Dynamic Pore-scale Reservoir-condition Imaging of Reaction in Carbonates Using Synchrotron Fast Tomography

The overall goal of this experiment is to observe the dynamic change in the fluid rock interface during reaction with acid brine in real rock under reservoir conditions. This method can help answer key questions in carbon storage, such as how to accurately predict subsurface fluid migration, and the effectiveness of surge permanence. The main advantage of this technique is that three-dimensional images can be taken rapidly and noninvasively.

Though this method can provide insight into geochemical systems, it can also be applied to other systems. Imaging multiple fluid phases during mechanical stress environments, or the functioning of batteries or biological systems such as insect eyes, are typical applications. Begin with calculating the X-ray spectra of the beam line at the highest pink beam energy and flux.

Then, predict imaging performance using the experimental tuning curve and measuring filtering transmissions. Next, it is critical to calibrate the beam spectrum with adequate filters to get a good image. It is time consuming, but essential.

Begin with filtering out the lower energy X-rays that heat the sample, and do not improve imaging. Calculate the theoretical filter transmission at the available light wavelengths, and select the appropriate filters. In this case, aluminum and gold filters are used.

Next, add a band pass filter. For the high pass X-ray filters, use a set of 0.2 millimeter pyrolytic carbon, and 0.2 millimeter aluminum filters. For the low pass filter, use an X-ray mirror operating near the critical angle.

A platinum-coated strip under an incident angle of 1.15 millirods is used here to reflect light below 30 kiloelectron volts. Next, choose a scintillator that scintillates abundantly at the beam lines'available light frequencies and flux. Here, a cadmium tungstenate stacked with lead tungstenate is used.

Then, choose an objective lens and a camera with an appropriate field of view, and snapping time resolution. For the imaging, use the fly-scan technique so the sample experiences less vibration. Begin with loading the core into the cell to prepare for the core flooding.

First, wrap the core in one layer of aluminum foil. Next, insert the core into a Viton sleeve that is cut so that it is two millimeters shorter than the combined length of the core and interior end fittings. Then, stretch the sleeve over the five millimeter end fittings to create a tight seal.

There should be no space between the end fittings in the core, or flow will get pinched off. Wrap the fittings and sleeve in two additional layers of aluminum to prevent carbon dioxide from diffusing into the confining fluid, and to keep the sleeve in place on the fittings. Now, put the core holder back together.

Slide on the tubing and seals, and replace the bolts. Then, mount the core holder on the stage, and connect the flow and the electrical lines. The flow and electrical lines must not inhibit free rotation of the stage over a 180-degree arc.

Now, take a dry scan of the entire core prior to beginning the experiment. Details are in the text protocol. Also take images of the scintillators as described in the text.

To start, load freshly-prepared brine into the reactor, and reassemble it. Tighten the bolts, rewrap it with heat tape, and insert the temperature probe. Now, load carbon dioxide from valve one into the injection pump until the pressure reaches 100 bar.

Then, open valve two to flood the reactor with carbon dioxide. Continuously stir the brine with an entrainment stirrer, and heat the reactor to 50 degrees celsius. Equilibrate the brine at 10 megapascals for two to six hours, to saturate it with carbon dioxide and fully dissolve the carbonate.

Once equilibrated, purge the system. First, connect the lines above and below the core holder to bypass the core holder. Second, set the receiving pump to refill to load the ionized water into the receiving pump through valve 11.

Thirdly, open valves seven, four, and three. Lastly, use the receiving pump on constant pressure mode to drive the water backwards through the system and out valve three, below the reactor. Use approximately 10 system volumes to ensure the lines are clear of air, and rinsed clean.

Now, empty the receiving pump and load a heavier brine into the receiving pump through valve 11. Use 25%by weight potassium iodide. Then, load the ionized water into the confining pump via valve 10.

Next, close valve 10, and open valves eight and six. Use the confining pump to confine the core at two megapascals. Now, close valve 11, and pressurize the receiving pump to 10 bar.

Then open valves nine, seven, four, and three. Use the resulting pressure drop to drive the brine through the core. Gradually step up the confining and poor pressures to get a reasonable flow rate.

Drive approximately two complete system volumes of brine to the core. Close valve three, and then incrementally increase the confining and poor pressures, until the core is confined at 12 megapascal, and the core pressure is 10 megapascal. The core must also re-equilibrate to 50 degrees celsius.

Now, stop the receiving pump, and open valve five at the base of the reactor to connect the reactor system to the core. This is a high temperature pressure experiment. To ensure success, be very careful assembling the equipment, and test it thoroughly before starting reactive flow.

Before starting the fluid flow, center the CMOS camera's field of view on the middle of the core, and start taking continuous 2D projections to track the flooding of the core. Next, adjust the receiving pump for the required flow rates through the core. Use the injection pump at the front end to regulate the system pressure.

Now, monitor the 2D projections for changes in attenuation that signal the arrival of reactive brine. The transmission of the core will increase, and the projections will brighten as more light hits the scintillator, as the highly X-ray transparent reactive fluid fills in. If there's no attenuation difference between reactive and nonreactive brine, then use a higher salt concentration brine, or a different highly absorbing salt.

When the reacting brine arrives, stop the 2D scans, and start taking successive 3D tomographies as fast as possible. Use around 1, 000 projections per scan, and scan the core using only 180 degrees of rotation. Scan until the time limit is hit, or the core looks so dissolved and there is an imminent danger of internal structural collapse.

Then, depressurize the system according to the text protocol, and carefully remove the core assembly from the core holder. Once removed, disconnect the sleeve from the interior end fittings, and place the sleeve-covered core in a beaker of deionized water to dilute any potentially reactive brine, and stop all reaction. Using the described method, a reaction was imaged between calcite and unbuffered super critical carbon dioxide-saturated brine in a Portland carbonate core.

The segmented images were analyze as a time series for porosity changes by counting the number of oxyls of pore and rock. During dissolution, porosity increased with time. Visual inspection of the segmented images show the presence of a channel in the direction of the flow.

Further investigation revealed that the channel formed in the first hour, and then widened as the experiment continued. The segmented images were then used as input into a network extraction model to analyze for permeability changes. There was a sharp increase in permeability during the initial hour, but then the permeability stabilized.

After watching this video, you should have a good understanding of how to image dynamic reaction using fast synchrotron tomography. One mastered, this technique can be done in four hours if it is performed properly. While attempting this procedure, it's important to remember to protect all equipment from liquid spills, and to thoroughly test it before installing it at the beam line.

We follow rigorous procedures to ensure very high standards of safety. When it comes to synchrotron science, health and safety are paramount.