We investigated the utility of a modified Washburn method, combining results of two sorption experiments, to study wettability alteration processes. The main objective in deploying this technique was to avoid the difficulty of direct contact angle measurements in surfactant solutions. In such cases, the ultra-low interfacial tension between the crude oil and surfactant tends to cause the oil drop to spread prematurely rendering the measurement unreliable if possible at all.
A carbonate-rock, dead crude oil, synthetic brines including SmartWater, and five surfactants were used in this study. The surfactants included: an anionic alfa olefin sulfonate, a cationic quaternary ammonium salt, an amphoteric surfactant, a nanosurfactant, and a nonionic ethoxylated alchohol. Hexane was also used as a reference completely wetting fluid. For sorption experiments, the rock was powdered. The powder with size between 80 and 100 mesh was compacted in the sample holder. In each experiment, a given fluid was raised to the bottom of the powder pack and allowed to rise into the powder by capillarity. A sensitive balance was used to measure fluid imbibition into the powder until no more fluid imbibes. Beside sorption experiments, surface and interfacial tension measurements were made. For benchmarking, we relied on previous findings reported in the literature.
Plotting the square of fluid mass against time gave a slope which enabled the calculation of contact angle in air. The results (slope) observed for the completely wetting fluid (hexane) provided the rock constant. Sorption results of crude oil coupled with the previously determined rock constant enabled estimation of the Wasburn oil/air contact angles using the modified Washburn equation. Sorption results with brine and surfactant solutions coupled with the rock constant and oil/air enabled estimation of the Washburn water/air contact angles using the modified Washburn equation. Based on water/air and oil/air contact angles, values of the conventional oil/water contact angle were estimated. For surfactant solutions, interfacial tension between oil and surfactant-free brine were used to approximate contact angles that were otherwise undefined. This approach, enabled a robust and rapid estimation of the effects of various processes on contact angles (hence wettability alteration). Based on the benchmark previous findings, the sorption approach yielded acceptable results especially for screening purposes. The results demonstrated the potential of the Smartwater recipe and the nonionic surfactant for wettability alteration. However, it is recommended to rely on the sorption method direct measurements including sorption rates to establish sorption-based wettability indices and eliminate the intermediary and probably unnecessary contact-angle estimation step.
In conclusion, the use of sorption to obtain contact-angle estimates provide a novel rapid and robust procedure for evaluation and screening of wettability-alteration agents. This eliminates direct contact angle measurements that are often cumbersome. It specifically eliminate the difficulties of attaching an oil drop onto a rock surface immersed in surfactant solutions.