The Synergistic Inhibition Mechanism of Interfacial Wetting Regulation and Film-Forming Plugging on the Invasion of Mudstone Micropore Throat Fluid

To address drilling-fluid filtrate invasion and the resulting wellbore instability associated with the well-developed micropore throats of hard–brittle shale in the Yangxia Formation, X-ray diffraction, contact-angle measurements, and low-field nuclear magnetic resonance were employed to investigate the effects of different drilling-fluid systems on shale wettability and fluid invasion into micropore throats. The compatibility and anti-invasion mechanisms of the hydrophobic-film-forming agent in different base fluids were also analyzed. The results show that the Yangxia Formation shale contains 50.1% clay minerals and exhibits strong hydrophilicity. In the absence of an externally applied pressure differential, fluid invasion is mainly driven by spontaneous capillary imbibition through micropore throats, with the newly generated NMR signals concentrated primarily in the short T2 region. The hydrophobic-film-forming agent exhibited pronounced compatibility differences among the base fluids. The organic-salt polysulfonate base fluid reduced the 24 h unit-volume NMR short-T2 signal to 375.0 a.u./cm3, corresponding to a 48.3% reduction relative to freshwater. After incorporation of the hydrophobic-film-forming treatment, the signal further decreased to 337.5 a.u./cm3, representing an additional reduction of 10.0% relative to the corresponding base fluid and an overall reduction of 53.5% relative to freshwater. The contact angle increased from 52° to 68° after treatment. These results indicate that the strong overall anti-invasion performance arises primarily from the organic-salt polysulfonate base system, while the hydrophobic-film-forming treatment provides an additional improvement through interfacial wettability regulation and restriction of pore-throat fluid entry. These results provide a basis for improving wellbore stability and optimizing drilling-fluid systems in the Yangxia Formation.