Combining Resistivity and Capture Sigma Logs for Formation Evaluation in Unknown Water Salinity--A Case Study in a Mature Carbonate Field

Abstract

Formation evaluation in unknown water salinity can be performed by solving simultaneously for S w and R w 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 and Archie m and n) are reasonably known, the technique works well and gives reliable S w and R w 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 S w 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 equisaturation and equisalinity lines superimposed on the crossplot of sigma vs. 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 S w 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 studied wells 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.). IntroductionTo manage existing fields effectively and efficiently, it is important to monitor the hydrocarbon saturations, fluid contacts, and formation pressures at all times. The accuracy of the computed water saturation at different times is critical to track reservoir depletion, detect injection-water breakthroughs, support workover programs, and design enhanced-oil-recovery projects.The examples discussed in this paper apply to a mature carbonate field discovered in the 1950s. The waterflooding project was