Surfactants play a crucial role in enhanced oil recovery by reducing interfacial tension (IFT) and promoting emulsification. However, for low-permeability reservoirs after water flooding, it remains unclear which ability is more crucial for improving recovery. To address this question, this study compared the oil displacement effects of three surfactants with different IFTs and emulsifying properties using a series of core displacement experiments. The results show that the strongly emulsifying surfactant S1# achieved an incremental oil displacement efficiency of 9.94%, which is higher than that of the ultra-low-IFT surfactant S3# (8.63%), while the composite surfactant S2# (strong emulsification + ultra-low-IFT) achieved the highest incremental oil displacement efficiency of 11.79%, representing an improvement of 3.16% compared with S3#. The three surfactants have the same effect on improving oil recovery and oil displacement efficiency in heterogeneous rock cores. Strong emulsification promotes abundant in situ emulsions, increases injection pressure (up to 0.85 MPa compared with 0.41 MPa for ultra-low-IFT flooding), and expands the swept volume, indicating that emulsification-driven sweep improvement dominates over capillary-force reduction under the investigated conditions. Furthermore, a composite displacement strategy combining emulsification-driven plugging and ultra-low-IFT oil washing achieved a 12.37% recovery improvement while maintaining relatively low injection pressure (~0.48 MPa). However, the effectiveness of this mechanism is strongly dependent on reservoir heterogeneity. When the permeability contrast increased from 5.4 to 17.3, incremental recovery decreased by up to 7.03%, demonstrating that strong heterogeneity significantly limits the effectiveness of emulsification-driven sweep expansion. This work advances the understanding of surfactant flooding mechanisms and provides guidance for designing and deploying surfactants in water-flooded low-permeability reservoirs.

Knowledge of the migration characteristics of polymer systems in pore throats is essential for the effective application of polymers as a profile-control oil-displacement agent for enhanced oil recovery. In this study, the effect of concentration on the viscosity and hydrodynamic radius of polymer systems was investigated using a rheometer and a dynamic light scattering instrument. Furthermore, pore-throat models, homogeneous cores, and multi-measuring-point sand-packed models were constructed to investigate pore-scale migration patterns and the effect of the throat–polymer ratio (defined as the ratio of throat size to polymer hydrodynamic radius) on the migration properties of polymers in porous media. The results showed that the transport of polymer systems in porous media is primarily related to the throat–polymer ratio. When this ratio is sufficiently small (i.e., no more than 18.94), the migration pattern of the polymer systems in the pore-throat model does not exhibit the characteristics of polymer solution flow, but rather, of discontinuous-dispersion retention, plugging-breakthrough migration, and stable-plugging retention. Upon increasing the injection rate, the polymer systems also exhibit the migration characteristics of discontinuous dispersion at a larger throat–polymer ratio. Moreover, polymer system migration resistance and improved sweep efficiency in porous media are influenced by not only the viscosity of polymer systems, but also the throat–polymer ratio. The smaller the throat–polymer ratio, the stronger the retention and plugging ability of the polymer systems. The outcomes of this study are significant for the design of polymer flooding operations in oilfields.

The physical state of water under extreme confinement can deviate from bulk behavior, yet such effects are typically considered limited to the sub-10 nm regime. Here we report a stable solid-like water phase under ambient conditions in SiO₂ microtubules spanning the submicron–micron scale. The confined water shows pronounced solid-like behavior: it can be sectioned by focused ion beam (FIB) milling and deforms plastically under stress rather than flowing, while maintaining a stable morphology from − 20 to 90 °C and from high vacuum (10⁻⁵ Pa) to atmospheric pressure. ¹H NMR, Raman, and IR spectra indicate strongly restricted molecular dynamics and a modified hydrogen-bonding environment distinct from bulk liquid water and crystalline ice. ¹H NMR quantification and surface-chemistry controls further identify an exceptionally high density of inner-wall silanol (SiOH) groups as the key factor governing formation and stability, with clear pH dependence and reversible transitions. These findings highlight a mesoscale confinement regime dominated by interfacial chemistry.