Feasibility Study of Thermally Enhanced Gas Recovery of Coal Seam Gas Reservoirs Using Geothermal Resources

Abstract

Thermally enhanced gas recovery of coal seam gas reservoirs is proposed as a new enhanced recovery technique. The endothermic nature of desorption process facilitates methane desorption of coal surface at elevated temperatures. Any thermal treatment of coal seam assists bond breakdown between methane molecules and coal surface and changes the sorption properties of the coal while the initial gas content of the coal is constant. It results in higher critical desorption pressure and higher gas recovery. The higher critical desorption pressure advances gas production and consequently shortens the dewatering stage. Coal seam gas production can be coupled with extensive underlying geothermal resources where a vast amount of hot water is available to thermally treat the coal seams. To investigate the feasibility of thermally enhanced gas recovery, the reservoir thermal simulator (CMG-STARS) and the coal seam gas simulator (CMG-GEM) were linked together. An inverted five spot pattern with a hot water injector well and four discharge wells was created to simulate hot water injection into the coal seam. When the injection phase was finished, the new reservoir temperature distribution from thermal simulator was imported into the coal bed methane simulator. Coal seam gas production was simulated at elevated reservoir temperatures while the seam temperature was updated every 2 years to account for the effect of heat loss to the neighboring formations. Hot water injection (80 °C) for a period of 2 years into the coal seam with an area of 40 acres successfully increases average reservoir temperature by 30 °C. Thermal treatment dramatically increases gas recovery by 58% during 12 years of production compared with conventional production. During thermally enhanced gas recovery, peak gas rate is almost 6.8 times higher than conventional recovery. In addition, the dewatering time, which was previously 9 months for conventional production, is eliminated when coal is thermally treated prior to gas production.