well logging project report_dileep p allavarapu


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summer ,winter training in well log interpretation_dileep p allavarapu

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well logging project report_dileep p allavarapu

  2. 2. 2 CERTIFICATEThis is to certify that this dissertation/summer training program entitled “CHARACTERIZING THE RESERVOIR BY OPEN HOLE WIRELINELOGGING” is the bonafide work of students submitted to theGeophysics Department, Andhra University in partial fulfillment of theM.Sc.(Tech)degree in Geophysics. (Proff N.V.B.S.S.PRASAD) Head –Department of Geophysics, Andhra University, Visakhapatnam. 2
  3. 3. 3 ACKNOWLEDGEMENT Its my privilege to thank Proff: N.V.B.S.S.Prasad, Head -Department of Geophysics, Andhra University, to all the pains he has takenin providing an opportunity to undergo training in a prestigious organizationlike Well Logging Services –O.N.G.C., Rajahmundry Asset, and I am verygrateful to him for his valuable guidance before and after the training for hiscritical discussion in bring out this field training report. I special thank to K.S. Prasad, G.M (W) and K.S.Murthy D.G.M(W), Well Logging Services –ONGC. Rajahmundry Asset, for grantingpermission to undergo project training. I would like to acknowledge the contributions of Dr.B.A.Rao, C.G(W) of Well Logging Services –O.N.G.C., in reviewing some of the materialin this document and esteemed guidance, kind cooperation during the entireperiod of training. I wish to express my gratitude to the staff members of Departmentof Geophysics and my colleagues for their cooperation and encouragementin the training program. While I can not possibly mention the names of all of those whocontributed to text in this document, I thank all of them for their time andefforts. (A.P.V.V.S.S.DILEEP) 3
  4. 4. 4 CONTENTSChapter 1:………………………………………………… 05INTRODUCTIONWire line loggingChapter 2:………………………………………………… 12PROPERTIES OF RESERVOIRProperties of rocks and fluidsChapter 3:………………………………………………… 22OPEN HOLE TOOLS andINSTRUMENTATIONElectrical, Radio Active, sonic and other toolsMeasurement PrinciplesChapter 4:…………………………………………………. 39OPEN HOLE LOGGING AND LOGSLogging operationPorosity, Lithology, Resistivity logs,Chapter 5:………………………………………………… 55INTERPREATION andCHARACTERIZATION OF RESERVOIRCalculation of R w, ø eff, V sh, S wPreparation of Para log 4
  5. 5. 5Chapter 1INTRODUCTION Wire line logging The complete evaluation of petroleum bearing reservoir includesdata from several sources – 3D-seismics, V.S.P., mud logging, coring,M.W.D-L.W.D, wire line logging, pressure tests, and sampling.The science of petroleum bearing reservoir evaluation encompasses ageneral knowledge of all these disciplines, while certain individuals may bespecialized in a specific discipline such as seismic interpretation ,loganalysis ,core analysis ……… In this connection research and development programs of an oil &gas company are devoted to the investigation of the properties of reservoirrocks and fluids and how they related to measurable properties. Wire line well logging operations provide measurements of bore holeand formation properties at accurately measured depth. With a few exceptions, petroleum company personnel (i.e., geologist,geochemist, geophysicist…etc.) are interested only in how wire linemeasurements are related to information they need: physical & chemicalproperties of reservoir not the tool functions. For locating the petroleum bearing reservoir require anunderstanding of nature of the subsurface sedimentary formations, and welllogs are important method of acquiring such information. Wire line welllogs are particularly useful in describing and characterizing reservoirs. 5
  6. 6. 6 Log measurements can define or at least infer these formationproperties such as porosity, shale volume, litho logy, and water, oil, or gassaturation. Estimation of permeability, prediction of water cut, selection ofover pressure zones and calculation of residual oil can also be made. Loganalysis is primarily used to describe formation properties in a single well. Quite normally, log and core data are often compared and used inconjunction to define reservoir properties. When cores are not available, logdata are often used as extension from core analysis and log comparisons onother wells. However, when a suite of logs in run is several wells representativeof a specific geological area, it can be used as a geological tool tounderstanding subsurface formations by describing local geology,stratigraphy, environment of deposition and reservoir geometry in ourpresent conditions. 6
  7. 7. 7 Chapter 2: PROPERTIES OF RESERVOIR Properties of rocks and fluids The characterization of reservoir requires reliable knowledge of certain fundamental reservoir properties. Log measurements can define or at least infer these properties: resistivity, porosity, shale volume, litho logy, and water, oil, or gas saturation and permeability.1) Resistivity: The opposition to flow of electrical current offered by amaterial 1m long, with a cross sectional area of 1sq mDenoted by:RUnits :ohm m. In combination with record depth, resistivity was the first formation parameter measured by wireline logging technique. The log -resistivity 7
  8. 8. 8 measurements are the function of compaction (amount of porosity) of rocks and fluid conduction (salinity). For ex: 1) Sand stone with low porosity : very high resistivity (high compaction ) 2) Sand stone with high porosity : (low compaction) pores filled with gas/oil : very high resistivity pores filled with fresh water : high resistivity pores filled with saline water : very low resistivity Then we can imagine the other Lithology conditions how affect the log measurement. The high resistivity is an indication of oil/gas or high compacted rocks. True resistivity: the resistivity of the true /un invaded zone that is beyond the transaction zone. It is denoted by Rt. Formation water resistivity: true resistivity of the formation water in the un invaded water bearing zone. it is also true resistivity It is denoted by Rw. Invaded zone resistivity: the resistivity of invaded zone between the mud cake and the transaction zone. It is denoted by Rxo. Filtrate resistivity: the resistivity of mud filtrate .that is in the invaded zone. It is denoted by Rmf. In general the resistivity means true resistivity. Information aboutformation water resistivity, invaded zone resistivity, filtrate resistivity isuseful in the calculation of true resistivity. 8
  9. 9. 92)Porosity: It is defined as total volume of sample that is occupied by pores or voids Or pore volume per unit volume of the formation. It denoted by ø. Its units are API units .It is measured as percentage (%). Porosities are classified according to the physical arrangement of the material that surround the pores and to the distribution and shape of the pores. 1) Primary porosity: The pore space exist between individual grains of rock matrix during deposition is called intergranular, or matrix porosity/ primary porosity. 2) Secondary porosity: The pore space created by the action of formation water or tectonic forces on the rock matrix after deposition is called facture porosity/ secondary porosity. These is another classification of porosity is effective porosity ,isolated or non effective porosity.it is shown in the figure 1 Fig1 Fig 2 9
  10. 10. 10 Unit cells of two systematic packings of uniform spheres are shown in Fig.2The porosity for cubical packing (the least compact arrangement) is47.6% and for rhombohedra packing (the most compact arrangement) is25.96%Effective porosity: the porosity that is due to inter connected pores in the formation.Non effective or isolate porosity: the porosity that is due to isolate /closed pores in the formation.3)Permeability: It is defined as measure of the ease with which fluids can flow through a formation. It is denoted by K. Its units are Darcys; which is very large, themillidarcy (md) is generally used. In order to permeable rock must have some interconnected pores,capillaries or fractures. Hence there exist some rough relation ship betweenporosity and permeability. Greater permeability in general, corresponds togreater porosity, but this is far from being an absolute rule. Shales and some sands have high porosities ,but the grains so smallthat the paths available for the moment of fluid are quite restricted andtheir permeability may be very low. Other formations ,such as lime stonecomposed of dense rock broken by a few small fissuies or fractures of greatextent .The porosity of a such a formations can be low ,but the permeabilityof a fracture can be enormous. 10
  11. 11. 11Permeability It is simply a measure of the capacity of a reservoir rock to transmitfluidsAbsolute permeability It refers to the permeability where in a reservoir rock only single fluidis present.Effective permeability This is defined as the permeability to one fluid in a multi fluid systemi.e. the permeability to a fluid when its saturation is less than 100%.Relative permeability Relative permeability indicate the ease with which one fluid of the twoor more fluids present will flow through connecting pore spaces in thepresence of each other as compared to the ease with which one fluid willflow when it alone is presen4)Saturation: It is defined as fraction of its pore volume occupied by the fluid. 11
  12. 12. 12Water saturation : It is a fraction of pore volume that contains formation water. The symbol for saturation is SwHydrocarbon saturation: it is a fraction of pore volume that contains hydrocarbons. The symbol for saturation is Sh Oil or gas saturation is a fraction of its pore volume that contains Oil or gas.The pores must be saturated with some fluid .thus the summation of all saturations in agiven formation rock must total to 100%. Sh, =( 1- Sw ).The water saturation is the most important parameter in cauterizing the reservoir forestimating hydrocarbon saturation.5)Shale volume: the volume of Shale(mixture clay minerals) present in thereservoir rock is the shale volune.Typical shale consists of 50% clay minerals, 25% silica, 10% feldspar, 10% carbonates,3% iron oxide, 1% organic material and also 2-40% water by volume.Modes of Occurrence shaleShale or clays minerals occur mainly as: Laminated ShaleDispersed Shale,Structural Shale 12
  13. 13. 13 Laminated ShaleLaminated shale refers to thin lamination of clay minerals of an inch to many inches inthicknesses that are inter-bedded with clean sand (Figure 2.1). The effective porosity andthe permeability of the shales are essentially zero so the overall porosity andpermeability of the reservoir rocks are reduced in proportion to the fractional volume ofthe shaleDispersed ShaleDispersed clay occurs as disseminated particles in the pore spaces of the sand andreplaces the pore fluid. This type of distribution is very damaging to the reservoir qualityas it chokes the pores and reducing the effective porosity and permeability of thereservoir unit (Figure 2.2).Structural Shale Here aggregates of the clay particles occurs and they takes the place of the sandgrains that is occur as framework grains of the reservoir along with the sand grains(Figure 2.3). Here the porosity and permeability of the reservoir rock is affected verylittle 13
  14. 14. 14Chapter 3:OPEN HOLE TOOLS andINSTRUMENTATIONElectrical, Radio Active, sonic and other toolsMeasurement Principles In the field operation wire line logging is done a mobilelaboratory, logging truck .It carries the down hole measurementinstruments, the electrical cable and winch needed to lower the instrumentsin to the bore hole, the surface instrumentation is needed to power the downhole instruments and to receive and process their signals, and the equipmentneeded to make a permanent recording of the Log.LOGGING UNIT: Logging service companies utilize a variety of logging units,depending on the location (onshore or offshore) and requirements of thelogging run. Each unit will contain the following components: Logging cable Winch to raise and lower the cable in the well Self-contained 120-volt AC generator Set of surface control panels Set of down hole tools (sondes and cartridges) Digital recording system 14
  15. 15. 15 Well logging is a wire line operation where the physicalparameters of various formations encountered in well, are measured bylowering the logging tools as a function of depth. These measurements help to understand the well/reservoir formationsbehavior.Finally  Well log is a continuous record of measurement made in bore hole respond to variation in some physical properties of rocks through which the bore hole is drilled.  Traditionally Logs are display on girded papers shown in figure.  Now a day the log may be taken as films, images, and in digital format. The down hole measurement instruments are composed of twocomponents. One component contains the sensor, called sonde. Thecomponent of the down hole tool is cartridge, contain the electronics thatpowers the sensors, process and transmit signals to the truck. The down holetool is attached to an electrical cable that is used to lower the tool into andremove from the well. the cable contain seven insulated copper conductorsor a fiber optic conductor along with six conductors. 15
  16. 16. 16LOGGING TOOLS: In practice the open hole wire line logging tools are used tomeasure the various parameters that influences the porosity, permeabilityand saturation of the formation. Various types of logging methods are usedto determine the formation properties.The logging tools are classified along with logging methods based on themeasurement principle. There are four principle logging methods are in use:Electrical logging,Radio Active logging,Sonic logging, andMiscellaneous logging.ELECTRICAL LOGGING TOOLS: 16
  17. 17. 17 In the electrical logging tools SP, Focused resistivity &micro resistivity and induction tools are in use. The SP tool together with Normal and Lateral devices is called conventional ES tools. The focused / non focused a resistivity &micro resistivity and induction tools are called non conventional ES tools.Conventional Electrical Survey Tools:SP ToolTool principle: The SP tool the records the naturally occurring electrical potential(voltage) which is produced by the interaction of formation connate water,conductive drilling fluid and certain ion-selective rocks (shale). It measuresthe potential difference between the movable electrode (A) in the boreholeand the fixed surface /reference electrode (B). It cannot record the potential difference in holes filled withnonconductive muds because such muds do not provide electrical continuitybetween the SP electrode and the formation. Furthermore, if the resistivitiesof the mud filtrate and formation water are about equal it will record the SPwith less significant features 17
  18. 18. 18Normal Device: In a normal device, current is passed between the two electrodes Aand B (Figure 3). The resultant potential difference is measured betweenthe two potential electrodes M and N. Electrodes A and M are on the sonde.Electrodes B and N are located at and infinite distance theoretically on thesurface. The distance AM is called spacing of the tool which is 16 inches(0.40 m) for short normal and 64 inches (1.62 m) for long normal. The pointof the reference is O, which is located at the center of the electrodes A andM.Lateral Device: In a lateral device, current is passed between the electrodes A and B.the resulting potential difference is measured between the electrodes M andN (Figure 4). These potential electrodes are located on the sonde. Here thepoint of inscription is O, which is the mid point of electrodes M and N. Thespacing AO is 18 ft. 8 inches (5.7 m). This device differs from the normal device in the sense that here theposition of the current and potential electrodes has been changed. Generallylonger the spacing the deeper the radius of investigation of the tool.Therefore the lateral device has deeper depth of investigation than of thenormal device.Non Conventional Electrical Survey Tools:Focused resistivity tools: The response of the conventional electrical logging systems canbe greatly affected by the borehole and adjacent formations. A family ofresistivity tools that uses focusing currents to control the path taken by themeasure current minimizes these influences. These current are emitted from 18
  19. 19. 19special electrodes on the sondes. The focusing electrode tools include theLaterolog (LL) and Spherically Focused Logs (SFL). These tools are much superior than the conventional logs and foralso highly resistive adjacent formations. They are also better for resolutionpoint of view for thin to moderately thick beds. Focusing electrode systemsare available with deep, medium and shallow depths of investigation. Focused electrical logging tool (DLL) The deep-reading devices include the Laterolog 7, the Laterolog 3,and the deep Laterolog and Dual Laterolog tool (DLL). The medium- toshallow-reading devices are the Laterolog 8 of the Dual Induction-Laterolog tool (DIL), the shallow Laterolog of the DLL tool. 19
  20. 20. 20Non -focused micro-resistivity tool: Micro resistivity devices are used to measure resistivity of the flushedzone, Rxo, and to delineate permeable beds by detecting the presence of mudcake. The example of such tool is micro log.Focused micro-resistivity tool: These are also used to measure resistivity of the flushed zone, Rxo,and to delineate permeable beds by detecting the presence of mud cake. Theexample of such tool is micro spherically focused logging (MSFL).TheMicro SFL is a pad-mounted spherically focused logging device that hasreplaced the micro laterolog and Proximity tools. It has two distinct advantages over the other Rxo devices. The first isits combinability with other logging tools, including the DIL and DLL tools.This eliminates the need for a separate logging run. The chief limitation ofthe micro laterolog measurement is its sensitivity limitation to mud cake.The proximity logs are insensitive to the mud cake.Tools principle: In order to minimizes mud cake effect select the electrodes spacingand bucking-current control. The surveying current flows outward from acentral electrode, Ao. Bucking currents, passing between the electrodes, Aoand A1, flow in the mud cake and, to some extent, in the formation. Themeasuring current, Io, is there by confined to a path directly into theformation, where it quickly “bells” out and returns to a remote electrode, B.To achieve this, the bucking current is adjusted to make the monitor voltageequal to zero. 20
  21. 21. 21 By forcing the measure current to flow directly into the formation, the effectof mud cake resistivity on tool response is minimized; yet, the tool still has avery shallow depth of investigationInduction tool:The induction-logging tool was originally measure formation resistivity inboreholes containing oil- base muds and in air-drilled boreholes. Inductiontools have many transmitter and receiver coils. The principle can beunderstood by considering a sonde with only one transmitter coil and onereceiver coil .Tools principle: The high frequency current is sent through a transmitter coil and ana.c magnetic field created induces currents in the formation surroundingthe borehole. These currents flow in circular ground loops coaxial with thetransmitter coil and create, in turn, a magnetic field that induces a voltagein the receiver coil. Because the alternating current in the transmitter coil isof constant frequency and amplitude, the ground loop currents are directlyproportional to the formation conductivity. The voltage induced in the receiver coil is proportional to the groundloop currents and, therefore, to the conductivity of the formation. There isalso a direct coupling between the transmitter and receiver coils. Using“bucking” coils eliminates the signal originating from this coupling. 21
  22. 22. 22 The induction tool also works well when the borehole containsconductive mud unless the mud is too salty, the formations are too resistive,or the borehole diameter is too large.RADIO ACTIVE LOGGING TOOLS:The GR together with Neutron and Density tools are called Conventional Radio ActiveTools. These tools detect either Neutron or Gamma ray.Scintillation counter: 22
  23. 23. 23GR TOOL:It measures the natural radioactivity of the formations. In sedimentaryformations its reading reflects the shale content of the formations. The mostradioactive elements tend to concentrate in clays and shales not in cleanformation.Tool principle: The gamma rays are emitted by the radioactive elements like U, Th, Kin the formations, and detected by the suitable gamma ray sensor (typicallyscintillation detector, 8 to 12 inches in active length). The detector gives adiscrete electrical pulse for each gamma ray detected. The parameterrecorded is the number of pulses recorded per unit of time by the detector.DENSITY TOOL:Tool principal: 23
  24. 24. 24In this technique a radioactive source is applied to emit medium-energygamma rays into the formations. These gamma rays may be thought of ashigh-velocity particles that collide with the electrons in the formation. Thistype of interaction is known as Compton scattering. The scattered gamma rays reaching the detector, at a fixed distancefrom the source, are translated in terms of formation density. The number ofCompton-scattering collisions is related directly to the number of electronsin the formation. . Electron density is related to the true bulk density, ρ b,which, in turn, depends upon the density of the rock matrix and density ofthe fluids and the density of the formation.The depth of investigation of density tool is quite shallow. Most of thedensity tool signal comes from a region less than 8” from the borehole wall.The CNL tool, gathers most of its signal from the region within 12” of theborehole wall. Thus, the density tool less affected by light hydrocarbonsthan the CNL tool.NEUTRON TOOL:Tool principal: Neutron tool respond primarily to the amount of hydrogen in theformation. Neutrons are electrically neutral particles, each having a massalmost identical to the mass of a hydrogen atom. High-energy (fast)neutrons are continuously emitted from a source in the sonde. Theseneutrons collide with nuclei of the formation materials in what may bethought of as elastic “billiard-ball” collisions. With each collision, theneutron loses some of its energy. 24
  25. 25. 25The amount of energy lost per collision depends on the relative mass of thenucleus with which the neutron collides. The greater energy loss occurswhen the neutron strikes a nucleus of practically equal mass - i.e., ahydrogen nucleus. Collisions with heavy nuclei do not slow the neutron verymuch. Thus, the slowing of neutrons depends largely on the amount ofhydrogen in the formation.SONIC LOGGING TOOLS:SONIC TOOL: A sonic tool consists of a transmitter that emits a sound-pulse and areceiver that picks up and records the pulse as it passes through theformation and reaches to receiver.Sonic- Tools: 25
  26. 26. 26 Tool principal: A transmitter sends compressional or longitudinal wavesinto the borehole fluid (mud). The compressional wave incident on wall getsrefracted into the formation. Acoustic pulse sent into the borehole mudreceived by the receiver is measured. These waves do not reach the receiversat the same time, but at different times depending upon the path traversedand the velocity of the medium. One path is 2ft longer than the other.Array sonic tool :It contains an array of 8 piezoelectric receivers 2piezoelectric transmitters ,receivers are spaced 6in apart with the closestreceiver 8 or 11.5feet from the upper transmitter. Two of these receivers,1and 5th spaced 2feet apart ,can be used for slandered LONG SPACEDSONIC(LSS) ,three of these receivers ,6,7and 8th spaced 2 feet apart ,can beused for DEPTH DERIVED BHC (DDBHC). Sonic- ToolIt record full wave form with the help of 8 receiver array. 26
  27. 27. 27CALIPER The wire line caliper measures continuously the diameter of theboreholes drilled. There are many types of caliper devices and design varieswidely. Some calipers are exclusively run for recording diameter. But quiteoften it is run in combination with other tools, especially with microresistivity tools.The diameter of the borehole is very useful parameter required to  Detect caving and decrease in hole size, so that amount of cement required for the casing can be calculated.  To select depth at which packer of DST tool can be set.  For log interpretation, mud cake thickness is important parameter required for correcting micro resistivity logs, sidewall neutron porosity log etc.,  Detection of mud cake thickness itself is a good indication of permeable nature of beds quite often this may be only way of detecting permeable beds 27
  28. 28. 28Checking the correction of caliper logs: This can be done best by recording the diameter inside the casingpresent at the top part of the well. Reading given by different calipers in thesame hole may be different depending on the caliper design combined withthe hole cross-section 28
  29. 29. 29 Chapter 4: OPEN HOLE LOGGING AND LOGS Logging operation Porosity, Lithology, Resistivity logs, What is logging? What is a log?Logging: The process of continuous recording of measurements correspondsto different properties of formation in a well To obtain comprehensive information about the formation, someelectrical instrument will be lowered into the well.Log: The record of comprehensive information about the formation in awell during logging process. Also print of all the data acquired in his well. The prestigious organization Well Logging Services –O.N.G.C.provides a wide range of service and information, allowing their ASSET todefine, reduces and manages their risk operations.It provides the following servicesOpen hole logging services.Cased hole logging services.Production logging services.Data Processing and Interpretation. There are three suits in Open hole logging services. Suit 1: Rt, Gamma Ray, SP, Caliper. Suit 2: Sonic, Neutron, Density, Gamma Ray. Suit 3: R.F.T, Side wall sampler, Dip meter.These three suits provide the information about Lithology, Porosity,and Resistivity of the Formations in the Bore hole. In these logs GR and SP provides the information about litho logy. So theseare called as litho logy logs. Some other logs infer the porosity andresistivity. 29
  30. 30. 30Litho logy logs: GR and SP.Porosity logs: Neutron, Formation density, Sonic.Resistivity logs: Conventional, Focused, Induction, Micro resistivity logs. LOGGING Wireline logging operations provide measurements of bore hole andformation properties at accurately measured depths. The measurements aremade under pseudo-dynamic conditions: bore hole fluid is static duringlogging operations, the measuring device is ascending the bore hole whilethe measurements are being recorded. there are few exceptions: some toolare held stationary while measurements made ,some tools are moving whileformation fluids are enter or exit the bore hole and some tools aredescending the bore hole while the measurements are made. LOG The Log must be containing the data acquired in well and all thenecessary information to make an accurate interpretation of reservoir.Itmust be in the form of standard presentation, known tool configuration,environmental conditionsIt must be contain corrected tool response inknown conditions and calibrated for specificationsIt must be documented correct operating procedures and deviations fromthat. Components of log: log header , log tool sketch, log remarks ,logcalibration ,log tail 30
  31. 31. 31. 31
  32. 32. 32.The content of a Log Header is all general information about the well andjob such as Company name, Well name, Tools run, Interval logged Mudrecord Deviation data…Log Tool sketch is that which tool has been used to record the data Must have tool numbers Sensor offsets… The content of a Log Main log is a record of the required data vs.depth over the whole well.Color codes & scales defined by client, presented in several scale(1/500 and 1/200) 32
  33. 33. 33 The content of a Log Remarks is disclaimer and additional informationthat will help for the interpretation The content of a Log Calibrations is a proof that all operational checkshave been performed before running in hole and after pulling out of hole.Tool was reading correctly in air. Last time we check the tool at the base inknown conditions, it was reading within toleranceThe content of a Tech Log is tool performance while logging and it tells theengineer ,if the tool was working fine. Monitoring of hardware (voltages, current) Green is good, Red is bad!The content of a Log Tail is end of the logIn this document our study limited to very few logs: GR, SP, Neutron,Formation density, Sonic,, Focused, Induction, Micro resistivity logs andCaliper.Understanding the Log Measurements: The logging instrumentation responds mostly to pore materialsand the chemical makeup (composition) of the rock matrix. As a result achemical rock classification is most suitable for use in log analysis. Rocks ina well have rather unique log responses that are usually identified easily. In order to understand log measurements and the methods ofobtaining these data, their must also be a general knowledge of other dateassociated with logs. The log is a record of the events leading up to and during thedrilling and completion of a bore hole. Some of the information of the logheader ,…….. is not a data from measurements taken by the wire linelogging tool. 33
  34. 34. 34 It is often useful in determining;Why some log responses are questionable,Why the logging instruments could not reach total driller depthorWhy a logging instrument became stuck at a certain depth. Some of the information is measured at the surface by thelogging crew and can have great importance in formation evaluation. Theinformation should be acquired and reported accurately. 34
  35. 35. 35 LITHO LOGY LOGS Spontaneous Potential (SP) log Gamma Ray (GR)logIntroduction: The lithology identification logs, Spontaneous Potential (SP) and GammaRay (GR) are recordings of naturally occurring phenomena in in-situconditions recorded as track 1 . SP curve records-the electrical potential produced by the interactionof formation connate water, conductive drilling fluid, and certain ion –selective rocks (shale). Natural GR Log indicates –the total natural radioactivity of theformations.THE SPONTANEOUS POTENTIAL (SP) LOG: It is a permeability indicator; the magnitude of the SP deflection andpermeability of a formation have no direct relationship. however ,when themud is saline than connate formation water, permeable beds are oftendelimited by negative SP excursions. R mf > R w or R xo > R t implies Negative SP. SP deflection normally occurs only if permeability exist to allowion migration between the mud and formation. In many cases, a good value of Rw can easily be found from the SP-curve recorded in clean water bearing formations.From the SP Curve: SSP = -K log (Rmf / Rw)Uses of SP:  Detect the permeable beds  Locate the bed boundaries and to permit correlation of such beds.  Determine Rw 35
  36. 36. 36  Give qualitative indication of bed shalenessTHE GAMMA RAY (GR) LOGIntroduction:It is a good shale indicator, it measures total gamma ray emissions form theformations. It is scaled in API units.It is also affected sometimes due toborehole conditions.The volume of shale (Vsh)in the reservoir rock can be estimated from thedeflection of the gamma ray curve.In cased holes, gamma ray log is used as a depth control. Also it is used toposition the formation testers and sidewall core guns.It is also used in radioactive tracer operations to locate pipe leaks,channeling behind casing. Uses:.  Correlation  Evaluation of the shale content analyses  Mineral analyses.Main use of SP &GR: - To differentiate porous and permeable reservoir rock (SST, LST, and DOLOMITE) from non permeable clays and shales. - Define bed boundaries and permit correlation of beds. 36
  37. 37. 37POROSITY LOGS Density log Neutron log Sonic logIntroduction: Rock porosity can be obtained from the sonic log, the density log orthe neutron log. Here the tool response is affected by formation porosity,fluid and matrix. If the fluid and matrix effects can be determined the toolresponse can be related to porosity. Here the depth of investigation is only afew inches generally with in the flushed zone.DENSITY LOG The density log is a measurement of scattered gamma rays reachingthe detector, at a fixed distance from the source, are translated in terms offormation density. The number of Compton-scattering collisions is related directly to thenumber of electrons in the formation. Consequently, the response is relatedto the true bulk density, ρb, which, in turn, depends upon the density of therock matrix and density of the fluids and the density of the formation.Uses:.  Calculation of porosity  Identification of hydrocarbon bearing zones  Estimation of bulk density of formation Bulk density: ρ b = Φ ρ f + (l - Φ) ρ ma Porosity: 37
  38. 38. 38NEUTRON LOGIntroduction: The Neutron log measurement of the slowdown neutron counts. Theneutron collides with formation, after sufficient number of collisions theneutron will reach a lower energy state where upon they are captured byformation nuclei. When a nucleus captures a thermal neutron, it dissipatesthe energy and slows down. The neutron tool responds to porosity but they are also influenced byother parameters and certain environmental effects: borehole fluid type,density, salinity, borehole size, mud cake, stand off temperature andpressure.Uses:  Identification of gas bearing formations  Estimation porosity (mainly liquid filled)  Determination of formation fluid type  Determination of lithologySpecial application: In cased hole it is used for correlation and depth control for perforationDepending on the device, these measurements may be made either in openor cased holes. 38
  39. 39. 39SONIC LOGIntroduction:The sonic log is a measurement the speed of sound waves in formations orthe interval transit time. These measurements are useful for a number ofreasons in many professional The interval transit time for a formation depends upon its litho logyand porosity. This dependence upon porosity, when the litho logy is known,makes the sonic log very useful as a porosity log. Integrated sonic transittimes are also helpful in interpreting seismic records. Porosity: (Wyllie- Time Average Equation) ∆t log = Φ∆tf + (1-Φ) ∆tma Φs = (Δtlog- Δtma)/ (Δtfl- Δtma)Where Δtma is the travel time of the sonic wave through the rock matrix,  For SST: 55.5μsec/ft  For LST: 47.5μsec/ft  For DOL: 43.5μsec/ft  Δtfl is the travel time of the sonic wave through the fluid; it is generally 189μsec/ft.Uses:  Estimation of porosity  Identification of lithology and factures  Integrated travel time and velocity for seismic interpretation  Identification of Cement behind the casing. 39
  40. 40. 40RESISTIVITY LOGS Normal log Lateral log Latro logsIntroduction: The resistivity log is a record of potential variation (or its equivalent,apparent resistivity) versus depth. the resistivity is a function of measuredpotential difference and sending current into the formation. It is sensitive torock properties such as porosity, shaliness, compaction or degree ofsedimentation, pore distribution and pore fluids. In general the formations encountered in oil wells are poor conductors,having resistivities in the range 0.2 to 1000 ohm-m.The resistivity of the formation depends on:  Resistivity of the formation water  Amount of water present  Pore structure geometry  Fluid type In addition to the deep responding resistivity tools, a number ofshallower responding resistivity devices are available for the measurementsof Rxo and Rt.DUAL LATRO LOG (DLL): he dual latro log is a set of record of resistivity, there are two records: DLL(latro log deep) and LLS (latro log shallow). Its response is mostlydependent upon the true formation resistivity.However LLS reading is useful to get true resistivity from LLD reading, andmost of the times LLD is very close to the true resistivity. 40
  41. 41. 41Uses:  Estimation of true resistivity  Identification of diameter of invasion.MICROLATRO LOG (MLL): The micro latro log is a record of measured resistivity of the flushedzone, Rxo, and to delineate permeable beds by detecting the presence of mudcake. Response of MLL depends upon the Rxo/Rmc ratio as current isprevented from flowing into mud cake. The depth of investigation of this toolis three to five inches, so even if invasion is low or moderate, MLL respondsto invaded zone. Effect of mud cake is negligible up to cake thickness of 3/8inches but increase rapidly with greater thickness of cake. MLLmeasurements are not preferred where mud cake thickness is (greater than3/8 inches).Uses:  Identification of permeable beds.  Information of flushed zone resistivity.MICRO SPHERICAL FOCUSED LOG (MSFL): The micro spherical focused log is a record of measured resistivity ofthe flushed zone, Rxo. is the shaping of the equipotent surface produced byresistivity device to approximately spherical form. A careful selection ofelectrode spacing achieves an optimum compromise between too much andtoo little depth of investigation. MSFL gives near true Rxo value in thickmud cake and low invasion conditions.Uses:  Information of flushed zone resistivity in low invasion conditions . 41
  42. 42. 42Chapter 5:INTERPREATION andCHARACTERIZATION OF RESERVOIRCalculation of R w, ø eff, V sh, S wPreparation of Para log Interpretation and characterization is an art of science through itssystematic application of rules based on past experience to assign and tovalidate the geologic framework and composition to the reservoir.Selection of control Parameters: Before the well log data interpretation The chosen log data intervalselect the control parameters, such as  Rsh → Resistivity value against shale from Resistivity log  Rlim → Maximum Resistivity observed on resistivity log  Rw → Resistivity of formation water (to be estimated)  ΦNs → Neutron porosity against shale from Neutron-Density log, Near the zone of interpretation  Rhob sh → Density value against shale from density log, near zone of Interpretation.  ΦNsh → Density porosity of the shale (calculated using empirical formula)  ΦS sh → sonic porosity of the shale (calculated using empirical formulae)  Rhobmat → Density of the matrix  GR min→ Minimum gamma ray count in the GR log, (form clean bed)  GR max → Maximum gamma ray count in the GR log (form shale bed)  SSP → Minimum SP curve deflection in SP-log (clean)  B.H.T → Borehole temperature at the bottom of the well. 42
  43. 43. 43Determination of Rw from the SP: Formation water, sometimes called connate water or interstitialwater, is the water uncontaminated by drilling mud that saturates the porousformation rock. The Resistivity of this formation water is an importantinterpretation parameter since it is required for the calculation ofsaturation’s (water and/or hydrocarbons) from basic logs. There are severalsources for formation water Resistivity information. These include watercatalogs, the spontaneous potential (SP) curve, and various Resistivity –porosity computations and cross-plots. In many cases, a good value of Rw can easily be found from the SP-curve recorded in clean water bearing formations. The static SP (SSP) valuein a clean formation is related to the chemical activities (a w and amf) of theformation water and mud filtrate through the formula: SSP = -K log (aw/amf)For Nacl solution, K=71 at 77°f (25°C); k varies in direct proportion totemperature: K=61+0.133T°F K=65=0.24 T°C For pure Nacl solution that not too concentrated, resistivities areinversely proportional to activities. However, this inverse proportionalitydoes not exactly at high concentrations or for all types of waters. Thereforeequivalent resistivities Rw and Rmf, which by definition are inverselyproportional to the activities, are used, Rw is the equivalent formation waterresistivity and Rmf is the equivalent mud filtrate Resistivity. SSP = -K log (Rmfe / Rwe) Knowing the formation temperature, the static SP value recordedopposite a porous, permeable, nonshaly formation can be transformed intothe resistivity ratio (Rmf / Rw). 43
  44. 44. 44The Rmf value at surface is given. The Rmf value at particular depth iscalculated from below formula. i.e. Temp gradient =(Td -Ts)(100)/depth difference.WhereTd=Temp in borehole at bottom depthTs=Temp at surfaceFormula:-Temp in particular depth=surface temp+(temp gradient*given depth)/100Rmf at given depth =( (Ts+6.77)/(temp at given depth+6.77))*Rmf atsurface For given SP value at particular depth plotted on SP chart-1 then wefind Rmfe/RweAfter that Rmf value plotted on SP chart-2 at temp of given depth then wecan estimate corresponding Rmfe valueFrom this Rmfe value we can find Rwe by using Rmfe/Rwe valueAfter that Rwe value plotted on SP chart-2 at temp of given depthcorresponding Rw value is estimated 44
  45. 45. 45Formation factor: Archie’s experiment show the resistivity of a clean formation isproportional to the resistivity of the brine saturating rock. The constant ofproportionality is known as formation resistivity factor (F).. Also hisexperiment concludes an empirical relationship between formation factorand porosity. and where m=cementation exponent a =Archie’s constantThe most widely used Archie’s relation between F and Φ for sands is,And other relationships for sands HUMBLE formula, TIXIER formularespectively below.The formation factor in term of resistivity as follows: 45
  46. 46. 46Determination of Rw from Hingle cross plot: In the late 1950’s Hingle proposed a method based on resistivity &porosity log data which allows the percent water saturation to bedetermined directly from a cross plot. The method is based on the wellknown archie’s equation, which in a rearranged form is plotted on specialgrid type graph paper.Plotting procedure is outlined as follows,  Select proper cross plot paper  Taking the x-axis in linear fashion for raw logging parameters (∆t, ρb ) and establish porosity scale. Porosity will be zero at the matrix point and increases to the right. Taking Y-axes axis in logerthemic fashion for raw log data (Deep resistivity,R t)  Plot the resistivity (Rt)Vs (∆t, ρb, ΦN). The resistivity scale can be changed by any order of magnitude to fit the log data. This is done without changing the validity of the graph paper grid.  The straight line drawn through the most north-westerly(clean)points defines Sw=1. Extrapolate this to the intersection with X-axis( Φ =0 ) .  At the intersection determine the matrix value (∆tma or ρma ) for a proper porosity scaling of the X-axis.  Calculate Rw from any corresponding set of Φ and Ro data along the water line such as Rw =Ro/F. 46
  47. 47. 47Estimation of water saturation:Resistivity ratio method: When a borehole is drilled, the formation close to the borehole isinvaded with mud filtrate. In an oil bearing zone, we will normally have azone of low resistivity close to the borehole, and one of higher resistivityfurther away. Thus a comparison of a deep resistivity device with a shallowresistivity device will detect hydrocarbons. Form the Archie’s equation wecan derive an expression for water saturation as a function of the ratio ofthese two curvesThe saturation in terms of porosity as follow : The saturation of water in term s of resistivity as follows:And 47
  48. 48. 48Shale corrected water saturation: (Indonesian equation)Shale Volume Calculation: The natural gamma ray log can beused to calculate volume of shale in porous reservoirs. The volume of shaleexpressed as a decimal fraction or percentage is called Vshale.Calculation of the gamma ray index is the first step needed to determine thevolume of shale from gamma ray log.The gamma ray log has several nonlinear empirical responses as well alinear responses. The non linear responses are based on geographic area orformation age. All non linear relationships are more optimistic that is theyproduce a shale volume value lower than that from the linear equation.Linear response (Vshale = IGR) : 48
  49. 49. 49There are several formula for shale volume calculations show in below:Estimation of Effective Porosity: A quick look estimate of porosity can be made simply by reading theneutron log and Density log using the limestone porosity scale, and takingthe average of the readings.For clean formation 49
  50. 50. 50In the presence of shale Where,  Φe is effective porosity  ΦN is neutron porosity  ΦNsh shale corrected neutron porosity  ΦD is density porosity  ΦD sh shale corrected densityThe minimum value of Rw is considered for Sw calculation based on theassumption Rw values estimated above are on the higher side. 50
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  52. 52. 52Reservoir Characterization Parameters The petroleum reservoir is that portion of the rock that contains thepool of the petroleum. Each reservoir is unique in its details. In order tocharacterize a reservoir, there are certain parameters which have to beestimated. The main reservoir characterization parameters being porosity,permeability and its saturation. 52
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  54. 54. 54 Conclusion Determination of Rw, Vsh, Φeff and Sw are estimated in waterbearing & hydrocarbon bearing formations taking sample data acquired inwell. The same sets of log data are processed using Geoframe softwarepackage available with workstations in Well Logging Services, GOC (GasOil Contact), OWC (Oil Water Contact) and GSC(Gas Shale Contact) whereever observed are marked in the parameter logs. The dissertation work on carried out during the period 19th May 2009to 19th Jun 2009 gave an insight into the details of formation evolution andthis provided valuable knowledge & experience . This will be useful furtherstudies and job assignments. 54