Effect of Supercritical Carbon Dioxide (SCCO2) on the mechanism of change in pore and fracture structure of coal with different moisture content

When the CO2 in a supercritical state (SCCO2) is released from deep coal seams, it reacts with minerals in coal, changing the porosity of coal, which in turn affects the effect of CO2 sequestration in coal seams and the effect of increasing methane production. In order to discover the influence characteristics of SCCO2-H2O-coal rock interaction on the porosity in coal, the experiments on the supercritical CO2 reforming coal under different water content conditions were carried out with coking coal as the research object. Based on the results of mineral composition and porosity measurements, the differences between the main minerals and the changes in pore cleavage at various scales in coal were compared, and the action mechanism of the SCCO2 fluid on the pore cleavage properties under various water content states was explored. The results show that: ① After the SCCO2 treatment, the surface of the coal body is rough and loose, some fissures are penetrated as a result of mineral dissolution, and the connectivity of micro fissure is improved. ② The SCCO2 fluid has a “pore-expanding”effect on coal, which is manifested by a decrease of the content of micro- and small pores, and an increase of the proportion of medium- and large pores, i.e., the conversion of micro- and small-sized pores to large pores, as well as the improvement of pore connectivity. Furthermore, it is discovered that the fractal dimension of the adsorption pores of the coal samples increases slightly in size and roughness, while the fractal dimension of the seepage pores decreases significantly, and the complexity and non-homogeneity are reduced. ③ The SCCO2 fluid has the best solubility on the carbonate minerals in coal, followed by clay minerals, and with the increase of water content, the proportion of carbonate minerals in the extracted coal first increases and then decreases. The SCCO2 fluid makes the carbonate minerals in the coal samples of the dry basal state and saturated water state dissolve significantly, which effectively improves the pore structure, and the effect on the coal samples of the saturated water state is better. After the air-dry basal coal samples were subjected to the SCCO2, the newly formed dolomite minerals gathered in the pore throats to cause the plugging effect, reducing the original large pore size, which is the main cause of the differential changes in the pore space of coals with different water content states.