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

Abstract

Structural integrity and mechanical stability of coal are important regarding the long-term safety of CO 2 sequestration in coal. To date, little understanding exists on long-term effects of CO 2 injection at presence of water on structural and mechanical properties of coal. In this study, bituminous coal samples were saturated with CO 2 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 CO 2 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 CO 2 phase state and slightly affected by saturation time. Rapid reduction of mechanical properties was found when CO 2 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 CO 2 injection in presence of water was sorption-induced cracking, which is similar to the case of pure CO 2 injection without water. However, the reduction extent after CO 2 injection in presence of water seems to be less at the same pressure conditions compared to that after pure CO 2 injection. This is because the presence of water may potentially hinder CO 2 sorption on coal. • The structural and mechanical properties of coal treated by CO 2 at presence of water for long-term were investigated. • The CO 2 +water treatment caused reductions of UCS values up to ~50% and the Young’s modulus up to ~65%. • The reductions of mechanical properties were closely related to the structural damage due to sorption-induced cracking. • The mechanical properties were significantly affected by CO 2 phase state and slightly affected by saturation time. • Rapid reductions in mechanical properties happened when CO 2 transitioned from subcritical to supercritical state. • The failure modes of coal changed from a brittle failure to a ductile failure during CO 2 phase transition.