SPE Journal
Volume 14, Number 1, March 2009, pp. 30-40

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.

© 2009. Society of Petroleum Engineers

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History

• Original manuscript received: 25 October 2006
• Meeting paper published: 24 September 2006
• Revised manuscript received: 29 September 2008
• Manuscript approved: 1 October 2008
• Published online: 16 March 2009
• Version of record: 1 March 2009