Effect of various coal constitutive models on coupled thermo-mechanical modeling of underground coal gasification

Abstract

Underground coal gasification (UCG) is an advanced method that can address the high energy demand in future by adding new resources to the proved reserves. The high temperature nature of the gasification reactions along with the formation of cavities within a coal seam necessitates a comprehensive analysis of mechanical and thermomechanical impacts on the UCG process. On the other hand, geomechanics plays a significant role in the efficiency of the UCG process by varying the reaction rates, porosity, and permeability in a flow model. More importantly, geomechanical modeling of UCG is necessary to mitigate the associated environmental hazards such as land subsidence and groundwater contamination.In this study, the results from a reservoir simulator are coupled with a geomechanical module to solve for the displacement and stress variations. A controlled retracting injection point (CRIP) method is applied in the reservoir simulator to model reactions and geochemistry. By performing sensitivity analyses using the linear elastic, hyperbolic, and elasto-plastic constitutive models, the two compelling factors of high temperature and cavity evolution, which have the highest contributions to the geomechanical responses of the UCG process, are comprehensively investigated. At the end, a stress rebalancing technique is imposed on the linear elastic model to mimic the elasto-plastic solution by capturing the shear softening and the post-failure behavior of coal.