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DOI:10.1306/05302322009
Effects of light hydrocarbons and extractable organic matter on the methane sorption capacity of shales
Qian Zhang12 , Reinhard Fink1 , Bernhard M. Krooss1 , Zhijun Jin2 , Rukai Zhu3 , Zhazha Hu1 , Garri Gaus1 , and Ralf Littke1
1 Institute of Geology and Geochemistry of Petroleum and Coal, Energy and Mineral Resources Group (EMR), RWTH Aachen University, Aachen, Germany
2 Institute of Energy, School of Earth and Space Sciences, Peking University, Beijing, China
3 PetroChina Research Institute of Petroleum Exploration and Development, Beijing, China
Ahead of Print Abstract
High-pressure methane (CH4) sorption measurements at 30°C and up to 20 MPa have been conducted on four carbonaceous shales with Total Organic Carbon (TOC) contents ranging from 8.52 to 11.73 wt.% and different maturities (0.53 to 1.45% vitrinite reflectance (VRr)). Excess sorption isotherms were measured on all four samples in the “dry”, “solvent-extracted”, “hexane-equilibrated” and “moisture-equilibrated” states. The isotherms of all samples, irrespective of thermal maturity, showed consistent effects of extraction, pre-adsorbed hexane, and moisture on methane sorption capacity. Removal of bitumen by solvent extraction generally increases the methane sorption capacity of the shales (at 1 MPa) by up to 63% compared to the dry state, most likely due to enhancing the accessibility of sorption sites. Moisture consistently reduces methane sorption capacity by about 23 to 48% as compared to the dry (unextracted) state. The effect of pre-adsorbed hexane on methane sorption capacity is strongly pressure-dependent: At low pressures its influence is negative and at high pressures positive. The significant increase of sorption capacity at high pressures is attributed to the almost linear increase of methane solubility in hexane with pressure while methane adsorption on the organic and mineral surfaces reaches saturation. The pre-adsorbed hexane reduces methane sorption capacity by about 20 to 40% if solubility effects are excluded. In view of these findings, the methane adsorption capacity of shales at the “wet gas” maturity level should be reconsidered. Our observations contribute to a better understanding of natural gas occurrence and producibility in liquid-bearing unconventional petroleum systems and a more accurate estimation of gas-in-place of shale gas reservoirs.
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