Mechanisms of Water-Lock Removal by a Nanoemulsion/Mud Acid Composite System in Low-Permeability Sandstone Reservoirs: Insights from Pore-Scale Characterization in the MB Block

Abstract Water-lock damage in low-permeability sandstone reservoirs is primarily caused by small pore-throat structures, strong capillary forces, and hydrophilic rock surfaces. Conventional treatment methods mainly target either fluid-interface modification or pore blockage problems separately. In this study, a nanoemulsion/mud acid composite system was developed to achieve a combined effect of fluid interfaces and localized mineral dissolution. Interfacial tension (IFT) measurements, contact angle analysis, constant-rate mercury intrusion (CRMI), nuclear magnetic resonance (NMR), and high-temperature high-pressure core flooding experiments were conducted to investigate the water-lock removal performance and underlying mechanisms of the composite system. The results demonstrate that the optimized 0.20 wt % nanoemulsion/mud acid system reduced the oil–water interfacial tension to 0.486 × 10–3 (mN·m–1) and increased the contact angle to 116.4°. The combined CRMI–NMR characterization revealed that fluid mobility in low-permeability sandstone can be classified into bound, transition, and movable zones, rather than being described by a single flow threshold. Increasing the production pressure differential intensified deep water-phase invasion and retention, weakened the production enhancement effect caused by increased driving force, and further aggravated water-lock damage. Mineralogical analysis, NMR measurements, and permeability-recovery experiments collectively indicate that water-lock removal is associated with coupled interfacial regulation and localized pore–throat modification. These findings provide a pore-scale theoretical basis for optimizing treatment-agent concentration and production pressure differential, while further field-scale validation remains necessary.