Sequestration in Geological Repositories

Geological repositories such as aquifers, petroleum fields, and carbon deposits can be the first long-range opportunity for massive carbon sequestration. The knowledge is mature due to the former experience in oil and natural gas extraction, the underground aquifer management, and the natural gas storage. Carbon can be sequestrated utilizing three basic mechanisms. To begin with, carbon can be "lodged" at its gaseous form, beneath a low permeability mineral, utilizing the same method used in natural gas storage. This technique is promising and it will, in a short term, be the dominant carbon sequestration method. Additionally, carbon can be dissolved in a liquid, such as oil. This is the basic operation for the majority of EOR oil extraction techniques. The oils' viscosity is minimized; thus, oil flows more easily, entailing economic benefits. At last, carbon reacts directly or indirectly with mineral and the organic matter and becomes a part of their matrix. The formation of calcium, magnesium, and carbonates will be the initial mineral storage procedure. On the other hand, the carbon reaction with the minerals is of a low rate and not understandable in its entirety.

16.5.1 The capacity of the geological repositories

There are three types of geological repositories that are able to store large quantities of carbon:

• Active or depleted oil and gas reservoirs

• High-depth water formations and salt formations

• High-depth carbon plunge and coal-bed methane formations

The storage capacity estimation for geological repositories in the USA is shown in Table 16.1.

Table 16.1 Storage capacity of geological repositions.

Geological repositions

Capacity estimation (GtC)

References

Salt and water formations

1-130

Bergman Kai Winter 1995

Natural gas reservoirs

25 a

R.C. Burruss 1977

10 b

Active reservoirs

0.3/yearc

Baes et al. 1980

Reinforced methane production "coal-bed"

10

Stevens, Kuuskraa and Spector 1998

a: The total carbon is sequestrated b: The excavate natural gas is replaced with carbon c: The excavate natural gas is replaced with carbon at same pressure

16.5.2 Active or depleted oil and gas reservoirs

Oil and gas reservoirs are a very promising technique of carbon sequestration. Oil and gas reservoirs are formations created naturally inside structural sinks that do not have leakage paths, and additionally, the general geological structure and physical properties are well studied and known, due to the oil and gas excavation industry. The first and more sustainable choice is the carbon sequestration through the EOR oil extraction technique (Fig. 16.4). Almost 80% of the commercially used CO2 is utilized in the oil industry. This technology is mature and economically applicable. Carbon can be sequestrated in depleted gas reservoirs and active gas reservoirs, when the natural gas extraction can be reinforced with the carbon gas injection.

Fig. 16.4 The EOR method.

The EOR flow diagram is presented in Fig. 16.5. Although the storage potential is the lowest of all, and its industrial compatibility is a matter of concern, this method can be implemented because of the economic feasibility.

Fig. 16.5 Basic flow diagram of EOR method.

Additionally the basic idea of the sequestration in depleted reservoirs (gas & oil) is presented in Fig. 16.6.

CO2 Tube (7,389 tpd) ^ Distribution ^ CO2 Injection

Fig. 16.6 Basic flow diagram of the sequestration in depleted reservoirs

This method has similarities with EOR, but it is much simpler because the reservoir is already depleted.

16.5.3 High-depth liquid formations and saline formations

Unutilized high-depth water formations (>60 m), usually saline water formations, could be a possible storage for carbon sequestration. This formation can be met globally. The knowledge gained from the natural gas storage at these formations can assist the carbon sequestration research. There are two major issues in these methods. Water formation is difficult to be spotted, comparatively with the gas or oil reservoirs, and the water is not removed with the CO2 injection, something that happens at EOR method. This can result in the pressure increase of the formation, with the possibility of a rock and surface distortion or the possibility of seismic activity. Additionally, the storage capacity is not known and there is always the possibility of CO2 leakage if the formation has low permeability in gases.

Cook Formation North Sea
Fig. 16.7 SACS - injection at Sleipner into the Utsira formation under the North Sea.

Saline Aquifer Carbon Dioxide Storage (SACS) (Fig. 16.7) is the first commercial effort for carbon sequestration. It was developed to store the CO2 co-product from the natural gas extraction. Carbon is injected in the Utsira formation at the North Sea, in a high-depth (800 m) saline storage.

Since the summer of 1996, when the SACS program has been running, almost 106 tons of carbon has been sequestrated. The SACS program can grant the theory into implementation.

16.5.4 High-depth carbon plunge and coal-bed methane formation

Carbon plunge gives the potential of carbon sequestration and natural gas production reinforcement. Methane production from high-depth carbon plunge can be reinforced with the CO2 injection, where the CO2 absorption came along with the methane desorption. This method is already under study in North America. In an experimental field, 85,000 m3 of carbon is injected daily through four injection wells. The initial results have shown that in the total implementation of the project, 75% of the "stored" methane is desorpted.

Coal-bed formations consist of several thick and slim coal seams and intermediate sandstone and shale layers.

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