The Presence of Conductive Solids in Reservoir Rocks as a Factor in Electric Log Interpretation

Abstract

Introduction In the quantitative interpretation of electric log data it is essential toknow the formation factor, defined as the ratio of the resistivity of theformation 100 per cent saturated with brine to the resistivity of the brine.This concept implies that the formation factor for any given rock is constantirrespective of the resistivity of the brine or solution with which it issaturated. It is apparent that this can be true only when the rock matrix iswholly non-conductive. As reservoir rocks may contain clay or shale having anappreciable conductance one may expect these constituents to influence thedetermination of true formation factor. The formation factor may be theoretically derived from log data by dividingthe resistivity of the mud filtrate invaded formation by the resistivity of themud filtrate. Such a method of calculation has implicit in it the assumptionthat the fluids in the invaded formation are wholly replaced by mud filtrate, that the solid matrix is non-conducting, and that the formation factor of theinvaded zone is identical with the formation factor of the uninvaded zone.Since the validity of the first two basic assumptions regarding the insulatingcharacter of porous matrices and the displacement of the interstitial fluidsin porous media do not appear to be fully established, the formation factors of anumber of cores and clay slurries have been measured under conditions suitablefor verifying the assumptions and the extent of any errors inherent inthem. Core Studies Apparatus and Procedure Resistivity measurements were made on short cylindrical permeable coreswhich had been previously mounted in lucite. The cores were clamped by means ofinsulated bolts between brass plates about 5 cm square and 1 cm in thickness.The plates were provided with a ring seal to prevent leakage between them andthe core mount, and had a centrally located orifice so that fluids could beflowed through the mounted core. The brass mounting plates were used aselectrodes and any current flow between them necessarily passed only throughthe constituents of the intervening core. Fluid saturation of the core, and fluid flow through the core, were effectedby a centrifugal pump connected to the outflow end of the core, thus providinga differential pressure across the core of approximately one atmosphere. T.P. 2797