Effects of pre-fracturing fluids on pore-fracture structure and mechanical properties of deep coal

Hydraulic fracturing is a key technology for enhancing coalbed methane recovery. Preconditioning the reservoir with acidizing or oxidative fluids can significantly improve stimulation outcomes; however, comparative studies evaluating these fluids in coal seams are still limited. In this work, deep coal samples (depth > 2500 m) were treated with five different pre-fracturing fluids—slick water (PAM), hydrochloric acid (HCl), hydrochloric and hydrofluoric (HCl + HF) acid, sodium hypochlorite (NaClO), and hydrogen peroxide (H2O2)—and their effects on porosity, permeability, and mechanical properties were systematically compared using low-field NMR, SEM, and mechanical testing. The potential relationships between changes in mechanical parameters and pore-fracture structure were also examined. Results show that all five fluids increased total porosity by 33.98 ~ 45.04%. The HCl + HF produced the most dramatic permeability enhancement (13,561.54%), whereas HCl alone reduced permeability by 78.01%. Moreover, HCl + HF led to the most pronounced reduction in both compressive and tensile strength. Kendall correlation analysis revealed several significant relationships: Young’s modulus (E) correlated negatively with total porosity (φt) and average roughness (Ra); Poisson’s ratio (ν) showed a positive correlation with φt; and tensile strength (Rt) was negatively correlated with Ra. These findings provide practical insights for selecting pre-fracturing fluids based on desired reservoir weakening or permeability enhancement objectives.