Long-term effects of CO2-water-coal interactions on structural and mechanical changes of bituminous coal

Structural integrity and mechanical stability of coal are important regarding the long-term safety of CO2 sequestration in coal. To date, little understanding exists on long-term effects of CO2 injection at presence of water on structural and mechanical properties of coal. In this study, bituminous coal samples were saturated with CO2 at pressures from 4 to 10 MPa, with presence of water, for saturation time up to 25 weeks. Uniaxial compression tests and X-ray computed tomography were conducted to study changes in mechanical properties and microstructures of coal. The results show, after CO2 injection with presence of water, the mechanical properties of coal changed, including the reduction of uniaxial compression strength (up to ~50%) and Young's modulus (up to ~65%). The reduction of mechanical properties was closely related to the structural damage due to sorption-induced cracking. The changes of mechanical properties were significantly affected by CO2 phase state and slightly affected by saturation time. Rapid reduction of mechanical properties was found when CO2 transitioned from subcritical to supercritical state and the failure modes changed from a brittle failure (shear fractures) to a ductile failure (multiple fractures) during this transition. When saturation duration increased from 3 up to 25 weeks, limited further reduction occurred and it was related to the wormhole formation due to coal matrix dissolution. Overall, the dominated cause for coal softening after CO2 injection in presence of water was sorption-induced cracking, which is similar to the case of pure CO2 injection without water. However, the reduction extent after CO2 injection in presence of water seems to be less at the same pressure conditions compared to that after pure CO2 injection. This is because the presence of water may potentially hinder CO2 sorption on coal.