Investigation of the mechanical damage of low rank coals under the impacts of cyclical liquid CO2 for coalbed methane recovery

Liquid CO2 enhanced coalbed methane recovery has been proposed for decades, and the accompanying effects of phase-transition and adsorption of liquid CO2 slightly influenced the coal structure strength. Studying the coal strength variation under the effects of liquid CO2 is of great significance for evaluating the selected liquid CO2 injection parameters, CO2 injectability, and coal cracking initiation capacity. However, the coupling effects of the temperature gradient and adsorption of cyclical liquid CO2 on the coal strength have not been systematically studied. In this study, based on the cyclical liquid CO2 fracturing experimental system, the low-rank coals were processed under the effects of liquid CO2 with different cyclical parameters. Subsequently, a uniaxial compression test was carried out to investigate the mechanical responses of the processed coals under the sole and coupled effect of liquid CO2 temperature and adsorption, respectively, using acoustic emission and strain gauge. The results showed that coals exhibited different destruction behaviours and fracture morphologies; for example, coals influenced by the sole effect showed an “axial separation” failure pattern, while liquid CO2 coupling affected coals showed a “separation and spallation” destruction form. Compared to raw coals, the affected coals contained a larger ring count and accumulated energy J, indicating that thermal cycling induced by uneven temperature distribution enhanced the generation of new cracks and expansion of the original cracks, and the enlarged crack volume provided sufficient adsorption sites for CO2 molecules to decrease the Gibbs free energy. The negative relationship between σc and cyclical parameters showed that the alternative temperature shock significantly degraded the cohesive strength among the grains with amounts of the damage accumulation, leading to tensile failure. The Je/Jt and Jr/Jt scatters positively and negatively correlated with the corresponding cyclical (cycles and time) and mechanical parameters (E and σc), respectively, indicating that the more liquid CO2 affected, the greater the damage accumulation, and the larger the destructive plastic region and lower the strain energy storage capacity, because of the synergy of the cold shock and gas adsorption. The research results provided theoretical guidance for optimizing liquid CO2 injection parameters for balancing the investment and output benefit of coalbed methane.