Seismic modeling of compositional and geochemical effects in CO 2 sequestration
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Time-lapse seismic monitoring of CO2 injection into a hydrocarbon reservoir can be important for either enhanced recovery or CO2 sequestration tasks. In the latter case, over long time periods, the interaction of CO2 with in-situ brine and host rock minerals generates a variety of geochemical reactions that significantly affect reservoir rock properties and reservoir fluid properties. Dissolution of CO2 in brine to attain gas-brine equilibria alters brine density, which changes the bulk properties of reservoir fluids. Furthermore, slow mineral reactions between CO2 reservoir fluids and host rock minerals change porosity and the salinity of the reservoir fluids. Here we present results of a modeling study that combines direct simulation of geochemical processes with fluid flow and seismic models. The results show that the CO2 injection leads to P-wave velocity reduction of up to 12% for the first 10 years, while chemical effects largely associated with salinity changes become observable seismically only after longer time periods of hundreds of years, producing small velocity changes of about 2%. In general, the results suggest that it will be difficult to distinguish mineral reactions and intra-aqueous reactions from the reduction in bulk modulus caused by injection of CO2 into brine, especially with noisy data. Therefore, this type of geochemical reaction may not be too important for monitoring of sequestration efforts. 2005 Society of Exploration Geophysicists.
