Analyzing CO2 phase and its influence on rock elasticity based on experimental investigation

Abstract During the long-term process of CO2 injection and storage, CO2 may undergo phase transition with changes in underground pressure and temperature. This requires a thorough understanding of CO2 and its impact on rock elastic parameters in order to effectively utilize seismic data to evaluate CO2 storage. This study monitored the changes in fluid parameters during CO2 injection into rocks in the laboratory and recorded the signals of P- and S- waves transmitting in rocks. Values of pore pressure (Pp), gas content (n) and compressibility factor (Z) during CO2 injection were analyzed, and the physical parameters of CO2 under temperature and pressure conditions were calculated according to the True Gas Law. Using CH4 injection data as a comparative analysis of the influence of pore pressure can help explain the phase changes during CO2 injection. The changes in velocity and amplitude were analyzed based on P- and S- wave signals, and the influence of pore pressure, fluid parameters on rock velocity was calibrated using a rock physics model. Through simultaneous monitoring of multiple physical fields and theoretical analysis, the behavior of CO2 phase transition is revealed by multiple physical parameters. The physical properties of CO2, such as density and bulk modulus, significantly increase after transitioning from a gaseous state to a supercritical state. This causes a significant decrease in rock velocity, but its impact on amplitude is not as significant as the impact of pore pressure. The rock physics models can accurately calculate the velocity of porous sandstone during CO2 injection and phase transition. Understanding the phase state of CO2 and its impact on rock elastic parameters is crucial for CO2 storage. This study can help to monitor and evaluate CO2 storage using time-lapse seismic data.