Professor Iraj Ershaghi and a team of researchers at the University of Southern California (USC) found a way to use idle oil and gas wells for energy storage — one of the major concerns for solar and wind energy generation.

Professor Iraj Ershaghi

Tech Briefs: Where did this idea come from?

Professor Iraj Ershaghi: A major problem in this country is what to do with the large number of oil and gas wells that have reached the end of their productive lives and have to be permanently abandoned. Well abandonment is a major issue that's facing all of the oil companies. I'm talking about the majors — smaller sized companies who sometimes cannot afford to pay for the cost of abandonment might just declare bankruptcy and walk away. It then becomes a responsibility for the state, and the cost of abandonment can be huge.

There are more than 37,000 Wells in California that are currently idle, and in the entire U.S., you're talking about more than a million. I always had some interest in the issue of well abandonment and thinking about what the engineering community can do to reduce abandonment cost and do it more effectively.

One problem is that when you improperly abandon a well, it could become a source of gas leakage. Not only is this bad because of its contribution to greenhouse gases, but it's not fun to unknowingly build a house on top of an abandoned well and then experience gas leaking into the garage.

I had been looking into the question of why this is happening. One of my thoughts was maybe it was not abandoned properly, maybe the cement they used was not proper. So, part of my interest has been how to do it more efficiently and more responsibly. In many cases, it’s difficult to even locate an abandoned well because when someone abandons it, they cut the wellhead casing so that it can be hidden.

Then a company approached us and said that they would be interested in working with USC to see if we could help them with a significant problem that the renewable energy industry is facing — energy storage. They had the impression that perhaps empty oil and gas reservoirs could be used as a storage. My first response was: don't even think about it. It Is not prudent to inject compressed air, with oxygen, into hydrocarbon reservoirs.

I was aware that the McIntosh Power Company in McIntosh, Alabama, had drilled into huge geological structures made of salt, called salt domes. They created cavities in these salt deposits to store compressed air during times when an energy source is producing excess un-needed power. When they need to use the stored energy, they release the compressed air to run a turbine that produces electricity.

Although we don’t have salt domes in California, we know that as you go from the surface all the way down to the sub-surface hydrocarbon zones, there are layers of sandstone full of ancient saline seawater. These are sediments that were deposited at the time California was covered by water, so the water content of these geologic layers is saline. We got the idea that perhaps we could use the McIntosh approach, but instead, store the compressed air in these saline water-containing sand deposits. One of our faculty members, Dr. Jha, calculated that if you could go down below 4000 feet, you could have enough storage to produce 5 to 10 megawatts of electricity in a 2000-foot radius around an idle well repurposed as an injection well. There are many acre-feet of layers around wells in California that can be used for a large amount of storage.

California regulations require utilities, by a certain date, to come up with large energy storage capacity. Utilities under this legislative mandate are struggling because they were counting on batteries, and as of now, battery technology is not yet good enough to satisfy this need. That’s when it dawned on us that a plausible solution in California could be to use the saline aquifer to store compressed air.

We heard from oil companies that they would welcome this use of some of their wells for energy storage, because it would give them carbon negative credits. In California, you have to be carbon neutral — for every barrel of oil they produce, operators have to show somehow that they are mitigating the CO2.issue.

We’re now talking to some of the operators that have shown interest in using their idle wells for a demonstration project.

My colleagues and I are very excited to think this is going to be transformational for facilitating and bringing renewables into the California energy market.

Tech Briefs: Where do you put the cement in relation to the aquifer?

Professor Ershaghi: A typical well has a series of pipes leading to the hydrocarbon deposits. In an idle well, a cement plug is placed above the hydrocarbon layer. However, the surface casing might be cut off 50 or 70 feet below the surface and filled in. This can sometimes become an environmental concern requiring costly remediation.

On the other hand, we are suggesting a solution that does not require cutting the wellhead. The oil and gas could be 9000 feet vertically below the surface, but there might be hundreds of feet of sandstone containing saltwater, perhaps 5000 feet below the surface. These deposits have previously been ignored because of the saline water.

If a solar energy source is producing excess power, it would be perfect if it could be stored to for use at night or to supply electric power when there is a brownout. My proposal is to store that excess power for use later, when it is needed, by using it to run an air compressor. The compressed air will then be injected into the salt water-bearing sandstone.

We do something similar to the water dam, where you make electricity because water releases its potential energy by flowing downward over the dam and turning a turbine. In this scenario, the kinetic energy of the compressed air is transferred to the water that energizes a turbine and subsequently a generator. When the stored energy is needed, the pressurized air rises to the surface and pressurizes a container of water to run a turbine.

In addition, we are proposing to put sensors at the surface of existing wells so that we will also be able to detect, in real time, any hydrocarbon leakage.

Because water has a low compressibility, its stored pressure of formation could be, for example, 3000 pounds per square inch. When you produce from that for a couple of days, the pressure drops very rapidly. When it drops to a certain level, say 500 psi, the compressor automatically starts up to bring the pressure back to 3000 psi by injecting more air.

By measuring the thickness and area of the wetland deposits, we know the volume and can calculate how much air one can store to be converted to power. Our calculations show that this wouldn’t add more than a few pennies, to the cost of the electricity.

Tech Briefs: Could you explain a bit more about the action between the air and water?

Professor Ershaghi: It’s similar to what we do when we store natural gas. You store natural gas in an oil reservoir. When you inject the gas, it pushes the oil back and when you produce, you produce. It’s just like a, yo-yo going back and forth, back and forth: inject then produce.

We have calculated that you could store 5 to 10 megawatts per well. Multiply that by the approximately 37,000 idle wells in California, and you have gigawatts. This would be a major source of electricity. The state could become self-sufficient, it would no longer have to import fuel. This would be a win-win because the production of oil and gas in California is going down, while the demand for electricity is not going anywhere.

Let's say that I’m an oil field operator, and I may have a thousand wells. I might know that a certain portion of the reservoir was depleted. Those wells would no longer be able to produce enough oil to make them economically viable. There might be 10 to 30 wells in that depleted area. With those, you easily build a 100-megawatt facility.

The beauty of this method is that it could be used in most parts of the United States — it’s not limited to oil-producing areas.

For example, in New York, while you don't have large-scale oil and gas production, you do have deposits of water sands above the Devonian shale, which could be used. In any part of the U.S. as you drill down, you're going to see saline aquifers. A million years ago much of the U.S. was covered by waterways, so there are now wet sediments all over. You could be in a state that never produced a drop of oil or gas, but it would still have these wet layers underground. The U.S. geological survey has maps that show you the location of underground deposits of wet sands.

To store energy in idle wells, sustainable power created by solar panels and windmills placed near them are converted into high-pressure air by a compressor. The air travels through pipes to subsurface saline aquifers, or shallow wet sands, between 1,000 and 8,000 feet beneath the surface. The high-pressure air remains safely underground until needed. (Image Credit: Iraj Ershaghi)

Tech Briefs: You mentioned that in California these sands contain saltwater; does it have to be, saline for this to work?

Professor Ershaghi: With the shortage of fresh water that we’re facing in many parts of the U.S. it would not be a good idea to use underground freshwater resources for this purpose. However, if we go deep enough, any water we discover is likely to be saline.

The explanation calls for a brief lesson in geology. If you plot the Earth's temperature for the last 300 million years, essentially, it shows that the temperature of the earth was sometimes very high and sometimes very low. When much of the surface of the earth was covered with water, it went through periods of freezing and thawing. This gradually broke down areas of rock formations, and the detrital materials accumulated.

The sediments formed by the rock erosion became saturated with seawater with the rise of the oceans. That is why the deepest deposits of water are saline, while the freshwater that is desperately needed in places like California is much shallower and usually replenished with rainwater.

As you know, there's been an effort going on for the last two decades for carbon capture and sequestration. The idea has been that if you have too much carbon dioxide, you just store it in subsurface geologic formations. But there have been concerns about carbon dioxide leaking. So, in our proposed energy storage concept, we are also benefiting from that experience, because there's a lot of research and modeling concerning subsurface CO2 sequestration. Our situation is much simpler because if you store air underground, even if it leaks out, who cares — you're just adding more air to the air, there's no carbon dioxide, there's no toxic fuel, it's just air.

Right now, the reason people aren’t building renewables as fast as is necessary, is because the economics don’t look good. The fact is, it costs a lot of money to build these facilities, and at this point you're not using every kilowatt that's being generated, a significant portion of this power could just be wasted without large-scale storage.

So, if you come up with a way to store the electricity and then use it when you need it, you're going to solve a major economic problem. That would make the expansion of renewable sources much more acceptable to investors and to society at large.

Tech Briefs: Are you basically saying that you’re using existing technology but applying it in a way that's more economical and useful?

Professor Ershaghi: I think our contribution is more the following. Number one, it’s a proven technology, that people have stored air in salt domes. Our first contribution is that you don’t require a salt dome — as long as you have a saline aquifer, that could do the job. Number two, the impression is that this is costly because you have to drill the well. But we’ve shown that we could use existing wells that are destined for abandonment. That's the idea. So essentially use the idle wells, use a saline aquifer, and expand the technology to make it widely available, while reducing the liability to the states and the public for the well abandonment costs.

Tech Briefs: What about the energy lost in running the compressor?

Professor Ershaghi: Let's assume that you’re using a hundred-megawatt solar source. By the time you take that hundred megawatts and use it to produce compressed air, if you look at the energy balance, of course, you lose energy. So, you may have, a fraction of the megawatts that are available.

But that's mother nature. The concept of entropy is that energy is always lost in any process where work is done. That's an area of research: how to minimize the losses.

So, after the 100 megawatts are converted to compressed air and it goes through the storage process and is returned, you might only have 60 megawatts that is usable. But it is better to go from 100 down to 60 than not generating any useful power at all.

Tech Briefs: Can you briefly describe how the overall operation would work.

Professor Ershaghi: In the Alabama facility, for a comparison, they use the return air to turn the compressor. When the air comes back, it has to be heated up to expand so they can use it. If you do that, you need to burn natural gas to produce the heat. We don't need that because we use a compressor to apply pressure to the water. It is the water pressure that operates the turbine.

Tech Briefs: How does the compressed air transfer its energy to the turbine?

Professor Ershaghi: The returned compressed air comes to the surface when needed, and it pressurizes the water in a high-pressure vertical container. The water pressure activates and runs the turbine. Kinetic energy for generating the electricity comes from the pressure of the compressed air.

Tech Briefs: What would be the economic incentives for building these systems?

Professor Ershaghi: First of all, we don't have to drill the well and go through the permitting process. In California, most of the oil and gas lands are fee lands. If I have title to a home on fee land, that means I own everything from the surface all the way down to the center of the earth. If it turns out that there is an oil field underneath my land, I can lease my rights for it to an oil company. This is generally in the form of a percentage of the income from what is produced each month.

The land owned by most oil companies is fee land. So, if I'm an operator, who owns a number of wells, some of which are economically unproductive, this could be a source of income. If I convert those abandoned wells to storage, I could charge a utility for the right to use them. The operator would then have a source of income from these non-producing wells. Meanwhile, the operators eliminate or postpone the abandonment cost.

This could also be a new source of income for landowners: leasing their subsurface rights to storage operators.

Utilities would find it worthwhile because they could reduce the amount of oil and gas they need to generate power.

Tech Briefs: Could there be underground disturbances, such as have occurred with fracking?

Professor Ershaghi: I'm glad you asked that question. There was a report from the National Resource Council summarizing research on the issue of whether fracking has caused earth tremors. Their conclusions were that there is no evidence that the actual practice of fracking caused the tremors. They concluded that minor earthquakes were caused by used waste fluids that are pumped into disposal wells. Continuously adding these fluids pressurizes the sand, thus changing the subsurface stress fields.

Tech Briefs: What are your next steps?

Professor Ershaghi: We will have a demonstration project, which will be funded by either utilities or oil companies. I expect that we could have a site by next fall. We have already identified some sites that would be ideal, at least for a demonstration. It might cost $22 million for a 5-megawatt generation facility, including further research on the interaction of oil and water. We will also be studying new materials, maybe putting in new tubing, maybe using composites, different types of cements. Our team includes electrical engineers, research scientists in groundwater, hydrologists, reservoir engineers, and composite engineers who will be working on optimizing the system. In addition, we will be studying site characterization, not only for proper geology but also for proximity to the electric power grid. We also intend to apply digital technologies for site monitoring.

An edited version of this interview appeared in the February 2021 issue of Tech Briefs.


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This article first appeared in the February, 2021 issue of Tech Briefs Magazine.

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