Silicon Micromodels Research Articles

Micro X-ray fluorescence reveals pore space details and spatially-resolved porosity of rock-based microfluidic devices.

Characterization of microscopic details of the fabric of mudstones and shales (i.e., structure and composition) is important to understand their storage and transport properties. Current characterization methods struggle to probe reliably multiple scales of interest (e.g., pore and fracture) and measure properties at the finest resolution under representative in situ conditions. Micro X-ray fluorescence (μXRF) is a high-performance imaging technique that produces elemental images at sub-10 μm spatial resolution and could offer insight into a diversity of shale properties, such as mineral composition, porosity, and in situ pressure gradients. This study designed and carried out a porosity mapping protocol using model and real-rock microfluidic devices and contrast fluids. Etched silicon micromodels with real-rock pore network patterns served as ideal models to establish a proof of concept. Measurements were performed on a novel μXRF microscope not powered by synchrotron radiation. We registered the μXRF datasets with the binary rock masks used for micromodel fabrication and applied segmentation algorithms to compare porosities. We assessed expected advantages and limitations through a sensitivity analysis and beam study. μXRF is an important new imaging technique for microfluidic applications.

Impact of Surface Roughness on Evaporation in 2D Micromodels

AbstractEvaporation—a key process for water exchange between soil and atmosphere—is controlled by internal water fluxes and surface vapor fluxes. Recent studies demonstrated that the dynamics of the water flow in pore corners and thick‐film flow on the rough pore‐solid interface determine the time behavior of the evaporation rate. A comprehensive experimental study of the combined effect of corner flow (CF) and thick‐film flow (TFF) on evaporation is lacking. Herein, we present a comparative micromodel study of the evaporation process using 2D‐porous media, which exhibit the same stochastic pore structure, but different degree of surface roughness (silicon micromodels with smooth surface and glass‐ceramic micromodels with rough surface). Our study proves both experimentally and theoretically that surface roughness and wettability play a key role for the time and temperature behaviors of the evaporation process. We found a similar behavior up to 42°C regarding the mass loss as a function of time in stage‐2 and a transition to linear t behavior for higher temperatures (61°C) with a high statistical significance (regression coefficients near 1) for both micromodels. Our experimental results elucidate the strong temporal correlation between mass loss and geometric pattern of the unsaturated CF and TFF region. Partial wetting in silicon micromodels causes a weak driving force for CF, whereas complete wetting in glass‐ceramic micromodels causes a strong driving force for TFF. For a consistent description of the time‐dependent mass loss and geometry of the CF/TFF region, the fractality of the evaporation front must be considered. Based on our data, we estimate the film thickness and discover some interesting film‐thinning effect with increasing temperature.

Quality Evaluation of Epoxy Pore Casts Using Silicon Micromodels: Application to Confocal Imaging of Carbonate Samples

Pore casting refers to filling the void spaces of porous materials with an extraneous fluid, usually epoxy resin, to obtain a high-strength composite material, stabilize a fragile porous structure, produce a three-dimensional replica of the pore space, or provide imaging contrast. Epoxy pore casting may be accompanied by additional procedures, such as etching, in which the material matrix is dissolved, leaving a clean cast. Moreover, an epoxy resin may be mixed with fluorophore substances to allow fluorescence imaging. Our work aims to investigate and optimize the epoxy pore casting procedure parameters, for example, impregnation pressure. We use silicon micromodels as a reference to validate the key parameters of high-pressure resin impregnation. We demonstrate possible artifacts and defects that might develop during impregnation with resin, e.g., resin shrinkage and gas trapping. In the end, we developed an optimized protocol to produce high-quality resin pore casts for high-resolution 3D imaging and the description of microporosity in micritic carbonates. In our applications, the high-quality pore casts were acid-etched to remove the non-transparent carbonate material, making the pore casts suitable for imaging with Confocal Laser Scanning Microscopy (CLSM). In addition, we evaluate the quality of our etching procedure using micro-computed tomography (micro-CT) scans of the pre- and post-etched samples and demonstrate that the etched epoxy pore casts represent the pore space of microporous carbonate rock samples with high fidelity.

Deposition of protein-coated multi-walled carbon nanotubes on oxide surfaces and the retention in a silicon micromodel.

The aggregation, deposition and porous retention of bovine serum albumin treated multi-walled carbon nanotubes (BSA-MWCNTs) are investigated using dynamic light scattering (DLS), quartz crystal microbalance with dissipation (QCM-D) and 2-dimensional silicon micromodel, respectively. The aggregation of BSA-MWCNTs is consistent with Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. The critical coagulation concentration (CCC) is 175 mM NaCl and 2.7 mM CaCl2, suggesting that Ca2+ causes stronger aggregation. The BSA-MWCNT deposition on SiO2 surface is unfavorable with critical deposition concentration (CDC) of 100 mM in NaCl and 0.9 mM in CaCl2. The deposition on the Al2O3 surface is favorable. Deposition rate is dominated by electrostatic forces at low ionic strength (IS), but electrostatic interaction is eliminated when IS is above CDC. Therefore the deposition rate on SiO2 or Al2O3 surface starts decreasing at the CDC point due to the reduced particle diffusion. In micromodel, the amount and position of attached BSA-MWCNTs in pore space can be observed by a microscope. The retention attachment efficiency increases at higher IS. The suspended BSA-MWCNTs approach to the collector through either diffusion or interception. The attached BSA-MWCNTs narrow the pore space and then clog the pore throats. The straining process happens on the clogged pore throats.

Pore-level hydrate formation mechanisms using realistic rock structures in high-pressure silicon micromodels

We report pore-level visualization of CO2 and CH4 gas hydrate formation with dynamic growth patterns within a 2D porous silicon wafer micromodel with realistic pore shapes and sizes. Direct observation of pore-scale hydrate growth patterns at reservoir conditions provides an improved framework for fundamental studies of sediment-hosted gas hydrates. The growth pattern and formation rate are discussed with respect to wetting films, fluid distribution, and PT conditions for CH4 and CO2 hydrate. Hydrates appeared initially on the interface between gas spheres and thin water films coating the grain surface, followed by a continuous hydrate growth throughout the pore space. Redistribution of hydrate was observed over several hours at stable conditions. Furthermore, repeated hydrate formation occurred more rapidly and at higher temperatures than during primary formation. The understanding of pore-scale growth of sedimentary gas hydrate has great value in describing hydrates in a geological setting and toward production of a vast energy resource. It may also be a key feature in CO2 storage schemes.

Quantifying the flow dynamics of supercritical CO2–water displacement in a 2D porous micromodel using fluorescent microscopy and microscopic PIV

The multi-phase flow of liquid/supercritical CO2 and water (non-wetting and wetting phases, respectively) in a two-dimensional silicon micromodel was investigated at reservoir conditions (80 bar, 24 °C and 40 °C). The fluorescent microscopy and microscopic particle image velocimetry (micro-PIV) techniques were combined to quantify the flow dynamics associated with displacement of water by CO2 (drainage) in the porous matrix. To this end, water was seeded with fluorescent tracer particles, CO2 was tagged with a fluorescent dye and each phase was imaged independently using spectral separation in conjunction with microscopic imaging. This approach allowed simultaneous measurement of the spatially-resolved instantaneous velocity field in the water and quantification of the spatial configuration of the two fluid phases. The results, acquired with sufficient time resolution to follow the dynamic progression of both phases, provide a comprehensive picture of the flow physics during the migration of the CO2 front, the temporal evolution of individual menisci, and the growth of fingers within the porous microstructure. During that growth process, velocity jumps 20–25 times larger in magnitude than the bulk velocity were measured in the water phase and these bursts of water flow occurred both in-line with and against the bulk flow direction. These unsteady velocity events support the notion of pressure bursts and Haines jumps during pore drainage events as previously reported in the literature [1–3]. After passage of the CO2 front, shear-induced flow was detected in the trapped water ganglia in the form of circulation zones near the CO2–water interfaces as well as in the thin water films wetting the surfaces of the silicon micromodel. To our knowledge, the results presented herein represent the first quantitative spatially and temporally resolved velocity-field measurements at high pressure for water displacement by liquid/supercritical CO2 injection in a porous micromodel.

Microvisual investigation of polymer retention on the homogeneous pore network of a micromodel

Abstract A new experimental technique is reported for visualization of polymer retention on the solid surfaces of porous media. Etched silicon micromodels with well-characterized pore networks were used during single-phase flow to examine the retention of 0.2 wt% partially hydrolyzed polyacrylamide (HPAM) solution. Image analysis included an image subtraction and an RGB-based global thresholding technique for quantification of polymer retention/adsorption. Results are reported as the percentage of porosity occupied by immobile polymer. Three factors were investigated including the salinity of displacing water, the change of wettability of the micromodel surface, and mechanical degradation of polymer. With respect to salinity in displacing water, the experiment confirmed that 5 wt% NaCl results in less polymer retention (6.3±0.3%) than without NaCl (7.5±0.3%). The increase in Na + concentration was sufficient to induce contraction of the size of the flexible HPAM molecules and, therefore, decrease the thickness of polymer adsorption on the grain. Two methods were used to alter the initially strongly water-wet surface of the micromodel. Wettability was changed by the deposition of crude oil and CTAB (cetyl trimethylammonium bromide) in aqueous solution. The micromodels treated using crude oil and CTAB showed polymer retention of 15.0±0.3%% and 5.0±0.3%%, respectively. Oil-wet micromodels aged by crude oil showed larger polymer retention than the polymer retention on water-wet micromodels (7.5±0.3%). Otherwise, the polymer retention on the CTAB treated micromodel was lower than the polymer retention on the water-wet micromodel. Finally, polymer solution flowed through a 7 µm filter was tested. Average polymer retention was 4.3±0.3% and this is 3.2±0.3% point lower than the value of unfiltered polymer solution. Further investigation of microgels within pore networks provided a chance to demonstrate that a size and structural flexibility of microgel leading to a transition from mobile to immobile conditions.

Silane modification of glass and silica surfaces to obtain equally oil‐wet surfaces in glass‐covered silicon micromodel applications

Wettability is a key parameter influencing capillary pressures, permeabilities, fingering mechanisms, and saturations in multiphase flow processes within porous media. Glass‐covered silicon micromodels provide precise structures in which pore‐scale displacement processes can be visualized. The wettability of silicon and glass surfaces can be modified by silanization. However, similar treatments of glass and silica surfaces using the same silane do not necessarily yield the same wettability as determined by the oil‐water contact angle. In this study, surface cleaning pretreatments were investigated to determine conditions that yield oil‐wet surfaces on glass with similar wettability to silica surfaces treated with the same silane, and both air‐water and oil‐water contact angles were determined. Borosilicate glass surfaces cleaned with standard cleaning solution 1 (SC1) yield intermediate‐wet surfaces when silanized with hexamethyldisilazane (HMDS), while the same cleaning and silanization yields oil‐wet surfaces on silica. However, cleaning glass in boiling concentrated nitric acid creates a surface that can be silanized to obtain oil‐wet surfaces using HMDS. Moreover, this method is effective on glass with prior thermal treatment at an elevated temperature of 400°C. In this way, silica and glass can be silanized to obtain equally oil‐wet surfaces using HMDS. It is demonstrated that pretreatment and silanization is feasible in silicon‐silica/glass micromodels previously assembled by anodic bonding, and that the change in wettability has a significant observable effect on immiscible fluid displacements in the pore network.

Colloid dispersion on the pore scale

Dispersion describes the spreading of a tracer or contaminant in an aquifer. Detailed knowledge of dispersion is the key to successful risk assessment in case of groundwater pollution or groundwater protection. The dispersion of colloids on the pore scale is controlled by flow velocity, ionic strength, colloid size, colloid concentration, and colloid–matrix interactions. The objective of this study was to provide quantitative data and to assess the scale dependency of colloid dispersion on the pore scale. The positions of carboxylated polystyrene microspheres (1 μm, 0.5 μm) were recorded during transport experiments in silicon micromodels with three pore topologies. The positions were combined into particle trajectories revealing the flow path of individual colloids. More than thousand trajectories were evaluated for each experiment to obtain the dispersivity of the colloids for flow distances between 10 and 1000 μm. All experiments were run at high Peclet numbers. The pore scale dispersivity was on the order of 8–30% of the flow distance with pure water, dependent on the heterogeneity of the pore topology. The dispersivity was positively correlated with the ionic strength and inversely correlated with the colloid size and the flow velocity. A coating of the micromodel surface with humic acid also increased dispersivity. The quantitative data set presented here supports the theoretical framework for colloid transport and allows to parametrize colloid transport on the pore scale.

Verification of Roof snap off as a foam-generation mechanism in porous media at steady state

An extensive microvisual analysis is reported of the hydrodynamics of simultaneous gas and water flow through homogeneous etched silicon micromodels. The object of this work is to prove, or to disprove, the relevance of Roof snap off during steady-state gas and water flow in homogeneous porous media. Significant evidence is found that repeated snap off of gas bubbles at pore throats is relevant to steady-state flow through porous media. Importantly, snap off is shown to be the dominant mechanism for generation of a discontinuous gas phase within this porous medium. Experiments are conducted under steady flow conditions in a relatively dry porous medium at significant capillary pressure. Smoothly constricted pores that exhibit small pore throat to pore body aspect ratios are found to be excellent sites for snap off even for low initial wetting liquid contents.

Experimental and modeling study of Newtonian and non-Newtonian fluid flow in pore network micromodels

The in situ rheology of polymeric solutions has been studied experimentally in etched silicon micromodels which are idealizations of porous media. The rectangular channels in these etched networks have dimensions typical of pore sizes in sandstone rocks. Pressure drop/flow rate relations have been measured for water and non-Newtonian hydrolyzed-polyacrylamide (HPAM) solutions in both individual straight rectangular capillaries and in networks of such capillaries. Results from these experiments have been analyzed using pore-scale network modeling incorporating the non-Newtonian fluid mechanics of a Carreau fluid. Quantitative agreement is seen between the experiments and the network calculations in the Newtonian and shear-thinning flow regions demonstrating that the ‘shift factor,’ α, can be calculated a priori. Shear-thickening behavior was observed at higher flow rates in the micromodel experiments as a result of elastic effects becoming important and this remains to be incorporated in the network model.

Visualization and Modeling of Polystyrol Colloid Transport in a Silicon Micromodel

A new experimental approach and complementary model analysis are presented for studying colloid transport and fate in porous media. The experimental approach combines high precision etching to create a controlled pore network in a silicon wafer (i.e., micromodel), with epifluorescent microscopy. Two different sizes of latex colloids were used; each was stained with a fluorescent dye. During an experiment, water with colloids was purged through a micromodel at different flow rates. Flow paths and particle velocities were determined and compared with flow paths calculated using a two‐dimensional (2D) lattice Boltzmann (LB) model. For 50% of the colloids evaluated, agreement between measured and calculated flow paths and velocities were excellent. For 20%, flow paths agreed, but calculated velocities were less. This is attributed to the parabolic velocity profile across the micromodel depth, which was not accounted for in the 2D flow model. For 12%, flow paths also agreed, but calculated velocities were less. These colloids were close to grain surfaces, where model errors increase. Also, particle–surface interactions were not accounted for in the model; this may have contributed to the discrepancy. For the remaining 18% of colloids evaluated, neither flow paths nor velocities agreed. The majority of colloids in this last case were observed after breakthrough, when concentrations were high. The discrepancies may be due to particle–particle interactions that were not accounted for in the model. Filtration efficiencies for all colloid sizes at different flow rates were calculated from filtration theory. Attachment rates were obtained from successive images during an experiment. With these, attachment efficiencies were calculated, and these agreed with literature values. The study demonstrates that excellent agreement between experimental and model results for colloid transport at the pore scale can be obtained. The results also demonstrate that when experimental and model results do not agree, mechanistic inferences and system limitations can be evaluated.

Visualization and Modeling of Polystyrol Colloid Transport in a Silicon Micromodel

Visualization and Modeling of Polystyrol Colloid Transport in a Silicon Micromodel