Microfluidic Devices for Characterizing Pore-scale Event Processes in Porous Media for Oil Recovery Applications
Summary
The goal of this procedure is to easily and rapidly produce a microfluidic device with customizable geometry and resistance to swelling by organic fluids for oil recovery studies. A polydimethylsiloxane mold is first generated, and then used to cast the epoxy-based device. A representative displacement study is reported.
Abstract
Microfluidic devices are versatile tools for studying transport processes at a microscopic scale. A demand exists for microfluidic devices that are resistant to low molecular-weight oil components, unlike traditional polydimethylsiloxane (PDMS) devices. Here, we demonstrate a facile method for making a device with this property, and we use the product of this protocol for examining the pore-scale mechanisms by which foam recovers crude oil. A pattern is first designed using computer-aided design (CAD) software and printed on a transparency with a high-resolution printer. This pattern is then transferred to a photoresist via a lithography procedure. PDMS is cast on the pattern, cured in an oven, and removed to obtain a mold. A thiol-ene crosslinking polymer, commonly used as an optical adhesive (OA), is then poured onto the mold and cured under UV light. The PDMS mold is peeled away from the optical adhesive cast. A glass substrate is then prepared, and the two halves of the device are bonded together. Optical adhesive-based devices are more robust than traditional PDMS microfluidic devices. The epoxy structure is resistant to swelling by many organic solvents, which opens new possibilities for experiments involving light organic liquids. Additionally, the surface wettability behavior of these devices is more stable than that of PDMS. The construction of optical adhesive microfluidic devices is simple, yet requires incrementally more effort than the making of PDMS-based devices. Also, though optical adhesive devices are stable in organic liquids, they may exhibit reduced bond-strength after a long time. Optical adhesive microfluidic devices can be made in geometries that act as 2-D micromodels for porous media. These devices are applied in the study of oil displacement to improve our understanding of the pore-scale mechanisms involved in enhanced oil recovery and aquifer remediation.
Introduction
The purpose of this method is to visualize and analyze multi-phase, multi-component fluid interactions and complex pore-scale dynamics in porous media. Fluid flow and transport in porous media have been of interest for many years because these systems are applicable to several subsurface processes such as oil recovery, aquifer remediation, and hydraulic fracturing1,2,3,4,5. Using micromodels to mimic these complex pore-structures, unique insights are gained by visualizing pore-level dynamic events between the different fluid phases and the media6,7,8,9,10,11.
The fabrication of traditional silica-based micromodels is expensive, time consuming, and challenging, yet constructing micromodels from optical adhesive offers a relatively inexpensive, fast, and easy alternative12,13,14,15. Compared with other polymer-based micromodels, optical adhesive exhibits more stable surface wetting properties. For example, polydimethylsiloxane (PDMS) micromodel surfaces will quickly become hydrophobic during the course of a typical displacement experiment16. Furthermore, the Young's modulus of PDMS is 2.5 MPa whereas that of optical adhesive is 325 MPa13,17,18. Thus, optical adhesive is less prone to pressure induced deformation and channel failure. Importantly, cured optical adhesive is much more resistant to swelling by low molecular weight organic components, which allows experiments involving crude oil and light solvents to be conducted18. Overall, optical adhesive is a superior alternative to PDMS for displacement studies involving crude oil when silica-based micromodels are prohibitively complex or expensive and high temperature and pressure studies are not required.
The protocol described in this publication provides the step-by-step fabrication instructions for optical adhesive micromodels and reports the subtle tricks that ensure success in the manipulation of small quantities of fluids. The design and fabrication of optical adhesive based micromodels with soft lithography is first described. Then, the fluid displacement strategy is given for ultra-low flow rates that are commonly unattainable with mass flow controllers. Next, a representative experimental result is given as an example. This experiment reveals foam destabilization and propagation behavior in the presence of crude oil and heterogeneous porous media. Lastly, typical image processing and data analysis is reported.
The method provided here is appropriate for visualization applications involving multi-phase flow and interactions in confined microchannel spaces. Specifically, this method is optimized for characteristic micro-feature resolutions greater than 5 and less than 700 µm. Typical flow rates are on the order of 0.1 to 1 mL/h. In studies of crude oil or light solvent displacement by aqueous or gaseous fluids on the order of these optimized parameters at ambient conditions, this protocol should be appropriate.
Protocol
Representative Results
Discussion
This protocol for studying oil recovery processes in optical adhesive micromodels strikes a balance between the robustness of non-polymeric micromodels – such as glass or silicon – and the facile fabrication of PDMS microfluidic devices. Unlike micromodels made of glass or optical adhesive, PDMS devices lack resistance to light organic species. PDMS micromodels are also not ideal for many experiments because the surfaces of these devices have unstable wetting properties, and the polymer matrix is permeable to…
Divulgations
The authors have nothing to disclose.
Acknowledgements
We acknowledge the financial support from the Rice University Consortium for Processes in Porous Media (Houston, TX, USA).
Materials
| 3 mL Leur-Lok Syringe | Fischer Scientific | 14-823-435 | |
| 10 mL Glass Syringe | Fischer Scientific | 1482698G | |
| Photomask | CAD/Art Services | ||
| Silicon Wafer | University Wafer | 452 | |
| Propylene-Glycol-Methyl-Ether-Acetate | Sigma Aldrich | 484431-4L | |
| 150 mm Glass Petri Dish | Carolina Biological Supply | #721134 | |
| 60 mm Plastic Petri Dish | Carolina Biological Supply | #741246 | |
| Mask Aligner | EV Group | EVG 620 | |
| 1 mm Biopsy Punch | Miltex, Plainsboro, NJ | 69031-01 | |
| Industrial Dispensing Tip | CML Supply | Gauge 23 | |
| Inverted Microscope | Olympus | IX-71 | |
| Plasma System | Harrick Plasma | PDC-32G | Plasma cleaner |
| Polydimehtylsiloxane (PDMS) | Dow Corning, Midland, MI | SYLGARD 184 | |
| Norland Optical Adhesive 81 (NOA81) or (OA) | Norland Products Inc. | 8116 | Optical adhesive |
| Quick-Set Epoxy | Fisher Scientific | 4001 | |
| Glass Slides | Globe Scientic Inc. | 1321 | |
| SU-8 2015 Photoresist | MicroChem | SU-8 2015 | Photo resist |
| Syringe Pump | Harvard Apparatus | Fusion 400 | |
| Glass Capillary Tubing | SGE Analytical Science | 1154710C | |
| High-Speed Camera | Vision Research | V 4.3 | |
| Polyethylene Tubing | Scientific Commodities Inc. | #BB31695-PE/3 |
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