Practically everyone has an opinion on climate change by now, although for the vast majority of scientists, the weight of evidence is overwhelming – emissions of carbon dioxide and other greenhouse gases are causing climate change, sometimes referred to as global warming. One possible technology for fighting climate change is Carbon Capture and Storage (CCS) in which geology plays an important role. In fact, future generations of geologists may be employed searching for CO2 storage sites in the subsurface, rather than for the more traditional search for oil and gas.
The aim of CCS is simple – to allow the continuing use of fossil fuels while reducing the emissions of greenhouse gases into the atmosphere. In the long term, the burning of fossil fuels will probably cease, but until we can rely on renewable sources of energy, we are stuck with these fuels as a cheap and reliable energy source. CO2 is emitted during many activities, including driving cars and heating homes, but the largest single sources are fossil fuel power plants, which generate electricity, followed by industries, such as steel works and cement plants. It is these that most research has been focussed on. And, in principle, the technology is simple – capture the CO2 from a source (such as a power plant; Fig. 1) before it gets into the atmosphere, then transport it to a suitable storage site and inject it into the ground where it will remain for tens of thousands of years.
Most power plants and industry do not emit pure CO2 – and to make best use of the underground stores, we need CO2 that is fairly pure. The technology of separating (‘capturing’) the CO2 from a gas has been the subject of much research, although some methods are well known – particularly ‘amine capture’, which has been used to produce CO2 for the fizzy drinks industry for decades. Transporting CO2 is fairly well established too – in the USA, there are thousands of kilometres of pipelines carrying CO2, mostly for use by the oil industry. There are even special ships for carrying CO2, again for use in manufacturing fizzy drinks.
Which brings us to geological storage. The idea is simple enough – we inject CO2 into the subsurface through boreholes that are similar to the ones that the oil companies use to extract oil and gas from the subsurface (Fig. 2). The geological requirements for storing CO2 in the subsurface are even similar to the requirements for an oil or gas field – although, in this case, we don’t need to find any oil or gas. Since the requirements for a storage site are so similar to an oil or gas field, one idea is to use old, depleted fields. This has the advantage of giving a breath of new life to these fields, and might use existing platforms, boreholes and pipelines. In some cases, it might be possible to extract extra oil from old oil fields, in a process called enhanced oil recovery (EOR). It has been suggested that EOR may help to pay to get CCS started in places such as the North Sea. But, for really large-scale storage, we will have to go beyond the known oil and gas fields, and into suitable geological formations that have become known as ‘saline aquifers’ – a term that reminds us that fluids at typical storage depths are salty, and have no known use (if you really want salty water, there is plenty in the sea). Many of these aquifers cover tens of thousands of square kilometres in locations, such as the North Sea, and have very large potential capacities for storage.
When searching for a suitable storage location, there are three essential elements: a reservoir, a seal and a trap, and we’ll take a look at each of these in turn.
A reservoir is a rock that is both porous and permeable – porosity is space within the rock. We need the porosity as this is where the CO2 will be injected. Importantly, below the water table, any porosity in rocks is filled with water, or brine at depths of hundreds of meters or more. As the CO2 is injected, the water or brine that was present initially will be pushed to one side (Fig. 3). We also need the reservoir to have good permeability – the ability of fluids to flow through the porosity. Most porous and permeable rocks are sedimentary – so most storage will take place within sedimentary strata, with which fossil collectors are of course familiar. Some sedimentary rocks are better than others – well-sorted sandstones (Fig. 4) and some limestones are best. Silty and muddy rocks are not good, but they have other uses (see below).
Once the CO2 is injected into a borehole, it will try to migrate upwards, driven by buoyancy (the CO2 is less dense than the water that is displaces). A rock above the reservoir is hence needed to prevent the CO2 from escaping from the reservoir and eventually reaching the surface – called a seal. These are either shales (Fig. 5) or evaporite rocks such as halite (rock salt).