Numerical study of ground deformation during underground coal gasification through coupled flow-geomechanical modelling

This paper presents a coupled flow-geomechanical model to study the ground deformation due to geoenergy applications involving complex thermo-hydraulic-chemical-mechanical coupled processes, such as underground coal gasification (UCG). The model is developed by coupling two sub-models, that is, the chemical reactions associated flow model that can predict temperature development and cavity growth and the geomechanical model that considers the temperature-dependent thermo-mechanical properties of geologic materials and the conversion of coal to the cavity. Numerical simulations of a field-scale UCG project with the linked vertical wells (LVW) were conducted through coupled flow-geomechanical modelling. The effects of the thermo-mechanical properties of the strata adjacent to the coal seam and the UCG operational conditions on the temperature distribution, cavity formation, and ground deformation in the operating stage of UCG were studied. Simulated results showed that temperature of over 1200 K would be obtained between the injection well and the production well in the coal seam while the 400 K isotherm only moved less than 2 m vertically in the strata near the UCG reactor. Cavity with long forward length and short backward length was formed between the injection well and the production well. Ground deformation during UCG was caused due to the combined effects of temperature and cavity evolution. The parametric sensitivity study showed that when the initial elastic modulus of the surrounding strata decreased from 4 GPa to 1 GPa, its effect on the ground deformation beyond the right side of the production well was not of significance. The ground deformation was not sensitive when the initial thermal expansion coefficient of the surrounding strata was reduced to the order of magnitude of 1.0(-6) (K-1). In addition, adjusting the supply rate of O-2 and modifying the location of the horizontal gasification channel can control the ground deformation during UCG. The coupled flow-geomechanical model presented in this paper can be helpful in the safety control of UCG.