Wettability of porous media after exposure to synthetic gasolines

Abstract

The wettability of a porous medium plays a critical role in the capillary phenomena governing the migration of a nonaqueous-phase liquid (NAPL) and subsequent efforts to recover this type of pollutant source from the subsurface. Although it is usually assumed that water-wetting conditions occur, limited field evidence at NAPL sites suggests that wettability characteristics can change to intermediate or organic-phase wetting, especially for complex NAPL's containing polar or surfactant molecules. The focus of this work was an assessment of potential wettability conditions for quartz mineral surfaces after exposure to synthetic gasolines. Many of the chemicals added to gasoline to increase engine performance have polar or surfactant characteristics. It is hypothesized that these additives could sorb to the quartz, causing the surface to become less hydrophilic. Four gasoline additives were added to isooctane, the base chemical for the synthetic gasoline. Following the exposure of the mineral surfaces to the organic phases, wettability was measured by three different techniques: oil-water contact angles, air-water imbibition rates and oil-water capillary pressure curves coupled with calculation of the USBM wettability index. Results show a change to intermediate-wetting conditions for two of the additives considered. Both concentration and the molecular structure of the additives affect the extent of these alterations.