Combined simulation and inversion of SP and resistivity logs for the estimation of connate-water resistivity and Archie’s cementation exponent

Knowledge of initial water saturation is necessary to estimate original hydrocarbon in place. A reliable assessment of this petrophysical property is possiblewhen rock-coremeasurements ofArchie’s parameters, such as saturation exponent n and cementation exponent m, are available. In addition, chemical analysis of formation water is necessary to measure connate-water resistivityRw. Suchmeasurements are seldomavailable inmost applications; if they are available, their reliability may be questionable.We describe a new inversion method to estimate Rw andArchie’s cementation exponent from the combined use of borehole spontaneous-potential SP and raw array-induction resistivity measurements acquired in water-bearing depth intervals. Combined inversion of resistivity and SPmeasurements is performed assuming a piston-like invasion profile. In so doing, the reservoir is divided into petrophysical layers to account for vertical heterogeneities. Inversion products are values of invaded and virgin formation resistivity, radius of invasion, and static spontaneous potential SSP . Connate-water resistivity is calculated by assuming membrane and diffusion potentials as the main contributors to the SSP.Archie’s or dual-water equations enable the estimation ofm. The newcombined estimationmethod has been successfully applied to a data set acquired in a clastic formation. Data were acquired in a high permeability and moderately highsalt-concentration reservoir. Values of Rw and m yielded by the inversion are consistent with those obtained with a traditional interpretationmethod, thereby confirming the reliability of the estimation. The method is an efficient, rigorous alternative to conventional interpretation techniques for performing petrophysical analysis of exploratory and appraisal wells wherein rock-core measurementsmay not be available.