A coupled thermal-mechanical numerical model of underground coal gasification (UCG) including spontaneous coal combustion and its effects

Underground coal gasification (UCG) is a promising option for extracting energy from coal in unworked or hard to access areas of the subsurface. From a geotechnical perspective, UCG involves various complex phenomena resulting from the elevated temperatures induced within the rock surrounding the UCG burn. This paper presents a coupled thermal-mechanical numerical model developed to represent a UCG trial in Wieczorek, Poland. Temperature dependent mechanical properties were assigned according to results obtained from laboratory experiments and data available in the literature. The coal burning process was simulated by modifying the energy balance equation with an additional term related to the calorific value of coal as a source. This source term was described using a time decay function to reflect the fact that the energy release from coal gradually decreases with time. The mechanical degradation of coal due to burning was simulated by removing the burned zone from the calculation after a specific time, which depended on zone size and type of coal. In this study, it was found that the maximum temperature at the burning zone was always < 1000 degrees C, which agrees with previous research carried out for other UCG trials. The size of the burning zone was predicted to spread about 15 m laterally after 20 days of burning. Ground subsidence was evaluated for single and multiple (parallel) panel simulations; subsidence at the top of the numerical mesh, corresponding to a depth of 395 m below the surface, ranged from 23 mm for a single panel to 85 mm for seven panels. The degradation of mechanical properties of the rock surrounding the burned zone due to heating was found to have a marginal effect on the ground subsidence when parallel burning was carried out. The numerical modelling results obtained from this study may provide guidance for the design and operation of UCG processes.