Why store CO2?


Our planet is undergoing a global warming: increase of average air and ocean temperatures, melting of snow and ice, rise of sea levels, modification of atmospheric and oceanic circulation…
Global warming results from too large greenhouse gases emissions in the atmosphere. As CO2 is one of the major gases responsible for climate change, the process of Carbon Capture and Storage (CCS) could reduce substantially those emissions.

Where does the stored CO2 come from?


CO2 can be captured from large point sources such as power plants and industrial processes (oil refineries, cement works, iron and steel production…). These sources are responsible for more than two thirds of global CO2 emissions.
Capture and storage of CO2 from dispersed sources (transport, small-scale heat or power production) is another option but less easily practicable.

How does CCS work?


Carbon is first captured, through separation from other gases produced. Several technologies are available depending on the CO2 state: chemical solvent scrubbing, physical solvent scrubbing, and adsorption/desorption, membrane separation, cryogenic separation... In the case of CO2 capture in power plants, several options can be chosen from: pre-combustion, post-combustion or oxycombustion.
Then, the CO2 is transported by pipeline or ship from the capture site to the storage site.
CO2 is finally stored for hundreds of years into depleted Oil & Gas reservoirs, deep saline formations or unminable coal seams, either onshore or offshore.

Where CO2 could be stored?


Geological storage processes are under development either into depleted oil & gas fields or into end-of- life fields for Enhanced Oil Recovery (EOR).
Other geological options are studied, for instance in unminable coal seams. CO2 injected into the seam could be trapped into the coal and locked up permanently. Moreover, CO2 injection into the seam would induce displacement of the existing methane, which can be then recovered and used.
Another option for geological storage of CO2 is to inject it into a deep saline aquifer, involving its partial dissolution in the water. Targeted aquifers contain water unsuitable for potable water supply (high salt and mineral content).
CO2 could also be stored on the deep ocean sea floor, but further research programmes are required to provide a better understanding of this technique.

Is CO2 storage currently implemented?


Storage of CO2 has taken place for many years within the framework of Enhanced Oil Recovery (EOR). Sleipner, a gas field located in the North sea operated by STATOIL, is the first example of commercial scale CCS site. CO2 separated from natural gas is injected into an underground saline reservoir since 1996.
Sleipner is joined by seven other large-scale CCS projects around the world to store about 23 million tons of CO2 per year. Eight other projects are currently under construction and about sixty additional projects are at the development stage (source : GCCSI « The Global Status of CCS: 2012 »).
It should be noted that geological analogue sites naturally contain CO2 for decades. For instance St. Johns (NM, USA), the Fizzy gas reservoir (North Sea) and other reservoirs in the North German Basin are natural laboratories which present a great opportunity to observe, measure and model the long-term behaviour of geologically stored CO2.
Moreover, underground storage of natural gas is also an established technique, widely practised for about 100 years. Natural gas is traditionally stored on a seasonal rhythm to meet the increased demand in winter, but also for commercial purpose (storing gas when prices are low and selling it when prices are high).
For more information about analogue fields and CO2 injection sites, click here.
For more information about natural gas storage, click here.

How are selected injection sites?


Geological formations considered for storage are located more than a thousand meters deep into the soil. These formations are much deeper than usable sources of freshwater.
CO2 injection sites are carefully selected and assessed by geologists over a number of years prior to the first commercial injection. For instance, a CO2 storage site will not be located where great earthquakes are likely to occur.

During and after the injection, how do we know the CO2 behaviour into the ground?


The status of the stored CO2 is supervised by the combination of:

  • Analogue investigation
    Natural geological analogues where CO2 has been stored for significant periods of time can be evaluated and inform understanding of storage processes.
  • Experimental investigation
    Processes operating during the storage of CO2 are simulated and evaluated in laboratories to ground truth assumptions of storage.
  • Computer modeling
    Mathematically based models are capable of simulating the behaviour of the stored CO2 during the injection and containment phases, as well as the mechanical, thermal and geochemical interactions with the environment.
  • Monitoring techniques
    3-dimensional geophysical surveying techniques as: remote sensing, surface geophysical surveys, core analysis, fluid sampling, interference and tracer tests…
These approaches are complementary techniques to manage any potential CO2 migration.

Is CCS already regulated?


To ensure safety and reliability of CCS activities on a CO2 storage site, an international regulatory framework is required. Specific legislations and standards for CO2 storage are emerging, for instance the European CCS directive 2009/31/EC of 23 April 2009 or the bi-national CCS standard for Canada and the US (CSA Z741). Moreover, Oil & Gas legislations for activities in deep geological formations are already available.
As a complement, certification provided by an external organization will ensure that legal requirements are met. For instance, certification can provide assurance that the site has been selected based on a transparent and recognized process, that it will be managed safely and responsibly according to industry practice, and that the activities are in compliance with regulations and relevant directives.

What are the potential impacts of CO2 storage?


CO2 is not toxic, nor flammable. A CO2 storage site can’t explode.
CO2 might be potentially dangerous if, in case of a sudden large-scale leakage, it would be collected at high concentration in an unventilated structure. In these exceptional circumstances, CO2 may become an asphyxiant.
After the injection, changes in the reservoir (in pressure, pH, and chemical reactions) may lead to unwanted migration of CO2 and/or brine to freshwater reservoirs and possible release of minerals trapped into the rock. It should be reminded that small amounts of CO2 or minerals in water are not harmful and often naturally contained in spring water. Nevertheless, a maximum acceptable level of these elements should not be exceeded.
These phenomena are only possible hazards identified and still need to be characterized in the context of CO2 storage (acceptable concentrations of CO2 or minerals in the water, time frame…). To improve the understanding of potential impacts, these topics are addressed in details in collaborative research projects as PANACEA.
Anyway, in the case of unwanted far field brine migration, the latter could be pumped out. In addition, CO2 could be removed from extracted fresh-water (aeration). In the case of release of trapped minerals, it would still be possible to treat the pumped water. Another option is to create a hydraulic barrier before the freshwater reservoir, thanks to new injection and extraction wells.

How to prevent the risks?


Preventive measures
The risk of sudden large-scale release of CO2 can be avoided as for any other natural gas storage sites, by a thorough site selection (for more information, see “How are selected injection sites?”) and a scrupulous monitoring of the site.
Regulation and certification will ensure that the CO2 storage activities meet all specific requirements.
In other words, a well-chosen and well-engineered site is not expected to leak.

What happens if there is a leakage?


Corrective measures As said above, sudden large-scale leakage of CO2 from a well-chosen site is highly unlikely. Leakage of CO2 from these in-depth reservoirs is expected to be a slow process and should not be a cause of safety concerns.
Slow migrating CO2 may not reach the atmosphere but rather stay trapped within the numerous porous layers of rock between the reservoir and the surface. For instance, suitable aquifers would have a cap rock of low permeability to trap the CO2 and prevent it from flowing towards the Earth’s surface. Moreover, many of the considered reservoirs have already stored gases and liquids for thousands of years (for instance: depleted oil & gas fields or deep saline aquifers).
If a CO2 storage site were leaking, it would be immediately detected thanks to the monitoring instrumentation. The project operators would apply methods used to manage fluid movements in oil and gas reservoirs, or leak mitigation and remediation techniques (ex: reduction of the injection pressure or stop of the operation, injection of cement or viscous slurries to seal the leak, controlled CO2/brine extraction, aeration of soil or groundwater…).

What does PANACEA do?


PANACEA is a research project funded by the European Union, aimed at providing a better understanding of CO2 storage mechanisms to ensure safer CCS operations.
In order to build a deep knowledge on the long term fate of geologically stored CO2, PANACEA objectives are:

  1. To identify and quantify the factors responsible for the long-term stability of the stored CO2.
  2. To provide measures for the assessment of the integrity and vulnerability of the reservoir (storage formation and cap-rock) and wells that penetrate it, to the CO2 stream.
  3. To quantify the impact of the stored CO2 on adjacent subsurface reservoirs.
  4. To identify and develop reliable monitoring, measurement and verification (MMV) technologies having the capacity to capture relevant information on the long-term behaviour of the stored CO2 both at the near and far field.
  5. Achieve an adequate degree of cooperation with other projects and initiatives in order to allow the collection of data necessary for validating the investigations and to allow the dissemination of findings.
For more information about PANACEA objectives, click here.