The Challenges of Carbon Sequestration

Meera - Lets now explore an area of research that could prove essential in our fight against climate change and that’s carbon sequestration. The technique hopes to capture carbon dioxide from Power Stations, compress it into a liquid and store it deep below the Earth’s surface. If proven to be viable, it’s estimated the technique could be used to store 1/5th of our total carbon dioxide emissions. Professor Martin Blunt from Imperial College London has been looking into the challenges of injecting these emissions into the ground...

Martin - Most of our research is concerned with carbon capture and storage as CO2 is separated from fossil fuel-burning power stations, compressed, transported, normally by a pipeline, and then injected deep underground and it is the injection and what happens to the CO2 deep underground that’s the focus of our research.

Meera - What are the challenges associated with injecting liquid carbon dioxide deep into the ground?

Martin - It is very high pressures, so it’s liquid-like, and what we’re injecting it into is porous rock. So I’ve go there a sample of a carbonate rock. The CO2 is injected into the tiny, micron scale, pore spaces of this rock that initially contain brine. So the rock is full of salty water, we inject CO2. It’s the same types of rock that we find oil and gas, indeed we could store the CO2in depleted oil and gas fields as well. The main concern that we have is when we inject the CO2, how can we ensure that it stays underground, that it doesn’t escape to the surface? So what we’re looking at is what happens to the CO2 when we inject it, will it stay underground?

Meera - This model of rock that you’ve got here, this is calcium carbonate? It’s a reasonably small cylinder or I guess, 10cm or so long, but why this particular rock?

Martin - The 2 major rocks in which we find both oil and gas and indeed the major storage of CO2 in aquifers, essentially carbonates, so calcium carbonate, the remains of seashells crushed and chemically altered over millions of years, and sandstones which are mainly silica.

Meera - You’ve mentioned that one of the key challenges or issues on people’s minds is whether this CO2 will stay down in the ground once it’s there, so how have you set about investigating how long it will stay there or whether it will be stable?

Martin - We’ve looked at the CO2 in the pore space of the rock at the micron scale. That’s why we’re using Diamond. So what I have here is the special flow cell we’ve used to study. So what we do is we take a very small sample of rock, I’m looking here now at a piece of rock that’s about 5mm across, about a 1cm/1.5cm long.

Meera - So a miniaturised version of your cylinder from earlier?

Martin - Yes, we’re not just imaging the rock, we want to image the fluids and what’s new about this setup is that we’re imaging the fluids at reservoir temperatures and pressures. So the temperatures are about 50°C, the pressures are about 100x more than atmospheric.

Meera - This must be quite hard then to simulate?

Martin - Yes, so what we do is we take the little bit of rock, we wrap it in a sleeve very tightly and we inject fluids at very high pressure. How do we contain all of this? What we do is have another jacket of water that’s at high temperature and pressure and we surround all of that with carbon fibres. Carbon fibre is extremely strong, thin and it’s x-ray transparent.

Meera - So just looking at the model here, you’ve got this long carbon fibre tube inside of which you will put your sample and on either end you’ve got steel, larger cylinders, what role are they playing? Are they providing, ‘cause they look like they’ve got connections coming out of them

Martin - Oh yes, that’s exactly what it is. So the bit in the middle, the carbon fibre, is where the x-rays go, the steel at the end is basically to have ports through which we inject the fluids.

Meera - And this is essentially what you take along to Diamond to image directly?

Martin - Indeed.

Meera - So what have you been able to see? So you’ve got these small samples of rock and you’ve injected various amounts of carbon dioxide into them, what have you been able to image?

Martin - Right, so we can see on the computer screen here – this is an example of a sandstone where we’ve had brine and we’ve injected the CO2, so we just look at the CO2 in the pore space and we can distinguish between grain, the brine and the CO2. And what we find is that the CO22 tends to move into the centres of the pore space and the brine clings int he narrow corners of the nooks and crannies and the small pores and the reason is that the water loves the rock. Rock, actually, will soak up water like a sponge, the CO2 doesn’t have such an affinity for the rock and tends to be in the wide pore spaces.

Meera - But then through your imaging, what you’ve seen is that you’re pushing this brine, this water, out and the CO2 is getting stored in, but then the water is coming back in.

Martin - That’s right. So we inject the CO2, CO2 goes in, can it escape? Well, the CO2 isn’t quite as dense as water, so what it’s going to do is move up. When it moves up, it moves out of a pore, what moves in? - brine. The brine, as I’ve said, like the narrow bits of the pore space, it fills the small pores and the nooks and crannies, it leaves the CO2 behind in the big pores and it surrounds the CO2 so you have bubbles, pore space 10 microns to a millimetre sized clusters of CO2 surrounded by brine and then you can push as much brine as you like, that CO2 is not moving. It’s not going anywhere. And that is what we can see directly on these images, you can see these trapped clusters of CO2. And the beauty about Diamond is that we don’t just have 2-Dimensional images, we can do this in 3-Dimensions and we can see the 3-D clusters but they fill about a quarter if the pore space. So what happens is, when the CO2 moves, it leaves behind a trail of trapped CO2 in about a quarter of the pore space. That CO2 can’t really escape because it leaves so much behind

Meera - I guess another challenge is that you aren’t able to see this over a period of another 1000 years, so although you know that the carbon dioxide becomes trapped in this way, how can you be assured that is stays in this way?

Martin - What we can do is we can inject a lot of brine. We can’t wait thousands of years, but we can inject so much brine that it’s equivalent to waiting that long. That is the amount of brine moving through is similar to waiting hundreds to thousands of years. And indeed we’ve done that in this experiment and nothing seems to happen.

Meera - What are some of the timescales of this? The rates in which carbon dioxide will be injected, the rates in which brine will naturally move in?

Martin - Typical power station burning coal at 2-3 gig watts is going to produce something around 9 million tonnes of CO2 a year. Chuck that underground for a period of say 30 years, that’s likely to create a plume of CO2 extending to over tens of kilometres. What we could do is extract brine from the aquifer and then re-inject it to enhance the movement to trap the CO2 and the design would be that within 4-5 years you’ve trapped virtually all the CO2. And you can walk away from that site assured that the CO2 is safely stored. Over hundreds to thousands of years, the CO2 will slowly dissolve, but when it dissolves in the brine, that CO2-laden brine is denser and it will sink in the aquifer. So it’s a kilometre underground to begin with, it’s just going to get deeper and deeper. And maybe over thousands to millions of years, the CO2 might react with the rock and form solid carbonate. What we’re showing is that is gets trapped, it will then dissolve, it will react. So it gets safer and safer, it gets less and less likely to escape over time.