Coupled thermal–oxygen–methane field evolution and coordinated prevention strategy in goaf under surface borehole drainage

Abstract

This study examines the multi-physics coupling effects and spontaneous combustion risks in the goaf of high-gas and combustion-prone coal seams under surface borehole drainage. Field monitoring of the spontaneous combustion “three-zone” structure, SF₆ tracer gas tests, and multi-field coupled simulations were conducted to analyze the evolution of flow, gas migration, and thermal fields, and to design a collaborative drainage–nitrogen injection strategy. Results show that surface borehole drainage establishes negative-pressure-driven seepage channels, enabling oxygen intrusion along the strike direction and promoting oxidation zone expansion toward the return-air side and deep regions, forming a high-risk pattern of “multi-directional leakage–deep activation.” While drainage reduces methane concentration, it also enhances oxygen supply and accelerates coal oxidation, enlarging high-temperature areas and increasing spontaneous combustion risk. Among the tested scenarios, Scheme III (three boreholes at 50 m, 130 m, and 210 m with graded suction of 5, 10, and 20 kPa) optimally balances methane drainage and oxidation suppression, achieving collaborative control of “efficient drainage–low-oxygen activation.” Building on this, a coordinated drainage–segmented nitrogen injection strategy (1000 m³/h at 70 m on the intake side and 500 m³/h at 110 m on the return side) reduced the oxidation zone by 80.8 %, restrained high-temperature development, and increased total methane extraction by 20 % compared with drainage alone. These results highlight a synergistic mechanism of “flow promotion–permeability enhancement–oxidation suppression,” offering a scientific and practical pathway for integrated fire prevention and gas control in goaf environments of high-gas, spontaneous combustion-prone coal seams.