Background  China possesses substantial resources of deep unmineable coal seams, which hold considerable potential for CO₂ sequestration. However, simply sequestrating CO₂ in deep coal seams incurs high economic costs while also wasting deep coal resources. The microbial conversion of sequestrated CO₂ into methane (CH₄) enables circular carbon capture, utilization, and storage (CCCUS), which is of great significance for the sustainable development of both resources and the environment.

Methods  Using a range of methods, including experimental study, theoretical analysis, and engineering simulations, CCCUS in deep unmineable coal seams is designed to (1) reveal the mechanisms behind efficient CO₂ fracturing and associated permeability enhancement, as well as CO₂ displacing CH₄, in deep unminable coal seams; (2) to investigate the mechanisms controlling the generation of complex fracture networks and efficient CH₄ displacement in the coal mines; (3) to develop methods for efficient hydrogen production from liquefied straw in deep coal seams under high-temperature and high-pressure conditions; (4) to propose a technical route for bioconversion of CO₂ in deep coal seams into CH₄ using liquefied straw; and (5) finally to evaluate the economic viability of CO₂ recycling.

Results and Conclusions CO₂ fracturing in deep coal seams can create more complex fracture networks, increase the connectivity of micro- and nano-scale pores and fractures, and enhance CH₄ displacement efficiency. Meanwhile, microorganisms enable the efficient conversion of CO₂ into CH₄, thereby improving the conversion efficiency of CO₂ in deep coal seams. This study investigates the evolution patterns of pore and fracture structures in coal reservoirs under the combined effects of CO₂ fracturing, CO₂ displacing CH₄, and CO₂ to CH₄ bioconversion. It improves the theory on the coupling of coal body structure evolution and fluid migration under CCCUS in deep unmineable coal seams and explores the influence patterns of CCCUS on surrounding rock stability and geological environment, along with associated controlling mechanisms. Furthermore, this study enriches and develops theories on multi-field coupling in transport within porous media, achieves the technical reliability of CO₂ recycling, and quantitatively characterizes the economic indicators of CO₂ recycling. The results of this study provide an important theoretical basis for the implementation of innovative CCCUS technology in deep unmineable coal seams and offer robust support for China’s energy revolution and the achievement of its goals of peak carbon dioxide emissions and carbon neutrality.