Microstructure evolution of bituminite and anthracite modified by different fracturing fluids

The intrusion of fracturing fluid into coalbed methane (CBM) reservoirs changes the microstructure of coal and affects CBM recovery. To study the microstructural evolution characteristics of bituminite and anthracite modified by different fracturing fluids, we prepared pure water (F1), slickwater (F2), and CO2 foam fracturing fluids (F3) to treat coals, and low-temperature N2 adsorption (LT-N2A), Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD) tests were conducted to characterize the variation in the microscopic pore structure, functional group structure, and mineral components of the modified coals, respectively. The results show that different fracturing fluids can significantly change the pore structure characteristics of bituminite and anthracite. The pore volume of bituminite and anthracite modified by F1, F2, and F3 decreases successively, while the average pore size and the percentage of macropore (>50 nm) volume increase. The average pore sizes of bituminite and anthracite modified by F3 are 16.78 nm and 19.38 nm, respectively, which increase by 18.59% and 30.89% compared with that of bituminite modified by F2 and F1, respectively; the average pore sizes of anthracite increases by 3.20% and 332.47%, respectively. The pore fractal dimension of coal was calculated based on the low-temperature N2 adsorption curve. It is found that the seepage pore fractal dimension (D1) of both bituminite and anthracite modified by F2 and F3 is smaller than that of F1 modification, with D1 of F2 modified coals being the smallest. Therefore, F2 and F3 can make the seepage pores of coal smoother and reduce the complexity. F2 and F3 can reduce the aromaticity (I) and the degree of condensation of aromatic rings (DOC) of bituminite and anthracite and increase the structural parameter of the oxygen-containing functional groups (‘C’). The three fracturing fluids have little effect on the aliphatic chain length (L). F2 and F3 can dissolve the carbonate and clay minerals of coal to generate new pores to increase porosity and permeability. The results provide theoretical guidance for the fracturing of medium- and high-rank CBM reservoirs.