Time-Varying Characteristics of CH 4 Displacement–Replacement Effect in Coal Seams During CO 2 -Enhanced Coalbed Methane Recovery

Jianchi Hao (), Shuangming Wang, Hu Wen, Zegong Liu and Xuezhao Zheng
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Jianchi Hao: College of Geology and Environment, Xi’an University of Science and Technology, Xi’an 710054, China
Shuangming Wang: College of Geology and Environment, Xi’an University of Science and Technology, Xi’an 710054, China
Hu Wen: College of Safety Science and Engineering, Xi’an University of Science and Technology, Xi’an 710054, China
Zegong Liu: College of Safety Science and Engineering, Anhui University of Science and Technology, Huainan 232002, China
Xuezhao Zheng: College of Safety Science and Engineering, Xi’an University of Science and Technology, Xi’an 710054, China

Energies, 2025, vol. 18, issue 20, 1-15

Abstract: Carbon dioxide (CO 2 )-enhanced coalbed methane recovery involves a complex process of mixed-gas adsorption, desorption, and diffusion–transport. The literature suggests that an appropriate range of CO 2 injection pressure and an optimal injection time window are critical for coal seams with varying reservoir conditions. That is, higher pressure and longer injection periods do not necessarily lead to better displacement performance. Therefore, in this study, experimental research was conducted on the time-varying characteristics of the displacement–replacement effect of CO 2 -enhanced methane (CH 4 ) extraction from coal seams, and the following results were obtained. (1) The process of displacement–replacement of CH 4 by CO 2 in coal seams can be divided into five stages: a stage of spontaneous CH 4 desorption caused by partial-pressure effects, a replacement-dominated stage, a stage where replacement and displacement act jointly, a displacement-dominated stage, and a stabilization stage. (2) For all three coal samples (anthracite, coking coal, and long-flame coal), cumulative CH 4 desorption increases with increasing CO 2 injection pressure below 5 MPa and finally stabilizes. However, when CO 2 injection pressure exceeds 5 MPa, the effect weakens, possibly due to the dynamic changes in CO 2 partial pressure. (3) The displacement–replacement ratio decreases with increasing CH 4 equilibrium pressure. Additionally, the larger the difference between the CO 2 injection pressure and the CH 4 equilibrium pressure, the better the displacement–replacement effect.

Keywords: gas management; gas extraction; CO 2 adsorption by coal; methane desorption (search for similar items in EconPapers)
JEL-codes: Q Q0 Q4 Q40 Q41 Q42 Q43 Q47 Q48 Q49 (search for similar items in EconPapers)
Date: 2025
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