Streaming Potential and the SP Log

Published in Petroleum Transactions, AIME, Volume 213, 1958, pages 170–179. Abstract Published laboratory data have established that very significant streaming potentials can exist across mud cakes subjected to pressure differentials such as exist between a mud column and formation fluids. Since shale is similar in kind and in properties to a well-formed mud cake, it appeared possible that streaming potentials might also arise across shales. A practical consequence would be that the streaming potential contribution to the SP would be smaller than might otherwise be assumed since the SP is measured with respect to the "shale base line". The resultant contribution would be the difference between the streaming potential across the shale and that across the mud cake. In order to investigate this possibility, a laboratory apparatus was devised for measuring streaming potentials under pressure condition normally encountered in wells. Reported data on streaming potentials across mud cakes were verified. More important, appreciable streaming potentials were found across shale samples. As in the case of mud cakes, the shale potentials varied with the concentration and nature of the impregnating electrolyte. Dependence on pressure was in a different relation than that for mud cakes, possibly because of the lower compressibility of shales. The differences in streaming potentials between the shales and mud cakes were generally small, although they might vary considerably in practice depending on the relative electrokinetic properties of the particular shale-mud cake combination. A "poorly effective" shale with a "very effective mud cake" would produce the largest negative addition to the electrochemical component of the SP.Results of field experiments with pressurized wells were considered in the light of the laboratory results. Introduction Field experiments wherein the pressure of the mud column in a borehole was varied have often indicated the presence of a streaming potential, but its magnitude was not easily predictable and its sign sometimes changed from one formation to another in the same well. In the interpretation of SP logs from standard surveys, nevertheless, the correct value of Rw is very frequently obtained, within the limits of field accuracy, using only the simple electrochemical expression of the SP provided that proper corrections are made for bed thickness, ion activity, etc.