Laboratory Measurements of Seismic Wave Velocity by CO2 Injection in Two Porous Sandstones

Abstract

This chapter presents a preliminary result of measurements on velocity changes while injecting CO2 into water-saturated Shirahama and Tako sandstone. Wave velocity and attenuation in porous sandstone are widely studied in fields of reservoir engineering and geo-engineering. Seismic survey provides substantial information concerning positions for new wells and modification of the existing depletion strategy. Cross-well seismic tomography is considered as a promising monitoring method to map the movement of CO2 in the subsurface. The formation water, which existed in pore spaces within reservoir rocks, will be partially displaced by the injected CO2. This process will affect the propagation characteristics of the seismic waves. Seismic properties depend on the mineralogical composition of the rock as well as factors such as porosity, fluid content, and in situ stress. Previous works on effects of CO2 flooding on seismic wave velocity clearly show that CO2 flooding caused compressional wave (P-wave) velocities to substantially decrease. Interpretation of seismic monitoring of CO2 flooding requires an understanding of the effects of pore pressure buildup caused by the CO2 injection and CO2 saturation. Experimental studies, such as converting field measurements of wave velocities and attenuations to CO2 saturation, support the interpretation of the survey results. A series of seismic tomography experiments on porous sandstone samples to demonstrate the use of cross-well seismic profiling for monitoring the migration of CO2 in geological sequestration projects have been conducted.