Injection of gas (CO₂) into coal seams is an effective method to benefit from both CO₂ geological storage and coalbed methane recovery. Based on the dual pore structure of coal mass, and the Weibull distribution of fracture permeability, a menmal-hydraulic-mechanical (THM) coupling mathematical model is proposed involving the non-isothermal adsorption of binary gases, dynamic gas diffusion between matrix and fractures, multiphase seepage, coal deformation, heat conduction and heat convection. This mathematical model is applied to study the process of CO₂-enhanced coalbed methane recovery (CO₂-ECBM). Results show that the CH₄ content of CO₂-ECBM in coal seam decreases significantly when compared with that of regular drainage, and decreases rapidly in the early stage but slowly in the later stage. Coal seam permeability evolution is triggered by changes in gas adsorption/desorption, temperature and effective stress. For regular drainage, the early permeability shows a decreasing trend dominated by the increase of effective stress, while the later permeability shows an increasing trend dominated by the CH₄ desorption caused shrinkage of coal matrix. For CO₂-ECBM, the permeability in coal seam generally shows a downward trend due to both matrix swelling induced by gas adsorption and thermal expansion, particularly near injection well. There appears an increased and delayed peak production rate of CH₄. The CH₄ production rate of CO₂-ECBM is always higher than that of regular drainage. The CH₄ cumulative production and CO₂ cumulative storage linearly increase with time, and the CH₄ cumulative production of CO₂-ECBM increased by 39.2% in the duration of 5000 d compared with regular drainage. Reasonable CO₂ injection starting time can overcome the issue of early CO₂ breakthrough and ineffective increase of CH₄ production. In the studied case, the optimal injection starting time is 2500 d. Compared with the simultaneous CH₄ extraction and CO₂ injection, the CH₄ cumulative production of optimal time has increased by 30.1%. The research provides a reference for determining the reasonable CO₂ injection time under similar conditions.

向煤层注入瓦斯(CO _{2 )是同时受益于CO 2}地质封存和煤层气回收的有效方法。基于煤体的双孔隙结构和裂缝渗透率的Weibull分布，提出了涉及二元气体非等温吸附、基质与裂缝间动态气体扩散、多相渗流、煤变形、热传导和热对流。_{该数学模型应用于研究CO 2}强化煤层气采收率（CO ₂ -ECBM）过程。结果表明CO _{2中CH 4含量}-煤层ECBM与常规抽采相比明显减少，且早期减少较快，后期减少较慢。煤层渗透率演化是由气体吸附/解吸、温度和有效应力的变化引发的。对于常规排水，早期渗透率呈现下降趋势，主要是有效应力的增加，而后期渗透率则呈现上升趋势，主要是CH ₄解吸导致煤基质收缩。对于CO ₂ -ECBM，由于气体吸附引起的基质膨胀和热膨胀，煤层的渗透率通常呈现下降趋势，特别是在注入井附近。_{CH 4}的峰值生产率出现增加和延迟。_{CO 2} -ECBM的CH ₄产率始终高于常规排水。CH ₄累计产量和CO ₂累计封存量随时间线性增加， CO ₂ -ECBM的CH ₄累计产量在5000 d的时间内比常规排水增加了39.2%。合理的CO ₂启动时间可以克服CO ₂过早突破和CH ₄增产效果不佳的问题。在研究的情况下，最佳注入开始时间为2500 d。与同时CH ₄萃取和CO _2相比注水后，最佳时间CH ₄累积产量增加了30.1%。_{该研究为类似条件下合理确定CO 2}注入时间提供参考。