Storage in deep saline aquifers

Storage in deep saline aquifers has the greatest potential in terms of storage capacity. Such aquifers are distributed in many regions of the world. They are located at depths between 1000 m and 3000 m. Due to the depth of these formations and their high salt content, they cannot be used as sources of drinking or irrigation water.

These aquifers can be open or closed. The configuration of closed aquifers is identical to that of oil and gas reservoirs, which ensures effective confinement vertically and laterally.

This is the type of aquifer used to store natural gas. It certainly provides a safe option in terms of confinement, but capacity is limited.

Open aquifers lie on a horizontal or slightly inclined plane. Since they do not ensure lateral confinement this would enable CO2 to migrate. This being said, their large size and low flow velocity can ensure a satisfactory confinement, assuming sufficient CO2 is present. In this case, the main trapping mechanism is the dissolution of gas in water.

The increase in density which results from this dissolution tends to carry CO2 towards the bottom of the aquifer. In the longer term, CO2 reacts with the surrounding rock and as a result of this mineralisation process the safety of CO2 trapping increases. The weak point of this solution is that little is known about the subject. This type of formation does not contain resources of interest, so it has hardly been studied. A major characterisation effort is required in order to qualify this type of aquifer for geological storage.

• Storage in unexploited coal seams

Storage in unexploited coal seams is based upon the capacity of coal to adsorb CO2 preferentially instead of the methane initially present. This trapping mechanism also enables the recovery of methane, which can be recovered in producing wells, and is a potentially attractive economic benefit. The main difficulty associated with this storage solution is the low permeability of this type of formation. As a result, it is not possible to inject large quantities of CO2 without multiplying the number of injecting wells. Such a solution is limited to unexploited coal seams in order to avoid any leakage of CO2 through mining galleries.

Table 8.1 shows the potential CO2 storage capacity for different geological structures (IEA data). Although current estimates are hardly precise, it appears that the potential storage capacity might be adequate for dealing with the overall world CO2 emissions.

Table 8.1 Potential storage capacity for different storage modes

Storage options

Total capacity

Gt CO2

Share of aggregate emissions in 2050 (%)

Depleted oil and gas

920

45

reservoirs

Deep aquifers

400-10 000

20-500

Unexploited coal seams

40

2

A more precise evaluation of storage capacities worldwide is needed in order to assess better the overall potential CO2 storage capacity.

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