Geological storage options for CO2.
1. Depleted oil and gas reservoirs
2. Use of CO2 in enhanced oil recovery
3. Deep unused saline water-saturated reservoir rocks
4. Deep unmineable coal seams
5. Use of CO2 in enhanced coal bed methane recovery
6. Other options (basalts, oil shales, cavities)
Carbon dioxide can be stored geologically:
- in depleted oil and gas reservoirs, returning the carbon (in the form of carbon dioxide) to where it came from (in the form of fossil fuel)
- through enhancing recovery of oil or natural gas, providing pressure to facilitate removal of the oil or gas to the surface
- in deep saline formations, both offshore and onshore, where the injected carbon dioxide dissolves in saline fluids
- through enhancing recovery of methane from coal seams which are unsuitable for mining either because they are too thin or too deep.
The main geological conditions required for secure storage of carbon dioxide are:
- reservoir rock which is both porous (having pore spaces in which carbon dioxide can reside) and permeable (having links between pore spaces allowing the carbon dioxide to permeate through the rock)
- a trapping mechanism, to stop the carbon dioxide migrating outside the target geological feature (Four basic mechanisms hold the CO2 in place: stratigraphical/structural, residual, solubility, and mineral trapping)
- an impermeable caprock to stop the carbon dioxide migrating upwards
Depleted oil and natural gas fields, which generally have proven geologic traps, reservoirs and seals, are potentially excellent sites for storing injected carbon dioxide.
The stages in storing carbon dioxide are:
