Lab on a Chip

PAPER

Cite this: DOI: 10.1039/c6lc01343k

Received 28th October 2016,Accepted 3rd March 2017

DOI: 10.1039/c6lc01343k

rsc.li/loc

Creation of a dual-porosity and dual-depthmicromodel for the study of multiphase flow incomplex porous media†‡

Wonjin Yun, Cynthia M. Ross, Sophie Roman and Anthony R. Kovscek*

Silicon-based microfluidic devices, so-called micromodels in this application, are particularly useful labora-

tory tools for the direct visualization of fluid flow revealing pore-scale mechanisms controlling flow and

transport phenomena in natural porous media. Current microfluidic devices with uniform etched depths,

however, are limited when representing complex geometries such as the multiple-scale pore sizes com-

mon in carbonate rocks. In this study, we successfully developed optimized sequential photolithography to

etch micropores (1.5 to 21 μm width) less deeply than the depth of wider macropores (>21 μm width) to

improve the structural realism of an existing single-depth micromodel with a carbonate-derived pore

structure. Surface profilimetry illustrates the configuration of the dual-depth dual-porosity micromodel and

is used to estimate the corresponding pore volume change for the dual-depth micromodel compared to

the equivalent uniform- or single-depth model. The flow characteristics of the dual-depth dual-porosity

micromodel were characterized using micro-particle image velocimetry (μ-PIV), relative permeability mea-

surements, and pore-scale observations during imbibition and drainage processes. The μ-PIV technique

provides insights into the fluid dynamics within microfluidic channels and relevant fluid velocities controlled

predominantly by changes in etching depth. In addition, the reduction of end-point relative permeability

for both oil and water in the new dual-depth dual-porosity micromodel compared to the equivalent

single-depth micromodel implies more realistic capillary forces occurring in the new dual-depth micro-

model. Throughout the imbibition and drainage experiments, the flow behaviors of single- and dual-depth

micromodels are further differentiated using direct visualization of the trapped non-wetting phase and the

preferential mobilization of the wetting phase in the dual-depth micromodel. The visual observations agree

with the relative permeability results. These findings indicate that dual-porosity and dual-depth micro-

models have enhanced physical realism that is pertinent to oil recovery processes in complex porous

media.

1. Introduction

Carbonate reservoirs hold substantial petroleum resources;yet, understanding the flow dynamics in these reservoirs ischallenging due to their propensity toward multi-scale hetero-geneity. For instance, the Ghawar Field of Saudi Arabia is theworld's largest oil field in terms of production (5.8 millionbarrels of oil per day) and total remaining proven oil reservesof 75 billion barrels as of January 1, 2014.1 Production in theGhawar Field as well as other Saudi Arabian fields is predomi-nantly from carbonate reservoir rocks including the prolific

Upper Jurassic Arab Formation.1 Characterizing carbonatereservoirs is often difficult due to multi-scale variability inpore structure and physical properties such as porosity andpermeability. The heterogeneity of the pore structure as wellas mixed surface wettability, typical of many carbonate reser-voirs, results in significant residual oil saturation comparedto most siliciclastic reservoirs. This phenomenon has beenwell documented.2,3

Microporosity, as well as large aspect ratios (that is, the ra-tio of pore-body size to throat size), contributes to the largeresidual oil saturations in carbonates.2,4 Cantrell and Hagerty(1999) report that microporosity (pores 10 μm or less in di-ameter for their study) ranges from 0 to 100% and typicallycomprise 25% to 50% of the total porosity in the most pro-ductive reservoir intervals of the Arab Formation.4 Alterna-tively, large aspect ratios (values >3) inhibit flow and contrib-ute to snap-off in which fluid phases become discontinuousor isolated.5,6

Lab ChipThis journal is © The Royal Society of Chemistry 2017

Stanford University, Energy Resources Engineering, 367 Panama St, room 50,

Stanford, California, USA. E-mail: kovscek@stanford.edu

† Time-resolved digital particle image velocimetry tool for MATLAB (version: 1.4)is an open source software and available from https://dx.doi.org/10.6084/m9.figshare.1092508.v6‡ Electronic supplementary information (ESI) available. See DOI: 10.1039/c6lc01343k

Publ

ishe

d on

06

Mar

ch 2

017.

Dow

nloa

ded

by S

tanf

ord

Uni

vers

ity o

n 23

/03/

2017

00:

21:3

4.

View Article OnlineView Journal

Lab Chip This journal is © The Royal Society of Chemistry 2017

The abundance of microporosity and the complexity of thepore structure coupled with large aspect ratios provide an on-going challenge to understanding the distribution of fluidsand the fluid flow behavior in carbonate reservoirs.7 Thisknowledge enables optimal field development and recoveryefficiencies. In light of this, a focused approach is needed tobetter understand the heterogeneous nature of carbonatescontaining multiple fluid phases and the flow propertieswithin these porous formations. This involves detailed char-acterization (including spatial distribution) of the pore struc-ture (pore-body and throat sizes, surface roughness, and con-nectivity), fluids (properties and saturations), permeability(absolute and relative), and capillarity.

2. Micromodels

Micromodels are 2D representations of porous media that al-low direct visualization of fluid flow behavior at the porescale. Pore-scale observations made using silicon-basedmicromodels have been widely used for understandingmultiphase flow and oil–water–solid interaction in enhancedoil recovery (EOR) studies.8–11 Micromodels have been usedas a tool for the visualization of behaviors such as capillarytrapping/fingering,12,13 observation of micro-emulsionphases,14 pore-level hydrate formation,15 foam coalescence,16

and fluid analysis such as the measurement of minimummiscibility pressure of CO2 in crude oils.17,18

Rock wettability is a key determining factor in the recover-ability of oil from the subsurface.19 Silicon-based photoli-thography creates strongly water-wet surfaces representativeof a clean, unaltered sandstone rock surface. Reservoir wetta-bility converts from strongly water-wet toward either oil-wetor mixed-wettability due to the interaction between mineralsurfaces, connate water, and specific components in the hy-drocarbons.20 To be able to replicate realistic surface interac-tions between reservoir fluids and reservoir rocks, previousstudies presented methods to modify the strongly water-wetsurfaces in silicon-based micromodels.21,22 Alternatively, asimple method has been introduced to fabricate syntheticCaCO3 reservoir micromodels that replicate the surface geo-chemistry of carbonate in microfluidic channels with relativesimple and homogenously aligned posts.23

In order to represent geometrically the complex pore struc-ture of the Arab-D carbonate reservoir in a micromodel,Buchgraber et al. (2012) used silicon-based photolithographywith a mask derived from images of a reservoir sample to cre-ate a dual-porosity micromodel with a carbonate pore net-work pattern and a single etch depth. The dual-porositymicromodel simulates typical Arab-D pore-size heterogeneityincluding both macro-pores (>21 μm) and microporosity (1.5to ≤21 μm).24,25 Using the realistic dual-porosity micromodel,the researchers observed pore-level mechanisms ofmultiphase flow and provided pore-scale quantification offluid phase dynamics by measuring porosity, permeability,fluid desaturation patterns, and recovery factors.

During two-phase flow, the capillary entry pressure (ΔPc,e)must be overcome for the non-wetting fluid (oil) to invadewetting-phase-filled pore throats as shown in Fig. 1. The re-quired capillary entry pressure (ΔPc,e) for a circular cross sec-tion is written as26

(1)

where Rth is pore throat diameter, and σow is the interfacialtension (IFT) between wetting and non-wetting fluids. Thus,the non-wetting fluid intrudes preferentially into pore bodiesaccessed by larger pore-throat diameters unless the drivingforce (ΔPc,e) is sufficient enough to penetrate smaller wetting-phase-filled pore throats.

Single-depth microfluidics devices use microchannelswhere variability in pore size is created by changes in chan-nel width.27 As presented by Wan et al. (1996), the etchingdepth (H) is constant regardless of the pore-size transitionand may cause either over- or underestimation of capillarityeffects.28 To overcome this limitation in micromodels,Trygstad et al. (1986) bonded two etched glass plates with allof the porosity etched onto one surface and only the widerpores onto the opposing surface to fabricate a glass micro-model representing microscopic rock pore heterogeneity.29

Wan et al. (1996) attempted to vary both width and etchingheight using a sequential-step, hydrofluoric (HF) acid etchingtechnique for relatively simple fractures and a homoge-neously patterned matrix structure. More recently, Xu et al.(2017) demonstrated a standard single-step lithography pro-cess using hydrofluoric (HF) acid to etch multilayered glassmicrochips with a homogeneous pattern.30

As yet, there has been no complete and rigorous silicon-based micromodel fabrication technique reported for a com-plex dual-porosity pore network system that mitigates thecapillarity limitation arising from single- or uniform-etchingdepths; hence, the contrast in capillary characteristics be-tween macro- and microporosity of dual-porosity pore net-works have not been fully realized within a micromodel.

Fig. 1 Schematic diagram of oil-phase invasion into water-filled porethroats. (a) Oil ganglia remain trapped when unable to surmount capil-lary entry pressure exerted by the pore throat size (Rth). (b) The drivingforce (left to right) overcomes capillary entry pressure and non-wetting phase (oil) flows into the water-filled pore throat. Adaptedfrom Rossen.42

Lab on a ChipPaper

Publ

ishe

d on

06

Mar

ch 2

017.

Dow

nloa

ded

by S

tanf

ord

Uni

vers

ity o

n 23

/03/

2017

00:

21:3

4.

View Article Online

Lab ChipThis journal is © The Royal Society of Chemistry 2017

Evidently, fluid flow behavior is more realistically repre-sented using contrasting pore sizes where certain 3D charac-teristics, namely wider pores, are etched more deeply thansmaller pores. Accordingly, we developed an optimizedsequential-etching approach to add 3D aspects to a 2D dual-porosity micromodel. Multiple etch depths correspondingwith macro- and micro-pore sizes are integral to the ap-proach. Characterization methods include pore-scale observa-tion, permeability measurements, and real-time velocity pro-files within the multi-depth dual-porosity micromodel. Thisnewly developed microfluidic device further improves thestudy of pore-scale single- and two-phase fluid dynamics aswell as fluid desaturation.

3. Methodology

This section discusses the detailed procedure to prepare twomasks based on the pore structures of real reservoir rocksand their usage in the sequential micromodel fabricationprocess. Furthermore, micromodel characterization methodsincluding μ-PIV, absolute and relative permeability measure-ments, and fluid saturation calculation are demonstrated.

3.1 Micromodel fabrication: complete- and partial-pore masks

Mask preparation for the dual-depth dual-porosity micro-model requires first constructing a mosaic consisting ofoverlapping high-resolution images from a polished epoxy-impregnated thin section of reservoir rock as described inBuchgraber et al. (2012).24 Images were collected with a pixelresolution of 0.235 μm using a JEOL JSM-5600LV scanningelectron microscope (SEM) in back-scattered mode (BSE). Thegrayscale mosaic compiled from these images was then seg-mented into pore and non-pore pixels. The resulting binaryimage was modified to fill pores with incomplete epoxy im-pregnation, reconnect pores in the 2D mosaic using throatsizes measured via mercury injection (Fig. 2), and alter theedges of the mosaic to ensure seamless connectivity betweenrepeated base images in the 3-by-3 arrayed mask in Fig. 3.The resulting base image (Fig. 2) has a porosity (ϕH) of 45.3%area. Due to hardware limitations, the base image wasprinted on the mask with a pixel size of 1.5 μm. Thisupscaling preserves the heterogeneity and relative pore sizescompared to the alternative of resampling the base image.The throat size distribution shown in Fig. 2 was rescaled tomatch. Distribution channels, injection ports, and registra-tion marks were added (Fig. 3). The resulting base image andmask is called “complete-pore”, because the mask containsboth micro- and macro-sized pore structures. This mask isused for the shallower etch depth.

A second mask for the deeper etch depth was then pre-pared by subjecting the “complete-pore” base image (Fig. 2)to seven cycles of erosion-dilation (E-D; 2-D Petrographic Im-age Analysis System v. 7.0). E-D strips off the outermost layerof pixels (erosion) and then adds a layer of pixels to theremaining surface (dilation). This process is repeated for two

layers, three layers, and so forth until seven layers have beeneroded and dilated. This process removes pores, throats, andsurface roughness features that are, in this particular case,less than or equal to 21 μm in diameter. This resulting baseimage has a porosity (ϕL) of 34.1% area and is denoted as the“partial-pore” image, because it contains only macro-porosity,that is pores and throats wider than 21 μm. Changes in thepore structure are observed in close-up examples of the origi-nal and eroded base images (Fig. 2). The difference in poros-ity between “complete-pore” and “partial-pore” base imagesis 11.2% and is hereafter referred to as microporosity (ϕmicro).Similar to the “complete-pore” mask, the “partial-pore” baseimage is arrayed as shown in Fig. 3. Registration marks iden-tical to those on the “complete-pore” mask were added foralignment. Injection ports and distribution channels werenot included in the “partial-pore” mask.

Fig. 2 Throat-size distribution from mercury injection (top) used toconnect pores in the “complete-pore” base image (ϕH; middle). Close-up corresponding examples (bottom) of “complete-pore” base image(left) and the “partial-pore” overlay (right). Black pixels represent poreswhereas white corresponds to the rock matrix. The base image wasadapted from Buchgraber et al. (2012).24

Lab on a Chip Paper

Publ

ishe

d on

06

Mar

ch 2

017.

Dow

nloa

ded

by S

tanf

ord

Uni

vers

ity o

n 23

/03/

2017

00:

21:3

4.

View Article Online

Lab Chip This journal is © The Royal Society of Chemistry 2017

3.2 Micromodel: sequential photolithography

Once the “complete-pore” and “partial-pore” masks were pre-pared, the masks were then used for sequential (two-step)photolithography. A detailed two-step fabrication procedurewas developed to generate precision dual-depth micromodelsbeginning with “partial-pore” single-depth etching and subse-quent “complete-pore” dual-depth etching as demonstratedin Fig. 4. During the “partial-pore” first- or single-etching pro-cess, the fresh 4 inch silicon wafers were primed anddehydrated at 150 °C for 30 minutes using HMDS (hexa-methyldisilazane) allowing better coverage and adhesion be-tween the wafer and photoresist. The primed wafer was thenspin-coated with 1.6 μm thick photoresist using a SVG (Sili-con Valley Group) spin coater with an automated track sys-tem for dispensing photoresist onto the silicon wafers. The

uniformity of spun photoresists is typically ±100 Å using theSVG spin coater on the flat surface of a fresh silicon wafer.After this coating process depicted in Fig. 4a, the wafers wereexposed to ultraviolet light through the “partial-pore” carbon-ate mask containing only the larger macro-pore network(Fig. 4b) using Karl Suss MA-6 Contact Aligner system.

After exposure, the wafers were processed through an au-tomated developing track system (SVG Developer) for devel-oping and post-baking the exposed photoresist-coated wafers.In order to finalize the pore network on the developed wafers,wafers were etched to about 7 μm using an inductivelycharged plasma deep reactive ion etcher with 1.8–4 μm min−1

etching rate (Fig. 4c). The remaining photoresist on the wa-fers was then removed by soaking the wafers in a chemicalbath of “piranha” (90% sulfuric acid/hydrogen peroxide) for20 minutes (Fig. 4d).

For the “complete-pore” etching process, the etched waferfrom the “partial-pore” or single-etch depth process wascoated with photoresist using an EVG101 Spray coater(Fig. 4e). Spray coating was used to cover the uneven, previ-ously etched wafer surfaces produced during the “partial-pore” procedure. Spray coating distributes photoresist moreevenly on the arbitrarily shaped and textured substrateswhereas spin coating proved inadequate with respect to filmthickness homogeneity and edge coverage.

Photoresist for spray coating was prepared by mixing con-ventional positive photoresist SPR 220-7 (Rohm and HaasElectronic Materials LLC, Marlborough, MA) with solventssuch as MEK (methyl-ethyl ketone) and PGMEA (metoxy-propyl acetate) to adjust the evaporation rate and the solidcontent of the photoresist.31 The photoresist mixture consistsof photoresist and solvents combined in various proportionssuch as 6.5 wt% SPR220-7, 68 wt% MEK, and 25.5 wt%PGMEA. A 1.6 μm thick layer of photoresist was spray-coatedon the etched silicon water using six passes at 80 °C followedby post baking at 90 °C for 200 s. Once the photoresist mix-ture was uniformly coated on the wafer by the spray-coatingtechnique and the “complete-pore” mask is aligned with pre-vious pattern, the photoresist-coated wafer was exposed to ul-traviolet radiation for six seconds at vacuum contact mode byfollowing the exposure protocol: 3 s exposure followed by 10s pause and another 3 s exposure. After the 6 s exposure pro-cess, a developing process of immersing the exposed wafer inconventional MF26A developer (mixture of >95% water, 2.3%tetramethylammonium hydroxide, and <1% polyglycol;Rohm and Haas Electronic Materials LLC, Marlborough, MA)for 50 s and then washing with deionized water was repeatedthree times (see ESI‡).

The “complete-pore” overlay mask pattern alignment dur-ing the exposure step (Fig. 4f) is critical to expose the previ-ously etched area in Fig. 4c. Once the “complete-pore” dual-depth wafer shown in Fig. 4g was etched up to H1, 1 mm di-ameter holes were drilled at the production and injectionports (I-A, I-B, I-C, and I-D in Fig. 3). After the ports weredrilled, the wafers went through a thorough cleaning process(Fig. 4h) to remove any remaining photoresist and

Fig. 3 (Top): Layout of the integrated experimental setup composedof an upstream pump for fluid injection (either n-decane solution (A)or fluorescent water (B)) and a downstream micromodel holder (D) forvisualization of fluid displacement. A syringe pump was connected toan inlet port of micromodel holder by Teflon tubes. Pressure dropacross the micromodel is monitored using pressure transducer (C), andthe back pressure regulator (E) minimizes possible pressure drop fluc-tuation. (Bottom): Micromodel inside of a holder (D) is displayed. A sy-ringe pump drives solutions into the micromodel through the injectionport (I-A). A flow distribution fracture or channel along the matrix con-nects to the other inlet port (I-B); a similar distribution channel con-nects the production ports (I-C and I-D). In the matrix, the rock-basedpore network was repeated in 3 by 3 array. Matrix size (grey area) is 6by 4.5 cm.

Lab on a ChipPaper

Publ

ishe

d on

06

Mar

ch 2

017.

Dow

nloa

ded

by S

tanf

ord

Uni

vers

ity o

n 23

/03/

2017

00:

21:3

4.

View Article Online

Lab ChipThis journal is © The Royal Society of Chemistry 2017

precipitants that might block the microchannels (pore bodiesand throats) on the wafers. Finally, a Schott Borofloat 33glass wafer (S.I. Howard Glass, Worcester, MA) was anodicallybonded to the top of the clean, twice-etched wafers at 1200volts after a thermal oxidation process at 300 °C for 45 mi-nutes to produce a water-wet silicon dioxide (SiO2) surface onthe silicon wafer.

3.3 Micro-particle image velocimetry (μ-PIV)

Particle Image Velocimetry (PIV) is a technique widely usedin hydrodynamics. The fluid is seeded with tracer particlesthat are assumed to follow faithfully the flow. PIV results inthe accurate quantitative measurement of fluid velocity vec-tors. In μ-PIV, the ability of the objective lens of the micro-scope to focus only on one plane at a time is used.32 The ap-plication of μ-PIV techniques for the study of flow in porousmedia has been reported previously.33–35 Flows may be steadyor transient.35 More importantly, for more complex and real-istic pore geometries, validation exercises of μ-PIV techniquesare necessary. Roman et al. (2015) have successfully appliedthe technique to the analysis of realistic sandstone geome-tries where converging and diverging pore structures exist.35

Our setup to perform μ-PIV is the same as that in Romanet al. (2015). Our optical velocimetry technique uses conven-tional microscopy and digital imaging methods for the quan-titative determination of two-component velocity data. In thiswork, μ-PIV was used to observe the fluid velocity field withinthe dual-porosity micromodels under steady-state flow condi-tions. Movies (640 by 480 pixels) recorded the movement ofparticles as the water phase flowed through the water-saturated pore space of both single- and dual-depth dual po-rosity micromodels. The particles in the water phase are car-

boxylate modified latex (CML) micro-particles (polybead car-boxylate microsphere 1 μm diameter, Polysciences) that havea hydrophilic surface due to carboxyl functional groups and anegative charge at pH values greater than 5. The density (1.05g ml−1) and hydrophilic properties of the particles minimizessedimentation and adsorption on the micromodel walls thatare water-wet. The particle seeding concentration in the waterphase is set at approximately 0.06% by volume.

The principle of PIV is to record two images of the flow ofparticles separated by a short time interval. Images aresubdivided into many small interrogation windows. The dis-placements of interrogation windows between the two imagesare determined through spatial cross-correlation. Velocity ismerely found by dividing the particle displacements by thetime between images. The quality of μ-PIV measurements isclosely connected to the implementation, post-experimentprocessing, and data analysis. As in Roman et al. (2015),35

image processing was conducted in order to obtain a se-quence of images that contains information related to themoving particles only. This final image sequence was usedfor μ-PIV analysis. An example is shown in Fig. 5. Imagesextracted from the video file (Fig. 5a) were processed usingnoise filtration, grayscale image transformation, and correc-tion of light intensity fluctuations (Fig. 5b). Each pixel of ev-ery three images was averaged over the duration of the se-quence to obtain a “reference image” that corresponds to theimage background (Fig. 5c). Grain edge detection from thereference image was conducted using the Canny method(Fig. 5d).36 The reference image was subtracted from each im-age of the sequence, (Fig. 5e) producing a binary image ofparticles only, in white (Fig. 5f).

In this work, more than 600 images were extracted fromthe original video and post-processed prior to use in a PIV

Fig. 4 Micromodel fabrication process: (a) spin coating of photoresist on a fresh Si wafer, (b) exposure in a single-etching process using a“partial-pore” carbonate mask on the photoresist coated wafer, (c) developed and etched wafer with the pore space represented by etched area,(d) photoresist removal, (e) spray coating on the etched wafer, (f) exposure in the second etching process using “complete-pore” carbonate maskwith precise mask alignment, (g) second-etching wafer for two different etching depths, and (h) photoresist removal.

Lab on a Chip Paper

Publ

ishe

d on

06

Mar

ch 2

017.

Dow

nloa

ded

by S

tanf

ord

Uni

vers

ity o

n 23

/03/

2017

00:

21:3

4.

View Article Online

Lab Chip This journal is © The Royal Society of Chemistry 2017

algorithm. Velocities of particles in the final images were cal-culated by PIVlab.37 The initial movie was recorded at 30 fps.To run PIVLab, one image for every three frames was used inorder to have enough particle displacement from one frameto the other such that there was at least a displacement ofone to two particle sizes per frame. This leads to a frame rateof 10 images per second. For running PIVlab, three passeswith windows of 64 × 64, 32 × 32 and 16 × 16 pixels were se-lected and arranged at 50% overlap. Based on the imagespatial resolution (0.4 μm per pixels), the smallest windowcontains at least three particle images.35,38 Micro-PIV mea-surements usually suffer from low particle image density,thus to ensure fully converged measurements, time averagingover the 600 image pairs was performed to increase the num-ber of particle images contributing to a micro-PIV evalua-tion.28 Moreover, the thickness over which moving particlesare imaged is on the order of the thickness of our micro-models (≈14 μm). Therefore, the velocities are measured foronly one focal plane that corresponds to the whole thickness.Thus, the velocities measured are assumed to be an averagevelocity for each depth.

3.4 Permeability and fluid saturation measurements

The micromodel was placed under a Nikon ME600 micro-scope. The objective lens used to track the motion of thefluids in the micromodel has a magnification of 10 and anumerical aperture of 0.3. The light source was a Metal Ha-lide lamp, whereas a CCD camera (Nikon Coolpix 5100, 640× 480 pixels at 30 fps, 8-bits) was used to acquire image se-quences. The recorded images have a spatial resolution of

0.4 μm per pixels. All the experiments are conducted atroom temperature.

Imbibition and drainage processes are crucial to oil recov-ery. Imbibition signifies non-wetting fluid removal from po-rous media by wetting fluid and drainage is the displacementof wetting fluid from pore space by non-wetting fluid. Hence,absolute and end point relative permeability measurementsthroughout the drainage and imbibition processes wereconducted sequentially to investigate the meaningful changein fluid dynamics in both the single- and dual-depthmicromodels.

First, the fluorescent deionized water was injected usingan 8 mL syringe (Harvard Stainless Steel Syringes) and a sy-ringe pump (Harvard Apparatus, Holliston, MA) setup, asshown in Fig. 3B. The pore space in the micromodel was fullysaturated with water in order to maintain micromodel sur-face wettability as strongly water-wet. A 10−3 M fluoresceindye solution gave a green fluorescence to the water phase forbetter color-based oil and water phase identification. Duringthe initial water injection, the absolute permeability (k) of themicromodel was calculated using Darcy's law:

(2)

where A is the cross sectional area (product of width, 4.5 cm,and etching depth (H2)), ΔP is the pressure drop in atm, μw isthe viscosity of water in cp, q is the volumetric flow rate inml s−1, L is the length (6 cm), and k is the absolute perme-ability in Darcys. Several steady-state pressure drops acrossthe micromodel were measured at different flow rates (0.001–

Fig. 5 Demonstration of image processing prior to μ-PIV analysis. (a) Original snapshot from the video clip, (b) filtered and sharpened grayscaleimage, (c) reference image without particles, (d) edges detected using the Canny method, (e) extraction of grains (white) and pore (black) from theedge-detected image, and (f) final binary image having only white pixels representing the particles. Scale bar in (a) is 100 μm.

Lab on a ChipPaper

Publ

ishe

d on

06

Mar

ch 2

017.

Dow

nloa

ded

by S

tanf

ord

Uni

vers

ity o

n 23

/03/

2017

00:

21:3

4.

View Article Online

Lab ChipThis journal is © The Royal Society of Chemistry 2017

0.02 ml min−1) as shown in Fig. 3(C) and (E). The capillarynumber (Nca), the ratio of viscous force to capillary force, isdefined as:

(3)

V is the interstitial velocity, μ is the displacing phase vis-cosity, and σow is the interfacial tension of n-decane and wa-ter phase, 52 mN m−1.26 The range of capillary numbers isfrom 1.3 × 10−6 to 2.6 × 10−5 indicating capillary-dominatedflow conditions in the micromodel. Using the dimensions ofthe micromodel, known constants, and measured variables,the product of q, μw, and L versus the product of A and ΔPwas plotted, and the slope, that is equal to k, was determined(plots are shown in Fig. S-10 of the ESI‡).

For forced water drainage experiments, injection port (I-B)in Fig. 3 is closed when injected oil from I-A filled the flowdistribution channel. Similarly, the n-decane solution with 1vol% of crude oil (μo = 0.859) was continuously injected todisplace the water phase at constant pressure, 100 psi (maxi-mum pressure limit of these micromodels), for 100 PVI (porevolume injected) until the residual water saturation (Swr) isreached. The residual water saturation was calculated by aver-aging the saturations of images at nine locations (Fig. 3)within the micromodel using image segmentation by theRGB-based threshold method in ImageJ.21 Phase saturationsare calculated based on the number of pixels correspondingto either the water phase (nw) or oil phase (no).

(4)

At the residual water saturation (Swr), the end point oilpermeability (ko) was calculated by monitoring the pressuredifference. Lastly, a forced water imbibition experiment wasconducted to flush out n-decane from the micromodel at 100psi for 100 PVI until the residual oil saturation (Sor) isreached. When steady state was obtained at Sor, the end pointwater permeability (kw) was measured. Image segmentationwas used to obtain the oil saturation corresponding to thekw. A schematic diagram for the entire process of permeabil-ity measurement is presented in Fig. S-9 of the ESI.‡

4. Results and discussion

Single- and dual-depth micromodels were fabricated as de-scribed above. Their topological and flow properties werethen compared.

4.1 Characterization of micromodel

A portion of the etched dual-depth wafer was processed usinga CCI HD-3D surface and film thickness optical profiler (Tay-

lor Hobson USA, West Chicago, IL). The profiler is used tomeasure the surface topography of etched wafers.

Fig. 6 shows the surface topology for corresponding re-gions of the single- and dual-depth wafers. The figure illus-trates the successful alignment and execution of thesequential-etching fabrication. Using the CCI HD step heightprofile in Fig. 6, etching depths were measured as H1 = 7 μmand H2 = 14 μm for the micro- and macro-porous regions, re-spectively. The etched depth of the single-depth wafer is thesame as the maximum depth (H2) of dual-depth case, 14 μm.Microporous regions (i.e., etch depth = H1) are apparent influid-filled micromodels having lesser luminescence com-pared to more deeply etched (H2) pores (Fig. 6a and b).

The porosity, including the single- and dual-etched depths,is calculated using a geometrical interpretation expressed as(see ESI‡):

(5)

where ϕdual-depth is the porosity of the dual-depth carbonatemicromodel and a is the ratio H2/H1. The dual-depth micro-model porosity was calculated as 39.7% volume whereas theporosity of the single-depth micromodel is 45.3% volume.The porosity of the dual-depth micromodel is 87.6% that ofthe single-depth micromodel.

4.2 Velocity profile from μ-PIV

The existing pressure versus flow rate relationship in themicrofluidic channels (rectangular conduits) of micromodels,as shown in Fig. 7, has been studied. Assuming the upperlimit of the width to height ratio is one and using a Fouriersum representation, the approximate solution (within 10%error) for the Navier–Stokes equation for a fully developed,steady-state flow with a Newtonian fluid between parallelrigid plates was expressed by Bruus (2008) and Cheung et al.(2012).39,40 The pressure drop (ΔP) across microchannellength (L) is represented as:

(6)

where q is the imposed flow rate, H is the height of themicrochannel, and μ is the viscosity of the Newtonian fluid.The pressure drop is a function of flow rate, height, andwidth of the cross sectional area. More importantly, given0.001 ml min−1 injection of water into a 6 cm long channelwith 500 μm width and 20 μm height, a reduction of bothchannel width (20 μm) and height (10 μm) increases the pres-sure drop roughly four times greater than the case in whichonly the width changes. Regardless of the importance of theheight and width on the pressure drop across a micro-channel, typical microfluidic studies use devices with micro-channels where the etching depth (H) is constant. Hence, the

Lab on a Chip Paper

Publ

ishe

d on

06

Mar

ch 2

017.

Dow

nloa

ded

by S

tanf

ord

Uni

vers

ity o

n 23

/03/

2017

00:

21:3

4.

View Article Online

Lab Chip This journal is © The Royal Society of Chemistry 2017

pressure drop in a microchannel is controlled only by thechange in channel width.

The μ-PIV technique provided velocity profiles for bothsingle- and dual-depth dual-porosity micromodels (Fig. 8) un-der single-phase flow condition. The velocity values are nor-malized by the maximum velocity in the area under study. Ac-cordingly, velocity profiles are used to evaluate the impact ofchange in the aspect ratio (height/width) of the channel onthe fluid flow velocity. Specifically, in the local region ofinterest shown in Fig. 8, the matrix with a microchannel is lo-cated in between two large pore spaces in the single-depthmicromodel. The aspect ratio of the fluid channel increasesdue to the reduction of width as the fluid meets the entrancethroat to the microchannel.

For the dual-depth carbonate micromodel, the increase inthe aspect ratio is less than the change observed in thesingle-depth micromodel, because both the height and widthof the same region are reduced. Dashed-circles in Fig. 8 indi-cate three transition zones where water phase transportsfrom the large pore to microchannel for both micromodels.

In practice, the limitation of using uniform-depth porousmedia is most obvious at pore-size transition zones. Theμ-PIV results demonstrate the sudden jump of fluid horizon-tal velocity at constrictions or pore throats within the single-depth micromodel (indicated by yellow and red regionswithin dashed circles; Fig. 8). On the other hand, as expected,these large flow velocities are not observed in the correspond-ing regions of the dual-depth micromodels. The dual-etched

Fig. 6 (Top) Images from CCI HD revealing the surface topography from the same location of the single-depth (left; macro porosity only) anddual-depth (right; micro and macro porosity) wafer surfaces. Differences, namely microporous regions, are highlighted in yellow. (Middle) CCI HDimage and the extracted profile from locations 1 to 2 shows different etching depths for micro (H1) and macro porosity (H2). (Bottom) Microscopicimages from the same location in water-saturated single- (a) and dual-depth (b) micromodels. Fluorescence intensity reduction is observed in theless deeply etched (H1) or microporous regions in the dual-depth micromodel (b). Scale bar is 100 μm. SEM image of cross-section of dual-depthmicromodel illustrating sharp pore corners is presented in Fig. S-5 of the ESI.‡

Lab on a ChipPaper

Publ

ishe

d on

06

Mar

ch 2

017.

Dow

nloa

ded

by S

tanf

ord

Uni

vers

ity o

n 23

/03/

2017

00:

21:3

4.

View Article Online

Lab ChipThis journal is © The Royal Society of Chemistry 2017

depths serve to reduce the aspect ratio change compared tothe single-etched depth model. The dual-depth micromodel hasthe smaller cross sectional areas at the pore throat imposing

the greater resistance against the flow than the resistance ofsingle-depth. In addition, depth differences (reduction byhalf) in the dual-etch case significantly impact the flow atconstrictions or pore throats more than only width reductionof the single-etch depth case by two orders of magnitude.

4.3 Permeability comparison

Absolute permeability (k) is used to quantify the capacity forflow through porous material with dimensions of lengthsquared, often expressed as Darcy (1 D = 10−12 m2). Absolutepermeability for the single- and dual-depth micromodels is0.298 D and 0.178 D, respectively. The permeability (kdual)with H2/H1 (=2) of the dual-depth micromodel is 60% of thesingle-depth micromodel permeability (ksingle). The reductionin dual-depth absolute permeability is related to the reduc-tion in cross-sectional area for flow. More importantly, thisresult closely mimics the observation of flow behavior fromthe μ-PIV velocity profile where water showed preferentialflow through macro-pores and experienced a barrier at theentrance to micropores in the dual-depth micromodel.

To further compare the single- and dual-depth micro-models, end point relative permeabilities were measured.End point relative permeability values are important parame-ters to quantify the modified transport through porous mediadue to the presence of trapped oil and water. The end pointpermeabilities of water (kw) and oil (ko) at residual phase sat-urations were evaluated using the best linear fit of datapoints. The value for ko is 0.155 D for the dual-depth micro-model and 0.069 D for the single-depth micromodels. kw atthe residual oil saturation was determined to be 0.131 D forthe dual-depth micromodel and 0.060 D for the single-depthmicromodel. The values for the relative permeability, krim =kim/km where i is either water or oil phase and m is denotedas single- (s) or dual- (d) depth micromodel, were calculatedand tabulated in Table 1. For the dual-depth water-wet micro-model, a drainage experiment (oil injection) left 7.6% moreconnate water in the porous medium than the single-depthcase (Table 1). This additional water was predominantlyspread over microporous regions and became an obstacle to

Fig. 7 (Left) Surface topography of dual-porosity carbonate micromodel. (Right) Simplified schematic diagram exemplifies dual-pore space in themicromodel (red-dashed circle) with the channel width (W1 and W2), a single channel or etching depth (H), and a channel length (L). Two channelsare connected by the pore throat with channel width (W3). The dark brown wall is the representation of the grains and the transparent channel oftwo different widths, W1 and W2, demonstrate the two different sizes of pore structure in a single-etch depth micromodel.

Fig. 8 Normalized magnitude of velocity from μ-PIV for single- (top)and dual-depth (bottom) dual-porosity micromodels at interstitial ve-locity of 1 m per day with water injection rate (0.0008 ml min−1). Flowdirection is from left to right. Single-depth micromodel has pore throatdimensions (width by depth) of the circled locations with constantetched-depth: a (30 μm by 14 μm), b (12 μm by 14 μm), and c (15 μmby 14 μm). Dual-depth micromodel has pore throat dimensions (widthby depth) of the circled locations where there is constant etch-depth,but b and c have reduced etched depth: a (30 μm by 14 μm), b (12 μmby 7 μm), and c (15 μm by 7 μm). Depth profiles are in Fig. 6b.

Lab on a Chip Paper

Publ

ishe

d on

06

Mar

ch 2

017.

Dow

nloa

ded

by S

tanf

ord

Uni

vers

ity o

n 23

/03/

2017

00:

21:3

4.

View Article Online

Lab Chip This journal is © The Royal Society of Chemistry 2017

oil flow. It was difficult to develop sufficient capillary entrypressure for the oil phase to enter these microporous water-saturated zones. Thus, the end point oil relative permeability(krod = 0.39) for the dual-depth micromodel is 0.13 lower thankros (=0.52) for the single-depth case.

The water end point relative permeability was measured atSor, which were 33.2% and 34.3% for single- and dual-depthmicromodel, respectively. Even though there is a relativelyinsignificant difference (±1.1%) of Sor between single- anddual-depth micromodels, the end point water permeability(krwd = 0.34) for the dual-depth case is 0.1 less than krws(=0.43) of the single-depth micromodel. This is mainly attrib-uted to residual oil trapping in macro pores for the dual-etched depth case and micro pores for the single-etcheddepth case. To be more specific, the dual-depth micromodelmay trap more oil in macro-pores, because the pore-throat as-pect ratio increases the capillary pressure threshold that isneeded to drive the oil phase into narrow water-wet micro-channels or throats.

The single-depth micromodel, however, has an insuffi-cient capillary pressure threshold across the pore throat toprevent the non-wetting fluid from entering microchannels.Eventually for the single-depth case, water bypassed the re-sidual non-wetting (oil) phase trapped in microporous zones(with narrower widths) and flowed through macro-pores andalong the pore walls due to capillary force and the adversemobility ratio between the oil and water phase. For dual-depth micromodels, the existence of the trapped oil gangliain macrochannels may cause the wetting fluid to flow aroundthe ganglia or bypass the microchannels.41 Hence, comparedto the single-depth micromodel, the reduction of availablechannel sizes for water transport is more significant and thusrepresented as a decrease in relative permeability for thedual-depth case.

In the next section, visualization of oil and water distribu-tions further confirms this interpretation of the relative perme-ability reduction, oil trapping, and preferential flow of water.

4.4 Visualization of fluid desaturation

The pore structure in many carbonate rocks has disorderedchannel networks unlike the relatively homogeneous pore ge-ometries with high pore-to-pore connectivity found in many

sandstones. Consequently, high permeability and porosityvalues associated with these disordered channel networks donot guarantee an increase in oil recovery.4 Understanding oilrecovery processes for complex pore networks, therefore, re-quires further investigation beyond porosity and relative per-meability analyses.

Throughout the experimental steps of the permeabilitymeasurements, water drainage and imbibition processes pro-vide evidence of significantly different capillary forces in thesingle- and dual-depth micromodels. The capillary force dom-inating immiscible displacement was expected to be larger inthe dual-depth micromodel, and the experimental result foroil recovery agreed with this preliminary estimation.

As noted in previous sections, visual observation of thepattern of phase saturation in the micromodel reveals the re-sidual oil phase trapping mechanisms and preferential flowof water. During the water drainage process by oil injectionfor dual-depth micromodel, Fig. 9(a) show that the oil

Table 1 Summary of end point relative permeability corresponding tothe residual water and oil saturations, Srw and Sro, respectively

End point oil relative permeability

Srw kro

Single-depth 32.5 0.52Dual-depth 40.1 0.39

End point relative water permeability

Sro krw

Single-depth 33.2 0.44Dual-depth 34.3 0.34

Fig. 9 For dual-depth micromodel: (a) water (green) drainage experi-ment shows the water-saturated shallow-depth (H1) microporosity(darker green) and deeper (H2) macro-porosity (bright green). Oil phase(blue) displaces the water phase in the macro pores. (b) Water imbibitionexperiment shows oil ganglia (blue) occupying macro pores after 100PVI water flooding at 100 psi. Oil ganglia meets pore throats with a crosssection of 20 μm width by 7 μm depth (X-box in a). Scale bar is 100 μm.

Lab on a ChipPaper

Publ

ishe

d on

06

Mar

ch 2

017.

Dow

nloa

ded

by S

tanf

ord

Uni

vers

ity o

n 23

/03/

2017

00:

21:3

4.

View Article Online

Lab ChipThis journal is © The Royal Society of Chemistry 2017

preferentially displaced water in the macro pore space; hence,a larger portion of the microporous space remained water sat-urated. As expected after the water imbibition experiment, theexistence of trapped oil ganglia in macro pores (Fig. 9b) is evi-dence of wetting fluid either flowing around the oil ganglia orbypassing along the pore wall into microporous regions.

In contrast, the oil phase flushed into the microporousarea of the single-depth micromodel during oil flooding, be-cause it had an insufficient capillary entry pressure barrier(Fig. 10a). Water bypassed the residual non-wetting phasetrapped in the microporous zone and displaced preferentiallythe oil remaining in the macro pores during the water imbi-bition experiment as shown in Fig. 10(b). These resultsconcur with the larger relative permeability values for thesingle-depth micromodel compared to the dual-depth model.The oil recovery values for single- and dual-depth micro-models were calculated by averaging the oil recovery values

from the nine regions of interest. The resulting oil recovery(50.8%) from the single-depth micromodel was 7.9% greaterthan the oil recovery (42.7%) in the dual-depth micromodel.Preferential displacement of oil from macropores by water inthe single-depth case yields greater oil recovery as comparedto the dual-depth model. The macropores comprise themajority of the pore structure, by volume, in the micromodeland hence changes in fluid occupancy of macropores domi-nate recovery.

5. Concluding remarks

For the purpose of investigating multiphase flow throughheterogeneous dual-porosity pore networks, a microfluidicdevice should have certain 3D characteristics to better repre-sent and mimic real rocks. To accomplish this, modificationof the conventional mask and fabrication steps for a dual-porosity micromodel generated multiple pore depths with asuitable change in cross-sectional height corresponding tothe width of the pores. Using multiple masks, an originalbase image and an overlay, created using E-D processing ofthe original base image, provided an easy method of preserv-ing selected pore sizes that are, in this case, macro poresgreater than 21 μm in diameter. The size threshold is deter-mined by the number of E-D cycles and the pixel size. Weestablished a reliable and repeatable fabrication routine fordual-depth silicon microfluidic devices using an effective ap-proach to the spray coating, exposure, and developing pro-cesses. This methodology could be extended to micromodelswith three and four etch depths.

Characterization methods including porosity and perme-ability measurements serve to compare the single- and dual-depth micromodels. The porosity of the dual-depth micro-model (H2/H1 = 2) was calculated as 39.7% compared to45.3% porosity of a single-depth micromodel. The absolutepermeability values of the single- and dual-depth micromodelwith 14 μm maximum depths were 0.298 D and 0.178 D,respectively.

We compared flow within the dual-depth networks to thatin the single-depth micromodels using various quantificationmethods. The observation of the change in velocity field usingμ-PIV provided an opportunity to evaluate the dynamics ofsingle-phase flow in the pore space of a dual-porosity micro-model for both single and dual etched depth cases. The μ-PIVmeasurements demonstrated differences in fluid flow behav-ior at the pore-size transition zone due to the modification ofheight-to-width aspect ratio of cross section of channel.

Finally, we investigated the relative permeability of single-and dual-depth micromodels at residual oil and water satura-tions to quantify the capacity for flow of the wetting and non-wetting phases. The end point relative permeability of oil(kro) is 0.39 for dual-depth and 0.52 for single-depth micro-models. The end point permeability of water (krw) was calcu-lated as 0.34 for dual-depth and 0.44 for single-depth micro-models. Visual observation of the oil and water phasedemonstrated enhanced capillarity whereby residual oil

Fig. 10 For single-depth micromodel. (a): Oil injection experimentshows that the oil phase (blue) occupies equal depth (H2) macro- andmicro-porosity regions (rectangles) after 100 PVI oil flooding at 100 psi.(b): Water (green) imbibition experiment shows preferential displace-ment of oil remaining in macro-porosity (rectangle) and oil phase (blue)remains in microporosity and along the pore walls after 100 PVI waterflooding at 100 psi. Oil ganglia meet a pore throat with cross section of20 μm width by 14 μm depth (O-box in a). Scale bar is 100 μm.

Lab on a Chip Paper

Publ

ishe

d on

06

Mar

ch 2

017.

Dow

nloa

ded

by S

tanf

ord

Uni

vers

ity o

n 23

/03/

2017

00:

21:3

4.

View Article Online

Lab Chip This journal is © The Royal Society of Chemistry 2017

remains in macropores and prevents preferential water flowthrough macropores. This causes the reduction of relativepermeability for the dual-depth micromodel. Due to the in-creased capillarity, oil recovery (50.8%) in the single-depthmicromodel was 7.9% greater than the oil recovery (42.7%)in the dual-depth micromodel.

The new methodology provides an optimized process todevelop and fabricate microfluidic devices with more realistic3D features and diverse approaches to qualify and quantifythe improvements provided by the dual-depth micromodel asa tool for the study of multiphase flow. The use of dual-depthdual-porosity micromodels with carbonate pore networksmay further knowledge of the fluid dynamics underlying thecomplex interactions among oil, water, steam, gas, surfac-tant, and foam for enhanced oil recovery processes (EOR). Inaddition, combining dual-depth dual-porosity micromodelswith surface treatments for wettability modification may helpelucidate the role of wettability and the surface-fluid interac-tion on the EOR mechanism.

Our fabrication method allows the height ratio (H2/H1) ofa dual-porosity pore network to be tuned; hence, it paves theway for the systematic investigation of the effect of various di-mensions in the pore throat on the fluid-flow and oil-desaturation. Especially, a tunable pore throat dimension isbeneficial to study effects of interfacial tension reducingagents on extracting hydrocarbon.

Acknowledgements

This work was supported under the trilateral agreement be-tween Stanford University, Saudi Aramco, and King Faud Uni-versity of Petroleum and Minerals. Additional support wasprovided by the Stanford University Petroleum Research Insti-tute (SUPRI-A) industrial affiliates. This support is gratefullyacknowledged. S. James Crabtree developed the erosion-dilation code and provided customized modifications. Part ofthis work was performed at the Stanford Nanofabrication Fa-cility (SNF). The authors also express their sincere gratitudeto S. Aldousary, M. Almajid, H. Guo, and T. W. Kim for in-valuable discussions and suggestions.

Notes and references

1 U. S. Energy Information Administration, Saudi Arabia's KeyEnergy Statistics, U.S. Energy Information Administration(EIA), September 10, 2014, Accessed June 10, 2016, http://www.eia.gov/beta/international/country.cfm?iso=SAU.

2 G. R. Jerauld and S. J. Salter, Transp. Porous Media, 1990, 5,103–151.

3 J. Kamath, R. F. Meyer and F. M. Nakagawa, in Proceedingsof SPE Annual Technical Conference and Exhibition, Society ofPetroleum Engineers, New Orleans, September, 2001.

4 D. L. Cantrell and R. M. Hagerty, GeoArabia, 1999, 4, 129–154.5 J. G. Roof, Soc. Pet. Eng. J., 1970, 10, 85–90.6 S. Akin, J. M. Schembre, S. K. Bhat and A. R. Kovscek, J. Pet.

Sci. Eng., 2000, 25, 149–165.

7 C. Soulaine, F. Gjetvaj, C. Garing, S. Roman, A. Russian, P. Gouzeand H. A. Tchelepi, Transp. Porous Media, 2016, 113, 227–243.

8 M. Buchgraber, T. Clemens, L. M. Castanier and A. R.Kovscek, SPE Reservoir Eval. Eng., 2011, 14, 269–280.

9 A. R. Kovscek, G.-Q. Tang and C. J. Radke, Colloids Surf., A,2007, 302, 251–260.

10 E. R. Rangel-German and A. R. Kovscek, Water Resour. Res.,2006, 42, W03401.

11 D. S. George, O. Hayat and A. R. Kovscek, J. Pet. Sci. Eng.,2005, 46, 101–119.

12 H. Geistlinger, I. Ataei-Dadavi and H. J. Vogel, Transp.Porous Media, 2016, 112, 207–227.

13 C. Zhang, M. Oostrom, T. W. Wietsma, J. W. Grate and M. G.Warner, Energy Fuels, 2011, 25, 3493–3505.

14 E. Unsal, M. Broens, M. Buijse, D. Boersma, A. Makurat, S.Global and S. International, in Proceedings of SPE AsiaPacific Enhanced Oil Recovery Conference, Society ofPetroleum Engineers, Kuala Lumpur, August, 2015.

15 L. P. Hauge, J. Gauteplass, M. D. Høyland, G. Ersland, A. R.Kovscek and M. A. Fernø, Int. J. Greenhouse Gas Control,2016, 53, 178–186.

16 M. M. Almajid and A. R. Kovscek, Adv. Colloid Interface Sci.,2015, 233, 65–82.

17 P. Nguyen, D. Mohaddes, J. Riordon, H. Fadaei, P. Lele andD. Sinton, Anal. Chem., 2015, 87, 3160–3164.

18 C. Zhang, M. Oostrom, J. W. Grate, T. W. Wietsma and M. G.Warner, Environ. Sci. Technol., 2011, 45, 7581–7588.

19 J. M. Schembre, G.-Q. Tang and A. R. Kovscek, J. Pet. Sci.Eng., 2006, 52, 131–148.

20 A. R. Kovscek, H. Wong and C. J. Radke, AIChE J., 1993, 39,1072–1085.

21 W. Yun and A. R. Kovscek, J. Pet. Sci. Eng., 2015, 128,115–127.

22 J. W. Grate, M. G. Warner, J. W. Pittman, K. J. Dehoff, T. W.Wietsma, C. Zhang and M. Oostrom, Water Resour. Res.,2013, 49, 4724–4729.

23 S. G. Lee, H. Lee, A. Gupta, S. Chang and P. S. Doyle, Adv.Funct. Mater., 2016, 26, 4896–4905.

24 M. Buchgraber, M. Al-Dossary, C. M. Ross and A. R. Kovscek,J. Pet. Sci. Eng., 2012, 86–87, 27–38.

25 C. M. Ross, C. A. Callender, J. B. Turbeville and J. J. Funk, inProceeding of SPE Annual Technical Conference andExhibition, Society of Petroleum Engineers, Dallas, October,1995.

26 L. W. Lake, R. Johns, B. Rossen and G. Pope, Fundamentalsof Enhanced Oil Recovery, Society of Petroleum Engineers,Richardson, 2014.

27 N. K. Karadimitriou and S. M. Hassanizadeh, Vadose Zone J.,2012, 11(3), DOI: 10.2136/vzj2011.0072.

28 J. Wan, T. K. Tokunaga, C.-F. Tsang and G. S. Bodvarsson,Water Resour. Res., 1996, 32, 1955–1964.

29 J. C. Trygstad, R. Ehrlich and N. C. Wardlaw, in Proceedingsof SPE Enhanced Oil Recovery Symposium, Society ofPetroleum Engineers, Tulsa, April, 1986.

30 K. Xu, T. Liang, P. Zhu, P. Qi, J. Lu, C. Huh and M. Balhoff,Lab Chip, 2017, 17, 640–646.

Lab on a ChipPaper

Publ

ishe

d on

06

Mar

ch 2

017.

Dow

nloa

ded

by S

tanf

ord

Uni

vers

ity o

n 23

/03/

2017

00:

21:3

4.

View Article Online

Lab ChipThis journal is © The Royal Society of Chemistry 2017

31 L. Yu, Y. Y. Lee, F. E. H. Tay and C. Iliescu, J. Phys.: Conf.Ser., 2006, 34, 937–942.

32 R. Lindken, M. Rossi, S. Grosse and J. Westerweel, Lab Chip,2009, 9, 2551–2567.

33 S. S. Datta, H. Chiang, T. S. Ramakrishnan and D. A. Weitz,Phys. Rev. Lett., 2013, 111, 64501.

34 G. Blois, J. M. Barros and K. T. Christensen, Microfluid.Nanofluid., 2015, 18, 1391–1406.

35 S. Roman, C. Soulaine, M. A. AlSaud, A. R. Kovscek and H.Tchelepi, Adv. Water Resour., 2015, 95, 199–211.

36 J. Canny, IEEE Trans. Pattern Anal. Mach. Intell., 1986, 8,679–698.

37 W. Thielicke and E. J. Stamhuis, J. Open Res. Softw., 2014, 2,e30.

38 R. J. Adrian and J. Westerweel, Particle ImageVelocimetry, Cambridge University Press, Cambridge,2011.

39 P. Cheung, K. Toda-Peters and A. Q. Shen, Biomicrofluidics,2012, 6, 26501–2650112.

40 H. Bruus, Theoretical microfluidics, Oxford University Press,Oxford, 2007.

41 S. S. Datta, T. S. Ramakrishnan and D. A. Weitz, Phys. Fluids,2014, 26, 22002.

42 W. R. Rossen, Colloids Surf., A, 2000, 166, 101–107.

Lab on a Chip Paper

Publ

ishe

d on

06

Mar

ch 2

017.

Dow

nloa

ded

by S

tanf

ord

Uni

vers

ity o

n 23

/03/

2017

00:

21:3

4.

View Article Online