Search

IEAGHG Technical review 2017-TR8. This report was prepared as an account of the work sponsored by IEAGHG, CLIMIT, Concawe and SINTEF.

Carbon dioxide Capture and Storage (CCS) is considered as one of the promising options for reducing atmospheric emissions of CO2 related to human activities. One of the main concerns associated with the geological storage of CO2 is that the CO2 may leak from the intended storage formation, migrate to the near-surface environment and, eventually, escape from the ground. This is a concern because such leakage may affect aquifers overlying the storage site and containing freshwater that may be used for drinking, industry and agriculture. The IEA Greenhouse Gas R&D Programme (IEAGHG) recently commissioned the CO2GeoNet Association to undertake a review of published and unpublished literature on this topic with the aim of summarizing 'state of the art' knowledge and identifying knowledge gaps and research priorities in this field. Work carried out by various CO2GeoNet members was also used in this study. This study identifies possible areas of conflict by combining available datasets to map the global and regional superposition of deep saline formations (DSF) suitable for CO2 storage and overlying fresh groundwater resources. A scenario classification is developed for the various geological settings where conflict could occur. The study proposes two approaches to address the potential impact mechanisms of CO2 storage projects on the hydrodynamics and chemistry of shallow groundwater. The first classifies and synthesizes changes of water quality observed in natural/industrial analogues and in laboratory experiments. The second reviews hydrodynamic and geochemical models, including coupled multiphase flow and reactive transport. Various models are discussed in terms of their advantages and limitations, with conclusions on possible impacts on groundwater resources. Possible mitigation options to stop or control CO2 leakage are assessed. The effect of CO2 pressure in the host DSF and the potential effects on shallow aquifers are also examined. The study provides a review of CO2 storage-specific regulations in the main countries undertaking CCS evaluation and research. It aims to identify the constraints imposed by existing regulations on the protection of groundwater resources and highlight the inconsistencies and gaps between CCS regulations and Water Protection regulations. The present paper focuses specifically on potential risks on groundwater quality caused by CO2 storage in DSF assessed via natural CO2 analogues from both the literature and detailed European case studies (France, Italy, Germany, Denmark).

To date, the deployment, integration, and utilization of intermittent renewable energy sources, such as wind and solar power, in the global energy system has been the cornerstone of efforts to combat climate change. At the same time, it is recognized that renewable power represents only one element of the portfolio of technologies that will be required to deliver a technically feasible and financially viable energy system. In this context, carbon capture and storage (CCS) is understood to play a uniquely important role, providing significant value through flexible operation. It is therefore of vital importance that CCS technology can operate synergistically with intermittent renewable power sources, and consequently ensuring that CCS does not inhibit the flexible and dispatchable nature of thermal power plants. This work analyses the intrinsic dynamic performance of the power and CO2 capture plants independently and as an integrated system. Since the power plant represents the fast dynamics of the system and the steam extraction is the main point of integration between the CO2 capture and power plants, disturbances with fast dynamics are imposed on the steam extraction valve during steady state and dynamic operation of a natural gas combined cycle (NGCC) to study the effects of the integration on power generation capacity. The results demonstrate that the integration of liquid-absorbent based post-combustion CO2 capture has negligible impact on the power generation dynamics of the NGCC.

Ambitions to produce electricity at low, zero, or negative carbon emissions are shifting the priorities and appreciation for new types of power generating technologies. Maintaining the balance between security of energy supply, carbon reduction, and electricity system cost during the transition of the electricity system is challenging. Few technology valuation tools consider the presence and interdependency of these three aspects, and nor do they appreciate the difference between firm and intermittent power generation. In this contribution, we present the results of a thought experiment and mathematical model wherein we conduct a systems analyses on the effects of gas-fired power plants equipped with Carbon Capture and Storage (CCS) technology in comparison with onshore wind power plants as main decarbonisation technologies. We find that while wind capacity integration is in its early stages of deployment an economic decarbonisation strategy, it ultimately results in an infrastructurally inefficient system with a required ratio of installed capacity to peak demand of nearly 2.. Due to the intermittent nature of wind power generation, its deployment requires a significant amount of reserve capacity in the form of firm capacity. While the integration of CCS-equipped capacity increases total system cost significantly, this strategy is able to achieve truly low-carbon power generation at 0.04 t CO2 /MWh. Via a simple example, this work elucidates how the changing system requirements necessitate a paradigm shift in the value perception of power generation technologies.

The UK is committed to the Paris Agreement and has a legally-binding target to reduce economy-wide greenhouse gas emissions by 80% relative to 1990 levels by 2050. Meeting these targets would require deep decarbonisation, including the deployment of negative emissions technologies. This study, via a power supply capacity expansion model, investigates the potential role of bio-energy with carbon capture and storage (BECCS) and direct air capture and storage (DACS) in meeting the UK's emissions reduction targets. We show that to achieve power sector decarbonisation, a system dominated by firm and dispatchable low-carbon generators with BECCS or DACS to compensate for their associated emissions is significantly cheaper than a system dominated by intermittent renewables and energy storage. By offsetting CO2 emissions from cheaper thermal plants, thereby allowing for their continued utilisation in a carbon-constrained electricity system, BECCS and DACS can reduce the cost of decarbonisation by 37–48%. Allowing some this value transferred to accrue to NETs offers a potential route for their commercial deployment.

International audience

The delayed deployment of low-carbon energy technologies is impeding energy system decarbonization. The continuing debate about the cost-competitiveness of low-carbon technologies has led to a strategy of waiting for a ‘unicorn technology’ to appear. Here, we show that myopic strategies that rely on the eventual manifestation of a unicorn technology result in either an oversized and underutilized power system when decarbonization objectives are achieved, or one that is far from being decarbonized, even if the unicorn technology becomes available. Under perfect foresight, disruptive technology innovation can reduce total system cost by 13%. However, a strategy of waiting for a unicorn technology that never appears could result in 61% higher cumulative total system cost by mid-century compared to deploying currently available low-carbon technologies early on. Investment and policy decisions about low-carbon technologies have been affected by the promise of a ‘unicorn technology’ that may reduce emissions at low costs. Researchers show that such investment delays can have high implications on cost and emissions whether the unicorn technology materializes or not.

Carbon capture and sequestration of CO2 from the combustion of fossil fuels in thermal power plants is expected to be important in the mitigation of climate change. Deployment however falls far short of what is required. A key barrier is the perception by developers and investors that these technologies are too inefficient, expensive and risky. To address these issues, we have developed a novel retrosynthetic approach to evaluate technologies and their design based on the demands of the system in which they would operate. We have applied it to chemical looping combustion (CLC), a promising technology, which enables carbon dioxide emissions to be inherently captured from the combustion of fossil fuels. Our approach has provided unique insight into the potential role and value of different CLC variants in future electricity systems and the likely impact of their integration on the optimal capacity mix, the operational and system cost, and dispatch patterns. The three variants investigated could all provide significant value by reducing the total investment and operational cost of a future electricity system. The minimisation of capital cost appears to be key for the attractiveness of CLC, rather than other factors such as higher efficiency or lower oxygen carrier costs.

© 2018 The Royal Society of Chemistry. Carbon capture and storage (CCS) is broadly recognised as having the potential to play a key role in meeting climate change targets, delivering low carbon heat and power, decarbonising industry and, more recently, its ability to facilitate the net removal of CO2 from the atmosphere. However, despite this broad consensus and its technical maturity, CCS has not yet been deployed on a scale commensurate with the ambitions articulated a decade ago. Thus, in this paper we review the current state-of-the-art of CO2 capture, transport, utilisation and storage from a multi-scale perspective, moving from the global to molecular scales. In light of the COP21 commitments to limit warming to less than 2 °C, we extend the remit of this study to include the key negative emissions technologies (NETs) of bioenergy with CCS (BECCS), and direct air capture (DAC). Cognisant of the non-technical barriers to deploying CCS, we reflect on recent experience from the UK's CCS commercialisation programme and consider the commercial and political barriers to the large-scale deployment of CCS. In all areas, we focus on identifying and clearly articulating the key research challenges that could usefully be addressed in the coming decade.

Sustainable energy & fuels 2(6), 1153-1169 (2018). doi:10.1039/C8SE00061A, Published by Royal Society of Chemistry, Cambridge

We present an power systems optimisation model for national-scale power supply capacity expansion considering endogenous technology cost reduction (ESO-XEL). The mixed-integer linear program minimises total system cost while complying with operational constraints, carbon emission targets, and ancillary service requirements. A data clustering technique and the relaxation of integer scheduling constraints is evaluated and applied to decrease the model solution time. Two cost learning curves for the different power technologies are derived: one assuming local learning effects, the other accounting for global knowledge spill-over. A piece-wise linear formulation allows the integration of the exponential learning curves into the ESO-XEL model. The model is applied to the UK power system in the time frame of 2015 to 2050. The consideration of cost learning effects moves optimal investment timings to earlier planning years and influences the competitiveness of technologies. In addition, the maximum capacity build rate parameter influences the share of power generation significantly; the possibility of rapid capacity build-up is more important for total system cost reduction by 2050 than accounting for technology cost reduction.

New cannot be measured with old. The transformation of the electricity system from a network of fossil-based dispatchable power plants to one with large amounts of intermittent renewable power generation, flexible loads and markets, requires a concurrent development of new evaluation tools and metrics. The focus of this research is to investigate the value of power technologies in order to support decision making on optimal power system design and operation. Technology valuation metrics need to consider the complexity and interdependency of environmental and security objectives, rather than focusing on individual cost-competitiveness of technologies outside of the power system. We present the System Value as a new technology valuation metric, based on a mixed-integer linear program (MILP) formulation of a national-scale electricity system. The Electricity System Optimization model is able to capture detailed technical operation of the individual power plants as well as environmental and security requirements on the system level. We present a case study on the System Value of onshore wind power plants in comparison with Carbon Capture and Storage (CCS) equipped gas-fired power plants in a 2035 UK electricity system. Under the given emission constraints, the deployment of both technologies reduce total system cost of electricity generation. In the case of CCS-equipped power plants the reductions in total system cost are 2 to 5 times higher than for the deployment of onshore wind capacity.

International audience; Since the very first European research project initiated in 1993, much progress has been made in Europe on the development ofCO2 geological storage as a key tool for combating climate change. A review of the main achievements and remaining obstaclesin Europe is proposed, with a glance at what is going on at global level. Scientific, technical, economic, regulatory and policyaspects are considered. The increasing role of the growing CO2GeoNet Association, a reference pan-European scientific body onCO2 geological storage, is emphasized, with actions both on the EU and global scene for enabling efficient and safe storage.

International audience; Leakage of CO2 from geological reservoirs is one of the most fearsome unexpected scenarios for CO2 storage activities. If a leakage reaches the ground level, exposure to high CO2 concentrations is more likely to occur in low ventilated spaces (pit dug in the ground, basement, building) where CO2 could accumulate to high concentrations. Significant literature and models about indoor exposure resulting from intrusion of soils gases in building are available in several domains (e.g., contaminated soils, radon, etc.). However, there is no guarantee that those approaches are appropriate for the assessment of consequences of CO2 leakage due the specificity of CO2 and due to the singularities of the source in case of leakage from anthropic reservoirs. Furthermore, another singularity compared to conventional approaches is that the risk due to CO2 exposure should be evaluated considering acute concentrations rather than long term exposure to low concentrations. Thus, a specific approach is needed to enable a quantitative assessment of the risk for health and living in indoor environment in case of leakage from a reservoir reaching the unsaturated zone below the buildings. We present the results of the IMPACT-CO2 project that aims at understanding the possible migration of CO2 to indoor environment and to develop an approach to evaluate the risks. The approach is based on modelling and experiments at laboratory scale and at field representative scale. The aim of the experiment is to capture the main phenomena that control the migration of CO2 through unsaturated zone, and its intrusion and accumulation in buildings. The experimental results will also enable numerical confrontation with tools used for risk assessment. Experiments at representative scale (Figure 1) are performed on the PISCO2 platform (Ponferrada, Spain) specifically instrumented and designed for understanding the impacts of CO2 migration towards the soil surface. The experiment is composed of a 2.2 m deep basin filled with sand upon which a specifically designed cylindrical device representing the indoor condition of a building (with controlled depressurization and ventilation) is set up. The device includes a calibrated interface that represents a cracked slab of a building. The injection of CO2 is performed at the bottom of the basin with a flow rate in the range of hundreds of g/d/m². The first results show that the presence of a building influences significantly the transport of CO2 in the surrounding soil leading to two competing phenomena: 1) seepage in the atmosphere mainly controlled by diffusion gradient and 2) advective/diffusive flux entering the building due to the depressurization. Models have been established to quantitatively assess the proportion of CO2

International audience; CO2GeoNet, the European Network of Excellence on CO2 geological storage, was created in 2004 as an EU PF6 project and became an association in 2008. The Association strives to enable efficient and safe CO2 storage in deep geological formations to combat climate change and ocean acidification. With a current membership of 26 research institutes spanning 19 European countries, activities include research, scientific advice, training, information & communication. CO2GeoNet, as a pan-European scientific body, has a valuable and independent role to play in enabling the deployment of the CO2 capture and storage (CCS) technology, when and where it is needed in Europe and in other parts of the World. CO2GeoNet is highly active on the international scene, through a cooperation agreement with IEAGHG and the Global CCS Institute, as a CSLF recognized network and a Liaison organization in the ISO CCS Technical Committee, and as a UNFCCC accredited Research NGO (RINGO).The COP21 Climate Conference in Paris in December 2015 was an important milestone and CO2GeoNet was deeply involved, both before and after this conference, in bringing the science behind CO2 storage and the rationale for CCS to a wide range of stakeholders including the general public. The main outcomes of these events will be presented, focusing on key messages.

International audience; The CO2-DISSOLVED project, funded by the ANR (French National Research Agency) is a techno-economic study assessing the feasibility of a new concept combining geothermal energy and CCS (Kervévan et al., 2013). This design combines capture, injection, and storage of dissolved CO2 (rather than supercritical) in a deep saline aquifer with geothermal heat recovery.

Many studies have quantified the cost of Carbon Capture and Storage (CCS) power plants, but relatively few discuss or appreciate the unique value this technology provides to the electricity system. CCS is routinely identified as a key factor in least-cost transitions to a low-carbon electricity system in 2050, one with significant value by providing dispatchable and low-carbon electricity. This paper investigates production, demand and stability characteristics of the current and future electricity system. We analyse the Carbon Intensity (CI) of electricity systems composed of unabated thermal (coal and gas), abated (CCS), and wind power plants for different levels of wind availability with a view to quantifying the value to the system of different generation mixes. As a thought experiment we consider the supply side of a UK-sized electricity system and compare the effect of combining wind and CCS capacity with unabated thermal power plants. The resulting capacity mix, system cost and CI are used to highlight the importance of differentiating between intermittent and firm low-carbon power generators. We observe that, in the absence of energy storage or demand side management, the deployment of intermittent renewable capacity cannot significantly displace unabated thermal power, and consequently can achieve only moderate reductions in overall CI. A system deploying sufficient wind capacity to meet peak demand can reduce CI from 0.78 tCO2 /MWh, a level according to unabated fossil power generation, to 0.38 tCO2 /MWh. The deployment of CCS power plants displaces unabated thermal plants, and whilst it is more costly than unabated thermal plus wind, this system can achieve an overall CI of 0.1 tCO2 /MWh. The need to evaluate CCS using a systemic perspective in order to appreciate its unique value is a core conclusion of this study.

International audience; Risk management for CO2 geological storage faces a number of challenges related to the very long time scales involved, to the uncertainties regarding the geological medium and the phenomena occurring, or to the lack of relevant experience for this technology. From a methodological point of view, the process relies upon classical risk management approaches applied to site-specific conditions. The ISO 31000:2009 standard, describing principles and guidelines for risk management, advocates for an iterative risk management process based upon the following steps:-Establishing the context;-Risk assessment, including the identification of risk scenarios and their analysis and evaluation through dedicated tools and models;-Risk treatment, i.e. the design and implementation of options modifying the risk level.Appropriate monitoring is required throughout this process as well as continuous communication with stakeholders.As specified for instance by the European Directive on CCS (2009/31/CE), any CO2 storage application requires both a description of measures to prevent significant irregularities and a proposed corrective measures plan. Such risk treatment measures have to match the risk scenarios seen as relevant for the considered site.GERICO (GEstion des RIsques CO2 or manaGEment of RIsks for CO2 storage) is a knowledge repository aiming at supporting site-specific management of risks, through the delivery of generic information relating to potential risk scenarios and to risk treatment measures. We have developed this repository from literature data and implemented a Web interface; a beta-version of the repository should be released by the end of Summer 2015 at http://gerico.brgm.fr. It is first oriented towards a public familiar with CCS, but not necessarily CO2 storage risks specialists: the CCS scientific community, private operators, or authorities.GERICO gathers a variety of information:•Potential risk scenarios for various phases of the storage lifecycle. These scenarios are figured through bow-tie diagrams displaying accidental sequences, from primary causes to impacting phenomena;•Potential impacts on vulnerable assets;•Risk treatment measures, placed with respect to the risk scenarios.It focuses on risk treatment measures with a synthesis sheet and a list of literature resources for each. In that respect, it may be more or less considered as a counterpart for risk treatment of the BGS / IEA GHG monitoring selection tool available at http://ieaghg.org/ccs-resources/monitoring-selection-tool1. This repository is an inventory, meant as generic as possible, of conceivable risk scenarios and risk treatment measures. It does not make any evaluation of the described risk scenarios, which likelihood and severity have to be specifically assessed on a case-by-case basis given local conditions of the site. In particular, the relevance of events sequences and of associated measures must be discussed for each site: some sequences or measures will not be adapted to the specific conditions encountered.The ambition of GERICO is to become a reference tool that may support the process of site-specific risk assessment and management, the documentation and communication of risk management, and guide further research where it is required. Its performance will improve with feedback from users to enrich information; especially field experience of risk management implementation would bring value in addition to the mainly theoretical data contained so far. This repository may then reflect best practices for risk management to support the deployment of CO2 geological storage.

The development and implementation of the CO2 Capture and Storage (CCS) technology is highly dependent on the assurance of the storage process safety with regards to a potential CO2 leakage from the target zone. Low permeability caprocks are viewed as critical element for a safe containment of CO2 in the target storage formation. As a result, the presence of any potential pathway is of a major concern since it may allow buoyant CO2 to migrate along and reach an overlying formation or be emitted at the surface, potentially impacting drinking water resources or sensitive stakes at the surface. Undesired CO2 migration may occur through man-made (abandoned and operational wells) and/or natural (caprock sealing defects including faults, fractures and high permeability areas) pathways. Managing a potential CO2 leakage risk scenario is of first importance and a dedicated, site-specific strategy has to be set up. The storage safety could be guaranteed through an adequate site selection and characterization leading to the choice of a site where the evolution of the CO2 plume and potential impacts of the storage are judged acceptable. Specifically to this given storage site, a proper risk-management process should be set up to anticipate the potential deviations from this acceptable behavior, including assessing the risks, monitoring the site to detect any potential loss of confinement, mitigating any potential leakage and remediating possible impacts. This study is focused on the risk treatment stage, more specifically on the methodologies and technologies for mitigation and remediation of unpredicted CO2 migration in the subsurface. To date this subject has not been addressed effectively although it is currently receiving more attention from the industrial and scientific communities. This evolution may be linked with the new regulations on CO2 geological storage, notably in Australia, Europe and in the USA, which specify requirements on mitigation and remediation methods. A comprehensive knowledge of mitigation techniques is needed to meet these requirements, which can be summarized with the three following categories: - Technical issue - the first challenge is to determine which ones of the existing technologies applied on the source, the transfer pathway or directly on the impacts may be adapted to avoid, reduce or correct any potential impacts induced by a CO2 migration. In other words, these are the scientifically conceivable measures; - Operational issue - from an operational perspective, the maturity of a technology is essential to ensure its feasibility. The achievability of one measure is dependent on additional criteria especially on the balance between the benefits (impact avoided) and the costs (economic costs and potential environmental negative impacts of the measure). The second challenge is therefore to specify properly those criteria for the measures and to develop tools to assess the relevance of each conceivable measures; - Implementation issue - the third challenge is, based on the knowledge available at a given time, to produce an intervention plan as required by the above-cited regulations. This plan should answer to the identified risk events, and also prepare the operator and the competent authority to make an informed decision for choosing the best mitigation and/or remediation option at the time of the detection of an abnormal behavior in the CO2 storage complex. The technical issue has already been tackled and with several technical papers devoted to specific techniques aiming at reducing the potential risks and impacts of CO2 based on either existing work conducted in other fields (e.g. oil and gas industry, soil or water remediation) or specifically developed for CO2 storage. However, new categories of remediation techniques have recently appeared based on newly emerging technologies. In addition there is a need of comparative data to help operators to select the most adapted measure. Decision-making tools are also required to balance the benefits gained by the implementation of a mitigation measure and its economic costs and potential environmental impacts. Finally, there is a lack of integrated studies on the mitigation plan setting-up process. For instance, no comparison between the different intervention strategies of existing and future CO2 storage projects has been published. There is thus a need of gathering the best practices for mitigation of undesired CO2 migration based on the scientific literature and experience gained from various CO2 geological projects. In line with these statements, BRGM and IRIS conducted a literature and experience review on the methodologies and technologies for mitigation of undesired CO2 migration in the subsurface on behalf of the IEA Greenhouse Gas R&D Program. The state of knowledge of CO2 leakage mitigation and remediation technologies has been first presented from a technical point of view. Thus, for different scenarios, the potential actions for avoiding, reducing or correcting impacts caused by an unwanted CO2 migration have been reviewed. The choice of the appropriate measure strongly depends on the nature of the leak. The intervention on leakage through man-made pathways (well remediation) stems from the oil and gas industry experience, and for some of them are considered as standard operations. However, in some cases of man-made leakage pathways, and more importantly in most cases of natural ones, the operator may not be able to rely on the well engineering experience, and may rather rely on either fluid management techniques or new breakthrough technologies for modifying the leaking paths or fluid properties. In case of an impacting CO2 migration, measures may be applied to remediate environmental impacts. The state of knowledge presented in this study integrates both measures that are standard and technically feasible at the present time as well as innovative and under development ones. For each measure and when possible, a technical description of the intervention technique, generic cost and time elements (intervention delay, efficiency time duration) as well as a discussion on the maturity of the technique are provided. Decision-making tools (cost-benefit, cost-effectiveness and multi-criteria analyses) integrating those criteria to help choosing the optimal series of remediation actions are presented in this study. The third part of the work is a review of the existing plans and guidelines for designing such intervention plans. A qualitative survey has been performed among a list of CO2 and natural gas storage sites. Recommendations - research and development directions or best practices - are proposed according to the outcomes of the entire study.

International audience; The development and implementation of the CO2 Capture and Storage (CCS) technology is highly dependent on the assurance of the storage process safety with regards to a potential CO2 leakage from the target zone. Low permeability caprocks are viewed as critical element for a safe containment of CO2 in the target storage formation. As a result, the presence of any potential pathway is of a major concern since it may allow buoyant CO2 to migrate along and reach an overlying formation or be emitted at the surface, potentially impacting drinking water resources or sensitive stakes at the surface. Undesired CO2 migration may occur through man-made (abandoned and operational wells) and/or natural (caprock sealing defects including faults, fractures and high permeability areas) pathways. Managing a potential CO2 leakage risk scenario is of first importance and a dedicated, site-specific strategy has to be set up. The storage safety could be guaranteed through an adequate site selection and characterization leading to the choice of a site where the evolution of the CO2 plume and potential impacts of the storage are judged acceptable. Specifically to this given storage site, a proper risk-management process should be set up to anticipate the potential deviations from this acceptable behavior, including assessing the risks, monitoring the site to detect any potential loss of confinement, mitigating any potential leakage and remediating possible impacts.This study is focused on the risk treatment stage, more specifically on the methodologies and technologies for mitigation and remediation of unpredicted CO2 migration in the subsurface. To date this subject has not been addressed effectively although it is currently receiving more attention from the industrial and scientific communities. This evolution may be linked with the new regulations on CO2 geological storage, notably in Australia, Europe and in the USA, which specify requirements on mitigation and remediation methods.A comprehensive knowledge of mitigation techniques is needed to meet these requirements, which can be summarized with the three following categories:- Technical issue – the first challenge is to determine which ones of the existing technologies applied on the source, the transfer pathway or directly on the impacts may be adapted to avoid, reduce or correct any potential impacts induced by a CO2 migration. In other words, these are the scientifically conceivable measures;- Operational issue – from an operational perspective, the maturity of a technology is essential to ensure its feasibility. The achievability of one measure is dependent on additional criteria especially on the balance between the benefits (impact avoided) and the costs (economiccosts and potential environmental negative impacts of the measure). The second challenge is therefore to specify properly those criteria for the measures and to develop tools to assess the relevance of each conceivable measures;- Implementation issue – the third challenge is, based on the knowledge available at a given time, to produce an intervention plan as required by the above-cited regulations. This plan should answer to the identified risk events, and also prepare the operator and the competent authority to make an informed decision for choosing the best mitigation and/or remediation option at the time of the detection of an abnormal behavior in the CO2 storage complex.The technical issue has already been tackled and with several technical papers devoted to specific techniques aiming at reducing the potential risks and impacts of CO2 based on either existing work conducted in other fields (e.g. oil and gas industry, soil or water remediation) or specifically developed for CO2 storage. However, new categories of remediation techniques have recently appeared based on newly emerging technologies. In addition there is a need of comparative data to help operators to select the most adapted measure. Decision-making tools are also required to balance the benefits gained by the implementation of a mitigation measure and its economic costs and potential environmental impacts. Finally, there is a lack of integrated studies on the mitigation plan setting-up process. For instance, no comparison between the different intervention strategies of existing and future CO2 storage projects has been published. There is thus a need of gathering the best practices for mitigation of undesired CO2 migration based on the scientific literature and experience gained from various CO2 geological projects.In line with these statements, BRGM and IRIS conducted a literature and experience review on the methodologies and technologies for mitigation of undesired CO2 migration in the subsurface on behalf of the IEA Greenhouse Gas R&D Program.The state of knowledge of CO2 leakage mitigation and remediation technologies has been first presented from a technical point of view. Thus, for different scenarios, the potential actions for avoiding, reducing or correcting impacts caused by an unwanted CO2 migration have been reviewed. The choice of the appropriate measure strongly depends on the nature of the leak. The intervention on leakage through man-made pathways (well remediation) stems from the oil and gas industry experience, and for some of them are considered as standard operations. However, in some cases of man-made leakage pathways, and more importantly in most cases of natural ones, the operator may not be able to rely on the well engineering experience, and may rather rely on either fluid management techniques or new breakthrough technologies for modifying the leaking paths or fluid properties. In case of an impacting CO2 migration, measures may be applied to remediate environmental impacts.The state of knowledge presented in this study integrates both measures that are standard and technically feasible at the present time as well as innovative and under development ones. For each measure and when possible, a technical description of the intervention technique, generic cost and time elements (intervention delay, efficiency time duration) as well as a discussion on the maturity of the technique are provided. Decision-making tools (cost-benefit, cost-effectiveness and multi-criteria analyses) integrating those criteria to help choosing the optimal series of remediation actions are presented in this study. The third part of the work is a review of the existing plans and guidelines for designing such intervention plans. A qualitative survey has been performed among a list of CO2 and natural gas storage sites. Recommendations – research and development directions or best practices - are proposed according to the outcomes of the entire study.