Abstract
Formation evaluation in unknown water salinity can be done by solving simultaneously for Sw and Rw from the logs of resistivity and sigma. This has not been used extensively in the past because the logs are often acquired on wireline at various times, leading to unknown invasion effects that complicate the integration of the deep-reading resistivity and the shallow-reading sigma logs. We will show that when the two measurements are free from invasion effect - and other interpretation parameters, such as porosity, Archie m and n, etc., are reasonably known - the technique works well and gives reliable Sw and Rw answers.
Both numerical and graphical solutions are presented. The numerical technique uses a minimization routine to solve for a salinity value that is consistent with Sw calculated from resistivity and from sigma. The salient point of the paper is the graphical technique that provides better insights into petrophysics than a set of equations. It uses an overlay of equi-saturation and equi-salinity lines superimposed on the crossplot of sigma versus resistivity data. The overlay dynamically changes as porosity, lithology, hydrocarbon type, temperature, and other parameters vary with depth. The graphical technique has several applications. For the interpretation, it is used to estimate Sw and salinity, and to identify zones of changing properties. For quality control, it is used to validate the input data. For processing, it is used to select parameters and analyze their sensitivities to the results. For job planning, it is used to validate (or invalidate) the application from input values of resistivity and sigma. The graphical technique provides a valuable aid to the petrophysicist and gives an independent verification of the numerical solution.
The proposed technique is illustrated with wireline induction and sigma logs acquired in several wells in a mature Middle East carbonate field. The wells under study are completed in open hole and are flowed during logging to ensure that both resistivity and sigma logs are free from invasion effect. The results of the resistivity-sigma technique compare very favorably with those of carbon-oxygen (C/O) logging. Moreover, production logging data confirmed the results and showed that the resistivity-sigma technique provides more robust answers than C/O in low-porosity formations (below 15 p.u.).
