Thermal-mechanical-numerical analysis of stress distribution in the vicinity of underground coal gasification (UCG) panels

During underground coal gasification (UCG), whereby coal is converted to syngas in situ, a panel (cavity) is formed in the coal seam. Unlike other underground mining methods, the coal and country rocks in the vicinity of a UCG panel are subjected to high temperatures which may be in excess of 1000 °C. UCG operation imposes significant geomechanical changes to the strata. In order to understand these changes, numerical modeling can provide a comprehensive and qualitative understanding of the UCG process. In this paper, a new approach, namely, Underground Coal Gasification-Pillar Stability Analysis (UCG-PSA) is presented to estimate the protection pillar width in UCG based on the Controlled Retraction Injection Point (CRIP) configuration. The UCG-PSA approach considers development load, side abutment load, front abutment load, thermal stress along with cavity syngas pressure as a lateral pressure to determine the protection pillar width. Furthermore, the role of the crushed zone around a pillar was considered in the estimation of the pillar strength. A new 3D Thermal-Mechanical (TM) model is developed for UCG based on the CRIP configuration to predict stress distribution in the vicinity of the UCG panel and to verify the UCG-PSA approach results using the FLAC3D software. In this model, a panel is subjected to high syngas pressure and high temperature. Moreover, after each step of gasification the modeling of caved material is performed. The results of the TM model compared with the Mechanical (M) model show that the stress distribution increased at the front of the face and pillar edge due to the thermal stress. In addition, the results show that the maximum side abutment stress at a depth of 600 m is 63.1 MPa and applied at a distance about 3.5 m away from the pillar edge while in the M model it is 59.2 MPa. The proposed methodology could provide a consistent and simple way for the stability analysis of pillars in UCG sites.