Abstract
Free gas saturation is a key parameter for calculating shale gas reserves. The complex conductivity mechanism of shale reservoirs restricts the application of Archie's formula and its extended form for the evaluation of free gas saturation. Instead, a number of non-resistivity-based saturation evaluation methods suitable for shale gas reservoirs have been established, including core calibration (TOC method, clay content method), gas porosity cutoff, excavation effect and four-pore modeling. These methods, together with adsorbed phase porosity correction, are used to calculate the free gas saturation. These methods are applied to shale reservoirs of the Upper Ordovician Wufeng Formation and the Lower Silurian Longmaxi Formation in the Sichuan Basin, southwestern China to test their applicability and accuracy. The results, when compared with measured data from core samples, show that the TOC-based core calibration is more accurate in evaluating free gas saturation in the entire shale gas interval, which is of great significance to the calculation of shale gas reserves.
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