Getting Rid Of CO2 – (A Guide To CO2 Sequestration)

7 February 2020

a guide to co2 sequestration

It was almost 10 years ago that we first got involved in CO2 sequestration studies – how to work out if it’s possible to pump CO2 into the subsurface and keep it down there.

Ironically, given the recent fires down under, it was the Australian Government that got us into it, albeit at one remove. Concerned about anthropological global warming and perhaps feeling slightly guilty at the amounts of Australian coal being shipped to China, they announced a large sum to be spent on research into CO2 sequestration. We worked with one university to get a sequestration module into REP.

It worked really nicely from a software point of view – we thought so anyway – but there was little actual CO2 being pumped underground anywhere – a few projects, but only a few. Then the oil price collapsed, and oil companies stopped even pretending to be interested. Then there was a change of government in Australia, and the new one (a bit like the current one) was unimpressed with global warming arguments and preferred to spend the money on polluting the Great Barrier Reef.

It’s all changed now.

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Governments, presidents and prime ministers are still happy to deny climate change. But nobody much is fooled, and even they are beginning to realise they will find it harder to be re-elected (or keep their people off the streets, in those places where election results are a matter of course) if they keep up the denial. Underneath the layer of lies you will find a lot of people working seriously and steadily on solutions – or at least mitigation. And among the things they are working on is CO2 sequestration.

Of course the best solution, as explicitly stated by the IPCC (which stands for Intergovernmental Panel on Climate Change, though some of my colleagues in the Society of Petroleum Engineers recommend a different rendition) is not to produce CO2 in the first place. But that’s not going to happen tremendously quickly, barring a catastrophe. And even if it does there’s still a lot of it in the atmosphere and it’s not going away very quickly, on its own.

While the transition from fossil fuels takes place we are going to have to learn to capture the CO2 we produce and put it somewhere, and perhaps build extractors to capture what’s there already – hard to imagine, but work is already being done.

Nature is good at storing carbon – in trees and plants, in soil and in rocks. But if you plant a load of trees there’s less room for crops to feed the hungry world – and if they then burn à la Oz you get the CO2 back. Rocks don’t burn (apart from coal, of course) but making them on the necessary scale is unproven.

Or you can inject it underground. And that’s what we can use REP to try to quantify.

As it happens, reservoir engineers have been injecting CO2 into oil reservoirs for years. The idea is to keep an aging oilfield going a bit longer by keeping up the pressure and sweeping the remaining oil towards the producing wells. Very often water and CO2 are alternately injected, one after the other. It’s called Water after Gas (“WAG”) and apparently it works pretty well with professional footballers as well.

For sequestration, the ideal scenario is to inject the CO2 back into a reservoir which used to hold hydrocarbon gas. Firstly, you know how much gas you got out, so with a few adjustments you can make a reasonable estimate of how much CO2 you could put back in. Secondly, you know that the trapping mechanism which held the hydrocarbon gas down there and stopped it rising to the surface should also work to keep the CO2 in place. And it’s likely, too, that you’ll have some wells already there, which can be turned from gas producers into CO2 injectors, and maybe even some pipelines to get the gas there.

Some gas reservoirs are a bit like a gas cylinder – as you drain it, the pressure goes down. That’s good for sequestration because you’ve got to pump the CO2 down. The lower the pressure you’ve got to pump against, the better. In other reservoirs the produced gas gets replaced by water (there’s a lot of salty water in the subsurface). This is not so good because the reservoir you are pumping into is at a higher pressure, so it’s more work and more expense. Also, there will be more gas left down there – reservoirs with pressure support usually produce significantly less hydrocarbon than reservoirs without, because the encroaching water traps gas behind it. If there’s less gas produced, there’s less you can put back.

Another type of reservoir into which you can try and inject CO2 is a “saline water” reservoir – one which contains only water, and has never had gas. You can use the same techniques as for hydrocarbon reservoir exploration to identify such a trap. In fact, at the pressures and temperatures you usually find “down there” CO2 is not a gas – it’s a “supercritical fluid” and acts more like a liquid. But it’s still lighter than water and so buoyancy will mean it will rise up until it is trapped or escapes to the surface. Saline reservoirs are a bit less attractive than depleted hydrocarbon reservoirs because you can never be absolutely sure that the expected trap will not leak. But there are a lot more of them and they can be huge. You just have to be careful of the geology.

There are engineering issues too. As everyone who orders sparkling water with lunch knows, CO2 is soluble in water. A nice way to store CO2 – as long as nature’s waiter doesn’t unscrew the cap.

There’s a third type of CO2 sequestration target – deep coal seams. These are much less common (not least because they are deep) and the trapping mechanisms are different.

When you mess about with nature you always take a risk, and there are certainly risks associated with CO2 sequestration. If your reservoir leaks and you get a plume coming to surface it could be rather dangerous. In 1986 there was a sudden and natural release of CO2 from Lake Nyos in Cameroon. It’s denser than air, and if there’s a lot of it around it displaces the air you need to be breathing. As a result over 1500 people died of suffocation, along with their animals. It’s not a gas you want to play about with.

The trouble is, we’ve been inadvertently playing around with it for a few hundred years. And now we’ve got to do something. Sequestration is not the full answer, but it may be a piece of the solution.

REP to the rescue!

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Image source: Unsplash

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