Coal and gas outburst mechanism: Research progress and prospect in China over the past 70 years

The effective prevention and control of coal and gas outbursts have always been a challenge for safe coal mining in China, and their prerequisite and basis are in-depth research on the coal and gas outburst mechanism. This study expatiated the efforts and achievements of China made in research on the coal and gas outburst mechanism over the past 70 years, which can be divided into four stages, namely active exploration (1955‒1977), the establishment of the theoretical foundation (1978‒2002), rapid development (2003‒2015), and stable development (2016 to the present). This study explored the hot research topics and frontier trends of the coal and gas outburst mechanism based on the bibliometric method and the analysis of knowledge graphs. Moreover, this study systematically summarized the research progress in the coal and gas outburst mechanism in China from the perspective of theoretical hypothesis, physical simulation, and numerical simulation. Regarding theoretical hypothesis, new viewpoints and theories represented by the rheological hypothesis, the spherical shell instability theory, and the mechanistic effect hypothesis have been formed based on the hypothesis of comprehensive effects and the actual conditions of coal mines in China, laying a foundation for the theoretical study of the coal and gas outburst mechanism. Regarding physical simulation, extensive experimental studies have been conducted focusing on the major factors controlling coal and gas outbursts, the evolution of multi-physical fields, energy conversion, instability conditions, geological structures and tectonically deformed coal, and disaster-causing mechanism of coal-gas two-phase flow. As a result, the occurrence conditions, development process, evolutionary patterns, and influencing factors of coal and gas outbursts have been roughly determined. Regarding numerical simulation, simulation studies have been carried out on coal seams suffering coal and gas outbursts, geological structures, and the coal-gas two-phase flow in coal and gas outbursts. However, the simulation analysis of the whole process of coal and gas outbursts is yet to be achieved due to the complex process and influencing factors of the outbursts. The theoretical system of coal and gas outbursts with Chinese characteristics has been initially formed. However, frequent coal and gas outbursts still indicate the severe situation of the prevention and control of the outbursts. By combining the study status and development trend, this study proposed further studying the whole-process, total-factor, and full-spatio-temporal coupling evolutionary process of coal and gas outbursts, analyzing the multi-scale rheological behavior of gas-bearing coal, and exploring the energy storage and dynamic release processes of coal seams. Moreover, it is necessary to reveal the forming mechanisms of complex and secondary disasters and, accordingly, to determine the coupling evolution mechanism, mechanical models, energy models, and catastrophe models of coal and gas outbursts. On this basis, it is suggested to carry out physical simulation experiments and whole-process numerical simulations and inversion of typical coal and gas outbursts under complex strata and to achieve real-time monitoring and continuous warning of the outbursts.