Well Log InterpretationSP LogEarth & Environmental Science University of Texas at Arlington

Spontaneous Potential LogThe SP log is the oldest type of log and is still common. It measures the D.C. voltage difference between surface and borehole electrodes

Spontaneous Potential LogThe millivoltages are generated by differences in salinity between the formation waters and the mud filtrate i.e. differences between Rw and Rmf

Cant be used with oil based mud.

Spontaneous Potential LogSP Log Used to:Detect porous & permeable bedsCalculate Rw (used in Archie Eq.)Estimate shale %

Spontaneous Potential LogFactors that affect SP response:Thicker beds: increasesHigher Rt: reducesMore shale: reducesHigher porosity/permeability: increases if Rw Rmf

Spontaneous Potential LogSome Definitions:SSP Static SP: Maximum SP response under ideal conditions (thick, permeable, porous layer) with assumed Rmf/RwShale Base Line: Minimum SP response produced by thick impermeable, nonporous shaleHydrocarbon Suppression reduced SP response because petroleum has no salinity.

Spontaneous Potential Log:Calculation of RwWARNING:Calculation of important parameters from well log data is messy because none of the equations are derived from basic physical/chemical equations. All the equations are determined experimentally, are approximations and involve many steps.

Spontaneous Potential Log:Calculation of RwSteps to calculate Rw are shown in Table 2.1 of text. Resistivities are temperature dependent.The first step is to find the formation temperature at the depth Rw is required. This is just a linear interpolation between surface and bottom hole temperature. The petroleum industry usually uses oF, not Celsius.Surface Temp (Ts), Formation Depth (FD), Bottom Hole Temp (BHT), and Total Depth (TD) are usually given on well header.

Spontaneous Potential Log:Calculation of RwThe 2nd step is to convert Rm & Rmf measured at the surface to Rmf at Tf, the formation temp. This is NOT a linear interpolation (see Fig 1.11 in text) The equation below is an approximation to the graphical solution in Fig 1.11. The constant 6.77 is 0F. For Celsius it is 21.5

Spontaneous Potential Log:Calculation of RwThe 3nd step is to convert find the ideal potential (SSP), correcting for bed thickness, using the Fig. 2.3Bed thickness is determined from the SP log by measuring the distance between inflection points of the SP curve (8 ft)To use Fig. 2.3, Must use Ri, from the SN (short reading resistivity) log next to the SP log in Fig 2.2 (33 Ohm-M).Calculate Ri/Rm (make sure you use the temperature corrected Rm), use Fig 2.3 to get the SP correction factor, then read off SSP from nomogram (71)

Spontaneous Potential Log:Calculation of Rw

Spontaneous Potential Log:Calculation of RwThe 4th step is to find the ideal formation water resistivity, Rwe, using the following equation (remember Rmf is at formation temperature):

Spontaneous Potential Log:Calculation of RwThe final step is to find the actual water resistivity, Rw, using the following equation:

Spontaneous Potential Log:Calculation of SwOnce Rw is calculated for the reservoir, use the Archie equation to calculate Sw.

Spontaneous Potential Log:AssignmentCreate a spreadsheet using the data in Fig. 2.2 and the formulas in Table 2.1 to calculate Rw, formation water resistivity, in the stratigraphic unit centered at 7446 in Fig 2.2. Email the results by next class period.You will need to use the graph in Fig 2.3 to estimate SSP.Some corrections to formulas in Table 2.1:SP in third equation should be SSPBHT in 3rd & 4th equations should be Tf, formation temperature.