Effects of light hydrocarbons and extractable organic matter on the methane sorption capacity of shales

AAPG Bulletin, vol. 108, pp. 327–349

Abstract

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 contents ranging from 8.52 to 11.73 wt. % and different maturities (0.53%–1.45% vitrinite reflectance). 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, preadsorbed 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 approximately 23% to 48% as compared to the dry (unextracted) state. The effect of preadsorbed 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, whereas methane adsorption on the organic and mineral surfaces reaches saturation. The preadsorbed hexane reduces methane sorption capacity by approximately 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.

Authors 8

1. Qian Zhang Aachen Energy and Mineral Resources Group (EMR)

   RWTH Aachen University · Peking University

   Affiliation as printed

   RWTH Aachen University 1 Institute of Geology and Geochemistry of Petroleum and Coal, Energy and Mineral Resources Group (EMR), , Aachen, Germany; Institute of Energy, School of Earth and Space Sciences, Peking University, Beijing, China; amadozhang@pku.edu.cn

2. Reinhard Fink Aachen

   RWTH Aachen University

   Affiliation as printed

   EMR, RWTH Aachen University 2 Institute of Geology and Geochemistry of Petroleum and Coal, , Aachen, Germany; reinhard.fink@bge.de

3. Bernhard M. Krooß Aachen

   RWTH Aachen University

   Affiliation as printed

   EMR, RWTH Aachen University 3 Institute of Geology and Geochemistry of Petroleum and Coal, , Aachen, Germany; bmkrooss@me.com

4. Zhijun Jin

   Peking University

   Affiliation as printed

   Peking University 4 Institute of Energy, School of Earth and Space Sciences, , Beijing, China; jinzj1957@pku.edu.cn

5. Rukai Zhu

   Research Institute of Petroleum Exploration and Development

   Affiliation as printed

   PetroChina Research Institute of Petroleum Exploration and Development 5 , Beijing, China; zrk@petrochina.com.cn

6. Zhazha Hu Aachen

   RWTH Aachen University

   Affiliation as printed

   EMR, RWTH Aachen University 6 Institute of Geology and Geochemistry of Petroleum and Coal, , Aachen, Germany; zhazha.hu@hpu.edu.cn

7. Garri Gaus Aachen

   RWTH Aachen University

   Affiliation as printed

   EMR, RWTH Aachen University 7 Institute of Geology and Geochemistry of Petroleum and Coal, , Aachen, Germany; garri.gaus@emr.rwth-aachen.de

8. Ralf Littke Aachen

   RWTH Aachen University

   Affiliation as printed

   EMR, RWTH Aachen University 8 Institute of Geology and Geochemistry of Petroleum and Coal, , Aachen, Germany; ralf.littke@emr.rwth-aachen.de