An Investigation of the Electrokinetic Component of the Self Potential Curve

Eight laboratory-prepared aqueous base drilling muds representing common mudtypes, and 15 aqueous base drilling muds sampled in the field, have been usedin an experimental investigation of the relationship between the streamingpotential developed across a mud filter cake and the differential pressurecausing filtrate flow. It has been established that the relationship is of the form, Es = kPy, where Es is the streaming potential, P the differential pressure and k and yare constants for any particular mud system at constant temperature. Beyond acertain minimum thickness this relationship is independent of filter cakethickness. Experiment has shown that both k and y are temperature dependentparameters and that in certain mud systems there is a critical value of P abovewhich the simple relationship Es = kPy ceases to hold. The constant k tends tobe a function of the mud resistivity at 77'F, but the exponent y has not beenfound to be related to any commonly measured mud property. Certain field experiments are described which serve to demonstrate that thetotal SP (self potential) developed opposite a formation may be quantitativelyaccounted for as the algebraic sum of an electrochemical potential and anelectrokinetic potential. A chart to permit the simple use of mean solutionactivities in the interpretation of electrochemical SP data is given and itsuse described. Various field methods for computing the electrokinetic SP component of thetotal SP are considered and rejected as being unduly complicated andinaccurate. It is shown that the employment of low resistivity drilling muds isessential if optimum accuracy is desired in the use of the S.P. curve for thepurpose of computing connate water salinities by the electrochemical SP method.The use of such low resistivity drilling muds is accordingly recommended. T.P. 2940