Opportunities for Particles and Particle Suspensions to Experience Enhanced Transport in Porous Media: A Review
Donath, A., Kantzas, A., and Bryant, S.
Transport in Porous Media, 128(3), March 2019.
Understanding the fate of colloids in porous media plays a vital role in applications such as enhanced oil recovery (EOR), colloid-facilitated transport or groundwater remediation. Although considerable research has been dedicated toward the retention characteristics of colloids over the last couple of years, less attention has been paid to their transport characteristics in porous media, and hence little knowledge is available on how colloids flow through a medium of complex channel alignment. This lack of understanding becomes apparent for nanoparticle applications in EOR, where relatively concentrated nanofluids are injected into low-permeable sedimentary rocks. Here, two flow phenomena arise: an earlier breakthrough of nanoparticles compared to a conservative tracer and an apparent slip effect, by which the measured pressure drop is less than that calculated via Darcy’s law. The underlying mechanism that couples both phenomena is typically attributed to the depleted layer effect—a theory that presumes that the nanoparticles are excluded from the low-velocity region near a wall at a distance typically larger than the nanoparticles. This depletion causes the particles to move on average faster than the bulk fluid containing them and causes that fluid to exhibit a viscosity lower than when measured in a viscometer. However, the depleted layer theory is not predicated on any direct observation during flow in porous media. In a broader sense, this review paper not only aims at finding answers as to why particles drive away from a wall so that a depleted layer occurs. It also questions the aforementioned hypothesis that early breakthrough and reduced apparent viscosity can be interrelated, and assesses whether each phenomenon should be treated independently. In a strict sense, this review presents both potential mechanisms of particles flow enhancement and how its manifestation can be hindered, as well as opportunities for particle suspensions to experience enhanced flow as a whole. It is meant to be an overview for readers who are not familiar with the characteristic transport features of particles. Accordingly, this review highlights also outstanding challenges in allocating the underlying mechanisms behind the flow enhancement phenomena and shows thereby future research opportunities.