Thermal Gas Recovery from Coal Seam Gas Reservoirs Using Underground Coal Gasification

Abstract

Abstract Gas production from coal seam gas reservoirs initiates by dewatering operation. Reservoir pressure reduction activates gas desorption and gas diffuses from micropores to macropores and consequently coal cleats. Dewatering operation provides the required driving force for the gas to desorb. In this research, in situ coal combustion is introduced as an enhanced gas recovery method. Methane desorption from coal matrix is an endothermic process. Heating disturbs thermal equilibrium of the coal and more gas is released at higher temperature. A numerical transport model is applied to investigate thermal effect on gas desorption and diffusion in coal. Desorption-diffusion properties of the coal are modeled at non isothermal condition. Due to extremely low thermal conductivity of coal, convection is the dominant mechanism responsible for heat transfer in coal seam. Physical properties of the coal, Langmuir pressure and volume, porosity, density and initial diffusivity control methane mobility in thermal treatment. Underground coal gasification which is a known technology in coal mining is proposed to be used as the source of heat. Controlled air injection into different wells in a coal seam reservoir can generate some hot spots in the reservoir. Heat flux generated from underground coal gasification sites has the potential to be transferred within the reservoir and locally increases reservoir temperature. It is shown that the combination of underground coal gasification and coal seam gas production has the potential to develop a new technology that not only reduces the dewatering operation but also enhances gas recovery. Results show that thermal recovery can significantly increase gas desorption and diffusion in coal seam gas reservoirs. Thermal treatment causes higher desorption rate and ultimate recovery factor, lower water production and higher wellhead pressure.