Module 8 Health, safety and environmental risks of CCS projects

Original text: J. Stephens & D. Keith, APEC Capacity Building in the APEC Region, Phase II Revised and updated by CO2CRC


Understanding the health, safety and environmental risks associated with geological storage of CO2 involves consideration of both the potential hazards of the technology, and the likelihood that those hazards will occur. The storage project can then be designed to control the risks. The potential hazards of a poorly designed CCS project can be understood through the examination of natural CO2 leakage events, although discussion must emphasize the difference between these events and the expected behavior of CO2 in long-term geological storage. This module mainly covers the hazards associated with CO2 storage, although some mention is made of transporting CO2.

Learning objectives

By the end of this module you will:

  • Understand the types and scales of risks associated with CO2 storage in geological reservoirs;
  • Be aware of the potential human and ecological health hazards which could result from CO2 leakage;
  • Gain an appreciation of how to assess the likelihood of hazard occurrence; and
  • Know the level of current risk attributed to CO2 storage leakage.

Types and scales of risk associated with CO2 storage

The risks associated with storing CO2 underground can be considered on two different scales: local and global (Figure 8.1).

Figure 8.1 Risks associated with storing CO2 underground.

At the local scale, potentially hazardous impacts may result from three mechanisms:

Leakage of CO2 from the storage location through the subsurface into the atmosphere – this leakage could occur through isolated, catastrophic events - such as an earthquake - or through sustained, slow venting of CO2 due to improper storage site selection or preparation. Either of these forms of leakage would result in elevated CO2 concentrations at the surface or in the shallow sub-surface that could negatively impact human health and safety as well as that of plants and animals living in the area. Figure 8.2 demonstrates the various pathways for CO2 leakage.

Alteration of groundwater chemistry resulting from CO2 dissolving in it – such a chemical change in groundwater that is used for drinking water could impact human health. Alterations in groundwater not used for human consumption may have impacts on the ecosystem it is in contact with.

Displacement of fluids previously occupying the underground space where the CO2 is injected – by injecting CO2 gas underground, salty brine water could be forced out into drinking water reserves. The increased pressure of this type of displacement could cause fractures or other physical changes in the subsurface rock.

Figure 8.2: Schematic diagram of possible pathways by which CO2 might leak to the surface (courtesy of Sally Benson).

The local risks of leakage are dependent on the location and timing of the leak. Continued and dispersed leakage will have very different impacts than episodic and isolated leakage events. For example, while slow but sustained leakage could gradually alter long-term soil ecosystems, a sudden, distinct leakage event could cause instantaneous disruption.

At a global scale, the major risk is that leakage of CO2 injected into geologic formations will limit the effectiveness of the initiative in reducing the global atmospheric CO2 concentration. This global risk, therefore, can be alternatively viewed as uncertainty in the effectiveness of CO2 containment and of CO2 storage as a climate change solution.

The global risk of leakage is dependent only on the average quantity of CO2 released from the storage site over time. This will be a reflection of the contribution of the project to reducing atmospheric CO2 emissions.

Finally, because of the energy penalty, the additional energy required to capture and store CO2, more fuel will be required per unit of delivered energy if CO2 is captured. Everything else being equal, therefore, there will be a corresponding increase in the various environmental impacts and risks associated with fuel production.

Potential human and ecological health hazards associated with leaked CO2

Both human health and safety impacts, as well as ecosystem impacts, must be considered in evaluating the potential risks associated with CO2 leaking from an underground storage site. In addition to the possibility of catastrophic leaks such as well blowouts or pipeline ruptures where large amounts are CO2 suddenly released, slow, less-obvious leaks also need to be considered.

Although there is currently minimal experience with engineered CO2 storage and no examples of leakage from existing projects to draw from, several naturally occurring CO2 underground reservoirs (natural analogues) that have released CO2 provide valuable insight into the types of hazards that could be anticipated at engineered sites. The primary natural analogues that have been studied are on the flanks of Mammoth Mountain, California (Fig. 8.3), at several locations in Italy and at Lake Nyos in Cameroon. Lake Nyos waters had been gradually saturated with CO2 from volcanic vents over a period of time. They suddenly released a huge amount of CO2 during the night which blanketed a local town killing 1,700 people. A fluctuating but constant flux of CO2 has been flowing from underground at Mammoth Mountain into the atmosphere for about 15 years, killing the trees in several distinct areas and altering the soil and water chemistry in the region. As case studies, they provide a useful basis for understanding both the ecosystem and human health hazards associated with CO2 leakage. It is important to note that these natural analogues are very different to the deep stable subsurface sedimentary storage basins that would be the preferred locations for engineered CO2 storage. Natural analogues are located in highly fractured volcanic zones and are not well suited to understanding the likelihood of leakage from a CO2 storage site.

In addition to these natural analogues, additional information relevant to both the potential hazards and the likelihood of occurrence can be gained through industrial experience of underground injection and in situations of humans operating in closed environments such as submarines and aircraft. This industrial experience includes the underground injection of CO2 to enhance oil recovery, store natural gas, and dispose of hazardous and non-hazardous waste.

Figure 8.3. Aerial view of Mammoth Mountain, California where a natural source of underground CO2 has been leaking through the soil into the atmosphere. The venting CO2 has killed the trees in several distinct areas, visible in the above photograph as the non-green areas close to the lake (courtesy of USGS).

Potential human health and safety hazards

Elevated CO2 concentrations in confined areas

The most serious human health and safety hazard associated with leaking CO2 from an underground storage site is injury or death caused by elevated CO2 concentrations in confined areas. Although CO2 gas generally disperses quickly in the open atmosphere, CO2 is denser than air so it will accumulate in confined environments including basements, tents, under snow-packs and in depressions or pits in the ground. Humans will suffer from unconsciousness and even death at CO2 concentrations above 10%. CO2 also causes significant respiratory and physiological effects in humans at concentrations over 2%. No adverse effects have been observed at concentrations below 1%.

Contamination of drinking water

Storage projects need to be designed to ensure that there is adequate protection of drinking water. The direct effects of dissolved CO2 in drinking water are probably minor, because drinking water is often carbonated with CO2 without any adverse health impacts. Dissolving CO2 in water, however, will increase the acidity of the water which could cause indirect effects including increased mobilization of toxic metals, sulfate or chloride and changes in the odor, color or taste of the water. Groundwater used for drinking water could also be contaminated by saline brine water that is displaced by the CO2 injection. This process could potentially render the drinking water too salty to drink. The infiltration of saline water into groundwater or the shallow subsurface also could pollute surface water and restrict or eliminate the use of some land for agricultural use.

Local heave and seismicity

A well charaterised storage site will consider the seismicity of the area as part of the site selection process (see Module 6). Underground injection of CO2 into porous rock under pressure can induce fracturing and movement of faults. This could cause potentially damaging earthquakes and could result in the creation of additional pathways for CO2 leakage. Several examples of induced seismicity resulting from the industrial practice of underground injection exist including the 1967 Denver earthquake and 1986 and 1987 Ohio earthquakes that are believed to have been induced from deep well injection of waste fluids.

Other potential hazards

A major pressure loss of dense phase CO2 could result in cryogenic burns, embrittlement of equipment and damage from dry ice. It is important to understand how CO2 will behave following a sudden release from pressurised containment. In assessing the safety hazards for pipelines to the storage site, fracture propagation in a pipeline needs to be considered.

Potential ecosystem hazards from CO2 storage

Elevated concentrations of CO2

Potential ecosystem impacts associated with CO2 storage and potential leakage from an underground storage reservoir include effects on plants and animals both below-ground and above-ground. Throughout the underground environment, even in the deep storage sites where CO2 could be injected, there are thriving microbial communities which rely on very specific conditions to live. Drastic changes, such as those associated with injecting CO2, would alter these ecosystems. In the shallower underground environment, elevated concentrations of CO2 could kill or weaken insects and burrowing animals as well as inhibit root respiration by displacing the soil oxygen needed for the roots of plants to function.

Acidification of soils and enhanced weathering

Other potential ecosystem hazards include the acidification of soils as CO2 gas forms an acid when it combines with water. This acidification may directly impact some wildlife, but a potentially more serious indirect impact is the increased release of toxic metals that can result from the enhanced mineral weathering rate as the result of increased acidity.

Alteration of groundwater chemistry

The infiltration of saline brines into groundwater or the subsurface caused by displacement from the CO2 injection also could impact many plants and animals who rely, directly or indirectly, on fresh rather than salty water.

Induced seismicity or ground heave

Ground heave and induced seismicity could also impact local ecosystems by the disruptions associated with earthquakes, including fractures and movement of the ground.

Impacts to off-shore benthic environments

Leakage from offshore geologic storage sites could impact benthic environments as the CO2 moves from deep geologic structures through benthic sediments to the ocean waters. Minimal research has been conducted to assess the potential impacts to benthic communities; however plants and animals in the benthic region that rely on specific CO2 concentrations could be threatened.

Evaluating the risks

To evaluate the risks associated with CO2 storage, the potential hazards associated with a specific event must be considered in conjunction with the probability of the event happening. This section describes what is known about the likelihood of leakage and the likelihood of the hazards associated with it.

Likelihood of hazards associated with leakage

Understanding the probability of CO2 leakage primarily involves assessing the effectiveness of the storage site being considered. Storage effectiveness is dependent on many different site specific factors including geological characteristics, the injection system being adopted, and the methods used to seal and contain CO2 within the injection site. There are no existing studies that systematically estimate storage effectiveness across a sample of different storage sites. Therefore, rough quantitative estimates of achievable storage effectiveness must be attempted drawing from other relevant knowledge and experience of the site.

Data from natural systems, such as natural analogues, demonstrate that large quantities of CO2, methane and oil can be trapped underground for geological timescales. CO2 trapped underground in the Jackson Dome in Mississippi, for example, is thought to have been generated more than 65 million years ago. This demonstrates that reservoir seals exist that are able to provide almost perfect confinement of CO2.

Engineered natural gas storage facilities provide additional insight into the likelihood of leakage. Among the approximately 470 natural gas storage facilities in the US and Canada, there have been nine incidents of significant leakage. Five of these were related to well bore integrity, three arose from leaks in the cap rocks, and the other was caused by early abandonment due to poor site selection. The performance of natural gas storage systems in North America suggests that the annual average gas leak rate is less than 0.01% of the stored gas. The leakage rates of natural gas storage facilities are expected to be higher than leakage from CO2 storage because natural gas systems are designed for rapid pressure/volume cycling. As such, it is anticipated that the risk of CO2 leakage is relatively small.

Off-shore geological storage leakage poses a less significant threat to the health and safety of humans because of its distance from human habitation. The probability of leakage in off-shore storage sites would also be reduced because there are fewer old abandoned wells off-shore than on-shore. Leakage off-shore also may not reduce overall CO2 storage effectiveness as much as it does on-shore because some of the CO2 that leaks out of an off-shore storage site will diffuse in the ocean rather than being re-released to the atmosphere. Although CO2 leakage into the oceans could have some impacts on the local area, the oceans are capable of naturally dissolving and absorbing large quantities of CO2 so leakage into ocean water does not pose the same risks as leakage into the atmosphere. Taken together, the experience with natural and engineered systems suggests that amount of leakage from well designed CO2 storage facilities will be very small. It is reasonable to expect that more than 99% of the CO2 would be retained for over 1,000 years. The ability to assess the health risks of a CCS project accurately is an area of rapid development.

Likelihood of hazards associated with groundwater contamination

The cumulative industrial experience with underground injection of other fluids (eg: oil, natural gas and waste) provides an empirical basis for assessing the likelihood of groundwater contamination and induced seismicity resulting from displacement caused by CO2 injection. The current rates of injection of these other fluids into the deep subsurface are roughly comparable to the rates at which CO2 would be injected if CO2 capture and storage technologies were widely adopted. Contamination of groundwater by brines displaced from injection wells is rare, so it is reasonable to assume contamination resulting from CO2 injection also would be rare.

Likelihood of hazards associated with seismicity

The injection of CO2 for enhanced oil recovery provides a direct basis for estimating seismic risk. Current experience suggests that the risks are very low, as no significant seismic effects have been attributed to the over 30 million tons per year of CO2 that is currently injected for enhanced oil recovery. Only a handful of individual seismic events have been associated with underground injection of other fluids suggesting that the risks of induced seismicity are generally low.

Risk assessment must be an integral component of CO2 storage site selection, site characterisation, storage system design, monitoring and, if necessary, remediation. Current risk assessment methodologies are being adapted to the meet the unique risks posed by CCS projects. Module 9 outlines some of the methodologies that are being applied to commercial and demonstration CCS projects.

The IEA GHG Risk Assessment Network as established in 2005 to address what the regulators are expecting and whether risk assessment can provide the answers they require. The Network is divided into a number of smaller and more specific subject areas, Data Management and Risk Analysis, Regulatory Engagement and Environmental Impacts. The CSLF has a Risk Assessment Taskforce. Another body set up to share expertise on risk assessment is the International Performance Assessment Centre for Geologic Storage of Carbon Dioxide (IPAC- CO2), managed from the University of Regina, Canada.


The risks associated with storing CO2 underground can be considered on two different scales: local and global and can affect both human health and safety, as well as that of ecosystems. Without direct experience to draw from, studies of natural analogues and engineered storage sites have largely provided the basis for understanding and quantifying the health, safety and environmental risks that could arise from CO2 that seeps from the shallow subsurface to the atmosphere.

Local scale risks include CO2 leakage from the storage location; alteration of ground and drinking water chemistry and displacement of potentially hazardous fluids formerly occupying the pore space being used to store the CO2.

At a global scale, CO2 storage could be a major contributor to reducing atmospheric levels of CO2 – one of the precursors to climate change. CO2 leakage into the atmosphere would limit the effectiveness of CO2 storage as a climate change solution.

Potential hazards of CO2 storage to human health include:

  • risk of death or unconsciousness from elevated CO2 concentrations;
  • contamination of drinking water as a result of increased acidity and mobilization of toxic metals; and
  • local ground heave and induced seismicity through fracturing.

Experience with natural analogues and engineered sites suggest that the likelihood of these risks occurring will be minimal.

Off-shore CO2 storage could cause risks to individuals on nearby ships or drilling rigs. However, there are no published studies on the issue and experience with industrial injection of other fuels suggests the risks will be minimal.

Ecosystems also face negative impacts from CO2 leakage including:

  • Damage or death from elevated CO2 concentrations;
  • Acidification of soils and enhanced weathering;
  • Alteration of groundwater chemistry;
  • Induced seismicity or ground heave; and
  • Impacts to off-shore benthic environments.

Proper site selection for CO2 storage is the single biggest factor determining the likelihood and magnitude of the risk. Risk evaluation is a young field and improvement in our understanding of these risks, as well as development of a methodology for risk evaluation on a site-by-site basis, are critical. Although natural analogues and the industrial practice of underground injection have provided valuable insight about potential risks, experience with and analysis of actual CO2 storage projects is needed to allow for accurate risk evaluation. In addition to pointing out the need for better understanding of the risks, it should be pointed out that recent efforts in developing effective tools for monitoring, verification and leakage remediation provide improved approaches for managing, and therefore minimizing, the risks associated with CO2 storage.


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The International Performance Assessment Centre for the Geological Storage of Carbon Dioxide (IPAC- CO2):

IEAGHG Risk Assessment Network: