**3.3 Risk of leakage to the atmosphere**

When injected, the CO2 is less dense than the saline fluids of the reservoirs, so it can migrate to other geological formations or to the surface. The escape to the atmosphere, besides causing risks to human health and the environment in nearby areas, also obviously reduces the effectiveness of the effort to control GHG emissions intended by the CCS project in the first place. The leakage of high concentrations to the atmosphere can have catastrophic effects on the local biota.


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Sonatrach, the Algerian national oil company (35%). The gas produced by the production wells in the Sahara Desert region has an average CO2 concentration of 7%, a level that needs to be lowered to under 0.3% for the gas to be exported to Europe. Therefore, a purification plant was built at the Krechba Oásis, 700 Km from Algiers (Iding & Ringrose, 2009). The purified methane is sent northward in a pipeline that connects to the Algerian gas exportation network, while the captured CO2 is pressurized, carried by pipeline and injected in a saline aquifer located below the gas field. The main risk of this undertaking is the possibility of migration of the CO2 toward a drinking water aquifer that lies above the gas reservoir. Investigations demonstrated that the upper part of the reservoir where the CO2 is being injected has a thick layer of schist that seals this reservoir. However this risk of groundwater contamination should be given priority attention, in this desert region, where

Another risk associated with underground movement is the possible generation of seismic events due to the alteration of the underground geophysical characteristics. Such seismic events, besides potentially generating geological fractures capable of releasing large amounts of stored CO2, can also unleash other catastrophic events that damage structures

The analysis of the risks of using depleted hydrocarbon reservoirs for geological sequestration of carbon or the employment of CO2 injection for enhanced oil recovery (EOR) is a complex process that must consider constant changes in the risk factors over time and

The status of a well can change, altering the set of instrumentation necessary and the ranking of the importance of the data necessary for risk management. Additionally, the change in the status of a determined well alters the entire system and affects the ability to monitor the system. Therefore, the risk management system must be adaptable to

Governments play an essential role in CCS, by setting safety standards and other requirements for operation and obtaining public support. The deployment of CCS projects relies on the approval of civil society, who must believe that the injected CO2 will stay stored in the reservoir for thousands of years. To this end, the analysis of possible risks associated with the escape of CO2 is an essential stage in the life cycle of the storage system and aims to promote and ensure the safety of the activity to the environment and to human

the various types of wells. In a given reservoir, there can be five basic well types:

there have historically been violent conflicts involving water rights.

**3.5 Risk of using hydrocarbon reservoirs for sequestration** 

 Sealed wells without monitoring instrumentation; Sealed wells with monitoring instrumentation; and

health, contributing to the technology's acceptance.

and endanger lives.

 Production well; Injection well;

Monitoring wells.

accompany the system's evolution.

**4. Policy and regulation** 


In an EOR project, the drilling of new injection wells continues until no longer economically feasible. The abandoned wells, although sealed with cement, can provide paths for CO2 to escape. This can happen due to degradation of the sealing materials. Contact with CO2 in brackish water increases the attack on cement by around tenfold in comparison with freshwater (Barlet-Goue´dard et al., 2009). The Weyburn project currently has over 1,000 wells along its extension. One of the assumptions of the studies conducted there is an increase in 100 years in the permeability of the sealing cement from an initial level of 0.001 md to 1 md (Zhou et al., 2004).

Changes in the porosity and permeability of the reservoir's rocks can be caused by the effect of the chemical interactions between the carbonic acid and the minerals forming the rocks. Carbonic acid is generated directly by the reaction of CO2 with the water present in the reservoir. This effect is stronger in storage projects that use saline aquifers, such as the Sleipner project, but is also occurs on a lesser scale in EOR projects, such as Weyburn.

A study carried out at the University of Nottingham (Patil et al., 2009) to assess the possible effects of CO2 employed injection at a controlled rate. The study utilized two types of ground: a pasture and a fallow plowed field awaiting planting. The results showed that the concentration of CO2 displaced the O2 from the soil and reduced its pH. The consequences of these alterations were impairment of the action of earthworms and reduced grass growth in the pasture and crop germination after planting, with consequent diminished productivity of both the pasture and planted field.
