Influence of injection-production parameters on CO₂ displacement of gas under thermal-hydraulic-mechanical coupling

Injection of CO₂ to displace gas is a key method to improve gas extraction efficiency in deep low-permeability coal seams, and can also realize CO₂ geological storage, delivering both economic and environmental benefits. To investigate the laws of CO₂ injection into coal seams for gas displacement, a thermal-hydraulic-mechanical coupling model considering binary gas seepage, diffusion, competitive adsorption, and coal deformation was proposed. With COMSOL numerical simulation software, dynamic changes in CO₂ concentration, coal permeability, and temperature under different gas injection pressures and injection-production spacings were systematically studied, and CH₄ recovery rate and CO₂ storage amount under different injection-production parameters were analyzed to evaluate the displacement effect. The results showed that: ① increases in gas injection pressure and injection-production spacing could significantly improve the diffusion capacity of CO₂ in coal, intensify coal deformation, and make fluctuations in permeability and temperature more obvious. ② When gas injection pressure increased from 0.5 MPa to 2.5 MPa, CH₄ recovery rate and CO₂ storage amount after 60 d of injection increased by 10.47% and 387.00%, respectively, and CO₂ storage amount was more sensitive to changes in gas injection pressure. ③ When injection-production spacing increased from 3 m to 7 m, CH₄ recovery rate and CO₂ storage amount after 60 d of injection increased by 48.34% and 9.19%, respectively, and CH₄ recovery rate was more sensitive to changes in injection-production spacing. ④ Engineering applications should optimize injection-production parameters according to actual objectives. If gas production enhancement was the main goal, injection-production spacing should be increased preferentially. If CO₂ storage was the main goal, gas injection pressure should be increased preferentially. Field process tests further confirmed that increasing gas injection pressure and injection-production spacing could effectively improve gas extraction rate and yield, verifying the reliability of the thermal-hydraulic-mechanical coupling model.