Inversion Modeling of the SP Log…Resurrecting and Quantifying a Critical Measurement for Predicting Permeability and Formation Water Resistivity
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