Ran Holtzman

Wettability Stabilizes Fluid Invasion into Porous Media via Nonlocal, Cooperative Pore Filling.

We study the impact of the wetting properties on the immiscible displacement of a viscous fluid in disordered porous media. We present a novel pore-scale model that captures wettability and dynamic effects, including the spatiotemporal nonlocality associated with interface readjustments. Our simulations show that increasing the wettability of the invading fluid (the contact angle) promotes cooperative pore filling that stabilizes the invasion and that this effect is suppressed as the flow rate increases, due to viscous instabilities. We use scaling analysis to derive two dimensionless numbers that predict the mode of displacement. By elucidating the underlying mechanisms, we explain classical yet intriguing experimental observations. These insights could be used to improve technologies such as hydraulic fracturing, CO2 geosequestration, and microfluidics.

Effects of Pore-Scale Disorder on Fluid Displacement in Partially-Wettable Porous Media

We present a systematic, quantitative assessment of the impact of pore size disorder and its interplay with flow rates and wettability on immiscible displacement of a viscous fluid. Pore-scale simulations and micromodel experiments show that reducing disorder increases the displacement efficiency and compactness, minimizing the fluid-fluid interfacial area, through (i) trapping at low rates and (ii) viscous fingering at high rates. Increasing the wetting angle suppresses both trapping and fingering, hence reducing the sensitivity of the displacement to the underlying disorder. A modified capillary number Ca* that includes the impact of disorder λ on viscous forces (through pore connectivity) is direct related to λ, in par with previous works. Our findings bear important consequences on sweep efficiency and fluid mixing and reactions, which are key in applications such as microfluidics to carbon geosequestration, energy recovery, and soil aeration and remediation.

Impact of spatially correlated pore‐scale heterogeneity on drying porous media

We study the effect of spatially-correlated heterogeneity on isothermal drying of porous media. We combine a minimal pore-scale model with microfluidic experiments with the same pore geometry. Our simulated drying behavior compares favorably with experiments, considering the large sensitivity of the emergent behavior to the uncertainty associated with even small manufacturing errors. We show that increasing the correlation length in particle sizes promotes preferential drying of clusters of large pores, prolonging liquid connectivity and surface wetness and thus higher drying rates for longer periods. Our findings improve our quantitative understanding of how pore-scale heterogeneity impacts drying, which plays a role in a wide range of processes ranging from fuel cells to curing of paints and cements to global budgets of energy, water and solutes in soils.

Drying in a microfluidic chip: experiments and simulations

We present an experimental micro-model of drying porous media, based on microfluidic cells made of arrays of pillars on a regular grid, and complement these experiments with a matching two-dimensional pore-network model of drying. Disorder, or small-scale heterogeneity, was introduced into the cells by randomly varying the radii of the pillars, around their average value. The microfluidic chips were filled with a volatile oil and then dried horizontally, such that gravitational effects were excluded. The experimental and simulated drying rates and drying patterns were then compared in detail, for various levels of disorder, in order to verify the predictive capabilities of our model. The geometrical features were reproduced well, while reproducing drying rates proved to be more challenging.We present an experimental micro-model of drying porous media, based on microfluidic cells made of arrays of pillars on a regular grid, and complement these experiments with a matching two-dimensional pore-network model of drying. Disorder, or small-scale heterogeneity, was introduced into the cells by randomly varying the radii of the pillars. The microfluidic chips were filled with a volatile oil and then dried horizontally, such that gravitational effects were excluded. The experimental and simulated drying rates and patterns were then compared in detail, for various levels of disorder. The geometrical features were reproduced well, although the model under-predicted the formation of trapped clusters of drying fluid. Reproducing drying rates proved to be more challenging, but improved if the additional trapped clusters were added to the model. The methods reported can be adapted to a wide range of multi-phase flow problems, and allow for the rapid development of high-precision micro-models containing tens of thousands of individual elements.

Micromechanics of Hydrate Dissociation in Marine Sediments by Grain-Scale Simulations

We seek to quantify the impact of hydrate dissociation on the strength of hydrate-bearing sediments. Dissociation of gas-hydrates in marine sediments converts the solid hydrate structure into liquid water and gas. Together with the associated pore pressure increase, this process reduces the stiffness of the sediments, which may fracture or be fluidized. If sediment failure occurs, seafloor subsidence and landslides can severely damage offshore infrastructure. To evaluate the mechanical properties of a sediment sample, we simulate loading of a disordered pack of spherical grains by incremental displacements of its boundaries. The deformation is described as a sequence of equilibrium configurations. Each configuration is characterized by a minimum of the total potential energy. This minimum is computed using a modification of the conjugate gradient algorithm. We verify our model against published data from experiments on glass beads. Our simulations capture the nonlinear, path-dependent behavior of granular materials observed in experiments. Hydrates are modeled as load-bearing solid particles within the pores. To simulate the consequences of dissociation, we reduce the solid fraction by shrinking the hydrate grains. The effect of the associated excess pore pressure is modeled by isotropic compression of the solid grains, and reduction in macroscopic effective stress. Weakening of the sediment is quantified as a reduction of the effective elastic moduli.

Water Sources and Quality along the Lower Jordan River, Regional Study

The Lower Jordan River received in the past a large volume of freshwater from Lake Tiberias, the Yarmouk River, and local runoffs. Currently, a much smaller flow-rate of mostly poor-quality fluids enters the river. The severe reduction of inflow and the poor-quality flows have led directly to the degradation of the water quality along the river. According to the regional peace agreements, both sewage and saline waters will be treated and used. Carrying out these agreements will result in a dramatic reduction of input flow-rates into the river. Under these circumstances, the almost sole available source will be drainage and groundwater. The objective of this study is to evaluate the different components that presently control the quality of water in the Lower Jordan River. In particular, the study is looking for ways to assess the role played by the subsurface contributions. We present here preliminary results of an ongoing research, which involves researchers from Israel, Jordan, and the Palestinian Authority. By means of water sampling, chemical analysis, isotope analysis, flow-rate measurements, and mass balance calculations, the study improves our understanding of the hydrology and hydrochemistry of the river system.

Dispersive transport and symmetry of the dispersion tensor in porous media

The macroscopic laws controlling the advection and diffusion of solute at the scale of the porous continuum are derived in a general manner that does not place limitations on the geometry and time evolution of the pore space. Special focus is given to the definition and symmetry of the dispersion tensor that is controlling how a solute plume spreads out. We show that the dispersion tensor is not symmetric and that the asymmetry derives from the advective derivative in the pore-scale advection-diffusion equation. When flow is spatially variable across a voxel, such as in the presence of a permeability gradient, the amount of asymmetry can be large. As first shown by Auriault [J.-L. Auriault et al. Transp. Porous Med. 85, 771 (2010)TPMEEI0169-391310.1007/s11242-010-9591-y] in the limit of low Péclet number, we show that at any Péclet number, the dispersion tensor D_{ij} satisfies the flow-reversal symmetry D_{ij}(+q)=D_{ji}(-q) where q is the mean flow in the voxel under analysis; however, Reynolds number must be sufficiently small that the flow is reversible when the force driving the flow changes sign. We also demonstrate these symmetries using lattice-Boltzmann simulations and discuss some subtle aspects of how to measure the dispersion tensor numerically. In particular, the numerical experiments demonstrate that the off-diagonal components of the dispersion tensor are antisymmetric which is consistent with the analytical dependence on the average flow gradients that we propose for these off-diagonal components.

Geochemical Processes During Managed Aquifer Recharge With Desalinated Seawater

We study geochemical processes along the variably-saturated zone during managed aquifer recharge (MAR) with reverse-osmosis desalinated seawater (DSW). The DSW, post-treated at the desalination plant by calcite dissolution (remineralization) to meet the Israeli water quality standards, is recharged into the Israeli Coastal Aquifer through an infiltration pond. Water quality monitoring during two MAR events using suction cups and wells inside the pond, indicates that cation exchange is the dominant subsurface reaction, driven by the high Ca 2+ concentration in the post-treated DSW. Stable isotope analysis shows that the shallow groundwater composition is similar to the recharged DSW, except for enrichment of Mg 2+ , Na + , Ca 2+ and HCO3 . A calibrated variably-saturated reactive transport model is used to predict the geochemical evolution during 50 years of MAR for two water quality scenarios: (i) post-treated DSW (current practice); and (ii) soft DSW (lacking the remineralization post-treatment process). The latter scenario was aimed to test soil-aquifer-treatment (SAT) as an alternative posttreatment technique. Both scenarios provide an enrichment of ~2.5 mg L -1 in Mg 2+ due to cationexchange, compared to practically zero Mg 2+ currently found in the Israeli DSW. Simulations of the alternative SAT scenario provide Ca 2+ and HCO3 remineralization due to calcite dissolution at levels that meet the Israeli standard for DSW. The simulated calcite content reduction in the sediments below the infiltration pond after 50 years of MAR was low (<1%). Our findings suggest that remineralization using SAT for DSW is a potentially sustainable practice at MAR sites overlying calcareous sandy aquifers. This article is protected by copyright. All rights reserved.

Reactive transport under stress: Permeability evolution in deformable porous media

We study reactive transport in a stressed porous media, where dissolution of the solid matrix causes two simultaneous, competing effects: pore enlargement (chemical deformation), and pore compaction due to mechanical weakening. A novel, mechanistic pore-scale model simulates flooding of a sample under fixed confining stress, showing that increasing stress inhibits permeability enhancement, increasing the injected volume required to reach a certain permeability, in agreement with recent experiments. We explain this behavior by stress concentration downstream, in the less dissolved (hence stiffer) region. As this region is also less conductive, even its small compaction has a strong bottleneck effect that curbs the permeability. Our results also elucidate that the impact of stress depends on the dissolution regime. Under wormholing conditions (slow injection, i.e. high Damkohler,

The origin and mechanisms of salinization of the Lower Jordan River

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