On Nov. 15, 2017, a 5.4-magnitude earthquake struck the seaside city of Pohang on South Korea’s southeastern coast.
The quake injured dozens of people and led to $52 million in damages. It was the second-strongest earthquake in the country’s history—behind September 2016’s Gyeongju earthquake—and a rarity in the relatively geologically inactive region.
Hiroki Sone
The event has since been linked to a nearby geothermal power plant, much like a 3.4-magnitude earthquake that hit Basel, Switzerland, in 2006 that had been the largest recorded earthquake tied to enhanced geothermal systems.
Enhanced geothermal systems work by injecting water two to three miles into the earth, creating fissures in the rock. This creates superheated underground reservoirs where the water warms before it is pumped to the surface. As the water returns to the surface, it turns into steam, which is used for power generation.
The rocks in regions where heat from deep within the earth rises close enough to the surface to use for these systems can be under large “stress,” direction-dependent pressure in rock, says Hiroki Sone, an assistant professor of civil and environmental engineering and geological engineering at the University of Wisconsin-Madison. There are two main types of stress: lithostatic stress, from the weight of material pushing down on rocks from above, and tectonic stresses, which is the mostly lateral compression caused by the collision and extension of rocks under plate tectonics.
Injection of water into geothermal systems can potentially reduce the rock’s resistance to slip by changing the stress, triggering earthquakes. However, not having an accurate knowledge of how much stress is there or what type it is can make it difficult for researchers to make an accurate prediction.
“Measuring stress in the earth is really difficult,” Sone says. “You can see strain, but not stress. So stress always needs to be inferred by indirect methods. Let’s say you’re holding a metal plate and you start to bend it. You can see the deformation, or the bending of the plate, but you never see the actual force that caused it. You can only imagine that there are forces acting on the plate from the fact that it is bending. The same holds true for stresses in the earth, but you have to do this miles below the surface”
Now, thanks to a $1.8 million grant from the U.S. Department of Energy’s Geothermal Technology Office, UW-Madison is partnering with RESPEC, an engineering firm in South Dakota, and the Lawrence-Berkeley National Laboratory to create a technology to help measure stress and reduce earthquake risk associated with enhanced geothermal systems.
Measuring an invisible force like stress can be hard enough on its own. With enhanced geothermal systems, depth and heat compound that difficulty. Enhanced geothermal systems run miles deep, and Sone says engineers can use imaging devices inserted into boreholes to look for stress-induced breaks or deformations along the borehole walls. However, the deeper the hole, the more difficult that becomes, as temperatures deep within geothermal systems can rise to more than 750 degrees Fahrenheit. Sone says that’s well beyond what most modern equipment can handle.
The research team will instead seek to determine stress from intentional fracturing of a borehole wall by cooling. To do this, they’ll burrow into the rock and flow coolant such as water at a target section in a borehole, which will make the surrounding rock contract and crack. By applying directional cooling, they will reveal how the stress varies around a borehole.
“Because we will relate the temperature decrease in the rock to the resulting critical stress that causes the cracking, you have to know when the cracking happens,” Sone says. “If borehole cameras don’t work, then we need another way to infer when this fracturing is induced by the cooling. The way we’re going to do that is by listening to the cracking sound of this induced fracture using miniature seismometers.”
Using several of these seismometers which can handle very high temperatures, the research team will be able to tell not only when the rocks fracture, but where.
The team has three years to work on the directional cooling induced fracture project, and Sone says most of that will focus on prototyping a lab-scale proof of concept. Sone says UW-Madison is uniquely positioned to handle that work, as it has a true triaxial apparatus—a machine that can stress rocks from all three dimensions and simulate the high pressures present within geothermal systems.
He says that by year three, the team plans to upscale its work to an actual borehole-size test to show that it works at full scale. From there, the researchers hope to be able to conduct further research at an actual geothermal site. Should the work bear fruit, it may prove instrumental in helping to prevent future earthquakes associated with enhanced geothermal systems, like the ones that struck South Korea and Switzerland.
“Stress drives these earthquakes,” Sone says. “You have to know the magnitude and direction of the stress within the earth if you want to do your homework and judge if you’re in a dangerous region to build these enhanced geothermal systems. Stress is very fundamental information that we need to know in order to evaluate how much water to inject or how much pressure we can use without triggering slips.”