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

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


254 Fossil Fuel and the Environment

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

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

One of the most important aspects that must be analyzed regarding injected CO2 is its

A comprehensive risk assessment must consider the main composition of the storage

The presence of saltwater, as in storage in saline aquifers, is important because it promotes the formation of carbonic acid, which reacts with the surrounding minerals and can carry the metals present in them. This transport can contaminate nearby potable water aquifers. In the case of silicate rocks, the carbonic acid reacts very slowly with the rock so there is practically no change in the porosity and permeability. In contrast, carbonate rocks react more quickly with the CO2, altering the porosity and permeability. This effect, however, is damped by the rapid increase of the pH of salt water, which leads to a decrease of acid

An example where the risk of underground movement is present is the project developed by In Salah Gas (ISG), a joint venture among British Petroleum (33%), Statoil (32%) and

capacity to carry metals in the underground that can contaminate groundwater.

reservoir's rock formation. There are basically two types of formations:

Carbonate rocks (calcite, argonite, dolomite. etc.); and

New geological fractures caused by seismic movements;

Long-term changes in the properties of the reservoir's rock formations.

Abandoned production or injection wells; and

productivity of both the pasture and planted field.

**3.4 Risk of underground movements** 

Silicate rocks (quartz, feldspar, etc.).

action on the rocks (Wilson et al., 2007).

md to 1 md (Zhou et al., 2004).


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 accompany the system's evolution.
