Combined Energy Recovery from Coal Seam Gas Reservoirs and Geothermal Resources (Simulation Study)

Abstract

Gas production from Australian Coal Seam Gas (CSG) reservoirs can be coupled by hot dry rocks (HDR) or wet geothermal resources. This study investigates the feasibility of heating the coal seams by application of available geothermal resources. Hot water or steam from geothermal resources can be circulated along a coal seam to increase the reservoir temperature. Since methane desorption from coal matrix is an endothermic process, coal adsorption affinity reduces significantly at the elevated temperatures. Therefore, when coal seam is heated, thermal equilibrium of coal is disturbed and consequently the pressure at which gas desorption starts increases. This pressure increase results in shortening of dewatering stage and also higher gas recovery from CSG reservoirs. A simulation study is carried out on Tiffany field, San Juan Basin, US, as a well developed CSG case to investigate the effect of temperature on gas recovery. Tiffany field is located in Southern Colorado, with very low permeability of 5 mD. The simulation has been examined at a constant abandonment pressure with varying reservoir temperature. Langmuir properties of the coal at higher temperatures are estimated from the reservoir temperature by Arrhenius equation. Primary gas recovery factor in absence of thermal treatment is calculated to be 28.6%. When the reservoir temperature is increased by 38 degrees of Celsius, the total gas recovery factor is raised to 44.8% showing 16.2% gain. This temperature increase reduces the dewatering time from 180 days to only 8 days. Simulation results suggest that temperature increase by 38 degrees of Celsius can extend the life of the coal seam gas reservoir for approximately 10 more years. In addition, total water production was reduced by 14%.