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Methane Re-Saturation in Barnett Shale Core Plugs and Determination of Post-Coring Gas Loss


Well cores and core plugs are considered to be the best approximation of in-situ reservoir conditions as they can retain trapped fluids such as gas and oil. However, these samples suffer from some loss of gas and light oils during coring, sampling, and core storage. The quantification and prediction of the amount of gas lost, and the characterization of the physiochemical mechanisms that control the loss of gas, is thus critical to accurately determine gas-in-place (GIP) and resource assessment of unconventional shale plays. The objectives of this study were to: (1) develop an experimental method for the re-saturation of core plugs with CH4 gas at different pressures and constant temperature condition, (2) experimentally define the key factors controlling gas loss including core size, core exposure time, and initial CH4-resaturation pressure to gas loss in post coring, (3) build a model based on gas degassing physics and mass balance calculations to predict gas loss in post coring. Core plug samples used in this study were collected from a single high maturity stratigraphic interval within the Barnet Shale Fm. of the Ft. Worth Basin. Texas. Four different gas saturation, exposure and degassing experiments were performed on 3/8” diameter plugs using high pressure gas adsorption equipment and a three phase experimental set up at a constant temperature. Gas adsorption experiments were performed at identical temperature conditions and Langmuir parameters were obtained through least squares fitting of experimental CH4 experimental isotherms. Experimental pressure data was interpreted using a mass balance method where amounts of gas in the free and sorbed phases were calculated at the saturation, initial degassing and peak degassing conditions. Initial saturation pressures of 177, 517, 1073 and 1699 psia result in lost gas amounts of 0.60, 1.33, 1.96, and 2.48 mmol respectively at 35.4°C. Linear extrapolation of pressure vs. time and pressure vs. sqrt (time) data measured during degassing was hypothesized as a viable method for determining pressure conditions at initial exposure. These extrapolated pressures can be used to calculate lost gas amounts via the previously mentioned mass balance method. Lost gas amounts calculated from extrapolated pressure vs. time data match experimentally derived amounts to 82-90% range, and lost gas amounts determined from extrapolated pressure vs. sqrt (time) data agree with experimental amounts to within 81-89%.