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

摘要

Knowledge of initial water saturation is necessary to estimate original hydrocarbon in place. A reliable assessment of this petrophysical property is possible when rock-core measurements of Archie'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 resistivity R-w. Such measurements are seldom available in most applications; if they are available, their reliability may be questionable. We describe a new inversion method to estimate R,, and Archie'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 SP measurements 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 of m. The new combined estimation method 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 R. and m yielded by the inversion are consistent with those obtained with a traditional interpretation method, 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 measurements may not be available.