The application of conventional surfactant-based enhanced oil recovery (EOR) suffers from either the adsorption of the surfactant on the rock near the wellbore or from diffusion into small water-filled pores. An approach to mitigate the loss of surfactant and enhance deliverability to the oil phase is achieved by formulating the surfactant molecules into nano-sized particles, which are referred to as NanoSurfactant (NS) in this study. An essential step to unleash the potential of NS in EOR is the accurate understanding of its performance under conditions reasonably close to actual conditions. In the current study, crude oil, synthetic brine, and petroleum sulfonate based NS were used. Eight limestone core plugs with a permeability range of about 125 millidarcies (mD) to 1282 mD were utilized in this study. Coreflooding was conducted under the near-reservoir condition. Advanced low-field nuclear magnetic resonance (NMR) techniques have been used to quantify the oil saturations by both secondary water injection and tertiary NS injection. The NMR techniques have also been used to assess the mechanisms underlying oil mobilization in carbonate core plugs. The protons in brine and NS solution were substituted with proton (H1) NMR invisible deuterium to enhance the contrast between oil and brine. The core samples were selected to investigate the effect of the NS soaking condition (soaking versus non-soaking), the NS injection rate, and the remaining oil saturation on the observed oil mobilization, respectively. All core plugs were initially saturated with oil and water, waterflooded, injected with NS (with and without soaking), followed by chase waterflooding. NMR measurements were conducted after each flooding stage. The results of the current study demonstrate that the investigated NS has a great potential for EOR applications in carbonate reservoirs, and that soaking NS before chase waterflooding enhances its efficiency. Significant oil mobilization was observed when soaking was applied before chase waterflooding. The oil mobilization was much lower without soaking of NS for the cores with low quality and less oil wetting tendency. The analysis indicated that NS flooding can produce both of the trapped oil and oil adsorbed to the rock surfaces.

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Wettability is an essential concept to understand oil trapping due to adhesion and capillary forces, and it depends on the fluid/rock molecular interactions within the thin liquid layer on the pore surface. The conventional methods for wettability determination exploit its macroscopic averaging effects during various displacement processes. These methods are sufficient for general categorization; however, they are not easy to be used in the wettability alteration study, which is one of the most import oil mobilization mechanisms for carbonate reservoir. The direct characterization of the microdynamics of liquid molecules on the pore surface would provide not only the measurement of wettability but also the understanding of how added materials affect fluid molecular dynamics and alter the wettability for oil mobilization. This study is to extend the application of the newly developed nuclear magnetic resonance technique and Fast Field Cycling (FFC) NMR relaxometry to more fluids and rock samples as the first stage for the above objectives. NMR dispersion (NMRD) profile by FFC NMR is a low field magnetic resonance technique, which measures the longitudinal spin relaxation over a wide range of Larmor frequencies determined by magnetic field strength. The dependency of spin-lattice relaxation rate on Larmor frequency allows to identify the liquid molecular dynamic patterns on the pore surface and determine the respective correlation times. NMRD affinity or wettability index was obtained from the time scales of the molecular translation and chemical exchange based on the dipolar interactions in the proximity of the paramagnetic spins. Only limited NMRD results of fluid/rock systems and saturation states were reported currently. This work utilizes a wide bore FFC NMR relaxometer to obtain NMRD on more fluids and rock samples of different mineralogy and analyzes their NMRD features with spin relaxation models. The current study obtains the NMRD curves and qualitative affinity/wettability estimation for different fluid/rock system, including brine, mineral oil, crude oil, sandstone, limestone, and carbonate reservoir rocks. The dynamic patterns of liquid molecules are evaluated through NMRD features for their adhesive degree one the pore surface. The comparison with supposed wettability property indicates that they correlate well, and the wettability states are better understood through their molecular dynamics. The NMRD measurement and interpretation of various fluid/rock systems extend its application; the determination of microdynamic patterns and parameters explains different wettability states from the molecular level. This study set the foundation for its application extended to fluid/rock systems at various saturation states and wettability alteration monitoring and optimizing during different oil mobilization processes.