Insights into ground response during underground coal gasification through thermo‐mechanical modeling

Abstract

This paper presents a coupled thermo-mechanical model to investigate the ground response during underground coal gasification (UCG). The model incorporated the temporal and spatial development of temperature, the gradual growth of the cavity, and temperature-dependent material properties. Model verification was made against two benchmarks to acquire the confidence for the predictive purpose. The first exercise demonstrated the correctness of the model implemented in COMPASS. The second exercise showed that using the ash-filled cavity to represent null or empty zones is a good option in the numerical modeling and provided highly comparable results to other models. Based on the Hanna UCG trial, different cases were simulated to investigate the effects of the cavity size in the coal seam and the thermal expansion coefficient of the caprock and base rock on key features that take place during the process of UCG. A maximum temperature in the range of 1200-1500 °C was induced by the gasification of coal, and a cavity with a maximum length of 13.5 m was formed after 30 days of simulation. Meanwhile, small vertical displacement in the range of -5-12 mm took place near the cavity because of the thermal expansion of the geologic materials and the reduction of the overall weight with the creation of the cavity. In addition, it was found the thermal expansion coefficients can influence the thermo-mechanical response of geologic materials, but the effects were insignificant when its order of magnitude was smaller than 10 -6 K -1 .

Thermo-Mechanical Numerical Analysis of Stress and Damage Distribution within the Surrounding Rock of Underground Coal Gasification Panels

“…Gao et al also conducted thermo-mechanical modeling of the underground coal gasification process to gain insights into the induced ground response, and the effects of cavity size and thermal expansion coefficients of the surrounding rocks were reported. It was found that the maximum deformation of the caprock and base rock were small [26]. Considering the possible risk of groundwater pollution, Otto and Kempka implemented thermo-mechanical simulations of rock behavior in UCG, with an emphasis on the damage-induced permeability enhancement of the surround rocks [27].…”