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After the capture and transportation of carbon dioxide it then must be stored. There are two main storage options for the UK and four main techniques to enable storage:
Carbon dioxide may be stored in:
- deep saline formations, and
- formations that contain hydrocarbons.
The storage may be:
via direct injection into a deep saline formation or depleted hydrocarbon formation:
- as a consequence of enhanced oil recovery;
- as a consequence of enhanced gas recovery, and/or
- for the purposes of enhanced coal bed methane production.
The availability of storage is determined mostly by geography, the proximity of an emitter to a potential sink and the nature of the site. Not all depleted hydrocarbon fields or EOR/enhanced gas recovery (EGR) schemes would be considered to be suitable storage sites.
Deep saline formations
Deep saline formations are geological formations located deep underground where highly saline water is present in porous rock formations. To get the carbon dioxide into the reservoir it is injected at pressure. This increases the ‘pressure’ within the reservoir which leads to a displacement of some of the in situ reservoir fluids which will offer resistance to movement.
Carbon dioxide is held in place in a storage reservoir through one or more of five basic trapping mechanisms: (1) stratigraphic, (2) structural, (3) residual, (4) solubility and (5) mineral trapping.
Initially, the dominant trapping mechanisms are stratigraphic or structural trapping or a combination of the two. Following injection and due to buoyancy forces carbon dioxide will rise to the top of the storage formation where it will be ‘trapped’ by an impermeable caprock which keeps the carbon dioxide within the reservoir. In structural trapping, impermeable rocks will contain the carbon dioxide within the reservoir as a result of faulting or some other geological mechanism where impermeable caprocks are juxtaposed over the more permeable reservoir rocks.
Residual trapping mechanisms generally dominate after injection ceases. Some carbon dioxide is trapped in pore spaces by the capillary pressure of water. Post injection, water from the surrounding rocks migrates back into the pore space containing the carbon dioxide which immobilises the carbon dioxide.
Over relatively short timescales carbon dioxide will dissolve into saline formation waters (solubility trapping). Carbon dioxide rich waters will be negatively buoyant and tend to sink to the base of the deep saline formation. Over much longer timescales the carbon dioxide m ay react chemically with the surrounding reservoir rocks to form stable minerals such as carbonates.
When storing carbon dioxide in deep saline formations, the most effective sites are those where the carbon dioxide becomes immobile. This happens when the carbon dioxide is: (1) trapped under a thick, seal of low permeability; (2) converted into solid minerals; (3) trapped through a combination of physical and chemical mechanisms.
These trapping mechanisms also apply in principle to carbon dioxide injection into hydrocarbon containing formations.
Abandoned oil and gas fields
Many abandoned oil and gas fields have very good potential for carbon dioxide storage as the hydrocarbons originally stored within the reservoirs did not escape into the atmosphere or overlying geological formations over millions of years prior to being extracted. They therefore provide a safe place to store the carbon dioxide without risking leakage into the atmosphere or overlying geological formations. A potential disadvantage, however, is that when hydrocarbon recovery was completed at these fields the production/monitoring wells were filled with mud-laden fluid and plugged with cement grout since it was not thought that one day they would be used as a storage facility for carbon dioxide. These man-made structures have the potential to provide preferential pathways for carbon dioxide leakage if their integrity is not maintained.
Enhanced oil recovery
There are two main methods of EOR which are thought to be suitable in the UK. Gravity stable gas injection typically takes place towards the end of a hydrocarbon field’s working life and involves the injection of gas into a reservoir pushing the oil downwards through the reservoir towards production wells. It is anticipated that significant volumes of carbon dioxide would have to be injected relatively slowly to maintain a carbon dioxide front moving through the reservoir. An alternative variant on this process is the injection of alternating slugs of carbon dioxide and water into an oil reservoir. The carbon dioxide mixes with the oil, decreasing its viscosity and swelling it, and because it is lighter than water, it may rise through the reservoir. The following slug of water helps move the carbon dioxide/oil mixture towards production wells. In both cases, between 50–70% of carbon dioxide returns with the oil; however, this can be separated and re-injected into the hydrocarbon reservoir to minimise operational costs. The remaining carbon dioxide is trapped in the reservoir formation and may be considered as permanently stored.
When recovering oil, the displacement created by the carbon dioxide depends on the phase behaviour of the carbon dioxide and the crude oil mixtures. These are strongly dependent on reservoir temperature and pressure. Once oil recovery has ceased, the reservoirs are in principle available for further carbon dioxide storage.
As carbon dioxide is denser than natural gas, injecting carbon dioxide into the base of a gas field would result in natural gas rising through the reservoir. This has the potential to increase gas recovery rates. To date EGR has only been carried out at pilot scale.
There is thought to be limited quantifiable carbon dioxide storage potential in UK coal seams due to their low permeability. Furthermore, as coal is an important energy mineral in the UK its use for carbon dioxide storage would limit the potential for future mining or underground gasification of the coal in the future. To date small scale pilot investigations have been attempted to ascertain the potential for carbon dioxide storage within coal beds throughout the world. At present this is still considered to be an immature technology.