Study on the Periodic Collapse of Suspended Sandstone Interlayer under Coupled Thermo–Hydro–Mechanical Environment

Abstract

In the process of in situ heat injection mining of coal seams, a circular combustion chamber will gradually be formed. However, for the interlayer coal seam, the interlayer will gradually suspend in the combustion chamber. When the critical distance is reached, the initial collapse and periodic collapse will occur, which will affect the flow field distribution and extraction efficiency of combustion gas. In order to study the initial and periodic collapse steps of interlayer in real-time high temperature and high-pressure thermo–hydro–mechanical (THM) coupling environments, sandstone specimens were collected from the field, and the macroscopic mechanical parameters of sandstone interlayer were tested using a self-developed thermo–hydro–mechanical coupling tester. The mechanical parameters (e.g., triaxial compressive strength, elastic modulus, Poisson’s ratio) of sandstone under different stress conditions were obtained. Then, based on the elastic thin plate theory, the mechanical modeling of the annular sandstone interlayer is carried out and the preceding mechanical parameters are brought into the generated analysis equation. Thus, the periodic collapse step of the suspended circular sandstone interlayer is obtained. The study results show that: (1) the macroscopic physicomechanical parameters of the sandstone interlayer do not vary monotonically with temperature—a slight increase in macroscopic parameters occurs at 400°C; and (2) the periodic caving pace of the sandstone interlayer decreases with the increase in the number of collapses. With the increase of temperature, the periodic caving pace of the sandstone interlayer decreases first, then increases, and then decreases. It gradually decreases with the increase of surrounding pressure. The research results of this paper can provide theoretical guidance and technical support for in situ heat injection mining technology.