Computing Realistic Effective Porosity and Fluid Saturations from Total Porosity and Fluid Saturation Calculations in the 26R Sand, Stevens Zone in the Elk Hills Field, California

Abstract A graphical method is presented for computing effective porosities from total porosities. This method references hydrocarbon pore volume. The conventional. method of defining shale properties was determined to be inappropriate and one which led to errors in computing effective porosity. The goal of the graphical method is to have a numerical representation of effective porosity that preserves the hydrocarbon pore volume component of the reservoir. The results are effective porosities adequate to contain the hydrocarbons from the total porosity, fluid saturation determination. Petrophysical evaluation of 26R Sands in the Elk Hills oil field in California to determine porosity and water saturation uses the Juhasz Normalized Waxman Smits routine. The porosity and water saturation results are expressed in terms of total porosity and total water saturation. Reservoir engineering and geostatistical analyses of voidage, recovery efficiency and reservoir distribution require effective porosity and water saturation. A conventional shale porosity determination was used in the conversion of calculated total porosity and fluid saturation to effective porosity and effective fluid saturation. The resulting effective porosity and effective water saturations did not compute the same hydrocarbon pore volume as computed from the total porosity, total water saturation computations. The equation selected for computing effective porosity () from total porosity is as follows: where = Effective porosity = Total porosity = Volume of shale = Porosity of shale Volume shale (Vsh) is computed using an average of the resistivity and SP curves. The results were consistent with the geologic model for turbidite overbank splays in a shale matrix. A conventional determination of shale porosity was used based on the mean value of a distribution of shale porosities in intervals with over 80% shale volume. This method assumes that shale properties within a sand interval are identical to shale properties in adjacent shale beds. This assumption is not always correct, as the deposition of clastic material generally is a random process. The shale porosity from this method generates too large of a reduction of total porosity to effective porosity. The results of this excess porosity reduction is negative effective water saturations, which are rounded to values of zero for water saturation.