Microfluidic Devices for Characterizing Pore-scale Event Processes in Porous Media for Oil Recovery Applications

The overall goal of this procedure is to rapidly produce a microfluidic device with customizable geometry for its use in oil recovery studies. This method allows us to be able to study multi-phase flows in porous media. By using microfluidic systems to be able to actually visualize these types of complex flows, we can design better enhanced oil recovery methods for large scale reservoir systems.

The main advantage of this technique is that it allows us to rapidly gather data and different enhanced oil recovery methods in a safe and cost-effective way. This method can provide insight into enhanced oil recovery mechanisms. It can also be applied to other systems such as CO2 sequestration and aquifer remediation.

To begin, design a photo mask consisting of a rectangular channel filled with an array of posts using CAD software. Expose this pattern on a silicone wafer coated with 20 microns of photo resist. And use this master to create a PDMS mold as described in the accompanying text protocol.

Place the clean PDMS mold pattern side up in the bottom of a dust free 150 millimeter plastic petri dish. Allow the PDMS to adhere to the plastic for 10 seconds and then protect the surface of the PDMS with clear plastic tape. The procedure may be paused at this point.

Next, remove the tape from the pattern surface and pour optical adhesive into the dish to a depth of approximately 0.9 centimeters above the top surface of the mold. Use a cotton swab to gently remove any bubbles that form. Now, cure the optical adhesive using a UV light curing system as described in the accompanying text protocol.

Next, use a box cutter to carefully break the optical adhesive out of the mold. Then, use a sturdy pair of scissors to remove excess optical adhesive from the edge of the design. Slowly peel the PDMS mold away from the optical adhesive puck.

With a 1 millimeter biopsy punch, create an inlet, an outlet, and drain holes in the device. Finally, use clear tape to protect the patterned portions of the optical adhesive and PDMS surfaces. Place a new glass slide onto a spin coater and dispense one millimeter of optical adhesive onto the slide.

Spin coat the slide in two steps. First, spin it at 500 RPM for five seconds and then increase the RPM to 4000 and spin it for 20 seconds. Quickly transfer the substrate to the UV light treatment and partially cure the thin optical adhesive layer under the UV light for 30 seconds.

Next, place the optical adhesive cast pattern-side up and the substrate coated-side up in an oxygen plasma cleaner. Pull a vacuum to 540 millitorr. And then plasma treat the surface for 20 seconds.

When finished, remove the pieces and firmly press the two treated surfaces together until all undesired air pockets have been minimized or removed. Then, place the device back under the UV light and fully cure it for 20 minutes. Next, place the device on a hot plate heated to 50 degrees celsius for 18 hours.

When finished, insert six inch long segments of 0.58 millimeter ID low density polyethylene tubing into each of the ports on the device. Then, add a quickset epoxy to secure the tubing into place. Use tape to secure the microfluidic device on an inverted microscope that is equipped with a high speed camera.

Select a 4x objective and focus on an area of interest. Here, the inlet region of the device is shown. Then, load three milliliters of crude or modal oil into a 10 milliliter glass syringe equipped with a 23 gauge industrial dispensing tip.

Secure the syringe to the syringe pump holder and set the appropriate diameter value on the syringe pump settings. Next, load one milliliter of the displacing fluid into a three milliliter plastic syringe equipped with a 23 gauge industrial dispensing tip. Secure the syringe in the syringe pump holder and again, set the appropriate diameter value on the syringe pump settings.

Connect the displacing fluid to the inlet of the device by inserting the needle tip into the tubing. Then, connect the oil-filled syringe to its port. Begin flowing the oil into the outlet port of the device at two milliliters per hour while simultaneously flowing the displacing fluid into the inlet port at 0.8 milliliters per hour.

The optional foam generator will be used for this demonstration. Collect the affluent into a 20 milliliter glass vial until the two fluids both flow out the drain port. The displacing fluid should not enter the porous media, but should instead go directly out the drain until the camera is in place and the filming has begun.

Begin filming the area of interest on the porous media device at a frame rate that is fast enough to capture the desired phenomenon. Additionally capture a still image of the 100%oil saturated area. Then, swiftly and simultaneously cut the tubing that is flowing in the oil while clamping the drain tube with a five centimeter binder clip.

Allow the displacing fluid to invade the device until either the oil displacement reaches steady state or the camera runs out of memory. Typical results from an oil saturated micro model are shown here. In the fracture region, the foam diverts into the matrixes of lower permeability as expected.

The foam is generated through two main mechanisms which can be described as pinch-off and lamella division. Foam destruction can be easily identified in the forms of coalescence, capillary suction, and diffusion coarsening. Following this method, we can actually be able to use these microfluidic systems to study other enhanced oil recovery processes, such as alkaline flooding, polymer flooding, surfactant flooding, as well as being able to use them to study other complex porous media processes such as aquifer remediation.

So another area of interest is actually using these microfluidic devices to study carbon capture and sequestration. We can actually see the mechanisms by which carbon dioxide is trapped within the porous media through these microfluidic systems.