Development of a multi-field coupled numerical simulation program for underground coal gasification and multi-field evolution laws

A numerical simulation procedure and method for underground coal gasification under the action of multi-field coupling has been developed, and the multi-field coupled thermal-force-chemical-displacement calculations are carried out simultaneously based on three subroutines DFLUX, HETVAL and USDFLD in ABAQUS software. Taking the first gasification working face of No.12 coal seam of the Shanjiaoshu coal mine in Panzhou, Guizhou Province as the engineering background, through this numerical simulation method, the simulation of heating the coal body by the ignition device at the initial stage of underground coal gasification is realized. The simulation of chemical heat change triggered by the spontaneous combustion of coal body under the continuous backward movement of the injection point is achieved. Also, the simulation of cavity formation after the coal gasification reaction is accomplished. Finally, the evolution law of temperature field-stress field-displacement field after the gasification reaction of No.12 coal seam of the Shanjiaoshu coal is analyzed. The results show that the overall trend of temperature field evolution increases first and then decreases, reaching the peak temperature in turn with the backward movement of the gasification point. The temperature conduction range of the overlying rock layer gradually expands under the influence of heat conduction. After 120 days of gasification, the overall temperature conduction range is similar to the teardrop shape, and there is a slight delay in the temperature change of the overlying rock layer and a gradual decrease in temperature as the height increases. After 120 days of gasification, the tensile stress area of the overlying rock layer is approximately “concave” in shape while that of the underlying rock layer is approximately “T” in shape. The stress evolution pattern of rock layers at different heights varies greatly, whereby the rock layers at the intersection of the combustion void area and the direct top are affected by the formation of cavity, and the stress concentration phenomenon occurs gradually as the gasification working face progresses. The overlying rock layer sinks as a whole, with the settlement amount increasing and then decreasing before stabilizing. The deformation of the rock layer decreases with the increase of height, while the overlying rock layer is thermally expanded under the influence of high temperature, which generates thermal stress to support the upward movement of the rock layer so that the settlement amount decreases. It can be inferred that the thermal stress generated by the high temperature environment can hinder the sinking of the overlying rock layer to a certain extent in the actual operation of large coal underground gasification projects. The present study aims to be closer to the actual working conditions of coal gasification, and the research results provide a new method to simulate coal underground gasification more practically.