Experimental investigation on permeability enhancement effect of improved true-triaxial tetrahydrofuran hydraulic fracturing: Perspectives of pore and fracture variations

To improve permeability enhancement in coal seams subjected to hydraulic fracturing, a true-triaxial solvent–hydraulic injection experimental system was developed. The evolution of pore and fracture networks during conventional hydraulic fracturing and tetrahydrofuran (THF)–enhanced fracturing was investigated using a suite of experimental techniques. Fracture development was characterized by box-counting fractal analysis and ultrasonic damage factors, while pore-structure fractal dimensions were quantified using Frenkel–Halsey–Hill theory in combination with the Pfeifer–Avnir formulation to assess multiscale pore changes. The mechanism by which THF promotes permeability enhancement was then elucidated. Results show that the breakdown pressure of the tested coal samples ranged from 12.08 to 19.73 MPa, and primary fractures produced by fracturing were oriented perpendicular to the minimum principal stress. Along the X-axis, P-wave velocity attenuation and ultrasonic damage factors were most pronounced. Fracturing with water and THF improved the pore structure, with a maximum increase of total pore volume by up to 15.83% and 37.39%, respectively. Both the pore-structure fractal dimension D ₁ and the D _Hg metric decreased, implying reduced methane adsorption space and enhanced pore connectivity, which together led to a marked increase in coal permeability. Owing to the combined effects of enhanced fracture development and dissolution of soluble molecules within pores, THF-assisted fracturing achieved a superior permeability enhancement, with a maximum increase of 106.933-fold. These findings offer new insights for optimizing permeability enhancement and enabling efficient clean-gas recovery from low-permeability reservoirs.