A numerical investigation into the environmental impact of underground coal gasification technology based on a coupled thermal-hydro-chemical model

This study aims to improve the understanding of the potential environmental impact of the underground coal gasification (UCG) process based on a numerical investigation of the gaseous and dissolved chemicals’ transport in the strata surrounding the UCG reactor. A coupled thermal-hydro-chemical (THC) model in the framework of COMPASS code is employed and further developed for this purpose. The model is compared against analytical solutions in the verification process and applied against a former UCG trial (Hoe Creek UCG programme) for validation purpose. The numerical model is then applied to investigate the potential environmental impact of UCG in three different geological materials (sandstone, coal, and shale) in two steps: the potential gas propagation during UCG process and the solute transport in UCG decommissioning stage. The results indicate that the gas propagation is limited to a near-cavity area in geological strata with low permeability and high air entry value such as shale, which is mainly attributed to the dry zone generated by the high temperature, while a larger gas plume is observed for materials with poor water retention characteristics, such as coal and sandstone. Besides, the gas propagation is retarded to some extent due to the reducing effect of high pressure on gas diffusion. The propagation of dissolved chemicals via diffusion is limited to near-cavity areas, i.e. 2 m, in three studied materials after 10 years. Based on the improved understanding of the various chemicals’ migration in different materials, by selecting the target coal seam surrounded by strata layers with high air entry value and low permeability, the gas losses from the reactor during the UCG reactor operation stage can be minimized and the potential negative environmental impact of ash leaching and solute propagation in UCG decommissioning stage can be avoided. This work also provides further insight and suggestions into the importance of investigating the adsorption capacities of different geological media to evaluate their performance as a natural cleaning system