The FRAC group is making strides in answering one of geosciences' longest standing problems — understanding how rock fractures form — by studying crystals inside the fractures themselves.
Big and small, rock fractures are everywhere. They give famous landmarks their character: Can you imagine Yosemite National Park’s El Capitan or Big Bend’s Casa Grande without their fractured rock faces? But the fractures we see at the surface are just the tip of the iceberg. The ground beneath our feet is riddled with them, with fractures serving as conduits for subsurface fluids. This includes water in aquifers that can be tapped for drinking or irrigation, and the natural fluids that humans depend on for energy, such as oil, gas, and the hot water and steam that serve as sources of hydrothermal energy.
It also includes fluids that humans introduce into the subsurface, such as carbon dioxide that’s injected into rocks to keep it out of the atmosphere and hydraulic fracturing fluid that taps into hydrocarbons by creating even more fractures.
The ubiquity of fractures means that any venture happening in the subsurface has to take them into account. But doing that is easier said than done, said Stephen Laubach, a senior research scientist at the Jackson School of Geosciences Bureau of Economic Geology. Despite fractures being an object of study for more than 100 years, scientists are still working out the basics of how they form.
“It’s not a new problem,” said Laubach. “But it is probably one of the most refractory problems in the sciences — which is why there are so few people who stick with it.”
Figuring out how fractures work is the driving force of the Fracture Research and Application Consortium, better known as FRAC.
Laubach is the leader of FRAC. Since the mid-1990s, the group has brought together faculty members, research scientists, postdoctoral researchers and graduate students from the Jackson School and the Cockrell School of Engineering to tackle the fracture question from multiple vantage points, including lab experiments, computational models and fieldwork around the globe.
But this is far from a see-what-sticks approach. All the research at FRAC is united by an overarching idea that Laubach calls “structural diagenesis.”
The term is meant to make structural and sedimentary geologists alike perk up. The two specialties focus on different aspects of geology, with structural geologists immersed in physics — the historical domain of fracture research — and sedimentary geologists, and others such as petrologists and geochemists, focused on the chemical reactions that, over time, turn sediments into stone, a process known as diagenesis.
Structural diagenesis is a combination approach. And it’s frequently applied to a long-overlooked domain: crystals that line fractures, creating coatings, bridges and other mineral structures that record a fracture’s life story while potentially influencing where it’s off to next.
As an industrial affiliates consortium, FRAC depends on funding from industry partners along with research grants. The fact that the group can count oil giants, international geothermal companies and the U.S. Department of Energy’s Office of Basic Energy Sciences (BES) — a research wing devoted to science at the atomic and molecular scale — among its funders speaks to the breadth of their work and its potential for advancing basic science and realworld applications.
“Steve’s really unbelievably unique,” said James Rustad, the geoscience division manager at the BES. “His observations do a lot to inform people looking mechanistically at laboratory samples.”
Wayne Narr, who attended consortia meetings on behalf of Chevron since the consortia was founded until retiring in 2019, said he valued the FRAC group in part as a sounding board.
“A benefit for me as a fracture specialist is to have somewhere to bounce my ideas off of,” Narr said. “I rarely came back from a meeting where I didn’t exchange a few emails — whether I was giving them my thoughts or soliciting ideas about something.”
What makes fractures so notoriously difficult to study in the first place?
A big part of the problem is their ubiquity. They show up in every rock formation and create patterns that can look more or less the same even in rocks with drastically different geologic histories. The situation is a textbook example of what’s known as “equifinality” — an end state that can be reached in a number of different ways.
Of course, you don’t necessarily need to know a fracture’s life story to judge how well it might transport fluids. When considering fractures as subsurface conduits, the critical questions often have to do with the present: Where are the fractures now? What sort of patterns do they form? Are they clear conduits for fluids or clogged with mineral deposits?
Unfortunately, the usual ways of interrogating the subsurface are a dead end when it comes to fractures, Laubach said. Most fracture networks are too fine to show up on seismic images. And he compares inferring large-scale fracture patterns from core samples as akin to “inferring the existence of Niagara Falls from a drop of water.” Analyses of core to understand fracture timing and diagenesis are fundamentally important, but there is just no way to know whether a single core sample is representative or not.
Outcrop fractures do have some predictive potential for fractures in buried rocks of the same age, and in the same area. However, when looking at the fracture patterns alone, it’s difficult to know whether they formed when the outcrop was at depth or something that occurred later, such as weathering. And it’s challenging to know what new fractures and fillings the outcrop have accumulated since it was first exposed, said Senior Research Scientist Julia Gale.
“Even if two rocks are sedimentologically identical, their fractures can be very different, depending on how deep they were buried, for how long, and whether they have been brought back up to the surface by tectonic processes,” she said.
The long list of issues makes it seem as if the ultimate problem with fractures is their complexity. But Laubach said that it’s quite the opposite. The problem at the root of fracture research is the apparent simplicity of fracture patterns; they look familiar wherever you go. As far as patterning is concerned, a fracture from a shale play and a fracture on the sidewalk could be one and the same.
Much of the research at FRAC is dedicated to searching out specifics in fracture samples — the characteristics that set a particular fracture apart from the others and help researchers pin down their history and the history of the rocks that host it. This is where structural diagenesis comes in.
The term came into use thanks in part to Jackson School Professor and inaugural Dean William Fisher, who in the early 2000s was serving as the school’s director when it was still part of the UT College of Natural Sciences. At this time, Fisher challenged researchers across the school’s three units — the Department of Geological Sciences, the Bureau of Economic Geology and the University of Texas Institute for Geophysics — to come up with big ideas in science that funds from the school’s newly established Jackson endowment could help get off the ground.
Laubach — working closely with then-Associate Professor Jon Olson (now professor and the chair of UT’s Hildebrand Department of Petroleum and Geosystems Engineering), Professor Randy Marrett (now professor emeritus) and Gale — responded to the call, with the team coming up with structural diagenesis as an all-encompassing term to describe their research goals.
“We came and said ‘we have got something. We’re going to found a new discipline,’” Laubach said. “We’re going to break through the walls, the silos, because there’s a fundamental problem, and this is the way to solve it — and we’re going to call it structural diagenesis.” Fisher accepted the pitch (contingent on the group ponying up half the funds from industry supporters), and the FRAC consortia, which had been running for four years on industry support alone, got its startup funds. “You had to establish a pretty good research record, and these folks had one,” Fisher said.
Put simply, structural diagenesis turns fractures from nonspecific patterns into objects with unique histories to be uncovered. It does that by investigating both the mechanics of fracture pattern formation and the chemical record preserved inside the fractures themselves.
This chemical perspective has historically been overlooked in fracture research — an outcome of fractures being primarily framed as a mechanics problem, Laubach said. However, the crystal coatings that line fracture walls offer a wealth of information that mechanics simply can’t provide. They serve as a chemical record, preserving details about timing, temperatures and fluids. They provide an opportunity for the problem of equifinality to be overcome.
Crystals form most readily in rocks buried anywhere from less than a halfmile to about nine miles underground, where hot temperatures, high pressures and reactive fluids catalyze chemical reactions that make minerals precipitate on fracture walls. The fact that these are the same rocks where a number of industries are most active — whether it’s pulling fluids out of the ground or looking for space to store them — makes getting specific about fractures in this area of the subsurface especially relevant.
Laubach is a big Sherlock Holmes fan, and he said that the story of the “Red- Headed League” has lessons for anyone looking to understand why FRAC focuses on structural diagenesis. In the story, Holmes remarks how the “simple crimes” are the hardest to solve because they offer few clues, while the bizarre crimes are easier because they offer more.
“Natural fractures are a simple crime,” Laubach said. “Structural diagenesis turns it into the ‘Red-Headed League’ [by providing more clues].”
Collectively, the FRAC group has produced hundreds of studies over the decades. The work is diverse and includes outcrop investigations from around the world, computational models, lab analyses and in many cases, a mix of all three.
Among the most notable studies are those that have uncovered insights on fracture behavior that can apply across environments.
One of these is a 2015 paper published in the Geological Society of America Bulletin and authored by Laubach and collaborator Rob Lander, a research fellow at the bureau and co-founder of the geosciences software company Geocosm. The paper describes how quartz crystal growth patterns inside sandstone correlate with fracturing events, and it presents a model for simulating how these patterns are expected to grow under different fracturing regimes. The research team uses the model to successfully reconstruct the formation of a quartz bridge that spans a fracture space in a core from the Cretaceous Travis Peak Formation in the East Texas Basin, with the conditions being confirmed with a detailed lab analysis of the quartz structure. All in all, the close matchup between the model and the experiments meant that it could serve as an alternative means for determining key attributes about fracture formation, including constraining fracture opening rates and the temperatures under which the fractures were active.
Oddly enough, the existence of the quartz bridge structure that helped demonstrate the model surprised Lander when Laubach first showed him the structure in 2004 when the two got together to talk about quartz structures. Laubach wasn’t fazed by it. As a structural geologist, the quartz filling seemed similar enough to quartz veins he had observed in metamorphic rocks. Lander, on the other hand, said that the structure should be impossible under the diagenetic conditions of a sedimentary basin. A study published later that year explained the chemistry that enabled this type of growth — and helped the FRAC group cinch their first big grant for the DOE’s Office of Basic Energy Science while laying the groundwork for the 2015 paper.
Laubach said that the experience illustrates the importance of structural diagenesis’s blended perspective.
“I didn’t have the right sort of theoretical underpinning to look at it and realize it was a problematic structure, and Rob did, but of course he was a diagenesis guy, and so he never looked at fractures before,” he said. “It was a crossdisciplinary moment.”
Quartz bridges have proved to be a key fracture feature when undertaking structural diagenetic investigations. That’s because they don’t form in a single instance. Instead, they grow a little more each time in response to microfracturing events — creating a record preserved as chronological segments. In addition, as the quartz grows, it frequently traps microscopic droplets of fluid from which the crystal precipitated. Corralled into segments across a quartz bridge, the droplets serve as a record within a record.
“Those are the fluid inclusions, little time capsules for the formation of the cement,” said Research Scientist Andr.s Fall, a fluid inclusion expert in the FRAC group.
Fluid inclusion analysis can reveal the type of fluid inside a fracture, and the temperature, pressure and formation history of the crystal that precipitated from it. This type of analysis was the key part of a 2010 study led by Steven Becker, who conducted the research in the FRAC group while he was a postdoctoral fellow at the bureau. It presented evidence for fractures forming over tens of millions of years — conflicting with the prevailing thought that they formed more or less instantly.
The study was conducted on the same bridge structure in the East Texas core sample examined by Laubach and Lander in 2004, and it found that the fateful quartz bridge formed over 48 million years — an interval lasting nearly as long as the entire Cambrian Period — and growing at a rate of 16 to 23 microns every million years.
The findings were not totally unexpected though. Years earlier, Olson had built a physics-based model that calculated some fracture patterns forming over tens of millions of years. At the time, the long formation time took a backseat to the patterns themselves. But the fluid inclusion findings showed that the model might have been on to something.
“I thought it was pretty cool,” Olson said. “I had been doing it in a purely physical space, using fracture mechanics principles it looked like it took tens of millions of years for the fractures to form. And then the fluid inclusion work totally independently came up with the observations.”
Whereas the quartz bridge took millions of years to form, FRAC’s hydrothermal lab is studying the conditions that control that growth all while staying in the same geological epoch; most structures take just a few days to grow.
Fall runs the lab, which was founded to experiment with the models for quartz growth first described by Lander and Laubach. The general process involves placing pre-fractured quartz and fluids in heated pressure vessels meant to simulate the depths of the subsurface. By studying the crystal structures and associated fluid inclusions that form inside fractures under these controlled settings, Fall is learning more about the conditions that steer their formation in the field.
From crystal linings, to segmented bridges, to the fluid inclusions inside of them, there’s an array of features that appear together inside of fracture deposits. The members of the FRAC group are pioneers in using a powerful tool to get a closer look: scanning electron microscopy combined with cathodoluminescence.
Scanning electron microscopes, or SEMs, capture high-resolution images by bombarding a sample with an electron beam. But the attributes that are visible in the final image analyzed by scientists depend on the type of detectors that are attached to the microscope. The cathodoluminescence (CL) detector turns the backsplash of photons emitted by rock samples into glowing, highdefinition mosaics. Here, fractures with crystal coatings go from looking more or less the same to no two looking alike.
“SEM-CL reveals the textures that you wouldn’t be able to see with any other imaging method,” said Sara Elliott, a research scientist associate in charge of the group’s SEM microstructural imaging.
Research Scientist Estibalitz Ukar is leading the way in applying the technology to fractures coated with carbonate cement deposits — which make up the majority of minerals found inside fractures outside quartz-rich sandstone, but have hardly been studied using high-resolution techniques due to their tendency to phosphoresce and create smudgy images under standard SEM-CL conditions.
When Ukar was hired in 2013, she used her startup funds to purchase the SEM that is now in use. She credits the amazing array of clear, crisp CL images of carbonate structures that the lab has been able to collect with being selective about the right scope — a model that could operate at low energy and still create a lot of luminescence — and the extensive array of samples the lab has on hand to study. By perusing the “FRAC Library,” she found a number of different textures and structures present in carbonate coatings.
“It was a hope, but really more of a surprise, to find such beautiful examples [of carbonate] showing the same types of textures we had already seen in quartz,” she said.
In 2016, Ukar published a paper describing different carbonate cement features she observed inside of fractures in an array of samples from all over the world. So far, they appear to have much in common with the quartz deposits that have been the primary focus at FRAC for so long, even spanning fractures in the form of crystal bridges. Research on quartz coatings is relevant to the oil and gas industry since it is the mineral that is most frequently present in fractures in sandstone. But Ukar said that carbonate cements are far more abundant, encompassing all rock types including carbonates, shale, sandstone and basement (crystalline) rock. In order to understand more about fractures in all types of rocks and settings, carbonate cements need to be just as much a part of the conversation.
“Quartz is the most abundant mineral in fractures within sandstones that are quartz rich, but in everything else it’s carbonate,” she said.
Analyzing the life history of individual fractures is just part of the story. The next step is to use SEM-CL and fluid inclusion analysis to determine the development and timing of multiple fractures within a set.
Jackson School doctoral student Stephanie Forstner is conducting research that applies these techniques, as well as burial history modeling, to investigate the geologic history of the highly fractured Cambrian Flathead sandstone exposed throughout the Teton Range in western Wyoming. Forstner’s fractures are exposed at the Earth’s surface in outcrops. But most were formed underground. By distinguishing how and when they formed, her work is helping illuminate the timescales and environments in which fractures create underground networks.
“My goal is to collect evidence in the field that ultimately provides insight into how fractures develop underground,” she said. “The Flathead is an ideal unit to study fracture development because it hosts multiple generations of subsurface-originating fractures that developed over a span of about 60 million years.”
A focus on structural diagenesis has prioritized natural fracture research in the FRAC group. Induced fractures — such as those made during hydraulic fracturing operations — don’t have crystal coatings to unpack.
Nevertheless, a growing interest in how induced fractures interact with natural ones has put FRAC researchers at the forefront of core analysis at the DOE-sponsored Hydraulic Fracturing Test Site. The goal of Gale’s research at the test site is to build up a public data repository on hydraulic fractures and their behavior in the subsurface. Since 2016, Gale and Elliott have been part of the team analyzing cores pulled from the Permian Basin, including samples from the Wolfcamp Shale in the Midland and Delaware basins.
The project results have attracted a huge amount of industry interest. In 2018, the American Association of Petroleum Geologists, the Society of Exploration Geophysicists and the Society of Petroleum Engineers held a special session in Houston at their joint annual conference on Unconventional Resource Technology dedicated to discussing unconventional hydrocarbons in which Gale and Elliott presented the preliminary results on the test site cores
“There was standing room only in really big rooms,” Gale said. Interpreting the cores is an ongoing effort, Gale said. But she and Elliott have already produced two papers characterizing core pulled from the different plays. Some interesting fracture features observed in both plays include hydraulic fractures appearing in closely packed pairs and triplets, a pattern that suggests bifurcation; proppant packs and patches; and, surprisingly, little induced fracturing appearing parallel with the rock layers.
Many of the natural fractures in the cores are sealed with calcite crystals, making them easy to distinguish from induced fractures. In contrast, fracture patterns made by drilling and injection of hydraulic fluid look very much the same. Gale said a big question is whether the hydraulic fracturing process is “reactivating” natural fractures — opening them up so they can serve as a passageway for hydrocarbons. But pinning down the opening of these natural fractures by hydraulic fracturing operations, let alone distinguishing between the role of drilling versus fluid injection in causing the opening, is something that remains out of grasp for now.
“I’ve been cautious of overinterpreting,” Gale said. “People want to grab the observations and run with them. I would say there’s a lot of uncertainty.”
As the cores from the Hydraulic Fracturing Test Site show, overcoming equifinality is an especially daunting challenge for fractures without crystal coatings. The patterns remain “simple,” and there’s nothing inside to interrogate for more clues.
Although unable to reconstruct specific fracture history, mechanical and statistical research has made progress in understanding how fractures form in response to stress and interpreting fracture arrangement. This type of work is part of the FRAC group too. Olson’s research centers on building physicsbased models of fracture pattern formation and fracture interaction. And much of Marrett’s work focused on how, in certain instances, fracture patterns in nature exhibited fractal behavior, with patterns maintaining a consistent arrangement over scales of magnitude — from microfractures to macro ones.
The newest member of FRAC, Michael Pyrcz, an associate professor in the Cockrell School and the Jackson School, has built a career on applying computational and statistical tools to problems in the geosciences. Before joining UT in 2017, he spent 13 years at Chevron building software for a host of geoscience environments — including fracture modeling and forecasting.
Now part of the FRAC group, he thinks that statistics and computing tools could mine complexity that exists in so-called simple patterns. He and his students use spatial data analytics and machine learning as new lenses to explore and model the subsurface.
“There’s no reason to bring in geostats if you can model it deterministically,” Pyrcz said. “But we have to augment our physics-based workflows with statistical, data-driven approaches to characterize and manage subsurface uncertainty.”
In August 2019, Laubach and others published a paper in the Reviews of Geophysics that gave an overview of how chemistry influences fracture pattern development, insights that structural diagenesis has enabled. The paper came out of a 2016 fracture workshop sponsored by the DOE, but it took so long to compile and write because searching the literature kept uncovering a wealth of research relevant to fracture science — from reactive transport modeling to studies in the engineering world.
The paper ends with a list of nine points of actions for fracture research in the future. But the heart of enabling these breakthroughs will involve getting more people in the structural diagenesis mindset, Laubach said.
“Changing perspective often is the most important thing that you can do in addressing a difficult problem,” he said.
Another important part is being adaptable. As part of the next generation of FRAC researchers, Fall and Ukar envision expanding the consortia’s membership while still serving as a critical resource for the oil and gas companies, which make up the majority of their members.
And there’s plenty of room to expand. Some of the biggest challenges facing society intersect with subsurface fractures. As sea levels rise, coastal aquifers are becoming vulnerable to seawater seeping up through fractures, while fighting climate change with carbon capture and storage means ensuring that fractures won’t serve as inadvertent escape hatches for greenhouse gas injected into geologic formations. Ukar and Fall see these as potential growth areas for future research.
Another area for research is geothermal and hydrothermal energy, an area where FRAC’s ongoing work puts it in an especially good position.
Senior Research Scientist Peter Eichhubl and postdoctoral researcher Owen Callahan are investigating how microfractures affect rock strength in hydrothermal environments, with the researchers investigating field samples from the Dixie Valley-Stillwater Fault Zone of Nevada. The same carbonate minerals that Ukar has been studying for years readily precipitate in geothermal settings, while Fall’s hydrothermal laboratory offers a ready-made setup for simulating the fluid-rock interactions in real time. And that’s not to mention the expertise offered by the bureau’s new associate director, Ken Wisian, who has experience applying geothermal research in industry settings via UT’s Geothermal Entrepreneurship Organization.
“The group has all these years of combined expertise,” Ukar said. “We’re in a good spot to apply our knowledge to addressing some of the biggest geological challenges of this century.”