In this research, we developed a double-permeability type compositional simulator (ECBM simulator) which deals with two phases of gas and water composed of four components of H2O, CH4, CO2 and N2. The phase behavior/equilibrium is calculated by solving the Equation of State (EOS). Hence in this simulator, phase behavior is determined in accordance with thermodynamic equilibrium. In addition, this simulator can predict the behavior of each component taking the Darcy flow, diffusion and adsorption/desorption into consideration.
After developing this in-house ECBM simulator, we validated the simulator by comparing the simulation results with those calculated by GEM (commercial simulator of Computer Modelling Group Ltd.). Through the comparison, we confirmed that the in-house ECBM simulator worked accurately. We then conducted case studies using the in-house ECBM simulator to examine the effects of the injection of CO2 and/or N2 gas on CH4 production. It was revealed that N2 gas enhanced the CH4 production from the early stage of the injection by reducing the partial pressure of CH4, while CO2 gas increased the CH4 production slowly by replacing CH4 adsorbed on the coal surface.