Carbon capture and storage

Levels of carbon dioxide (CO₂) in the Earth’s atmosphere have increased dramatically since the start of the Industrial Revolution. This is because CO₂ is a by-product of burning fossil fuels such as coal, oil and methane gas, which are the main source of energy in our modern societies. There is now so much man-made CO₂ in the atmosphere that it acts like a blanket, raising global temperatures. Global warming is changing weather patterns and climate. It is also causing sea level rise, which will greatly affect people’s lives around the world.

One way to reduce global warming is to stop CO₂ getting into the atmosphere. For example, the CO₂ from power stations and other industrial sources could be collected, piped, and then injected into underground reservoirs, never reaching the atmosphere. Scientists and engineers think that if enough of these so-called carbon capture and storage (CCS) schemes are built around the world, then this will slow down global warming.

How does carbon capture and storage work?

The exhausts of power stations burning coal, oil and gas can be fitted with special collectors that capture CO₂. The CO₂ can then be transported by pipeline to a geological storage site, which can be either on land or under the sea. The advantage of geological storage is that it uses the same industrial technologies as conventional hydrocarbon exploration.

CO₂ storage reservoirs for enhanced oil recovery

The biggest problem is knowing where it is safe to locate the geological CO₂ storage sites. They require geological formations, known as reservoir rocks, that are both porous and permeable, such as sandstones and limestones. They also need a cap rock, such as shale, that provides an impermeable barrier to the upward migration of CO₂ from the reservoir.

In fact, these requirements are identical to those for hydrocarbon reservoirs. Some scientists and engineers have suggested using depleted gas fields in the North Sea, for example, to store CO₂ as they are known to have the right properties. An advantage of this approach is that by injecting CO₂ it should be possible to extract even more methane gas from these reservoirs.

The main disadvantage of using depleted oil and gas fields is that many hydrocarbon reservoirs are too deep for optimal CO₂ storage. The optimum storage depth for CO₂ is about 1 km below the seabed because the temperatures and pressures there are just right for CO₂ to form a dense fluid, known as 'supercritical' CO₂. Supercritical CO₂ has the density of a liquid but the viscosity of a gas, and hence is very efficient for pumping into geological reservoir rocks.

CO₂ storage in saline aquifers

Another approach is to find suitable geological formations where there are no hydrocarbons, saturated with natural brines. The same geophysical methods as those used in hydrocarbon exploration can be used to identify suitable reservoir units,. However, iit may be more difficult to guarantee a cap rock seal over the reservoir. Unlike oil and gas reservoirs where the very presence of oil and gas guarantees a seal, the seal can only be tested by injecting CO₂ or other fluids into a saline aquifer reservoir. Also, injection of CO₂ requires that the brine is displaced to some other locality. However, reservoir engineers can predict how the reservoir will behave once CO₂ injection starts and the CO₂ and brine can be monitored using geophysics and other techniques.

Seabed CO₂ seepage

Once the CO₂ has been injected into the sub-seafloor reservoir, it is important to monitor the site to ensure that there is no seepage of CO₂ into the overlying seawater, and potentially into the atmosphere. This can be done in a number of different ways.

Firstly, geophysical techniques can be used to image the structure of the seafloor and shallow sub-seafloor, in order to identify potential seepage pathways to the seafloor. These pathways can then be closely monitored.

Geophysical techniques can also be used to image pockets of gas within the sub-seafloor sediments, as well as bubbles seeping from the seafloor in the overlying seawater.

Secondly, potential seepage of CO₂ could be monitored using chemical techniques. During the injection procedure, fluids and gases already within the sub-seafloor storage reservoir may be displaced, and their detection within the sub-seafloor sediments would enable a leak to be stopped at the earliest opportunity, before any CO₂ seepage. Mapping the distribution of chemical constituents that are tracers of CO₂ seepage is an important part of any monitoring strategy.

Finally, many of the tiny animals that inhabit the shallow sub-seafloor, or live on the seafloor itself, have different tolerances to levels of CO₂. Experiments can be done to assess how different organisms respond to different levels of CO₂, in terms of changes in their mortality, physiology, behaviour and gene expression. This information can be used to interpret the results of biological surveys above the sub-seafloor storage sites in terms of potential CO₂ seepage. Importantly, biological measurements are vital for the detection and monitoring very low levels of seepage, that are not easily detectable by chemical and physical sensors.

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