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Thermal-hydraulic-mechanical coupling analysis of N2-ECBM recovery efficiency: Pressure and temperature dependence

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  • Long, Hang
  • Lin, Hai-fei
  • Ma, Dong-min
  • Bai, Yang
  • Ji, Peng-fei
  • Li, Bai

Abstract

Understanding the impact of N2 injection on CH4 displacement efficiency was critical for advancing N2-enhanced coalbed methane (N2-ECBM) recovery technology. This study developed a thermal-hydraulic-mechanical (THM) coupling model incorporating the dynamic dispersion effect between N2 and CH4, followed by validation against experimental data. Systematic analyses evaluated the effects of N2 injection pressure (2.0–4.0 MPa), initial CH4 pressure (1.0–2.0 MPa), and injection temperature (293.15–333.15 K) on CH4 concentration distribution, coal permeability evolution, and displacement efficiency. Key findings revealed that CH4 displacement dynamics were governed by the synergistic interplay of convective transport and seepage processes. Notably, significant CH4 accumulation (14–29 % above the initial concentration) was observed proximal to the injection side during the early stage of N2 injection, demonstrating pronounced pressure sensitivity. The temporal evolution of CH4 recovery efficiency consistently exhibited Langmuir-type behavior across varying conditions. Permeability enhancement correlated strongly with higher N2 injection pressure (k/k0 = 1.17 at 4.0 MPa), higher initial CH4 pressure (k/k0 = 1.23 at 2.0 MPa), and lower injection temperatures (k/k0 = 1.19 at 293.15 K). Correspondingly, CH4 recovery efficiency increased to 0.524, 0.551, and 0.526 under these respective conditions, surpassing the natural recovery efficiency of 0.426. Overall, CH4 recovery efficiency exhibited a positive correlation with N2 injection pressure and initial CH4 pressure, and a negative correlation with injection temperature. Mechanistic analysis indicated that permeability evolution during N2-ECBM primarily arisen from porosity modifications induced by the coupled hydraulic, thermal, and mechanical effects. These findings provided critical insights for optimizing ECBM operational parameters and inform strategies for engineered reservoir modification in carbon sequestration applications.

Suggested Citation

  • Long, Hang & Lin, Hai-fei & Ma, Dong-min & Bai, Yang & Ji, Peng-fei & Li, Bai, 2025. "Thermal-hydraulic-mechanical coupling analysis of N2-ECBM recovery efficiency: Pressure and temperature dependence," Energy, Elsevier, vol. 335(C).
    • Handle: RePEc:eee:energy:v:335:y:2025:i:c:s0360544225039891
    DOI: 10.1016/j.energy.2025.138347
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