3.3 Leakage pathways

Five different leakage events are identified in Guidance Document 1 of the EU CCS Directive. These include caprock deficiency, faults and fractures, structural spill of the trap, updip leakage through high permeable intervals and transport of dissolved CO2 out of the storage complex. These are all very different in nature and are effects of the geological and anthropogenic conditions of the storage complex. There is, therefore, no single solution to all these possible leakage events, hence the importance of treating each possible leakage event separately.

Not all potential leakage scenarios will be relevant to the various geological storage options of CO2, since the trapping mechanisms of CO2 are different depending on the geological conditions of the chosen storage site. Within the FRAM, potential leakage pathways have also been identified and their potential impacts during the life cycle of a storage project have been delineated (Fig 3-1).

Leakage can be predicted and also possibly avoided using numerical modelling, and flow modelling in particular, of the storage complex. The many processes, e.g. hydrogeological and geochemical processes, acting within a reservoir, are commonly interlinked and dependent on each other. Therefore, all dependent processes must be considered during modelling in order to create the most accurate model, including rock property data, e.g. porosity, permeability, relative permeability, capillary pressures, fluid saturation and mineralogy. A list of different numerical codes that are used in modelling are listed in NETL (2011).

There are technologies to be used in order to avoid leakage, and decrease the effects of potential leakage from a geological storage of CO2. Benson and Hepple (2005) summarised a number of actions that can be taken to mitigate leakage from the storage reservoir in different scenarios. As the nature of a geological storage of CO2 is that it is defined by natural geological boundaries, these are not likely to be "fixed" if leakage occurs. Therefore leakage remediation actions are about controlling the CO2 within the storage reservoir and surrounding formations. Controlling actions include reduction of pressure within the storage reservoir, increasing the pressure in the geological formation into which CO2 is leaking and interception and extraction of the injected CO2 before it leaks out of the storage reservoir.

 

in depth

3.3.1 Permeable caprock

The caprock is the impermeable part of the storage complex which will prevent CO2 from migrating and finally leaking out...

3.3.2 Faults and fractures

The site characterisation should include mapping and modelling of existing faults and fractures of the storage complex i...

3.3.3 Wells

Wells are known possible anthropogenic leakage pathways from a CO2 storage site. In the risk assessment and site charact...

3.3.4 Other anthropogenic leakage events

Hydraulic fracturing is a method which is commonly used in shale gas production and is also used in methane production f...