Modelling of free and adsorbed CO2-induced mechanical property alterations in coal

Abstract

CO2 interaction-induced mechanical alteration in coal is imperative in the context of CO2 geo-sequestration, as it governs the project safety, in general. Accurate estimations of mechanical characteristics are thus essential prior to the implementation of field projects. Through this study, we numerically model the alterations in coal compressive strength, caused by CO2 interaction. The combined effect of both free and adsorbed CO2 on coal strength is evaluated based on the Mohr-Coulomb failure criterion, in which a modified-cohesion model is derived to define the adjusted-failure envelope, by integrating the CO2 adsorption-induced surface energy change. The model is initially validated with two experimental studies conducted under confined- and unconfined-stress conditions. The validation results confirm that the model is capable of successfully simulating the effect of confining stress, and the effect of free and adsorbed CO2 on the alterations of coal compressive strength. The calibrated model is used to estimate the strength alterations at relatively large confining stresses (i.e. Pb ≈ 25 MPa) and at incremental pore pressures (i.e. 0.1 MPa < PCO2 < 24 MPa), in which a sensitivity analysis is carried out to evaluate the model's sensitivity on different influential parameters. The results show that coal compressive strength decreases substantially with CO2 pore pressure and starts to slightly increase beyond a certain pore pressure (i.e. PCO2 > 12 MPa) due to the matrix-compression, caused by the elevated pore pressure, and the developed modified-cohesion model is capable of capturing this behaviour. The model is highly sensitive to the Biot's coefficient, as it defines the fraction of pore pressure that contributes to the effective stress, and ultimately to both free and adsorbed CO2-induced strength alteration. The sensitivity analysis highlights the significance of the Langmuir constants, as they play a critical role in modelling the fully-coupled process, thus should be precisely determined for a given coal type, in order to accurately simulate the overall strength alterations.