Carbon capture and storage technologies are essential tools to reduce industrial carbon dioxide emissions and will play a pivotal role in decarbonizing the global economy. For carbon capture to be effective, storage of the captured emissions must be permanent. Geologic storage of captured carbon dioxide provides a means to permanently store captured carbon dioxide deep underground.

Carbon capture is rapidly gaining momentum in Europe, with more than 50 proposed carbon capture projects already announced. This momentum has spurred questions about geologic storage options and the relative permanence of sequestered carbon dioxide. In this blog, we will answer frequently asked questions to illustrate how geologic storage does indeed provide long-term, low-risk, and permanent reduction of carbon dioxide emissions. When done correctly, it is extremely unlikely that injection of carbon dioxide into deep geologic formations will ever reach the atmosphere in significant quantities or pose a risk to public health.

Why store carbon dioxide deep underground?

Geologic storage of captured carbon dioxide can help facilitate the transition to net-zero by permanently storing significant emissions from existing assets in hard-to-abate sectors. Heavy industry accounts for ~20% of total emissions in the EU and there are few to no options for abating these emissions other than carbon capture. Important industries like chemical production, cement, and steel must be future-proofed for a carbon-neutral world and carbon capture can provide a proven, scalable solution for addressing greenhouse gas emissions from these industries. For carbon capture to be effective, the captured carbon must be permanently stored or utilized. While there are ways to utilize captured carbon emissions by transforming them into products, these technologies are mostly experimental and not yet proven at the scale required for deep decarbonization. Geologic storage of carbon dioxide has been safely and effectively implemented at scale for nearly 50 years and according to the EIA’s Net Zero by 2050 report, around 95% of captured carbon will need to be stored permanently via geologic storage.

How much carbon dioxide can be stored in Europe?

In terms of geologic storage availability, Europe is well positioned to deploy large-scale geologic storage to help deliver on climate goals. Significant geologic storage resources exist in Europe across multiple sedimentary basins and storage fairways. According to the EU Geological CO2 Storage Summary report by the Geological Survey of Denmark and Greenland (GEUS) using the CO2StoP dataset, the total saline storage capacity of prospective geologic units in Europe is estimated to be 482 gigatonnes (Gt), which equates to over 300 years’ worth of EU emissions from stationary sources at current rates. This estimate was derived from 28 countries in which sufficient geologic data was available. The map below shows the location of storage areas and associated storage capacities for all the storage units in Europe that were considered for this estimate (Figure 1). A more detailed summary of EU geologic storage resources can be found in this CATF blog.

Figure 1: Location of storage areas and associated capacities across Europe. Source: GEUS, 2021

As geologic storage resources are unequally distributed across Europe, not all countries will be able to store their carbon dioxide within their own borders. Consequently, carbon dioxide storage will require cross-border cooperation and coordination. It is unlikely that geologic storage clusters will be developed in every country. Depending on their saline geologic storage resources, some countries will store carbon dioxide from other countries.

Europe is fortunate to have significant geologic resources for storing carbon dioxide, and next we will discuss how the geologic storage of carbon dioxide works.

How does geologic storage of carbon dioxide work?

Geologic storage of carbon dioxide involves the injection of captured carbon dioxide into deep, porous rock reservoirs that are overlain by sealing rock layers (i.e, caprocks). This prevents the carbon dioxide from getting into the atmosphere and permanently stores it in the subsurface.

In its simplest form, carbon dioxide is injected into a porous rock capped by an impermeable rock, trapping it in place. To achieve the most efficient use of space, captured carbon dioxide is compressed and made denser which reduces the volume of the carbon dioxide and therefore occupies less space in the rock when injected. Prior to starting injection operations, geologists must locate suitable geologic formations capable of safely and efficiently storing carbon dioxide. The following characteristics are that of an ideal storage reservoir system:

• Presence of a suitable reservoir rock formation that is very porous, like a sponge, that can host the injected carbon dioxide
• Reservoir rock with sufficiently interconnected pore space, known as permeability, that allows for carbon dioxide to easily flow into and through the reservoir
• An impermeable “caprock” that overlies the reservoir which acts as a barrier to fluid flow, containing the injected carbon dioxide within the underlying reservoir
• A reservoir that is of sufficient depth, typically ~800m, such that the natural temperature and pressure conditions of the reservoir can keep the injected carbon dioxide dense and in place
• A reservoir system that has excellent mechanical integrity and is free of major faults that carbon dioxide could leak through

The most ideal candidate storage options for large volumes of carbon dioxide are deep saline formations, which refers to any deep rock formations that are filled with non-potable, highly saline (i.e., “salty”) water. Depleted oil and gas fields that are no longer economic for production can also be ideal storage reservoirs provided they have an established caprock. Saline aquifers that are suitable for geologic storage are very deep in the subsurface and are separated from the surface by hundreds to thousands of meters of solid rock formations. Figure 2 illustrates the subsurface scale of a typical saline storage project.

Figure 2: Schematic of a saline storage project with heights of commonly known buildings for reference. Source: EPA

To find subsurface options for storing large volumes of carbon dioxide, scientists must perform reconnaissance work in the form of feasibility studies to collect subsurface geologic data and identify areas that have potential to host suitable geologic formations. For most storage projects, this includes collecting and analyzing geophysical and geochemical data from existing wells, creating 3D models of the subsurface geology and performing injection simulations, drilling a characterization well to collect new geophysical data and rock core samples to confirm the presence of a reservoir, and running geophysical surveys to confirm that the geologic formations are present across the whole study area. Once a potential injection site has been determined to be feasible for injecting carbon dioxide, then the injection facility can be built, and injection operations can commence.

Carbon dioxide storage operations follow very strict engineering and safety protocols to ensure safe and efficient storage. A more detailed explanation of this is provided in the next section.

Is geologic storage safe?

While carbon dioxide is a pollutant in the atmosphere and in drinking water, it is not a pollutant in the deep subsurface where it commonly already exists naturally. Carbon dioxide can be stored in the deep subsurface safely and efficiently by following strict engineering and safety protocols with extremely limited risk of carbon dioxide leaking back into the atmosphere or into shallow drinking water aquifers. Significant volumes of carbon dioxide are already safely being stored in deep geologic formations around the world.

There are more than 20 commercial-scale carbon capture facilities operating globally, permanently capturing and storing around 40 Mt of carbon dioxide annually which is equivalent to the annual emissions from over 8 million passenger cars. The safety and permanence of geologic storage is realized through proper site selection and engineering protocols. Rigorous subsurface geologic modeling and simulation is performed prior to injection to demonstrate that the geologic formations can handle the injected carbon dioxide and that the injected carbon dioxide will stay in place.

Injection wells are engineered to extremely high standards to prevent any leakage of the injected carbon dioxide. Injection well operators are required to not exceed a determined pressure that would cause the rock to fracture, ensuring that the mechanical integrity of the geologic formations is preserved. During carbon dioxide injection operations, the injected carbon dioxide plume is monitored closely using sophisticated geophysical and geochemical monitoring techniques to track the location and size of the plume throughout the entire project. This monitoring ensures that the plume is not traveling anywhere that it shouldn’t, such as near faults. After injection ceases, the plume continues to be monitored to ensure that it has stabilized, and it is not migrating. Once an operator can demonstrate that the plume has stabilized, the injection well is plugged to prevent any carbon dioxide migration up the open hole.

The rigorous engineering and design process summarized above are routine measures taken at every carbon dioxide storage project to ensure the safety, efficiency, and longevity of geologic storage. The technology and methods for injecting, storing, and monitoring carbon dioxide underground are mature and have been practiced since 1972. Multiple commercial scale carbon dioxide storage projects around the world have successfully demonstrated that carbon dioxide can be safely stored in geologic formations. These storage projects undergo rigorous risk and mitigation planning so that in the event of a carbon dioxide leak, injection ceases immediately and appropriate mitigation techniques are applied to identify the leakage mechanism and remedy it. In 2005, the Intergovernmental Panel on Climate Change published a report about the limited risk of carbon dioxide leakage associated with geologic storage.

Regulations in the EU also exist to ensure that sites are adequately monitored for safety. The European Commission’s CCS Directive is the main legislative package that details how carbon dioxide storage can be done in the EU. It provides strict rules to ensure that carbon dioxide storage sites are strictly monitored on a regular basis. Under Article 13 of the CCS Directive, Member States must ensure that storage sites are monitored, while Article 14 outlines that storage operators must report to the competent authorities at least once per year, how well the storage site is operating. Furthermore, Article 15 provides that Member States shall ensure that storage sites are subject to frequent inspections.

Despite the overwhelming data that demonstrates the safety of geologic storage, concerns remain. A common concern is the risk of human-induced earthquakes. When fluids are injected into rock at a pressure that exceed the strength of the rock, such as in shale gas exploration, small cracks may form. This process, commonly known as “fracking,” occasionally results in microseismicity- mini temblors measurable only by extremely sensitive instruments, at levels rarely noticed by humans. However, unlike in gas exploration, geologic storage practices are designed to avoid fracturing. This is because microfractures reduce rather than enhance the volumes of carbon dioxide that can be stored in the reservoir rock.

As mentioned above, operators of geologic storage projects continuously monitor reservoir and injection pressures and are required to keep pressures below that which would fracture the rock. The Illinois Basin Decatur Project in the United States successfully injected over 1 million tonnes of carbon dioxide into a deep saline reservoir and performed an extensive microseismic monitoring study during injection. The results of the seismic monitoring showed that minimal microseismic events occurred that were far smaller than what would be required to potentially compromise the caprock or be felt by humans at the surface.

Proper engineering design and controls ensure that geologic storage can be performed safely and various storage projects around the world have demonstrated this in practice. Next, we will get into how carbon dioxide is actually trapped and securely stored in geologic formations.

How is injected carbon dioxide trapped and how long can injected carbon dioxide remain securely stored?

Ancient fluids like brine and hydrocarbons, as well as carbon dioxide, occur naturally in subsurface geologic formations and have been trapped in the subsurface, in some cases, for hundreds of millions of years. These ancient, naturally occurring fluids provide a valid analog for the ability of subsurface geologic reservoirs to securely retain carbon dioxide over geologic timescales. The oil and gas that we extract from geologic formations has been trapped for millions of years in the subsurface and would remain there for millions of years in the future if we were not actively extracting them.

So how is carbon dioxide trapped? Let’s continue to use hydrocarbons as an analog. Like carbon dioxide, hydrocarbons are less dense than water and thus have migrated buoyantly upward through rocks and fractures over geologic time following their generation until they encountered a geologic “trap.” Simply put, a geologic trap consists of an overlying impermeable rock, which we refer to as a caprock or seal, with an underlying porous rock formation that acts as a reservoir for fluids. Geologic traps can occur in the form of geologic structures like anticlines, which are dome-like structures, faults that have been sealed by mineralization, and stratigraphic traps (Figure 3). In all cases, geologic traps are capable of “trapping” buoyant fluids, preventing them from migrating upward, where they can remain in the subsurface over geologic timescales. When prospecting for oil and gas plays, geologists look to identify areas in the subsurface where geologic traps exist that have accumulated hydrocarbons over time.

Figure 3: Diagram illustrating the different types of geologic traps that exist in the subsurface. Source: University of Calgary

Similarly, when planning a carbon dioxide storage project, a geologist carefully “prospects” for geologic traps that, instead of trapping hydrocarbons, would be capable of structurally trapping injected carbon dioxide.

While structural trapping, as outlined above, is the most dominant trapping mechanism, other trapping mechanisms also occur in geologic formations that provide further security (Figure 4). These mechanisms include solution trapping, residual trapping, and mineral trapping. When injecting into saline aquifers, some of the injected carbon dioxide will slowly dissolve into the brine that exists in the pore space of the rock and becomes permanently trapped via solution trapping. Some portion of the injected carbon dioxide will become trapped in small pore spaces as residual carbon dioxide that is immobile, similar to water in a sponge that is leftover after wringing. This mechanism is referred to as residual trapping. The final mechanism for carbon dioxide trapping occurs when dissolved carbon dioxide reacts with the reservoir rock to form a new mineral, which is called mineral trapping. This mechanism of trapping effectively traps the carbon dioxide permanently in the form of a solid mineral.

Figure 4: Plot illustrating the relative contribution of each trapping mechanism as a function of time following the cessation of injection. Source: IPCC, 2005

When using naturally occurring hydrocarbon deposits as an analog, carbon dioxide, when properly injected into geologic traps, can remain stored in the subsurface for much longer than modern civilization has existed on this planet (i.e., millions of years).

Can faults and fractures allow injected carbon dioxide to migrate to the surface?

Faulting and fracturing may pose a risk to stored carbon dioxide if they provide a route around traps to the surface. In older rocks, however, existing faults and fractures can be filled in with minerals, or mineralized, such that they actually provide excellent traps where impermeable formations have been tectonically juxtaposed against permeable ones. So, where a fault crops up in a geologic review in preparation for a storage project, it is important to determine what type of a fault it is. Robust regulations requiring such detailed geologic study is essential to identify any such features, and to determine whether they are transmissive or impermeable under expected injection conditions.

If permeability is likely, or if there is nearby earthquake risk, a site may not – and should not – qualify for a storage permit. Regulations provide these protections, requiring identification and monitoring of any potentially transmissive faults and fractures. We will discuss how the safe storage of carbon dioxide is regulated in Europe in the next section.

How is the safe storage of carbon dioxide regulated in Europe?

To ensure good governance, with clear rules and responsibilities for the safe storage of carbon dioxide, while ensuring the removal of legal barriers, a strong legal framework is necessary. Fortunately, a legal framework for the safe geological storage of carbon dioxide already exists in Europe. In 2009, the European Union adopted the Carbon Capture and Storage Directive (CCS Directive) as part of the 2009 climate and energy package. The CCS Directive provides a legal framework to ensure the environmentally safe geologic storage of carbon dioxideand creates the necessary legal certainty for investors to construct large-scale carbon dioxide storage sites (including capture and transportation). As described in the 2019 European Commission Report on Implementation of the CCS Directive, this framework aims to ensure that, for any given geologic storage project; 1) there is no significant risk of leakage of carbon dioxide from the geologic reservoir, 2) there is no significant risk of damage to public health or the environment, and 3) there are no adverse effects on the security of the transport network or storage sites. The CCS Directive focuses mostly on the selection of sites for carbon dioxide storage as well as on monitoring, permitting, closure and post-closure obligations. The CCS Directive has been transposed into national law by all EU and EEA Member States, albeit to different extents. Some Member States have directly transposed it into their national legislation and others, like Germany and the Netherlands, have implemented additional and stricter measures.

Who decides where carbon dioxide is to be stored?

Although the CCS Directive provides for a legal framework for the capture, transport and storage of carbon dioxide in the EU and EEA, it is for the Member States to decide how this is to be done.

Article 4 of the CCS Directive makes clear it is for the Member States to decide whether the storage of carbon dioxide is permitted in parts or in the whole of their territory. This will be based on assessments which deem certain areas suitable for the storage of carbon dioxide, such as the EU Geological carbon dioxide Storage Summary report as described above. Under Articles 5 and 6, if an area is deemed suitable for storage, Member States may then permit certain plots for exploration and storage, giving the holder exclusive rights to explore and store carbon dioxide in those designated areas. Article 7 provides strict rules as to how these permits are to be licensed to storage operators, including demonstration of technical expertise and ability to meet the financial requirements which are outlined in more detail in the following sections. Ultimately, it is the competent authority – usually a Ministry for the Environment or Climate – which decides who may explore and store carbon dioxide in a particular area of land and what the terms of those permits are.

Who is liable for possible leakage of carbon dioxide from storage sites?

In the case of potential leakage of carbon dioxide from a storage site, clear rules regarding the liability for breaching carbon dioxide storage regulations as well as potential environmental and climate damages are needed. The CCS Directive, as well as the Environmental Liability Directive (ELD) and Emissions Trading Directive (ETS Directive) provides for three types of liability: liability for corrective measures, liability for environmental damage, and liability for climate damage arising from leakage.

Article 16 of the CCS Directive outlines these forms of liability. The operator must take corrective measures in case of leakages and significant irregularities. Similarly, the operator must take preventive action or remedial action under the ELD where there is an imminent threat or actual environmental damage. Furthermore, emissions trading allowances have to be surrendered in accordance with the ETS Directive for any leaked carbon dioxide.

While storage operators must comply with the rules under the CCS Directive and are liable for carbon dioxide leakages at storage sites in operation, these responsibilities eventually transfer to the State after the storage site has closed and a certain amount of time has elapsed. Article 18 of the CCS Directive provides that these liabilities are transferred to the State after a minimum of 20 years have elapsed since the closure of the storage site. This transfer of responsibility has strict requirements, in particular, that all available evidence indicates that the stored carbon dioxide will be completely and permanently contained. In addition, storage operators must also provide a financial contribution, to be used for monitoring and any other expense arising from the maintenance of the storage site, which is provided for under Article 20.

In sum, the inherent characteristics of rock formation and carbon storage analogs, accompanied by regulatory requirements for planning, injecting, storing and monitoring injected carbon dioxide, strongly suggest that well-sited geologic storage is low risk for the long-term, and is permanent. Project leakage will be a rare exception and limited in magnitude due to what we know about the physics of geologic trapping of injected carbon dioxide, decades of experience with injection of carbon dioxide and other analogs, and existing regulatory requirements for selecting and operating carbon dioxide injection and storage sites, including requirements for injection well construction and mechanical integrity. This is the permanence needed to meet carbon dioxide reduction goals that geologic storage provides offers for captured carbon dioxide emissions.

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