Solving geothermal energy's earthquake problem

Solving geothermal energy’s earthquake problem
Conventional geothermal resources have been generating commercial power for decades in places where heat and water from burble up through naturally permeable rock. Credit: Shutterstock

On a November afternoon in 2017, a magnitude 5.5 earthquake shook Pohang, South Korea, injuring dozens and forcing more than 1,700 of the city's residents into emergency housing. Research now shows that development of a geothermal energy project shoulders the blame.

"There is no doubt," said Stanford geophysicist William Ellsworth. "Usually we don't say that in science, but in this case, the evidence is overwhelming." Ellsworth is among a group of scientists, including Kang-Kun Lee of Seoul National University, who published a perspective piece May 24 in Science outlining lessons from Pohang's failure.

The Pohang earthquake stands out as by far the largest ever linked directly to development of what's known as an enhanced geothermal system, which typically involves forcing open new underground pathways for Earth's heat to reach the surface and generate power. And it comes at a time when the technology could provide a stable, ever-present complement to more finicky wind and solar power as a growing number of nations and U.S. states push to develop low-carbon energy sources. By some estimates, it could amount to as much as 10 percent of current U.S. electric capacity. Understanding what went wrong in Pohang could allow other regions to more safely develop this promising energy source.

Conventional geothermal resources have been generating power for decades in places where heat and water from deep underground can burble up through naturally permeable rock. In Pohang, as in other enhanced geothermal projects, injections cracked open impermeable rocks to create conduits for heat from the Earth that would otherwise remain inaccessible for making electricity.

"We have understood for half a century that this process of pumping up the Earth with can cause earthquakes," said Ellsworth, who co-directs the Stanford Center for Induced and Triggered Seismicity and is a professor in the School of Earth, Energy & Environmental Sciences (Stanford Earth).

Here, Ellsworth explains what failed in Pohang and how their analysis could help lower risks for not only the next generation of geothermal plants, but also fracking projects that rely on similar technology. He also discusses why, despite these risks, he still believes enhanced geothermal can play a role in providing renewable energy.

How does enhanced geothermal technology work?

The goal of an enhanced is to create a network of fractures in hot rock that is otherwise too impermeable for water to flow through. If you can create that network of fractures, then you can use two wells to create a heat exchanger. You pump cold water down one, the Earth warms it up, and you extract hot water at the other end.

Operators drilling a geothermal well line it with a steel tube using the same process and technology used to construct an oil well. A section of bare rock is left open at the bottom of the well. They pump water into the well at high pressure, forcing open existing fractures or creating new ones.

Sometimes these tiny fractures make tiny little earthquakes. The problem is when the earthquakes get too big.

What led to the big earthquake in Pohang, South Korea?

When they began injecting fluids at high pressure, one well produced a network of fractures as planned. But water injected in the other well began to activate a previously unknown fault that crossed right through the well.

Pressure migrating into the fault zone reduced the forces that would normally make it difficult for the fault to move. Small earthquakes lingered for weeks after the operators turned the pumps off or backed off the pressure. And the earthquakes kept getting bigger as time went by.

That should have been recognized as a sign that it wouldn't take a very big kick to trigger a strong earthquake. This was a particularly dangerous place. Pressure from the fluid injections ended up providing the kick.

What are the current methods for monitoring and minimizing the threat of earthquakes related to fluid injection for geothermal or other types of energy projects?

Civil authorities worldwide generally don't want drilling and injection to cause earthquakes big enough to disturb people. In practice, authorities and drillers tend to focus more on preventing small earthquakes that can be felt rather than on avoiding the much less likely event of an earthquake strong enough to do serious harm.

With this in mind, many projects are managed by using a so-called traffic light system. As long as the earthquakes are small, then you have a green light and you go ahead. If earthquakes begin to get larger, then you adjust operations. And if they get too big then you stop, at least temporarily. That's the red light.

Many geothermal, oil and gas projects have also been guided by a hypothesis that as long as you don't put more than a certain volume of fluid into a well, you won't get earthquakes beyond a certain size. There may be some truth to that in some places, but the experience in Pohang tells us it's not the whole story.

What would a better approach look like?

The potential for a runaway or triggered earthquake always has to be considered. And it's important to consider it through the lens of evolving risk rather than hazard. Hazard is a potential source of harm or danger. Risk is the possibility of loss caused by harm or danger. Think of it this way: An earthquake as large as Pohang poses the same hazard whether it strikes in a densely populated city or an uninhabited desert. But the risk is very much higher in the city.

The probability of a serious event may be small, but it needs to be acknowledged and factored into decisions. Maybe you would decide that this is not such a good idea at all.

For example, if there's a possibility of a magnitude 5.0 earthquake before the project starts, then you can estimate the damages and injuries that might be expected. If we can assign a probability to earthquakes of different magnitudes, then civil authorities can decide whether or not they want to accept the risk and under what terms.

As the project proceeds, those conversations need to continue. If a fault ends up being activated and the chance of a damaging increases, civil authorities and project managers might say, "we're done."

From everything you've learned about what happened at Pohang, do you think enhanced geothermal development should slow down?

Natural geothermal systems are an important source of clean energy. But they are rare and pretty much tapped out. If we can figure out how to safely develop power plants based on enhanced geothermal systems technology, it's going to have huge benefits for all of us as a low-carbon option for electricity and space heating.

Explore further

Geothermal plant 'triggered earthquake' in S. Korea

More information: Kang-Kun Lee et al. Managing injection-induced seismic risks, Science (2019). DOI: 10.1126/science.aax1878
Journal information: Science

Citation: Solving geothermal energy's earthquake problem (2019, May 24) retrieved 18 August 2019 from https://phys.org/news/2019-05-geothermal-energy-earthquake-problem.html

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User comments

May 24, 2019
This technique of forcing water down into the Earth and hoping it to shoot its way back out in a controlled manner seems like Russian roulette. Surely there are more worthwhile endeavors out there that don't involve fracking -- which lets be honest, is what this really is. Next we'll be hearing about how much oil they are able to get out of the ground as an "added benefit".

May 26, 2019

Your right, I am sure this research is simply another conspiracy, just like the one that tries to tell us the world is round like a ball!

May 27, 2019
EVERY energy source has its own unique limitations and potential problems.
All this temporary setback means is that we now have become painfully aware of one of those potential problems for geothermal energy and we must now take that into account.
The correct response would NOT be to now go against geothermal energy but rather to LEARN from this past mistake of exploiting it incorrectly and then adapt the way we exploit it in such a way so that not to let it cause the same kind of disaster to happen again.

Despite this temporary setback for geothermal energy, the average number of human deaths geothermal energy has caused per unit energy generated from it is DWARFED by that from fossil fuels and mainly because of the local air pollution generated from burning fossil fuels kills MILLIONS of people. But I bet now we will get the usual morons here to use this disaster to moronically argue we shouldn't have renewables but should just keep burning fossil fuels.

May 27, 2019
There are two different things you can learn from any event. The right thing or the wrong thing. If you generalize a conclusion it is usually the wrong thing.
