A new mathematical simulation model for gas injection enhanced coalbed methane recovery

The main purpose of this study is to improve the numerical evaluation of gas injection enhanced coalbed methane (ECBM) recovery by establishing a new mathematical coalbed methane (CBM) simulation model, in which the two-dimensional Peng-Robinson equation of state (2D PR EOS) model and the Maxwell-Stefan bidisperse pore diffusion (MS-BPD) model for multicomponent gas adsorption and diffusion are implemented. First, the extended Langmuir, ideal adsorbed solution and 2D PR-EOS models were investigated and compared with the experimental data of pure and mixed gas for modeling multicomponent gas adsorption behavior. Then, the MS-BPD model was developed based on the 2D PR-EOS model, bidisperse diffusion mechanisms, and Maxwell-Stefan diffusion theory, which provided an improved simulation of multicomponent gas diffusion behavior. On the basis of previous research, we developed a new CBM simulation model to analyze the effects of injection gas composition on the gas injection ECBM recovery process. The results indicate that mixed-gas injections are able to enhance methane recovery. The optimum composition of injected gas is 75% N2 and 25% CO2, which increases methane recovery by 25% over the pressure depletion production process. This CBM simulation model will provide an effective technical approach for research and development of CBM by gas injection.