Wettability Evaluation by Fast Field Cycling NMR Relaxometry

Abstract

Abstract 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.