If you want to engineer better underground systems, it’s important to understand how fractured rocks deep underground allow fluid to move through them.
That’s exactly what geological engineering sophomore Porter Garst is studying under Hiroki Sone, an assistant professor of geological engineering at the University of Wisconsin-Madison.
Garst is one of 28 students who earned support for their work from UW-Madison as 2021 sophomore research fellows.
Rock permeability is crucially important for industries and systems that rely on fluid movement beneath the earth’s surface, such as enhanced geothermal systems. Such systems inject water deep underground, creating superheated reservoirs, and pump the superheated water back up to the surface. The heated water gives off steam, which can be used for power generation or to heat buildings.
Garst’s project will build upon foundational research that’s already been conducted on various factors at play for fluid permeability in underground sites. He says that, while reviewing previous research papers on the topic, he and Sone noticed that some researchers studied the permeability of fractured rocks, while others cracked rocks under stress or studied the roughness profiles created when rocks break.
Now, Garst and Sone hope to combine all of those factors into a unique and comprehensive approach: fracturing rock samples, flowing water through them while they’re under the same types of pressure that would be expected in the field, and shearing the rock samples again. By doing all of these steps under conditions that are as realistic as possible, they hope to better understand how rock permeability behaves in conditions that could be expected in geothermal or other underground systems.
“When we were looking at the research papers, Hiroki pointed out that none of them were really going from step one to the final step,” Garst says. “We realized there’s research that touches on all of these foundations—how the fractures form, how roughness forms, and so on. We’re going to build on all of that.”
Garst’s research will focus on shale gas and enhanced geothermal systems—the two primary industries that rely on rock permeability. He and Sone will crack shale with similar characteristics to that found in shale gas fields, and they’ll use granite to study rock permeability for enhanced geothermal systems. They’ll use a triaxial rock deformation apparatus, in which hydraulics put rocks under pressure until they break.
In addition to observing rock fractures and how fluid flows through the broken samples, Garst and Sone will monitor how much the rocks slip—or move along the fracture—and the overall impact that movement has on permeability.
“Rock type is one variable, but the other big one is how much slip occurs,” Garst says. “As a rock slips, for example, it might grind itself down to become flat, or it might create other particles that could either create new gaps or clog up the system.”
Sone says all of the research is important because engineers who design systems for shale gas extraction or enhanced geothermal systems want to model how fluid flows through rocks in underground reservoirs. To do that, they have to also take into account how rock moves along fractures over time and the effect that has on overall rock permeability. Ultimately, he and Garst hope the project will lay the groundwork for further research.
“This is very fundamental-level research, but we are trying to overcome the shortcomings of past datasets so we can improve the knowledge of how permeability changes,” Sone says. “That can then be implemented into models and improve the overall engineering approach to these systems in the field. There’s a very clear application in mind. We may not, in one year, come up with an equation that people can use, but this is a great foundation for future work.”