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formation evaluation chapter 1

  2. 2. 1 Introduction
  3. 3. Outline Logging History Openhole Logging Acquisition Principles of Openhole Logging Tools – GR, SP, Neutron-Density, Resistivity, Sonic
  4. 4. Objectives After completing this chapter, you should be able to: - Understand the basic principle of formation evaluation and well logging - Describe the surface and downhole equipments/ tools to conduct a logging operation - Describe the principle and log response of each of logging tool.
  6. 6. Overview Formation Evaluation… Process/method to determine or identify if a potential oil or gas field is commercially viable by using all available data (e.g. well log data, core data, mud log, RFT data etc.) for interpretation of reservoir formation
  7. 7. Overview Well Logging… A method or in situ measurement or recordings (vs depth) to determine the physical, chemical and petrophysical properties of the reservoir rocks and fluids.
  9. 9. Overview Logging While Drilling… Advanced logging operation allowing acquisition of log data via tools placed in the actual drilling assembly, which transmit the data to the surface on a real-time basis or store the data in a downhole memory from which it may be downloaded when the assembly is brought back to the surface. Their use may be justified when: – real time information is required for operational reason, e.g. steering a well – acquiring data prior to the hole washing out or invasion occurring – safeguarding information if there is a risk of losing the hole – the trajectory where wireline acquisition is difficult
  10. 10. Overview EVALUATION SEQUENCE Rock Hydrocarbons Gas Evaluate Reservoir Water Oil EvaluateNon ReservoirLocate the Detect DifferentiateReservoir Hydrocarbons Between gas/oil
  11. 11. What subsurface information is important? • Hydrocarbon thicknessWhat is value of hydrocarbon in place? • Porosity(Potential value) • Saturation • Area • Hydrocarbon typeHow easily can the hydrocarbon flow • Permeabilityout the well? • PressureHow easy is it to drill to the reservoir? • Lithology(Cost of drilling, completing…) • Depth, pressure, temperature
  12. 12. Formation evaluation is critical to understanding the reservoirWhat is value of hydrocarbon in place?(Potential value)How easily can the hydrocarbon flowout the well?How easy is it to drill to the reservoir?(Cost of drilling, completing…)
  13. 13. Logging History Electrical LoggingYear Description1927 • First electrical log was recorded in a well in the small oil field of Pechelbronn, in Alsace, a province of north-eastern france. • Single graph of electrical resistivity of rock formations was recorded by “station” method. • “sonde” was stopped at periodic intervals in borehole, measurements made, and calculated resistivity was hand-plotted on a graph – this procedures was carried out from station to station until entire log was recorded. • Resistivity log was used to detect HC present in the formation.1929 • Electrical resistivity logging was introduced on a commercial basis in Venuzuela, US, Russia and Dutch East Indies. • Usefulness: for well to well correlation and identification of potential HC-bearing strata.1931 • Include SP measurement with Resistivity curve on electrical log. • Schlumberger brothers (Marcel & Conrad) perfected a method of continuous recording1936 • Photographic-film recorder was introduced • Electrical log consisted of SP curve, short normal, long normal & long lateral resistivity curves, was predominant in logging activity from 1936 to late 1950’s (curves were recorded simultaneously after about 1946).
  14. 14. Logging History Dipmeter LogYear Description1930’s • The development of dipmeter began with the anisotropy dipmeter tool.1943 • Three-arm dipmeter device, with an associated photoclinometer was introduced – permitted both direction and angle of formation dip to be determined (SP sensor at each arm).1946 • SP sensors were replaced by short resistivity devices – made dip measurements possible in wells where SP had little correlatable detail.Mid- • First continuously recording electrical dipmeter sonde (used 3 microresistivity arrays and1950’s contained a fluxgate compass) was introduced.Today • A 4-arm dipmeter tool records 10 microresistivity curves simultaneously, and a triaxial accelerometer and magnetometers provide highly accurate info on tool and deviation azimuth. • Processing data done exclusively with electronic computers.
  15. 15. Logging History GR and Neutron Tools (first use of radioactive properties in well logging)Year Description1941 • Neutron log was first described by Pontecovo. • In combination with GR log, neutron log enhanced lithological interpretations and well-to-well stratigraphic correlations.1949 • Attention to neutron log as a porosity indicator.1962 • SNP sidewall neutron porosity tool was introduced.1936 • CNL* compensated neutron tool was introduced. • Dual Porosity neutron tool combines those 2 neutron measurements into a single tool.
  16. 16. Logging History Early Porosity Determination & Microresistivity MeasurementYear Description1950’s • Microlog tool was introduced – used a miniature linear array of 3 electrodes imbedded in the face of an insulating pad, which is applied to the borehole wall. • Microlog recording is also useful to delineate permeable beds, and other microresistivity devices help establish resistivity profile from the invaded zone near the borehole to the non- invaded virgin formation.1951 • Laterolog tool was introduced (the first focused deep-investigating resistivity device) – focused resistivity logs are well adapted for investigating of thin beds drilled with low- resistivity muds (eg. Salt muds & highly resistive formations)1953 • Microlaterolog tool was developed for salt muds. • The MicroProximity log and MicroSFL* log have followed.Today • DLL* dual lateral log tool (deep laterolog and shallow laterolog measurements) is the standard. • Usually run with a MicroSFL device as well
  17. 17. Logging History Induction Log (replace original electrical log in freshwater muds)Year Description1949 • Induction log was developed, as an outgrowth of wartime work with mine detectors, for use in oil-based mud. • However, its superiority over electrical log in freshwater muds was soon recognized.1956 • Combine a five-coil induction device with SP curve and a 16-in normal to make induction electrical tool.1959 • Five-coil device was replaced by one with a six-coil array with deeper investigation.1963 • DIL* dual induction log was introduced, now is the standard – deep induction, medium induction, and shallow resistivity-measurements. • The shallow resistivity-measuring device is now a focused resistivity device – a Laterolog 8 on the 1963 tool and an SFL device on current tools • A new dual induction log, the Phasor* induction, provides improved thin-bed response, deeper depth of investigation, and greater dynamic resistivity range.
  18. 18. Logging History Sonic LogYear DescriptionSince • Logging cables have been used to lower geophones into wells to measure long-interval1930 acoustic travel times from sound sources at the surface.Late • Sonic log was accepted as a reliable porosity logs – its measurement responds primarily to1950’s porosity and is essentially independent of saturation. • Sonic log, coupled with focused resistivity logs (laterolog and induction) – made possible modern formation evaluation from well logs. • Sonic log – measure porosity; focused resistivity logs – measure true resistivity of non- invaded virgin formation. • Subsequent improvements in sonic logging – BHC borehole compensated sonic, LLS* long-space sonic, and the Array-sonic* tools.
  19. 19. Logging History Density LogYear DescriptionEarly • Logging of formation bulk density (measurement of formation porosity), was commercially1960’s introduced.1964 • An FDC* compensated formation density log (compensated for the mudcake), was quickly followed.1981 • Litho-Density* log provided an improved bulk density measurement and a lithology-sensitive photoelectric absorption cross section measurement.
  20. 20. Logging HistoryRecovery of Physical Rock Samples & Formation Fluid Samples with Wireline ToolsYear Description1937 • Sidewall coring, using a hollow, cylindrical “bullet” shot into formation and retrieved by pulling it out, has existed since 1937.1957 • A formation tester was introduced – recovered a sample of formation fluids and pore presure was measured during the sampling process. • FIT formation interval tester and RFT* repeat formation tester have followed (RFT tool can make unlimited number of pressure measurements and recover two fluid samples per trip.1978 • Dielectric measurements have been developed to handle formation with freshwater & formation, or varies in salinity, or in which salinity is unknown.1985 • EPT* electromagnetic propagation log was introduced in 1978 • DPT* deep propagation log was followed in 1985.
  21. 21. Wireline LoggingIntroductionWell logs or wireline logs are continuous recordings of well depth versusdifferent petrophysical characteristics of the rocks through which the well isdrilled. There are many types of well logs, depending upon the characteristicsof the rock being measured.Logging ObjectivesThe main purpose of well logging is: - to provide data for evaluating petroleum reservoirs. - to aid in testing, completion and repairing of the well.To calculate the oil reserve in an oil pool we need to know the following. • Thickness of the oil bearing formation. • Porosity of the formation. • Oil saturation. • Lateral extent of the pool.Logs should always be calibrated with core data to improveinterpretations.
  22. 22. Wireline Logging• In situ meas. (vs. depth) of – Rock properties – Fluid properties• When – Openhole (before casing) Casing • While drilling (LWD / MWD). • After drilling (wireline). – Cased hole (C/O, sigma)• Interpretation for: Open hole – Geological properties. – Petrophysical properties. – Production properties.
  23. 23. Types of Well LoggingWell logging is classified into three broadcategories: Open Hole Logging Cased Hole Logging Production Logging
  24. 24. Open Hole LoggingLogging surveys taken before the hole is cased are called openhole logs. The logs included in this group are: Electrical surveys (induction, laterolog and microlog logs). Sonic logs. Caliper Logs. Dipmeter Logs. SP logs Radioactive surveys (density, neutron and gamma ray logs).
  25. 25. Electrical LogsElectrical logs (Induction, laterolog, and microlog)measure the electrical properties of the formationalongwith the formation fluids.Sonic/ Acoustic LogsSonic logs measure the elastic or (sound) waveproperties of the formation.Caliper LogsCaliper logs measure the size or geometry of the hole.
  26. 26. Dipmeter LogsDipmeter logs measure dip of the formations.SP LogsSP logs measure potential different between a shale-sand orshale-carbonate due to difference salinity of formation waterand mud filtrate.Radioactive LogsGamma ray & neutron logs measure radioactive and neutronabsorption properties. Density logs measure electron density ofthe formation which is related to formation density.
  28. 28. Cased Hole LoggingLogging surveys taken after the casing is lowered are usuallycategorized as cased hole logs. The surveys included in this group are: Gamma Ray Neutron Temperature Pulsed Neutron Cement Bond Log C/O and sigma LogSome of these surveys like the gamma ray, neutron and temperaturelogs can be run in both open and cased hole wells.
  30. 30. Production LoggingWell logging surveys taken to improve production or repair the well aretermed as production logs. Surveys included in this category are: Flowmeter Pressure Temperature Fluid Density
  31. 31. VALUE AND LIMITATIONS OF WELL LOG DATAStrengths• Provides remotely sensed values of reservoir properties and fluids.• Among the most abundant reservoir data.• Presentation results fairly well standardized.• Allows evaluation of lateral (map) and vertical (cross section) changes in reservoir properties and fluids.Limitations• Indirect measurements.• Vertical resolution.• Depth of investigation.
  32. 32. Petrophysical Logging Tools - PrimaryLog Type Tool Type Physical Derived Interpreted Measurement Parameter ParameterResistivity-Induction Array Voltage (V) Rt Sw-Laterolog Array V and Current (I) Rt Sw-Micro laterolog Pad Current Rxo SxoAcoustic- Sonic Array Transit Time PHIs LithologyNuclear-GR (Density) Pad Gamma Ray RHOB, PHID Lithology- Neutron Mandrel Neutron RHON LithologyAuxiliary-Natural GR Mandrel Gamma Ray None Vsh-SP Electrode mV None Vsh-Caliper (*various) Dh, Volume 32
  33. 33. SOME QUESTIONS ADDRESSED BY LOG INTERPRETATION• Geophysicist / Geologist • Reservoir Engineer – How thick is the pay zone? – Are the tops as predicted? – How homogeneous is the zone? – Are potential zones porous? – Porosity? – Formation intervals? – Permeability? – Lithology? – Hydrocarbons? • Production Engineer – What type of hydrocarbons? – Which zone(s) to complete? – Commercial quantities? – What production rates? – Any water production? – Is zone hydraulically isolated? – Will well need stimulation? – What stimulation would be best?
  34. 34. Fig. 3.1: A Logging Truck
  36. 36. Computerized Logging UnitsComputer-based units offer the following features: Computer control of the data allows logs to be recorded either logging up or down with all curves on depth. Calibration are performed under programme control and can be performed more quickly, consistently and accurately. Logs can be played back from the data tapes on many different formats. Basic wellsite, processing/analysis of data is available.
  38. 38. LOGGING CABLE 39
  39. 39. Log Presentation• Heading.• Curves related to some physical property of rock/casing surrounding the wellbore.
  40. 40. LOG PRESENTATION - THE HEADING• Well location• Depth references• Date of log• Well depth• Casing shoe depth• Bit size• Mud data – Type – Properties – Resistivities• Max. Temperature 41
  42. 42. LOG PRESENTATION - LINEAR GRID Depth Track 1 track Track 2 Track 3 43
  44. 44. TYPES OF LOGS TO BE RUN• Logging suites generally include one resistivity and one porosity device.• The logging string will also have other tools like the gamma ray, SP and caliper tools.• However, logging suites usually have two porosity devices to give more information about rock type, hydrocarbon type and porosity.• Other considerations – to estimate permeability or to take fluid samples – require other special tools like the formation testers.
  45. 45. MUD FILTRATE INVASION Uninvaded Zone (Rt) Invaded Zone (Rxo) Wellbore Mud (Rm)Uninvaded Mud Cake Zone (Rmc) (Rt)
  47. 47. COMMON TERMINOLOGYBoreholeRm : Borehole mud resistivityRmc : Mudcake resistivityInvaded zoneRmf : Mud filtrate resistivityRxo : Invaded zone resistivitySxo : Invaded zone water saturationUninvaded zoneRw : Interstitial water resistivityRt : Uninvaded zone resistivitySw : Uninvaded zone water saturation
  48. 48. Radial Fluid and Resistivity Distribution Rx0 Rt Rx0 RtResistivity Resistivity Rxo Rxo Rt Rt Water Based Muds Qualitative Distribution of Resistivity (Rmf > Rw)
  49. 49. Fresh mud, salt water zoneSalty mud, Hydrocarbon zone
  51. 51. Sources of subsurface dataData collected during drilling Penetration rate Drill cuttings analysis Drill mud analysis Mud gains/losses Shows of gas/oil/waterCore analysis Lithology Presence of shows Porosity Permeability Special core analysisWireline log analysis Electric logs Acoustic logs Radioactivity logs Pressure measurements Special logsProductivity tests Formation tester Drill stem test Production test
  52. 52. Sources of subsurface dataData needed: Data source:Hydrocarbon thicknessPorositySaturation Cuttings, Mud logArea CoringHydrocarbon type Logging • LWD – Logging while drillingPermeability • WL – Wireline (usually open hole)PressureLithology
  53. 53. Mud Log• Immediate interpretation of what the drill bit has penetrated and whether there are any hydrocarbons present (a show).• Making maps of the subsurface geology.
  54. 54. Sources of data – Mud log
  55. 55. Mud log
  56. 56. Sources of subsurface dataData needed: Data source:Hydrocarbon thicknessPorositySaturation Cuttings, Mud logArea CoringHydrocarbon type Logging • LWD – Logging while drillingPermeability • WL – Wireline (usually open hole)PressureLithology
  57. 57. Coring - Conventional• Taking a core requires that the regular drill bit be removed from the hole. It is replaced with a "core bit", which is capable of grinding out and retrieving the heavy cylinder of rock.• The core bit is usually coated with small, sharp diamonds that can grind through the hardest rock. A core bit cuts very slowly.• A core is a solid cylinder of rock about 4-5 inches in diameter, and a single core will usually be about 30 feet long.
  58. 58. Coring - ConventionalWhole Core Slab Core
  59. 59. Sources of data – Core
  60. 60. Coring - Sidewall• This method is cheaper than the conventional coring.• Cores can be taken in hours, instead of days.• In sidewall coring, a slim wireline coring tool is run into the hole. The tool may be of two general types; either "rotary sidewall" or "percussion".• Typically, cores about 1" in diameter and 1" to 2" long can be retrieved with this method.
  61. 61. Coring - Sidewall
  62. 62. Sources of subsurface dataData needed: Data source:Hydrocarbon thicknessPorositySaturation Cuttings, Mud logArea CoringHydrocarbon type Logging • LWD – Logging while drillingPermeability • WL – Wireline (usually open hole)PressureLithology
  63. 63. Sources of data – Logs