Could sequestered hydrogen be clean fuel for the future?

New model predictions suggest that Earth’s subsurface may hold billions of tonnes of natural hydrogen. Hydrogen is the most abundant element in the universe and can be a source of carbon-free energy. Burning it releases mostly water. But teasing it apart from natural compounds that contain it, like water or natural gas can be expensive and carry a carbon cost, which has been a deterrent to wide scale exploitation of hydrogen previously. But, geoscientists think that there may be vast reservoirs of the gas trapped beneath our feet, and a lot of it within reach. If they’re right, their findings suggest that geologic hydrogen could produce twice as much energy as all the proven natural gas reserves on Earth. I’ve been speaking to Geoff Ellis from the US Geological Survey…

Geoff - We had generally assumed that natural accumulations of hydrogen just couldn't form because it's a very small molecule and it's very diffusive. It leaks out through rocks in the subsurface and it's also readily consumed by microorganisms. There was an accidental discovery of an accumulation of almost pure hydrogen gas about a dozen years ago in the country of Mali in West Africa. As I learned about this discovery, I started to question this notion that maybe accumulations of natural hydrogen could form.

Chris - Presumably, if we were to go burrowing after hydrogen like that, it's not got the same implications as digging up fossil fuels, petroleum, coal, gas because there isn't a carbon footprint, apart from the extraction process, associated with it. It would be a clean form of, I suppose, fossil fuel.

Geoff - That's exactly right. Today the hydrogen that we get is actually predominantly made from fossil fuels. In the process of making a kilogram of hydrogen, we actually release about 10 kilograms of CO2 into the atmosphere.

Chris - I suppose we should point out that there are of course ways of making hydrogen in a non-carbon releasing way, that is, if you use a sustainable energy source to do it. But that is a drop in the ocean compared to where most comes from, isn't it?

Geoff - That's right. Today, very little of this clean hydrogen is being made. The technologies that we have do exist, but they're quite expensive. It's not really clear how we're going to get all this low carbon hydrogen that we think we're going to need.

Chris - How have you turned this into a tractable question/problem then?

Geoff - We looked at what we do know about natural occurrence of hydrogen on Earth. This is something that has been studied for many, many decades, mostly by biologists that are interested in the deep biosphere and life on other planets. We know that there are microorganisms that are living on hydrogen in the subsurface. Then there were some knowledge gaps that we had to fill in, so we used analogues from other things that have been well studied, things like petroleum systems. Through studies of petroleum, we know how gases migrate and trap and get accumulated in the subsurface. We were able to then borrow knowledge from those fields to fill in gaps to put together a global model of just how much hydrogen might actually be trapped in accumulations in the entire Earth's crust.

Chris - Before we come to how much, just tell us where then we would anticipate this hydrogen might be.

Geoff - The processes that are thought to be capable of generating the largest amounts of hydrogen tend to be associated with crystalline rock settings, iron rich rocks that are associated with the mantle underneath the Earth's crust that have been brought up near the surface, and also radioactive rocks. The radioactive decay of radiogenic minerals can actually split water and generate hydrogen. Actually at the mid ocean ridges today, we see the sea water being reduced and forming hydrogen, and also in many hydrothermal systems, hot springs and so forth, we find hydrogen gases there. These are, by the way, very different places from where we explore for oil and gas, so this can explain why we don't find hydrogen associated with petroleum.

Chris - And they're also, by the sound of it, within reach, some of these sources, potentially.

Geoff - Yes, absolutely. And in fact, this accidental discovery in Mali was only a hundred meters below the surface. So in fact, it's very possible that it could be very accessible.

Chris - Now, notwithstanding the fact this is a model, therefore it's got strengths and weaknesses - there are going to be knowns, there are gonna be unknowns - how much hydrogen might there be lurking within reach beneath the Earth's surface?

Geoff - The units that we refer to are million metric tons or megatons. We estimate that there could be anywhere from maybe just a few thousand megatons to potentially billions of megatons in the Earth's crust. The median, or most likely value, is on the order of around 5 million megatons. On the order of millions of megatons seems to be probably most likely.

Chris - And in energy terms, is that a meaningful amount? How would that relate to, for instance, the amount of energy that's locked up in natural gas that's exploitable at the moment?

Geoff - The other thing that we have to be aware of is that these millions of megatons are actually across the entire Earth's crust. We make no prediction about the distribution of that hydrogen, and almost certainly most of this is going to be inaccessible, it's just simply going to be too deep or too far offshore or in accumulations that are much too small that it could never be economically recovered. But as a thought exercise, I say let's assume that maybe if there are 5 million megatons in the subsurface, and if we could recover just 2% of that, that would represent 100,000 megatons. I did a quick calculation to see how much energy is in that, and it's roughly about twice as much energy as all of the proven reserves of natural gas on Earth. It seems that if we could recover just a fraction of what we think is down in the subsurface, it could actually be a substantial amount of energy.

Chris - It sounds to me like you're saying that your predictions are, the world actually is quite replete with hydrogen, but there's enormous uncertainty as to whether we can actually get at this, what would be a wonderful resource if we could access it. Is it even worth pursuing this, do you think?

Geoff - From my perspective, the answer is clearly yes, I think it is. There's enough potential here, and given what we know about other gases, about natural gas, about CO2, helium, hydrogen, it seems that it's quite likely that we could have accumulations of natural hydrogen in places that could be accessible and economically recovered. The key point is that we just simply have never looked for it.