Numerical simulation of surface subsidence after UCG including groundwater effect

Underground Coal Gasification (UCG) is a method that allows for the extraction of energy from hard to access coal seams (i.e. very thin or deep underground). One of the greatest advantages of UCG is that it eliminates the need to put humans underground, thereby reducing the risk of injury and fatality, which are significant concerns during conventional underground coal excavations. UCG currently has two main hurdles that are affecting its potential industrialization: the risk of surface subsidence and groundwater pollution. The existence of groundwater is particularly a challenge because of implications to both pollution and its influence on mechanical processes underground. Two mechanical effects of groundwater should be considered: changes in effective stress and in volumetric strain. Additionally, at any UCG site, the influence of groundwater should be analyzed together with the thermal impact of coal combustion. To couple the fluid, thermal and mechanical analyses, the commercial software FLAC3D by Itasca is utilized. The modelling results are compared with the field measurements at the Shatsk UCG station in the Moscow Basin. The site at hand is complicated by six aquifers in the overburden above the UCG reactor. The relative performance of the models with and without groundwater and thermal effects is evaluated based on predictions of the surface subsidence. The groundwater increases the depth and narrows the width of surface subsidence closing the modelling results to the measurements, whereas the high-temperature decreases the depth without changing the shape of the surface subsidence.