Effects of Thermal Evolution Degree and Industrial Components on Pore Fracture Distribution Heterogeneity in Deep Coal Reservoirs

Many studies have shown that the thermal evolution degree is the main factor affecting the micropore structure of coal reservoirs. However, within the same thick coal seam, the Ro,max of the entire coal seam is not much different, which affects the determination of the main controlling factors of pore structure heterogeneity. Therefore, No. 8 coal collected from Benxi Formation in the eastern margin of Ordos was taken as an example, and 16 samples were selected for low-temperature liquid nitrogen, carbon dioxide adsorption, and industrial component tests. Based on heterogeneity differences of Ro,max, industrial components and pore volume distribution of adsorption pores (pore diameter is less than 100 nm), the main controlling factors affecting the micropore structure of ultra-thick coal seams, were discussed. Then, the surface free energy theory was used to study the influencing factors affecting surface free energy variations during coal adsorption. First of all, Ro,max is not the main controlling factor affecting the micropore-fracture structure, as the effects of industrial components on the micropore structure are obvious, which indicates that industrial components are the main factors affecting vertical differences in the micropore structure within the same thick coal seam. Second of all, Ro,max and industrial components affect the adsorption process. When the adsorption pressure is lower, the adsorption volume and adsorption potential increase rapidly. When the adsorption pressure is higher (pressure is larger than 15 Mpa), the adsorption capacity and potential tend to be stable. Moreover, the maximum surface free energy increases with the increase in coal rank, which indicates that the degree of thermal evolution is the core factor affecting the adsorption free energy, but it is also controlled by the influence of industrial components (ash content). Lastly, micropores affect the adsorption capacity, and mesopores have little effect on the adsorption capacity, since micropores restrict the adsorption capacity and change the adsorption process by affecting surface free energy variations. The refined characterization of pore-fracture structures in deep coal reservoirs plays a crucial role in the occurrence and seepage of coalbed gas. This research can provide a theoretical basis for the efficient development of deep coalbed gas in the target area. This study aims to identify the primary factors controlling micropore structures in No. 8 coal from the Benxi Formation and to analyze the role of industrial components, which has been overlooked in previous research.