**3.4 Risk of underground movements**

One of the most important aspects that must be analyzed regarding injected CO2 is its capacity to carry metals in the underground that can contaminate groundwater.

A comprehensive risk assessment must consider the main composition of the storage reservoir's rock formation. There are basically two types of formations:


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

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 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 there have historically been violent conflicts involving water rights.

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 and endanger lives.
