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Breadcrumb Main Story Geological hydrogen’s potential for next-generation energy Smallest element may hold big promise for clean energy
“hydrogen’s Potential As A Clean Energy Vector In The Gas-electricity Nexus”
A previously overlooked potential geological energy source could increase the renewables and reduce the carbon footprint of our nation’s energy portfolio: natural hydrogen.
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Hydrogen, as you may remember from your school days, is a gas. It is considered the cleanest fuel because its combustion produces only heat and clean water. Engineers have even created a way to use it to generate electricity in a hydrogen fuel cell. In a nutshell, it works because a fuel cell binds hydrogen and oxygen together to form water and generate electricity in the process.
Although its primary use as an energy source today is in rocket fuel, hydrogen is expected to play an important role in future energy systems. It can offer solutions for reducing the carbon footprint of processes that cannot be easily electrified, such as long-haul flights and industrial heating. The catch is that the vast majority of hydrogen is produced using natural gas in a process that consumes energy and releases large amounts of carbon dioxide into the atmosphere.
Scientists have known for some time that hydrogen also occurs naturally, it is formed by geological processes. Using natural resources would eliminate the problem that dogs produced hydrogen because it would not release such large amounts of carbon into the atmosphere. There’s just one problem: there’s little scientific information available about how much hydrogen is out there or where it might be.
Diagram of how a hydrogen fuel cell works with inputs and outputs. Courtesy of the Department of Energy’s Energy Information Administration.
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To get an idea of how much hydrogen gas the Earth can store, research geologist Geoffrey Ellis enlisted the help of his Energy Resources Program colleague Sarah Gelman to develop a global resource model. Before they could use the model to estimate the amount of hydrogen available, they had to advance scientific knowledge about the behavior of hydrogen in the subsurface. The pair used existing knowledge of analogues such as natural gas to fill gaps in existing knowledge and develop their hydrogen model.
“Using a conservative range of input values, the model predicts a mean volume of hydrogen that could meet projected global hydrogen demand for thousands of years,” Ellis said.
However, he is quick to warn: “However, we have to be very careful when interpreting this number. Based on what we know about the distribution of oil and other gases in the subsurface, most of this hydrogen is probably inaccessible.”
In other words, hydrogen reserves are buried too deep or too far offshore or in accumulations that are too small to make it highly unlikely that they can ever be economically recovered.
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The good news is that if even a small fraction of this estimated volume could be recovered, there would likely be enough hydrogen in all global deposits to last for hundreds of years. Ellis believes that the amount of hydrogen in the Earth’s interior could potentially represent a primary source of energy.
“The key,” he said, “is to understand whether hydrogen exists in significant accumulations that can be economically accessible, and if so, how to find those sources.”
To begin to understand the potential for hydrogen storage, scientists need a better geological model to understand how hydrogen forms, where it comes from in rock layers, and where it ends up.
Drawing on his background in petroleum geology, he is working to develop a model that uses a petroleum system approach.
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A petroleum system is a conceptual model designed to understand the occurrence of petroleum in geological basins. Petroleum geologists have used it for decades to efficiently guide oil and gas exploration and to derive accurate estimates of undiscovered oil resources.
The model helps geologists analyze the geological factors that must come together to effectively form an oil accumulation. Imagine a geologist following a series of clues to solve a puzzle. First, the source rock must contain organic material capable of generating oil. Then the geologist must consider all the paths the oil could take as it escapes from the source rock and migrates through the next layers of rock. In addition, the geologist must identify any porous reservoir rocks where oil could accumulate. Finally, the geologist must evaluate whether there are rocks nearby that may have sealed the fluid in place, often for millions of years. If any of these components fail, the geologist can infer that the oil accumulation would not form.
Forsterite, an olivine mineral. The interaction of groundwater with olivine can lead to the accumulation of hydrogen in the surrounding rock layers. Image credit: Smithsonian National Museum of Natural History.
To fit a petroleum system model to hydrogen accumulation, geologists must identify how natural hydrogen forms in rock layers, what types of natural processes can affect the hydrogen formed, and how hydrogen can become trapped in rock layers along its path to the surface.
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Geologists already know that there are dozens of natural processes that generate hydrogen, but understanding the potential of hydrogen sources requires identifying which of these mechanisms are capable of generating hydrogen.
Amount of gas. One such process that scientists generally agree on occurs when groundwater interacts with iron-rich minerals such as olivine. (Olivine is a magnesium-iron silicate that has a green hue not unlike that of—you guessed it—olive.) This interaction can cause water to be reduced to oxygen, which binds with the iron in the minerals, and hydrogen, which then escapes into the surrounding rocks.
Once hydrogen is formed, the gas can be consumed by a number of natural processes. In particular, many microbes survive on hydrogen, and microbiologists have now described a vast, deep hydrogen-powered biosphere. Moreover, the process by which oil is formed from organic-rich rocks uses up all available hydrogen. This is one reason why hydrogen is rarely found in hydrocarbon gases such as methane or propane.
Any hydrogen not consumed by these processes can enter porous rocks where the gas could accumulate. But for the accumulation to persist, an effective seal rock must be present to hold the gas in place. For decades, geoscientists assumed that seal rocks could not effectively contain hydrogen accumulation because the small size of hydrogen would allow it to escape through even the tightest rocks. However, studies show that the molecular diameter of two hydrogen atoms is about the same as that of one helium atom, and that the two gases are likely to be trapped by similar rock layers. Accumulations of helium are known to have persisted for up to 100 million years, so it is reasonable to assume that hydrogen could be trapped for similar time spans.
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Scientists incorporate all of these factors into their model, which will improve our understanding of the potential for natural hydrogen resources on Earth.
As principal scientist in an effort to evaluate the potential for geologic hydrogen resources, Ellis is leading efforts to map areas in the U.S. most likely to contain geologic hydrogen. His team uses a model of the hydrogen system as the basis for this work. By mapping the distribution of each of the components of the hydrogen system and assessing how well they align, they can provide an initial estimate of geologic hydrogen potential across the country.
There are at least two major regions of the earth that have favorable geology for generating significant volumes of hydrogen. These lie along the Atlantic Coastal Plain and in the central part of the US, in the subsoil of parts of the Great Plains and the Upper Midwest.
The Atlantic Area of Interest stretches along most of the eastern seaboard and is associated with a belt of iron-rich rock layers buried deep beneath the ocean floor. These rocks were deposited during the formation of the Atlantic Ocean basin. Geophysical surveys confirmed that some of the iron in these rocks reacted with water to produce hydrogen, which most likely escaped from the iron-rich rocks and migrated along the sedimentary layers toward the shore.
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The central US area of interest is related to the rocks that formed when an ancient rift nearly split North America in two. The failed rift, known as the Midcontinent Rift, occurred about 1.1 billion years ago and lies beneath Lake Superior and much of Iowa, Minnesota, and Michigan. Although the rift failed to split the continent, it brought a huge amount of minerals into the upper crust, including iron-rich minerals that could form hydrogen.
Despite the considerable potential for hydrogen production in these regions, this does not necessarily equate to the high potential of geological hydrogen resources.
Ellis explained, “Remember that we have to have all the components of the hydrogen system to make the system work. We still need to do more work to determine the extent to which other components such as reservoirs and seals are present in these areas before we know how likely they are to contain significant amounts of geological material.
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