NMR evaluation of multiscale pore-structure alteration in coal induced by ultrasonic-assisted fracturing

• Ultrasonic-assisted fracturing modified pore–fracture structures in intact coal. • NMR revealed multiscale pore–throat upgrading from micro- to macropores. • Seepage-effective pore proportion increased by 1.24–9.69%, improving connectivity. • Permeability increased by 46.74% under coupled ultrasonic–hydraulic treatment. • Ultrasonic power and pressure synergistically enhanced pore–throat enlargement. Ultrasonic-assisted fracturing shows strong potential for improving pore–fracture structures and enhancing permeability in coal. To investigate its modification effects on the non-hydraulically fractured zones between hydraulic fractures, raw coal specimens were subjected to high-pressure soaking and ultrasonic treatments. Low-field nuclear magnetic resonance (NMR) was employed to quantitatively evaluate pore-scale evolution before and after treatment. Results show that all treatments reduced micropores while increasing mesopores and macropores, along with a pronounced expansion of pore throat structures. The proportion of seepage pores increased by 1.24–9.69%, indicating significantly improved connectivity. Based on the Coates model, permeability increased by 6.38%, 16.60%, and 46.74% after ultrasonic, hydraulic fracturing, and ultrasonic-assisted fracturing treatments, respectively; the coupled method achieved improvements 2.8–7.3 times greater than single treatments. Increasing ultrasonic power from 600 to 1800 W enhanced permeability by 62.44%, whereas raising soaking pressure from 5 to 15 MPa improved permeability by 138.07%. Ultrasonic stimulation upgrades pore scales through cavitation and mechanical effects, whereas hydraulic fracturing compresses the matrix yet dilates larger pores and fractures. When combined, medium and large pores grow most significantly, outperforming either method alone. This study clarifies multi-scale pore throat evolution and connectivity-enhancement mechanisms under different treatments and provides both experimental evidence and theoretical support for permeability enhancement and efficient CBM extraction in non-hydraulically fractured zones.