Presentation on theme: "Basic well Logging Analysis – The Spontaneous Potential (SP) Log"— Presentation transcript:
1Basic well Logging Analysis – The Spontaneous Potential (SP) Log Hsieh, Bieng-ZihFall 2009
2The 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.
3The 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.
4The 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.
5The 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.
6The 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.
8Functions of SP logThe 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.
10Static 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.
11Shale Base LineThe 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.
12Resistivity of Formation Water (Rw) Calculates from the SP Curve
13Resistivity of Formation Water (Rw) Calculates from the SP Curve Step 0:Setup Shale-base-lineread SP log recording and 16” Normal Resistivity log reading every 2 ft from 7430 to 7460
14Resistivity of Formation Water (Rw) Calculates from the SP Curve (Cont Step 1:determine the formation temperature (Tf)
15Resistivity 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)
16Resistivity of Formation Water (Rw) Calculates from the SP Curve (Cont Step 3:Determine the SP reading (in water-bearing zone or wet-formation)
17Resistivity of Formation Water (Rw) Calculates from the SP Curve (Cont Step 4:Correct SP to SSP
18Resistivity of Formation Water (Rw) Calculates from the SP Curve (Cont Step 5:Determine Rmf/Rwe ratioStep 6:Determine Rwe
19Resistivity of Formation Water (Rw) Calculates from the SP Curve (Cont Step 7:Correct Rwe to Rw
20Volume 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.
21Calculate Vsh from the SP Curve Use SP log recording (PSP) every 2 ft from 7430 to 7460SSP is from step 4
22Review – SP LogThe 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 .
23Review – 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.