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A Multicomponent, Two-Phase-Flow Model For CO2 Storage And Enhanced Coalbed-Methane Recovery

Carolyn J. Seto, Kristian Jessen, Franklin M. Orr

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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. Introduction Atmospheric concentrations of CO2 have increased significantly from preindustrial levels of 280 ppm to current concentrations of 385 ppm (Carbon Dioxide Information Analysis Center 2003). This increase is attributed to human activity, the majority of which is ascribed to fossil-fuel combustion, and is believed to be responsible for current global warming trends (Metz et al. 2005). ECBM recovery is a promising technology that could contribute to reduction of greenhouse-gas emissions (Stevens et al. 1998, Stevens 2001). Simultaneous recovery of CH4 while CO2 is sequestered in coal seams is an attractive option because it addresses the issue of increasing atmospheric CO2 concentrations while offsetting some of the costs of capture, compression, transportation, and storage of CO2 by production of CH4, the fossil fuel that has the lowest CO2 emissions per unit of energy made available for conversion. In this paper, we use the method of characteristics to obtain new analytical solutions for two-phase flow in ternary and quaternary systems with adsorption. A number of researchers have applied this technique to solve problems relevant to the oil industry (Isaacson 1980; Monroe et al. 1990; Dindoruk et al. 1992; Johns and Orr 1996; Wang 1998; Orr 2007). Related problems for multicomponent flow of incompressible fluids with adsorption have been investigated by Johansen and Winther (1988, 1989), Dahl et al. (1992), and Shapiro et al. (2004). Zhu et al. (2003) applied this method to model single-phase gas flows with adsorption to investigate ECBM in a dry coal. We consider isothermal, 1D flow in a homogeneous porous medium. The effects of gravity and capillarity are neglected to isolate the interplay of sorption and flow. We use an extended Langmuir isotherm (Yang 1987) to describe the adsorption and desorption of gas species on the coal surface and assume that water does not adsorb on the coal surface. In this analysis, no chemical reactions occur between the injected CO2 and in-situ elements of the coalbed reservoir system. The analytical solutions presented are used to delineate the interactions of convection, phase behavior, and adsorption and desorption of gas components from the coal surface as gas mixtures propagate through the system. These solutions are also used to predict the effect of mixed-gas injection on CH4 recovery. They provide a guide to optimizing injection-gas composition, depending on the objectives of the gas-injection scheme.