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B ASIC WELL L OGGING A NALYSIS – T HE S PONTANEOUS P OTENTIAL (SP) L OG Hsieh, Bieng-Zih Fall 2009 1.

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Presentation on theme: "B ASIC WELL L OGGING A NALYSIS – T HE S PONTANEOUS P OTENTIAL (SP) L OG Hsieh, Bieng-Zih Fall 2009 1."— Presentation transcript:

1 B ASIC WELL L OGGING A NALYSIS – T HE S PONTANEOUS P OTENTIAL (SP) L OG Hsieh, Bieng-Zih Fall

2 T HE SPONTANEOUS POTENTIAL (SP) LOG The spontaneous potential (SP) log was one of the earliest electric logs used in the petroleum industry, and has continued to play a significant role in well log interpretation. By far the largest number of wells today have this type of log included in their log suites. 2

3 T HE SPONTANEOUS POTENTIAL (SP) LOG (C ONT.) Primarily the spontaneous potential log is used to identify impermeable zones such as shale, and permeable zones such as sand. However, the SP log has several other uses perhaps equally important. 3

4 T HE SPONTANEOUS POTENTIAL (SP) LOG (C ONT.) The spontaneous potential log is a record of direct current (DC) voltage differences between the naturally occurring potential of a moveable electrode in the well bore, and the potential of a fixed electrode located at the surface (Doll, 1948). It is measured in millivolts. 4

5 T HE SPONTANEOUS POTENTIAL (SP) LOG (C ONT.) Electric currents arising primarily from electrochemical factors within the borehole create the SP log response. These electrochemical factors are brought about by differences in salinities between mud filtrate (R mf ) and formation water resistivity (R w ) within permeable beds. 5

6 T HE SPONTANEOUS POTENTIAL (SP) LOG (C ONT.) Because a conductive fluid is needed in the borehole for the SP log to operate, it cannot be used in non-conductive (i.e. oil- based) drilling muds. 6

7 SP DEFLECTION 7

8 F UNCTIONS OF SP LOG The SP log is recorded on the left hand track of the log in track #1 and is used to: (1) detect permeable beds, (2) detect boundaries of permeable beds, (3) determine formation water resistivity (R w ), (4) determine the volume of shale in permeable beds. An auxiliary use of the SP curve is in the detection of hydrocarbons by the suppression of the SP response. 8

9 F UNCTIONS OF SP LOG 9

10 S TATIC SPONTANEOUS POTENTIAL (SSP) The concept of static spontaneous potential (SSP) is important because SSP represents the maximum SP that a thick, shale-free, porous and permeable formation can have for a given ratio between R mf /R w. SSP is determined by formula or chart and is a necessary element for determining accurate values of R w and volume of shale. 10

11 S HALE B ASE L INE The SP response of shales is relatively constant and follows a straight line called a shale baseline. SP curve deflections are measured from this shale baseline. Permeable zones are indicated where there is SP deflection from the shale baseline. 11

12 R ESISTIVITY OF F ORMATION W ATER (R W ) C ALCULATES FROM THE SP C URVE 12

13 R ESISTIVITY OF F ORMATION W ATER (R W ) C ALCULATES FROM THE SP C URVE 13 Step 0: Setup Shale-base-line read SP log recording and 16” Normal Resistivity log reading every 2 ft from 7430 to 7460

14 R ESISTIVITY OF F ORMATION W ATER (R W ) C ALCULATES FROM THE SP C URVE (C ONT.) 14 Step 1: determine the formation temperature (T f )

15 R ESISTIVITY OF F ORMATION W ATER (R W ) C ALCULATES FROM THE SP C URVE (C ONT.) 15 Step 2: correct the resistivities of the mud filtrate (R mf ) and drilling mud (R m ) to formation temperature (T f )

16 R ESISTIVITY OF F ORMATION W ATER (R W ) C ALCULATES FROM THE SP C URVE (C ONT.) 16 Step 3: Determine the SP reading (in water-bearing zone or wet-formation)

17 R ESISTIVITY OF F ORMATION W ATER (R W ) C ALCULATES FROM THE SP C URVE (C ONT.) 17 Step 4: Correct SP to SSP

18 R ESISTIVITY OF F ORMATION W ATER (R W ) C ALCULATES FROM THE SP C URVE (C ONT.) 18 Step 5: Determine R mf /R we ratio Step 6: Determine R we

19 R ESISTIVITY OF F ORMATION W ATER (R W ) C ALCULATES FROM THE SP C URVE (C ONT.) 19 Step 7: Correct R we to R w

20 V OLUME OF S HALE C ALCULATION The SP log can be used to calculate the volume of shale in a permeable zone by the following formula: The volume of shale in a sand can be used in the evaluation of shaly sand reservoirs. 20

21 C ALCULATE V SH FROM THE SP C URVE 21 Use SP log recording (PSP) every 2 ft from 7430 to 7460 SSP is from step 4

22 R EVIEW – SP L OG The spontaneous potential log (SP) can be used to: (1) detect permeable beds; (2) detect boundaries of permeable beds; (3) determine formation water resistivity (R w ); and (4) determine volume of shale (V sh ) in a permeable bed. The variations in the SP are the result of an electric potential that is present between the well bore and the formation as a result of differences in salinities between R mf and R w. 22

23 R EVIEW – SP L OG (C ONT.) The SP response in shales is relatively constant and its continuity of amplitude is referred to as the shale baseline. In permeable beds the SP will do the following relative to the shale baseline: (1) negative deflection to the left of the shale baseline where R mf > R w ; (2) positive deflection to the right of the shale baseline where R mf < R w ; (3) no deflection where R mf = R w. The SP curve can be suppressed by thin beds, shaliness, and the presence of gas. 23


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