Macroscopic permeability and microscopic structural evolution of anthracite under synergistic injection of steam and oxygen in gasification-pyrolysis

To investigate the fracture patterns and flow evolution characteristics of coal seams in different regions between injection and production wells during the steam-oxygen gasification–pyrolysis process, a long-distance reaction experiment was conducted on anthracite under synergistic steam and oxygen gasification-pyrolysis. Macroscopic permeability tests were performed on coal samples at varying distances from the heating injection end. High-resolution micro-CT technology was employed to characterize the microscopic physical structures of the samples, and specialized image processing software was used to reconstruct and quantify the three-dimensional pore–fracture networks, thereby elucidating the physical modification characteristics of anthracite during steam–oxygen gasification–pyrolysis. The study shows that near the injection end, the weight loss of anthracite can reach up to 60%, with a permeability of 3.42×10⁻¹⁵ m², significantly exceeding the initial permeability (on the order of 10⁻¹⁹ m²), while at a greater distance from the injection end, the weight loss is less than 20%, and the permeability is 1.26×10⁻¹⁵ m². Overall, the permeability of anthracite gradually decreases from the injection end to the production end, following a pronounced logistic nonlinear trend. Nevertheless, the minimum permeability still reaches 8×10³ times the initial permeability of the anthracite. This provides a better reaction interface for the subsequent gasification-pyrolysis reactions. The gasification or pyrolysis reactions of anthracite, the thermal stress effects within the coal body, changes in mineral content, and the extraction of decomposition products all contribute to the development of numerous pore and flow channels within the anthracite. The microscopic fracture structure within the coal body is classified into three levels: large fractures, medium fractures, and small fractures. The number of large fractures is far fewer than that of medium and small fractures, but their volume is much higher than the other two types of fractures. Overall, the proportion of large fractures within different regions of the coal body ranges from 88% to 97%, playing a dominant role. From the high-temperature (715–820 ℃) zone near the injection end to the low-temperature (410–535 ℃) zone near the production end, the porosity of the coal body gradually decreases, and this trend is consistent with the change in permeability. The highly developed high-temperature fracture system increases the porosity of the coal body, further enhancing permeability by improving pore connectivity. This study conducted a laboratory-scale simulation of the full process from thermal fluid injection at the heating well, coal oxidation–reduction reactions, and pyrolysis reactions, to product extraction at the production well, providing valuable insights for understanding and optimizing the in-situ steam–oxygen gasification of deep coal seams. Future work will focus on enhancing gasification temperatures, implementing multiple oxygen injection cycles, and conducting comparative experiments on different coal types and particle sizes, aiming to further elucidate the mechanisms of physical modification and seepage evolution of coal under high-temperature steam–oxygen conditions.