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Zezo research14

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Zezo research14

  1. 1. Science Benha Faculty ofscience Benha University Geophysicsdepartment Fourthlevel 2013 Electrical well logging Resistivity and Spontaneous Potential Abdel Aziz Hamed
  2. 2. Electrical Well Logs AbdelAzizHamedElectrical Well Logs 1 Content Page No What is the logging…………………….………………..……….……………..….. (2) What is the wire line logging……………………………..….…………………. (2-3) Why log…………………………………………………………………..………… (5- 7) Uses of logs……………………………………………………………..……………(8) Pre face of electrical logs SP…………………………………………………………………..………..… (9-10) Resistivity……………………………….…………………………..……… (11-14) Spontaneous Potential Log Uses of SP Log……………………………………………………....……(15-16) Applications of SP Log………………………....………………………. (17-23) Instrumentation……………………….……………………………………… (24) Calibration and standardization……………………………………………(24) Radius of Investigation ……………………………………...….........….… (25) Unwanted Effects ……………………………………...……….…...….... (26-27) Factors affected on SP Deflection …………………………………………(28) Quantitative Uses of SP ……………………………………………….... (29-30) Limitation ……………………………………………...……………...……… (31) ResistivityLog Resistivity……………………………………………..…………...…..... (32-33) Formation Factor ……………………………………….....………...…. (33-34) Application of Resistivity……………………………………………….... (34) Zone of Invasion and Resistivity ……………………………………..….. (35) Factors affecting on Resistivity…………………………………….…. (36-37) ResistivityTools………………………………………….………….……(37-43) Quantitative Uses of ResistivityLog …..……………………….…….. (44-45) Qualitative Uses of ResistivityLog …………………......................... (45, 47) References………………………………………………………...………………... (48)
  3. 3. Electrical Well Logs AbdelAzizHamedElectrical Well Logs 2 What is Logging? The birth of logging can be dated to the first recorded event [1] at Pechelbronn on September 5, 1927 where H. Doll and the Schlumberger brothers (and a few others) made a semi continuous resistivity measurement in that tired old field in Alsace. The operation was performed with a rudimentary device (a sonde) consisting of a Bakelite cylinder with a couple of metallic electrodes on its exterior. Connecting the device to the surface was a cable/wire, thus providing us with the term wire line Logging. Wire line refers to the armored cable by which the measuring devices are lowered and retrieved from the well and, by a number of shielded insulated wires in the interior of the cable, provide for the electrical power of the device and a means for the transmission of data to the surface. More recently, the devices have been encapsulated in a drill collar, and the transmission effected through the mud column. This procedureis known as logging while drilling (LWD). What is Wireline Logging? The process oflogging involves a number of elements, which are schematically illustrated in Fig. 1.1. Our primary interest is the measurement device, or sonde. Currently, over fifty different types of these logging tools exist in order to meet various information needs and functions. Some of them are passive measurement devices; others exert fsome influence on the formation being traversed. Their measurements are transmitted to the surface by means of the wire line. Much of what follows in succeeding chapters is devoted to the basic principles exploited by the measurement sondes, without much regard to details of the actual devices. It is worthwhile to mention a few general points regarding the construction of the measurement sondes. Superficially, they all resemble one another. They are generally cylindrical devices with an outside diameter on the order of 4 in. or less; this is to accommodateoperation in boreholes as small as 6 inch. in diameter. Their length varies depending on the sensorarray used and the complexity of associated electronics required. It is possible to connect a number of devices concurrently, forming tool Strings as long as 100 ft. Some sondes are designed to be operated in a centralized position in the borehole. This operation is achieved by the use of bow-springs attached to the exterior, or by more sophisticated hydraulically actuated “arms.” Some measurements require that the sensor package (in this casecalled a pad) be in intimate contact with the formation.
  4. 4. Electrical Well Logs AbdelAzizHamedElectrical Well Logs 3 This is also achieved by the use of a hydraulically actuated back-up arm. Figure 1.2 illustrates the measurement portion of four different sondes. On the right is an example of a centralized device which uses four actuated arms. There is a measurement pad at the extremity of each arm. Second from the right is a more sophisticated pad device, showing the actuated back-up arm in its fully extended position. Third from the right is an example of a toolwhich is generally kept centered in the borehole by external bow-springs, which are not shown in the photo. The toolon the left is similar to the first device but has an additional sensorpad which is kept in close contact with the formation being measured. These specially designed instruments, which are sensitive to one or more formation parameters of interest, are lowered into a borehole by a surface instrumentation truck. Fig. 1.1 The elements of well logging: a measurement sonde in a borehole, the wire line, and a mobile laboratory. Courtesy of Schlumberger. This mobile laboratory provides the downhole power to the instrument package. It provides the cable and winch for the lowering and raising of the sonde, and is equipped with computers for data processing, interpretation of measurements, and permanent storage of the data. Most of the measurements which will be discussed in succeeding chapters are continuous measurements. They are made as the toolis slowly raised toward the surface. The actual logging speeds vary depending on the nature of the device. Measurements which are subject to statistical precision errors or require mechanical contactbetween sensor and formation tend to be run more slowly between 600 ft and 1800 ft/h – newer tools run as fast as 3600ft/h. Some acoustic and electrical devices can be withdrawn from the well, while recording their measurements, at much greater speeds.
  5. 5. Electrical Well Logs AbdelAzizHamedElectrical Well Logs 4 The traditional sampling provides one averaged measurement for every 6inch. of tool travel. Forsome devices that have good vertical resolution, the sampling interval is 1.2 inch. There are special devices with geological applications (such as the determination of depositional environment) which have a much smaller vertical resolution; their data are sampled so as to resolve details on the scale of millimeters. In the narrowest sense, logging is an alternate or supplement to the analysis of cores, side-wall samples, and cuttings. Although often preferred because of the possibility of continuous analysis of the rock formation over a given interval, economic and technical problems limit the use of cores.∗ Side-wall cores obtained from another phase of wire line operations give the possibility of obtaining samples at discrete depths after drilling has been completed. Side-wall cores have the disadvantage of returning small sample sizes, as well as the problem of discontinuous sampling. Cuttings, extracted from the drilling mud return, are one of the largest sources of subsurfacesampling. However, the reconstitution of the lithological sequence from cuttings is imprecise due to the problem of associating a depth with any given sample. Although well logging techniques (with the exception of side-wall sampling) do not give direct access to the physical rock specimens, they do, through indirect means, supplement the knowledge gained from the three preceding techniques. Well logs provide continuous, in situ measurements of parameters related to porosity, lithology, presence of hydrocarbons, and other rock properties of interest. Fig. 1.2 Examples of four logging tools. The dipmeter, on theleft, has sensors on four actuated arms, which are shown in their fully extended position. Attached to the bottomof one of its four arms is an additional electrode array embedded in a rubber “pad.”It is followed by a sonic logging tool, characterized by a slotted housing, and then a density device with its hydraulically activated back-up arm fully extended. The toolon the extreme right is another version of a dipmeter with multiple electrodes on each pad. Courtesy of Schlumberger.
  6. 6. Electrical Well Logs AbdelAzizHamedElectrical Well Logs 5 ?Why log Exploratory drill holes or wells are the only means of direct access to the subsurface.Drilling is an expensive means of access to the lithosphere, and sampling of the rocks and fluids penetrated and geophysicalwell logging are the only ways information can be derived from these holes. Valid well logs, correctlyinterpreted, can be used to reduce future drilling costs by guiding the location, properdrilling, and construction of test holes and productionor disposalwells. Welllogging also enables the vertical and horizontal extrapolation of data derived from drill holes. ,the lithologyGeophysicallogs can be interpreted to determine ,bulk density,resistivity factor-formation,resistivity,geometry yield ofspecific, andmoisture content,permeability,porosity , andmovement,to define the source, andbearing rocks-water . Quantitativechemical and physical characteristics of water interpretation of logs will provide numerical values for some of the rock characteristics necessaryto designanalog or digital models of ground-water systems.Log data aids in the testing and economic developmentof ground-water supplies and of recharge and disposalsystems and can be of considerable value in the designand interpretation of surface geophysicalsurveys. Stallman (1967)pointed out that if the following pretestinformation is not obtained , failure of pumping tests is invited. Hydraulic conditions along the well bore ; storage characteristics of the aquifer; and depth to, and thickness of, the aquifer being tested,as well as changes in either within the area of the test. He also suggested that changes in transmissivity should be mapped.Estimates of pertinent hydraulic properties of the aquifer can be provided by borehole,geophysicalstudies. , Geophysical well logging can provide continuous objective records with values that are consistentfrom well to well and from time to time, if the equipment is properlycalibrated and standardized. In contrast, the widely used geologist’s or driller’s log of cuttings is subjective, greatly dependentupon personal skills and terminology, and is limited to the characteristics being sought.
  7. 7. Electrical Well Logs AbdelAzizHamedElectrical Well Logs 6 Geophysicallogs can be reinterpreted in a postmortem investigation of some geologicor hydrologic factor that was not considered while the hole was being drilled. Serendipity -the gift of finding agreeable or valuable things accidentally-has resulted in the discoveryof uranium, phosphate,potash, and other minerals from the interpretation of well logs. Each year many more wells are drilled for water than for petroleum. Although mostof the water wells are shallow, each is a valuable sample of the geologic environment, and logs of these holes can aid in the definition and developmentof water supplies,sand and gravel deposits,other nonmetallic and metallic mineral deposits,petroleum,and waste storage or disposaland artificial-recharge sites and can provide the engineering data necessary for construction. In contrast to uninterrupted geophysicallogs, samples of rock or fluid almost never provide continuous data. Even if a hole is entirely cored,with loo-percentrecovery, laboratory analysis of the core involves the selectionof point samples.Continuous coring and subsequent analysis of enough samples to be statistically meaningful costs much more than most geophysical-logging programs.In addition, the volume of material investigated by mostlogging sondes may be more than 100 times as large as the volume of most core samples extracted from the hole. Although geophysicallogging should partly supplant routine sampling of every drill hole, some samples,properly taken and analyzed, are essential to the interpretation of logs in each new geologic environment. Sidewall sampling techniques are available for poorly consolidated sediments and can be utilized after logs have been run, to provide the most representative samples (Morrison, 1969).Sidewall samples can also be taken in hard rocks by commerciallogging service companies; however, they are relatively expensive.One well, adequately sampled and logged,can serve as a guide for the horizontal and vertical extrapolation of data through borehole geophysics.Furthermore, well logging provides the only means for obtaining information from existing wells for which there is no data and from wells where casing prevents sampling.
  8. 8. Electrical Well Logs AbdelAzizHamedElectrical Well Logs 7 Geophysicallogs can be digitized in the field or office by commercialservice companies and are then amenable to computeranalysis and the collation of many logs.They can be very economically stored on, and retrieved from, magnetic tape. Digitized geophysicallogs of oil wells are transmitted by radio and telephone for interpretation by log analysts in response to the need for rapid answers at the well site. Logging techniques also permit time-lapse measurements to observe changes in a dynamic system. Changes in both fluid and rock characteristics and well construction caused by pumping or injection can be determined by periodic logging.Radiation logs and, under some conditions, acoustic logs are unique in providing data on aquifers through casing. This permits logging at any time during or after the reestablishmentof native fluids behind the pipe. The graphic presentation of geophysicallogs allows rapid visual interpretation and comparisonat the well site. Decisions on where to set screen and on testing procedurescan be made immediately, rather than after time-consuming sample study or laboratory analyses. 1. Plan the logging program on the basis of the data needed. 2. Carry out drilling operations in a manner that produces the most uniform hole and the least disturbance of the environment. 3. Take representative formation and water samples where necessary,using logs as a guide, if possible. 4. Insist on quality logs made with calibrated and standardized equipment. 5. Logs should be interpreted collectively, on the basis of a thorough understanding of the principles and limitations of each type of logging technique, and some knowledge of the geohydrologicenvironment under study.
  9. 9. Electrical Well Logs AbdelAzizHamedElectrical Well Logs 8 USES OF LOGS A set of logs run on a well will usually mean differentthings to different People.Letus examine the questions asked–and/or answers sought by a variety of people. The Geophysicist: As a Geophysicistwhat do you look for? '' Are the tops where you predicted? '' Are the potential zones porous as you have assumed from seismic data? '' What does a synthetic seismic sectionshow? The Geologist: The Geologistmay ask: '' What depths are the formation tops? '' Is the environment suitable for accumulation of Hydrocarbons? '' Is there evidence of Hydrocarbon in this well? '' What type of Hydrocarbon? '' Are Hydrocarbons presentin commercialquantities? '' How good a well is it? '' What are the reserves? '' Could the formation be commercialin an offsetwell? The Drilling Engineer: "What is the hole volume for cementing? "Are there any Key-Seats or severe Dog-legs in the well? "Where can you get a good packer seat for testing? "Where is the bestplace to set a Whip stock? The Reservoir Engineer: The ReservoirEngineer needs to know: "How thick is the pay zone? "How Homogeneous is the section? "What is the volume of Hydrocarbon per cubic meter? "Will the well pay-out? "How long will it take? The Production Engineer: The ProductionEngineer is more concerned with: "Where should the well be completed (in what zone(s))? "What kind of productionrate can be expected? "Will there be any water production? "How should the well be completed? "Is the potential pay zone hydraulically isolated?
  10. 10. Electrical Well Logs AbdelAzizHamedElectrical Well Logs 9 Spontaneous potential& Resistivity Log 1) Spontaneous potential: (SP) The spontaneouspotential log (SP) measures the natural or spontaneouspotential difference (sometimes called self-potential) that exists between the borehole and the surface in the absence of any artificially applied current. It is a very simple log that requires only an electrode in the borehole and a reference electrode at the surface. These spontaneous potentials arise from the different access that different formations provide for charge carriers in the borehole and formation fluids, which lead to a spontaneous current flow, and hence to a spontaneous potential difference. The spontaneous potential log is given the generic acronym SP.
  11. 11. Electrical Well Logs AbdelAzizHamedElectrical Well Logs 10 Origin of SP current: • Electrochemical components • Electro kinetic components Tool Operation: The tool is extremely simple, consisting of a single electrode that is connected to a good surface earth point via a galvanometer for the measurement of DC potential. A small 1.5 V battery is also included commonly to ensure that the overall signal is measured on the correct scale. Uses of SP: • The detection of permeable beds • The determination of Rw • The indication of the shale lines of a formation • Correlation. Notes: • The SP tool has a poorresolution. So it can be used for correlation. • The drilling mud salinity will affect the strength of the electromotive forces (EMF) which give the SP deflections. If the salinity of the mud is similar to the formation water then the SP curve may give little or no responseoppositea permeable formation; if the mud is more saline, then the curve has a positive voltage with respect to the baseline opposite permeable formations; if it is less, the voltage deflection is negative. In rare cases the baseline of the SP can shift suddenly if the salinity of the mud changes part way down hole. • Mud invasion into the permeable formation can cause the deflections in the SP curve to be rounded off and to reduce the amplitude of thin beds. • A larger wellbore will cause, like a mud filtrate invasion, the deflections on the SP Curve to be rounded off and decrease the amplitude opposite thin beds, while a smaller diameter wellbore has the opposite effect.
  12. 12. Electrical Well Logs AbdelAzizHamedElectrical Well Logs 11 2) ResistivityTools: Resistivity logging is a method of well logging that works by characterizing the rock or Sediment in a borehole by measuring its electrical resistivity. Resistivity is a fundamental material property which represents how strongly a Material opposes the flow of electric current. The log must run in holes containing electrically conductive mud or water. 1-Electrode tools: Modern Resistivity Log: Laterologs: (LL) It is a type of modern electrodes which have a number of electrodes. • LL3 has 3 current emitting electrodes (vertical resolution is (1ft). • LL7 has 7 current emitting electrodes (vertical resolution is (3ft). • LL8 is similar to the LL7, but has the current return electrode (Vertical resolution is 1ft). Dual Laterologs:(DLL) It is the latest version of the later log. As its name implies, it is a combination of two tools, and can be run in a deep penetration (LLd) and shallow penetration (LLs) mode. These are now commonly run simultaneously and together with an additional very shallow penetration device. The tool has 9 electrodes. Both modes of the Dual later log have a bed resolution of 2 feet. The resistivity readings from this toolcan and should be corrected for borehole effects and thin beds, and invasion corrections can be applied. The dual later log is equipped with centralizes to reduce the borehole effect on the LLs. A micro resistivity device, usually the MSFL, is mounted on one of the four pads of the lower of the two centralists.
  13. 13. Electrical Well Logs AbdelAzizHamedElectrical Well Logs 12 NOTE: Separation of the LLs and LLd. from each other and from the MSFL is indicating the presence of a permeable formation with hydrocarbons. Spherically FocusedLog:(SFL) The spherically focused log (SFL) has an electrode arrangement that ensures the current is focused quasi- spherically. It is useful as it is sensitive only to the resistivity of the invaded zone. Micro-Resistivity Logs Micro Log: (ML) It is a rubber pad with three button electrodes placed in a line with 1 inch spacing The result from this toolis two logs called the 2”normal curve (ML) &the1½“inverse curve (MIV). The difference between the two curves is an indicator of mud cake (so it is used in making sand counts). Micro laterolog:(MLL) It is the micro-scale version of the laterolog. The toolis pad mounted, and has a central button current electrode. The depth of investigation of the MLL is about 4 inches.
  14. 14. Electrical Well Logs AbdelAzizHamedElectrical Well Logs 13 Proximity Log: (PL) This toolwas developed from the MLL. It is used to measure RXO. It has a depth of penetration of 1½ ft., and is not affected by mud cake. It may, however, be affected by (Rt) when the invasion depth is small. Micro Spherically FocusedLog:(MSFL) It is commonly run with the DLL on one of its stabilizing pads for the purposeof measuring RXO. It is based on the premise that the best resistivity data is obtained when the current flow is spherical around the current emitting electrode. The current beam emitted by this device is initially very narrow (1”), but rapidly diverges. It has a depth of penetration of about 4” (similar to the MLL). 2- Induction Tools: These logs were originally designed for use in boreholes where the drilling fluid was very resistive (oil-based muds or even gas). It can, however, be used reasonably also in water-based muds of high salinity, but has found its greatest use in wells drilled with fresh water-based muds. The sondeconsists of 2 wire coils, a transmitter (Tx) and a receiver (Rx). High frequency alternating current (20 kHz) of constant amplitude is applied to the transmitter coil. This gives rise to an alternating magnetic field around the sonde that induces secondary currents in the formation.
  15. 15. Electrical Well Logs AbdelAzizHamedElectrical Well Logs 14 These currents flow in coaxial loops around the sonde, and in turn create their own alternating magnetic field, which induces currents in the receiver coil of the sonde. The received signal is measured, and its size is proportional to the conductivity of the formation. Calibration: Induction logs are calibrated at the well site in air (zero conductivity) and using a 400ms test loop that is placed around the sonde. The calibration is subsequently checked in the well oppositezero conductivity formations (e.g., anhydrite), if available. 1- The 6FF40 Induction-ElectricalSurvey Log (IES-40) It is a 6 coil device with a nominal 40 inch Tx-Rx distance, a 16 inch short normal device and an SP electrode. 2- The 6FF28 Induction-ElectricalSurvey Log (IES-28) It is a smaller scale version of the IES-40. It is a 6 coil device with a nominal 28 inch Tx-Rx distance, a 16 inch short normal device and an SP electrode. 3- The Dual Induction-Laterolog (DIL) It has several parts: (i) a deep penetrating induction log (ILd) that is similar to the IES-40. (ii) a medium penetration induction log (ILm). (iii) a shallow investigation laterolog (LLs). and an SP electrode. The ILm has a vertical resolution about the same as the ILd (and the IES-40), but about half the penetration depth. 4- The Induction SphericallyFocusedLog (ISF) It combines (i) IES-40, (ii) a SFL, and (iii) an SP electrode. It is often run in combination with a sonic log. 5- Array Induction Tools (AIS, HDIL) It consists of one Tx and four Rx coils. Intensive mathematical reconstruction of the signal enables the resistivity at a range of penetration depths to be calculated, which allows the complete invasion profile to be mapped.
  16. 16. Electrical Well Logs AbdelAzizHamedElectrical Well Logs 15 Spontaneous-potentiallogs are records of the natural potentials developed betweenthe borehole fluid and the surrounding rock materials. (No artificial currents are applied) The spontaneous potential is used chiefly for: 1) Geologic correlation(indicate Facies). 2) Determination of bed thickness. 3) Separating nonporous from porous rocks in shale-sandstone and shale-carbonate sequences (impermeable zones such as Shale ,and permeable zones such as Sand) maximum deflectionis clean Sand and minimum is shale . 4) Determination Formation water Resistivity (Rw) in salt and fresh mud. 5) Calculation the volume of Shale in permeable beds. Because the electric log is a measure of natural potentials and resistivities, it can be run only in open (uncased) holes that are filled with a conducting fluid such as mud or water. Presentation of an SP curve in a Sand-Shale Sequence.
  17. 17. Electrical Well Logs AbdelAzizHamedElectrical Well Logs 16 The chart paper commonlyused for the electric log is divided into two vertical columns called tracks. Under the API (American Petroleum Institute) system, the left-hand track is 2.5inches wide, with four divisions per inch and major divisions at 1.25-inch increments across the width of the track. The right-hand track is 5 inches wide, with four divisions per inch, and also has major divisions at 1.25Lb-inchintervals across the width of the track. The footage scale is divided into 10 divisions per inch, with major divisions at 0.5inch and 2.5-inch intervals. The API chart paper allows for a wide choice in SP, resistivity, and footage scales. The electric log usually includes the spontaneous potential in the left-hand track and one or more resistivity curves in the right-hand track. Some of the commonlyused resistivity curves are the single point, short normal, long normal, lateral devices, micro log, micro focused log,and the guard, or laterolog. Each of these devices has its application, depending on the lithology, depth of mud invasion, and other borehole conditions.Table 2 shows the applicability of various electric logging methods to the solution of typical hydrologic problems.
  18. 18. Electrical Well Logs AbdelAzizHamedElectrical Well Logs 17 Principles and applications The spontaneous-potentiallog is a graphic plot of the small differences in voltage, measured in millivolts that develop at the contacts between the borehole fluid, the shaleor clay, and the water in the aquifer. :Two sources of potential are recognized The firstsource, and least important to the magnitude of SP, is the phenomena. Thiselectro kineticstreaming potential caused by electromotive force(emf) develops when an electrolyte moves through a permeable medium. The emf appears in the borehole at places where mud is being forced into permeable beds, although in water wells, streaming potentials may be generated in zones gaining or losing water. Streaming potentials can sometimes be detected on the SP curveby sudden oscillations or by departures fromthe more typical responsein a particular environment. (See section on “Extraneous Effects “) A discussion of the streaming potential and its effects on SP was given by Gondouin and Scala (1958). .in theThe second and most importantsourceof SP arises emf produced at the junction of dissimilar materials inElectrochemical the borehole. The junctions are between the following materials: (Mud-mud filtrate), (mud filtrate-formation water), (formation water- shale), and (shale-mud). Becausethe mud filtrate is derived fromthe mud, it generally has a similar electrochemical activity, and any emf developed across this junction will be minimal and can be disregarded. The Potential developed across the junction from formation water to shale to mud is called the membrane potential, and the potential developed across the junction frommud filtrate to formation water is .liquid junction potentialthecalled
  19. 19. Electrical Well Logs AbdelAzizHamedElectrical Well Logs 18 The potentials arising fromthese junctions causea currentto flow near shale-aquifer boundaries in the mud column in the borehole. Figure 9 is a schematic diagram of the circulation of currentacross the various junctions and through the borehole (Doll, 1948). When theformation water is much more saline than the mud, the currentfollows the paths shown by the arrows, entering the mud column fromthe shale and moving into the sandstone. As the SP electrode moves upward through the bottom shale (fig. 9), it senses a decreasing potential becausethe currentflows parallel to the well bore. At the bed boundary the current density is maximum and the SP curveexhibits an inflection at this level. As the SP electrode moves toward the midpoint of the sandstonebed, the currentdensity decreases, the curvatureof the SP curveis reversed, and the SP attains its maximum negative potential at midpoint in the sand. As the SP electrode moves beyond the midpoint of the sandstone, the curverecorded is a mirror image of the lower half if the beds are uniform. The SP log, therefore, is a measureof the potential drop that occurs in the mud, and only approaches the static spontaneous potential (SSP) under favorableconditions. (See “Glossary” for definitions.) If, on the other hand, the formation water is fresh compared with the mud, the polarity of the SP curveis reversed, and the reciprocalof the log in figure 9 is produced. The SP is thereforemore positive oppositethe sands and is more negative opposite the shales. This condition occurs in hydrologic regimes where ground water contains very few dissolved solids and results in an electric log on which both the SP and the resistivity deflect in the samedirection, opposite the sand and shale beds. An example of this is illustrated in figure 10. The water in the formations above500 feet contain dissolved solids of the drilling mud lie somewhereFormations below 800 feet havea dissolved-solids content of about 250 mg/l. The dissolved solids of the drilling mud lie somewherebetween these two extremes. The resistivities above450 feet are all off-scaleto the right, and lie between 100 and 200 ohm- meters.
  20. 20. Electrical Well Logs AbdelAzizHamedElectrical Well Logs 19 Spontaneous-potentialdeflections are recorded on the left-hand track of the electric log, with deflections to the left considered as negative and those to the right as positive. In a sand-shalesequence containing formation water that is more saline than the mud, the greatest positive deflections can be expected oppositethe shales, and the greatestnegative deflections cart be expected opposite the sands. A shale line can be constructed to fit through as many of the extreme positive deflections as possible, and a sand line through as many of the extreme negative deflections as possible. If the ionic concentration of the borehole fluid and the aquifer water are constantthroughoutthe length of the borehole, the shale line and the sand line will generally be parallel to the vertical axis of the log. , and all potentialhe shale line is considered to be the baselineT measurements are made perpendicular to this line using the scale of SP in millivolts on the log heading. When carefully applied this method can be used to estimate sand-shaleratios and is applicable to aquifers with water of high salinity and, in somecases, to fresh-water aquifers. Unfortunately, the method fails in many water wells in sand-shale formations becausethe SP deflections are not necessarily negative opposite the sands. If the borehole fluid has a very low resistivity compared with the sand beds, the SP deflections opposite the sands may actually be more positive than the SP deflections opposite the shales. Accordingly, the sand-shaleratios become meaningless for formations that do not contain shaleor clay beds, even though large positiveSP deflections can be seen. Perhaps the most significant, but often misapplied, useof the SP in ground-water hydrology is the determination of water quality fromSP deflections. In petroleum exploration, whereNaCl is dominant.
  21. 21. Electrical Well Logs AbdelAzizHamedElectrical Well Logs 20 The following equation is usefulto calculate the quality of the formation water: 𝑆𝑃 = −𝐾 𝑙𝑜𝑔 𝑅𝑚 𝑅𝑤 Where SP = log deflection, in millivolts K = 60 +0.133T T = borehole temperature, in degrees Fahrenheit Rm = resistivity of borehole fluid, in ohm-meters Rw = resistivity of formation water, in ohm-meters. There are three cases of SP deflection: 1) Rm =Rw there is no deflection(shale base line). 2) Rm >Rw (–ve) deflectionand the deflectionwill be to the left of the shale line (shale free). At thick, water bearing sand free shale (static SP) & at thin beds shale or gas (pseudo SP) 3) Rm <Rw (+ve) deflectionand the deflectionwill be to the right of the shale line. In actual practice the SP deflection oppositea sand bed is read from the log and the Rm is measured with a mud kit or a fluid-resistivity tool. Inserting thesevalues in the equation determines the formation fluid, Rw which can be related back to milligrams per liter of NaCl from salinity-resistivity charts. Theequation is predicated on the following important assumptions, which may or may not hold true in water wells: (1) Both the borehole fluid and the formation water are sodiumchloride solutions. (2) The shale formations are ideal ion-selective membranes, and the
  22. 22. Electrical Well Logs AbdelAzizHamedElectrical Well Logs 21 sand formations haveno ion-sieving properties (no clayey sand or sandy clays in the zone of investigation). (3) The borehole fluid has a much greater resistivity than the combined resistivity of the sand and the shale. In petroleum logging the sand formations are generally saturated with brines of low resistivity, and the conditions in assumption 3, above, are generally satisfied. In hydrologic logging where the sand formations are saturated with fresh water, the resistivity of the sands may be many times that of the borehole fluids, and the conditions in assumption 3, above, may no longer be valid. However, the combined resistivity of the sands and shales can be less than that of the borehole fluid if the fresh water used in the drilling fluid has a lower ionic concentration than the formation water. The divalent ions Ca (+2) and Mg (+2) commonly found in fresh water havea different effect on the SP than Na (+1). The calcium and magnesiumsolutions affect the SP as though the water weresaltier than its resistivity indicates. Alger (1966) described a method for finding the .)m/RwK log (R-SP =of fresh waters by useof the equationwR In order to correlate SP deflections with water resistivity and ionic concentration fromsalinity-resistivity charts, hefound it necessary to convertall anions and cations in the water to an equivalent sodium chloride solution. Alger assumed that, within one ground-water regime, the relative ionic concentrations and ratios are approximately constant. The relations between SP, Rw and total dissolved solids - once determined froma borehole having an electric log and water analyses - were extrapolated by Alger to other bore-holes in the regime exclusively fromthe SP deflections. In an unpublished paper by HubertGuyod, Consultant, entitled “Fundamentals of Electrical Logging and their Application to Water Wells,” presented at the Second Advanced Seminar on Borehole Geophysics, U.S. Geological Survey, Denver, Colo., December 1968.
  23. 23. Electrical Well Logs AbdelAzizHamedElectrical Well Logs 22 can be usedRw)Rm/log (k-SP=Guyod pointed out that the formula only when the following conditions aresimultaneously satisfied: (1) The formation water is very saline. (2) NaCl is the predominantsalt. (3) The mud is relatively fresh and contains no unusualadditives. Guyod further stated that “the three conditions specified aboveare probably never met by fresh water aquifers. Therefore, it is not possible to determine, not even estimate, from the SP curvethe resistivity of formation water, that is, the total dissolved solids in fresh water sands.” Patten and Bennett (1963) also emphasized the unreliability of the SP formula for determining dissolved solids. Considering the unreliability of the method, especially for dissolved solids of less than about 10,000 mg/l, the method probably should not be used in the analyses of fresh-water-bearing aquifers. Vonhoff (1966)) however, found thata “workableempirical relationship exists be-tween the spontaneous- potential deflection on the electric log and the water quality in the glacial aquifers.” His conclusions arebased on a study of test wells in Saskatchewan, Canada, in which the ionic composition of the drilling fluid and formation water are similar, and the resistivity of the drilling fluid is much greater than that of the formation water. The actual dissolved solids in the test wells ranged from1,191 to 3,700 mg/l. Figure(9)
  24. 24. Electrical Well Logs AbdelAzizHamedElectrical Well Logs 23 Figure (10) Figure (11)
  25. 25. Electrical Well Logs AbdelAzizHamedElectrical Well Logs 24 Instrumentation In its simplest form an SP logging device consists of a movablelead electrode which traverses the borehole on an insulated wirea ground electrode also made of lead and a device for measuring potential such as a millivolt meter. The movable electrode senses the ohmic-potential drop caused by the currents flowing into the mud column near the shale-aquifer boundaries. Becausepotential at the ground electrode remains constant the potential read fromthe meter represents the change in potential in the mud between the shales and the permeable formations. Figure11 shows theSP measuring circuit and, also, the electrical equivalent. In this diagramthe upper Ek represents the shale streaming potential, the lower Ek is the streaming potential developed across the sand, Ep is electrochemical potential developed at the liquid junction (mud filtrate-formation water), and Es is the membrane or shale potential. The terms Rsh, Rss, and Rm are the resistances of the shale, sandstone, and mud, respectively. Calibrationandstandardization Spontaneous potential is measured in millivolts, and the millivolts per horizontalchart inch, or full scale of the chartpaper (sensitivity), is shown on the log heading of the left track. The smallsingle-conductor cable loggers used in ground-water studies commonly haveSP sensitivities of 10, 25, 100, and 200 mv per inch. Some loggers are furnished with an auxiliary SP calibrator. The main purposeof the calibrator is to check logger function on all ranges of SP sensitivity. The calibrator contains a battery and resistors so as to providea predetermined potential, in millivolts, which is applied to the SP electrodes of the logger. Each rangeon the calibrator should produce a 1-inch deflection of the recorder pens when the SP sensitivity control on the logger panel is set in the corresponding range. Calibrators supplied with the single-conductor loggers arenot precision devices, and deflections on corresponding recorder ranges may or may not exactly equal 1 inch (or one division). Accuracies aregenerally on the order of ±10 percent.
  26. 26. Electrical Well Logs AbdelAzizHamedElectrical Well Logs 25 Radiusof investigation The potential drop along the mud column results from currents that originate away fromthe borehole along formation boundaries. For example, in a lithologic section, such as that in figure 12 (Schlumberger Well Surveying Corp., 1958),the currents tend to flow fromthe borehole into the permeable beds until a sufficient cross- sectional area of the compact (very resistive) formation is encountered to carry the current. The currentthen flows across thelarge area of resistiveformation until an impervious conductiveshale bed is intersected. The currents then travel with greater density along the conductive beds until the borehole is intersected again. Therefore, the radius of investigation is highly variable. Also, the SP may not relate directly to the permeable beds becausethe effect of these beds spreads the SP above and below the bed boundaries, as shown by the SP curveon the left-hand side in figure 12.
  27. 27. Electrical Well Logs AbdelAzizHamedElectrical Well Logs 26 (Unwanted logging effects)Extraneous effects connected to a surface earth to workThe SP tool must be .effectively nd an iron probe canthis causes no problem a:on land wellsFor directly into soilbe pushed the SP will not bewithout an effective earth:offshore wellsFor recorded Other unwanted SP potentials: (Heavy rain _ Noise_ Logging drum and Sheave Magnetism _Disruption to the ground reference). SP noise can be defined as any spurious or unwanted signals that are not correlated with the actual SP in the borehole. Noise and anomalous potentials are relatively common problems in SP logs. Some of the early model loggers used insulated cable with a single conductor, and the SP was relatively free fromnoise. With the introduction of steel armored cable, noise became troublesome. Steel is electrochemically active, and when the cable is immersed in an electrolyte (water or drilling mud), a battery effect develops along its entire wetted length. If the cable remains motionless in the drill hole, the batteries become polarized, and their output remains constant. This small current impresses on the SP electrode a potential that merely shifts the SP curve left or right. When the cable begins to movein the hole, however, the polarization film is wiped off intermittently, and the currentoutput from the battery effect is thereforevaried. The varying potentials resulting fromcable motion are thus impressed on the SP to producenoise.
  28. 28. Electrical Well Logs AbdelAzizHamedElectrical Well Logs 27 The sourceand effect of this noise is shown in figure13 (Electra Technical Laboratories, 1959). Noisefromthe partof the armored cable near the surfacein the borehole can also be coupled into the SP ground electrode. If the well is cased to considerabledepth, the casing may act as a shield around the wetted upper part of the cable to effectively screen the SP ground electrode fromcable noise. This type of noise is mosttroublesome wheresurfaceformations havehigh resistivities. Corrective procedures to lessen cable noise caused from battery effect are based on the electrical isolation of the SP electrode and the SP ground electrode from the cable. Wrapping the armored cable with insulating tape for 10 feet or more above the SP electrode may displacethe battery effect far enough up hole to reduce the magnitude of this sourceof potential. Moving the SP ground electrode as far as possiblefrom the well head may also help. Most noise- reduction procedures arelargely trial-and-error processes becausethe sourceof noisegenerally is not evident. Other sources of noise and anomalous SP are magnetization of armored cable, currents set up by casing corrosion in the Logged well or in nearby wells, magnetic storms fromsolar flares flow of ground water through the well bore, and manmade effects. In this last category fall such influences as circulating ground currents near electrical switch yards and transformer stations, electrochemical action along buried pipelines, Cathodic protection of buried pipelines, and potentials set up along large or long metallic objects, such as railroad tracks. If theSP ground electrode is in the mud pit, even the operation of the rig mud pumps, or generator, or the dragline cleaning of the pits. Water moving into the hole may be located by SP noise. The oscillating SP between 700-750 feetin figure 14 is probably due to streaming potentials caused by water moving into the well bore.
  29. 29. Electrical Well Logs AbdelAzizHamedElectrical Well Logs 28 Factors affected on SP deflection 1) Bed thickness: SP decreases when bed thickness decreases 2) Bed resistivity: higher resistivity reduces the deflectionof the SP curves 3) Borehole and invasion: the effectof borehole diameterand invasion on the sp log is very small, in general can be ignored. 4) Shale content: the presence ofthe shale in a permeable formation reduces the SP deflection. 5) Hydrocarbon content: in hydrocarbon bearing zones the SP deflectionis reduced,this effectcalled (hydrocarbon suppression). 6) Salinity effect:(Rm/Rw) fresh mud (-ve) SP, saline mud (+ve) SP. 7) Mud filtrate: the magnitude and direction of SP deflectionfrom base line depends on Rm, Rw. 8) Permeable beds:depends on differencein salinity of mud and formation where there is no deflectionin permeable beds when no differencein salinity.
  30. 30. Electrical Well Logs AbdelAzizHamedElectrical Well Logs 29 :Quantitative uses of SP Rwwater resistivityFormation1) 𝑆𝑆𝑃 = −𝐾 𝑙𝑜𝑔 (𝑅𝑚) 𝑒 (𝑅𝑤) 𝑒 SSP: static spontaneous potential. K: temperature dependentcoefficient. (Rm)e: equivalent mud filtrate resistivity. (Rw)e: equivalent formation resistivity. 1) Negative deflectionthe formation water more saline than mud filtrate. 2) Positive deflectionthe formation water fresherthan mud filtrate. of shale contentVolume2) 𝑉𝑠ℎ(%) = 1 − 100 𝑃𝑠𝑝 𝑆𝑠𝑝 Vsh: volume of shale. Psp: pseudo static spontaneous potential. (Water bearing shaly sand zone) Ssp: static spontaneous potential. (Max SP in clean sand zone).
  31. 31. Electrical Well Logs AbdelAzizHamedElectrical Well Logs 30 𝑉𝑠ℎ = 𝑆𝑃 − 𝑆𝑃𝑐𝑙 𝑆𝑃𝑠ℎ − 𝑆𝑃𝑐𝑙 Vsh: shale volume. SP: SP log reading. SPsh: SP log reading in shale zone. SPcl: SP log reading in clean sand zone. (FT)temperatureFormation3) 𝐹𝑇 = 𝑆𝑇 + ( 𝐵𝐻𝑇 − 𝑆𝑇 𝑇𝐷 ) 𝐹𝐷 FT: formation temperature. ST: the mean annual surface temperature. BHT: bottom hole temperature. TD: total depth of the well. FD: formation depth.
  32. 32. Electrical Well Logs AbdelAzizHamedElectrical Well Logs 31 :Limitations 1) Borehole mud must be conductive. 2) Formation water must be water bearing and conductive. 3) A sequence of permeable and non-permeable zones must exist. 4) A small deflectionoccurs if Rm=Rw. 5) No fully developed infront of thin beds.
  33. 33. Electrical Well Logs AbdelAzizHamedElectrical Well Logs 32 measurement of a formationa: the resistivity log isThe log resistivity that is its resistance to the passage of an electric current. It’s measured by resistivity tools. Conductivity tools measure formation conductivity or its ability to conductive an electric current. It’s measured by induction tools. Most rock materials are essentially insulators, while there enclosed fluids (except hydrocarbons) are conductors. Conductivity is generally converted directly and plotted as resistivity on log plot. &. Meterhm(OUnits of measurements are:Units and presentation Mhos). Presentation: tracks 2 and 3 on logarithmic scale.fig.1 Resistivity The electrical resistivity of a rock depends on physicalproperties of the rock and the fluids it contains. Most sedimentary rocks are composed of particles having a very high resistanceto the flow of electrical current. When these rocks aresaturated, the water filling the pore spaces is relatively conductive compared with the rock particles or matrix. The resistivity of a rock, therefore, is a function of the amount of fluid contained in the porespaces, the salinity of that fluid, and how the porespaces are interconnected.
  34. 34. Electrical Well Logs AbdelAzizHamedElectrical Well Logs 33 The main factor in pore geometry is probably tortuosity, which is defined as the squareof the ratio of the actual path length of the currentto the length of the sample through which it flows. Resistivity is measured in ohm-meters, which is the resistanceof a cube of material that is one meter on a side, and the resistance of that cube, in ohms, is numerically equal to the resistivity, in ohm-meters. The resistivity of a rock that is 100-percentsaturated with formation water is Ro and the resistivity of the water is Rw True resistivity, Rt, is distinguished from Ro be-causecorrection for partial saturation by hydrocarbons is necessary in petroleumexploration. Correction for partial saturation would also be necessary in hydrology if we made resistivity logs above the water table. Using Ro fromlogs and F, it is simple to calculate Rw which is a function of the temperature and quality of water in an aquifer: 𝑅𝑤 = 𝑅𝑜 𝐹 Formation factor The formation resistivity factor (F) is de-fined as the ratio of the electrical resistivity of a rock 100- percent saturated with water to the resistivity of the water with which it is saturated, F=Ro/Rw (Archie, 1942). Becausemost rock grains have a very high resistancerelative to water, the formation factor is always greater than 1.
  35. 35. Electrical Well Logs AbdelAzizHamedElectrical Well Logs 34 Formation factor is roughly related to effective porosity in poorly consolidated rocks as follows: 𝐹 = .62∅−2.15 . Guyod (1966) showed how porosity and resistivity data fromlogs can be used to determine the salinity of the interstitial water (fig. 3). The figure is based on average values for clay-freegranular aquifers and is therefore approximate when applied to a particular aquifer. Thus, if all other factors are equal (fig. 3), the higher the porosity and salinity, the lower the aquifer resistivity. Principal uses (application of resistivity logs): The resistivity logs wereused in the following (table 6.1): 1- To find hydrocarbons. 2- Petro physicalcalculations. 3- Give information about lithology, texture, facies, over pressureand sourcerock aspects. 4- Determination the volume of shale. 5- Correlate between wells.
  36. 36. Electrical Well Logs AbdelAzizHamedElectrical Well Logs 35 Zones of invasion and resistivity: There are three zones: There is small layer of mud cake on the wall of bore hole. And it’s the firstlayer in the borehole. 1-Flushed zoneclosestthe borehole, behind the mud cake. 2-Transition zonebetween flushed zoneand UN invaded zone(virgin formation).or called invaded zone. 3-Un invaded zone(virgin formation) where the essential target of resistivity logging is that of the true resistivity of the formation (Rt). Distribution of porefluids in zones around a well which initially contained hydrocarbons. From Dewan [2].
  37. 37. Electrical Well Logs AbdelAzizHamedElectrical Well Logs 36 Factors affecting on resistivity of reservoirs: 1-Salinty: 1/R 2-Temprature: 1/R 3-Saturation: 1/R 4-Presence of Hydrocarbon: R 4-Lithology plays an indirect role for Controlling Resistivity where: - Most rock forming mls are insulator. - Pore content oil or gas insulator. - Pore content water conductor. - Clay and shale conductors. Rock resistivity decreaseswith: - Increasing porosity or fracturing. - Increasing water saturation and water salinity. - Increasing shale and clay content. g on resistivity logging:Factors affectin 1- Borehole Size. 2- Mud Cake type and thickness. 3- Invasion Diameter. 4- Bed thickness and tool Resolution. 5- Depth of Investigation.
  38. 38. Electrical Well Logs AbdelAzizHamedElectrical Well Logs 37 rocesses:PeologicalGluenced byesistivity is infRRock -Clay alteration (decrease). -Dissolution (decrease). Faulting (decrease).- -Salt water intrusion (decrease). -Shearing (decrease). Weathering (decrease).- -Metamorphism (increase or decrease). -Induration (increase). -Carbonate precipitation (increase). -Silification (increase). 2) Resistivity Tools: Resistivity logging is a method of well logging that works by characterizing the rockor Sediment in a borehole by measuring its electrical resistivity. Resistivity is a fundamental material property which represents how strongly a Material opposesthe flow of electric current. The log must run in holes containing electrically conductive mud or water. 1-Electrode tools: Modern ResistivityLog: Laterologs:(LL) It is a type of modern electrodes which have a number of electrodes. • LL3 has 3 current emitting electrodes (vertical resolution is 1ft). Two guard electrode to keep central (bucking current) more focused. Potential of central electrode is measured relative to the potential at infinity to give a potential difference. From (potential difference & current) we can calculate the resistivity. • LL7 has 7 current emitting electrodes (vertical resolution is 3ft).
  39. 39. Electrical Well Logs AbdelAzizHamedElectrical Well Logs 38 Main current from central electrode, bucking current from far electrodes (80) inch apart. The main current focused in a thin disk far out into the formation. The potential between one of the monitoring electrodes and the potential at infinity measured and known resistivity from (potential difference and current) to provide geo metrical factor of the arrangement. Strongly focused beam a little affected with (hole size), and penetrates the invaded zone, and measures (Rt) true resistivity. • LL8 is similar to the LL7, but has the current return electrode (Vertical resolution is 1ft). Measured (Rxo) rather than (Rt). Dual Laterologs: (DLL) It is the latest version of the later log. As its name implies, it is a combination of two tools, and can be run in a deep penetration (LLd). And shallow penetration (LLs) mode. These are now commonly run simultaneously and together with an additional very shallow penetration device. The toolhas 9 electrodes (four potential and five current). Both modes of the dual later log have a bed resolution of 2 feet. The resistivity readings from this tool can and should be corrected for borehole effects and thin beds, and invasion corrections can be applied. The dual later log is equipped with centralizes to reduce the borehole effect on the LLs. A micro resistivity device, usually the MSFL, is mounted on one of the four pads of the lower of the two centralists.
  40. 40. Electrical Well Logs AbdelAzizHamedElectrical Well Logs 39 The difference between (LLD)and (LLS) LLD: the main currentemits a currentwith intensity equal10, And the five electrodes emits currentwith the same polarity but differentin intensity to focus main current. LLS: only three currentelectrodes emit current, this currentreturn to reversely polarizedelectrodes. The two measurements have differentdepth of investigationare called (Rd) deep resistivity,and (Rs) shallow resistivity. NOTE: Separation of the LLs and LLd. from each other and from the MSFL is indicating the presence of a permeable formation with hydrocarbons. Spherically FocusedLog:(SFL) The spherically focused log (SFL) has an electrode arrangement that ensures the current is focused quasi-spherically. It is useful as it is sensitive only to the resistivity of the invaded zone (Ri).
  41. 41. Electrical Well Logs AbdelAzizHamedElectrical Well Logs 40 Micro-Resistivity Logs Micro Log: (ML) It is a rubber pad with three button electrodes placed in a line with 1 inch Spacing. The result from this tool is two logs called the 2”normal curve (ML) &the1½“inverse curve (MIV). The difference between the two curves is an indicator of mud cake (so it is used in making sand counts). A known current is emitted from electrode A, and the potential differences between electrodes M1 and M2 and between M2 and surface electrode are measured. Micro laterolog:(MLL) It is the micro-scale version of the laterolog. The tool is pad mounted, and has a central button current electrode that emits known measurements current surrounded coaxially by two rings shaped monitoring electrodes, and a ring shaped guard electrode that produces a bucking current as in DLL. The depth of investigation of the MLL is about 4 inches. Proximity Log: (PL) This tool was developed from the MLL. It is used to measure RXO. It has a depth of penetration of 1½ ft., and is not affected by mud cake. It may, however, be affected by Rt when the invasion depth is small.
  42. 42. Electrical Well Logs AbdelAzizHamedElectrical Well Logs 41 Micro Spherically FocusedLog:(MSFL) It is commonly run with the DLL on one of its stabilizing pads for the purposeof measuring RXO. It is based on the premise that the best resistivity data is obtained when the current flow is spherical around the current emitting electrode. The current beam emitted by this device is initially very narrow (1”), but rapidly diverges. It has a depth of penetration of about 4” (similar to the MLL). 2- Induction Tools: These logs were originally designed for use in boreholes where the drilling fluid was very resistive (oil-based muds or even gas). It can, however, be used reasonably also in water-based muds of high salinity, but has found its greatest use in wells drilled with fresh water-based muds. The sondeconsists of 2 wire coils, a transmitter (Tx) and a receiver (Rx). High frequency alternating current (20 kHz) of constant amplitude is applied to the transmitter coil. This gives rise to an alternating magnetic field around the sonde that induces secondary currents in the formation. These currents flow in coaxial loops around the sonde, and in turn create their own alternating magnetic field, which induces currents in the receiver coil of the sonde. The received signal is measured, and its size is proportional to the conductivity of the formation. The induction tools are important because they provide the only resistivity measurement in wells drilled with oil base mud.
  43. 43. Electrical Well Logs AbdelAzizHamedElectrical Well Logs 42 Applications: 1- Reservoir delineation. 2-Determination of true formation resistivity. 3-Determination of water saturation. 4-Hydrocarbone identification. 5-Determination of movable hydrocarbon. 6-Invation profiling. 7-Thin bed analysis.
  44. 44. Electrical Well Logs AbdelAzizHamedElectrical Well Logs 43 Calibration: Induction logs are calibrated at the well site in air (zero conductivity) and using a 400ms test loop that is placed around the sonde. The calibration is subsequently checked in the well opposite zero conductivity formations (e.g., anhydrite), if available. 1- The 6FF40 Induction-ElectricalSurvey Log (IES-40) It is a 6 coil device with a nominal 40 inch Tx-Rx distance, a 16 inch short normal device and an SP electrode. 2- The 6FF28 Induction-ElectricalSurvey Log (IES-28) It is a smaller scale version of the IES-40. It is a 6 coil device with a nominal 28 inch Tx-Rx distance, a 16 inch short normal device and an SP electrode. 3- The Dual Induction-Laterolog (DIL) It has several parts: (i) Deep penetrating induction log (ILd). That is similar to the IES-40. (ii) Medium penetration induction log (ILm). (iii) Shallow investigation laterolog (LLs) And an SP electrode. The ILm has a vertical resolution about the same as the ILd (and the IES-40), but about half the penetration depth. 4- The Induction SphericallyFocusedLog (ISF) It combines (i) IES-40. (ii) SFL. (iii) SP electrode. It is often run in combination with a sonic log. 5- Array Induction Tools (AIS, HDIL) It consists of one Tx and four Rx coils. Intensive mathematical reconstruction of the signal enables the resistivity at a range of penetration depths to be calculated, which allows the complete invasion profile to be mapped.
  45. 45. Electrical Well Logs AbdelAzizHamedElectrical Well Logs 44 Quantitative uses of the resistivity logs: The resistivity logs are used to give the volume of oil in a particular reservoir, or in petro physical terms, to define the water saturation. Sh+Sw=1, where, Sh=Sg+So Sw: water saturation. Sh: hydrocarbonsaturation. So: oil saturation. Sg: gas saturation. Shr=1-Sxo Shr: residual hydrocarbonsaturation. Sxo: water saturation in invaded zone. Shm=Sh-Shr, or, Shm=Sxo-Sw Shm: movable hydrocarbonsaturation. RF: Shr/Sh RF: recovery factor. The basic equation of petro physics (Archie equation) Sw= ( 𝑎 𝑅 𝑤/∅ 𝑚 𝑅𝑡)1/n Where: SW:water saturation of un invaded zone. Rw: formation water resistivity. Rt: resistivity of un invaded zone. ∅: Formation porosity. n: saturation exponent and its equal 2. a&m: these values are varied according to the lithology and equal to the formation factor.
  46. 46. Electrical Well Logs AbdelAzizHamedElectrical Well Logs 45 Rsh: resistivity reading in front shale. Rt: resistivityof un invaded zone. b: constant which equal 1. The ratio (Rsh/Rt) range from (0.5 to 1). Qualitativeuses :exturesT-1 The simplest expressionof resistivity variation with texture is that variation of resistivity with porosity changes. When porosity decreases the resistivity increase, these changes indicate to change in water saturation and the presence of hydrocarbons. :ariationsVthologicalLi-2 Although resistivity logs do not allow the direct identification of commonlithologies, they are very sensitive lithology indicators. The resistivity logs are in fact responding to two things (changes in texture and changes in composition). :Facies-3 Facies change can be followed on resistivity logs; one of the principle uses of resistivity log in facies analysis is its ability to registerchanges in (sand-shale) mixture.
  47. 47. Electrical Well Logs AbdelAzizHamedElectrical Well Logs 46 :Correlation-4 The sensitivity of the resistivity logs to lithological changes is the bases for their use in correlation. Logs which correlate well are those which are more sensitive to vertical changes than to lateral changes. :Compaction, shale porosity and over pressure-5 There is relation between conductivity and shale porosity. Where the resistivity increase with increasing of the compaction of shale along the pore hole, and this relation plotted between ./m) and Depth (km)2 shale resistivity (Ωm :Source rock investigation-6 The resistivity log may be used both qualitatively and quantitatively to investigate source rocks, the effectof a source rock on the resistivity log depends onthe maturity of the organic matter, when the organic matter immature there is a little effect, but when the organic matter is mature there is a large effect. :ithologyLGross-7 Resistivity logs cannot be used for a first recognition of the commonlithologies.There are no characteristic resistivity limits for shale, or limestone or sandstone.The values depend on many variables such as compaction,composition,and fluid content ……., however, in any restricted zone, gross characteristics tend to be constant and the resistivity log may be used as a discriminator. In certain specificcases the resistivity logs can be used to indicate a lithology. These cases are clearly where certain minerals have distinctive resistivity values (salts, anhydrite, gypsum and coal) all have unusually high. But in case of limestone and dolomite the resistivity usually high.
  48. 48. Electrical Well Logs AbdelAzizHamedElectrical Well Logs 47 Ohm. mResistivityLithology /mineral Variable(0.5-1000) Depend on porosityand salinity Depend on porosityand salinity Depend on porosityand salinity 10000-infinity 10000-infinity 1000 10-1000000 0.0001-0.1 Moderate Generally high Generally high Moderate-low Very high Very high High High Very low Shale Limestone Dolomite Sandstone Salt Anhydrite Gypsum Coal pyrite
  49. 49. Electrical Well Logs AbdelAzizHamedElectrical Well Logs 48 REFERENCES 1-Dr M.Sharaf text Book (WellLogging)for Fourth Level Faculty of Science, Benha University, Geophysics department.(2012) 2- WellLogging for Earth Scientists 2nd Edition. Darwin V. Ellis and Julian M. Singer. 3- An Introduction to GeophysicalExploration 2nd Edition (Philip Kearey and Michael Brooks). 4- Applicationof Borehole Geophysics to Water-Resources investigations by W. ScottKeys and L. M. MacCary. (1985) 5- Applied Geophysics.(W.M.Telford,L.P.Geldart,R.E.Sheriff). (1990) 6- Well Logging (Data Acquisitionand Applications) Oberto Serra and Lorenzo Serra. (2004) 7- The geological Interpretation of Well Logs.Second Edition. Rider (1996) 8- Geology& Geophysics in Oil Exploration. (2010) 9- An Introduction to GeophysicalExploration. Third Edition. Philip Kearey, Michael Brooks and Ian Hill. (2002) 10- Applied Geophysics.Second Edition. (W.M.Telford, L.P.Geldart, R.E.Sheriff).(2004) By: Geophysicist: Abdel Aziz hamed Abdel Aziz Fourth level Mob: 01144299361-01024719740. E-mail: zezo0_1992@yahoo.com.

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