mud cake, the for- mation completely flushed by the invading fluids, the invaded formation, which may be considered as the 65
  • 72. transinon zone between the flushed formation andundisturbed formation and in some cases by the undis- MINILOG .... PROXIMITY LOGturbed formation. .., I The focusing of the Micro Laterolog and Proximity IJ ,.Log forces a narrow beam of current through the mud - -cake into the formation. The current enters the forma- RESISTIVITY " o&J J. . . ,CAl r--ftl ~tion perpendicular to the mud cake with no appreci-able change in the shape of the current beam. Since the - .:resistivity of the mud cake is generally less than theresistivity of the formation, the contribution of themud cake to the total response of the instrument is I ,.• ...... ··j·;fL I I II I I ~ ~~ III , IIsmall and can generally be neglected .. This is true as I! , I,long as the thickness of the mud cake does not exceed I I .~ ) I i~ ! jlthe limits of the logging device. I I "r. 1<> I I~~~ I I The amount of flushing that occurs in a formation I j ;flt III I I I II 1 I I Iis dependent upon the porosity and permeability of I I I I I 1 I 11 I I lU I I! : I ILl ~ M...... PROX II l~~ Ithat formation and the quantity of fluids that filter out I I I II I J I II 1 ~~of the mud. The amount of formation flushing varies I tf4 M , I IJ ~; r II I I I, I ~continuously from the borehole wall through the I 1 I ~ I I j ..formation, until it reaches the undisturbed formation. I I I I I I I 1,IGenerally, it is considered that complete flushing only ........4t+ .- I -~i ~I I I "occurs in the first (0 or three inches of an invaded j I Iformation. Since the Micro Laterolog receives very 1 ! I -~ I rr , I j I !;little of its response beyond a distance of three inches I l c: I I ( I I i I I, Ifrom the pad, it essentially measures the resistivity of I I ~- II I i I I H~the flushed formation. I, II I I I I I ~ ---))1 I iI II The Proximity Log, as a result of improved focus- I 1 1 1 I 1 I IIing through the mud cake, has a slightly greater depth I II III _. - ... -~ I I Ii Iof investigation. It receives most of its response within .. -.4- ,; ·.. .-..-. · . , .. ,a distance of six to 10 inches from the pad. Eventhough the radial investigation of this instrument isdeeper than the Micro Laterolog, field tests indicate .. - . . -~ ~~. ~. ~--- -_.t-~ ----1 · , ~..:~ .. --~. -+~~- I ..- ... ....-.- I ... ~ ...... · ...... -that where moderate to deep invasion exists and suffi- ~~ · .~cient flushing has occurred, reliable values may be .:~ -e. t> I . .... , .. - .........obtained from the resistivity of the flushed formation. - .. K _. • . . . . . .-j .~ · .. --. r:: .~ " . · ......... .......Vertical Resolution "5 ~ ~-- c:;~ I ,-;J: ~. -. -....-..-7 · ...... ... - ... ~~~/;It -:;:::;> Both the Micro Laterolog and Proximity Log, being ~.:-. ~ .t-pad-type focused resistivity devices, have high resolu-tion for thin beds and respond to rapid changes in for- ..~ ~ --- ~_.~ ~ ~:~ ....... ... ..-.. .... t~ __ .... ~ - .----...... -..... ~ ..mation resistivity. When formations are one ft thickand greater, the adjacent bed effect is negligible for ~-=.+~ ~---- ---~ ~ .......the Proximity Log. Effects of adjacent beds are less ---. - ... .~ - . [Ill j~~ij ~for the Micro Laterolog, giving this instrument a very j I .l--~ I I Ihigh degree of accuracy in defining bed boundaries. RGURE 8.8 Figure 8.8 shows a proximity log recorded in com- Proximity log recorded in combination with a Minilog.bination with a Minilog", tion resistivity values. As the contrast and mud cake thickness increase, the effect of the mud cakeMud Cake Effects increases. Mud cake effects on the Proximity Log are negli- The effect of the mud cake on the Micro Laterolog gible for mud cakes 3/4-in. thick, even when the con-is negligible when the thickness of the mud cake is less trast between the resistivity of the formation and thethan 3/8-in. When the contrast between the resistivity mud cake is high.of the formation and the mud cake is small, the thick- Corrections for the effects of mud cake are shownness of the mud cake has very little effect on forma- for both devices by Figures 8.9 and 8.10. 66
  • 73. with deep investigation resistivity measurements. 100 The Micro Laterolog is most widely used in wells ( 14... ~~ ~~ drilled with 10 resistivity muds. When the resistivity 3/8" ~vv of the mud filtrate is approximately equal to or less 50 ~ v1l2 than the formation water resistivity, the contrast bet- 40 30 ~V 3/4r ween the resistivity of the formation and mud cake ~ v~ 11 resistivity is high. For these conditions mud cakes are 20 v Mud Cake ~ Thickness generally not too thick, and mud cake contribution to (Inches) the total resistivity measurement is small. The instru- ment will perform equally well where the resistivity of , ., ~ .lII1II the mud filtrate is greater than the resistivity of the formation water as long as the thickness of the mud 5 cake does not exceed instrument limitations. To ac.. 4 V / curately define the resistivity of the flushed formation, 3 invasion should be greater than six inches so that the / 2 formation beyond the flushed zone will not influence the resistivity measurement. 1 The Proximity Log is recommended for wells 1 2 3 4 5 20 30 4050 100 having thicker mud cakes. The radial investigation of a larger volume of formation by this instrument requires deeper invasion and more formation flushingFIGURE 8.9 for this log to read the resistivity of the flushed forma-Micro Laterolog determination of Rxo . tion. As discussed previously) the Rxo obtained by either log is used to correct the apparent R, (from the Dual Induction or Dual Laterolog) for invasion through use 100 ( I J of the tornado charts. 1/4 in.~ ~~rz The Proximity Log and Micro Laterolog can be ..tv 1 50 ~ f------ combined with a deep investigation resistivity log and .~ 3/4 in. 40 a porosity log to calculate movable hydrocarbons, 30 / such as oil. 20 I I / , , I / 1/ iV----_. / ~ 5 4 / --- 3 V 2 i/ 1 1 V 2 3 4 5 10 Rxo 20 30 4050 100 Am,RGURE 8.10Proximity Log determination of Rxo .APPLICATION The Micro Laterolog and Proximity Log are used tomeasure the resistivity of the flushed lone. Theresistivity of the flushed zone, Rxo , may be used todetermine porosity and estimate water saturation. Toestimate water saturation, the log values are combined 67
  • 74. QUESTIONS 8(1) Which micro-resistivity device has the best vertical resolution and provides the most positive indica- tion of mud cake along the borehole? a) Proximity Log b) Minilog" c) Micro Laterolog •(2) Which of the factors below decrease the information available from the Minilog" service? a) rugose borehole b) air-filled borehole c) Rmc/R xo =1 d) none of the above e) all of the above(3) A Micro Laterolog is most useful in formations drilled with mud. a) fresh b) saline c) oil base mud d) none of the above(4) I f the resistivity of the flushed zone is 32 Q-m, the residual oil saturation is 22070 and Rm f = 2 Q-m. What is the porosity in a clean carbonate? (a = 1; m = n = 2) a) 32f1Jo b) 28010 c) 22070 d) not enough in formation 68
  • 75. SPONTANEOUSPOTENTLAL(SP)LOG 9INTRODUCTION The variations of the measured SP indicate that The Spontaneous Potential (SP) log is a record of there are currents flowing within the wellbore. Thesethe naturally occurring potentials in the wellbore as a currents are primarily of an electrochemical nature.function of depth. The SP log involves a single,moving electrode in the borehole and a reference elec- Electrochemical Component of the SPtrode, usually located at the surface in the mud pit orsome other suitable location. (Fig. 9.1) Mounce and Rust used a simple experiment to prove The recording is a relative measurement of the DC that two waters of different salinities, together withvoltage in the borehole without a zero being recorded. shale and a permeable inert membrane between the(Fig. 9.1) Readings opposite shales are relatively cons- two fluids, generate an electromotive force and cur- rent flow in the cell. (Fig. 9.2) The current flows from the fresh to the salty water and then through the shale. Removal of the shale stops the current flow. Inter- changing the two liquids reverses the direction of cur- rent flow. SP -H+ Fresh Water Shale Salty Water t Permeable Shale Salty Water Shale FIGURE 9.2 Salty Water Electrochemcial component of the SP.FIGURE 9.1 ShaleSP curve responses. The cell in Figure 9.2 proves very similar to condi- tions existing in the borehole, where the drilling mudtant and are referred to as "the shale base line." Op- salinity is different from the formation water salinity.posite permeable formations (typically the SP curve) The measured SP is the voltage observed in theshow excursions to the left (negative polarity) or to the borehole caused by the potential drop as the currentsright, depending upon the salinity of the drilling mud flow through the mud. Generally, the potential drop isand formation waters. The logging engineer sets the larger in the borehole than in the shale or permeableposition and sensitivity so that the logged deflections formations.opposite permeable beds stay within the plotting Assuming the solutions contain only NaCl and thelimits. Normally, the shale base line on the plotting mud activity is limited to the free fluid within it, thentrack is set two chart divisions from the right of the the following simplified analysis applies. This discus-track. sion is based on fresh mud and salty formation water. 69
  • 76. (Fig. 9.3) The shale, due to its predominant clay con- The SP Curvetent, acts as a cationic membrane. That is, it ispermeable to cations (Na ~ ), but not to anions (Cl ) The formation contammg salt water must bedue to an apparently high negative charge on the clay permeable for an SP to develop. (Fig. 9.4) Perme-lattice. The sodium ions (Na +) can then move through ability does not quantitatively influence the SP. Thethe shale from the high concentration salt water to the minimum permeability necessary for generation of SPlower concentration fresh water or mud. This move- is not that normally considered for liquid flow at com-ment of cations generates a membrane potential. mercial rates, but that which is sufficient to allow the flow of ions. The SP may develop opposite beds which yield little fluid on a test. The SP will not develop Fresh opposite impermeable beds. Mud (Water) Shale SP Salty Water in Shale Sandstone Bore- (Permeable) hole Permeable Bed + Shale + + Fresh Mud Salty SP Water .--- ShaleFIGURE 9.3Fresh mud and salty formation water effects. Impermeable Limestone At the salt water (formation water) and fresh water(mud filtrate) contact, the Cl - ions have greater Shalemobility than the Na -+ ions and thus move morerapidly. This rapid movement generates a negativepotential across the "liquid junction." In the mud column opposite the shale, a positive FIGURE 9.4 Development of SP.potential is generated by the Na + ions while at thejunction between the formation water and the mud Porosity does not have independent influence on thefiltrate a negative potential is developed. These spon- SP. The influence of permeability is binary (yes/no) intaneous potential differences cause current flow in the which the only applicable question is: does the bedmud column. have permeability ... yes or no? If yes, a SP may The magnitude of this potential (E e) is develop; if no, the SP will not develop. The SP has a remarkable ability to repeat during aw Rmf E e = -K log- = -K l o g - (1) subsequent logging runs. Every small variation of the amf R wc SP may be physically significant. This becomes ap- parent when the many small variations between wellswhere a w and amf are the mean activities of the two and, in many cases, over wide areas correlate to a highNaCl solutions at the given temperature. K is a coeffi- degree.cient which is proportional to temperature. These acti-vities are related to the salinity of the solutions and The Static SPthus to the resistivities. The Rwe to R". correction isobtained by empirical chart which corrects for high The Static SP (SSP) refers to the maximum SP thatsalinities and divalent ions. can be obtained given a shale and two waters of dif.. 70
  • 78. ferent salinity. It is essentially the SP that would be rshatoobtained if no current flowed. The SSP has no bedboundary effects. Since current flows and the actualSP is the measurement of the potential changes in the r mudwellbore, there are variations between the idealizedSSP and the actual SP. (Fig. 9.5) The bed boundaries rsa.,dof the actual SP are at the in flection points, (max-imum slope on the curve). (Fig. 9.6) Inflection pointsoccur where the greatest potential drop occurs, i.e., atthe bed boundaries. The finite magnitude of the SPelectrode also tends to elongate the bed boundary ef- Shalefeet. The in tlection point always occurs at the bedboundary though its horizontal position may vary, oc- Bore-curring near the shale base line, or even close to the hole Sandtop of the permeable bed anomaly. Current FIGURE 9.7 Current for SP. It is apparent the measured SP is approximately equal to the SSP under conditions where the effective resistance of the shale and permeable bed are very small in relation to the borehole resistance. Remem- bering that resistance is: L r R- (3) A where: r resistance Bed R resistivity Boundary L lengthFIGURE 9.6Bed boundaries of the actual SP. A = area The SP currents flowing through the shale, thepermeable bed and the mud column are alI subject to The analysis of the effect of the environmentalpotential drops due to the electrical resistance present. geometry is relatively easy at least on a qualitative tA schematic circuit for the SP currents using effective basis. From Equation 2, it can be observed that maximumresistances encountered by the SP due to permeable SP is obtained when the borehole resistance is large.formation and shale is shown in Figure 9.7. Applica- This can be accomplished by having a high resistivity,tion of ohms law gives: (fresh water) mud and a small diameter borehole in which both the shale beds and permeable beds are very thick. This makes the respective area terms large. Vm Vsh Normally, the borehole diameter is relatively constant; = --- = - - = Vsd the important variable thus becomes the length of the r"h rsd mud column in the borehole. As the beds get thicker the mud column length opposite them increases and the and Vrn + Vsh + V~ = SSP area terms for the shale and permeable beds increase.then: This increases the voltage drop in the borehole and reduces the voltage drops in the shale and permeable beds. Thick shale beds with a thin permeable bed give SP SSP (2) a small borehole resistance opposite the permeable rm + rsh + r~d bed, which according to Equation 2 reduces the SP. 72
  • 79. As the permeable bed thickness increases, the depth and maximum temperature reading is used; orresistance ratio term in Equation 2 approaches unity 6 b) by the BHT, total depth and average surfaceSo in very thick beds, the SP measured will be very temperature as outlined in Lesson 4.close to the SSP. After obtaining formation temperature, the mud Invasion of the mud filtrate into the formation resistivity and mud filtrate resistivity measured at thetends to reduce the SP by moving the triple point, surface on the log heading are corrected to thiswhere the shale, salt water and fresh water meet, away temperature using the charts in Lesson 3.from the borehole, which reduces the number of cur- Establish the shale base line on the SP. The shalerent lines flowing in the borehole. The effect of inva- base line in fresh mud environments will generally besion appears (0 be less severe than envisioned initially. the line established by the maximum SP deflections toGenerally, invasion is not a piston-type mechanism in the right.which all the formation water is displaced at the inva- The inflection points on the SP curve represent thesion front. This long transition zone may be compared bed boundaries of the formation. The inflection pointto a series of liquid junction potentials that are in (for maximum slope) on the curve is generated byseries. Thus, the effect of invasion on SP appears to be maximum current flow in the well bore at the bedmuch less than expected from the theory. boundary. The apparent bed thickness from the SP is used; not the bed thickness indicated on some otherHydrocarbon Bearing Zones log. If the bed appears shaly, a shaliness correction must be applied to the SP. The SP usually has a lesser amplitude in hydrocar- The SP value is the millivolt reading indicated onbon zones, (Fig. 9.8) Hydrocarbons (non-conductors) the log, from the shale base line to the maximumin the permeable zone give higher permeable zone deflection on the SP in the permeable bed. It is advis-resistivity and correspondingly reduced SP. Thus, it is able to use the SP in a water-bearing zone. Oil andbetter to use the SP (when Rw is needed) in the water- gas-bearing zones generally depress the SP and thisbearing section of the reservoir. lesser deflection w ill calculate to a higher Rw than actually exists. In a shaly zone) the SP is depressed due Shate to the shale content and a reasonable R w may not be obtained from the SP unless a shale correction is Hydrocarbons applied to the SP. Chart 9.1 is a generalized correction chart for the SP. It corrects the bed thickness and resistivity effects Water on the SP amplitude. More exotic, complete and diffi- cult charts are available; however, for most applica- tions they are not significantly better. Low resistivity, Shale thick beds require little or no correction. If the bed is thick enough, the SP will reach maximum applitude.FIGURE 9.8 (Fig. 9.9) On Chart 9.1, the resistivity value from theSP amplitude behavior In GeneralShale Effect SP Shale, in a permeable bed, generates a secondary SP Needsof opposite polarity to the main SP reducing the Correctionmeasured SP. This reduction is directly proportionalto the percentage of shale present.R w From the SP A procedure to determine R w from the SP is out-lined below. This is a simplified but adequate ap .. Needs Noproach for normal interpretation. The data and charts Correctionneeded arc shown in the text. The zone or permeable bed in which water resistivitywill be determined is selected on the log. FIGURE 9.9 The formation temperature can be determined Bed thickness effecteither by: a) direct measurement, if the zone is at total 73
  • 80. short normal is used as Rr, A shallow, focused log, mined from Chart 9.2 by dividing Rmr, at formation(from the dual induction), as well as any microresis- temperature, by the Rmr/Rweq ratio.tivity log, can be used. Use Chart 9.3 to correct R weq for the average devia- Chart 9.2 relates the corrected SP deflection, for- tion from sodium chloride solutions found in forma-mation temper ature and the ratio of the resistivity of tion water. This corrects for high salinities and normalthe mud filtrate to apparent formation water for a concentrations of calcium, magnesium and othersodium chloride solution (Rweq ) . From this chart divalent ions as well as the influence of formationdetermine Rmr/Rwc..q. temperatures. The final result is R w which should be R weq is obtained from the Rn1r/RWC ratio deter- used with other data for the interpretation. 30 ,~ SP from Log 120 SS~O 110 30 20 ~l 100 90 40 50 " ~ 1 ~~ 80 60 . SP Correction Factor ,, , 70 ... 70 15 0 11 7< , 60 80 11 1;] , /.4 .s 1 ~ ~ ~ i 50 90 ,, ~ ~O ! ,1 40 100 s: <S ~ 10 ~ ·0 G) e .>t:. o 1 ~ ~ 30 a,S 4·0 10 , 9 ~ ~ ~"" 1: to- 20 50 ·0 120 ~ ~ 8 7 " ~ " ~ <, 6 , ~ ll.. ~ ~ r-, r-, ~ ~ I~ r 5 1, 1 " -, ~ ~ ~~ "-", r-, ~~ 4 5 , I 20 ~ ..... sor-, f 100 , ,, ~ <, "200~ Am ~ ~~ -~ "~ ~ r----... 3 r-, ~ ~ 10 1.2 1.5 2.0 2.5 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 SP Correction FactorCHART 9.1SP correction. 74
  • 81. 0.5 0.4 -- I 100 0F ~/ 0.3 150 0F /t~~ 200 0F .-~~ 0.2 I -: ~ - ,.... 300 0F ~~ t ~~~ / .", 400 0F ~~ ~ / I ~" ~ V 500°F 0.1 1 I Lt:V / ~/ /" "" I I I ~~, ~-" ~~ »: I ~V ~ ~ ~ 0.05 01 ~i I ~ ~ ~V ~ I .L, a: o A ~I I I I I I I i rl 0.02 I I /. WI / i I f J~VI V ! 0.01 /J ~/l 1 7 , // ~7 j I- 1/ / / 7 r, I I , / / I {J ) J V/ J 100 ·F I I I 0.005 I / I I I r I 150cF J J J 200 I ~ 0 0.002 Vv soo ~FI a<lpoF I( I I ! 4bO~ F I 0.001 1 0.005 0.01 0.02 0.05 0.10 0.20 0.50 Aw or R ml • gemCHART 9.3Rw from Awe· 76
  • 82. QUESTIONS 9Questions(I) The water resistivity for the Problem log (Example 9.1) is about a) 0.017 b) 0.027 c) 0.29 d) 0.19(2) Shale in a reservoir containing saline formation water and drilled with a fresh mud system a) reduces SP amplitude b) rounds the shoulders c) has no effect d) reverses(3) If sand #1 has a 100 mV SP and sand #2 has a 90 mV SP, sand #1 has a) a higher permeability b) a lower permeability c) the same permeability d) not enough information(4) If the SP correction factor is 1.7 and the SP from the log is 50 millivolts, what is SSP? t a) 91 b) 85 c) 76 d) 50 77
  • 83. SP DEPTH RESISTIVITY CONDUCTIVITY Ohms m2/m Millimhos/m SP (MillivOlts) 16" NORMAL INDUCTION CONDUCTIVITY 20 0 2 40" SPACING - UI I + 0 10 4000 0 0 100 Am = 0.7 (fJ. 78° 8000 4000