From The Ground Up: Evolving Geomechanics to Meet Energy Needs

Tomasz Hueckel has witnessed a lot of change at Duke Engineering since joining the faculty in 1987: the leadership of five deans, the construction of two buildings with more than 400,000 square feet of space dedicated to engineering, the number of tenure-track faculty expanding to more than 125.

But having been a pioneer in the field of geomechanics for nearly a half-century, he’s seen his chosen field change even more.

“Geomechanics grew out of what used to be a very narrow context of civil engineering that was basically constrained to a simple understanding of foundations and tunnels,” said Hueckel, professor of civil and environmental engineering at Duke. “But it has grown to include a wide subset of fields intended to deepen our knowledge of how the infrastructure we build today can serve us decades or even centuries into the future.”

"Geomechanics grew out of what used to be a very narrow context of civil engineering that was basically constrained to a simple understanding of foundations and tunnels."

Tomasz HUeckel, Duke professor of civil and environmental engineering

Today, he says, geomechanics is a conglomerate of many disciplines, drawing on chemistry, fluid mechanics, physics, geology, computational modeling and more. And much of what has driven his discipline forward is the global demand for energy—and the complexities of the subterranean world where much of it is drawn from and stored.

“Energy is a hot topic and heavily reliant on geomechanical knowledge,” said Manolis Veveakis, assistant professor of civil and environmental engineering at Duke. “Whether it’s extracting clean energy from the ground, storing or burying energy and its byproducts in the ground, or even engineering the most efficient foundations for wind farms, a modern approach to geomechanics is essential to meet tomorrow’s clean energy needs.”

At Duke’s Department of Civil and Environmental Engineering (CEE), the field’s evolution into a kaleidoscope of disciplines pushing the boundaries of what’s possible can be seen in how four different—but not that different—faculty are using their research to meet the energy needs of the future.

In the early 2010s, the Australian government encountered a problem. They’d discovered one of the largest reservoirs of geothermal energy on Earth. Sitting several kilometers beneath the surface of the Outback interior at almost 600 degrees Fahrenheit, the reservoir would be extremely difficult to tap into. Still, it was also too compelling to ignore.

But after several years and a half a billion dollars, they couldn’t produce enough energy from the site to run a refrigerator. The standard techniques for geothermal energy weren’t working. In a typical geothermal energy setup, two wells are drilled into the ground—one to send water in and one to receive the heated water coming out. But first, high-pressure fluids are used to open new channels within the rock for the water to flow through. While the engineers could initially create the tunnels, they were closing back up within a matter of days.

“It was the very definition of burning your money,” said Veveakis, who was called in among other experts to help figure out what was going wrong. “At the temperatures they were working at, trying to crack fissures in the rock is like cutting butter. Once you stop, it will just melt back together.”

“At the temperatures they were working at, trying to crack fissures in the rock is like cutting butter. Once you stop, it will just melt back together.”

Manolis Veveakis, Duke Assistant professor of Civil and Environmental Engineering

The water was only flowing along natural cracks in the underground rock, otherwise known as fault lines. And pumping water through fault lines is inherently dangerous, as the process can activate the faults and cause earthquakes.

The situation, Veveakis said, illustrates how the push to tap underground energy resources has driven his field forward. In this case, with a fundamental understanding of how the rock was reacting, the research experts suggested the government team inject water slowly within the faults at low pressures but continuous rates. While monitoring the system’s response to ensure the process was progressing safely, this would enhance the process of creating natural fractures for the water to flow through.

While initial attempts at his suggestion worked, it was too late. With the natural gas fracking boom beginning to take hold, the Australian government decided it had already spent too much money for too little reward and pulled the plug on the project.

“There’s no off-the-shelf answer as to how to engineer these systems because they are so complex,” said Veveakis. “We only had a chance to try to mitigate their mistakes at the very end of the project when things got desperate. If they had taken this complexity into account from the beginning, the results showed that our ideas might have allowed it to work on a commercial scale.”

The United States has known for more than seven decades that it is going to have to do something with the nuclear waste created at nuclear power plants. But that hasn’t stopped administration after administration from kicking the can down the proverbial road.

Most experts agree that the best solution is to somehow encase and bury the radioactive material somewhere deep within a desert mountain, far away from any cities or thriving habitats that a leak could destroy. But even the most remote regions of the country have underground water tables that flow for hundreds of miles and are teeming with their own unique fauna and flora.

To try to ensure no nuclear waste can escape no matter where it’s buried, researcher have recently turned to clay. By using clay as a buffer zone between the walls of the chamber and the dangerous container drums themselves, researchers believe an acceptable solution can be found. This is because clay is capable of self-healing its own cracks to prevent any avenues of escape as the centuries drag on.

Or at least it usually can.

“Storage of nuclear waste was originally theorized to be limited to 80 degrees Celsius (180 Fahrenheit),” Hueckel explained. “But now researchers are finding out that, in reality, they may actually need to push to 100 or 150 degrees Celsius. And at those temperatures, evaporation of moisture in the clays begin to make them more prone to cracking. And you cannot have cracked barriers.

”Four years ago, the Department of Energy issued a grant to Hueckel and Veveakis to try to understand the mechanisms causing the drying and cracking of the clay in question—and potentially methods for preventing it. To do that, the team must take into account dozens of micro-scale processes that are happening all the time including evaporation, deformation, thermal expansion or collapse, and perhaps even issues that are not yet known.

The challenge, Hueckel says, is putting old knowledge and equations to very new uses, such as understanding the evolving capillary processes in drying clays that cause them to crack.
“It’s funny how a relatively practical objective such as nuclear waste disposal leads to very fundamental questions about the behavior of materials that I never would have thought would be very important to the task,” Hueckel said. “Having been in the field for so long, however, I find a certain excitement of seeing so many different elements of geomechanics coming together in very unexpected configurations.”

Although it doesn’t enjoy the same level of attention in Hollywood and people’s imaginations as nuclear waste, the remnants of coal burned in power plants is also a major health and environmental concern. This became especially clear in North Carolina in 2014 when approximately 39,000 tons of coal ash and 27 million gallons of ash pond water were released into the environment near the Dan River.

For decades the most common practice for disposing of coal ash has been to stick it in a liquid landfill. Besides the obvious dangers of keeping these wastes in open ponds within a state that regularly sees major hurricanes and floods, there are many concerns about how safe these storage structures are even in the best of conditions.

Heileen Hsu-Kim, Professor of Civil and Environmental Engineering at Duke, has spent a lot of time researching the chemistry of coal ash. For example, when Duke Energy was considering cleaning up their coal ash ponds by drying them, turning them into solid landfills and sealing them off, Hsu-Kim pointed out that cutting the waste off from oxygen could accelerate the leaching of elements such as arsenic and selenium, leading to more contamination than expected.

Rather than trying to bury it and forget about it, Hsu-Kim argues that one of the best ways of getting rid of coal ash might just be to recycle it. While the material is already used in many recipes for concrete, Hsu-Kim argues that its chemical composition could make it suitable for much more extreme environments than a sidewalk.

“One idea is to use (coal ash) in barriers for nuclear waste storage. If coal ash has the right particle size, shape and chemical characteristics, it ends up solidifying into an extremely hard and impermeable barrier.”

Heileen Hsu-Kim, Duke Professor of Civil and Environmental engineering

“One idea is to use it in barriers for nuclear waste storage,” said Hsu-Kim, noting that Veveakis already has a student working on a related project. “If coal ash has the right particle size, shape and chemical characteristics, it ends up solidifying into an extremely hard and impermeable barrier.”

One obstacle to recycling coal ash, however, is that not all burnt coal is created equally. Different ash from different regions have different chemical and physical compositions. And if you’re going to use these residues in a new application, you have to be extremely sure that it won’t end up polluting the environment itself.

“In the past couple decades, we’ve developed new ways to assess whether or not the toxic elements found in coal ash might be dangerous if the ash is recycled for building materials,” said Hsu-Kim. “For example, advanced spectroscopy tools can help us understand what kind of selenium or arsenic are in the ash and how they’re distributed in the grains. And both of those variables play a big role in how safe it is to use coal ash for concrete and other building materials.”

With all of the advances in the field of geomechanics over the past few decades, the quantum leap forward in computational modeling might be the most impactful. You can discover new chemical properties of soils down to the micrometer. You can experimentally approximate high temperatures and pressures on a material over long periods of time. And you can take pressure and composition measurements from effluent coming out of a fracking operation. But only today’s advanced modeling techniques can put them all together in a single, cohesive simulation.

"While the salt may seem solid enough, over the course of millions of years, it behaves like a liquid and can form a wide variety of structures. And these rock salt structures reflect sound waves in a way that makes it difficult to tell whether or not there is oil present.”

GUGLIELMO SCOVAZZI, DUKE PROFESSOR OF CIVIL AND ENVIRONMENTAL ENGINEERING

One of the leaders in the field of computational modeling is Guglielmo Scovazzi, professor of civil and environmental engineering at Duke. Scovazzi works on a wide range of problems in fluid flows and solid mechanics that benefit from his novel approach to breaking a large problem down into small, discrete sub-cells for a computer to solve.

One recent example of Scovazzi’s work involved helping petroleum companies locate new reservoirs. When searching for new places to drill, petroleum companies send sound waves deep into the earth and measure how the waves bounce back to the surface. Almost like an ultrasound for the ground, this technique can help identify areas within dense earth and rock that might contain liquid. But these searchers were running into a salt problem.

“Because so much of today’s land masses was once covered by oceans, there are many places where a layer of salt sediment is covered over mud,” said Scovazzi. “While the salt may seem solid enough, over the course of millions of years, it behaves like a liquid and can form a wide variety of structures. And these rock salt structures reflect sound waves in a way that makes it difficult to tell whether or not there is oil present.”

Part of the problem is that these salt formations come in a wide variety of shapes and sizes. They can form gigantic pillars, mushrooms, floating islands, walls and canopies, just to name a few. There are even salt glaciers in Iran and huge ridges throughout northern Europe.

To help in their search for black gold, Exxon Mobil asked Scovazzi to develop a computational model that could explain how all of these various salt formations are made. After working on multiple iterations of a model that simulates how layers of salt and mudrock interact over hundreds of thousands of years, Scovazzi eventually discovered that the key lay in getting the hardness of the mudrock just right.

“If you make the mudrock on top of the salt too strong, nothing happens even after a million years,” said Scovazzi. “But if you get the strength of that layer just right, the simulations begin to reveal the mushroom formations that we see in nature. These findings explain why sometimes complex salt structures are found in nature—and sometimes not, with deceptively similar initial conditions.”

While Scovazzi says that Exxon is happy with the results and plans to use his computational model in the future, he also says that the problem he’s addressing is much more fundamental in nature than a search for oil. It is for this reasons that, towards the end of this project, he saw an opportunity of a broader collaboration with his CEE colleague Manolis Veveakis. This can be said for all of the geomechanical projects Duke CEE faculty are tackling.

The energy industry wants to be able to tap into rich deposits trapped within shale miles beneath the surface. Governments want to store nuclear waste within invulnerable vaults for tens of thousands of years. Coal plants want to recycle their ash into concrete and other building materials that aren’t just safe, but better than what’s currently on the market. Oil companies want to know where to expect salt rock formations so they’re not fooled into thinking salt is oil.

“It wasn’t too long ago that our students were mainly recruited by petroleum companies,” said Hueckel, who together with Veveakis and adjunct CEE professor Lyesse Laloui are the editors of the Elsevier journal Geomechanics for Energy and the Environment. “And while they’re still recruited by petroleum companies today, there’s now a much wider demand for their expertise in a wide range of fields, especially when it comes to energy.”

Duke has long been at the leading edge of this trend by implementing new programs that promote interdisciplinary research and learning, and these CEE faculty are making the most of these opportunities.

For example, Hsu-Kim has long led projects with Bass Connections, a program built to put undergraduates, graduates and faculty from disparate fields to work together on imaginative projects. Several of her classes involve environmental epidemiology, where students track contaminants and their effects through ecosystems and communities both at home and abroad. Veveakis is working on a project with the Nicholas School of Environment involving the Duke University Energy Initiative—a university-wide interdisciplinary collaboration focused on advancing an accessible, affordable, reliable, and clean energy system—to power the university’s cooling from geothermal wells on campus.

 “The moral of the story is that interdisciplinary research is the only way to solve these complex problems,” echoed Veveakis. “We need to both act from these many points of view and educate from these many points of view. And that’s a big strength of Duke’s CEE department, both in its faculty and its students.”