Pore Structure Evolution of Bituminous Coal Modified by CO 2 Foam Fracturing Fluid under Different Treatment Durations

Injecting CO2 foam fracturing fluid into coal seams leads to modifications in the coal’s pore structure, which directly affects coalbed methane (CBM) recovery. Among the influencing factors, the duration for which the CO2 foam remains within the coal seam plays a critical role. This study examines how the pore structure of Zhangminggou bituminous coal evolves after treatment with a CO2 foam fracturing fluid for different durations. Experiments were conducted at 40 °C and 5 MPa, with treatment durations of 3, 6, 12, 24, and 48 h. The evolution of the pore structure in modified coal was analyzed through nuclear magnetic resonance and low-temperature nitrogen adsorption techniques. Results show that pore structure evolution is strongly time-dependent. Relative to untreated coal, foam-treated coal exhibits an increase in micropores but a decrease in average pore size, alongside a moderate rise in mesopores and a marked increase in macropores and fractures. Fractal analysis of NMR images indicates that the opening size of the generalized fractal dimension spectrum and the width of the multifractal singularity spectrum both increase with treatment time, which reflects growing spatial complexity and heterogeneity in pore distribution. Low-temperature nitrogen adsorption results reveal that the specific surface area, total pore volume, and average pore diameter increase with treatment time, following a power-law relationship. The pore volume distribution shifts from 2–10 nm toward >50 nm, with a rising proportion of pores larger than 50 nm. These findings provide valuable insights for optimizing the CO2 foam fracturing parameters in practical applications and enhancing CBM recovery.