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Injection and storage

Before injection into a deep geological formation for long-term storage, the gas is concentrated to a minimum of 90 per cent carbon dioxide, and compressed to a supercritical state (a temperature and pressure at which it shows properties of both liquids and gases).  Provided the carbon dioxide is injected to a depth of at least 800 metres, it will remain in a supercritical state, enabling each reservoir to store much more carbon dioxide. Coal-bed storage of carbon dioxide, which relies on the adsorption of the carbon dioxide on the coal, can take place at shallower depths.

Geological storage options
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 suggested 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
  • 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.

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