Modelling and optimization of enhanced coalbed methane recovery using CO₂/N₂ mixtures

Injection of gas mixtures (CO₂, N₂)into coal seams is an efficient method to both reduce CO₂ emissions and increase the recovery of coalbed methane. This process involves a series of complex interactions between ternary gases (CH₄, CO₂, and N₂)co-adsorption on coals, mass transport of two-phase flow, together with heat transfer and coal deformation. We develop an improved thermo-hydro-mechanical (THM)model coupling these responses for gas mixture enhanced CBM recovery (GM-ECBM). The model is first validated, and then applied to simulate and explore the evolution of key parameters during GM-ECBM recovery. Schedules of constant- and variable-composition injection are optimized to maximize CH₄ recovery and CO₂ sequestration. Result shows that the injected gas mixture displaces CH₄ through competitive sorption and accelerates the transport of CH₄ within the coal seam. The consistency between the modelling and field results verifies the feasibility and fidelity of the THM model for effective simulation key processes in GM-ECBM. Permeability evolution is strongly influenced by the combined effects of CH₄ desorption induced matrix shrinkage, CO₂/N₂ adsorption induced matrix swelling, thermal strains, and compaction induced by changes in effective stress. During ECBM, reservoir permeability first increases due to pressure depletion and CH₄ desorption, then dramatically decreases due to matrix swelling activated by the arrival of the CO₂/N₂ mixture. CH₄ pressure decreases rapidly at early time due to displacement by the injected gas mixture, and then deceases slowly in the later stage. The sweep of N₂ accelerates CH₄ desorption and subsequent transport, and hence promotes a decrease in reservoir temperatures distant from the injection well even prior to the arrival of CO₂. CH₄ production rate during GM-ECBM exhibits a decline-increase-decline trend and usually has an elevated but delayed CH₄ production peak compared to primary recovery. A higher CO₂ Langmuir strain constant reduces the critical CO₂ composition in the injected mixture when reaching the threshold of well shut down. An improved balance between early threshold (N₂)and large matrix swelling (CO₂)can be achieved by injection beginning with low CO₂ composition and following with a sequential increase of CO₂ composition. In studied cases, the gas recovery ratio of the optimal variable-composition case reaches 68.4% compared to of 59.4% pure CO₂ and 64.2% of optimal constant-composition cases, indicating a higher efficiency of variable-composition injection.