- CO2 storage in saline aquifers (Type 1) has the highest capacity potential for long-term sequestration
- Storage in depleted oil and gas pools (Type 2) is suitable to sequester CO2 for geological (long) time frames
- In enhanced oil recovery (EOR) schemes (Type 3), CO2 is injected into a depleted oil reservoir. The CO2 dissolves in the previously unrecoverable oil, reducing the oil’s viscosity, which allows it to flow to a production well. EOR schemes maximize oil recovery while eventually sequestering CO2, which minimizes environmental impacts.
Pressure plays a significant role in carbon dioxide (CO₂). Physical state, solubility, and behavior in subsurface environments are all effected by pressure.
- At low pressures, CO₂ exists as a gas.
- At high pressures and moderate temperatures, CO₂ becomes liquid.
- At high pressures (1,070 psi) and high temperatures (31.1+ °C), CO₂ enters a supercritical state, where it behaves like both a gas and a liquid.
- Higher pressure increases CO₂ solubility in both water and hydrocarbons, an important factor in geological carbon storage.
There are several criteria that are either critical, essential or desirable to ensure that a depleted pool or saline aquifer is suitable to safely and economically store CO₂ in the subsurface. These include:
- Reservoir-seal pairs (i.e. the reservoir is effectively sealed from leakage)
- Being in a suitable pressure regime
- The ability to monitor CO₂ injection and flow
- Protecting groundwater quality
The table below summarizes screening criteria for subsurface CO₂ storage.
Data from IEAGHG, 2009, Table 2
