Formation evaluation and well log correlation

Formation evaluation and well log correlationSwapnil PalIMT Geological Technology Indian Institute of Technology RoorkeeOverviewWell log definition and its importanceFundamentals of qualitative and quantitative log interpretationFormation evaluation with well logsProcessing and interpretation of well log data Conclusion

2Well log

The continuous recording of a geophysical parameter along a borehole produces a geophysical well log.The value of the measurement is plotted continuously against depth in the well. 3ObjectiveThe fundamental questions that has to find answers during Formation evaluation. What kind of rock is present?reservoir or non-reservoir rock?If Reservoir rock exists.Are any hydrocarbon present?Type of hydrocarbon present whether oil or gas? How much hydrocarbon is there?(pay thickness, porosity, saturation etc. and finally the estimation of reserve).

Basic Logs Tools and their MeasurementsGamma Ray log

Basic Logs Tools and their MeasurementsCalliper log

Basic Logs Tools and their MeasurementsElectrical log

Basic Logs Tools and their MeasurementsDensity log

Basic Logs Tools and their MeasurementsNeutron log

Petrophysical InterpretationQualitative assessment Quantitative assessment Assessment of reservoir properties, fluid type form log pattern.Numerical estimation of reservoir properties viz. % of oil, water etc.Basic steps for quick look evaluationROCKReservoir Non-Reservoir Hydrocarbon bearingWater bearingGas bearingOil bearing

Qualitative InterpretationIdentification of Reservoir or Non-reservoir

Low gamma ray(Reservoir rock)12

Identification of hydrocarbon or water bearing zoneQualitative InterpretationLow gamma ray +High Resistivity valueQualitative Interpretation

Low gamma ray +High Resistivity value +Large deviation in RHOB and NPHI value(cross-over region)Identification of oil or gas bearing zoneQualitative Interpretation

Gas bearing zoneOil bearing zoneWater bearing zoneQuantitative InterpretationEstimation of effective porosity & permeability.Estimation of volume of clay fraction.Estimation of hydrocarbon saturation.Determination of the depth and thickness of net pay.Estimation of reserves of hydrocarbon.Quantitative InterpretationEstimation of porosity

Quantitative InterpretationEstimation of porosity

Depth (m)VshPHIDPHIdcPHINPHInceffective (Gas)19400.7105260.2787880.1065390.460.1757890.14534919450.0789470.4181820.3990430.090.0584210.28517419480.0526320.3818180.3690590.070.0489470.26324919500.0394740.4242420.4146730.060.0442110.2948819530.0394740.4848480.4752790.140.1242110.3473619570.6973680.3272730.1582140.330.0510530.11755419580.6447370.2666670.1103670.410.1521050.132885gas bearing zoneQuantitative InterpretationEstimation of porosity

Depth (m)VshPHIDPHIdcPHINPHInceffective (Gas)19700.6578950.2363640.0768740.360.0968420.08685819720.6447370.2181820.0618820.3250.0671050.06449419740.1184210.2727270.2440190.190.1426320.19332519760.4605260.2727270.1610850.340.1557890.15843719780.2763160.2545450.187560.220.1094740.14851719800.4342110.2727270.1674640.2750.1013160.1343919820.2631580.2848480.2210530.240.1347370.17789519840.7236840.2545450.0791070.430.1405260.109817oil bearing zoneQuantitative InterpretationEstimation of porosity

Depth (m)VshPHIDPHIdc20250.2368420.2787880.21275920280.5657890.2303030.120290.3289470.2121210.14234420300.2763160.2606060.18859620310.3421050.2424240.1594920340.3947370.2787880.1687420350.2236840.2606060.20231320360.2236840.2606060.20231320370.1973680.2363640.189713water bearing zoneQuantitative InterpretationEstimation of hydrocarbon saturationCan not be measured directly but inferred from determination of WATER SATURATION (Sw) from RESISTIVITY and POROSITY logs.Sw Fraction of pore space occupied by water.Sh Fraction of pore space occupied by hydrocarbon.Sh + Sw = 1

Oil Water Quantitative InterpretationWater saturation estimationObjective: whether the pores of the formation is completely saturated with formation water or the pore space is partially saturated with oil/gas.

waterSand grainsRoCurrent travels along the path of least resistance which is measured as wet resistivity (Ro).Rw: Formation water resistivitySw = 100%Quantitative Interpretation

Archies equationQuantitative InterpretationCalculation of formation water resistivity : RwUsing Inverse Archies equationRt0.950.70.10.80.90.70.70.70.85Rw0.0738370.0371280.00450.0543320.0528930.0544060.0475410.0475410.047488Depth202520282029203020312034203520362037Water bearing zoneRw= 0.05, this is used in Archies equation to calculate water saturation in oil and gas bearing zoneQuantitative InterpretationDepth194019451948195019531957195819701972197419761978198019821984PHId0.2787880.4181820.3818180.4242420.4848480.3272730.2666670.2363640.2121210.2727270.2727270.2545450.2727270.2909090.254545Rt0.65153548330.90.8511.5242.931.20.8Sw (Archie)0.9948430.1380620.0989910.0760760.0802830.7202010.9095090.9460290.8607070.5797510.4099460.5158470.4733650.7016770.982143Calculation of water saturation in oil and gas zonesQualitative Interpretation

Gas bearing zoneOil bearing zoneWater bearing zone

Hurray!Oil found!! V/sQuantitative InterpretationConclusion Well logs contains key information about the formation drilled in different petro-physical measurements. i.e.Prospective zones of hydrocarbon.Reservoir type and thickness.Estimation of Porosity, permeability.Fluid type present in the pores and saturation level.To economically establish the existence of producible hydrocarbon reservoirs (oil & gas).References Archie II: Electrical conduction in hydrocarbon bearing zone. (n.d.). In Rock Physics (Vol. 36).Archie III: Electrical conduction in shaly sand. (n.d.). In Rock Physics (Vol. 1).Archie's law: Electrical conduction in clean, water bearing rock. (n.d.). In Rock physics/History (Vol. 36).Halliburton. (n.d.). Log Interpretation Charts. M H Rider. (1991). The Geological Interpretation of well logs. Glasgow: Whittes Publishing.(1989). Open hole well logging Interpretation. Texas: Schlumberger Wireline & Testing.Serra, O. (1984). Fundamentals of well log interpretation. Amsterdam: Elsevier.Thank you