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There are several options for the long-term storage of CO2 in geological formations including injection into depleted oil reservoirs, depleted natural gas fields, deep saline aquifers and unmineable coal seams. Together these are estimated to have a global storage capacity of 1000-10,000 GtCO2 according to the IPCC, therefore with current world energy-related CO2 emissions of about 27GtCO2 per year there is sufficient storage capacity for CCS to play a major role in emissions abatement.
Most of the world’s carbon is held in geological formations, and is either locked in minerals or hydrocarbons, or dissolves in seawater.
Naturally occurring CO2 is frequently found with petroleum accumulations, having been trapped either separately, or together with hydrocarbons, for millions of years.
Several types of geological formations can be used to store CO2. Those with the greatest potential capacity and security are:
- deep saline-water saturated formations,
- depleted oil and gas fields, and
- unmineable coal beds.
Deep saline formations
Based on current knowledge, deep saline formations provide the largest potential volumes for geological storage of CO2.
These brine-filled sedimentary reservoir rocks (e.g. sandstones) are found in sedimentary basins and provinces around the world, although their quality and capacity to store CO2 varies depending on their geological characteristics.
To be suitable for CO2 storage, saline formations need be:
- sufficiently porous and permeable to allow large volumes of CO2 to be injected in a supercritical state; and
- overlain by an impermeable cap rock, or seal, to prevent CO2 migration into overlying fresh water aquifers, other formations, or the atmosphere.
The chief advantages of deep saline formations for CO2 storage are their widespread nature and potentially huge available volumes.
Depleted oil and gas reservoirs
Oil and gas reservoirs generally have similar properties to saline formations, that is:
- a permeable rock formation (reservoir); and
- an impermeable cap rock (seal).
The reservoir is that part of the saline formation that is generally contained within a structural closure (e.g. an anticline or dome), and was therefore able to physically trap and store a concentrated amount of oil and/or gas.
Conversion of depleted oil and gas fields for CO2 storage should be possible as the fields approach the end of economic production. There is high certainty in the integrity of the reservoirs with respect to CO2 storage, as they have held oil and gas for millions of years.
A major drawback of oil and gas reservoirs compared with deep saline aquifers is that they are penetrated by many wells of variable quality and integrity, which themselves may constitute leakage paths for the stored CO2.
It is important to note that the storage capacity of depleted oil and gas fields is small relative to the potential capacity of deep saline formations and to CO2emissions, however they do present an early opportunity for CO2 storage.
Unmineable coal beds
Coal beds below economic mining depth could be used to store CO2. Carbon dioxide injected into unmineable coal beds may react and be absorbed by the coal, providing permanent storage as long as the coal is not mined or otherwise disturbed.
Carbon dioxide storage in coal is limited to a relatively narrow depth range, between 600m and 1000m. Shallow beds less than 600m deep have economic viability and beds at depths greater than 1000m have decreased permeability for viable injection.
Other Geological Storage Options
Other geological CO2 storage options include injection into:
- oil shale;
- salt caverns and cavities;
- geothermal reservoirs;
- lignite seams; or
- methano-genesis in coal seams or saline formations.
These are in early stages of development, and appear to have limited capacity except, possibly, as niche opportunities for emissions sources located far from higher capacity storage options.
Nature’s way of geologically storing CO2 is the very slow reaction between CO2 and naturally-occurring minerals, such as magnesium silicate, to form the corresponding mineral carbonate.
Of all forms of carbon, carbonates possess the lowest energy, and are therefore the most stable. CO2 stored as a mineral carbonate would be permanently removed from the atmosphere.
Research is underway to increase the carbonation rate, however, the mass of mineral that would have to be quarried would be many times the mass of CO2captured. At present, this option would be considerably more expensive than others.