Experimental comparison of agent-enhanced flushing for the recovery of crude oil from saturated porous media

The subsurface remediation of nonaqueous liquid (NAPL) has proven to be challenging even when implementing more aggressive enhanced-flushing techniques. The objective of this study was to evaluate the effectiveness of a combination of alkaline- and surfactant-based enhanced flushing for the removal of crude oil (medium fraction) from saturated porous media. 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. Results showed that 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 porous medium mixture composition and type of fluid (i.e. surfactant vs. alkaline) used for flushing. The results of this study can be used to aid in the understanding of physical and chemical parameters/properties that control mobilization of crude oil in saturated porous 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.