Background To curb coal mine gas accidents from sources, exploit low-carbon and clean coalbed methane (CBM) resources, and protect the atmospheric environment (achieved by methane emission reduction in coal mining areas), the authors’ team, by working with Shanxi Jincheng Anthracite Mining Group and cooperating closely with relevant research institutes, colleges, and universities, has progressively developed theories with clear concepts and scientific connotations, as well as relevant technical systems and industrial paradigms, for coal and CBM co-production through over two decades of original, integrated, and cross-industry innovations (spanning across fluid and solid mineral industries), along with theoretical exploration, technological research and development, and engineering practice. The mining theories, technical systems, and industrial paradigms of the coal and CBM co-production have undergone constant development. Influenced by national demands, economic conditions, industrial base, scientific and technological levels, professional talent, and cognitive level, among others, coal and CBM co-production has experienced several major development stages sequentially: (1) coal mining without gas extraction, (2) gas drainage followed by coal mining, and (3) integrated gas extraction and coal mining, with the former followed by the latter. This study provides a systematic summary of achievements obtained through over two decades of exploration and practices.

Advances  Primary innovations and advances are introduced as follows. The purpose of the coal and CBM co-production has been clarified. Specifically, through the full-time extraction of CBM across all vertical horizons throughout a coal mining area, the coal and CBM co-production is aimed at exploiting both coal (solid) and CBM (fluid)—two paragenetic or associated mineral resource types that are cogenetic, contemporaneous, and hosted by the same geological structure in a safe, efficient, and collaborative manner. This will help curb coal mine gas accidents from sources and practically gain multiple benefits including safety, resource utilization, and environmental protection. The concept of the coal and CBM co-production has been established as integrated gas extraction and coal mining, with the former followed by the latter. This means that the spatiotemporal cooperation between gas extraction, roadway tunneling, and coal mining should be planned and achieved based on the full lifecycle of the coal and CBM co-production. The core nodes of the collaborative co-production have been determined, referring to several key thresholds for safe production in coal mines, which consist of the maximum permissible gas content and gas pressure for safe tunneling and production. A technical logic and industrial paradigm of CBM extraction have been summarized, defined as surface-underground combined full-time gas extraction integrating four zones (i.e., planning, preparation, mining, and mining-affected or goaf zones) across all vertical horizons throughout a mining area. A technical system for CBM extraction has been innovatively developed, encompassing large-scale surface gas extraction in the planning zone of a coal mine (through commercial CBM development or advance surface pre-drainage of coal mine gas), surface-underground combined gas drainage in the preparation zone, large-scale and precise underground gas drainage in the mining zone, and surface or underground gas drainage in the mining-affected or goaf zone through pressure relief (CBM drainage in a mining-affected zone using an underground high-level borehole group is referred to as the technology of replacing roadways with boreholes). Significant theoretical innovations include the theory on two fluid seepage regimes in coal reservoirs (non-Darcy flow in coal matrix pores and Darcy flow in fractures and large and medium pores), the theory on multi-source gas gush in a mining-affected zone, and the disturbance effect of mining activities on the physical properties of CBM reservoirs (causing CBM reservoirs in different zones to occur in original, disturbed, and semi-open states).

Prospects  Based on the authors’ exploration and engineering practices in the in situ fluidized mining of coal and gas in recent years, this study proposes that in the future, the coal and CBM co-production should focus on the in situ fluidized mining of coal and CBM, involving physical, chemical, biological, and thermodynamic technical pathways. The authors have conducted innovative explorations and engineering tests on the theory and technology of the in situ fluidized mining of coal and CBM. The results indicate that in situ fluidized mining of coal and CBM represents a feasible technical pathway, signifying the coal and CBM co-production in the true sense.