Modelling the local environmental impact of underground coal gasification

Underground coal gasification (UCG) has the potential to access vast resources of stored fossil energy in a safe, clean and environmentally sound manner. Previous experiments have however led to concerns around surface subsidence, groundwater pollution and water table lowering. These issues can be prevented through the use of appropriate site selection and an understanding of the processes which cause these effects. Numerical simulations provide a cost effective means of predicting these issues without the need for costly and publically opposed field trials.\ud \ud This work uses a commercially available discrete element code to simulate the coupled thermal, hydraulic and mechanical phenomena which cause environmental damage. Surface subsidence is predicted through the displacements of fully deformable discrete elements separated by a network of fractures. The flow of groundwater through these fractures is simulated in order to predict the effects of water table lowering and the inflow of groundwater into the UCG cavity. Heat conduction from the cavity walls is simulated using an explicit finite difference algorithm which predicts both thermal expansion effects and the influence of temperature on rock material properties.\ud \ud Comparison of results with experimental observations in the literature show good agreement for subsidence and groundwater behaviour, while initial predictions for a range of designs show clear relationships between environmental effects and operating conditions. Additional work is suggested to incorporate groundwater contaminant transport effects, and it is envisioned that the overall model will provide a valuable screening tool for the selection of appropriate site designs for the future development of UCG as an economically viable and environmentally sound source of energy