Multi-field coupling theory and research progress of methane extraction in deep coal seams: A review

Methane extraction in deep coal seams is a multi-field coupling process affected by multiple factors, and a systematic multi-field coupling theory is of guiding significance for ensuring the safety and efficiency of such extraction. In this study, the theoretical framework was constructed for the multi-field coupling theory of methane extraction in coal mines. First, the key scientific issues were identified. Subsequently, based on the stress distribution and structure of the coal seam, the longwall panel was divided into three zones, i.e., the virgin zone, the excavation disturbed zone and the goaf. Furthermore, the research progress of multi-field coupling during methane extraction were analyzed for the three zones, respectively. Finally, the deficiencies of current researches on multi-field coupling theory for methane extraction were revealed, and the future research directions were pointed out. The following research findings were obtained: Deep coal mining is accompanied by a notable multi-field coupling phenomenon, and the occurrence of compound disasters promote the difficulty in disaster control. However, we can adopt appropriate measures to decouple compound disasters after fully grasping the triggering mechanism and development process of the fields. In this way, the disasters can be controlled separately. In addition, from the perspective of the whole longwall panel, the main development directions are to reveal the gas flow in cross-scale structures and to develop a unified model for gas migration in the multi-scale structure.