Thermo-fluid–mechanical numerical simulations of surface subsidence at the site of underground coal gasification

Abstract

This work deals with modelling surface subsidence that aims to help industrialize underground coal gasification (UCG). UCG is a long-known, but poorly industrialized, method of energy extraction from coal. Risks of surface subsidence and groundwater pollution are two main hurdles that are affecting the potential industrialization of UCG. The particular challenge is the existence of groundwater because of implications to both its pollution and its influence on surface subsidence. Additionally, the coal combustion and the complex geometry of the UCG reactors impacts surface subsidence. To meet these challenges, the thermal and fluid analyses should be included in the model, and surface subsidence should be modelled in three dimensions to capture the collapsed shape of the UCG reactor. Based on the nature of these challenges and an earlier successful implementation, Itasca's commercial FLAC3D software with intrinsic thermal and fluid models was chosen to model surface subsidence. This study determines that the inclusion of fluid analysis improves the predictions of surface subsidence when compared with measurements at the highly watered Shatsk UCG site. In turn, thermal analysis mildly influences the modelled surface subsidence. The fluid analysis shows that the flow in the upper aquifer influences surface subsidence to a greater extent than the flow in the lower aquifers. High temperatures cause an upward flow in the lower aquifer located above the UCG reactor but does not change the flow pattern in the upper aquifer. The fluid analysis also reveals that if the UCG reactor is filled with water, surface subsidence does not occur.