Study on Water Imbibition and Wettability Characteristics of Marine Shale: A Novel Method for Macro-Scale Wettability Evaluation Based on Micro-Scale Water Distribution

Accurate assessment of shale wettability is crucial for optimizing fracturing design and enhancing shale gas recovery. However, conventional evaluation methods are often unreliable due to shale’s complex mineral composition and heterogeneous pore structure. This study investigated marine shale from the Sichuan Basin by establishing a multi-scale research framework that integrates macro-scale spontaneous imbibition evolution, micro-scale dynamic video observation, and interfacial property characterization. Using techniques including X-ray diffraction (XRD), nuclear magnetic resonance (NMR), and deep-field microscopy, we investigated the water distribution patterns and imbibition mechanisms. The results indicate that the mixed-wettability characteristics of shale are governed by the synergistic effects of mineral composition and pore structure. Specifically, hydrophilic surfaces facilitate stable adsorbed water film formation via hydrogen bonding and van der Waals forces, whereas hydrophobic surfaces inhibit water spreading. At the macro-scale, a distinctive “water ring” was observed immediately upon immersion. This phenomenon reveals a physical correlation between the mass per unit length of the water ring and the contact angle at the gas–solid–liquid interface. Based on this correlation, an innovative standard curve method was developed to evaluate rock wettability. This method allows for the inversion of the apparent contact angle by simply measuring the mass per unit length of the water ring, thereby overcoming the limitations of traditional optical methods that are constrained by surface roughness and pore structure. Consequently, a logical chain of “wettability → occurrence characteristics → imbibition patterns” was established. This work provides new insights and theoretical support for understanding fluid dynamics and optimizing fracturing fluids in unconventional reservoirs.