Abstract:We study the evolution of binary mixtures far from equilibrium, and show that the interplay between phase separation and hydrodynamic instability can arrest the Ostwald ripening process characteristic of nonflowing mixtures. We describe a model binary system in a Hele-Shaw cell using a phase-field approach with explicit dependence of both phase fraction and mass concentration. When the viscosity contrast between phases is large (as is the case for gas and liquid phases), an imposed background flow leads to vis… Show more
“…6c. A similar phenomenon has been observed in immiscible two phase systems, where the thermodynamics couples to the flow [35]. For C ≥ 200 mM, we observe longer and narrower fingers, essentially the same as Fig.…”
We present an experimental study of pattern formation during the penetration of an aqueous surfactant solution into a liquid fatty acid in a Hele-Shaw cell. When a solution of the cationic surfactant cetylpyridinium chloride is injected into oleic acid, a wide variety of fingering patterns are observed as a function of surfactant concentration and flow rate, which are strikingly different than the classic Saffman-Taylor (ST) instability. We observe evidence of interfacial material forming between the two liquids, causing these instabilities. Moreover, the number of fingers decreases with increasing flow rate Q, while the average finger width increases with Q, both trends opposite to the ST case. Bulk rheology on related mixtures indicates a gel-like state. Comparison of experiments using other oils indicates the importance of pH and the carboxylic head group in the formation of the surfactant-fatty acid material.
“…6c. A similar phenomenon has been observed in immiscible two phase systems, where the thermodynamics couples to the flow [35]. For C ≥ 200 mM, we observe longer and narrower fingers, essentially the same as Fig.…”
We present an experimental study of pattern formation during the penetration of an aqueous surfactant solution into a liquid fatty acid in a Hele-Shaw cell. When a solution of the cationic surfactant cetylpyridinium chloride is injected into oleic acid, a wide variety of fingering patterns are observed as a function of surfactant concentration and flow rate, which are strikingly different than the classic Saffman-Taylor (ST) instability. We observe evidence of interfacial material forming between the two liquids, causing these instabilities. Moreover, the number of fingers decreases with increasing flow rate Q, while the average finger width increases with Q, both trends opposite to the ST case. Bulk rheology on related mixtures indicates a gel-like state. Comparison of experiments using other oils indicates the importance of pH and the carboxylic head group in the formation of the surfactant-fatty acid material.
“…In the bulk, the scaling regimes that ensue at long times are relatively well understood [21]. While it has been shown that hydrodynamic instabilities may arrest the coarsening process [76], the role of wetting conditions on liquid-vapor transitions in porous media remains largely unexplored. The practical relevance of these phase ordering processes lies in the fact that they may control the initial distribution of fluids within the pore space as well as phase 084302-13 connectivity.…”
Section: Spinodal Decomposition Of a Van Der Waals Fluid In A Poromentioning
confidence: 99%
“…After the initial separation phase the pore domains occupied by the different phases will grow in a coarsening process that evolves towards the minimization of interfacial energy. In the bulk, the scaling regimes that ensue at long times are relatively well understood [21], although hydrodynamic instabilities may arrest the classical coarsening process [76].…”
The combination of high-resolution visualization techniques and pore-scale flow modeling is a powerful tool used to understand multiphase flow mechanisms in porous media and their impact on reservoir-scale processes. One of the main open challenges in pore-scale modeling is the direct simulation of flows involving multicomponent mixtures with complex phase behavior. Reservoir fluid mixtures are often described through cubic equations of state, which makes diffuse-interface, or phase-field, theories particularly appealing as a modeling framework. What is still unclear is whether equation-of-state-driven diffuse-interface models can adequately describe processes where surface tension and wetting phenomena play important roles. Here we present a diffuse-interface model of single-component two-phase flow (a van der Waals fluid) in a porous medium under different wetting conditions. We propose a simplified Darcy-Korteweg model that is appropriate to describe flow in a Hele-Shaw cell or a micromodel, with a gap-averaged velocity. We study the ability of the diffuse-interface model to capture capillary pressure and the dynamics of vaporization-condensation fronts and show that the model reproduces pressure fluctuations that emerge from abrupt interface displacements (Haines jumps) and from the breakup of wetting films.
“…The driving force is the capillary pressure difference between the bubbles (Figures 1a-1c). At equilibrium, Ostwald ripening leads to the formation of a single large bubble that may become hydrodynamically unstable (Fu et al, 2016). In the presence of confinement, however, the behavior of Ostwald ripening is modulated such that stable bubble configurations, with uniform capillary pressure, can exist.…”
Section: 1029/2019gl085175mentioning
confidence: 99%
“…Correspondence to: K. Xu While it is known that capillary trapping is hydrodynamically stable, its thermodynamic stability has been the subject of investigation only recently (Xu et al, 2017, de Chalendar et al, 2018, Garing et al, 2017. In the absence of geometric confinement, Ostwald ripening (Fu et al, 2016, Voorhees, 1985, Voorhees, 1992 is the mechanism that causes the diffusion of CO 2 from small bubbles towards large bubbles. The driving force is the capillary pressure difference between the bubbles (Figures 1a-1c).…”
Capillary or residual trapping is considered one of the safest geologic CO 2 storage mechanisms due to its hydrodynamic stability. We present a first study of the impact of gravity on Ostwald ripening in porous media and show it may render capillary trapping thermodynamically unstable. Gravity induces a vertical chemical potential gradient that leads to the upwards diffusion of CO 2 . Thus, bubbles at shallower depths grow at the expense of bubbles in deeper strata, leading to the formation of an overriding gas cap. We first develop a pore-scale model for two bubbles trapped within adjacent pores and then upscale it to obtain a one-dimensional continuum model. We use the latter to predict the macroscopic evolution of a trapped bubble population. Factors controlling the ripening process are isolated to assist in selecting CO 2 storage sites. Gravity-induced ripening may also play a role in geologic fluid emplacement and migration over millions of years.Plain Language Summary Capillary trapping is a mechanism that ensures that the CO 2 injected during geologic carbon sequestration remains stable and safe underground. However, the stability of sequestered CO 2 can be disrupted by another competing mechanism called Ostwald ripening. In Ostwald ripening, CO 2 is transported from smaller bubbles towards larger bubbles. This is undesirable since large bubbles may become remobilized and potentially leak towards the surface. In this study, we show that gravity plays a crucial role in modifying the behavior of Ostwald ripening. Specifically, gravity causes an upwards migration of CO 2 towards shallower depths, which over long periods of time can lead to the formation of a mobile gas cap at the risk of leakage. The models we develop herein reveal that it is possible to decelerate or even prevent CO 2 migration if we carefully select sequestration sites that have the right kind of geology and rock type.
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