1964 Paper A

AN INVESTIGATION OF THE FACTORS AFFECTING THE SP IN SOFT FORMATIONS

By Hubert Guyod, Consultant Well Instrument Division Mandrel Industries, Inc.

ABSTRACT

A simple analysis of the working of SP in boreholes shows that the SP can be considered as an apparent resistivity obtained with a particular four-electrode arrangement. Because of this similarity, some of our basic knowledge on apparent resistivities measured with Normal and Lateral devices is directly applicable to the SP. A few applications of this analogy are outlined for the case of a single uniform reservoir bed situated in a thick shale formation. The analogy explains, in particular, why it is essential to take into account all the usual formation and borehole parameters to accurately obtain the SSP from the SP. This, in turn, calls for a large number of analysis charts.

Typical charts illustrating the influence of formation and borehole parameters on the amplitude of the SP curve and on the area under the curve are presented. It is shown that the area generally does not give accurate sand counts but that it can nevertheless be used in certain cases.

Data on the shape of the SP curve are also presented to illustrate that the point of inflection can be either on, inside or outside the boundaries, depending upon the resistivity distribution.

Most of the basic data used for the investigation were obtained with a resistor network analogue.