Seeing Inside Rocks: Multi-scale X-ray Imaging at NETL

Coal, oil, and natural gas provide most of the power for our nation and the world, so understanding geology is very important. Geological formations may play an additional role in future energy use by providing storage sites for our carbon dioxide (CO₂) emissions. Thus, understanding the small-scale structure of geomaterials and the phenomena that occur within them at the sub-millimeter or pore level is important to continued improvement of energy-related technologies.

Samples of geomaterials, such as sandstone, limestone, coal, etc., are obtained as cores while drilling into and through these formations. Many different types of informative tests can be performed on these core samples. At NETL, x-ray computed tomography (CT) permits noninvasive, sub-millimeter to micron scale observations of these materials, similar to the way that the medical profession uses CT scans to examine the human body.

NETL now has three x-ray CT scanners that complement each other in terms of resolution and the size of samples that can be analyzed. Experiments are also possible in two of the CT scanners. Temperature-controlled pressurized vessels are used to simulate the temperatures and pressures deep underground. The purpose of this Labnote is to briefly describe the CT systems at NETL and how they are currently being used in energy-related research.

Universal Systems Medical CT Scanner with pressurized core holder

The workhorse of the NETL CT scanners has been a Universal Systems upgraded medical CT scanner that has been operating since 2005. In this scanner, the sample lies horizontally and the x-ray source and detector rotate around it. Two-dimensional images (slices) are collected and are mathematically combined into a three-dimensional view of the sample. The medical CT scanner has a spatial resolution of 250 microns, a slice thickness as low as 1 mm, and the capacity to analyze core samples of geologic materials up to 15 cm (6 inches) in diameter and 1.2 m (4 ft) long. Pressure vessels and flow systems are routinely used to maintain samples under in-situ conditions and to inject fluids during the scanning process. The adjacent picture shows the medical CT scanner during a scan of a pressurized core holder, with important components labeled.

Xradia MicroXCT-400 X-ray scanner

Two additional CT scanners have been procured to complement the medical scanner. One is the Xradia Inc. x-ray micro-CT scanner, shown in the adjacent figure, which is capable of imaging smaller samples with a resolution near 1 micron. This instrument also is capable of high contrast, which enables materials of similar density to be distinguished. Unlike the medical CT scanner, the x-ray source and detector remain stationary while the sample, which is mounted vertically, rotates during the scanning process. It can accommodate samples with diameters up to 20 cm (8 inches); however, higher resolution scans are best obtained on samples that are less than 10 cm (4 inches) in diameter. The lead-lined enclosure on this instrument can also potentially accommodate small pressure vessels with core capacities up to about 4 cm (1.5 inches) in diameter. This scanner has been extensively used since 2010.

North Star Imaging Industrial CT Scanner

The other new addition, the North Star Imaging industrial-scale x-ray CT scanner shown in the adjacent figure, extends the sizes of samples that can be analyzed at NETL. Cores up to 76 cm (30 inches) in diameter and 122 cm (48 inches) long can be analyzed. Resolutions near 5 microns are possible on small samples, which is much better than the medical scanner. Like the micro-CT scanner, the x-ray source and detector in the industrial CT scanner remains stationary and the sample rotates on a base. Like the medical-CT scanner, pressure, temperature, and flow controls are being built into the system to enable researchers to study dynamic flow in geomaterials. Unique to this scanner is the capability to tilt the sample to study gravitational effects of fluid flow through porous media. This new scanner became operational in 2011.

The three complementary x-ray CT scanners at NETL permit extensive characterization of a variety of geomaterials, including sandstones, limestones, carbonates, coals, etc. The in-situ and flow capabilities permit complex processes, such as oil recovery and CO₂ storage, to be examined in actual core samples at pore-scale resolutions. This ensemble of equipment provides NETL with a world-class facility for providing critical information that will help improve energy extraction and CO₂ storage technologies.

Seeing CO₂ Push Fluid through Rock

When no surfactant was added, the CO₂ flowed through the core in a finger-like pattern, displacing little of the brine. When the surfactant was added, the CO₂ moved through the core as a stable plug, displacing a much higher percentage of the in-place fluid. This is shown in the figure below, in which the CO₂ is represented by the darker colors. In each montage of this figure, the circular images are of slices along the core length from the inlet in the top left of each montage to the outlet in the bottom right of each montage. The top montages were taken earlier in the experiments than those at the bottom and show that the observed patterns persisted throughout the experiments.

To further examine what happens when fluids of different viscosities interact within porous rocks, micro-CT scans of Berea sandstone were performed, and a segment of the pore geometry was isolated.

The data from both sets of experiments were used to perform multiphase flow simulations. These computational fluid dynamics simulations allow the researchers to set the fluid properties to what would be expected in the subsurface. Initial simulations have shown that the amount of CO₂ that fills the pore space is greater when the CO₂ viscosity is higher; the pore-scale simulations showed similar results to the core-scale experiments.

The results from the experimental measurements at the core scale were then combined with numerical simulations at the pore scale using information on pore structure obtained in the micro-CT scanner. The results are depicted in the figure below.

Computational fluid dynamics simulation of CO2 (blue) moving through a micro-CT Scanner scanned pore space initially filled with brine.

This combination of techniques promotes a much greater understanding of fluid flow phenomena. This evaluation at multiple scales is a small step towards the grand challenge of applying what we learn in the lab to what occurs in reservoirs at the kilometer scale. Upscaling of results across orders of magnitude is required to make the best use of our laboratory-scale studies. Similar studies are planned using the industrial CT scanner on large core samples.

Seeing the Effects of Rock Heterogeneity on CO₂ Movement

Dry sample scans from the medical, industrial, and micro-CT scanners of the CAS sandstone core.

A small sub-core was analyzed with the micro-CT scanner to determine the pore scale features of these bedding planes. A region was scanned that contained both a calcite-rich bedding plane and a calcite-lean region next to it. This scan is shown in the right-most image in the above figure. The resolution of this image is 2 microns/pixel and shows that some of the calcite (lighter gray) is not bonded to the sand grains (darker gray) and that there is some porosity (black), even in the calcite-rich region.

A CO₂ flood of a brine-saturated core was then performed in the medical scanner and the motion of the fluids was captured by multiple scans. The results, shown below, illustrates that the CO₂ preferentially flowed through the bedding planes that contained less of the calcite in-fill.

Sequence of 3D reconstructions of CO2 within the core preferentially filling the open bedding planes during the flow experiment. Flow is from left to right.

This multi-scale tomography clearly showed that preferential flow paths existed in the sandstone sample that would impact its use as a geologic formation for storage of CO₂. Such information is vital for deciding on the applicability of this formation for storage and helps to guide estimates of what percentage of the pore space could be filled if sequestration were to occur. Combining NETL imaging capabilities with traditional core-scale petrographic analysis enables researchers at NETL to characterize samples and understand how they will behave when in a CO₂-rich environment.

Seeing the Impact of CO₂ - saturated Brine on Cap-Rock Fractures

A 1” diameter core of the carbonate cap rock was mechanically fractured, reassembled, and stabilized with a small amount of epoxy on the core diameter. The core was scanned using the micro-CT scanner before and after treatment with the brine solution, which had been acidified with CO₂. The medical CT scanner was then used to view the core during treatment with the CO₂/brine solution.

The results from the medical CT scanner showed erosion occurring in the fracture during the treatment. This is evidenced in slices from the scans shown below, which show the fracture widening as the flow of CO₂/brine proceeded. The voxel resolution in these images is 250 microns.

Scans of the same region in a carbonate core showing erosion of a fracture by CO2/brine flow.

Whole core scans performed with the micro-CT at a pixel size of 27 microns showed preferential dissolution of certain minerals, creating non-uniform increases in the fracture aperture. This phenomenon is shown in the adjacent figure. The artificial white region represents the original fracture, while the black is the fracture after

Micro-CT scans showing fracture erosion in carbonate cap rock.

treatment with the CO₂/brine solution. The upper left inset in this figure is an enlargement of Box 1 in the figure and shows the uneven dissolution pattern caused by the CO₂/brine solution. Other subsequent analyses of portions of the treated core determined that calcite was dissolving faster than other minerals in the sample. Box 2 represents one such area that was analyzed. Dolomite in the sample did not dissolve as rapidly and clay-rich areas became more micro-porous as mineral phases were leached from them.

The results of this study provide information that can be used in the selection of CO₂ storage sites and demonstrate how the mineralogy of the cap rock can be an important factor in long-term containment of the CO₂.

Keeping an Eye on CO₂ Storage in Geologic Formations

In this research, core samples were obtained from a limestone field in which large volumes of CO₂ have been used to enhance oil recovery. Small cores were scanned to determine the three-dimensional pore connectivity, pore orientation, and reactive surface area. These data are being used in the development of a flow model. The figure below shows images from micro-CT scanner scans at different levels of resolution.

Micro-CT scan slices of a limestone sample from an oil field. The left image is a higher resolution scan of the center of the right image.

The right image in the above figure was obtained using a 4X detector objective that gave a pixel size of 3.9 microns and a field of view of 3.9 mm. A second scan of the central portion of this core at higher magnification (20X) is shown on the left. The corresponding pixel size and field of view are 1.3 microns and 1.3 mm, respectively. The results illustrate the complex porosity in this sample.

Information at larger scales is important in scaling up the models developed using higher-resolution images on smaller samples. Five-inch cores were scanned in the medical CT scanner to obtain information on porosity at a larger scale. Zones of high and low porosity were identified. In the images below, zones of higher porosity are colored, whereas, lower porosity regions are black. This information, combined with actual seismic measurements on well-characterized samples, is facilitating the development of seismic techniques to monitor the movement of CO₂ stored in geologic formations.

Images from the Medical Scanner showing zones of high and low porosity in a limestone core.

In summary, the multi-scale, x-ray CT tomographic resources at NETL allow researchers to study complex geochemical and geophysical problems that will facilitate the development of energy-related technologies.