Elastic wave velocity behavior of sandstone during CO₂ injection

Abstract

To realize carbon dioxide capture and storage（CCS）, it is essential to have technology to monitor changes in the subsurface when CO₂ is injected deep underground. To this end, the physical properties of rocks in CO₂ injection reservoirs must be properly evaluated, and the application of rock physics experiments in a laboratory environment that reproduces the high temperature and pressure environment of the subsurface will contribute to the reliability and effectiveness of monitoring. In this rock physics experiment, P- and S-wave velocities are measured in core samples taken from oil reservoirs during injection of brine, liquid CO₂, or supercritical CO₂ at controlled effective pressure and temperature. As a result, the P-wave velocity（Vp）was generally in agreement with the logging data, and the velocity variation with effective pressure（Pe）was obtained. The P-wave velocity（Vp）before CO₂ injection in the reservoir was estimated based on the experimental equation of P-wave velocity depending on the effective pressure（Pe）during brine injection. The trend of the P-wave velocity（Vp）by the type of injection fluid was also obtained, and the trend of velocity decrease due to CO₂ saturation was characterized for the cases of liquid CO₂ and supercritical CO₂ as pore fluids. For S-wave velocity（Vs）, the same velocity variation with effective pressure（Pe）was obtained as for P-wave velocity（Vp）. On the other hand, the S-wave velocity（Vs）decreased during supercritical CO₂ injection, showing the opposite trend to that assumed by Gassmann’s S-wave velocity（Vs）formula. XRD analysis confirmed the presence of siderite as a cause of the higher velocity. The change in mineral composition after CO₂ injection was also observed.