Susan Hovorka on carbon capture and storage

In a world that is getting warmer, scientists are studying a technique known as carbon capture and storage to prevent the release of the greenhouse gas CO₂ into Earth’s atmosphere from coal-burning power plants and other industries. The idea is to capture the CO₂ (carbon dixoide), and pump it underground. It’s a new technology, which would need to be implemented on a global scale to make a difference in atmospheric CO₂ levels warming the planet. But where on Earth can CO₂ from power plants be stored underground? And is the process safe and effective? Researcher Susan Hovorka of the University of Texas Bureau of Economic Geology has studied many sites on Earth for their potential for carbon storage. She talked with EarthSky about the latest science on this emerging technology. This interview was made possible in part by the Bureau of Economic Geology at the University of Texas at Austin.

You’ve been studying carbon capture and storage for more than a decade. What is it, and why is it being studied?

Currently, when we extract energy from fossil fuels, we emit the byproducts CO₂ and water vapor into the atmosphere. The water vapor doesn’t bother us. But the CO₂ doesn’t cycle as fast as the water. In fact, it takes decades or centuries to come back to equilibrium. And we’ve been extracting more and more energy from fossil fuel.

One of our options – instead of emitting the CO₂ to the atmosphere – is to capture the CO₂ and put it back underground, where the fossil fuel came from, making more of a closed loop and avoiding adding CO₂ to the atmosphere.

We love fossil fuels. I myself enjoy fossil fuel in many ways: in my car, in my stove, to make my electricity. But there are so many of us on the planet who need and use energy. The cumulative effect of the CO₂ emissions on the atmosphere is negative in terms of climate impacts and ocean impacts. So if we want our energy, but we don’t want to suffer the repercussions of putting the CO₂ in the atmosphere, we need to make a choice to change.

That’s where carbon capture and storage comes in. Instead of emitting the CO₂ into the atmosphere, we can capture it through a number of different chemical processes. You do it at a point source, such as a power station or a refinery that’s handling a lot of carbon emissions. You capture it by a chemical process and compress the CO₂ to high density. And then you ship it to a safe permitted place to inject it into the subsurface.

Much of our research at the University of Texas Bureau of Economic Geology is in identifying those safe places. And we provide the information that regulators and investors and policymakers need to make sure that the place is safe.

Are there enough places underground to store carbon at the scale needed to make a difference in atmospheric CO₂ concentrations?

There is definitely enough space underground. Many people think of Earth as completely solid, and that there wouldn’t be space in a solid Earth. People think that the injection needs a space like a cavern or an excavation. But the spaces we’re dealing with here are the spaces between sand grains.

So this is like the parable of the elephant and the ants. A lot of ants can move an elephant. The spaces between sand grains are tiny spaces, but there are a lot of them — in the kilometers-thick crust of the Earth at many places. We know these spaces really well because we get resources like water, oil, and gas from this storage in the Earth.

So we know how fast these resources can come out of the Earth. We also know quite a lot about putting things back into the Earth. In many places, we’ve already returned fluids to the subsurface. For example, if water is extracted from the subsurface during oil field operations or from industrial and municipal wastes, and we don’t want to perturb the surface, we recycle or put back the water. We know how to do this.

In the same way, when we extract the carbon as fossil fuel, we need to learn how to put the carbon, as carbon dioxide back into the same spaces it came from.

There have been extensive studies done, funded by the U.S. Department of Energy and other governments such as Australia, European Union, Japan and China. The answers from all these governments, supported by many studies, is that there is space underground for carbon storage. We scientists may fight about exactly how much and exactly what the best space is. But the issue isn’t that there’s not enough space.

How well do scientists know what’s going to happen to CO₂ stored underground?

This question is the focus of our research. We do experiments where we inject small or large volumes of CO₂ into these densely instrumented arrays, like the ones pictured at Cranfield, Mississippi, where we observe exactly what happens. The short answer is we know very well what happens to fluids in the subsurface.

We can make some predictions. When CO₂ is injected in the subsurface at sufficient pressure, it moves the water out in the pore spaces — the spaces between the sand grains. How much energy it takes to move the water depends on what we call permeability, how easily the fluids can move around. This is something we can measure in the lab or we can measure by testing a well.

Then we know how much energy we need to put it in, and we can plan for it and make sure that it’s safe. We put in an amount of energy that’s below the strength of the rock, just like any other engineering problem. We use an engineering approach to measure the strength of the rock and to find out how much pressure would be too much.

The CO₂ moves underground. It moves mostly sideways, laterally through bedded rocks. It tries to rise buoyantly, it’s less dense than the water. It’ll rise upward like oil and gas does, but it’s trapped against low permeability layers. You could think about these layers as being impermeable, like the plate that you eat your dinner on. Fluids won’t go through it. Those layers trap the CO₂ underneath them.

Is it safe to store large quantities of CO₂ underground? What does the science say?

Any significant engineering problem like injecting large volumes of CO₂ underground requires a rigorous assessment. It could be unsafe if it was done thoughtlessly, or ignorantly, or without correct oversight in engineering and geology. It’s not particularly difficult to do correctly. Injecting fluids underground has been done for about a century.

We here at the Bureau of Economic Geology have been involved in five finished projects where we did extensive research with big international teams. We did a test at the oldest CO₂ injection site in the world, the SACROC Field in Scurry County, Texas. My colleagues Katherine Romanak and Rebecca Smyth went out and measured groundwater quality to see if groundwater had been damaged by decades of deep injection. Their answers were, no, there has been no harm. In fact, the groundwater at SACROC is slightly better than the surrounding areas, partly because of the investments made for the injection activity. It’s a clean operation, and the groundwater is undamaged.

We’ve also been working with the company Denbury Resources, which is injecting CO₂ at a site in Mississippi called Cranfield. And we’ve done a large-scale monitoring project. There have now been 3.5 million tons injected over about four years. We have intensive, deep measurements from the subsurface, from the groundwater, from the surface that show the CO₂ is retained. No harm is being done.

If people want to reduce their emissions of CO₂ to Earth’s atmosphere – while still enjoying the benefits of fossil fuels – one of the real-world possibilities is that, instead of emitting, you can capture and store.

All you have to do is pay for it.

It’s a personal and financial decision we need to make it as a community of energy consumers. But the possibility is completely available to us, to move forward on this option.