Coal mine methane leaks could be hidden emissions nightmare

Escaping methane is a major climate change risk, because it’s much more potent as a greenhouse gas than CO2. So documenting where it’s coming from, and at what sorts of rates, is a high priority for understanding the atmospheric carbon budget. And it turns out that, in Australia, there’s a particularly numerous and under-appreciated source of the gas in the form of old exploration boreholes sunk by coal prospectors; these allow gas trapped in underground coal deposits to seep out. One such borehole, in Queensland, produces the methane equivalent of the yearly emissions of around 10,000 cars, and there are tens of thousands of these holes across eastern Australia, many uncapped and unmonitored. And this means the problem could be far reaching. Here’s the University of Queensland’s Phil Hayes, who’s been studying the problem using a new bit of kit he developed...

Phil - What we've done here is we've taken a piece of technology out that enables us to accurately quantify fugitive methane emissions. And what we've actually found is coal exploration holes that have been drilled historically that are leaking fugitive methane into the atmosphere.

Chris - What actually is fugitive methane though?

Phil - So this is methane that is not being captured from industrial operations, from digesters, from the oil and gas industry. Really, it's gas that has escaped. And in this case, we're saying it's fugitive coming out of these coal holes because it's gas that's escaping from underground.

Chris - And presumably the relevance here is that methane is a potent greenhouse gas, and we want to make sure we know exactly how much of it is getting up into the atmosphere at any one time, and from where?

Phil - Yes, absolutely. So this is an emission source that is not accounted for in emission estimates. These exploration holes have not been identified previously as a significant source.

Chris - How are you doing it then? Tell us about the tech. Before we look at how much methane's coming out, tell us how you're actually finding it. How does that work?

Phil - Okay, the actual system we use is a laser-based LiDAR camera, and that's tuned in the near infrared such that certain times it would, if there's methane in the air, that laser will be absorbed, and other times that laser is not absorbed. And it's the difference between those two signals which goes forward into calculations of how much methane is in the air. That laser is then scanned around a field of view, and over a minute it picks up a three- and a four-dimensional picture of methane in the atmosphere to answer the question of how much methane must be being released.

Chris - What do you do then? Drive around with this on the back of a four-by-four or something?

Phil - It's more of a static system. What we've done is we've integrated this into a trailer, so that trailer can be towed around, and we've operationalised that for an Australian environment, so we can go out into rural areas. So we've got solar panels, batteries, communications, and compute facilities on board that trailer. So that means we can set up in locations and survey over longer periods of time, and that's actually what we did with this coal hole. So one of the interesting factors here, and part of the reason we've written the paper about it, is that the emission rate actually varies over time, and it's dependent on the weather. If the weather is bad, as in pressure is low, it's rainy, you actually get more emission than if it's in fine weather.

Chris - When you said it had been Australian operationalised, I thought you were going to say it's got a beer cooler and a barbie on the back as well. But dwelling on the actual places you've explored, where did you test this then, and how did the fact that you've got all this methane coming out of the ground come to light?

Phil - So Australia's big. On the eastern portion of the country, we have a very large system of geological sedimentary basins. So these are areas of sedimentary rock, they go down many kilometres, and the largest system is referred to as the Great Artesian Basin, that covers around 22% of Australia. The Surat is a smaller sub-basin of that, but it's still the size of a large European country, sort of the size of France or Spain. We've been out there for a number of months, and we were given a tip-off about these coal holes through negotiation with landholders. We managed to get out there and survey a few of these, so pretty extensive area.

Chris - And these are holes that people prospecting for natural resources, historically, they've drilled down to do the geology and see what might be down there, and then they've just left the hole there. That's what you're examining?

Phil - Yes, these are coal exploration holes, so they'll typically be drilled 100, 200 metres down, and what they're after is to get samples of, in this case, coal to find out about the distribution, the quality, the thickness, how much there is. These holes are supposed to be backfilled, but historically we know that certainly some of them have not been filled adequately or adequately sealed. And the one that we surveyed, you couldn't actually see a hole on the surface, it was covered in dirt. There was a very large amount of methane coming out through the soil.

Chris - How many of these holes are there then?

Phil - We're not entirely sure. There is an estimate that in this particular basin, the Surat, there's around 30,000 exploration holes and probably another 100,000 in another basin, the Bowen Basin to the north.

Chris - And how much methane, based on your measurements, is coming out of holes like this?

Phil - We surveyed a couple of holes which had similar rates, and these numbers might not sound huge, but around 235 tonnes a year of methane, but that's the equivalent of at least 6,500 tonnes of CO2 a year. That's comparable to the distance driven by an average Australian car in a year of around 10,000 cars.

Chris - So now you can put some numbers on the scale of emissions from sources like this. Is the motivation that now we can quantify this, we know how big the threat is, and this helps to persuade people to come up with mitigation strategies. Is that where you hope this is going to lead?

Phil - Yes. What I'd be hoping from this work is that we move into a more proactive period where these holes are found and sealed, rather than a slightly more reactive mode that only wants real problems that have been seen, before anything is done about them. And there is an opportunity here. This is the low-hanging fruit of fugitive emissions. Finding these holes is not hard, and sealing them is actually technically quite straightforward. There would be some money involved, but it's not tens of millions.

Chris - Do we have to stick with just coal holes, though? Because there are lots of places where methane comes from. For instance, near London, there are decades of London waste piled up making artificial mountains that have been grassed over, but they're spewing out loads of methane. I mean, could we use this sort of technology to quantify possible overlooked sources and mitigate those as well, beyond just fossil fuels?

Phil - The same technology absolutely can be used to scan across landfills to identify where leaks are occurring and to quantify those rates. And that's an application that we're looking at taking further here in Australia, but I'm also aware that it's being done overseas.