FAQ

According to the EC Directive 2009/31/EC - Guidance Document 3, the life cycle of a CO₂ storage site can be generally subdivided into 6 phases as it is shown in the figure below.

 

CO₂ Storage Life Cycle Framework (Source: European Commission: Implementation of Directive 2009/31/EC on the Geological Storage of Carbon Dioxide - Guidance Document 3)

 

CO₂CARE deals with phases 5 and 6, post-closure/pre-transfer and post transfer of liability. The main goals of the project are closely linked to the three high-level requirements of the EU Directive 2009/31/EC, Article 18 for CO₂ storage which are: 1. absence of any detectable leakage, 2. conformity of actual behaviour of the injected CO2 with the modelled behaviour, and 3. evolving of the storage site towards a situation of long-term stability.
These criteria have to be fulfilled prior to subsequent transfer of responsibility to the competent authorities, typically 20 or 30 years after site closure.
Considering this, CO₂CARE aims formulating robust procedures for site abandonment which will meet the regulatory requirements and ensure long-term integrity of the storage complex.

 

 Project brochure (pdf)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

In 2005 the EC introduced its CO₂ Emissions Trading Scheme (ETS), a key instrument for the EU climate policy. This initiative includes all large industrial point CO2 sources (any from 20 MW capacity (thermal) upwards). The scheme applies only to CO₂ emissions directly to the atmosphere. The trading scheme comprises between 10.000 and 12.000 industrial companies in Europe belonging to energy, steel, glass, and cement production. All these companies are responsible for about 40% greenhouse gas emissions in the EU. From 2013 onwards, ETS will include the aviation sector as well.
Each industrial plant receives permission to release a certain amount of CO₂ per year, also called EU allowances (EUA, emission right for 1 tonne of CO₂). Any excess allowances can be sold via some of the Carbon Trading Exchanges. If a company exceed the limit, they in turn have to buy extra allowances. Tradable units are the so-called Certified Emission Reductions, abbreviated CER. One CER is equivalent to one metric ton of CO₂ emissions (tCO₂e). Companies taking part in the CO₂ Emissions Trading Scheme can buy or sell directly to other companies or at several Carbon Trading Exchanges, for instance the European Energy Exchange, the Asian Carbon Exchange (ACX-change) or the NASDAQ OMX Commodities Europe.
 

Before CO₂ storage becomes operational, the location has to be selected carefully in terms of geological suitability, storage capacity, long-term stability, and long-term safety (Phase 1-3 = assessment, characterisation and development). This is probably the most important step in the lifecycle of a CO₂ storage site (also see “What are the main goals of CO₂CARE?”).  ..
In order to ensure negligible impacts on humans or the environment in the long term, specific criteria needs to be defined for each individual CO₂ storage site. In this respect, international requirements and guidelines for CO₂ storage must provide rules for safety. Both are key issues of the CO₂CARE project.
Development, operational phase, as well as a transfer of responsibility after site closure will always be escorted, supported, and controlled by the responsible authorities. 
Potential leakage pathways for CO₂ have to be considered from the beginning of a project in order to ensure long-term safety and anticipating any remediation action. Such potential pathways are: through and along wellbores, improperly abandoned old wells within the storage complex, and unsealed fault or fracture zones through the natural caprock. Another potential threat for the environment is the displacement of brines (salty water) in storage formations caused by CO₂ injection. This could eventually lead to salinisation of potable shallow aquifers in some quite specific circumstances.
All these potential CO₂ and saltwater pathways need to be monitored (see FAQ 10) during the injection phase and for some period of time after site closure. Therefore, risk management and the assessment of suitable monitoring tools or methodologies are further key issues in CO₂CARE.
Each potential risk scenario of a storage site candidate is analysed in detail in order to ensure the safety of the storage site. The probability of the occurrence of irregularities and related consequences for humans and environment are evaluated so that a safe operation can be carried out.

 

After CO₂ has been injected for instance into a deep lying saline aquifer, the gas is trapped in different ways by physical and chemical mechanisms strongly dependent on time and on geological settings.
Generally, four basic trapping mechanisms for long-term stabilisation and immobilisation of CO₂ are: structural and stratigraphic, capillary (residual), dissolution, and mineral trapping.

Structural and stratigraphic trapping
This type of physical trapping of CO₂ happens at the top of an anticline or in a tilted fault block below low-permeability seals (caprocks), such as shale or salt beds. It is the principal means to store CO₂ in geological formations. Sedimentary basins have such closed, physically bound traps or structures, which are occupied mainly by saline water, oil and/or gas including natural CO₂ such as in Montmiral. Structural traps include those formed by folded or fractured rocks. Faults can act as permeability barriers in some circumstances and as preferential pathways for fluid flow in others. Structural/stratigraphic trapping takes place immediately after injection. The CO₂ is less dense than the saline water and therefore it rises upwards into the trap.

Capillary (residual) trapping
Simply explained, porous sedimentary rocks behave like a sponge in which the injected CO₂ is trapped within the pore spaces initially brine saturated. However, the trapping mechanism is strongly influenced by the geometry of the sedimentary rocks. The arrangement of pore spaces, the interconnectivity of the pore space, and the local capillary forces are responsible for this type of trapping. The most important mechanism for residual capillary trapping is the snap off, which is a disconnection of a small amount of CO₂ of a continuous stream of the non-wetting CO₂ when it passes through pore constrictions. CO₂ is trapped in pore spaces due to surface tension. Over time, the CO₂ trapped in pore spaces can dissolve into the formation water.


Solubility trapping
When CO₂ comes in contact with brine, as it is the case in a reservoir, part of the gas dissolves in the formation water (CO₂(aq) + H₂O  ↔  H⁺ + HCO₃- ) at specific conditions of pressure, temperature and salinity. This process is commonly called solubility trapping and is very beneficial because groundwater saturated with CO₂ is denser than before and so will tend to sink in the reservoir. CO₂ dissolution is a significant trapping mechanism therefore and saturating formation brine with CO₂ would create huge CO₂ storage capacities. However, complete solubility trapping is a long-term process operating on timescales of hundreds to thousands of years depending on convection effects, reservoir petrophysical properties and aquifer dynamism.

Mineral trapping
Over time, chemical reactions between the surrounding minerals of the reservoir rock and the CO₂-rich water will take place and leads to precipitation of new mineral phases such as  calcite (CaCO₃), dolomite (CaMg(CO₃)₂), siderite (FeCO₃), or dawsonite (NaAlCO₃(OH)₂). This is the most stable form of CO₂ trapping. However, under favourable conditions this conversion process of CO₂ into carbonate minerals is extremely slow and needs thousands of years.