Scientists are searching for a formation that is porous and permeable and filled with salty water. Aquifers filled with fresh water are often considered unsuitable as they may represent potential future groundwater resources.
Furthermore, the formation should be superimposed on good sealing rocks, often slate or clay stones, of sufficient thickness to minimize the risk of leaks.
It is also important that the rock is not cracked or has faults that can allow CO2 to migrate out of the storage.
At the same time, the reservoir must be bounded so that CO2 does not migrate; there must be traps present.
Such traps may be either stratigraphic or structural, of which the structural may be mainly fault-controlled or deformation-controlled (see figure above).
The pressure in the reservoir also plays an important role. A good CO2 reservoir is deeper than 800 meters, resulting in a high pressure environment that compress the CO2 gas into a liquid, thus taking up less space.
On the other hand, very deep reservoirs will be more expensive to drill, as well as generally having lower porosity and permeability.
When it comes to risk, it is also important that there is a lot of available data from the formation, including seismic and drill cores. At the same time, it is desirable to avoid “pierced” reservoirs (many old wells) due to the risk of leaks.
There are several reservoirs around the Troll field and at Utsira Hill that are considered well suited for CO2 storage.
Calculating the potential storage capacity of CO2 for a given reservoir:
MCO2 = A x h x Φ x ρCO2 xSeff
MCO2 = Number of tonnes of CO2 that can potentially be stored
A = Area of reservoir
h = Average thickness of the reservoir
Φ = Mean porosity in the reservoir
ρCO2 = Density of CO2 which depends on depth of reservoir
Seff = Storage efficiency (Strongly varies, 2% is the norm for high uncertainty)