CO2 Capture and Storage (CCS) is recognised as a potentially important corner stone amongst the climate change mitigation technologies in Europe and worldwide. Although individual components of the CCS value chain are proven technologies, as a whole-chain process, CCS is a new technology which was expected to reach fast implementation and at a very large scale in the energy and other industrial sectors. The concern that a rapid implementation could result in a regulatory vacuum, lead the European Commission, the USEPA and other international organisations to proactively work on relevant legislation and directives. Legislation specifically and CO2 geological storage was also implemented at national and regional level in several regions of the world. This legal context forms the focus of this report, but is approached from the practical point in which the storage project and the related operational and safety risks are the starting point.

At time of publication, nearly all EU members have used the EU CCS Directive to implement appropriate legislation that allows the safe and uniform rollout of CCS throughout Europe, especially regarding the geological storage of CO2. Although implementation of the specific regulation is mandatory, member states can autonomously decide whether or not to allow CCS activities on their territory.

Geological storage of CO2 deserves specific attention, and is as such also the focus of this report. This key report is at the same time a highly practical and scientifically sound document, that provides a thorough overview of the legislation and regulation in place in Europe, and compare it with that of other leading CCS countries and regions.

Rather than taking the structure of a legal document as starting point, this report approaches this topic from the following five, very practical angles:

  • Storage site operation
  • Leakage events
  • Monitoring
  • Remediation
  • Closure and post-closure

These form the main chapters of this report, and care was taken that each of them can be read largely independently from the others, allowing the reader to approach the topic from the angle that is best suited to them.

During the lifetime of a storage site, the risks associated with storing CO2 depend on many factors, including the infrastructure used, the type of reservoir, experience gained with a specific reservoir, and also the different stages of project development. It is from the different types and levels of risk that legislation and regulations are summarised and evaluated in chapters 2 and 6 focusing on storage site operation and closure respectively.

Early detection of leakage and other irregularities requires a correctly tailored monitoring plan, although this has also many other purposes, including optimising the understanding of reservoir dynamics and, with time, reliably predicting the long-term stability of a reservoir. Therefore, monitoring is a crucial part of any storage project, and thus a point of focus for directives and regulations. The salient points with regards to monitoring and regulations are outlined in chapter 4.

In the unlikely event that leakage occurs, despite risk minimisation efforts, the CO2 storage project enters an unexpected and undesired stage. It is a situation which is typically thoroughly dealt with in the different regulations and guidelines. In any case, a project should be well-prepared for such contingency, in order to respond properly, as is discussed in chapter 3.

If CO2 leakage is detected, direct containment of the incident usually covers only part of the actions that need to be taken. Wherever adverse effects have occurred or can be expected, remediation actions are necessary. Compared to monitoring and leakage, the focus of relevant directives and regulations is much more on liability, rather than prescribing exact actions or obligations, as outlined in chapter 5.

The following paragraphs discuss the approach taken in this report with regards to the main aspects that are considered for CO2 storage site operational and safety risks in each of the chapters.

Storage site

A storage site is constructed to continuously inject large amounts of CO2 in the storage reservoir. The operational phase is preceded by several other stages, which can largely be grouped under exploration, development and testing. Such activities are only briefly discussed in this report, because from a regulatory point of view they fall under existing national laws, which regulate the general activities for the appraisal of the subsurface.

The run-up to the full-scale operational phase of a CO2 geological storage project is essential, because it is aimed to maximise the knowledge of the reservoir and the sealing structures, sets baseline values used in the monitoring campaign, and/or tests the expected behaviour of the reservoir through injections tests. A geological reservoir, nevertheless, remains a natural system of which the details can never be fully mapped. The residual lack of knowledge is the main cause of reservoir related risks.

The engineering aspects of the site infrastructure are the second source of risk. CO2 is transported to an injection site, where it is first handled (local transport, compression, buffer storage, temperature preconditioning, etc.) using infrastructure that is located mainly at the surface, or above the sea level for off-shore installations. This operation and its related risks are not unlike that of large industrial installations where fluids are handled at large scale. Nevertheless, even though CO2 is a relatively harmless substance when handled is small quantities, it is worthwhile considering the risks related to a full-scale industrial project. The actual injection infrastructure is the link between the surface installation and the geological reservoir.

CO2 is generally considered as a non-hazardous substance, however this might not be always the case, e.g. when used at high-pressure, and may be corrosive to some materials. Minor additional substances in the CO2 stream may also introduce an additional concern. Also, external risk factors, such as potential accidents due to damage to pipelines, need to be taken into account.

There is a large amount of relevant guidelines and regulations specifically designed to properly regulate the handling of CO2 during the operation of CO2 geological storage projects, both on- and off-shore. In addition to these ad-hoc regulations, there exists a significant amount of indirect regulations that have to be taken into account in storage projects. Some of those more general conventions even contain clauses that may be incompatible with some CCS projects, and could therefore be fundamental obstacles.


Identification and regulation of risks and risk-related activities will minimise, but not absolutely prevent the leakage of CO2 from a reservoir. Therefore, a second set of regulations seems necessary in order to ensure that proper actions are taken in case of leakage events.

Leakage-specific regulation indeed includes, but is usually not restricted to designing an action plan in case of leakage. In addition to this, the regulation refers for the important apects of risk evaluation and gathering of appropriate monitoring data. This is considered both logical and useful, since the identification of risks allows to anticipate the different potential scenarios under which CO2 can leak from a specific reservoir.

Similarly, monitoring data is of essential importance, because the follow-up of the evolution of a storage reservoir provides essential insights as to whether a reservoir behaves as expected, which potential leakage scenarios become more or less likely, and what potential amounts of CO2 can leak from specific parts of the reservoir.

However, the final target of the majority of these regulations is indeed to maximally prevent adverse effects in case of leakage. This usually requires that a fully prepared action plan is ready for deployment. Especially in the European context where the ETS forms an important part of the financial balance sheet of a storage project, regulations also need to be in place to compensate for the loss of CO2 from a storage reservoir. In this, but also in a more general context of proper supervision, the reporting obligation to the national competent authority is strictly embedded in the European legislation and guidelines.

In order to mitigate the adverse effects of potential leakage, the effects of CO2 in the environment into which it has leaked need to be properly understood. This again is a vast discipline in its own right, and is relatively well studied. One of the typical examples discussed is the leakage of either CO2 or displaced brine into an aquifer that is exploited for drinking water. Although the most direct effects seem to relate to pH changes that do not necessarily negatively affect the quality of the drinking water, field experiments have demonstrated that it are mainly the secondary effects (e.g. dissolution of minerals) that result in potentially hazardous chemical changes.

In case leakage occurs through geological boundaries, this leakage is potentially a spatially diffuse process. This means that leakage mitigation actions will usually consist of controlling the stored CO2, rather than enhance or repair the impermeable barriers. Such actions often involve reservoir engineering schemes such as the depressurisation of the reservoir, or the injection of water to steer the CO2 plume away from a spill point.


Throughout the lifetime of a reservoir, and also before CO2 injection is started, monitoring of the reservoir properties and the injected CO2 is a crucial element for the successful completion of a storage project. At the same time, it adds significantly to the operational costs, and therefore proper regulations are useful as additional motivation for the storage operator.

Since many monitoring techniques are designed to detect relative changes, e.g. regarding densities in 3D seismic profiles, groundwater composition, or increase of CO2 in soil gas, it is important to establish a proper baseline. Such a baseline is often the definition of the natural background values, which may by themselves be variable through time. This should be taken into account to avoid later disputes (e.g. claims regarding alleged leakage of CO2).

A crucial aspect of monitoring that is particularly emphasised in most regulatory documents, is that active monitoring will allow to verify that the CO2 plume is migrating as expected. Where deviations are observed, the reservoir model is to be adjusted accordingly. As such, monitoring will lead to an increasingly better understanding of the reservoir during operation, and improve the accuracy of the long and short term predictions of the reservoir in response to injection activities. This aspect has direct consequences to the appreciation of the different risks related to the geological storage of CO2.

As a final major element, monitoring of the CO2 plume should be performed to maximise the early detection of CO2 leakage. Guidelines have been set up to provide an evaluation framework for the techniques that can be used. In conjunction with understanding of migration and potential leakage pathways, it is possible to decide which techniques can or should be deployed.

Due to the intrinsic variability of geological reservoirs and storage scenarios, it is difficult to turn such guidelines into absolute obligations. The approach is, therefore rather, that the proposed site operator designs a monitoring plan, in line with the objectives of the guidelines, which is then to be evaluated by an independent governmental body. The guidelines provide a reference framework for both the design and the evaluation (followed by a motivated approval or rejection) of a monitoring plan. A monitoring plan is not a final document, but will frequently be updated to reflect the increasing knowledge of the reservoir. This report illustrates how such a process works in practice by discussing the few early projects that have partly or fully undergone through the process of setting up and submitting a monitoring plan.


The chapter on how remediation is regulated focusses on which actions are required in case significant irregularities occur in a CO2 geological storage project. A significant irregularity covers situations where there is direct (threat of) economic or environmental damage or endangerment of a human population.

It is useful to distinguish different categories of such situations, simply because they often relate to which remediation actions can be considered. Although, as is shown in the chapter on risks related to site operation, irregularities may also relate to the surface installations, the focus of this chapter is on subsurface problems in or around a reservoir. In that context, the cause of the problem is either natural (geological) or anthropogenic pathway.

A typical case of leakage along an anthropogenic pathway is leakage along an abandoned well. In such case, remediation is taken in two steps. The first is the identification (localisation) and resealing (potentially including a work-over) of the well. The second involves the remediation of the damage done. Again taking a typical situation, leakage may have resulted in the contamination of an aquifer. In such cases, remediation may involve pump-and-treat to actively remove primary and secondary contaminants, and possibly also the restoration of the pH condition in the aquifer.

When an irregularity has a geological origin, the identification step is likely more complex because the cause of the irregularity is generally less localised. In such instances, reservoir engineering solutions can still offer a way out. The report discusses a variety of situations that may occur, and remediation actions that can be considered, before discussing the relevant regulatory regimes and guidelines.

Especially in the European context, the implementation of remediation actions is explicitly embedded in the legislation. The operator needs to report any irregularity immediately, resulting in a direct involvement and supervision of the situation by the national competent authority, who has large freedom in ordering remediation actions as well as including those not foreseen in a remediation plan. In case the operator fails to comply, they can ultimately be relieved of site operation duties.

Closure and post closure

An essential step in any project that envisages permanent CO2 storage is the post-operational phase of the project. As a general rule, the risk on leakage decreases over time with different geological processes that slowly but steadily increase the stability of the stored CO2.

The level of risk is, however, determined by the abandonment procedure, e.g. the sealing or otherwise final closure of the wells that were used during the operational phase. Actually, well abandonment is considered as one of the most crucial points of the site closure process. The process is not unlike standard well-abandonment of oil and gas fields, but specific precautions should be considered for CO2 storage sites.

Until the injected CO2 is fully stabilised, or at least until its behaviour can be fully predicted, monitoring will remain necessary, be it according to a scheme modified to a situation where there is no longer active injection.

In Europe, a system in which the responsibility for a reservoir, once the operator has convincingly been able to show that it will evolve to stability, is handed over to the state authorities. This has the advantage that long-term responsibility is guaranteed. Naturally, the hand-over of such previous injection sites is a point of attention for the regulator.

In general, proving the safety of a post-closure project involves a specific risk assessment in relation to modelling, an evaluation of the historical monitoring record, the demonstration that mainly wells are adequately abandoned, and of course the absence of environmental problems. In normal situations, transfer of liability is not foreseen to be problematic. Naturally, no industrial scale project has reached the point where site closure is practically being considered or prepared.