Abstract:
A comprehensive study was performed to evaluate the mechanisms controlling mobilization of trapped nonaqueous phase liquid (NAPL) crude oil as a function of wetting phase dynamics and geochemistry during enhanced-flushing conditions. Synchrotron X-ray microtomography (SXM) was used to perform pore-scale examination of NAPL fragmentation and changes in blob morphology, and recovery using three different advective flushing methods: surface-active agent (surfactant) flushing, alkaline flushing, and sequential alkaline-surfactant flushing. This set of experiments was conducted to understand effects on such processes (fragmentation and recovery) as a function of media composition (geochemical/mineralogical) and pH alterations due to calcium-carbonate fraction. The sequential flushing technique (alkaline→ surfactant) yielded the highest recovery, 32% after 5 pore volumes (PV) of flushing. The crude oil (NAPL) distribution varied due to differences in heterogeneity and type of fluid (i.e. surfactant vs. alkaline) used for flushing. Drop shape analysis was also performed to measure contact angle across the solid-liquid-liquid (S-L-L) junction for a multiphase system for three different API° gravity (or density) crude oils within aqueous solutions of varying pH and in contact with two different types of media including silica (glass plate) and limestone (calcium carbonate plate). Interfacial tension (IFT) was also measured during drop shape analysis for the three different density oils, as a function of pH. Higher pH values showed the greatest change in contact angle (i.e. high to low) and subsequently the lowest IFT measurements. During alkaline flushing, buoyancy forces predominantly controlled mobilization of NAPL (oil), and during surfactant flushing, viscous forces yielded relative greater control on crude oil (NAPL) mobilization. Calculated capillary force values showed significant variation for the set of experiments due to the changes in interfacial tension between the three crude oils tested and the alkaline aqueous phase (of varying pH). The results from this research can be used to aid in the understanding of physical and chemical parameters/properties that control mobilization of crude in porous saturated media. This can help reduce time and cost during remediation of contaminated sites that contain crude oil or less dense NAPL derivatives consistent with fuel-type petroleum hydrocarbons.
