Analysis of Spontaneous and Dynamic Imbibition Characteristics of Silica-Based Nanofluid in Microscopic Pore Structure of Tight Oil Reservoirs

Imbibition behavior plays a crucial role in tight oil reservoir development, to increase tight oil production, a novel sodium lauryl ether sulfate (SLES) nanofluid was developed. Spontaneous and dynamic imbibition performance of nanofluid and the relative influencing factors were systematically investigated using the online low-field nuclear magnetic resonance (LF-NMR) technique. The microscopic mobilization characteristics and mechanism of nanofluid-enhanced matrix crude oil recovery in tight oil reservoirs were further explored. The experimental results showed that the proposed SLES nanofluid achieved the highest imbibition efficiency, and appropriate concentrations of nanoparticle and surfactant were helpful in enhancing the imbibition recovery. Excessive concentrations of chemical agents may block the tiny pores. A reasonable injection rate and shut-in time should be determined to fully leverage capillary forces for water phase imbibition and oil phase displacement as well as the displacement effect of the viscous force and increased water wetness of the rock surface provided by the nanofluid. In addition, NMR T₂ cutoff was applied to distinguish the imbibition and displacement regions during dynamic imbibition process. The pore sizes corresponding to the T₂ cutoff values after nanofluid and deionized (DI) water flooding were 0.6 and 5 μm, indicating that nanofluid considerably increased the movable crude oil in pores of tight oil reservoirs compared with DI water. Furthermore, imbibition behavior was dominant in micropores (<0.1 μm) and mesopores (0.1-1 μm), and displacement principally occurred in microfractures and macropores. Finally, the underlying mechanisms responsible for nanofluid enhanced oil recovery were related to wettability alteration, capillary pressure, and viscous force. This work provides further understanding of spontaneous and dynamic imbibition when employing nanofluid in tight oil reservoirs.