Simulation of underground coal gasification (UCG) is an integrated approach involving a Thermo-Hydro-Chemical-Mechanical process (THCM). The key is to understand the surface subsidence in the UCG process associated with the cavity growth and roof rock collapse during the coal seam combustion. However, the interaction between the mass transports and thermal-mechanical induced cavity and spalling cannot be fully captured by a conventional flow simulator without coupling geomechanics.
The literature has documented numerous methods of modeling the coupled Thermal-Chemical-Geomechanical process for UCG. The objective of this paper is to summarize the current research status and future developments of coupled geomechanical modeling approach for the UCG process, followed by a case study. Geochemistry and chemical reactions are modeled in the thermal reservoir simulator with a well-defined Controlled Retraction Injection Point (CRIP) configuration. A modular coupling approach is utilized in which geomechanical module calculates the changing of the stress and strain due to the changing of pressure and temperature, updating the porosity and permeability simultaneously. Finally, surface subsidence is investigated and limitations are also identified for future development.