Selective SP Logging

Abstract

An earlier paper on the general subject of the SP log has analyzed thelimitations of that log, in particular when dealing with thin permeable bedsenclosed in thick highly resistive ones. Under such conditions, the SP log doesnot give a good definition of the permeable beds.

Selective SP logging is a new method which gives an accurate determinationof the permeable beds, even in highly resistive formations, except when thesalinity of the mud is extremely high. It also permits a close approximation ofthe static SP to be obtained.

The method consists generally in controlling the potential of auxiliaryelectrodes located in the drill holes in the vicinity of the measuringelectrode. In this manner, the flow of SP current along the drill hole ismodified to counterbalance the effect of highly resistive strata.

Several procedures and arrangements used to record selective SP curves andstatic SP curves are explained.
Field examples illustrating these methods are given and discussed.

Introduction

In an earlier paper, the shape of the SP curves and the amplitude of the SPpeaks were analyzed, considering how the circulation of the SP current isinfluenced by the subsurface media as a function of their geometricalconfiguration and their resistivities. It was shown, in particular, that thepermeable beds situated between highly resistive formations are sometimesindistinctly indicated by the deflections of the SP log.

It is the purpose of this paper to discuss a new method now being developed,namely selective SP logging, whose principal feature is to enable a moreaccurate determination of the depths of the permeable beds present in highlyresistive formations, such as occur in limestone fields.

It will be shown also how an additional curve called the static SP log can hederived from the selective SP log, or recorded directly, to facilitate theinterpretation.

T.P. 2850