Modeling Time Lapse Seismic Monitoring of CO2 Sequestration in Hydrocarbon Reservoirs Including Compositional and Geochemical Effects

The sequestration of CO₂ into geologic formations, specifically existing and depleted oil and gas reservoirs, is a promising solution for reducing environmental hazards from the release of greenhouse gases into the earth's atmosphere. A critical component of long-term sequestration will be our ability to adequately monitor the movement of CO₂ fronts in the subsurface. In this article, we examine the viability of time-lapse seismic monitoring using an integrated modeling of fluid flow, including chemical reactions and seismic response. Modeling of CO₂ injection is complicated by the various interactions between CO₂, reservoir fluids, and the minerals in the formation. These interactions change fluid and bulk rock properties with time, which in turn impact the seismic signatures. We perform a comprehensive simulation of the gas injection process accounting for the phase behavior of CO₂-reservoir fluids, the associated precipitation/dissolution reactions, and the accompanying changes in porosity and permeability. The simulation results are then used to model the changes in seismic response with time. The general observation is that gas injection decreases bulk density and wave velocity of the host rock system. Seismic amplitude attributes therefore change with time as well, and these effects provide a tool for tracking the movement of the CO₂ front. Analysis of the results also confirms that much of the change can be attributed to chemical effects that should therefore be considered in studies of long-term sequestration projects.