What are the options for storage?
The CO2 would be stored in porous rock, such as sandstones, underneath an impervious cap rock. In nature, such rock structures usually contain salty (undrinkable) water, in much the same way that sand on a beach contains seawater. These are called saline aquifers.
In some cases, oil or gas has become trapped inside the structures, resulting in an oil or gas field. And in rare cases, they already contain naturally occurring carbon dioxide.
There are three main ways of storing carbon dioxide:
1. Storage in depleted gas/oil fields
Our proximity to North Sea gas fields means the UK is particularly well suited to this type of storage. It’s also, in many ways, the most attractive option because we already know so much about such fields.
2. Enhanced Oil Recovery
This process is already in use, notably in West Texas, and sees the carbon dioxide injected into an oil reservoir to increase the amount of oil recovered while also storing the CO2.
3. Storage in aquifers
An alternative to depleted oil and gas fields would be to store the gas in deep saline formations, known as aquifers. These can be either on- or offshore and have the advantage of a much larger capacity.
Will the carbon dioxide stay where it’s put?
It is believed that, over 1,000 years, storage would be at least 99% effective.
For such storage projects, the vast majority of the CO2 will gradually be immobilised by a combination of physical and geochemical trapping mechanisms which could retain the gas for millions of years.
Should any carbon dioxide leak from the initial rock formation, it is likely that it would be trapped well before reaching the surface. Furthermore, it is predicted that over time, much of the CO2 will dissolve in the water present in the rock formation. This increases the density of the water, at which point it will tend to sink, preventing any further escape of carbon dioxide.