Abstract

Published in Petroleum Transactions, AIME, Volume 210, 1957, pages 58–72. Abstract In the quantitative interpretation of the S.P. logs, the electrochemical component is generally taken equal to -K log Rmf/Rw, where K has the theoretical value corresponding to solutions of pure sodium chloride. This method may be misleading when relatively large quantities of salts other than NaCl are present in formation waters, - as is generally the case for low salinities, - or in gyp-base muds. In such cases recourse is made in the field to changing the K value or to adding a correction term to the equation on the basis of local experience. An investigation has been made in the laboratory of the influence of HCO3, SO4, Ca++ and Mg++ on the amplitude of the S.P. deflection, which included in particular determinations of the activity coefficients of Ca++ and Mg++. The theory, the experimental techniques and a tentative method for applying the results are described. So far, applications to actual field cases where chemical analyses of waters were available have provided an excellent confirmation of the proposed method. The case of the S.P. in very salty brines is also considered and it is shown that the use of activity data for sodium gives satisfactory results. Historical In early times of electrical logging, the SP curve was used exclusively as a tool for the location of permeable beds and the definition of their boundaries. Later, with the introduction of methods of quantitative analysis, attention was called to the possibility of deriving from the SP log some information on the formation water resistivity, which constitutes one essential element for the computation of water saturation from log data. The first laboratory experiments in 1932 had shown that, as a result of electrochemical phenomena arising at the contact of the mud and the formation, the amplitude of the deflection of the SP curve was, at least partly, a function of the salt concentration of interstitial water and accordingly of its resistivity. It was indicated that the amplitude of the electrochemical component of the SP should be equal to -K log R1/R2, R1 and R2 being the resistivities of the mud and of the formation water respectively.

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