Experimental investigation of seismic velocity behavior of CO2 saturated sandstones under varying temperature and pressure conditions.

Subsurface storage of CO₂ into geological formations is considered an important strategy to mitigate increasing atmospheric CO₂. Time-lapse seismic monitoring is an integral component of a geological CO₂ 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, CO₂ can be present at gaseous, liquid, and supercritical states, with the supercritical and liquid states preferred in CO₂ 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 CO₂-saturated sandstone core plugs (Red Wildmoor and Knorringfjellet formations) have been measured under a range of temperatures and pressures in which CO₂ 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 CO₂ pressure above the critical point led to a gradual increasing of V_p while the V_s remained unchanged. The effect of CO₂ on the seismic velocity of the sandstone was compared with the effects of N₂ and distilled water at the same conditions. It was further indicated that the seismic velocity changes were mainly connected to significant changes of CO₂ 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 [ABSTRACT FROM AUTHOR]