Imbibition mechanism analysis of modified black nanosheet and low salinity water composite system for enhanced oil recovery by NMR method

Black nanosheets (BN, with a specific chemical composition of molybdenum disulfide) have been widely studied for low-permeability reservoir development due to their unique nanoscale dimensions and lamellar structure. Our prior research demonstrated that cationically modified BN combined with low-salinity water (LSW) significantly enhances oil displacement. This study compared the imbibition adaptability of the composite system under both ambient-pressure and pressurized conditions, while combining nuclear magnetic resonance (NMR) techniques to analyze related imbibition mechanisms and reservoir permeability adaptability. Results showed that the modified BN-LSW composited system achieved an imbibition recovery efficiency of 43.92% under ambient pressure. The mechanism was capillary force-dominated, preferentially displacing oil from small pores by improving core wettability and emulsification. Under pressurized conditions, the driving force became dominant, further increasing recovery efficiency to 56.52%, with the system displacing oil from both large and small pores. Additionally, the system showed optimal adaptability in cores with 0.05 × 10⁻³ μm² permeability. Imbibition efficiency declined at higher/lower permeabilities due to weakened capillary forces or nanoparticle aggregation-induced clogging. This study confirmed that the modified BN-LSW composite system enhanced imbibition stability and recovery efficiency, and combined with nuclear magnetic resonance (NMR) technology, its mechanism was revealed at the microscale. This provided theoretical and technical support for the efficient development of low-permeability reservoirs, with significant engineering value.