Coupled Numerical Simulation of Coal Failure Caused by Thermal Impacting in Underground Coal Gasification Zone

ABSTRACT: Underground Coal Gasification (UCG) is an environmentally friendly way to utilize coal resources. The cavity evolution process, especially the thermal impact on coal rock, is an important topic of UCG. The thermally induced coal rock failure processes in gasification environment are rarely investigated, but they are important to the gas production space and the flow from the injection well to the production well. In this study, a coupled numerical simulation program based on PFC3D and FLAC3D was established to realize multi-field coupled thermal impact conditions. The numerical model of burning-induced coal surface failure was established, and the basic cavity evolution mechanism was revealed. The main results show that (1) the redistributed temperature field in the coal rock is the most important parameter for the coupling model. The mechanical properties of the model element are assigned in accordance with its temperature state. (2) Taking into account the effects of heating, microscopic fracture and deformation are generated through PFC3D model, inhomogeneous temperature distribution is simultaneously achieved through FLAC3D model. (3) The evolution law of the expansion of the gasification cavity has been obtained by the coupled numerical simulation. (4) Eventually, the height of the gasification cavity increases, and the upper coal seams expand to an approximate equilibrium, the height affected by the temperature field becomes stable, and the vertical expansion rate of the gasification cavity gradually decreases. 1. INTRODUCTION Underground Coal Gasification (UCG) is a revolutionary technology for clean coal utilization that integrates the three key processes of well construction, coal mining and gasification for efficient gas production. This technology allows coal to undergo controlled in-situ combustion, pyrolysis and gasification reactions to produce syngas with methane, hydrogen, carbon monoxide, carbon dioxide and other components for surface recovery by injecting air, oxygen-enriched air, oxygen/water and other gasification agents into underground coal seams[1–4]. Currently, the underground gasification of middle-deep coal seams has no practical experience, especially for the dynamic study of the deforming and destroying process of the surrounding stone (coal) in the gasification cavity under high temperature and high pressure. The numerical simulation technology of gasification cavity development dynamics is still in the initial stage. The simulation technology of gasification cavity controllable development control is not deep enough, and the analysis of the effect of dynamic process parameters in the process of cavity formation is still insufficient.