Prolonged Acid Stimulation Drives a Nonmonotonic Evolution of Coal Wettability from Hydrophilization to Hydrophobic Recovery

Wettability fundamentally governs the transport, displacement, and retention of fracturing fluids in deep coalbed methane (CBM) reservoirs, yet its long-term evolution under acidic reservoir conditions remains insufficiently constrained. To address this gap, this study elucidates the coupled microstructural and chemical pathways through which hydrochloric acid alters coal wettability over extended reaction periods. Benxi Formation coal from the Ordos Basin was subjected to continuous exposure to 5% HCl at 75 °C and 25 MPa for five weeks, and its transformation was characterized using an integrated suite of X-ray diffraction, scanning electron microscope, low-temperature nitrogen adsorption, and fourier transform infrared spectroscop (XRD, SEM, LTNA, and FTIR) analyses, together with dynamic contact angle measurements. The results reveal a temporally partitioned dissolution–precipitation regime that reshapes both the mineralogical composition and pore architecture. Hydrophilic clay minerals initially accumulate before diminishing, as secondary precipitates progressively emerge. Concomitantly, the pore system experiences irreversible coarsening, manifested by a 63.92% decline in the specific surface area, a 42.73% reduction in pore volume, and a 47.29% increase in the average pore diameter. Chemical spectroscopy indicates sustained generation of C–O–C linkages, depletion of ring hydroxyl groups, and a nonlinear evolution of OH···O interactions, collectively signaling substantial alteration of surface functional chemistry. These coupled processes drive a pronounced wettability reversal: the coal surface becomes increasingly water-wet until week 3 (minimum contact angle of 62.8°), followed by a shift toward reduced wettability by week 5 (75.1°). Overall, this work clarifies the time-dependent mechanisms controlling acid-driven wettability modification in deep coal seams and provides a mechanistic foundation for optimizing acid stimulation strategies in CBM reservoir engineering.