Calculation method of safe width of gasification cavity for medium-deep underground coal gasification

Underground coal gasification is currently the highest-temperature (over 1200 ℃) unconventional development method for fossil energy. Underground gasification of coal in the medium-deep (refer to the depth of 800～1500 m in this paper) has obvious advantages in improving the gasification pressure and reducing the geological safety risks. Scientific prediction of safe width of gasification cavity is important to ensure stable gasification operation. Since the method of calculating the safe width of gasification cavity based on Controlled Retreat Injection Point (CRIP) process has not yet been established, targeted research is needed to ensure the smooth implementation of field tests. Once the top plate of the gasification cavity is “exposed” to high temperature behind the gasification cavity. The generation position of thermal stresses inside the rock and the influence law of differences in the thermal expansion coefficient of particles and matrix on the magnitude of thermal stresses were studied by numerical simulation under compressive stress constraints. the thermal damage mechanism of rock under high temperature was clarified combined with the scanning electron microscope results of rock after high temperature treatment. According to the characteristics of CRIP gasification process, a thin plate model of gasification cavity roof considering the influence of high temperature was established, and a method for calculating the safe width of the gasification cavity was proposed in combination with the theory of “key layer”. The study revealed that, rock thermal damage was caused by the interaction and synergy of rock physicochemical reactions and thermal stress, and the rock undergone irregular deformation at high temperatures. Microstructural changes of rock caused by the thermal damage were the fundamental cause of changes in rock mechanical and physical properties. The maximum tensile thermal stress in rock occurred at the grain interfaces or in grains with small coefficient of thermal expansion. The maximum tensile thermal stress increased rapidly with decreasing the thermal expansion coefficient of the grain when the ratio of the thermal expansion coefficient of the grain to that of the matrix is in the range of 0.01−1). The microcracks of mudstone was developed when heated to 200 ℃, the crack development was more obvious when heated to 400 ℃, which mainly along the edge of the particles rupture. The number of cracks was increased and the size was larger when heated to 600−800 ℃. The larger cracks and numerous pores were formed when heated to 1000 ℃. The crack connectivity increased significantly at 1200 °C, and the stomatal development was larger. Due to the effect of high temperature, the step constant for the thin plate model is no longer a constant, and specific values need to be determined based on the extent of thermal damage to the roof of gasification cavity and the location of the hard rock layer on the top plate. The safe width of gasification cavity was affected by temperature. In the study, the calculated safe widths of sandstone roof at 35 and 1000 °C were 34.3 m and 14.1 m, respectively, with a difference of 58.9%. while the safe widths at 35 and 1000 °C were 16.7 m and 15.9 m, respectively, with a 4.8% difference. Lastly, a method of determining the longitudinal target area of coal seam was proposed from the perspective of reducing the risk of roof collapse and improving gasification control. When the coal seam thickness exceeded half of the gasification cavity safe width, it was suggested to design the longitudinal target area of horizontal well at a location not more than half of the safe width of gasification cavity from the top of the coal.