Effect of CO2 on coal P-wave velocity under triaxial stress

As P-wave velocity is sensitive to the variations in coal reservoir parameters, it is possible to monitor the injected CO2 through P-wave velocity during CO2 sequestration in coal. However, the effects of CO2 on the coal P-wave velocity under triaxial stress are not clearly discerned. In the present study, different boundary conditions and gases were utilised to investigate the factors affecting the P-wave velocity after the interaction of coal with CO2. Experiments with helium indicated that the pore pressure primarily affected the P-wave velocity by altering the effective stress. Experiments with CH4 and CO2 indicated that matrix swelling induced-cleats porosity decline significantly promoted P-wave velocity. Moreover, CO2 caused a wider scale and severe weakening of coal matrix than CH4, thereby significantly decreasing the P-wave velocity, and the decline in P-wave velocity increases with vitrinite content. Furthermore, experiments under different boundary conditions showed that with the boundary condition having more constraints, the decrement of pore pressure on P-wave velocity is more weaken, whereas the improvement of matrix swelling on P-wave velocity is more evident. This study contributes to understanding the mechanism of effect of CO2 on P-wave velocity under triaxial stress condition and provides guidance for monitoring CO2 sequestration in coal.