Geological Carbon Storage

Carbon Dioxide Transport:

The safe and reliable transport of carbon dioxide, from where it is captured to a storage site, is an essential step in the overall Carbon Capture and Storage process. It is the link between large industrial facilities and geological reservoirs which will develop with time into an infrastructure comparable to that of natural gas transport today. CO2 transport is a well understood activity and is already a reality at significant scale, in many parts of the world.

How is CO2 transported? Extensive networks consisting of millions of kilometres of pipelines already exist around the world, both on land and under the sea, to transport various gases. Of this extensive network, 6,500 kilometres of pipelines actively transport CO2 today. So pipelines are the most common method to transport millions of tonnes of CO2. They are likely to be the backbone of our CCS infrastructure.

It is possible to transport CO2 as a gas or as a liquid. Liquid CO2 pipelines are the preferred option for new infrastructure, because of the higher density of liquid compared to gas, which allows greater volumes to be transported. In some cases, it will be preferable to transport CO2 as a gas though, because it allows the re-use of the existing natural gas infrastructure at the end of its life. With minimal modifications, it is possible to reverse the flow of some pipelines and replace CO2 with natural gas. Transportation by ships can be an alternative option for certain regions of the world to export their CO2 to other regions with a more suited geology for CO2 storage. Ships offer flexibility to match capture with storage, and requires lower investment costs than pipelines. However, for the volumes expected at the scale of an industrial region, the unit transport cost of ship is higher than with pipelines.

Shipping is already taking place for food quality CO2, using ships about a tenth of the size of the larger vessels required for a single CCS facility. Larger-scale vessels are likely to have much in common with vessels we use today for liquefied petroleum gas and liquefied natural gas.

These are two well established, worldwide industries, with expertise that can be adapted to CO2. Transport of CO2 by truck and rail is, in principle, possible for small quantities, typically for small-scale research projects. But it is unlikely, that truck and trains will be a significant part of a future CCS infrastructure, given that the volumes of CO2 of a commercial CCS facility are thousands of times larger than what they can handle.

Is the transport of CO2 safe? The transport by pipelines and ships is an industrial activity, and like any industrial activity it abides to safety standards and regulations. It poses no higher risk than the transport of hydrocarbons, natural gas and oil, which are safely managed. In the USA alone, there are about 800,000 kilometres of hazardous liquid and natural gas pipelines, in addition to 3.5 million kilometres of natural gas distribution lines. So the safety record and experience of CO2 pipelines is based on 36 pipelines currently in operation in the USA. They safely transport 50 to 60 million tonnes of CO2 each year, over a total 6500 kilometres of pipelines, from natural sources and industrial facilities. This is an industry with significant safety experience. The scale of the transport infrastructure needed to support CCS globally is vast.

Why Geological Storage?

We have no idea what Earth or our society will be like in 10,000 years’ time. Ten thousand years ago, the human race was using stone tools, and the last ice age had only just finished. So, as we have no idea who will be around in 10,000 years’ time, and what type of society they will live in, the CO2 storage that we design now must work without maintenance or intervention by future societies. If we were to build a complicated storage system, and leave an instruction manual - what language would we write the manual in? Which languages will people speak in 10,000 years' time? As we have no idea of the answer to these questions, we need to design a storage system that is maintenance free. Geological storage offers this.

Storage Volumes:

The volumes of CO2 that we plan to store are also very large. As an example, a large coal-fired power station might emit 10 million tonnes of CO2 every year – but how large is this? We can work this out, using the density of gas – about 2 kilograms for each cubic metre (2 kg / m3) at surface conditions. So we would need to store around 5000 million cubic metres of gas.

Compare this to the Great Pyramid of Giza in Egypt, which has a volume of around 2.5 million cubic metres. So the carbon dioxide from one large coal-fired power station would have the same volume as two thousand ‘Great Pyramids’, for each year of operation.

Yet there are more than 6,000 coal-fired power plants in the world today. And, as you learned earlier in the course, there are many other sources of carbon dioxide, such as gas-fired power stations, cement plants and steel works. So building tanks to hold the carbon dioxide is impractical – we would need far too many.

Of course, we could compress the CO2, and store it under pressure, as this would decrease the volume of the gas. But the tanks would have to last for 10,000 years with no maintenance, as we saw above. So, it is impractical – could you build a pressurised tank and be sure that it would still be gas-tight in 10,000 years? Or we could use some chemical reaction to fix the CO2 as a solid. But we quickly run into a problem - the sheer volume of material that we are talking about, and what to do with the solid once we have made it.

Deep Storage:

The one way that we know of to store fluids such as carbon dioxide for long periods of time is to use geology. The reason that we are confident that this will work is that natural geological systems have stored fluids for very long periods of time. A good example is the oil in the North Sea (between the UK and mainland Europe). This oil was generated at least 20 million years ago, and has been held underground ever since. Some of the very earliest oil generated here may have been stored for even longer, since when dinosaurs were running around on the surface, unsuspecting that a meteorite impact was going to end their reign forever.

Because of the arguments that we have just discussed, most experts think that the only feasible way to store carbon dioxide is deep underground, in what is commonly termed ‘geological storage’. By deep, we mean at least 800m below the surface, and usually to between 1 and 3 km deep. The diagram below gives some idea of the depths involved. At these depths, the pressure keeps the carbon dioxide at a fairly high density, so we can fit plenty in to the available storage volume. It is also comfortingly far from the surface where we all live! Geological storage also has the pleasing symmetry that the carbon dioxide is derived by burning fossil fuels which were originally obtained from underground: as coal from coal mines, or as oil and gas from wells. Hence, in a very real way, we are simply putting the carbon dioxide back where it originally came from. 

In the Reservoir:

When the carbon dioxide is injected into the reservoir, it will displace the water that is already in within the reservoir. In the image below, the carbon dioxide (red) displaces water (blue) from between the sand grains (brown). Note that this image is not to scale, and that the sand grains would be much smaller than the injection pipe.

Folded Traps:

To prevent the carbon dioxide from moving too far sideways, we need a trap. The most simple example of a trap is a dome, illustrated in cartoon form below. Don't forget that the dome shown would not be at the surface - it would be buried at least 800 m deep below the ground's surface. 

The photograph below shows a real dome that can be seen at Berwick, close to the border between England and Scotland. As it is exposed at the surface, it is of no use for storing carbon dioxide, but it does show the concentric arrangement of the sedimentary layers in a horizontal plane. Try comparing this real example to the top surface of the sketch above. The dome in the photograph is about 500 m across, so is also rather too small to be of use for storage, even if it were buried to a suitable depth.

Faulted traps:

Another common geological structure that forms traps is faults, where rocks have fractured along a more or less planar surface and then slide past each other. The most famous example is, of course, the San Andreas fault in California, USA. The diagram below shows what is termed a 'normal fault', offsetting a seal (green) and reservoir (yellow) which traps carbon dioxide (blue). Note how the fault (in red) places seal rock (green) against the reservoir (yellow) - preventing the carbon dioxide from escaping to the right of the diagram. Many real traps include elements of both faulting and folding.

The photograph below shows an exposed example of a real fault, again at Berwick on the border between England and Scotland. The rocks on the left are light and dark grey shales, which are analogues for seal rocks. The pale pink rocks on the right are a sandstone, and are analogues for a reservoir. The sloping surface running from top right to lower left is the fault itself. The rucksack shows the scale, it is 70 cm high.

Storage in Aquifiers & Depleted Oil Fields:

There are two main ways of using underground geology to store carbon dioxide. One of them is to use old oil and gas fields and we'll talk about that first. Old oil and gas fields have several advantages. When a company decides a field is no longer economic, it's been working in that field for a number of years, and it will have collected a lot of information. They would have drilled bore holes down through the field and as they drill the bore holes, they collect information about the rocks as they go. So we have that information. We've got a good idea of how the rocks are arranged in the subsurface. The company will have brought some rocks up to the surface and they will have measured porosity and permeability. So we actually have some data about what the rocks are like, which is useful. And the company will have shot what are known as seismic surveys over the field. Now seismic surveys, what you do is, you send a sound wave down into the subsurface and you listen to the echoes as they come back. You collect those and you process them into an image of the subsurface, giving you a 3D picture of what the rocks are like. So its actually the same process, that we use to make ultrasound scans.

Of course, if you want to send sound waves kilometres down into solid rock, you need rather more sound than if you want to make an image of an unborn baby, but it is exactly the same technique. So the great advantage of oil and gas field is, we know what's down there. They do have a disadvantage. The oil company will have drilled bore holes down into the reservoir and those go through the seal. Now we know that seal held oil and gas for perhaps millions of years before we came along, but we've drilled some holes in it. Those holes are potential pathways for leakage. So CO2 could follow the bore holes back through that seal and get potentially up to the surface. If we've only got a few bore holes that's not a huge problem. Some old fields have literally hundreds of bore holes and in some cases we don't even know where they are.

So there's no way we can use those particular fields for CO2 storage. Old oil and gas fields are also good, because we've got lots of experience of injecting carbon dioxide into oil and gas fields. When an oil company walks away from an oil field and says it's finished, it's empty, actually they've left a lot of the oil in the ground. Sometimes in excess of 50%. So a lot of ingenuity has been spent in thinking of techniques to get extra oil out of the ground. And one of those techniques is to use is to inject carbon dioxide into the oilfield. The carbon dioxide mixes with the oil down in the reservoir and the extra pressure helps to push the oil towards the producing bore holes. This technique is known as CO2 EOR, which stands for Carbon Dioxide Enhanced Oil Recovery. What this means is, that when we're dealing with old oil and gas fields we've actually got forty or fifty years of experience injecting CO2 into these fields, which is obviously very useful. But there are only a limited number of oil and gas fields in the world.

The reservoir and seal rocks, that we find in an oil and gas field, are not unique to that field. In many cases those reservoir and seal rocks extend sideways for tens of sometimes even hundreds of kilometres. What that means is, we've got huge volumes of rock down there, that we could potentially use. These storage sites have become known as saline aquifers. Aquifers, because inside the porosity in the rock we only have water. Saline to remind us of the fact, that we're not trying to use drinking water aquifers. They dwarf oil and gas fields by orders of magnitude. The disadvantage is, that we don't know much about them. Oil companies only invest money in generating data for the subsurface, where they find oil and gas. If they drill and there's no oil and gas they stopped collecting data. So the aquifers we have very little data about them. That's their big disadvantage.

The diagram below shows storage in aquifers and old oil and gas fields. Note that this diagram shows storage at very shallow depths - far too shallow to be realistic. But it does give some idea of how oil and gas fields are part of larger saline aquifers. The figure also shows storage in unmineable coal seams, which we will not consider here, though they might be important in some parts of the world. The figure was made by Scottish Carbon Capture and Storage and you might want to check out their website for lots of CCS-related information.