Experimental investigation of seismic velocity behavior of CO 2 saturated sandstones under varying temperature and pressure conditions

Subsurface storage of CO 2 into geological formations is considered an important strategy to mitigate increasing atmospheric CO 2 . Time‐lapse seismic monitoring is an integral component of a geological CO 2 sequestration project because the seismic behavior of the rock is a function of both mineralogical composition and pore fluid properties. At the uppermost kilometer of the sedimentary basin, CO 2 can be present at gaseous, liquid, and supercritical states, with the supercritical and liquid states preferred in CO 2 storage operations due to the higher sweep efficiency. In this study, the seismic velocities [both compressional (V p ) and shear (V s ) waves] of two CO 2 ‐saturated sandstone core plugs (Red Wildmoor and Knorringfjellet formations) have been measured under a range of temperatures and pressures in which CO 2 phase transitions occur. The experiments were done using a uniaxial hydrostatic cell equipped with seismic wave transmitting and receiving transducers. The experimental investigation illustrated that seismic velocities (both V p and V s ) decreased until the critical point was reached. Further increases in the CO 2 pressure above the critical point led to a gradual increasing of V p while the V s remained unchanged. The effect of CO 2 on the seismic velocity of the sandstone was compared with the effects of N 2 and distilled water at the same conditions. It was further indicated that the seismic velocity changes were mainly connected to significant changes of CO 2 density and the corresponding bulk rock moduli over the critical point. The observed velocities are in good agreement with Gassmann‐predicted velocities as well as literature data. © 2016 Society of Chemical Industry and John Wiley & Sons, Ltd