Abstract
Summary Injection of CO2 into deep unminable coal seams is an option for geological storage of CO2. Moreover, injection of CO2 may enhance the recovery of CH4 in these systems, making coal reservoirs interesting candidates for sequestration. New analytical solutions are presented for two-phase, three-and four-component flow with volume change on mixing in adsorbing systems. We analyze the simultaneous flow of water and gas containing multiple adsorbing components. The displacement problem is solved by the method of characteristics. Mixtures of N2, CH4, CO2, and H2O are used to represent enhanced coalbed-methane (ECBM) recovery processes. The displacement behavior is demonstrated to be strongly dependent on the relative adsorption strength of the gas components. In ternary systems, two types of solutions result. When a gas rich in CO2 displaces a less strongly adsorbing gas (such as CH4), a shock solution is obtained. As the injected gas propagates through the system, CO2 is removed from the mobile phase by adsorption, while desorbed gas propagates ahead of the CO2 front. The adsorption of CO2 reduces the flow velocity of the injected gas, delaying breakthrough and allowing for more CO2 to be sequestered per volume of CH4 produced. For injection gases rich in N2, a decrease in partial pressure is required to displace the preferentially adsorbed CH4 and a rarefaction solution results. In quaternary displacements with injection-gas mixtures of CO2 and N2, the relative adsorption strength of the components results in solutions that exhibit features of both the N2-rich and CO2-rich ternary displacements. Analytical solutions for ECBM recovery processes provide insight into the complex interplay of adsorption, phase behavior, and convection. Improved understanding of the physics of these displacements will aid in developing more efficient and physically accurate techniques for predicting the fate of injected CO2 in the subsurface.
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