Well logging in_7_ [ SP-log]_052629.pptx

Well logging in_7_ [ SP-log]_052629

1. PE 3 Well Logging Lecture 7 Spontaneous Potential Log Dr. Ali Mohammed Al Tameemi

2. SP Measurement М N The SP is measurement against depth of the potential difference between the voltage in the wellbore and an electrode at the surface

3. Liquid-Junction Potential (Diffusion Potential, E j ) : The negatively charged chloride ions have greater mobility than the positive sodium ions and an excess negative charge tends to cross the boundary, Membrane Potential ( E m ) : The shale bed acts as a cationic membrane, permitting the sodium cations to flow through it but not the chloride anions SP Origin

4. SP Origin

5. Diffusion Potential The diffusion potential is caused by the contact of two fluids of different salinity. Ions diffuse from the more concentrated solution (typically formation water) to the more dilute. The ion flow constitutes electrical current, which generates a small natural potential.

6. Membrane Potential Shale has an electrical double layer: pore walls of a shale are negatively charged. Passage of Cl - ions is greatly restricted. Na + ions can enter the membrane freely. Concentrated solution will be negatively charged and the result is a potential E m across the membrane.

7. SP Development The combination of membrane and liquid junction potential creates a closed circuit.

8. The resistivity of formation water must be significantly different from that of mud filtrate Drilling fluid must be conductive. Shale must be present next to a permeable zone like a sandstone in order to have an SP SP is generally an indicator of permeability Spontaneous Potential (SP)

9. SP - Example Shale baseline: SP response for shale follows a straight line, which is assumed to be zero. SP deflections are measured from this line. No deflection for R mf = R w Negative deflection for R mf > R w Positive deflection for R mf < R w (fresh water formation). Permeable zones are indicated where there is a deflection from the shale baseline. Inflection points give bed thickness.

10. SP: R mf >> R w sdfds Shaly sand formations: SSP: static SP = max SP for a thick, shale free, wet (water bearing) sandstone (clean) (for a given ratio of R mf to R w ) PSP: pseudo-static SP = max SP of shaly formation The formula for the theoretical calculated value of SSP is given by:

11. SPP The theoretical SSP is dependent on the chemical activity of NaCL solutions. For pure NaCl sloutions that are not too concentrated resistivities are inversely proportional to activities.

12. Influences Bed thickness: in a thin formation SP is less than SSP. Curve is pointed and narrow. High resistivity: reduce the deflection. Shale content: shale reduces SP deflection. Hydrocarbon presence suppresses SP deflection. Qualitative phenomena, cannot be used to determine hydrocarbon saturation. Ratio of R mf to R w

13. Bed thickness and true resistivity

14. Bed-thickness correction to determine SSP from SP Fig 2.3. SP bed-thickness correction to determine SSP from SP Procedure: Calculate R i / R m Locate a bed thickness on the vertical scale Follow the bed-thickness value horizontally across until it intersects the R i / R m curve Drop vertically from this intersection and read the SP correction factor on the scale across the bottom Multiply SP by the SP correction factor to find SSP.

15. Example: Determine SPP from SP Given: R m =0.94 W m at reservoir temperature Bed thickness from log: 8ft R i =34 W m; R i / R m =36  SSP=1.3SP

16. Example: Shale content Calculate shale content for zone D: Givens: Zone A represents a thick clean sand, i.e. SP = SSP

17. Reading the SP Log SP Scale: -90 to 10 mV Solid Line Example: Determine the thickness, shale content and R w of interested zone using SP Log. Determine gross lithology (reservoir vs. non-reservoir); determine permeable bed boundaries Estimate shale content Estimate formation water resistivity, R w Identify depositional environments Correlation of formations from well to well Interpretation Goals

18. Qualitative Indication of Permeable Formation The presence of an SP (+ or -) opposite a bed indicates permeability Only minimal amount of permeability is required to develop an SP and therefore ,there is no technique to determine the magnitude of the permeability from the SPE Determine permeable bed boundaries: Determine the thickness of the zones A, B, C, and D A B C D 1. Gross lithology

19. Determine the shale content of the zones A, B, C, and D 1. Draw shale baseline. 2. Determine SSP (clean formation) 3. Calculate V sh for zone of interest 2. Shale content B D A C

20. Identify shale baseline Determine SP for zone of interest Calculate formation temperature Determine R mf at formation temperature If necessary correct SP for bed thickness influence; obtain SSP Determine R we (equivalent water resistivity) using charts Convert R we to R w using chart B D 3. Estimate R w A C

21. Correction charts for R w from SP

22. Identify shale baseline Determine SP for zone of interest If necessary correct SP for bed thickness influence; obtain SSP Calculate formation temperature Determine R mf at formation temperature Determine R we (equivalent water resistivity) using charts Convert R we to R w using chart B D 3. Estimate R w A C

23. B D 3. Estimate R w A C Given: Total depth: 11,192 ft Bottom hole temperature (BHT)=175 F Annual mean surface temperature (AMST)=80 F R mf =0.58 ohm-m@70 F Step 1: Identify a zone (A, B, C or D?) on the logs which is clean, wet and permeable The identified zone is ________. Step 2: Determine SP value at the depth of maximum deflection So SP = _____ mV at ________ feet

24. B D 3. Estimate R w A C Given: Total depth: 11,192 ft Bottom hole temperature (BHT)=175 F Annual mean surface temperature (AMST)=80 F R mf =0.58 ohm-m@70 F Step 3: Calculate T f at the depth of Sp value. Step 4: Convert R mf from surface temperature to R mf at formation temperature.

25. 3. Estimate R w Step 5: Determine R mf / R we using chart Step 6: Determine R w using chart

26. 4. Identification of Depositional Environment s SP shape is one of the best facies indicators since it is sensitive to grain size, however this is complicated because it is sensitivity to many borehole and fluid effects.

27. Curves are scanned for similarity in shape and magnitude. 5. Correlations of Formations When comparing the SP curves in a variety of wells, remember that The location of the shale baseline on the log grid is set by the logging engineer, and has no interpretive meaning. Differences in SP magnitude between wells could be due to A change of the shaliness of the formation A change in mud filtrate resistivity, Rmf , in different wells The presence of hydrocarbons in one of the wells A change in the formation water resistivity, Rw