NANOPARTICLE TRANSPORTATION IN UNSATURATED POROUS MEDIA

Characterizing multiphase flow behavior in-situ in the subsurface is of paramount importance in many fields but especially in oil reservoirs where variations in gas saturation reflect and impact oil recovery. Very few methods are currently available to probe in-situ gas saturation. The ~3 nm diameter C-dot nanoparticle tracer which, as described in other talks in this session is remarkably inert in water-saturated porous media, could be useful in this regard. Previous studies have shown the air-solid-water interface plays an important role in retaining colloidal size particles. This is true also for nanoparticles. In laboratory experiments we have carried out involving both C-dots and chloride in unsaturated sand columns, the chloride recovery is 100% but the C-dot recovery ~90% (whereas under saturated column conditions it is 100%). The C-dot retention depends on the air saturation in the column, which suggests the nanoparticles are trapped at the air- water solid- interface where the meniscus attaches to the sand grain, as has been observed by confocal microscopy for colloidal particles. If the relationship between air saturation and particle retention is regular, particle tracers might be able to characterize variations in gas saturation along a flow path, as well as diffusion from the flow path.