Analytical Modeling of CO2 Storage and Enhanced Coal Bed Methane Recovery

Injection of CO2 into deep unminable coal seams is an option for geological storage of CO2. In these systems, adsorption of gas on the internal surfaces of the coal is the storage mechanism. In many of these reservoirs, large amounts of CH4 are adsorbed onto the coal. CO2 is preferentially adsorbed compared to CH4, which offers the possibility of desorption and increased recovery of CH4 from coal without lowering pressure in the coal bed. The interaction between CO2 sequestration and enhanced CH4 production makes coal reservoirs interesting candidates for sequestration. We report analytical solutions for one-dimensional, two-phase, three-component flow with adsorption of gas components onto coal surfaces in the absence of dispersion in the flowing phases. The solutions were obtained by the method of characteristics. We present solutions for combined flow of brine, CO2, and CH4, with solubility of both gas components in the brine and with adsorption represented by constant equilibrium partition coefficients. The solutions consist of shocks and continuous variations. We show that the arrangements of shocks and rarefactions depends on the relative strength of the adsorption of the gas components. These solutions provide insight into the complex interplay of adsorption, phase behavior and convection in these systems. Improved understanding of the physics of these displacements will aid in developing more efficient and physically accurate techniques for predicting the movement and fate of injected CO2 in the subsurface.