Research on variation law of coal seam temperature field with different borehole heat injection timing

Analyzing and evaluating the dynamic evolution law of coal seam temperature field with different borehole heat injection timing has important theoretical significance for optimizing the thermal injection and gas extraction process and plan, and further improving the efficiency of thermal injection and gas extraction in coal mines. Based on the theories of seepage mechanics and heat transfer, this paper establishes a coupled numerical calculation model of flow and heat, and conducts numerical simulation research on the change law of coal seam temperature field under the conditions of intermittent heat injection in a single borehole and alternate heat injection in adjacent boreholes. The research shows that the coal seam temperature field presents a single peak shape in the process of intermittent heat injection in a single borehole. When the total duration of heat injection is the same, within a certain range around the borehole, the longer the intermittent heat injection cycle, the lower the average temperature of the coal body near the borehole, and the weaker the non-uniformity of the temperature field. When adjacent boreholes inject heat alternately, the temperature field of the coal seam exhibits a bimodal pattern. When the total duration of heat injection is the same, within a certain range around the borehole, the longer the alternating heat injection cycle, the lower the average temperature of the coal body near the borehole, and the weaker the non-uniformity of the temperature field. Within a certain range near the borehole, when the alternating heat injection period is the same, the larger the borehole spacing, the higher the coal temperature, and the stronger the non-uniformity of the temperature field. The heating efficiency of coal seams for different heat injection methods and drilling arrangements is intermittent heat injection between single drilling holes<alternating heat injection between adjacent drilling holes of 2 m hole spacing<alternating heat injection between adjacent drilling holes of 4 m hole spacing. The research results are of great significance for the optimization and efficiency improvement of gas injection mining technology.