Name: reservoir condition pore-scale imaging of multiple fluid phases using x-ray microtomography
Uploaded: 2015-02-25T14:00:00
Duration: 8 min 2 s
Description: Watch this Scientific Journal Video about Reservoir Condition Pore-scale Imaging of Multiple Fluid Phases Using X-ray Microtomography at JoVE.com

We gratefully acknowledge funding from the Qatar Carbonates and Carbon Storage Research Centre (QCCSRC), provided jointly by Qatar Petroleum, Shell, and Qatar Science &#38; Technology Park. We also acknowledge funding from the Imperial College Consortium on Pore-Scale Modelling.

1. Imaging Strategy Design
<ol>
	<li>In order to predict the imaging performance of different solute choices for the brine, calculate the x-ray spectrum of the incident x-rays29-31. Include the impact of the core-holder, core assembly and confining fluids on x-ray spectrum. An example incident x-ray spectrum using an acceleration voltage of 80 kV and electron current of 87 &#181;A is shown in Figure 1.</li>
	<li>Compare this spectrum to the transmission factors of the sample containing different pore-fluids. Simulate changes in the transmission factor due to changes in the pore-fluid using the Beer-Lambert law, assuming an effective optical length of the species within the sample, and calculated x-ray attenuation coefficients (Figure 3)32. Determine the overall transmission factor by integrating over all incident x-ray energies. An example of the resulting effective transmission factors for the rock matrix and pore-space material, and the changes in transmission factors relative to the case when the pore-space is filled with vacuum can be seen in Table 1.</li>
	<li>Choose a brine solute and concentration such that the change in transmission factor associated with brine is approximately half the change in transmission factor associated with the solid. This will maximize three phase contrast in the reconstructed image. Weigh out the required amount of salt (7% (w/w) KI was used for the experiment from which the representative results were taken) and vigorously mix with deionized water.
	<ol>
		<li>Alternatively, if a specific brine composition is desired, change the x-ray source acceleration voltage to change the spectrum of the incident x-rays.</li>
	</ol>
	</li>
</ol>
2. Assembly of Equipment and Cell
<ol>
	<li>Assemble the equipment as shown below in Figure 2. Use PEEK flowlines to reduce lateral sample load on the flow cell. Test each connection carefully for any fluid leaks.
	<ol>
		<li>Place the brine, of composition decided during steps 1.1-1.3, in the base of the reactor. Wrap the flexible heater around the flow cell.</li>
	</ol>
	</li>
	<li>Construct metal end fittings. Remove the thread from the 1/8&#8221; end of a 1/16&#8221; to 1/8&#8221; reducer fitting. Then cut small grooves into the face of the 1/8&#8221; end of the fitting to distribute the injected CO2 over the entire face of the core.</li>
	<li>Pass the high pressure thermocouple through the metal end parts of the micro-flow cell and seal using &#188;&#8221; ferrules and nut, so the hot junction of the thermocouple sits adjacent to the inlet face of the core, within the confining annulus of the cell.</li>
	<li>Drill desired sample into a core 6.5 mm in diameter and 30 mm to 50 mm in length. Grind down the ends of the core flat, to ensure a good connection with the metal end pieces. Wrap this core in aluminum foil then place within a flouro-polymer elastomer sleeve.
	<ol>
		<li>Connect the ends of the elastomer sleeve to the metal end fittings. Add another wrap of aluminum foil to the exterior of the elastomer sleeve before placing the hot junction of the thermocouple next to the confining annulus of the cell and adding a final wrap of aluminum foil. This forms the core assembly (Figure 4).</li>
	</ol>
	</li>
	<li>Assemble the micro-flow cell with the core assembly sealed within it and connect the cell to the stage within the micro-CT enclosure (Figure 5) using a clamp mounted on top of the rotational CT stage.</li>
</ol>
3. System Pressurization
<ol>
	<li>Close all valves apart from valve 1, 2 and 3, as defined in Figure 3. Load CO2 from the cylinder into pump 1 and the reactor, then close valve 1. Slowly raise the temperature and pressure within the reactor to that desired for the pore fluid during the experiment.</li>
	<li>Vigorously mix the reactor for at least 12 hr to ensure all phases are in chemical equilibrium prior to injection.</li>
	<li>Open valve 14 and load the confining fluid into pump 3. Close valve 14. Open valves 12 and 13. Pressurize the confining annulus of the cell to at least 10% higher than the proposed pore-fluid pressure.</li>
	<li>Open valve 11. Load brine into pump 2. Close valve 11 and open valves 9, 8 and 6.
	<ol>
		<li>Slowly pressurize the pore-space of the rock until it is at the desired pore-fluid pressure, filling the pore-space of the sample with brine that has not been equilibrated with scCO2.</li>
	</ol>
	</li>
	<li>Open valve 4. Flush more than 1,000 pore volumes of equilibrated brine through the core by refilling pump 2 at a constant flow rate. Pore volume is found by multiplying the core volume by the porosity found using helium porosimetry. 
	NOTE: This will miscibly displace the un-equilibrated brine, ensuring 100% initial brine saturation and creating conditions in the core akin to the subsurface conditions in an aquifer at a point slightly ahead of the front of a scCO2 plume.</li>
</ol>
4. Fluid Flow and Image Acquisition
<ol>
	<li>Pass through 10 pore volumes (around 1 ml) of scCO2 through the core at very low flow rates (1.67 &#215; 10-9 m3/sec), ensuring a low capillary number of around 10-6. Continually take 2D projections in order to accurately measure the total injected volume by observing the point when scCO2 displaces brine in the pore space.</li>
	<li>Pass through 10 pore volumes (around 1 ml) of equilibrated brine through the core at the same low flow rate, causing scCO2 to become trapped as a residual phase in the pore-space.</li>
	<li>After steps 4.1 or 4.2, take scans of the sample to image drainage or imbibition respectively. Use a voxel size such that the entire diameter of the core fits within the field of view.</li>
	<li>Reconstruct the scans using a tomographic reconstruction program. To scan the entire length of the core while retain a small voxel size, reconstruct composite volumes by stitching together multiple overlapping sections, acquired sequentially. 
	NOTE: Each section required around 400 projections, taking 15-20 min to acquire, so the scanning of an entire composite volume took around 90 min.</li>
</ol>
5. Image Processing and Segmentation
<ol>
	<li>Apply a non-local means edge preserving filter33,34 to the dataset and correct the images for any beam hardening or softening artifacts created during image reconstruction by modeling these artifacts as radially symmetric Gaussian functions35.</li>
	<li>Segment the data (turn the greyscale information into a binary representation of the CO2 within the image) by the use of a watershed algorithm with a seed generated using a 2D histogram36, treating the CO2 as one phase and the brine and the rock together as the other phase.</li>
	<li>Analyze this segmented image to find both the total number of CO2 voxels and also the sizes of each connected cluster of residual CO2.</li>
</ol>

The authors have nothing to disclose.

The most critical steps for successful imaging of multiphase fluids at elevated pressures and temperatures are: 1) The successful isolation of the pore fluid from the surrounding confining fluid; 2) the effective equilibration of the fluids and rock prior to injection; 3) effective temperature control throughout the experiment; and 4) the effective segmentation of the resulting images.
The use of the aluminum wraps is critical for the successful isolation of the pore-fluid from the surrounding confining fluid as in its absence diffusive exchange across the sleeve is rapid, and saturation within the core does not remain constant for the duration of the scan. This problem can also be evident when fluid remains in the PEEK flowlines for extended periods of time (&#62; 2 hr) prior to injection into the core in step 4.1 and 4.2. Once again, CO2 diffusively exchanges across the plastic, causing the brine to desaturate. If this desaturated brine is injected into the core, the saturation in the core will decrease as residual clusters are dissolved by the injected brine.
Other methods for the equilibration of fluids and rocks, including fluid recirculation50, have been proposed in the literature. These methods increase the complexity of the experimental setup, which in turn would have increased the amount of time for each experiment, which would have in turn increased the likelihood that the brine in flow lines would have diffusively desaturated.
Effective temperature control is essential, and the presence of a thermocouple within the confining annulus of the flow cell is critical for this. Temperature is only measured at a single point, meaning there may be some gradient across the sample, leading to solubility imbalance and dissolution or exsolution. This can be minimized by locating the hot junction of the thermocouple as close as possible to the inlet face of the rock core.
The effective segmentation of the resulting images can be a real challenge with these systems, as the segmentation of images containing a partial saturation of multiple fluids is significantly more challenging that the segmentation of dry images, so the use of simple grey-scale universal thresholding is insufficient51. The use of watershed segmentation not only gives the most reliable results, compared to other algorithms in the literature, but is also the most effective at dealing with ring and partial volume artifacts35.
One of the most significant limitations of this technique is that it can only access the macro-pore space of a rock. The microporosity (on scales smaller than the image resolution) remains inaccessible, and may be important for multiphase flow. Higher resolutions reveal a greater proportion of these parts of pore space, but also correspond to a decrease in the field of view. The applicability of the technique to a specific rock type could be addressed by comparing the resolution of the scan to the pore throat size distribution obtained using independent method such as mercury injection capillary pressure.
This method is a leading technique for the pore-scale imaging of multiple fluids at reservoir conditions in realistic systems, with existing applications including a cross comparative study of capillary trapping24 and the measurement of contact angle26, and the method is easily applicable to a large range of porous systems. Future work could study, at the pore scale, a wide variety of single phase and multiphase flow in porous media problems at conditions representative of subsurface aquifers, oil and gas fields and other deep geological systems.

Carbon Capture and Storage is the process where CO2 is captured from large point sources and stored in porous rock, displacing resident brines so that it remains in the subsurface for hundreds to thousands of years1. The CO2 resides in the subsurface as a dense super-critical phase (scCO2), with properties radically different to CO2 at ambient conditions. There are four principal mechanisms by which scCO2 might be immobilized in the subsurface: stratigraphic, solubility, mineral and residual trapping. Stratigraphic trapping is where CO2 is held underneath impermeable seal rocks; solubility trapping is where CO2 dissolves into the resident brine surrounding the injected CO22-4; mineral trapping is where carbonate mineral phases are precipitated into the rock5; and residual or capillary trapping is where CO2 is held by surface forces as tiny droplets (ganglia) in the pore-space of the rock6. This can occur either naturally, by the migration of the CO2 plume7-9, or can be induced by the injection of chase brines10. In order to understand the processes governing the flow and trapping of this CO2 in the subsurface a new suite of experiments must be conducted, harnessing new advances in technology to better understand the fundamental physics associated with multi-phase flow.
X-ray microtomography has developed as a technique over the past 25 years from early attempts to visualize both dry geological samples11 and multiple fluid phases12 to the primary method for the non-invasive imaging of rock cores, both for modeling purposes and for experimental implementation13-15. Because microtomography is non-invasive, it has the ability to study systems at representative conditions, which is particularly attractive for the CO2-brine-rock system, as the multiphase flow behavior of scCO2 is highly dependent on thermo-physical properties, such as interfacial tension and contact angle, which are in turn a strong function of system conditions such as temperature, pressure and salinity16-18. In such a complex system, with such an extensive and poorly understood set of inter-dependent variables, experiments using idealized pore structures19 or analogue fluids20,21 may not be applicable to flow processes in the subsurface. Imaging multiple fluids at conditions representative of a prospective CO2 injection formation has, however, remained a challenge22. In this study we outline a methodology for the examination of multi-fluid behavior at reservoir conditions, focusing on the examination of capillary trapping23,24. This will include the designing of an imaging strategy, the assembly of the fluid cell, the injection strategy and subsequent image processing.
The experimental examination of pore-scale multiphase flow behavior in real rock systems focuses on the imaging of partially saturated rock cores after both non-wetting phase injection (drainage) and wetting phase injection (imbibition). These fluids are injected by connecting the cores to fluid injection pumps using flexible flow lines, while confining the core using a Hassler-type coreholder design25. To successfully image the in-situ arrangement of scCO2 and brine, a novel and highly sensitive experimental setup was used, primarily focusing on the use of a high resolution x-ray microscope23,24,26. The requirements for conducting experiments at elevated temperatures and pressures are very stringent, and require recent developments in both materials technology and micro-CT facilities. The key requirements that have to be fulfilled are that any core/sample holder needs to be able to withstand high pressure high temperature (HPHT) conditions while remaining sufficiently x-ray transparent to allow for effective imaging. Lab-based instruments impose an additional constraint, as the core-holder must be small enough such that the x-ray source can be placed close to the sample and that sufficiently large geometric x-ray magnification can be achieved such that the pore space is effectively resolved. Although this constraint has been relaxed somewhat with the introduction of secondary optics in newer lab-based micro-CT machines, it has not been completely removed, especially if rapid acquisition times are desired, as higher optical magnifications tend to increase the time required to acquire images.
Experiments with soluble fluids provide an additional challenge when using lengthy acquisition times, as CO2 will diffuse through the polymeric portions of the experimental assembly, reducing the in-situ fluid saturation. All these issues meant that scan times longer than around 2 hr were impractical. In order to keep scan times below this requirement, particularly stringent for lab based sources, the core-holder must be around 1 cm in diameter. A larger coreholder size would have required the detector to be much further from the source to achieve the same geometric magnification, reducing the x-ray flux incident on the detector and therefore increasing required projection exposure times. The flow cell used in these experiments was based on a traditional Hassler cell design, built around a carbon fiber sleeve, with a sleeve design similar to that used by Iglauer et al.27, but with two significant alterations: 1) The carbon fiber composite used in the sleeve manufacture was changed from T700 fibers, with a stiffness of 230 GPa, to M55 fibers, with a stiffness of 550 GPa. This not only reduced the amount of sample movement during tomography acquisition, but also increased the maximum working pressure of the cell from 20 MPa to 50 MPa. 2) The sleeve has been elongated from 212 mm to 262 mm to allow the source and detector to be as close to the sample as possible.
A major experimental shortcoming in the first study to use micro-CT to examine CO2 at reservoir conditions was the use of metal lines to control the flow to and from the core-holder27. As the sample is rotated relative to the pumps, the flow lines also need to be rotated. Stiff flow lines can cause the sample to move, reducing effective image resolution or making some or all of the dataset unusable. To prevent this we replaced all the flow lines close to the rotation stage with flexible polyether ether ketone (PEEK) tubing. These flow lines were flexible, providing very small lateral forces (load) to the core-holder during acquisition. We also attached the flow lines to valves attached to the sample stage, rather than attaching the flow lines to the coreholder. This meant that any existing flow-line load was transmitted directly to the stage, rather than to the sample, reducing the probability of sample motion. A major disadvantage of using the PEEK tubing was that CO2 was able to slowly diffuse through it, over a timescale of around 24 hr. This meant that CO2 saturated brine left in the flow lines would gradually desaturate.
Another major experimental shortcoming of previous studies was inaccurate control of temperature. This can impact results in a number of ways. Firstly, temperature is a strong control on both interfacial tension and contact angle16-18. Moreover, the solubility of both scCO2 and carbonate rock in brine is also highly temperature dependent28. Solubility control is critical, as when scCO2 is injected into a saline carbonate aquifer it will dissolve into the resident brine, forming a highly reactive carbonic acid, which will in turn start to dissolve any calcite present. Any inaccuracy in solubility control can therefore lead to scCO2 dissolution/exsolution or solid dissolution/precipitation.
Previous studies27 used a heated confining fluid to heat the coreholder; however this was problematic. It has the disadvantages associated with the difficulty of accurately maintaining a constant confining pressure using a recirculating water supply, requiring extra heating baths for that supply. Furthermore, this system only maintains an accurate control of temperature at the point of the heating bath (not at the point of the core holder, and the confining fluid would cool between the water bath and the core holder). It also requires both an inlet and an outlet port for the confining fluid, increasing the number of fluid lines attached to the coreholder and so increasing flow line load.
Instead of using a heated confining fluid, a flexible heating jacket was used to surround the core holder. This very simple heating method resulted in very little coreholder load, and allowed for the precise and accurate heating. An extremely thin polyimide heating film was used, in order to minimize sample size. The construction of this film consists of an etched copper foil element 0.0127 mm thick, encapsulated between two layers of 0.0508 mm polyimide film. The copper elements present in the jacket did not noticeably affect image quality. Temperature was measured using a thermocouple sitting in the confining annulus of the cell. It was positioned on the outside of the confining sleeve, as close as possible to the core, ensuring an accurate, reliable and stable reading of the pore-fluid temperature. The thermocouple and heating film were connected to a custom built Proportional Integral Derivative (PID) controller, and temperatures were controlled within &#177; 1 &#176;C.
To maintain complete control over inter-phase solubility, and represent conditions present in the aquifer far away from the injection site, prior to injection the brine was equilibrated with scCO2 by vigorously mixing the two fluids together with small particles (1-2 mm) of the host rock in a stirred and heated reactor. All wetted components within this reactor are made of Hastelloy to minimize corrosion. The reactor contains a filtered dip tube to allow for denser fluid to be extracted from the base of the reactor (brine) and less dense fluid to be extracted from the top of the reactor (scCO2). High pressure syringe pumps were used to maintain pressure and control flow in the pore-space of the rock and in the reactor, with a displacement accuracy of 25.4 nl. The experimental apparatus used in this study is shown in Figure 1. The ionic salt used for the experiment from which the representative results were drawn was Potassium Iodide (KI), as it has a high atomic weight and so a high x-ray attenuation coefficient, making it an effective contrast agent. Less attenuating salts (such as NaCl) or mixtures could be used, however larger salinities would be required to achieve the same x-ray attenuation.

<table border="0" cellpadding="0" cellspacing="0" width="608">
	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Name of Reagent/ Equipment</td>
			<td style="width:107px;">Company</td>
			<td style="width:119px;">Catalog Number</td>
			<td style="width:167px;">Comments/Description</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">High Pressure Syringe Pump</td>
			<td style="width:107px;">Teledyne ISCO</td>
			<td style="width:119px;">1000D</td>
			<td></td>
		</tr>
		<tr height="63">
			<td height="63" style="height:63px;width:216px;">Parr Reactor</td>
			<td style="width:107px;">Parr Instrument Company</td>
			<td style="width:119px;">4547A - hastelloy</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">PEEK Tubing</td>
			<td style="width:107px;">Kinesis</td>
			<td style="width:119px;">1560xL</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Potassium Iodide Salt</td>
			<td style="width:107px;">Sigma Aldrich</td>
			<td style="width:119px;">30315-1KG</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Carbon Dioxide</td>
			<td style="width:107px;">BOC</td>
			<td style="width:119px;">CO2 - size E</td>
			<td></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Thermocouple</td>
			<td style="width:107px;">Omega Engineering</td>
			<td style="width:119px;">KMTSS-IM300U-150</td>
			<td></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Kapton Flexible Heater</td>
			<td style="width:107px;">Omega Engineering</td>
			<td style="width:119px;">KH-112/10-P</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">X-Ray Microscope</td>
			<td style="width:107px;">Zeiss</td>
			<td style="width:119px;">Versa XRM 500</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Snoop Leak Detector</td>
			<td style="width:107px;">Swagelok</td>
			<td style="width:119px;">MS-SNOOP-8OZ</td>
			<td></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Flouro-Elastomer Polymer (Viton) Sleeve</td>
			<td style="width:107px;">Fisher Scientific</td>
			<td align="right" style="width:119px;">11572583</td>
			<td></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Micro-CT Coreholder</td>
			<td style="width:107px;">Airborne Composites</td>
			<td style="width:119px;">262mm Coreholder</td>
			<td>Constructed in conjunction with Imperial College</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Tomographic program</td>
			<td style="width:107px;">Zeiss</td>
			<td style="width:119px;">XM-Reconstructor</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">ImageJ - image processing</td>
			<td style="width:107px;">NIH</td>
			<td style="width:119px;">ImageJ</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Matlab</td>
			<td style="width:107px;">Mathworks</td>
			<td style="width:119px;">Matlab</td>
			<td>Used for regression analysis</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Avizo</td>
			<td style="width:107px;">FEI</td>
			<td style="width:119px;">Avizo</td>
			<td></td>
		</tr>
	</tbody>
</table>

reservoir condition pore-scale imaging of multiple fluid phases using x-ray microtomography

X-ray microtomography was used to image, at a resolution of 6.6 &#181;m, the pore-scale arrangement of residual carbon dioxide ganglia in the pore-space of a carbonate rock at pressures and temperatures representative of typical formations used for CO2 storage. Chemical equilibrium between the CO2, brine and rock phases was maintained using a high pressure high temperature reactor, replicating conditions far away from the injection site. Fluid flow was controlled using high pressure high temperature syringe pumps. To maintain representative in-situ conditions within the micro-CT scanner a carbon fiber high pressure micro-CT coreholder was used. Diffusive CO2 exchange across the confining sleeve from the pore-space of the rock to the confining fluid was prevented by surrounding the core with a triple wrap of aluminum foil. Reconstructed brine contrast was modeled using a polychromatic x-ray source, and brine composition was chosen to maximize the three phase contrast between the two fluids and the rock. Flexible flow lines were used to reduce forces on the sample during image acquisition, potentially causing unwanted sample motion, a major shortcoming in previous techniques. An internal thermocouple, placed directly adjacent to the rock core, coupled with an external flexible heating wrap and a PID controller was used to maintain a constant temperature within the flow cell. Substantial amounts of CO2 were trapped, with a residual saturation of 0.203 &#177; 0.013, and the sizes of larger volume ganglia obey power law distributions, consistent with percolation theory.

The results for a single carbonate, Ketton limestone, an oolite from the upper Lincolnshire Limestone Member were analyzed in 3D in order to identify and measure the volume of each unique disconnected ganglion, which was then labeled (Figure 6). All processing was conducted within the Avizo Fire 8.0 and ImageJ programs37.
The segmented partially saturated images were analyzed by counting the number of voxels of residually trapped scCO2 to find the proportion of the rock volume occupied by trapped scCO2&#160;&#8212; the capillary trapping capacity. This can then be converted to a residual saturation (Sr) by dividing this value by the porosity as obtained using helium porosimetry. Significant proportions of scCO2 were trapped as a residual saturation, with a residual saturation of 0.203 &#177; 0.013. This agrees with the results found in previous studies using micro-CT23. Larger core-scale studies of residual trapping in this rock type showed a lower residual saturation of 0.137 &#177; 0.01238.
The ingress of brine into a scCO2 saturated core is an imbibition process where a wetting fluid (brine) invades each pore, displacing non wetting fluid (scCO2). In a strongly water-wet rock we expect the water to fill areas of the pore space in order of size39,40, trapping disconnected ganglia in the process called snap-off. This process should be percolation like41 so predictions can be made about the size distribution of the isolated clusters. The number n of clusters of volume s (measured in voxels) should scale as , where &#964; is the Fisher exponent42. Network modeling has shown that in three-dimensional cubic regular lattices the value of this exponent is around &#964;=2.18943. One natural way of extracting this exponent from real data is to plot the binned quantity, as defined by Dias and Wilkinson41.
<img alt="Equation 1" fo:content-width="2in" src="/files/ftp_upload/52440/52440eq1.jpg" />
which should scale as:
<img alt="Equation 2" fo:content-width="2in" src="/files/ftp_upload/52440/52440eq2.jpg" />
This is then plotted on a log-log plot as a function of s (Figure 7), showing power-law behavior for large ganglia, but an under-representation of smaller ganglia compared to the power law model. The exponent was calculated by excluding ganglia smaller than 105 voxels (approximately the start of the power-law behavior) and performing Levenberg-Marquardt regression44,45 using a least absolute residual robust fitting algorithm46,47. This was performed using a commercial software package. The Fisher exponent for this system was 2.287 &#177; 0.009, close to the theoretical value of 2.189, indicating that imbibition in this system is indeed percolation like. More generally these results confirm conclusions in larger core-flood experiments38,48,49 that scCO2 acts as the non-wetting phase in carbonates.
<img alt="Figure 1" src="/files/ftp_upload/52440/52440fig1.jpg" /> 
Figure 1. Experimental apparatus, showing the pumps, valves and reactor used to control flow and the seating of the coreholder within the micro-CT enclosure. <a href="https://www.jove.com/files/ftp_upload/52440/52440fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" src="/files/ftp_upload/52440/52440fig2.jpg" /> 
Figure 2. The normalized energy spectrum for x-rays incident on the core, filtered through the coreholder, confining sleeve and confining fluid. Calculated using SpekCALC29-31.
<img alt="Figure 3" src="/files/ftp_upload/52440/52440fig3.jpg" /> 
Figure 3. The linear attenuation coefficients of different fluids and rock materials as a function of photon energy.
<img alt="Figure 4" src="/files/ftp_upload/52440/52440fig4.jpg" /> 
Figure 4. Detail of the core assembly, showing a triple aluminum wrap around the core, preventing diffusive CO2 exchange across the flouro-elastomer sleeve. <a href="https://www.jove.com/files/ftp_upload/52440/52440fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" src="/files/ftp_upload/52440/52440fig5.jpg" /> 
Figure 5. Detail of the flow cell, heating apparatus and siting of the core assembly within the flow cell. The thermocouple must be placed as close as possible to the inlet face of the core. <a href="https://www.jove.com/files/ftp_upload/52440/52440fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 6" src="/files/ftp_upload/52440/52440fig6.jpg" /> 
Figure 6. Image of the carbonate after drainage and imbibition. (A) A 3D rendering of the core after drainage where each non-wetting phase cluster is given a different color. (B-F) A 3D rendering of the core after five imbibition experiments, colored as described for (A). The large range of colors indicates a poorly connected residual phase. (G) A cross-section of the core after drainage. The darkest phase is the scCO2, the intermediate phase is brine and the lightest phase is rock grain. (H) A cross-section of the core after imbibition. <a href="https://www.jove.com/files/ftp_upload/52440/52440fig6large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 7" src="/files/ftp_upload/52440/52440fig7.jpg" /> 
Figure 7. The size distribution of the residual ganglia shown in Figure 6.
<table border="0" cellpadding="0" cellspacing="0">
	<tbody>
		<tr>
			<td>Material Filling Porosity</td>
			<td>Vacuum</td>
			<td>CO2</td>
			<td>H2O</td>
			<td>H2O &#8211; 7 wt% NaCl</td>
			<td>H2O &#8211; 7 wt% KI</td>
			<td>Solid (CaCO3)</td>
		</tr>
		<tr>
			<td>Transmission Factor</td>
			<td>0.25</td>
			<td>0.247</td>
			<td>0.243</td>
			<td>0.242</td>
			<td>0.224</td>
			<td>0.202</td>
		</tr>
		<tr>
			<td>Change in transmission factor relative to vacuum</td>
			<td>N/A</td>
			<td>-0.003</td>
			<td>-0.007</td>
			<td>-0.008</td>
			<td>-0.026</td>
			<td>-0.048</td>
		</tr>
	</tbody>
</table>
Table 1. Summary of results (transmission factor and change in transmission factor relative to the vacuum filled case) from simulation of the x-ray optical properties of the rock and pore-space filling material imaged during this study. Each column represents a different material filling the pore-space of the rock within the coreholder.

Watch this Scientific Journal Video about Reservoir Condition Pore-scale Imaging of Multiple Fluid Phases Using X-ray Microtomography at JoVE.com

Reservoir Condition Pore-scale Imaging of Multiple Fluid Phases Using X-ray Microtomography

We present a methodology for the imaging of multiple fluid phases at reservoir conditions by the use of x-ray microtomography. We show some representative results of capillary trapping in a carbonate rock sample.

X-ray microtomography was used to image, at a resolution of 6.6 &#181;m, the pore-scale arrangement of residual carbon dioxide ganglia in the pore-space ...

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JoVE Journal

Engineering

Energy Dispersive X-ray Tomography for 3D Elemental Mapping of Individual Nanoparticles

Energy dispersive X-ray spectroscopy within the scanning transmission electron microscope (STEM) provides accurate elemental analysis with high spatial resolution, and is even capable of providing atomically resolved elemental maps. In this technique, a highly focused electron beam is incident upon a thin sample and the energy of emitted X-rays is measured in order to determine the atomic species of material within the beam path. This elementally sensitive spectroscopy technique can be extended to three dimensional tomographic imaging by acquiring multiple spectrum images with the sample tilted along an axis perpendicular to the electron beam direction.
Elemental distributions within single nanoparticles are often important for determining their optical, catalytic and magnetic properties. Techniques such as X-ray tomography and slice and view energy dispersive X-ray mapping in the scanning electron microscope provide elementally sensitive three dimensional imaging but are typically limited to spatial resolutions of &gt; 20 nm. Atom probe tomography provides near atomic resolution but preparing nanoparticle samples for atom probe analysis is often challenging. Thus, elementally sensitive techniques applied within the scanning transmission electron microscope are uniquely placed to study elemental distributions within nanoparticles of dimensions 10-100 nm.
Here, energy dispersive X-ray (EDX) spectroscopy within the STEM is applied to investigate the distribution of elements in single AgAu nanoparticles. The surface segregation of both Ag and Au, at different nanoparticle compositions, has been observed.

The use of energy dispersive X-ray tomography in the scanning transmission electron microscope to characterize elemental distributions within single nanoparticles in three dimensions is described.

Energy dispersive X-ray spectroscopy within the scanning transmission electron microscope (STEM) provides accurate elemental analysis with high spatial ...

Failure Analysis of Batteries Using Synchrotron-based Hard X-ray Microtomography

Imaging morphological changes that occur during the lifetime of rechargeable batteries is necessary to understand how these devices fail. Since the advent of lithium-ion batteries, researchers have known that the lithium metal anode has the highest theoretical energy density of any anode material. However, rechargeable batteries containing a lithium metal anode are not widely used in consumer products because the growth of lithium dendrites from the anode upon charging of the battery causes premature cell failure by short circuit. Lithium dendrites can also form in commercial lithium-ion batteries with graphite anodes if they are improperly charged. We demonstrate that lithium dendrite growth can be studied using synchrotron-based hard X-ray microtomography. This non-destructive imaging technique allows researchers to study the growth of lithium dendrites, in addition to other morphological changes inside batteries, and subsequently develop methods to extend battery life.

Synchrotron-based hard X-ray microtomography is used to image the electrochemical growth of dendrites from a lithium metal electrode through a solid polymer electrolyte membrane.

Imaging morphological changes that occur during the lifetime of rechargeable batteries is necessary to understand how these devices fail. Since the advent ...

Measurement of X-ray Beam Coherence along Multiple Directions Using 2-D Checkerboard Phase Grating

A procedure for a technique to measure the transverse coherence of synchrotron radiation X-ray sources using a single phase grating interferometer is reported. The measurements were demonstrated at the 1-BM bending magnet beamline of the Advanced Photon Source (APS) at Argonne National Laboratory (ANL). By using a 2-D checkerboard &#960;/2 phase-shift grating, transverse coherence lengths were obtained along the vertical and horizontal directions as well as along the 45&#176; and 135&#176; directions to the horizontal direction. Following the technical details specified in this paper, interferograms were measured at different positions downstream of the phase grating along the beam propagation direction. Visibility values of each interferogram were extracted from analyzing harmonic peaks in its Fourier Transformed image. Consequently, the coherence length along each direction can be extracted from the evolution of visibility as a function of the grating-to-detector distance. The simultaneous measurement of coherence lengths in four directions helped identify the elliptical shape of the coherence area of the Gaussian-shaped X-ray source. The reported technique for multiple-direction coherence characterization is important for selecting the appropriate sample size and orientation as well as for correcting the partial coherence effects in coherence scattering experiments. This technique can also be applied for assessing coherence preserving capabilities of X-ray optics.

The measurement protocol and data analysis procedure are given for obtaining transverse coherence of a synchrotron radiation X-ray source along four directions simultaneously using a single 2-D checkerboard phase grating. This simple technique can be applied for complete transverse coherence characterization of X-ray sources and X-ray optics.

A procedure for a technique to measure the transverse coherence of synchrotron radiation X-ray sources using a single phase grating interferometer is ...

Using Synchrotron Radiation Microtomography to Investigate Multi-scale Three-dimensional Microelectronic Packages

Synchrotron radiation micro-tomography (SR&#181;T) is a non-destructive three-dimensional (3D) imaging technique that offers high flux for fast data acquisition times with high spatial resolution. In the electronics industry there is serious interest in performing failure analysis on 3D microelectronic packages, many which contain multiple levels of high-density interconnections. Often in tomography there is a trade-off between image resolution and the volume of a sample that can be imaged. This inverse relationship limits the usefulness of conventional computed tomography (CT) systems since a microelectronic package is often large in cross sectional area 100-3,600 mm2, but has important features on the micron scale. The micro-tomography beamline at the Advanced Light Source (ALS), in Berkeley, CA USA, has a setup which is adaptable and can be tailored to a sample&#39;s properties, i.e., density, thickness, etc., with a maximum allowable cross-section of 36 x 36 mm. This setup also has the option of being either monochromatic in the energy range ~7-43 keV or operating with maximum flux in white light mode using a polychromatic beam. Presented here are details of the experimental steps taken to image an entire 16 x 16 mm system within a package, in order to obtain 3D images of the system with a spatial resolution of 8.7 &#181;m all within a scan time of less than 3 min. Also shown are results from packages scanned in different orientations and a sectioned package for higher resolution imaging. In contrast a conventional CT system would take hours to record data with potentially poorer resolution. Indeed, the ratio of field-of-view to throughput time is much higher when using the synchrotron radiation tomography setup. The description below of the experimental setup can be implemented and adapted for use with many other multi-materials.

For this study synchrotron radiation micro-tomography, a non-destructive three-dimensional imaging technique, is employed to investigate an entire microelectronic package with a cross-sectional area of 16 x 16 mm. Due to the synchrotron&#39;s high flux and brightness the sample was imaged in just 3 min&#160;with an 8.7 &#181;m spatial resolution.

Synchrotron radiation micro-tomography (SR&#181;T) is a non-destructive three-dimensional (3D) imaging technique that offers high flux for fast data ...

Protocol of Electrochemical Test and Characterization of Aprotic Li-O2 Battery

We demonstrate a method for electrochemical testing of an aprotic Li-O2 battery. An aprotic Li-O2 battery is made of a Li-metal anode, an aprotic electrolyte, and an O2-breathing cathode. The aprotic electrolyte is a solution of lithium salt with aprotic solvent; and porous carbon is commonly used as the cathode substrate. To improve the performance, an electrocatalyst is deposited onto the porous carbon substrate by certain deposition methods, such as atomic layer deposition (ALD) and wet-chemistry reaction. The as-prepared cathode materials are characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray absorption near edge structure (XANES). A Swagelok-type cell, sealed in a glass chamber filled with pure O2, is used for the electrochemical test on a battery test system. The cells are tested under either capacity-controlled mode or voltage controlled mode. The reaction products are investigated by electron microscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy, and Raman spectroscopy to study the possible pathway of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). This protocol demonstrates a systematic and efficient arrangement of routine tests of the aprotic Li-O2 battery, including the electrochemical test and characterization of battery materials.

Protocol of Electrochemical Test and Characterization of Aprotic Li-O2 Battery

A protocol for the electrochemical testing of an aprotic Li-O2 battery with the preparation of electrodes and electrolytes and an introduction of the frequently used methods of characterization is presented here.

We demonstrate a method for electrochemical testing of an aprotic Li-O2 battery. An aprotic Li-O2 battery is made of a Li-metal anode, an aprotic ...

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

Underground storage permanence is a major concern for carbon capture and storage. Pumping CO2 into carbonate reservoirs has the potential to dissolve geologic seals and allow CO2 to escape. However, the dissolution processes at reservoir conditions are poorly understood. Thus, time-resolved experiments are needed to observe and predict the nature and rate of dissolution at the pore scale. Synchrotron fast tomography is a method of taking high-resolution time-resolved images of complex pore structures much more quickly than traditional &#181;-CT. The Diamond Lightsource Pink Beam was used to dynamically image dissolution of limestone in the presence of CO2-saturated brine at reservoir conditions. 100 scans were taken at a 6.1 &#181;m resolution over a period of 2 hours. The images were segmented and the porosity and permeability were measured using image analysis and network extraction. Porosity increased uniformly along the length of the sample; however, the rate of increase of both porosity and permeability slowed at later times.

Synchrotron fast tomography was used to dynamically image dissolution of limestone in the presence of CO2-saturated brine at reservoir conditions. 100 scans were taken at a 6.1 &#181;m resolution over a period of 2 h.

Underground storage permanence is a major concern for carbon capture and storage. Pumping CO2 into carbonate reservoirs has the potential to dissolve ...

In Depth Analyses of LEDs by a Combination of X-ray Computed Tomography (CT) and Light Microscopy (LM) Correlated with Scanning Electron Microscopy (SEM)

In failure analysis, device characterization and reverse engineering of light emitting diodes (LEDs), and similar electronic components of micro-characterization, plays an important role. Commonly, different techniques like X-ray computed tomography (CT), light microscopy (LM) and scanning electron microscopy (SEM) are used separately. Similarly, the results have to be treated for each technique independently. Here a comprehensive study is shown which demonstrates the potentials leveraged by linking CT, LM and SEM. In depth characterization is performed on a white emitting LED, which can be operated throughout all characterization steps. Major advantages are: planned preparation of defined cross sections, correlation of optical properties to structural and compositional information, as well as reliable identification of different functional regions. This results from the breadth of information available from identical regions of interest (ROIs): polarization contrast, bright and dark-field LM images, as well as optical images of the LED cross section in operation. This is supplemented by SEM imaging techniques and micro-analysis using energy dispersive X-ray spectroscopy.

In Depth Analyses of LEDs by a Combination of X-ray Computed Tomography CT and Light Microscopy LM Correlated with Scanning Electron Microscopy SEM

A workflow for comprehensive micro-characterization of active optical devices is outlined. It contains structural as well as functional investigations by means of CT, LM and SEM. The method is demonstrated for a white LED which can be still be operated during characterization.

In failure analysis, device characterization and reverse engineering of light emitting diodes (LEDs), and similar electronic components of ...

Applying X-ray Imaging Crystal Spectroscopy for Use as a High Temperature Plasma Diagnostic

X-ray spectra provide a wealth of information on high temperature plasmas; for example electron temperature and density can be inferred from line intensity ratios. By using a Johann spectrometer viewing the plasma, it is possible to construct profiles of plasma parameters such as density, temperature, and velocity with good spatial and time resolution. However, benchmarking atomic code modeling of X-ray spectra obtained from well-diagnosed laboratory plasmas is important to justify use of such spectra to determine plasma parameters when other independent diagnostics are not available. This manuscript presents the operation of the High Resolution X-ray Crystal Imaging Spectrometer with Spatial Resolution (HIREXSR), a high wavelength resolution spatially imaging X-ray spectrometer used to view hydrogen- and helium-like ions of medium atomic number elements in a tokamak plasma. In addition, this manuscript covers a laser blow-off system that can introduce such ions to the plasma with precise timing to allow for perturbative studies of transport in the plasma.

X-ray spectra provide a wealth of information on high temperature plasmas. This manuscript presents the operation of a high wavelength resolution spatially imaging X-ray spectrometer used to view hydrogen- and helium-like ions of medium atomic number elements in a tokamak plasma.

X-ray spectra provide a wealth of information on high temperature plasmas; for example electron temperature and density can be inferred from line ...

Visually Based Characterization of the Incipient Particle Motion in Regular Substrates: From Laminar to Turbulent Conditions

Two different experimental methods for determining the threshold of particle motion as a function of geometrical properties of the bed from laminar to turbulent flow conditions are presented. For that purpose, the incipient motion of a single bead is studied on regular substrates that consist of a monolayer of fixed spheres of uniform size that are regularly arranged in triangular and quadratic symmetries. The threshold is characterized by the critical Shields number. The criterion for the onset of motion is defined as the displacement from the original equilibrium position to the neighboring one. The displacement and the mode of motion are identified with an imaging system. The laminar flow is induced using a rotational rheometer with a parallel disk configuration. The shear Reynolds number remains below 1. The turbulent flow is induced in a low-speed wind tunnel with open jet test section. The air velocity is regulated with a frequency converter on the blower fan. The velocity profile is measured with a hot wire probe connected to a hot film anemometer. The shear Reynolds number ranges between 40 and 150. The logarithmic velocity law and the modified wall law presented by Rotta are used to infer the shear velocity from the experimental data. The latter is of special interest when the mobile bead is partially exposed to the turbulent flow in the so-called hydraulically transitional flow regime. The shear stress is estimated at onset of motion. Some illustrative results showing the strong impact of the angle of repose, and the exposure of the bead to shear flow are represented in both regimes.

Two different methods for characterizing the incipient particle motion of a single bead as a function of the sediment bed geometry from laminar to turbulent flow are presented.

Two different experimental methods for determining the threshold of particle motion as a function of geometrical properties of the bed from laminar to ...

Fabrication of Superhydrophobic Metal Surfaces for Anti-Icing Applications

Several ways to produce superhydrophobic metal surfaces are presented in this work. Aluminum was chosen as the metal substrate due to its wide use in industry. The wettability of the produced surface was analyzed by bouncing drop experiments and the topography was analyzed by confocal microscopy. In addition, we show various methodologies to measure its durability and anti-icing properties. Superhydrophobic surfaces hold a special texture that must be preserved to keep their water-repellency. To fabricate durable surfaces, we followed two strategies to incorporate a resistant texture. The first strategy is a direct incorporation of roughness to the metal substrate by acid etching. After this surface texturization, the surface energy was decreased by silanization or fluoropolymer deposition. The second strategy is the growth of a ceria layer (after surface texturization) that should enhance the surface hardness and corrosion resistance. The surface energy was decreased with a stearic acid film.
The durability of the superhydrophobic surfaces was examined by a particle impact test, mechanical wear by lateral abrasion, and UV-ozone resistance. The anti-icing properties were explored by studying the ability to repeal subcooled water, freezing delay, and ice adhesion.

We illustrate several methodologies to produce superhydrophobic metal surfaces and to explore their durability and anti-icing properties.

Several ways to produce superhydrophobic metal surfaces are presented in this work. Aluminum was chosen as the metal substrate due to its wide use in ...