Pedro Silva and Dr. Zaki Bas8iouni
Petroleum Engineering Department
Louisiana State University
ABSTRACT
Formation water resistivity (Rw) is an important interpretation parameter. When water samples are available, Rw is determined by direct measurement or calculated from the water chemical analysis. It is more practical, however, to estimate Rw from the SP log. Poor correlations between the values obtained from water samples and those estimated from the SP are common.
The basic SP equation SP = -K log (aw/amf) was examined, and two modifications deemed necessary. First, the chemical activities aw and amf were directly expressed in terms of the resistivities Rw and Rmf. The use of the artificially-defined equivalent resistivities, Rwe and Rmfe, is no longer necessary. Second, the parameter K was found to vary with the shale resistivity (Rsh). A new chart relating SP to Rsh, Rmf and Rw was obtained. The use of the new chart resulted in a tremendous improvement in the correlation between Rw values obtained from water samples and those estimated from the SP log.