How geologic hydrogen went from fringe science to potential green fuel

Hydrogen, you may recall from your school days, is a gas. It is considered the cleanest fuel, because burning it only produces heat and pure water. Engineers have even created a way to use it to generate electricity in the hydrogen fuel cell. In brief, this works because the fuel cell binds hydrogen and oxygen together to make water, generating 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 may offer a solution for reducing the carbon footprint of processes that cannot easily be electrified, such as long-distance flights and industrial heating. The catch is, the vast majority of hydrogen is manufactured using natural gas through 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, generated through geologic processes. Tapping into natural sources would eliminate the problem that dogs manufactured hydrogen, because it wouldn’t release those 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 found.

A Global Perspective

To get a sense of the amount of hydrogen gas that the Earth may be storing, USGS 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 a model to estimate the amount of hydrogen available, they had to advance scientific understanding about the behavior of hydrogen in the subsurface. The pair used existing knowledge of analogues such as natural gas to fill the 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 supply the projected global hydrogen demand for thousands of years,” Ellis said.

However, he quickly cautions, “We have to be very careful in interpreting this number, though.  Based on what we know about the distribution of petroleum and other gases in the subsurface, most of this hydrogen is probably inaccessible.”

In other words, hydrogen supplies are too deeply buried, or too far offshore, or in accumulations that are too small, making it highly unlikely they could ever be economically recovered.

The good news is, if even a small fraction of this estimated volume could be recovered, there would likely be enough hydrogen across all the global deposits to last for hundreds of years. Ellis is convinced that the amount of hydrogen in the Earth’s interior could potentially constitute a primary energy resource.

“The key,” he said, “is to understand if hydrogen exists in significant accumulations that can be economically accessed, and if so, how to find these resources.”

Sources/Usage: Public Domain. Visit Media to see details. How hydrogen forms underground. Source: “HIDDEN HYDROGEN: Does Earth hold vast stores of a renewable, carbon-free fuel?” from Science

Taking Lessons from Oil and Gas

To begin to understand the potential for hydrogen accumulation, scientists need a better geologic model to understand how the hydrogen forms, where it comes from within the rock layers, and where it ends up.

Ellis notes, “We’re fortunate that we are not starting from scratch here.”

Relying on his background in petroleum geology, he is working to create a model that uses the petroleum system approach.

The petroleum system is a conceptual model designed for understanding the occurrence of petroleum within geologic basins. It has been used by petroleum geologists for decades to effectively guide oil and gas exploration and to derive accurate assessments of undiscovered petroleum resources.

The model helps geologists analyze the geologic factors that must come together to effectively form a petroleum accumulation.  Picture a geologist who is following a series of clues to solve a puzzle. First, a source rock must contain organic material capable of generating petroleum.  Then, the geologist must consider any pathways the petroleum could follow as it escapes the source rock and migrates through other rock layers. In addition, the geologist must identify any porous reservoir rocks where the petroleum could accumulate. Last, the geologist must evaluate whether there are rocks in the vicinity that could have sealed the fluid in place, often for millions of years.  If any of these components fail, then the geologist can deduce that a petroleum accumulation wouldn’t form.

Sources/Usage: Public Domain. Visit Media to see details. Forsterite, an olivine mineral. Groundwater interacting with olivine can result in hydrogen building up in the surrounding rock layers. Image credit: Smithsonian National Museum of Natural History.

The Hydrogen System

To adapt the petroleum system model for hydrogen accumulations, geologists must identify how natural hydrogen forms within rock layers, what types of natural processes might affect the hydrogen once formed, and how the hydrogen can get trapped in rock layers along its way to the surface.

Geologists already know that there are dozens of natural processes that generate hydrogen but understanding hydrogen resource potential requires identifying which of those mechanisms are capable of generating large quantities of the gas. One such process that scientists generally agree upon happens when groundwater interacts with iron-rich minerals like olivine. (Olivine is a magnesium iron silicate that has a green hue not unlike that of—you guessed it—olives.) This interaction can cause the water to be reduced to oxygen, which bonds with the iron in the minerals, and hydrogen, which then escapes into the surrounding rock.

Once hydrogen has formed, a variety of natural processes can consume the gas.  In particular, many microbes survive on hydrogen, and microbiologists have now described a vast, deep biosphere fueled by hydrogen. Additionally, the process by which petroleum forms from organic-rich rocks consumes any available hydrogen. This is one of the reasons why hydrogen is rarely found with hydrocarbon gases like methane or propane.

Any hydrogen that isn’t consumed by these processes may reach porous rocks, where it could form a gas accumulation. But in order for the accumulation to persist, an effective seal rock must be present to hold the gas in place. For decades, geoscientists have assumed that seal rocks could not effectively contain hydrogen accumulations, because hydrogen’s small size would allow it to escape through even the tightest rocks. However, studies show that the diameter of a molecule of two hydrogen atoms is about equal to that of a single helium atom and that the two gases are likely to get trapped by similar rock layers. There are known helium accumulations that have been preserved for as long as 100 million years, so it is reasonable to assume that hydrogen could be trapped for similar time spans.

USGS scientists are incorporating all these factors into their model, which will improve our understanding of the resource potential of natural hydrogen on Earth.

Sources/Usage: Public Domain. Visit Media to see details. Meet the Mid-Continent Rift, one of the most geologically fascinating regions in the United States and Canada.

Mapping It Out

As the principal scientist in the USGS effort to evaluate the potential for geologic hydrogen resources, Ellis is leading the USGS effort to map the regions of the conterminous U.S. most likely to contain geologic hydrogen. His team is using the hydrogen system model as the basis of 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 the potential for geologic hydrogen across the nation.

There are at least two major areas of the country that have favorable geology for the generation of significant volumes of hydrogen. These lie along the Atlantic coastal plain and in the central U.S., underlying parts of the Great Plains and the Upper Midwest.

The Atlantic area of interest stretches along most of the East Coast and is associated with a band of iron-rich rock layers buried deep beneath the ocean floor. These rocks were deposited as the Atlantic Ocean basin formed. Geophysical surveys have confirmed that some of the iron in these rocks has reacted with water and produced hydrogen, which most likely escaped from the iron-rich rocks and migrated along sedimentary layers toward the shore.

The central U.S. area of interest is related to rocks that were formed when an ancient rift almost split North America in two. The failed rift, known as the Midcontinent Rift, occurred about 1.1 billion years ago, and underlies Lake Superior and much of Iowa, Minnesota, and Michigan. Although the rift did not succeed in dividing the continent, it did bring vast quantities of minerals to the upper layers of the Earth’s crust, including iron-rich minerals that could form hydrogen.

Despite the significant potential for generation of hydrogen in these regions, this does not necessarily equate to high potential for geologic hydrogen resources.

Ellis explained, “Remember, we have to have all of the hydrogen system components present in order for the system to work. We still have more work to do to determine the extent that other components, such as reservoirs and seals, are present in these areas before we will know how likely they will be to contain significant amounts of geologic hydrogen.”

Exploring for and Producing Hydrogen

Exploration for geologic hydrogen resources is likely to employ many of the same strategies and technologies that are currently used in petroleum exploration, with some added elements taken from mineral and geothermal resource exploration. Because of the potential for hydrogen to cause steel to become brittle, production of hydrogen trapped in reservoirs will require slightly different materials. Otherwise, the same drilling and completion equipment that is currently in use for natural gas development can be used.

However, unlike natural gas fields, some of the gas in natural hydrogen fields may be renewable given the rapid rate of hydrogen generation via water reduction. Moreover, some researchers have proposed that reservoirs, traps and seals may not even be necessary to produce geologic hydrogen. They suggest that we might be able to tap into rocks that are generating hydrogen, or have hydrogen migrating through them, and produce the hydrogen gas as it is being generated. Other scientists go even further and propose that hot water could be injected into iron-rich rocks that are not currently generating hydrogen in order to stimulate generation, somewhat similar to enhanced geothermal energy production.

“If you add up the amount of hydrogen we think might be trapped in reservoirs, plus the amount that might be produced directly as it is generated, and the amount that could be made through stimulation, you get a very large potential resource”, Ellis said.

What’s Next for the Science

While geologic hydrogen offers a lot of promise, realizing its actual resource potential still requires further investigation to reduce the uncertainty surrounding the components of the hydrogen system and to develop exploration strategies. At the USGS, we’re using our expertise in fields such as petroleum, geothermal, and mineral resources to advance our understanding of geologic hydrogen resource potential.

Next steps include publishing the global resource potential and hydrogen system models, as well as releasing a preliminary map of the areas that are most likely to contain geologic hydrogen resources. But there is more work to be done beyond that. The USGS is directing ongoing research efforts to develop exploration tools and strategies that will improve our understanding of this previously unrecognized energy resource to help meet the nation’s future energy needs.

More information can be found by visiting the USGS Energy Resources Program website. To keep up-to-date with USGS energy science, sign up for our Newsletter and follow us on Twitter.

Source: USGS.gov