Benzhong Zhao

Capillary pinning and blunting of immiscible gravity currents in porous media

Gravity-driven flows in the subsurface have attracted recent interest in the context of geological carbon dioxide (CO2) storage, where supercritical CO2 is captured from the flue gas of power plants and injected underground into deep saline aquifers. After injection, the CO2 will spread and migrate as a buoyant gravity current relative to the denser, ambient brine. Although the CO2 and the brine are immiscible, the impact of capillarity on CO2 spreading and migration is poorly understood. We previously studied the early time evolution of an immiscible gravity current, showing that capillary pressure hysteresis pins a portion of the macroscopic fluid-fluid interface and that this can eventually stop the flow. Here we study the full lifetime of such a gravity current. Using tabletop experiments in packings of glass beads, we show that the horizontal extent of the pinned region grows with time and that this is ultimately responsible for limiting the migration of the current to a finite distance. We also find that capillarity blunts the leading edge of the current, which contributes to further limiting the migration distance. Using experiments in etched micromodels, we show that the thickness of the blunted nose is controlled by the distribution of pore-throat sizes and the strength of capillarity relative to buoyancy. We develop a theoretical model that captures the evolution of immiscible gravity currents and predicts the maximum migration distance. By applying this model to representative aquifers, we show that capillary pinning and blunting can exert an important control on gravity currents in the context of geological CO2 storage.

Forced Wetting Transition and Bubble Pinch-Off in a Capillary Tube

Immiscible fluid-fluid displacement in partial wetting continues to challenge our microscopic and macroscopic descriptions. Here, we study the displacement of a viscous fluid by a less viscous fluid in a circular capillary tube in the partial wetting regime. In contrast with the classic results for complete wetting, we show that the presence of a moving contact line induces a wetting transition at a critical capillary number that is contact angle dependent. At small displacement rates, the fluid-fluid interface deforms slightly from its equilibrium state and moves downstream at a constant velocity, without changing its shape. As the displacement rate increases, however, a wetting transition occurs: the interface becomes unstable and forms a finger that advances along the axis of the tube, leaving the contact line behind, separated from the meniscus by a macroscopic film of the viscous fluid on the tube wall. We describe the dewetting of the entrained film, and show that it universally leads to bubble pinch-off, therefore demonstrating that the hydrodynamics of contact line motion generate bubbles in microfluidic devices, even in the absence of geometric constraints.

Pore geometry control of apparent wetting in porous media

Wettability, or preferential affinity of a fluid to a solid substrate in the presence of another fluid, plays a critical role in the statics and dynamics of fluid-fluid displacement in porous media. The complex confined geometry of porous media, however, makes upscaling of microscopic wettability to the macroscale a nontrivial task. Here, we elucidate the contribution of pore geometry in controlling the apparent wettability characteristics of a porous medium. Using direct numerical simulations of fluid-fluid displacement, we study the reversal of interface curvature in a single converging-diverging capillary, and demonstrate the co-existence of concave and convex interfaces in a porous medium—a phenomenon that we also observe in laboratory micromodel experiments. We show that under intermediate contact angles the sign of interface curvature is strongly influenced by the pore geometry. We capture the interplay between surface chemical properties and pore geometry in the form of a dimensionless quantity, the apparent wettability number, which predicts the conditions under which concave and convex interfaces co-exist. Our findings advance the fundamental understanding of wettability in confined geometries, with implications to macroscopic multiphase-flow processes in porous media, from fuel cells to enhanced oil recovery.