Supercritical CO2
In general, CO2 is preferably stored as a supercritical fluid. As a supercritical fluid, CO2 is much denser than it is as a gas (thus more carbon can be stored). In addition, the supercritical CO2 can also diffuse within the aquifer like a gas and react like a liquid.
CO2 becomes a supercritical fluid when conditions in the aquifer are:
- Temperature > 31 C and
- Hydrostatic Pressure > 7.38 MPa
As a rule of thumb, these conditions are met when the depth to the aquifer exceeds ~800 metres. However, this is very dependent on the geothermal gradient and the hydrostatic pressure within the aquifer.
An example of an analysis a potential storage site was carried out in the Otway Basin, Australia using information built from the geofabric and the geothermal gradient developed for the Otway Basin Hot Sedimentary Aquifers & SEEBASETM project. Copies of the full report and the associated datasets can be downloaded from Victoria DPI or South Australia P.I.R.S.A.
As a means of comparison, the approximate location of the CCS test site in the Otway Basin is shown, note that the target aquifer for the CCS project lies above the Eumeralla Formation.
Depth to the top of the Eumeralla Formation
Using the geothermal gradient for the region, the temperature of the top of the Eumeralla Formation can quickly be calculated.
Aquifer Temperature = (Depth * Gradient) + Air Temperature
For ease of viewing, the results have been categorised as always supercritical (> 33 C), may be supercritical (29-32 C) and never supercritical (<= 29 C).
State of CO2 based on Temperature in the Eumeralla FormationA similar approach can be used to calculate the hydrostatic pressure within the Eumeralla Formation. For the purposes of this example, the hydrostatic gradient was derived by averaging the hydrostatic pressure gradient from all of the available drillstem tests (DST) in the region. This came up with the equation.
Aquifer Pressure = Depth * 0.00937
Again for ease of comparison, the results were categorised as always supercritical (> 7.45 MPa), may be supercritical (7.32-7.45 MPa) and never supercritical (< 7.32 MPa).
State of CO2 based Hydrostatic Pressure in the Eumeralla Formation
The two figures were then combined using the logic as proposed by Korose et al. for the Illinois Basin, USA.
- Never Supercritical = Never + Never
- Maybe Supercritical = Sometimes + Always/Sometimes/Never
- Always Supercritical = Always + Always
Combined rating for CO2 supercriticality in the Eumeralla FormationPlease note that the above is an example only. FrOG Tech neither endorses or condemns the siting of any CCS project. Full analysis would need better control on the hydrostatic pressure gradient and the geothermal gradient.
However, through this quick example, it is clear that simple, high level screening tools can be built using products derived from the geofabric.