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Modelling surface subsidence during underground coal gasification

Underground coal gasification (UCG) is an alternative method of extracting energy from coal whereby the coal is burnt within an in situ UCG reactor. The method has been established for almost a century, but it has not been widely used despite its advantages, which include the circumvention of underground human presence and the possibility to work with low quality coal that is deep underground. One of the main difficulties associated with the implementation of UCG on an industrial scale is the prediction of surface subsidence, which is required to assess potential damage to surface infrastructure, UCG equipment, and boreholes. This work considers the numerical modelling of surface subsidence during UCG. For this, the finite difference numerical modelling software FLAC3D by Itasca is utilized. Historically, this tool has been used for modelling surface settlement caused by traditional coal mining activities. The mechanism of surface subsidence during conventional coal mining and UCG are almost identical; however, the UCG reactor has some distinguishing features, for example, thermal strains and the resulting altered mechanical properties of the soil-rock. In this work, firstly, a thermal analysis is run to impose the thermal fields. Secondly, the engineering properties relationship with temperature is implemented in the model. Finally, model results are compared with field observations and discussed. The conclusion is drawn that updating the mechanical properties, i.e. elastic stiffness, friction angle and cohesion, in correlation with the elevated temperatures improves the surface subsidence predictions.