A Study of Electric Log Interpretation Methods in Shaly Formations

Abstract

Published in Petroleum Transactions, AIME, Volume 204, 1955, pages 103–110. Abstract A review is given of the principles on which recently proposed methods of electric log interpretation in shaly sands are based and of the evidence brought up in support of the theoretical derivations. It is pointed out that the qualitative techniques suggested to date have a tendency to be too pessimistic in the prediction of the presence of commercial hydrocarbon accumulation. In the quantitative treatment, a new concept is introduced, namely that of strongly reduced activity of the double layer counter ions which are present near the negatively charged rock surfaces. This lemma when applied to the calculation of the equilibrium concentrations of interstitial waters yields a set of very simple relations. The resulting expressions for electrochemical potentials across rock samples appear in satisfactory agreement with laboratory experiments. The formulae obtained for the electrical conductivity of shaly formations are of the same form as those arrived at empirically by previous workers. Combination of the expressions for potentials and conductivities gives a direct proof of the Tixier relation which states that for shaly water sands the product of apparent formation water resistivity and apparent formation factor equals the resistivity of the sand 100 per cent saturated with formation water and which was verified by the work of Wyllie and Perkins and their co-workers. The relations for the resistivities and spontaneous potentials have been extended to the case of hydrocarbon bearing shaly formations, thus laying a formal basis forthe quantitative interpretation of electric logs in shaly oil sands. Introduction The presence of disseminated clays in porous rocks saturated with electrolytic solutions has a strong influence on the transference of ions taking place under electrical or chemical potential gradients. Both the electrochemical emf's in evidence on the SP curves of electric logs and the electrical conductivity measured on resistivity logs are directly dependent on the ionic transference in the interstitial waters of the formations traversed by boreholes.