Three Fundamental Challenges in Deep Coal and Gas Outburst: Progress, Controversies, and Future Perspectives

Abstract

Coal and gas outburst (CGO) remains one of the most severe dynamic hazards in deep coal mining. Increasing burial depth challenges conventional single-factor criteria. This critical review synthesizes advances in CGO mechanisms reported between 2015 and 2025, focusing on three fundamental challenges: the limited applicability of the 0.74 MPa gas-pressure threshold under deep conditions, the nonmonotonic thermal regulation of gas adsorption–desorption and transport, and the apparent high-storage–low-trigger paradox of tectonically deformed coal (TDC). Comparative assessment of elastic-energy- and adsorbed-gas-dominated theories indicates that deep outbursts are better interpreted as condition-dependent competition among elastic strain energy, gas expansion work, fracture-energy consumption, and dissipative processes. Reported values further show that coal deformation/fracture energy and gas expansion energy can become comparable in magnitude, whereas the fraction of adsorbed-gas energy converted into dynamic work remains highly uncertain. Temperature exerts competing influence on sorption, transport, deformation, and energy redistribution, while TDC combines high gas-storage capacity, low permeability, and low mechanical strength, favoring pressure disequilibrium and structural instability. Three priority research questions emerge: quantifying adsorbed-gas energy conversion into dynamic work, establishing cross-scale similarity and validation for thermo-hydro-mechanical (THM) coupling, and developing physics-constrained prediction of nonlinear instability. These advances provide a pathway from single-index hazard identification toward mechanism-informed, multiparameter early warning.