Numerical simulation of CO₂-ECBM for deep coal reservoir with strong stress sensitivity

CH₄ production rate of coalbed methane (CBM) well decreases rapidly during primary recovery in the deeply buried coal seam, resulting in a lot of CH₄ residues. CO₂ pour into deep coal seam with high stress sensitivity is available for enhancing CH₄ recovery by improving permeability for reservoir fracture and displacing CH₄ 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 CO₂ and CH₄, multi-phase flow, unsteady diffusion, heat transmission and in-situ stress variety. The model is verified by historical production and then used for CO₂ enhanced CBM (CO₂-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 CH₄. The permeability of the reservoir decreases from 7.89 × 10^-16 m² to less than 1.50 × 10^-16 m², CH₄ production rate in CBM well reduces to below 2000 m³/d, and the average total CH₄ content of coal reservoir is reduced by 5.49 m³/t with the decrease of only 1.12 m³/t of average adsorbed CH₄ in a production duration of 2000 d (2) With 10 MPa CO₂ continuous injection into coal seam after 700d of primary, the permeability for reservoir and CH₄ production rate increase while the total CH₄ content and adsorption CH₄ content in reservoir decrease compared with the primary recovery. (3) CO₂ pouring into coal reservoir increases the CH₄ production time and rate, which improves CH₄ recovery of coal reservoir. And it increases by 23.36 %, 23.07 % and 22.46 % with shut-in thresholds of CH₄ production rate of 1000 m³/d, 800 m³/d and 600 m³/d, respectively. The investigation is of great significance for the development of deep coalbed methane.