Spatial deterioration responses of coals under the thermo-mechanical effects from liquid CO2 interaction

The technology of CO₂ fracturing and enhanced coalbed methane recovery has garnered significant attention worldwide because of its potentials of CH₄ resource exploitation and geological sequestration. The Joule-Thomson effect during the liquid CO₂ injection process along the borehole induces rapid changes in temperature and pressure, which might have some impacts on the mechanical responses of coals. However, these effects have not been visualized through physical experiments. This paper focused on the thermo-mechanical impacts of cyclic liquid CO₂ injection on the deterioration behaviors of coals, by unsealing and sealing borehole, respectively, under the conditions of different confining pressure. Several non-contacting monitoring technologies were employed to document the spatial deterioration and fracturing behaviors of coals during the liquid CO₂ injection process. When the unsealed samples were subjected to cyclical cryogenic CO₂, the temperature inner borehole steadily decreased and remained at a low-temperature of −22 °C, only beginning to rise when the injection was completed. The acoustic emission (AE) events initially manifested at the bottom of borehole and subsequently dispersed along the borehole axial direction within the cyclical CO₂ injection. The presence of repeatability and multiple steps in AE events indicated that the thermal fractures were generated under the effects of alternative changes of negative/positive temperatures. Compared to the straightforward crack morphology observed in the free-pressured sample, a higher number of cracks were produced in the samples subjected to the confining stress, and these cracks had greater toughness and good fractal characteristics. The crack density, crack number and fragments had positive relations with the fractal dimension, respectively. The confining stress and physical features of coals jointly affected the crack spatial distribution, and the reduction of ultrasonic velocity reflected that matrix had the anisotropy of the mechanical responses. Finally, a cracking initiation model was established considering the thermal cycling, damage accumulation and expansion pressure by phase-transition. The experimental results might have some theoretical significances on the field application.