In-situ SAXS study on deformation behavior of nanopores during CO2 adsorption in cyclically water-immersed coal based on multifractal theory

Abstract The multifractal characteristics of coal nanopore structure have important implications for the occurrence and migration of coalbed methane. Based on the in-situ adsorption device, this paper introduced the multifractal theory into the in-situ small angle X-ray scattering (SAXS) method, and synchrotron radiation SAXS was used to study the evolution of nanopore structures and their multifractal characteristics during CO 2 adsorption in cyclically water-immersed coal. The results show that in the above process, both the mesopores and macropores have multifractal characteristics, and the multifractal characteristics of the macropores are more obvious; the pore size distributions of mesopores and macropores are mainly dominated by the dense area, and this dominance is more significant in the macropores. Cyclic water immersion significantly reduces the effect of CO 2 adsorption on 3~20 nm pores in lean coal and long-flame coal, but enhances the effect of CO 2 adsorption on 3~20 nm pores in anthracite. As the number of water immersions increases, the heterogeneity of pore distribution within the 3~83 nm range during CO 2 adsorption by anthracite decreases, and the complexity of its pore structure also decreases. Among them, the heterogeneity of pore size distribution in smaller-sized pores decreases, and the pore size distribution in larger-sized pores is always more uniform. The error ranges between the model and experimental values for the average pore diameter, porosity, and specific surface area of anthracite are basically within −0.576 nm to 0.756 nm, −0.137% to 0.101%, and −4.746 to 3.270 m 2 /cm 3 .