Basic well Logging Analysis – The Spontaneous Potential (SP) Log Hsieh, Bieng-Zih Fall 2009

The 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.

The spontaneous potential (SP) log (Cont.) 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.

The spontaneous potential (SP) log (Cont.) 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.

The spontaneous potential (SP) log (Cont.) 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 (Rmf) and formation water resistivity (Rw) within permeable beds.

The spontaneous potential (SP) log (Cont.) 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.

SP deflection

Functions 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 (Rw), (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.

Functions of SP log

Static 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 Rmf /Rw . SSP is determined by formula or chart and is a necessary element for determining accurate values of Rw and volume of shale.

Shale Base Line 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.

Resistivity of Formation Water (Rw) Calculates from the SP Curve

Resistivity of Formation Water (Rw) Calculates from the SP Curve Step 0: Setup Shale-base-line read SP log recording and 16” Normal Resistivity log reading every 2 ft from 7430 to 7460

Resistivity of Formation Water (Rw) Calculates from the SP Curve (Cont Step 1: determine the formation temperature (Tf)

Resistivity of Formation Water (Rw) Calculates from the SP Curve (Cont Step 2: correct the resistivities of the mud filtrate (Rmf) and drilling mud (Rm) to formation temperature (Tf)

Resistivity of Formation Water (Rw) Calculates from the SP Curve (Cont Step 3: Determine the SP reading (in water-bearing zone or wet-formation)

Resistivity of Formation Water (Rw) Calculates from the SP Curve (Cont Step 4: Correct SP to SSP

Resistivity of Formation Water (Rw) Calculates from the SP Curve (Cont Step 5: Determine Rmf/Rwe ratio Step 6: Determine Rwe

Resistivity of Formation Water (Rw) Calculates from the SP Curve (Cont Step 7: Correct Rwe to Rw

Volume of Shale Calculation 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.

Calculate Vsh from the SP Curve Use SP log recording (PSP) every 2 ft from 7430 to 7460 SSP is from step 4

Review – SP Log The spontaneous potential log (SP) can be used to: (1) detect permeable beds; (2) detect boundaries of permeable beds; (3) determine formation water resistivity (Rw); and (4) determine volume of shale (Vsh) 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 Rmf and Rw .

Review – SP Log (Cont.) 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 Rmf ＞ Rw ; (2) positive deflection to the right of the shale baseline where Rmf ＜ Rw ; (3) no deflection where Rmf ＝ Rw . The SP curve can be suppressed by thin beds, shaliness, and the presence of gas.