Abstract

Abstract In recent years, coalbed methane has become an important source of energy in the United States. Since primary production techniques typically recover less than half of the methane in a coalbed, enhanced coalbed methane (ECBM) recovery processes are needed in which N2 and/or CO2 are injected into the coalbed to recover more CH4. CO2 injection into coalbeds also provides additional benefit of sequestering carbon in the subsurface. One of the main mechanisms that govern the dynamics of ECBM recovery processes is the sorption of gases onto the coal surfaces. Despite the well-documented complexity of multicomponent sorption phenomena, adsorption and desorption of CH4/CO2/N2 mixtures in the porous coal is commonly modeled with the extended Langmuir model. The extended Langmuir model has been proven unable to accurately describe the multicomponent sorption that is central to ECBM recovery processes and, therefore, more sophisticated sorption models are needed. In this paper we apply potential theory to describe the multicomponent sorption of relevance to ECBM processes. In this approach for modeling multicomponent sorption, each component is assumed to be affected by a characteristic potential field emitted by the coal surface. We discuss the implementation of potential theory with emphasis on the simulation of ECBM processes where computational efficiency and accuracy must be balanced. The model must be solved by an iterative scheme, and is hence more computationally expensive than the extended Langmuir approach. The results and analysis presented in this paper demonstrate that the application of potential theory of sorption to modeling of ECBM recovery processes can improve the accuracy of calculations. However, the additional complexity of the model and the associated increase in the computational efforts may not balance the gain in accuracy sufficiently to warrant application in general purpose reservoir simulation. The presented algorithms and modeling results are directly applicable in the development and utilization of coalbed methane resources.

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