Predicting the change in the seismic response as a consequence of CO2 sequestration into saline aquifers

Abstract

Geological sequestration of CO2 into brine-saturated reservoirs is an immediate option to reduce anthropogenic CO2 emissions into the atmosphere. It is anticipated that time-lapse 3-D seismic technology will form the foundation for monitoring CO2 migration within the subsurface. The success of seismic monitoring will be determined by the magnitude of the change in the elastic properties of the reservoir during the lifecycle of CO2 storage. In the short-term, there will be a strong contrast in density and compressibility between `free' CO2 and brine. The contrast between these fluids is greater at shallower depth and higher temperature. The change in the elastic moduli of the reservoir will enable time-lapse seismic methods to readily monitor structural trapping of CO2 below an impermeable seal. However, because the acoustic contrast between brine saturated with CO2 and brine containing no dissolved CO2 is very slight, dissolved CO2 is unlikely to be detected by any seismic technology, including high-resolution borehole seismic. The detection of porosity increases associated with dissolution of susceptible minerals within the reservoir may provide a means for qualitative monitoring of CO2 dissolution. Conversion of aqueous CO2 into carbonate minerals should cause a detectable rise in the elastic moduli of the rock frame, especially the shear moduli. The magnitude of this rise increases with depth. Forward modelling suggests that the optimal reservoir depth for seismic monitoring is between 1000 and 2500 meters. Higher reservoir temperature is also preferred so that `free' CO2 will resemble a vapour.