Background

Site characterisation for each potential geological storage site is carried out to identify those where it is extremely unlikely that any CO₂ leakage would occur. Extensive Risk Assessments and MMV plans will be performed and designed for each selected site.  However, it is also important to have a mitigation and remediation plan in place in the unlikely event that migration of CO₂ out of the storage complex occurs.

A primary role of risk management is to drive the development of the monitoring program to be best equipped to identify unexpected movement of CO₂ in the subsurface, either within the target zone or beyond, but prior to any potential migration to the near-surface.  As part of the risk management structure, methods for mitigating, preventing and, if needed, remediating, adverse effects related to any unexpected behaviour will be part of the overall MMV and operating plans.

Many of the methods for mitigation span a range of specialties, and have not often been part of ongoing discussions on aspects of geologic storage; however, regulators, operators, and some laypersons are interested in methods that can mitigate unpredicted CO₂ movement.

Mechanisms that could lead to migration out of the storage complex and potentially leakage to the atmosphere or seepage into potable aquifers could include equipment failure e.g. wells, fault activation due to over-pressurisation, geochemical reactions between the CO₂ and the caprock and migration through weak points in the caprock.  There are therefore a number of leakage/ migration scenarios that will need to be considered.

In the case of migration up wells and leakage to the surface, there are known methods for reparation that are used in other industries, such as the oil and gas industry, these include replacing the injection tubing and packers and plugging leaks behind the casing with cement. Mud can be pumped down an interception well in case of well blow out. Wells that cannot be repaired may be plugged and abandoned.

CO₂ may also migrate out of the storage formation, either from fractures in the caprock or migration through the caprock if the capillary threshold pressure is exceeded. There are a number of possible solutions to this, including reduction of pressure in the storage formation by stopping/ reducing injection or increasing the number of wells. Extracting formation water from the storage reservoir may allow steering of the CO₂ plume and will reduce the pressure. The pressure could also be increased in the overlying aquifer or upstream by water injection, thus forming a pressure barrier. It may also be possible to plug with low permeability materials.

In case of migration out of the confining structure from an unknown cause, the first step would be to stop injection, then begin investigation into the source of the migrating CO₂, by checking pressure and well logs and reviewing the local geology. Using this information, shallower zones can be drilled to locate the source and migrating CO₂ can be controlled by lowering the pressure in the storage zone or creating a hydraulic barrier. It may also be able to be plugged and the storage operation may have to be reconfigured to take account of the new information.

If CO₂ were to migrate into potable groundwater, any accumulations of CO₂ can be removed by drilling wells to intersect and extract them. CO₂ can also be extracted in the dissolved phase using extraction wells and aerating it. If the groundwater has been contaminated by other substances that have been mobilised by CO₂, then pump and treat methods may need to be applied. Hydraulic barriers could also be used to immobilise and contain the contaminants.

Leakage of CO₂ could adversely affect the vadose zone, ecosystem and surface water all of which would need remediating. CO₂ can be extracted from soil-gas by vapour extraction techniques by drilling wells. As CO₂ is a dense gas, it could be collected in subsurface trenches, extracted and reinjected or vented. Acidification of the soils from contact with CO₂ could be remediated by irrigation or drainage.

There also needs to be a consideration of leakage into the atmosphere. For large releases spread over a large area, dilution may occur from natural atmospheric mixing, otherwise fans could be used.

There have recently been modelling studies, looking at using extraction wells to remove CO₂. Some preliminary results show that this method may work fairly well on smaller plumes, but appear less effective with larger plumes. However, remediation of larger plumes may be more effectively carried out by simultaneous CO₂ extraction and injection of water.

CO₂GeoNet, a consortium based in Europe, was commissioned by IEAGHG to undertake this study.

Key Messages

• Migration of CO₂ can occur via engineered or natural pathways. Mitigation methods related to wells are the best established.

• Measures related to natural pathways are likely to be via pressure management methods.

• There are many novel technologies available, though current mitigation plans for existing projects include only mature technologies. However, mitigation plans should be flexible to take account of newer technologies when they become available and allow holisitic decision making.

• All mitigation and remediation plans should be site-specific and risk-based.