Geological controls on the methane storage capacity in organic-rich shales

Abstract High-pressure methane sorption isotherms were measured on selected Paleozoic and Mesozoic organic-rich shales, considered as shale gas targets in Europe. The samples include the Upper Cambrian–Lower Ordovician Alum Shale, Carboniferous (Mississippian–Pennsylvanian) shales and Lower Toarcian Posidonia Shale. In addition, samples from producing shale gas formations in the USA (Barnett, Haynesville and Eagle Ford) were studied for comparison. Excess sorption measurements were performed over an extended range of pressures (up to 25 MPa) and temperatures (up to 150 °C) on dry samples and at 38 °C on moisture-equilibrated samples to study the effect of organic matter content (TOC), maturity, mineralogy and moisture content on the methane sorption capacity. Additionally, water isotherms were measured at 24 °C and at relative humidities (RH) from 8 to 97%. A 3-parameter ( n L , p L , ρ a ) excess sorption function based on the Langmuir equation for absolute sorption was used to fit the measured methane sorption isotherms. The water sorption isotherms were parameterized by the Guggenheim–Anderson–de Boer (GAB) function. In both cases, excellent fits to the measured data were achieved. The methane sorption capacities of the dry shales show a positive correlation with TOC but significant deviations from this trend exist for individual samples. The TOC-normalized sorption capacities correlate positively with maturity in terms of Vitrinite Reflectance (VR r ) up to a certain value of VR r (~ 2.5%) above which an opposite trend is observed. No correlation was observed between the clay content and the TOC-normalized sorption capacity to methane, indicating that clay minerals do not significantly contribute to methane sorption in these organic-rich shales. The shape of the excess isotherms changes systematically with temperature and maturity. The Langmuir pressure ( p L ) increases exponentially with temperature and follows a negative power-law trend with maturity. Compared to dry samples, the sorption capacity in moisture-equilibrated samples (at 97% RH) is reduced by 40 to 60%. No difference is observed between 97% and 75% RH, indicating that the critical moisture content is at or below 75% RH. The monolayer sorption capacities for water obtained from the GAB fit are 0.5 to 3 times those for methane, derived from the Langmuir fit. There is a weak positive correlation between the methane and the water sorption capacity, suggesting that methane and water molecules share some of the sorption sites and these reside partly within the organic matter.

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