Mechanisms, Means and Duration of CO2 Sequestration in Geological Media

Deep Saline Aquifers

Oil and Gas Reservoirs

Coalbeds

Salt Caverns

The fundamental trapping mechanisms for CO2 sequestration in geological media (geosphere) can be divided into two categories:

• Physical
• Chemical

Physical mechanisms involve trapping of CO₂ in its gaseous, liquid or supercritical state as a free-phase substance within a volume of a geological medium. The following fall in this category:

• geological trapping
• hydrodynamic trapping
• cavern trapping

Chemical mechanisms involve trapping of CO₂ as a result of various chemical processes between CO₂ and the fluids and/or rocks in the geosphere. In this case, CO₂ generally loses its state as free CO₂ and transforms into or becomes attached to another substance. The following fall into this category:

• solubility trapping in formation water or reservoir oil
• ionic trapping by which CO₂ decomposes into its ionic components
• mineral trapping as CO₂ may precipitate into a stable mineral phase
• adsorption trapping

The fundamental physico-chemical mechanisms for CO₂ storage in underground geological media basically translate into the following trapping means:

1. Volumetric, whereby pure-phase CO₂ is trapped in a rock volume and cannot rise to the surface due to physical and/or hydrodynamic barriers. The storage volume can be provided by:

a) large, man-made cavities, such as caverns and mines; or

b) the pore space present in geological media. If trapped in the pore space, CO₂ can be at saturations greater or less than the irreducible saturation. If the latter, the interfacial tension keeps the residual gas in place. If the former, pure CO₂ can be trapped:

• in stratigraphic and structural traps in depleted oil and gas reservoirs and in aquifers (static accumulations); or
• as a migrating plume in large-scale flow systems (hydrodynamic trapping).

2. In solution, whereby CO₂ is dissolved into fluids that saturate the pore space in geological media, such as formation water and reservoir oil.

3. Adsorbed onto coal matrix. Adsorption trapping is achieved by preferential adsorption of gaseous CO₂ onto the coal matrix because of its higher affinity to coal than that of the methane that is usually found in coal beds.

4. Chemically bound as a mineral precipitate.

These means of CO₂ storage are found in the following geological media:

• oil and gas reservoirs, either at depletion or for enhanced oil, and possibly gas, recovery;
• uneconomic coalbeds, with the possibility of producing coalbed methane;
• deep aquifers saturated with brackish water or brine; and
• salt caverns.

Although injection into deep saline aquifers and salt caverns are pure forms of storage, injection into oil and gas reservoirs and coalbeds, in conjunction with oil and gas recovery, will realize an economic benefit that will partially offset the cost of CO₂ capture and storage.

Various means of CO2 geological sequestration.

The geological media that provide the space and the means for the underground geological storage of CO₂ are found in sedimentary basins, which are the place where fossil fuel resources were generated, have accumulated and are being produced, and are fortuitously the place where most of the large power generating facilities based on fossil fuels are located. Crystalline and metamorphic rocks, such as granite, are not suitable for CO2 storage because of their fractured nature and because they lack the porosity and permeability needed for CO₂ injection. Volcanic areas and orogenic belts (mountains) are also unsuitable for CO2 sequestration mainly because they lack capacity and are unsafe due to faults and fractures created during mountain-building events.

Precipitation of carbonate minerals ( mineral trapping ) ensures sequestration permanency, which is also achieved in CO₂ dissolution in formation water (solubility trapping). In geological traps (structural or stratigraphic), cavern traps and hydrodynamic traps, the same seals (aquitards or aquicludes, or caprock) that impeded cross-formational flow and/or the escape and migration of hydrocarbons over geological time should retain CO₂ 'permanently' (i.e., for tens of thousands to millions of years), as long as faults, fractures or improperly completed or abandoned wells do not create pathways to the surface. If CO₂ is used in oil reservoirs to enhance oil recovery (EOR), then the dissolved CO₂ is produced at the wellbore within several months, after which it is usually re-circulated into the system. In the case of adsorption trapping, sequestration permanency is ensured as long as the coal is not mined. If CO₂ sequestration in coalbeds is used in conjunction with methane production, then the retention time is months to years, similarly to EOR operations.

Thus, assuming there are no leaks from the geosphere, the timeframe of CO₂ storage in underground geological media varies between several months for enhanced oil and gas recovery operations, including coalbed methane, to hundreds to millions of years for other means, and even permanently, as in the case of mineral precipitation.