Research on shear strength of rock has implications for carbon sequestration

With a $1.1 million grant from the Department of Energy, seismologist Emily Brodsky will address seismic challenges involved in carbon sequestration, geothermal energy, and other areas

Emily Brodsky (photo by Carolyn Lagattuta)

Emily Brodsky, professor of Earth and planetary sciences at UC Santa Cruz, has received a $1.1 million grant from the U.S. Department of Energy to study the shear strength of rocks in the Earth’s crust and the conditions in which they break, which generates earthquakes.

Any activities that involve pumping fluids into or out of the ground have the potential to induce earthquakes by changing the stresses within the Earth’s crust. Examples include hydraulic fracturing (“fracking”) in the oil and gas industry, geothermal wells, groundwater management, and carbon sequestration by injecting carbon dioxide deep underground. Characterizing and predicting the conditions in which rocks fail is key to the sustainability of these activities.

“It’s important to understand what’s going to make the rocks break,” Brodsky said.

Geologic carbon sequestration (storing carbon dioxide in underground geologic formations) could be a major tool for fighting global climate change, but induced earthquakes could potentially scuttle the technology, she said.

“Induced earthquakes could be the Achilles heel of carbon sequestration. If the rocks fail you no longer have sequestration—it breaks the confinement of the carbon dioxide, and you’ve just wasted time and money while getting no closer to a climate change solution,” Brodsky explained.

Much of the current understanding of shear failure in rocks is based on laboratory experiments commonly performed on centimeter-scale samples. These results are then extrapolated to field settings where failure can occur on scales from meters to kilometers, and that worries Brodsky.

“There are a lot of ways in which that might be a bad extrapolation,” she said. “Our research will focus on this scalability problem—how do we take knowledge based on laboratory samples and scale it up to field scales.”

The project combines theoretical, experimental, and observational approaches toward a better description of shear strength in the crust at the scale of relevance for earthquakes and other failure events. The grant from the DOE’s Office of Basic Energy Sciences will support two postdoctoral researchers and a graduate student.

The observational studies will involve analyzing the roughness of fault surfaces and the role of friction in the shear strength of faults. The research will also include statistical analyses of the distribution and abundance of induced earthquakes as a function of applied stress.

The DOE grant is enabling a pivot of Brodsky’s research towards laboratory work. For the laboratory studies, Brodsky’s team has developed a method using a transparent material that models the elastic properties of rock on a small scale to capture the dynamics of fully confined ruptures. The experiments provide insight into the correct interpretation of stress drops and failure stresses for events that only partially break an interface.

“Rocks are elastic—they deform—and only part of the fault ruptures in an earthquake. By using a material that’s squishier than rock, we can model that on a smaller scale, and because it’s a transparent material, we can get movies of the rupture process,” Brodsky explained. “It’s fun for me that I’m becoming an experimentalist 20 years into my career.”