As-Received Core Electrical Properties Tests for Determining Formation Factor FF and Resistivity Index RI

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Abstract We introduce a new core analysis workflow for determining Resistivity Index (RI), Formation Factor (FF), and other petrophysical properties, directly from an "as-received" (AR) set of core samples. Unlike common practices that require lengthy core cleaning and wettability restoration, the new workflow does not introduce external liquids or alter the wettability of the matrix. The new workflow starts with AR cores in the laboratory. Initial porosity and water saturation (Sw) are measured using nuclear magnetic resonance (NMR). We measure formation water resistivity (Rw) from a patented method based on resistivity dispersion. A two-electrode system provides the bulk resistivity (Rt) measurement. Sample de-saturation is by centrifuge at two drainage pressures. Sw and Rt measurement occurs at each de-saturation state. Sw measurement at each subsequent de-saturation step is by the gravimetric method. Regression analysis of the Rt and Sw data series yields Ro, the resistivity that would be expected for a 100% water-saturated sample, the Formation Factor (FF), the Resistivity Index (RI) and the Archie parameters of ‘m’ and ‘n’. With the new workflow, cleaning, drying, and re-saturating the sample are no longer required. The risk of altering the initial wetting state of the sample is minimal. Another advantage is the reduction of lab analysis time from weeks to days. This is particularly valuable since FF and RI play an essential role in estimating hydrocarbon in place using Archie’s equation, and in providing insight into partial oil-wetting conditions. Samples used in the method are from the Avalon and Wolfcamp formations. The method is also applicable to permeable, conventional rocks. The best estimation of Ro is from the paired Sw and Rt results measured at the AR state and after the first de-saturation step. As in the conventional method of measuring electric properties, a series of cross-plots provide a graphic solution for ‘m’ and ‘n’. A cross-plot of FF and porosity provides a graphical solution of the porosity exponent, ‘m’. A cross-plot of Sw and RI is the source for the solution of the saturation exponent, ‘n’. These graphic results compare well to results using a numerical solution. Finally, using the electric properties within the Archie equation, log based Sw values were found to be consistent with the measured Sw from core extraction.