Optimisation of gas mixture injection for enhanced coalbed methane recovery using a parallel genetic algorithm

Abstract Gas injection in coalbed Methane reservoirs is an environmentally friendly and economically viable enhanced recovery technique. Promising results can be obtained by injecting a mixture of CO2 and N2. The optimum composition is a function of geomechanical and sorption characteristics of the coal. In the current study, it is sought to optimise the composition of the injected gas, based on an economic objective function. The decision variables are the composition of the injected gas and the injection rate that can be both subjected to change over a continuous injection. In the formulated objective function, the OPEX costs (e.g., separation, compression for injection and injectant supply) and the income, resulting from, CH4 production and CO2 sequestration are considered. This optimisation problem is nonlinear, and the corresponding search space is high-dimensional. Therefore, a sophisticated optimisation algorithm is required. For this study, a parallel real-value genetic algorithm is coded in MatLab and coupled to a commercial coalbed simulator (ECLIPSE-E300). This interface allows us to measure the goodness of each solution-candidate and also to perform optimisation automatically. The algorithm is used to optimise rates and compositions of a semi-synthetic ECBM case study, and the optimum scenario is compared with the optimum scenario of a constant composition injection. The comparison confirms that a varying-composition strategy results in more revenue from an ECBM project. In this study, also, the optimum solution for different economic conditions are approximated and then the optimum solutions are compared with each other to investigate the effect of carbon credit and Methane price on the injection schedule. In those economic conditions that carbon credit is higher than CO2 supply costs, the optimum scenarios tend to yield a higher amount of sequestrated CO2, and in all of them, the optimum schedules are the ones that start with a very low fraction of CO2 in the injected gas and continue by a gradual increase of CO2 fraction. In other economic conditions, the optimum scenarios move towards the ones that less CO2 is injected.

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