Gianluca Blois

A methodology for velocity field measurement in multiphase high‐pressure flow of CO2 and water in micromodels

This paper presents a novel methodology for capturing instantaneous, temporally and spatially resolved velocity fields in an immiscible multiphase flow of liquid/supercritical CO2 and water through a porous micromodel. Of interest is quantifying pore-scale flow processes relevant to geological CO2 sequestration and enhanced oil recovery, and in particular, at thermodynamic conditions relevant to geological reservoirs. A previously developed two-color microscopic particle image velocimetry approach is combined with a high-pressure apparatus, facilitating flow quantification of water interacting with supercritical CO2. This technique simultaneously resolves (in space and time) the aqueous phase velocity field as well as the dynamics of the menisci. The method and the experimental apparatus are detailed, and the results are presented to demonstrate its unique capabilities for studying pore-scale dynamics of CO2-water interactions. Simultaneous identification of the boundary between the two fluid phases and quantification of the instantaneous velocity field in the aqueous phase provides a step change in capability for investigating multiphase flow physics at the pore scale at reservoir-relevant conditions.

Effect of bed permeability and hyporheic flow on turbulent flow over bed forms.

This paper uses particle imaging velocimetry to provide the first measurements detailing the flow field over a porous bed in the presence of bed forms. The results demonstrate that flow downstream of coarse-grained bed forms on permeable beds is fundamentally different to that over impermeable beds. Most significantly, the leeside flow separation cell is greatly modified by jets of fluid emerging from the subsurface, such that reattachment of the separated flow does not occur and the Reynolds stresses bounding the separation zone are substantially lessened. These results shed new light on the underlying flow physics and advance our understanding of both ecological and geomorphological processes associated with permeable bed forms. Water fluxes at the bed interface are critically important for biogeochemical cycling in all rivers, yet mass and momentum exchanges across the bed interface are not routinely incorporated into flow models. Our observations suggest that ignoring such exchange processes in coarse-grained rivers may overlook important implications. These new results also provide insight to explain the distinctive morphology of coarse-grained bed forms, the production of openwork textures in gravels, and the absence of ripples in coarse sands, all of which have implications for modeling and prediction of sediment entrainment and flow resistance.

Micro‐PIV measurements of multiphase flow of water and liquid CO2 in 2‐D heterogeneous porous micromodels

We present an experimental study of pore-scale flow dynamics of liquid CO2 and water in a two-dimensional heterogeneous porous micromodel, inspired by the structure of a reservoir rock, at reservoir-relevant conditions (80 bar, 21 °C). The entire process of CO2 infiltration into a water-saturated micromodel was captured using fluorescence microscopy and the micro-PIV method, which together reveal complex fluid displacement patterns and abrupt changes in velocity. The CO2 front migrated through the resident water in an intermittent manner, forming dendritic structures, termed fingers, in directions along, normal to, and even opposing the bulk pressure gradient. Such characteristics indicate the dominance of capillary fingering through the micromodel. Velocity burst events, termed Haines jumps, were also captured in the heterogeneous micromodel, during which the local Reynolds number was estimated to be ∼21 in the CO2 phase, exceeding the range of validity of Darcys law. Furthermore, these drainage events were observed to be cooperative (i.e., across multiple pores simultaneously), with the zone of influence of such events extending beyond tens of pores, confirming, in a quantitative manner, that Haines jumps are non-local phenomena. After CO2 completely breaks through the porous section, shear-induced circulations caused by flowing CO2 were also observed, in agreement with previous studies using a homogeneous porous micromodel. To our knowledge, this study is the first quantitative measurement that incorporates both reservoir-relevant conditions and rock-inspired heterogeneity, and thus will be useful for pore-scale model development and validation.

A numerical investigation into the importance of bed permeability on determining flow structures over river dunes

Although permeable sediments dominate the majority of natural environments past work concerning bedform dynamics has considered the bed to be impermeable, and has generally neglected flow between the hyporheic zone and boundary layer. Herein, we present results detailing numerically modelled flow which allow the effects of bed permeability on bedform dynamics to be assessed. Simulation of an isolated impermeable bedform over a permeable bed shows that flow is forced into the bed upstream of the dune and returns to the boundary layer at the leeside, in the form of returning jets that generate horseshoe-shaped vortices. The returning flow significantly influences the leeside flow, modifying the separation zone, lifting the shear layer adjoining the separation zone away from the bed. Simulation of a permeable dune on a permeable bed reveals even greater modifications as the flow through the dune negates the formation of any flow separation in the leeside. With two dunes placed in series the flow over the downstream dune is influenced by the developing boundary layer on the leeside of the upstream dune. For the permeable bed case the upwelling flow lifts the separated flow from the bed, modifies the shear layer through the coalescence with vortices generated, and causes the shear layer to undulate rather than be parallel to the bed. These results demonstrate the significant effect that bed permeability has on the flow over bedforms that may be critical in affecting the flux of water and nutrients.

Volumetric Velocity Measurements in the Wake of a Hemispherical Roughness Element

Plenoptic particle image velocimetry was used to perform instantaneous three-dimensional velocity measurements in the near wake of a wall-mounted hemispherical roughness element at a Reynolds number (based on roughness height) of 4.57×103 and boundary layer to roughness height ratio of 2.4. The experiment was performed in a refractive index matched flow facility to mitigate laser reflections from the hemispherical surface. Data gathered from this experiment represented one of the first applications of plenoptic particle image velocimetry. The ensemble-averaged flow is characterized by a separated shear layer and a symmetric recirculation region. In the instantaneous three-dimensional velocity fields, a separated shear layer and recirculation region, both with asymmetric characteristics, are present. Additionally, arch vortices are found that are detached to the hemispherical surface. The proper orthogonal decomposition was applied to both the three-dimensional velocity and three-dimensional vorticity fiel...

Turbulence Links Momentum and Solute Exchange in Coarse‐Grained Streambeds

The exchange of solutes between surface and pore waters is an important control over stream ecology and biogeochemistry. Free-stream turbulence is known to enhance transport across the sediment-water interface (SWI), but the link between turbulent momentum and solute transport within the hyporheic zone remains undetermined due to a lack of in situ observations. Here, we relate turbulent momentum and solute transport using measurements within a streambed with 0.04 m diameter sediment. Pore water velocities were measured using endoscopic particle image velocimetry and used to generate depth profiles of turbulence statistics. Solute transport was observed directly within the hyporheic zone using an array of microsensors. Solute injection experiments were used to assess turbulent fluxes across the SWI and patterns of hyporheic mixing. Depth profiles of fluctuations in solute concentration were compared with profiles of turbulence statistics, and profiles of mean solute concentration were compared to an effective dispersion model. Fluorescent visualization experiments at a Reynolds number of Re 27,000 revealed the presence of large-scale motions that ejected tracer from the pore waters, and that these events were not present at Re 5 13,000. Turbulent shear stresses and high-frequency concentration fluctuations decayed greatly within 1–2 grain diameters below the SWI. However, low-frequency concentration fluctuations penetrated to greater depths than high-frequency fluctuations. Comparison with a constant-coefficient dispersion model showed that hyporheic mixing was enhanced in regions where turbulent stresses were observed. Together, these results show that the penetration of turbulence into the bed directly controls both interfacial exchange and mixing within a transition layer below the SWI. Plain Language Summary Streams and rivers continuously exchange water with their underlying sediments in a region called the hyporheic zone. This zone is a hotspot of transformation for many societally relevant chemicals, including carbon, nutrients, and contaminants. Accurate predictions for how much transformation occurs in the hyporheic zone requires an improved understanding of how reactive chemicals are transported into, and within, this region of a riverbed. Although fluid turbulence can be the dominant process controlling surface-subsurface exchange in gravel-bed streams, its influence is poorly understood due to the difficulty of measuring turbulent fluid velocities and concentrations within the streambed. In this experimental study, we show that turbulence strongly couples surface waters with hyporheic waters in a thin layer where the water column and stream sediments meet. As a result, fluid transport and mixing are enhanced several centimeters into the hyporheic zone of gravel-bed streams. These findings support recent theoretical arguments that surface and subsurface waters are not independent and must instead be treated as a single unit to accurately model solute, particulate and pollutant transport in streams and rivers.