Investigation of varying-composition gas injection for coalbed methane recovery enhancement: A simulation-based study

Abstract

Gas-injection in coalbed methane resources is a well-studied enhanced recovery technique. Through several experimental, simulation and pilot studies, it has been shown that the composition and type of injectant have a significant impact on the ultimate Methane recovery and the production rate. The commonly used gases are Carbon Dioxide (CO2), Nitrogen (N2) and a mixture of N2 and CO2. Pure CO2 (CO2-enriched gas) injection may cause irretrievable well injectivity reduction, because of high adsorption affinity of coal to CO2, which result in matrix swelling. By injecting pure N2 (N2-enriched gas), an early breakthrough of the injected gas may occur, which degrades the quality of the produced gas. Studies have shown that a better performance is obtained, when a mixture of CO2 and N2 is injected, and there is an optimum composition for the mixture, which depends on the geomechanical and sorption characteristics of the coal. In all of these studies, the composition of the injected gas is kept constant within the period of injection.In this study, a varying-composition injection is proposed, as an alternative technique to the constant-composition injection. In a varying-composition injection, the composition of the injected gas is altered during the injection period through several steps. Such a scenario can postpone the breakthrough time, and meanwhile avoid the deterioration of well injectivity. To assess the proposed method and find an optimal and practical injection schedule, a semi-synthetic simulation 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. By carrying out 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 technique has the following benefits, in comparison with the optimum scenario of constant-composition injection: 1- greater Methane recovery, 2- higher Methane production rate 3- deferment in permeability reduction, 4- later N2 breakthrough.