Reasonable start time of carbon dioxide injection in enhanced coalbed methane recovery involving thermal-hydraulic-mechanical couplings

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.