Study of the impact of fluid–rock interface interactions in shale oil reservoirs on imbibition

During hydraulic fracturing in shale oil reservoirs, fracturing fluid imbibition can enhance oil recovery. However, the mechanisms by which the displacement of fluids in multiscale pores and the physicochemical reactions at the fluid–rock interface enhance imbibition remain unclear. This study employed a combination of Raman spectroscopy and nuclear magnetic resonance (NMR) to investigate fluid flow during the imbibition process. Comparing Raman spectra before and after imbibition revealed differences in peak shape and position, indicating that the fracturing fluid penetrated and was retained within the rock core. During imbibition, driven by capillary forces, the fracturing fluid preferentially entered smaller pores, while oil was displaced from larger pores due to capillary force differences between adjacent pores and gravity. The mobilization of oil in larger pores was relatively slow and low. This paper proposes a combined approach utilizing multiple experimental methods (NMR, X-Ray Diffraction, Scanning Electron Microscope, etc.) to investigate the pore structure, mineral composition, and wettability of shale. The results showed that the fractures/large pores in shale samples were mainly intergranular pores composed of quartz and feldspar, while the medium, micro-, and nano-pores were mainly intragranular pores. One of the main reasons that fracturing fluid imbibition enhances oil recovery is that the minerals composing the fractures/large pores are prone to wettability reversal in the fracturing fluid, which promotes the extraction of crude oil from clay pores through imbibition. This study is of great significance for the later development of oil fields.