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Modeling of the SP 
Log
…Resurrecting and Quantifying a Critical Measurement for
Predicting Permeability and Formation Water Resistivity*
By
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2Goolsby Brothers and Associates, Inc, 8174 South Holly Street, #507, Centennial, CO 80122
Abstract
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.
Log, National Petroleum Reserve, Elk Hills, California
