Reservoir Modeling, Rock Physics, and Seismic Simulation of CO2 Sequestration in Coal Beds

We present results of an integrated simulation study to assess the effectiveness of surface seismic imaging for monitoring CO2 sequestration. We considered two scenarios. In the first, injected CO2 remained confined within a shallow coal formation. In the second, the sequestered CO2 gas leaked through a semi-permeable shale layer to an overlying sand unit sealed above. The reservoir and seismic simulations required the construction of 3D geologic and facies models, the estimations of seismic velocities based on rock physics correlations, and the development of geostatistical dual-porosity reservoir descriptions of the coal and overlying shale and sand units. We ran the 3D reservoir simulations with an equation-of-state compositional reservoir simulator with the capability to model adsorption of injected CO2 in coal, desorption of CH4, and matrix shrinkage-swelling effects. We generated synthetic seismograms by simulating the propagation of acoustic waves through the 3D heterogeneous media, and used reverse time migration to create 3D seismic images corresponding to the state of the reservoir at the beginning and end of ten years of CO2 injection. The resulting seismic images clearly identified the regions of CO2 gas saturation, closely matching the gas saturation profiles predicted by the reservoir simulator.