Determination of Thermo-mechanical Coal Deformations and Implication for CO₂ Storage in Deep Coal Formations

CO₂ sequestration along with enhanced coalbed methane (ECBM) has received considerable attention for energy recovery and CO₂ storage. CO₂ injection not only reduces permeability due to the swelling effect but modifies the temperature field, resulting in coupled thermo-mechanical coal deformations. Limited efforts have been devoted towards the understanding of thermo-mechanical deformations in the coalbed methane (CBM) industry. This study proposed a theoretical model of thermal expansion coefficients through energy principle. Direct measurements of coal deformations with variations in temperature and pressure were carried out. The results indicate that thermo-induced deformation linearly correlates with temperature variations with estimated thermo-deformative coefficients between 8×10^-5/K to 10×10^-5/K, falling within the theoretical bounds. Additionally, the coal matrix retains its elastic properties after thermal cycling. The mechanical compression at different temperatures exhibits similar trends, increasing linearly with pressure. The matrix bulk modulus increases with pressure cycles at elevated temperatures, indicating that the coal becomes stiffer due to residual strain and gradually increases with pressure depletion. Anisotropic matrix deformation was observed when the temperature was above 273.15 K. The deformation of the coal can have significant implications on the evolution of effective stress, permeability, and localized failure, ultimately controlling CO₂ sequestration and long-term CBM production.