User:Jesikatrese/sandbox

= '''For my CBE 195 final group project, please go to my user page. This page shows my extra credit assignment contribution to the "carbon sequestration" article.''' = Editing: MOF

= Carbon sequestration =

Geological sequestration
Geological sequestration refers to the storage of CO2 underground in depleted oil and gas reservoirs, saline formations, or deep, un-minable coal beds.

Once   CO2 is captured from a gas or coal-fired power plant, it would be compressed to ~100 bar so that it would be a supercritical fluid. In this fluid form, the CO2 would be easy to transport via pipeline to the place of storage. The CO2 would then be injected deep underground, typically around 1 km, where it would be stable for hundreds to millions of years. At these storage conditions, the density of supercritical CO2 is 600 to 800 kg / m3. For consumers, the cost of electricity from a coal-fired power plant with carbon capture and storage (CCS) is estimated to be 0.01 - 0.05 $ / kWh higher than without CCS. For reference, the average cost of electricity in the US in 2004 was 0.0762 $ / kWh. In other terms, the cost of CCS would be 20 - 70 $/ton of CO2 captured. The transportation and injection of CO2 is relatively cheap, with the capture costs accounting for 70 - 80 % of CCS costs.

The important parameters in determining a good site for carbon storage are: rock porosity, rock permeability, presence of fault lines, and geometry of rock layers. The medium in which the CO2 is to be stored ideally has a high porosity and permeability, such as sandstone or limestone. Sandstone can have a permeability ranging from 1 to 10^-5 Darcy, and can have a porosity as high as ~30%. The porous rock must be capped by a layer of low permeability which acts as a seal, or caprock, for the CO2. Shale is an example of a very good caprock, with a permeability of 10-5 to 10-9 Darcy. Once injected, the CO2 plume will rise through buoyant forces, since it is less dense than its surroundings. Once it encounters a caprock, it will spread laterally until it encounters a gap. If there are fault lines near the injection zone, there is a possibility the CO2 could leak into the atmosphere, which would be potentially dangerous to life in the surrounding area. Another danger related to carbon sequestration is induced seismicity. If the injection of CO2 creates pressures that are too high underground, the formation will fracture, causing an earthquake.

While trapped in a rock formation, CO2 can be in the supercritical fluid phase or dissolve in groundwater/brine. It can also react with minerals in the geologic formation to precipitate carbonates.

Worldwide storage capacity in oil and gas reservoirs is estimated to be 675 - 900 Gt CO2, and in un-minable coal seams is estimated to be 15 - 200 Gt CO2. Deep saline formations have the largest capacity, which is estimated to be 1,000 - 10,000 Gt CO2. In the US, there is an estimated 160 Gt CO2 storage capacity.

There are a number of large-scale carbon capture and sequestration projects that have demonstrated the viability and safety of this method of carbon storage, which are summarized here by the Global CCS Institute. The dominant monitoring technique is seismic imaging, where vibrations are generated that propagate through the subsurface. The geologic structure can be imaged from the refracted/reflected waves.

Ocean storage
If CO2 were to be injected to the ocean bottom, the pressures would be great enough for CO2 to be in its liquid phase. The idea behind ocean injection would be to have stable, stationary pools of CO2 at the ocean floor. The ocean could potentially hold over a thousand billion tons of CO2. However, this avenue of sequestration isn’t being as actively pursued because of concerns about the impact on ocean life, and concerns about its stability.

= References =