Monitoring site performance is critical to ensure that the principal goal of CO₂ geological storage is attained, namely the long-term isolation from the atmosphere of anthropic CO₂. There are several reasons for monitoring storage sites, such as:

• Operational: to control and optimize the injection process;

• Safety and environmental: to ensure the mitigation of global climate change, and to minimize or prevent any impact on people, wildlife and ecosystems in the vicinity of a storage site;

• Societal: to provide the public with the information needed to understand the safety of the storage site and to help gain their confidence;

• Financial: to build market confidence in CCS technology and to verify the stored volumes of CO₂ so that they are credited as avoided emissions in the European Union’s Emission Trading Scheme (ETS).

Monitoring of both the initial state of the environment (i.e. baseline) and the subsequent site performance is an important regulatory requirement in the Directive 2009/31/EC on CCS, published on the 23^th of April 2009. Operators need to be able to demonstrate that the storage performance is conforming the regulations and will continue to be over the long term. Monitoring is an important component that will reduce uncertainties in site performance, and thus it should be strongly linked to safety management activities.

Monitoring can be focused on various targets and processes in different parts of the site, such as:

• Plume imaging: tracking of the CO₂ as it migrates from the injection point. This provides key data for released will have to be quantified for accounting purposes within national greenhouse gas inventories and theETS schemes.

• Quantity of stored CO₂ for regulatory and fiscal calibrating models that predict the future distribution of CO₂ at the site. Many mature techniques are available, most notably repeat seismic surveys, which have been successfully applied at several demonstration and pilot-scale projects (Fig. 16).

• Cap-rock integrity: necessary to evaluate if the CO₂ is isolated within the storage reservoir and to enable early warning of any unexpected upward CO₂ migration. This can be especially important during the injection phase of a project, when reservoir pressures are significantly, but temporarily, increased.

• Well integrity. this is an important issue as deep wells could potentially provide a direct pathway for CO₂ migration to the surface. CO₂ injection wells plus any observation wells or pre-existing abandoned wells must be carefully monitored during the injection phase and beyond to prevent sudden escape of CO₂. Monitoring is also used to verify that all wells have been efficiently sealed once they are no longer required. Existing geophysical and geochemical monitoring systems, which are standard practice in the oil and gas industry, can be installed within or above wells to provide early warning and ensure safety.

• Migration in the overburden: at storage sites where additional, shallower rock units have properties that are similar to those of the cap rock, the overburden may form a key component in reducing the risk of CO₂ escape into the sea or the atmosphere. If monitoring in the reservoir or around the cap rock indicates an unexpected migration through the cap rock, monitoring of the overburden will be necessary. Many of the techniques used in plume imaging or monitoring cap-rock integrity can be used within the overburden.

• Surface leakage and atmospheric detection and measurement: to ensure that the injected CO₂ has not migrated to the surface, a range of geochemical, biochemical and remote sensing techniques is available to locate leaks, assess and monitor CO₂ distribution in the soil and its dispersion in the atmosphere or the marine environment (Fig. 17).

• Quantity of stored CO₂ for regulatory and fiscal purposes: although the amount of CO₂ injected can be readily measured at the wellhead, quantification in the reservoir is technically very challenging. If leakage to the near-surface occurs, then the amounts being release will have to be quantified for accounting purpose within national greenhouses gas inventories and the ETS scheme.
  
  
• Ground movements and microseismicity*: the increased reservoir pressure due to CO₂ injection can, in specific cases, increase the potential for microseismicity and small-scale ground movements. Microseismic monitoring techniques and remote methods (surveys from aircraft or satellites) able to measure even small ground distortion are available.

A wide range of monitoring techniques has already been applied at existing demonstration and research projects. These include methods that directly monitor the CO₂, and those that indirectly measure its effects on rocks, fluids and the environment. Direct measurements include the analysis of fluids from deep wells or the measurement of gas concentrations in the soil or atmosphere. Indirect methods include geophysical surveys, and monitoring pressure changes in wells or pH changes in groundwater. 

Monitoring is required for storage sites whether they are offshore or onshore. The selection of appropriate monitoring techniques depends on the technical and geological characteristics of the site and the monitoring objectives. A wide range of monitoring techniques is already available (Fig. 18), many of which are well established in the oil and gas industries, but adapted to a CO₂ context. Research to optimize e existing methods or the development of innovative techniques is also underway with the goal of improving resolution and reliability, reducing costs, automating operation, and demonstrating effectiveness.

When designing a monitoring strategy, many decisions must be based on the geological and engineering conditions specific to each individual site. Some examples are reservoir geometry and depth, expected spread of the CO2 plume, potential leakage pathways, overburden geology, injection time and flow rate, and surface characteristics, such as topography, population density, infrastructure and ecosystems. Once decisions have been made regarding the most appropriate measurement techniques and locations, baseline surveys must be conducted prior to injection operations to serve as a reference for all future measurements. Finally, each monitoring programme must be flexible so that it can evolve as the storage project itself evolves. A monitoring strategy capable of integrating all these issues, while at the same time improving cost effectiveness, will form a critical component in risk analysis and the verification of site safety and efficiency.

The monitoring of a CO₂ storage site is already feasible with the many techniques that are available on the market or under development. Research is currently underway, not only to develop new tools (particularly for sea-floor use), but also to optimize monitoring performance and reduce the costs.