school energy&environment

Geologic sequestration of CO2

Since fossil sources are destined to be the protagonists of the world energy scenario for many more years, we  must act immediately, directly on their utilization, in order to reduce CO₂ emissions with regard to their combustion. CO₂ Capture & Storage (CSS) technology enables the capture and sequestration of the CO₂ generated by the use of fossil sources, reducing the emissions in the atmosphere.
With regard to the capture of CO₂, there are technologies that are already well known and utilized by the petrochemical industries and others are being developed. At present, CO₂ can be captured in three principal ways.

• post-combustion: in post-combustion capture,  CO₂ is separated from the combustion fumes, first purified from the pollutants with modern treatment systems. This separation takes place using a solvent that absorbs CO₂ at low temperatures, that it subsequently releases  for heating, generating a practically pure CO₂ current;
• pre-combustion: in pre-combustion capture, CO₂ is removed  before combustion. The fossil fuel is gasified with oxygen to generate hydrogen and CO₂. The CO₂ is separated, while the hydrogen is utilized to  generate electricity in a combined cycle, or for other uses as an energy vector;
• oxy-combustion: with this method, combustion of fossil fuels is fed with oxygen instead of air, thus generating a gaseous current that consists mainly of CO₂ and water vapour. The water vapour is separated through condensation and the concentrated CO₂ current can be compressed and stocked.

Once it has been captured and compressed, CO₂ is transported in pipes up to the storage site, and injected  to a depth of about one kilometre underground. Depleted hydrocarbon deposits and saline aquifers (deep bodies of water with an enormous capacity to absorb CO₂) are considered suited reservoirs for permanent geological confinement of carbon dioxide.
The CO₂ is injected at high pressures, so that it reaches the so-called “supercritical” behaviour, in other words a  state that is similar to a gas, due to  its capacity to spread rapidly in the porous surfaces of the geological formation, and that is also similar to a liquid, in terms of density, and therefore a volume that can be stored. In the depleted oil or gas fields, the CO₂ fills the pores in which the hydrocarbons were trapped. In case relevant amounts of hydrocarbons are still in the deposit at the time of injection, the CO₂ can also favour the additional production of oil or gas (Enhanced Oil Recovery – EOR e Enhanced Gas Recovery – EGR processes).
Costs and feasibility
In the practical application of CCS there still are difficulties to be overcome, that are related prevalently to the costs. The initial stage of CO₂ capture has a significant energetic and economic cost, that covers approximately 80% of the total costs of the technology. In order to act positively on this phase, it is necessary to operate on plants that emit large quantities of CO₂. Once the CO₂ is separated, it is transferred to the storage site, which must not be very distant  in order to minimize the costs. For distances of a few dozen kilometres,  transportation accounts for about 15% of the total cost. The final stage in which the CO₂ is injected into the ground accounts for 5% of the total cost. This, however, is the most delicate stage, from the point of view of safety, and it significantly affects the sustainability of the entire process. The injection of CO₂, however, is a process that is well known in the oil drilling world, that knows its technological and geological characteristics well.  In fact, for decades the oil companies have been re-injecting CO₂ derived from the treatment of acid gases into the hydrocarbon deposits in order to maintain the pressure and to support production.
The knowledge and experience matured in the oil sector can be applied to  CO₂ Capture and Storage technologies, for example in the choice of the most suitable sites for carbon dioxide sequestration. In fact the oil sector has a good knowledge of characteristics such as the porosity of the storage site, that define the potential volume that can be stored, and help to evaluate the consequences on the mechanical stability of the geological formation and any seismic effects; and to identify the characteristics of the caprocks which guarantee well sealed sites for the injected CO₂ over the years.
Use of saline aquifers as CO₂ reservoirs is an option that is less mature at present, it requires the development of more know-how as  these basins have not been as widely studied as the hydrocarbon deposits. On the other hand aquifer deposits are also present in areas where oil  and gas are not produced and they offer a potential storage  that is considerably greater than deposits that have become depleted or are on the decline.