Influence of surfactants on wettability alteration during hydraulic fracturing in sandstone reservoirs: NMR analysis in spontaneous imbibition experiments

During hydraulic fracturing in tight sandstone reservoirs, surfactants are extensively employed to modify wettability, thereby enhancing the final gas recovery rate. However, the mechanism of wettability alteration due to the migration of imbibition fluids within multiscale pores as well as their physicochemical reactions throughout this process, remains unclear. Therefore, this study proposed an improved nuclear magnetic resonance (NMR) method that integrates spontaneous imbibition (SI) experiments to quantitatively characterize dynamic wettability. The influences of surfactants with different concentrations were investigated through mineral reactions, imbibition mass and movable fluids. Firstly, mineral fractions and contact angles were measured before and after the SI experiments. Subsequently, the relationship between normalized SI volume and imbibition time was established to define the pore wettability index (PWI). Movable fluid porosity was then determined by the fluid migration within multiscale pores. Finally, the PWI was refined by incorporating movable fluid porosity and characteristic pore radius to characterize dynamic wettability during the SI process. The experimental results indicate that higher concentrations of surfactants promote the conversion process of potassium feldspar to hydrophilic minerals (kaolinite and muscovite), which significantly contributes to increased hydrophilicity. The application of surfactants significantly increases the imbibition rate, although further increases in concentration have a diminishing effect. Compared to static contact angles, the dynamic wettability characterization using the improved NMR method captures details of interfacial properties, offering novel insights into wettability modification.