Multiscale Pore-Fracture Reconstruction and Seepability Enhancement of Coal by Hydraulic Fracturing: A Field-Laboratory Integrated Investigation

Abstract Clarifying the transformation mechanism of hydraulic fracturing on coal-rock pore-fracture structures and the associated seepage response is fundamental to the efficient development of low-permeability coalbed methane reservoirs. However, existing studies are hindered by a disconnect between field observations and laboratory evaluations, and the use of single characterization methods fails to capture the full-scale pore structures. To address these limitations, this study establishes a novel multiscale comparative framework that integrates primary coal, field-fractured coal, and laboratory-simulated fractured coal. This systematic approach was not previously implemented. A comprehensive suite of characterization techniques, including scanning electron microscopy, high-pressure mercury intrusion, low-temperature nitrogen adsorption, carbon dioxide adsorption, nuclear magnetic resonance, and overburden-pressure pore permeability measurements, is employed to elucidate the fracturing-induced alterations across the full pore size spectrum. The cross-scale fractal dimension evolution is quantitatively characterized, seepage differences among sample groups are compared, and the laboratory simulation method is rigorously validated against field-fractured samples. The results demonstrate that hydraulic fracturing shifts the pore structure from micropore-dominated to mesopore-macropore synergistic development with substantial enhancement of meso- and macropores. Meanwhile, stress-induced irreversible collapse of key adsorption micropores of 0.5 to 0.6 nm leads to a reduction in the CO2 adsorption capacity. The fractal dimension increases with pore size, confirming that fracturing promotes structural complexity through pore merging and improved connectivity. Field-fractured samples exhibit an increased movable fluid porosity, enhanced permeability, and pronounced stress sensitivity. Notably, the laboratory-simulated method effectively reproduces the coupled mechanical-hydraulic effects, yielding pore characteristics and seepage responses that are consistent with field samples, thereby validating its reliability as a predictive tool for fracturing optimization. Collectively, these findings provide both theoretical insights and experimental evidence to support the quantitative evaluation of fracturing effectiveness and process optimization in low-permeability coalbed methane reservoirs.