Enhancing Coal Bed Methane Recovery by Varying-Composition Gas Injection

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Abstract Gas injection in coalbed methane (CBM) resources is a well-studied method. Findings indicate that although Carbon-Dioxide (CO2) injection expedites the Methane desorption process, its high adsorption affinity causes irretrievable permeability reduction. Nitrogen (N2) injection does not have the swelling issue, however, an early breakthrough of the injected gas occurs. Recently, through several simulation-based and experimental studies, it has been shown that a better result is obtained, when a mixture of CO2 and N2 is injected. In these studies, an optimum composition for the injected gas was found, by carrying out a series of sensitivity analyses for the given CBM with known geomechanical and sorption characteristics. In all of these studies, the composition of the injected mixture remains constant within the period of injection. In the current work, in order to obtain a better performance, an alternative method is proposed in which the composition of the injected gas is adaptively updated during the injection through several steps. This method can postpone the breakthrough time and also keep well injectivity high. To evaluate the proposed method and find an optimal and practical injection schedule, a semi-synthetic model is constructed. Different injection scenarios are compared with each other, using a compositional simulator (ECLIPSE-300), which uses the extended Langmuir isotherm and the modified Palmer-Mansoori model. The compositional simulation allows us to investigate the sorption competition between each component through the whole system. By a series of sensitivity analyses, an optimum scenario is found. The best obtained scenario, is the one that begins by injecting a mixture with less CO2, and continues by a sequential rise in the CO2 fraction. The outcomes confirm that the proposed method has the following benefits, in comparison with the continuous injection: 1- higher Methane recovery, 2- deferment in permeability reduction, 3- later N2 breakthrough.