Inversion Modeling of the SP Log…Resurrecting and Quantifying a Critical Measurement for Predicting Permeability and Formation Water Resistivity*
2Goolsby Brothers and Associates, Inc, 8174 South Holly Street, #507, Centennial, CO 80122
The Spontaneous Potential (SP) curve is an important geophysical log measurement; however, it is difficult to use quantitatively due to poor vertical resolution and the effects of borehole and formation fluid resistivity. Tedious and cumbersome chart book corrections are only valid for “ideal” beds (permeable beds encased in conductive shale) that are greater than 4 or 5 ft thick. Service companies do not charge for the SP curve and, as a result, have been less concerned with quality control on the SP measurement in recent years. As a result, the SP log is often ignored quantitatively in favor of the Gamma Ray measurement (which may be unrelated to reservoir quality).
Inversion modeling can make the SP measurement quantitatively useful from zone to zone and from well to well. Inversion modeling yields a pseudo-static SP (PSP) curve with a vertical resolution of 1-2 ft and removes the effects of thin beds, formation resistivity, and borehole size. The PSP can be normalized to accommodate changes in formation water resistivity (Rw) and mud filtrate resistivity (Rmf). The normalized PSP is a clay volume (Vshale) curve which can be used quantitatively in multiple well projects to accurately determine connate water resistivity (Rw) and rock quality (permeability).
Because the SP curve has been acquired universally for more than 70 years, the modeled results provide a standard measurement for comparing rock properties in areas with mixed vintage log suites. Examples include data from the Pinedale area of Wyoming and the National Petroleum Reserve located in California.