Mathematical model of coal seam methane displacement by injecting CO2/nitrogen
In order to understand the physical mechanism of coal seam methane replacement by injecting CO2/N2, this paper analyses the flow mechanisms of methane and injected gas during replacement and the corresponding controlling factors. Based on Langmuir model, Langmuir-like sorption swelling model, P&M permeability model and variable Klinkenberg coefficient model, the sorption model, sorption swelling model of mixed gas were proposed, and the absolute and apparent permeability models of gas injection process were improved as well. Based on above equations, a coupling mathematical model of coal seam methane replacement by injecting CO2/N2 was proposed by using mass conversation law. The proposed model reflects the adsorption-desorption, diffusion, Darcy flow and the additional flow from Klinkenberg phenomenon, and it can be used to calculate the gas content and pressure change, the injected and produced gas volume and the gas composition of the produced gas. This study provides theoretical foundation for the design of coal seam methane replacement by injecting CO2/N2.