Numerical investigations to assess ground subsidence and fault reactivation during underground coal gasification

This paper aims to assess potential environmental impacts associated with commercial-scale application of in situ coal conversion in a target area in Hungary. The site is an environmentally protected forested area. Parametric numerical modelling techniques were employed using a discrete element method (DEM) to evaluate the surface subsidence and fault activation during mining processes. The Mohr–Coulomb elastic–plastic material model adopted to simulate rock formations. Zero-thickness interfaces with friction and cohesive characteristics were employed to simulate geological faults. A sensitivity study on rock formations and fault properties was conducted to address the importance of geological parameters that have an impact on surface subsidence as well as fault activation, which could in turn result in pollutant migration from the deep coal seams to the surface. The analysis of the results demonstrated that surface subsidence is affected by the average Young's modulus of the geological strata, whereby the activation of the faults is influenced by the friction angle between faults. Also, it was found that shallower seams are more likely to produce surface subsidence; i.e. as excavation depth increases, the surface subsidence decreases. Finally, computational outputs from this work were used to develop the web-based and interactive Environmental Hazards and Risk Management Toolkit (EHRM) for planning and decision-making processes during in situ coal conversion.