Mechanical responses of coals under the effects of cyclical liquid CO₂ during coalbed methane recovery process

Liquid CO₂ fracturing technology has been introduced to enhance coalbed methane (CBM) recovery for the decades, under its multiple-effects of phase transition and flooding, by complicating the crack distribution and improving the reservoir permeability. When huge amounts of cryogenic fluids were cyclically injected into the reservoirs, the accompanying effects of temperature shock and adsorption might have some potential impacts on the fracturing results, and their influence degree and the related mechanical deterioration were required to systematically studied. Three different rank coals were chosen and cyclically treated by liquid CO₂, and the uniaxial compression behaviors of these affected coals were studied to investigate the destruction patterns, the evolutions of mechanical parameters, and the mechanism of damage failure. Compared to the raw coals, the ones affected by temperature-adsorption coupling effects of liquid CO₂ displayed a more complex “separation + spallation” destruction pattern, while the ones treated by the sole effect had a destruction pattern of “spallation” and “separation” respectively. The more complex the destruction pattern, the larger the fractal dimension, and the greater the structure degradation. The affected coals exhibited diverse mechanical responses, for example, the compression strength σ_c and elastic modulus E had negative correlations with the increasing cyclical parameters, while Poisson's ratio μ and damage variable D_v positively correlated with the increasing affecting parameters, respectively, which manifested that the accompanying cyclical temperature shock could facilitate the crack growth and the CO₂ adsorption aggravated the strength deterioration, finally destroying the coals with the smaller yield strength. A new damage model was established by jointly considering the grain anisotropy from the microscopic aspect and the diversity of mechanical responses from the macroscopic perspective, and the induced unrecoverable deformations greatly weakened the cohesive stress among the grains or matrix skeleton, finally destroying the coal structure by producing amounts of new cracks. Furthermore, the produced new-raw cracks provided amounts of adsorption sites for CO₂ molecules, which further decreased the surface energy and accelerated shear failure. The results were helpful to the injection parameters optimization, the low-cost of CO₂ injection invests and the potential profit maximization of CBM utilization.