Numerical simulations of fracture propagation in overlying strata for deep underground coal gasification using controlled retraction injection point technology

Underground coal gasification (UCG) enables the in situ clean conversion of coal seams, making it highly suitable for the development and utilization of deep coal seams. This study focused on the UCG of deep coal of the Xishanyao Formation in the Baikouquan area of Xinjiang, China. A multi-field coupled evolution model based on the finite element method was established for deep UCG to simulate the propagation of induced fractures and multi-physical fields during UCG. The following conclusions were obtained: (1) At burnout distances of 200, 500, and 1000 m, the maximum temperature influence ranges were 20, 23.75, and 25 m, respectively. (2) When the burnout distance reached 200 m, no natural fractures propagated due to the high stress of the deep overlying strata. However, when the burnout distances increased to 500 and 1000 m, the maximum heights of the propagated fractures reached 232.5 and 270 m, with average heights of 157.5 and 172.5 m, respectively. (3) At burnout distances of 200, 500, and 1000 m, the maximum subsidence displacements in the roof rock were 1.68, 5.60, and 17.40 m, respectively. (4) In the actual design process of UCG projects, excessively long gasification channels should be avoided to prevent interaction between aquifers and the UCG cavity, which could lead to the failure of the UCG project. Results of this paper contribute to controlling the scope of the temperature field in practical UCG projects, and predicting the propagation heights of induced fractures as well as the displacement characteristics of the overlying strata. With these as references, a more reasonable UCG scheme considering the scope of induced fractures and stress concentration can be designed, which helps avoid groundwater influx risks and improve the stability of UCG project.