- Email: [email protected]
Integrated path towards geological storage
Energy 29 (2004) 1339–1346 www.elsevier.com/locate/energy
Integrated path towards geological storage R. Bouchard , A. Delaytermoz TOTALFINAELF Exploration and Production, Research and Development CSTJF, Avenue Larribau, 64018 Pau cedex, France
Abstract Among solutions to contribute to CO2 emissions mitigation, sequestration is a promising path that presents the main advantage of being able to cope with the large volume at stake when considering the growing energy demand. Of particular importance, geological storage has widely been seen as an effective solution for large CO2 sources like power plants or refineries. Many R&D projects have been initiated, whereby research institutes, government agencies and end-users achieve an effective collaboration. So far, progress has been made towards reinjection of CO2, in understanding and then predicting the phenomenon and fluid dynamics inside the geological target, while monitoring the expansion of the CO2 bubble in the case of demonstration projects. A question arises however when talking about sequestration, namely the time scale to be taken into account. Time is indeed of the essence, and points out the need to understand leakage as well as trapping mechanisms. It is therefore of prime importance to be able to predict the fate of the injected fluids, in an accurate manner and over a relevant period of time. On the grounds of geology, four items are involved in geological storage reliability: the matrix itself, which is the recipient of the injected fluids; the seal, that is the mechanistic trap preventing the injected fluids to flow upward and escape; the lower part of the concerned structure, usually an aquifer, that can be a migration way for dissolved fluids; and the manmade injecting hole, the well, whose characteristics should be as good as the geological formation itself. These issues call for specific competencies such as reservoir engineering, geology and hydrodynamics, mineral chemistry, geomechanics, and well engineering. These competencies, even if put to use to a large extent in the oil industry, have never been connected with the reliability of geological storage as ultimate goal. This paper aims at providing an introduction to these interactions and examining the consequences of storing CO2 underground over long period of time. It is indeed of tremendous importance, if geological storage is to become an accepted solution from both a technical and social point of view, to focus from now on long term issues. # 2004 Published by Elsevier Ltd.
Corresponding author. Tel.: +33-55983-6055; fax: +33-55983-4481. E-mail address: [email protected] (R. Bouchard).
0360-5442/$ - see front matter # 2004 Published by Elsevier Ltd. doi:10.1016/j.energy.2004.03.069
1340
R. Bouchard, A. Delaytermoz / Energy 29 (2004) 1339–1346
1. Introduction Geological storage of carbon dioxide is more and more considered as an option for greenhouse gas emission mitigation. When large flowrates of CO2 are to be handled, like power plants, refineries or any other facilities requiring large energy production, an effective option consists of capturing CO2 from flue gases and injecting it underground into appropriate geological formations. Oil and gas reservoirs, depleted or not, are usually considered for this purpose, but also deep saline aquifers like in Sleipner case, or else saline cavities. 2. Fundamental principles of geological storage In Fig. 1, CO2 injection, and its subsequent storage, is conveniently pictured by a three-axis diagram: the fluids are on one axis, the geological formation on the second one, and time, maybe the most important parameter, on the third one. . Injected fluids are not composed of only pure CO2. They are a mixture containing a majority of CO2 in which other components can strongly impact the behaviour of the considered fluid, for instance modifying density, viscosity, and chemical reactivity. . The geological formation accepting the injected fluids comprises the matrix itself, its closure and the fluids previously in place. The matrix is composed of a porous media characterised by its physical parameters and mineral composition. The closure is defined by the caprock
Fig. 1. CO2 injection, and its subsequent storage.
R. Bouchard, A. Delaytermoz / Energy 29 (2004) 1339–1346
1341
sealing the formation on its top and laterally, although sometimes closing on a fault. The lower part of the structure is set either by a change in quality of the porous material or by the presence of a bottom aquifer. The formation fluid is at equilibrium (chemical, physical and mechanical) with the trap. . Then time, for which four main phases can be determined. Prior to injection the characteristics of the formation condition directly the potential of a geological storage. They are the result of the geological history and possible human activities, therefore including depletion of oil and gas reservoirs. Second, the injection phase has a duration of 30–50 years. During this period, substantial modifications of the system will take place: drilling of wells, injection of extraneous substances, modification of pressure conditions. Then stabilisation occurs over the post-injection period at the end of which a new equilibrium between fluid and formation is reached. The last phase is the geological storage itself which will last for a very long period of time. This characteristic makes it very specific among any other man-derived activities.
Fig. 2. Possible leakage paths from a geological structure.
The illustration displayed above aims at exposing the problem in simple terms. However, the reality is far more complex because of the interference between all parameters pertaining to each axis. They cannot be taken into account in an independent manner because they influence each other. It is therefore essential to understand all these interactions in order to select an appropriate site for geological storage of CO2.
1342
R. Bouchard, A. Delaytermoz / Energy 29 (2004) 1339–1346
2.1. Time-dependent analysis of geological storage The three main technical issues for which time constitutes a prevalent factor can be summarised under the form of questions such as: what is the injectivity of fluids during the injection phase? How will the re-injected fluids evolve in the formation, while stabilisation is taking place? How will the fluids remain trapped in the formation after stabilisation? During the injection phase, the main concern for the operator is to guarantee that the flowrate of injected fluid will be accepted by the formation and that technical shortcomings, possibly detrimental to the continuity of the operations, are thus avoided. Injectivity can, however, be affected by a downgrading of the physical properties of the porous material (pore plugging) in the vicinity of the well bore, caused by: 1. formation of emulsions resulting from incompatibility between the injected fluid and the fluids in place; 2. precipitation and deposition of organic or mineral substances (e.g. asphaltenes, mineral scales, etc.); 3. Transportation and settling of fines (corrosion products, solid particles, etc.) coming along with the injected fluids; 4. Impact of relative permeability when the mixture injected into the formation is under multiphase flow conditions. We are thus facing a very conventional problem of reservoir engineering and management that the oil industry is already handling on a day to day basis. The stabilisation phase will last as long as the movements of the injected fluids in the formation can be monitored. It will therefore require the implementation of a fully comprehensive monitoring policy based on non-intrusive techniques. Four-dimensional seismics, as in Sleipner, will certainly be used in spite of its cost, but will be supplemented by other techniques allowing for monitoring on a continuous basis. Special attention will be given to the behaviour of decommissioned injection wells, considering that remedial intervention is still possible, should any trouble occur, before natural ageing of the storage system. The main issue to be addressed during the geological storage phase will be its integrity so as to ensure reliability and safety towards the public and the environment. Considering the duration of the period during which geological storage will be effective (several thousands of years is commonly heard of), abandonment—and all its consequences—must be accepted as a concept principle. The industry must therefore clarify all pending questions in relation with very slow phenomena, like chemical reactions, and punctual (and generally brutal) events like earthquakes. 2.2. A view across the technical disciplines involved in geological storage Phenomena involved in geological storage can be connected in a transverse approach through technical disciplines. Fundamentals needed to describe and explain phenomenology, and then to model and predict the phenomena, derive from a set of competencies that have been broadly developed for the needs of the oil and gas industry. Table 1 provides a list of the technical dis-
R. Bouchard, A. Delaytermoz / Energy 29 (2004) 1339–1346
1343
Table 1 Technical disciplines involved and their interaction with the geological storage system
ciplines potentially involved, and illustrates the way they interact within each main element of the geological storage system. These issues cannot be addressed only by discipline specialists, it is indeed absolutely vital to gather a multi-disciplinary team of specialists who will pool their talents in order to: (1) isolate and resolve major concerns within their respective field of expertise, (2) identify problems where fundamentals must be disentangled and (3) highlight unresolved convoluted topics upon which further work is required.
3. Identification of leakage sources Fig. 2 summarises the possible leakage paths in a symbolic geological structure, assumed to be a depleted oil reservoir in this particular case. v The critical point of CO2 is 73.82 bars and 31.04 C, which can be encountered at a depth of about 800–1500 m, dependent on local land surface temperature, heat flow and lithology. Therefore, in most cases CO2 will be stored under supercritical conditions in the formation, i.e. with a density low enough to ensure that it will migrate upwards. Short term consequence is a lightening of the fluid column in the formation inducing additional mechanical stress of the cap rock. On a longer term perspective, this accumulation of injected fluid in the upper part of the storage could favour leakage by migration though the caprock. These mechanisms are not common knowledge spread through the industry and must be further investigated. Chemical reactivity, apart from dissolution along the injection path as noted in EOR CO2 operations and known as ‘‘worm-holing’’, may also impact the matrix wherever CO2 has been trapped and dissolved into water. Highly complex chemical reactions will therefore take place, whereby some minerals will be dissolved while others precipitate, that are likely to affect the mechanical integrity of the reservoir.
1344
R. Bouchard, A. Delaytermoz / Energy 29 (2004) 1339–1346
Presence of faults and fractures can be in some cases very detrimental to the reliability of CO2 storage. The very low viscosity of super-critical CO2, as well as the type of operations envisaged, make CO2 storage very similar to natural gas storage operation in this respect. Therefore, technologies used in this last industry are likely to be valid for this specific application. Whether or not caprock permeability to CO2 remains questionable (see above), the assessment of fracture network is central to the assessment of leakage potential. An argument frequently put forward by experts and favourable to long term integrity of geological storage is that oil and gas reservoirs have contained pressurised hydrocarbon fluids, for millions of years and in a very reliable way. Even if injection operations are conducted carefully and in a professional manner (to avoid spillage by high points of the formation), a major issue arises nevertheless from the injection well itself. Whether we like it or not, these man-made elements have breached the integrity of the storage and will necessitate careful consideration in relation to their design, positioning and trajectory, with all possible consequences on the development architecture of the field. Drilling and completion technology is well known within the oil and gas industry, however, limited to relatively short periods of time, in line with typical hydrocarbon production operations. When CO2 geological storage becomes the objective, then the long term behaviour of the wells becomes all-important, with a special emphasis on abandonment technologies. Fortunately topics raised by well abandonment are mostly addressing cement integrity issues and are very similar to those which have been mentioned when dealing with geological formation: flow properties, chemical reactivity, mechanical behaviour, and ageing of these characteristics. Special care will be given to the development of the industry’s ability to manage the materials and their physical interfaces over time, including those involving metal products. The general philosophy underlying well abandonment is indeed the replication of the cap rock properties and its continuity breached by the well.
4. The risk management approach 4.1. General The general objective of risk management is to insure that all precautions have been taken in order to avoid unexpected damages to a project; most industries evolving in a competitive and technological context tend to apply its recommendations. Risk management should be implemented as soon as possible in any project, because the opportunity to influence risks decrease with time as original choices are made. Risk management methodology consists in a four-step approach as follows: identification of hazards and probability of occurrence, assessment of the magnitude of their consequences, definition of mitigation measures (so that the final situation would be acceptable within the envelope of residual risks) and monitoring of the effectiveness of implemented actions. The major difficulty encountered when adhering to this methodology comes from the lack of statistical data, replaced by qualitative estimates based on development of scenarios.
R. Bouchard, A. Delaytermoz / Energy 29 (2004) 1339–1346
1345
4.2. Risk management and geological storage The objectives of risk management, very commonly shared, are to ensure the health and safety of people, respect of the environment, compatibility with applicable regulations (to be defined), gaining public acceptance (local population and NGOs) and promotion of the company image and reputation. However its implementation in the context of geological storage is specific for the following reasons: . the technology has not proved its efficiency yet; . all technical issues have not been addressed and players do not share the same terms of reference; . stakes are large and go beyond mere industrial objectives (greenhouse gas emissions abatement); . storage reliability is related to large time scale; . the public is barely aware of geological storage possibility and its correlated issues; . there is no existing appropriate regulation. 4.2.1. Risk modeling Risk can be defined as impact multiplied by probability, however, this calculation is not adequate for risk resulting from small impact but high probability, or high impact but small probability, as for geological storage purposes. The evolution of a geological storage system will indeed require new and complex numerical modelling techniques, that might be suspicious to other parties: to be convincing, the proof of storage reliability must be simple enough and accessible to many. When quantitative data are not available, a first useful screening tool is the risk acceptability matrix. The objective is to determine what could be the acceptable impacts and probabilities of the different risks to the public and the environment. In Table 2, the value of the risk is simply the greatest value between the impact and the probability: this calculation then expresses the greatest form of risk aversion (risk propensity would be the converse). 4.2.2. Risk trend The approach discussed above is, however, limited by our theoretical and mathematical capacity to compute absolute thresholds, worsened by the impact of the time parameter that down-grades even further the accuracy of computations. The difficulty to provide positive assessments, either for favourable or unfavourable cases, reduces the domain of favourable cases where geological storage could be envisaged and shrinks it even further when the effect of time-dependent parameters and phenomena is contemplated. Table 2 Risk acceptability matrix Risk acceptability matrix
Impact
Probability
High Medium Low
Low
Medium
High
High Medium Low
High Medium Medium
High High High
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R. Bouchard, A. Delaytermoz / Energy 29 (2004) 1339–1346
A way to overcome the former difficulty is provided by the risk trend concept. This is the system’s propensity to evolve towards a risky or not risky status, in other words the comparison of the next period’s risk level with the present period. Thus, even if a risk analysis is valid for a given period of time, and cannot therefore forecast future risks, the computation of a favourable risk trend can give confidence in the fate of the system. 4.2.3. Duration over which risks must be forecast Taking into account the above considerations and keeping in mind that CO2 remains potentially dangerous over time, it is necessary to determine the period over which risks will have to be predicted. If the system can reach equilibrium (the phase of stabilisation) the time it will take to attain this status can be contemplated as the basis for determining the duration of the period for risk assessment, as long as the equilibrium will be independent of time. This approach does not preclude the work on low probability, external disruptive events such as earthquakes or changes in the water table. Finally, the essential key is to know the level of risk that our society can accept, particularly those that can be passed onto future generations. If indeed no monitoring or maintenance obligations could be left to our descendants, then it will be necessary to prove geological storage reliability over a period of acceptable duration corresponding to the stabilisation phase. This issue needs tight co-ordination between industrialists, researchers and governments. 5. Conclusion As current studies and research projects should confirm the ability of geological reservoirs to store carbon dioxide, other studies must focus on associated risks (deviance from the expected processes and disturbances from external events) in order to check if storage remains reliable in the long term, i.e. if carbon dioxide does not leave the reservoir to leak eventually into the atmosphere. We must be fully aware that decisions made today will inevitably lead to the transfer of risks onto future generations. This is a very serious issue, not only limited to the technical players, that needs to be carefully addressed by all stake holders who have to agree on grounding principles, regardless of their affiliation.
Integrated path towards geological storage R. Bouchard , A. Delaytermoz TOTALFINAELF Exploration and Production, Research and Development CSTJF, Avenue Larribau, 64018 Pau cedex, France
Abstract Among solutions to contribute to CO2 emissions mitigation, sequestration is a promising path that presents the main advantage of being able to cope with the large volume at stake when considering the growing energy demand. Of particular importance, geological storage has widely been seen as an effective solution for large CO2 sources like power plants or refineries. Many R&D projects have been initiated, whereby research institutes, government agencies and end-users achieve an effective collaboration. So far, progress has been made towards reinjection of CO2, in understanding and then predicting the phenomenon and fluid dynamics inside the geological target, while monitoring the expansion of the CO2 bubble in the case of demonstration projects. A question arises however when talking about sequestration, namely the time scale to be taken into account. Time is indeed of the essence, and points out the need to understand leakage as well as trapping mechanisms. It is therefore of prime importance to be able to predict the fate of the injected fluids, in an accurate manner and over a relevant period of time. On the grounds of geology, four items are involved in geological storage reliability: the matrix itself, which is the recipient of the injected fluids; the seal, that is the mechanistic trap preventing the injected fluids to flow upward and escape; the lower part of the concerned structure, usually an aquifer, that can be a migration way for dissolved fluids; and the manmade injecting hole, the well, whose characteristics should be as good as the geological formation itself. These issues call for specific competencies such as reservoir engineering, geology and hydrodynamics, mineral chemistry, geomechanics, and well engineering. These competencies, even if put to use to a large extent in the oil industry, have never been connected with the reliability of geological storage as ultimate goal. This paper aims at providing an introduction to these interactions and examining the consequences of storing CO2 underground over long period of time. It is indeed of tremendous importance, if geological storage is to become an accepted solution from both a technical and social point of view, to focus from now on long term issues. # 2004 Published by Elsevier Ltd.
Corresponding author. Tel.: +33-55983-6055; fax: +33-55983-4481. E-mail address: [email protected] (R. Bouchard).
0360-5442/$ - see front matter # 2004 Published by Elsevier Ltd. doi:10.1016/j.energy.2004.03.069
1340
R. Bouchard, A. Delaytermoz / Energy 29 (2004) 1339–1346
1. Introduction Geological storage of carbon dioxide is more and more considered as an option for greenhouse gas emission mitigation. When large flowrates of CO2 are to be handled, like power plants, refineries or any other facilities requiring large energy production, an effective option consists of capturing CO2 from flue gases and injecting it underground into appropriate geological formations. Oil and gas reservoirs, depleted or not, are usually considered for this purpose, but also deep saline aquifers like in Sleipner case, or else saline cavities. 2. Fundamental principles of geological storage In Fig. 1, CO2 injection, and its subsequent storage, is conveniently pictured by a three-axis diagram: the fluids are on one axis, the geological formation on the second one, and time, maybe the most important parameter, on the third one. . Injected fluids are not composed of only pure CO2. They are a mixture containing a majority of CO2 in which other components can strongly impact the behaviour of the considered fluid, for instance modifying density, viscosity, and chemical reactivity. . The geological formation accepting the injected fluids comprises the matrix itself, its closure and the fluids previously in place. The matrix is composed of a porous media characterised by its physical parameters and mineral composition. The closure is defined by the caprock
Fig. 1. CO2 injection, and its subsequent storage.
R. Bouchard, A. Delaytermoz / Energy 29 (2004) 1339–1346
1341
sealing the formation on its top and laterally, although sometimes closing on a fault. The lower part of the structure is set either by a change in quality of the porous material or by the presence of a bottom aquifer. The formation fluid is at equilibrium (chemical, physical and mechanical) with the trap. . Then time, for which four main phases can be determined. Prior to injection the characteristics of the formation condition directly the potential of a geological storage. They are the result of the geological history and possible human activities, therefore including depletion of oil and gas reservoirs. Second, the injection phase has a duration of 30–50 years. During this period, substantial modifications of the system will take place: drilling of wells, injection of extraneous substances, modification of pressure conditions. Then stabilisation occurs over the post-injection period at the end of which a new equilibrium between fluid and formation is reached. The last phase is the geological storage itself which will last for a very long period of time. This characteristic makes it very specific among any other man-derived activities.
Fig. 2. Possible leakage paths from a geological structure.
The illustration displayed above aims at exposing the problem in simple terms. However, the reality is far more complex because of the interference between all parameters pertaining to each axis. They cannot be taken into account in an independent manner because they influence each other. It is therefore essential to understand all these interactions in order to select an appropriate site for geological storage of CO2.
1342
R. Bouchard, A. Delaytermoz / Energy 29 (2004) 1339–1346
2.1. Time-dependent analysis of geological storage The three main technical issues for which time constitutes a prevalent factor can be summarised under the form of questions such as: what is the injectivity of fluids during the injection phase? How will the re-injected fluids evolve in the formation, while stabilisation is taking place? How will the fluids remain trapped in the formation after stabilisation? During the injection phase, the main concern for the operator is to guarantee that the flowrate of injected fluid will be accepted by the formation and that technical shortcomings, possibly detrimental to the continuity of the operations, are thus avoided. Injectivity can, however, be affected by a downgrading of the physical properties of the porous material (pore plugging) in the vicinity of the well bore, caused by: 1. formation of emulsions resulting from incompatibility between the injected fluid and the fluids in place; 2. precipitation and deposition of organic or mineral substances (e.g. asphaltenes, mineral scales, etc.); 3. Transportation and settling of fines (corrosion products, solid particles, etc.) coming along with the injected fluids; 4. Impact of relative permeability when the mixture injected into the formation is under multiphase flow conditions. We are thus facing a very conventional problem of reservoir engineering and management that the oil industry is already handling on a day to day basis. The stabilisation phase will last as long as the movements of the injected fluids in the formation can be monitored. It will therefore require the implementation of a fully comprehensive monitoring policy based on non-intrusive techniques. Four-dimensional seismics, as in Sleipner, will certainly be used in spite of its cost, but will be supplemented by other techniques allowing for monitoring on a continuous basis. Special attention will be given to the behaviour of decommissioned injection wells, considering that remedial intervention is still possible, should any trouble occur, before natural ageing of the storage system. The main issue to be addressed during the geological storage phase will be its integrity so as to ensure reliability and safety towards the public and the environment. Considering the duration of the period during which geological storage will be effective (several thousands of years is commonly heard of), abandonment—and all its consequences—must be accepted as a concept principle. The industry must therefore clarify all pending questions in relation with very slow phenomena, like chemical reactions, and punctual (and generally brutal) events like earthquakes. 2.2. A view across the technical disciplines involved in geological storage Phenomena involved in geological storage can be connected in a transverse approach through technical disciplines. Fundamentals needed to describe and explain phenomenology, and then to model and predict the phenomena, derive from a set of competencies that have been broadly developed for the needs of the oil and gas industry. Table 1 provides a list of the technical dis-
R. Bouchard, A. Delaytermoz / Energy 29 (2004) 1339–1346
1343
Table 1 Technical disciplines involved and their interaction with the geological storage system
ciplines potentially involved, and illustrates the way they interact within each main element of the geological storage system. These issues cannot be addressed only by discipline specialists, it is indeed absolutely vital to gather a multi-disciplinary team of specialists who will pool their talents in order to: (1) isolate and resolve major concerns within their respective field of expertise, (2) identify problems where fundamentals must be disentangled and (3) highlight unresolved convoluted topics upon which further work is required.
3. Identification of leakage sources Fig. 2 summarises the possible leakage paths in a symbolic geological structure, assumed to be a depleted oil reservoir in this particular case. v The critical point of CO2 is 73.82 bars and 31.04 C, which can be encountered at a depth of about 800–1500 m, dependent on local land surface temperature, heat flow and lithology. Therefore, in most cases CO2 will be stored under supercritical conditions in the formation, i.e. with a density low enough to ensure that it will migrate upwards. Short term consequence is a lightening of the fluid column in the formation inducing additional mechanical stress of the cap rock. On a longer term perspective, this accumulation of injected fluid in the upper part of the storage could favour leakage by migration though the caprock. These mechanisms are not common knowledge spread through the industry and must be further investigated. Chemical reactivity, apart from dissolution along the injection path as noted in EOR CO2 operations and known as ‘‘worm-holing’’, may also impact the matrix wherever CO2 has been trapped and dissolved into water. Highly complex chemical reactions will therefore take place, whereby some minerals will be dissolved while others precipitate, that are likely to affect the mechanical integrity of the reservoir.
1344
R. Bouchard, A. Delaytermoz / Energy 29 (2004) 1339–1346
Presence of faults and fractures can be in some cases very detrimental to the reliability of CO2 storage. The very low viscosity of super-critical CO2, as well as the type of operations envisaged, make CO2 storage very similar to natural gas storage operation in this respect. Therefore, technologies used in this last industry are likely to be valid for this specific application. Whether or not caprock permeability to CO2 remains questionable (see above), the assessment of fracture network is central to the assessment of leakage potential. An argument frequently put forward by experts and favourable to long term integrity of geological storage is that oil and gas reservoirs have contained pressurised hydrocarbon fluids, for millions of years and in a very reliable way. Even if injection operations are conducted carefully and in a professional manner (to avoid spillage by high points of the formation), a major issue arises nevertheless from the injection well itself. Whether we like it or not, these man-made elements have breached the integrity of the storage and will necessitate careful consideration in relation to their design, positioning and trajectory, with all possible consequences on the development architecture of the field. Drilling and completion technology is well known within the oil and gas industry, however, limited to relatively short periods of time, in line with typical hydrocarbon production operations. When CO2 geological storage becomes the objective, then the long term behaviour of the wells becomes all-important, with a special emphasis on abandonment technologies. Fortunately topics raised by well abandonment are mostly addressing cement integrity issues and are very similar to those which have been mentioned when dealing with geological formation: flow properties, chemical reactivity, mechanical behaviour, and ageing of these characteristics. Special care will be given to the development of the industry’s ability to manage the materials and their physical interfaces over time, including those involving metal products. The general philosophy underlying well abandonment is indeed the replication of the cap rock properties and its continuity breached by the well.
4. The risk management approach 4.1. General The general objective of risk management is to insure that all precautions have been taken in order to avoid unexpected damages to a project; most industries evolving in a competitive and technological context tend to apply its recommendations. Risk management should be implemented as soon as possible in any project, because the opportunity to influence risks decrease with time as original choices are made. Risk management methodology consists in a four-step approach as follows: identification of hazards and probability of occurrence, assessment of the magnitude of their consequences, definition of mitigation measures (so that the final situation would be acceptable within the envelope of residual risks) and monitoring of the effectiveness of implemented actions. The major difficulty encountered when adhering to this methodology comes from the lack of statistical data, replaced by qualitative estimates based on development of scenarios.
R. Bouchard, A. Delaytermoz / Energy 29 (2004) 1339–1346
1345
4.2. Risk management and geological storage The objectives of risk management, very commonly shared, are to ensure the health and safety of people, respect of the environment, compatibility with applicable regulations (to be defined), gaining public acceptance (local population and NGOs) and promotion of the company image and reputation. However its implementation in the context of geological storage is specific for the following reasons: . the technology has not proved its efficiency yet; . all technical issues have not been addressed and players do not share the same terms of reference; . stakes are large and go beyond mere industrial objectives (greenhouse gas emissions abatement); . storage reliability is related to large time scale; . the public is barely aware of geological storage possibility and its correlated issues; . there is no existing appropriate regulation. 4.2.1. Risk modeling Risk can be defined as impact multiplied by probability, however, this calculation is not adequate for risk resulting from small impact but high probability, or high impact but small probability, as for geological storage purposes. The evolution of a geological storage system will indeed require new and complex numerical modelling techniques, that might be suspicious to other parties: to be convincing, the proof of storage reliability must be simple enough and accessible to many. When quantitative data are not available, a first useful screening tool is the risk acceptability matrix. The objective is to determine what could be the acceptable impacts and probabilities of the different risks to the public and the environment. In Table 2, the value of the risk is simply the greatest value between the impact and the probability: this calculation then expresses the greatest form of risk aversion (risk propensity would be the converse). 4.2.2. Risk trend The approach discussed above is, however, limited by our theoretical and mathematical capacity to compute absolute thresholds, worsened by the impact of the time parameter that down-grades even further the accuracy of computations. The difficulty to provide positive assessments, either for favourable or unfavourable cases, reduces the domain of favourable cases where geological storage could be envisaged and shrinks it even further when the effect of time-dependent parameters and phenomena is contemplated. Table 2 Risk acceptability matrix Risk acceptability matrix
Impact
Probability
High Medium Low
Low
Medium
High
High Medium Low
High Medium Medium
High High High
1346
R. Bouchard, A. Delaytermoz / Energy 29 (2004) 1339–1346
A way to overcome the former difficulty is provided by the risk trend concept. This is the system’s propensity to evolve towards a risky or not risky status, in other words the comparison of the next period’s risk level with the present period. Thus, even if a risk analysis is valid for a given period of time, and cannot therefore forecast future risks, the computation of a favourable risk trend can give confidence in the fate of the system. 4.2.3. Duration over which risks must be forecast Taking into account the above considerations and keeping in mind that CO2 remains potentially dangerous over time, it is necessary to determine the period over which risks will have to be predicted. If the system can reach equilibrium (the phase of stabilisation) the time it will take to attain this status can be contemplated as the basis for determining the duration of the period for risk assessment, as long as the equilibrium will be independent of time. This approach does not preclude the work on low probability, external disruptive events such as earthquakes or changes in the water table. Finally, the essential key is to know the level of risk that our society can accept, particularly those that can be passed onto future generations. If indeed no monitoring or maintenance obligations could be left to our descendants, then it will be necessary to prove geological storage reliability over a period of acceptable duration corresponding to the stabilisation phase. This issue needs tight co-ordination between industrialists, researchers and governments. 5. Conclusion As current studies and research projects should confirm the ability of geological reservoirs to store carbon dioxide, other studies must focus on associated risks (deviance from the expected processes and disturbances from external events) in order to check if storage remains reliable in the long term, i.e. if carbon dioxide does not leave the reservoir to leak eventually into the atmosphere. We must be fully aware that decisions made today will inevitably lead to the transfer of risks onto future generations. This is a very serious issue, not only limited to the technical players, that needs to be carefully addressed by all stake holders who have to agree on grounding principles, regardless of their affiliation.