Estimation of Interstitial Water From the Electric Log

Abstract

A comparison is made between the amount of interstitial water found by analysisin cores and that estimated from the resistivity curve of the electric log forthe corresponding strata of producing reservoirs. It is shown that while thedetermined and the estimated water saturations are in good agreement in somecases, this agreement may be due to a fortuitous compensation of errors.Estimates of water saturations made from the electric log by use of existingrelations should, therefore, be employed with caution. Also presented are dataon the variation of salinity of interstitial waters in reservoir sands, and onthe effect of variation of salinity and porosity on the validity of estimatesof water content made from the electric log.

Introduction

Throughout the history of electric logging there have been recurrent cyclesof optimism and pessimism as to the value of the electric log in thequantitative estimation of reservoir fluid content. The very early days ofelectric logging were an optimistic period, in which there was widespread faithin the ability of the resistivity curve to reflect fluid content, and in theability of the potential curve to indicate textural characteristics. In timethe accumulation of exceptions to this idealistic behavior brought about ageneral disillusionment, and caused the more pessimistic of those dealing withelectric logs to believe that the sole importance of the log lay instratigraphic correlation.

Within the last several years optimism as to the quantitative use of theelectric log has been revived, primarily as the result of the stimulating workof G. E. Archie on the estimation of interstitial water in sands fromresistivity data. Even though the general trend of thought on the quantitativevalue of the electric log reflects optimism, it is inevitable that there aresome widely divergent opinions of the validity of quantitative estimates basedon the electric log.

T.P. 2936