Pore-scale Imaging and Characterization of Hydrocarbon Reservoir Rock Wettability at Subsurface Conditions Using X-ray Microtomography

The overall goal of this work is to provide a thorough protocol to characterize the in situ wettability of reservoir rocks at subsurface conditions. This method can help answer key questions in multiphase flow porous media with application to oil recovery, contaminant removal and carbon dioxide storage. The main advantage of this technique is that we can study displacement processes and wettability in natural systems.

This protocol shows how to determine the in situ wettability of hydrocarbon reservoir rocks at subsurface conditions or from segmented three-dimensional x-ray images. The protocol requires a mini sample of rock. Obtain mini samples from core samples and flatten each end to facilitate good contact.

Determine the sample's total porosity and assess its internal pore structure. During x-ray microtomography, use a Hassler-type core holder. First work with the core holder's top to thread polyether ether ketone tubing.

Then attach the tubing to a custom-made end piece that accommodates the sample. Thread the holder's base with tubing that goes to the other end piece. Next, get rubber tubing long enough to cover the sample and the end pieces.

Slide the sample into the tubing. Once it is inside, connect the end pieces to the sample's top and bottom. Now place a thermocouple on the base end piece with its tip next to the sample's base.

Secure it with aluminum tape. Carefully complete the assembly of the core holder. This core holder assembly is ready for use in the protocol.

This schematic provides details of its construction. Shown in cutaway are the layers that surround the sample including a heating jacket, a carbon fiber sleeve, and a confining fluid. Use a carbon fiber sleeve with a small diameter to allow the x-ray source to be close to the sample.

The thermocouple measures the confining fluid temperature. Make use of a clamp to hold and transport the core holder assembly. Take the core holder assembly to the x-ray microtomography scanner.

With the clamp, support the core holder vertically on the rotation stage. Once the core holder is in place, connect the tubing from its top and bottom. Tubing from the top and bottom of the core holder goes to different open three-way valves.

In addition, connect tubing from the core holder's confining line to a syringe pump containing deionized water. Use the syringe pump to apply 1.5 megapascals of confining pressure. Now connect the carbon dioxide cylinder to the base three-way valve.

Flush CO2 through the sample at a low rate for one hour. Then disconnect the carbon dioxide cylinder. Once this is done, connect the syringe pump with doped brine, the brine pump, to the base three-way valve.

Initially set the valves so flow does not enter the sample to flush air out of the injection line. Then inject brine into the sample at 0.3 milliliters per minute for one hour to fully saturate it. Connect the heating jacket and the thermocouple to a PID controller.

Connect the receiving pump, a syringe pump filled with doped brine, to the base three-way valve. Use the receiving and confining pumps to increase the pore and confining pressures in one megapascal increments to 10 and 11.5 megapascals respectively. At the PID controller, set a heating jacket target temperature of 60 degrees Celsius to complete mimicking subsurface conditions.

Flush air from the line and connect an oil pump to the closed top three-way valve. Increase the pressure to the correct equivalent pressure. Then stop the oil pump and open the top three-way valve to inject 20 pore volumes of oil at a constant flow rate.

After two hours, prepare to acquire x-ray images. For high resolution images, select a 4X objective. Then adjust the positions of the source and detector.

Check the rotation of the core holder assembly and begin the scan. Start the rotation of the core holder assembly. Ensure that the tubing attached to the cell does not interfere with the rotation.

When all is in order, begin the x-ray tomography scan with a high number of projections. After the scan, disconnect the core holder assembly and remove it from the scanner. Move the core holder to an oven at 80 degrees Celsius.

There re-establish the flow rates, pressures, and perform aging over at least three weeks. Once the aging process is complete, move the core holder assembly back to the scanner. First, connect the confining pump to apply the same confining pressure.

Then connect the brine line to the base three-way valve. Also, connect the receiving pump to the top of the core holder via its three-way valve. When the target pressure is reached, open the top three-way valve to have the receiving pump apply the pore pressure to the core.

Continue making connections to re-establish subsurface conditions for the sample. Then with the brine pump off, open the bottom three-way valve to perform water flooding of 20 pore volumes at a low flow rate. After the system reaches equilibrium, again acquire high resolution scans at the same location.

Reconstruct the x-ray tomography data using reconstruction software. Save the image and open it in Trainable Weka Segmentation software. Select the free hand drawing tool.

Use the free hand drawing tool to highlight instances of one of the phases throughout the image, in this case oil. When done, click Add to Class. Try to follow the shape of a phase while labeling the pixels.

Then add the region to the appropriate class. When examples of all three phases have been labeled, click Train Classifier to segment the entire image. Review the segmented image.

Repeat the training and segmentation steps as necessary to obtain good results. Choose Create Results to get the final segmented image. Save the image for later use in analysis software.

Use the segmented image to measure the in situ contact angle distribution. The automated method produces a spreadsheet with the measured angles and their coordinates. These data allow a plot of the distribution of the contact angles.

These are distributions for a weekly water wet sample and two mixed wet samples. For quality check, crop and segment a subvolume of the mini sample. Select a subvolume with one or more oil ganglia for manual contact angle measurement.

Find the in situ contact angle distribution with the automated code. Load the generated VTK file into the data visualization software. Select the region option to view the oil and brine phases.

Click on Probe Location. Then enter the coordinates of a randomly selected contact angle from the data generate by the automated method. Locate its spatial location at the three-phase contact line.

Now load the segmented subvolume image into the data analysis software. In the software, search for the arithmetic module. Within the module, find the expression field.

Type the expression required to isolate the oil and brine phases. Next, search for the generate surface module. Use it to generate the oil and brine surfaces.

Then visually search the surfaces for the previously identified point. Find and open the slice module from the filtered raw x-ray image. Change the translate value to bring the x-ray image slice to the level of the point on the surface.

Find the label surfaces module. Within it, enter three in the number of phases box. Move to only black voxels and select no.

After applying the changes and altering the color map, return to the slice module. Select the set plain option. In the options, select show dragger.

Move the dragger to the location where the contact angle will be measured. Bring up display options. There select the rotate option.

Rotate the slice to be perpendicular to the three-phase contact line. When done, select the angle measurement tool. Use the tool to measure the angle at the selected point.

A test of the automated contact angle measurement is to plot its results against angle measurements made manually. As in this case, the results should be approximately equal. These horizontal cross-sections are of raw x-ray images and their segmented images of three samples.

The segmented images allow measuring contact angles determining the remaining oil saturation and finding the shape of the remaining oil ganglia. These are the measured in situ distributions of the contact angle for the three different samples found using this method. Sample one has a weekly water wet condition from static aging at 60 degrees Celsius with no oil injection.

Sample two is an example of a mixed wet condition with more oil wet surfaces due to aging at 80 degrees Celsius with oil injection during aging. Sample three is similar to sample two, but not strongly oil wet as a result of a lower aging temperature and different oil composition. The oil morphology remaining after water flooding varies under the different wetting conditions.

For the weekly water wet sample one, the brine percolated through the small pore corners leaving the oil trapped in the center of the pore spaces. In contrast, for the mixed wet cases of samples two and three, the brine entered the center of the pores as a non-wetting phase leaving oil connected in sheet-like layers in small pores and crevices. It's important to avoid having any air in the system that can act as a fourth phase when you look at the images.

Following this protocol, you could look the pore scale distribution of fluids and wettability in other systems such as leaves, roots, or fuel cells. But remember, we're dealing with x-rays and high-pressure fluids so it's very important that there is a thorough risk assessment and the people doing the experiments have had appropriate training. This technique of characterizing in situ wettability paved the way for researchers in the field of enhanced oil recovery to explore the associated extra oil recovery due to the change in wettability.