Numerical analysis of permeability rebound and recovery evolution with THM multi-physical field models during CBM extraction in crushed soft coal with low permeability and its indicative significance to CO2 geological sequestration

The permeability rebound and recovery are the key factors affecting the production efficiency of CBM and the storage capacity of CO2 during the CO2-ECBM process. In this study, fully coupled mathematical models of multi-physical fields with thermal-hydraulic-mechanical fields were established. The phenomenon of permeability rebound and recovery is accurately illustrated, and the influence of initial geological factors and later engineering practice on its evolution is analyzed. The indicative significance of permeability evolution to CO2 geological sequestration is further discussed. The results show that the pressure near the extraction boreholes is firstly affected and the dropping rate is large. The rate of pressure drop in reservoir away from the extraction boreholes is low to high in the early stage and high to low at the late stage. The net effect of effective stress and gas adsorption/desorption controls the rebound and recovery of permeability. When the CO2 injected does not spread to the production well, the permeability near the injection well is mainly affected by the effective stress and the gas competitive adsorption, while the permeability near the production well is mainly affected by the effective stress and CH4 desorption. As CO2 injected spreads to the production well, the competitive adsorption effect gradually replaces the effective stress effect on permeability near the production well. The periodic pressurization is a new method in CO2-ECBM engineering practice, which is a process of gradual pressurization and gas injection. Nonetheless, a pressure limit must be considered during CO2 injection. This study has important theoretical and practical significance for explaining the production efficiency of CBM and the storage capacity of CO2.