Numerical simulation of CO 2 -ECBM for deep coal reservoir with strong stress sensitivity.

CH ₄ production rate of coalbed methane (CBM) well decreases rapidly during primary recovery in the deeply buried coal seam, resulting in a lot of CH ₄ residues. CO ₂ pour into deep coal seam with high stress sensitivity is available for enhancing CH ₄ recovery by improving permeability for reservoir fracture and displacing CH ₄ adsorbed in matrix. A coupled adsorp-hydro-thermo-mechanical (AHTM) model for deep methane development is established by considering the coupling relationships of non-isothermal and non-constant pressure competitive adsorption between CO ₂ and CH ₄ , multi-phase flow, unsteady diffusion, heat transmission and in-situ stress variety. The model is verified by historical production and then used for CO ₂ enhanced CBM (CO ₂ -ECBM) of deep coal reservoir in a sedimentary basin in Northwest China. The simulation results show that: (1) For primary recovery, permeability in coal reservoir drops rapidly with the development of CBM, which seriously restricts the production of CH ₄ . The permeability of the reservoir decreases from 7.89 × 10 ^-16  m ² to less than 1.50 × 10 ^-16  m ² , CH ₄ production rate in CBM well reduces to below 2000 m ³ /d, and the average total CH ₄ content of coal reservoir is reduced by 5.49 m ³ /t with the decrease of only 1.12 m ³ /t of average adsorbed CH ₄ in a production duration of 2000 d (2) With 10 MPa CO ₂ continuous injection into coal seam after 700d of primary, the permeability for reservoir and CH ₄ production rate increase while the total CH ₄ content and adsorption CH ₄ content in reservoir decrease compared with the primary recovery. (3) CO ₂ pouring into coal reservoir increases the CH ₄ production time and rate, which improves CH ₄ recovery of coal reservoir. And it increases by 23.36 %, 23.07 % and 22.46 % with shut-in thresholds of CH ₄ production rate of 1000 m ³ /d, 800 m ³ /d and 600 m ³ /d, respectively. The investigation is of great significance for the development of deep coalbed methane.
Competing Interests: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
(© 2024 The Authors.)