Reviewing the implications of unlikely but potential CO2 migration to the surface or shallow subsurface

Overview

CO2 leakage from geological storage is considered unlikely from properly selected sites and the potential impacts small when compared to other anthropogenic and natural stressors. However, it is important to predict and understand potential environmental impacts and risks to human health from a range of leak scenarios in order to undertake appropriate monitoring and mitigation necessary to meet both regulatory and societal expectations.

Migration of CO2 to the surface, subsurface or into potable water reservoirs poses a risk, and although is predicted to be rare and limited in quantity, cannot be excluded completely, especially via abandoned wells, along fault surfaces or via gas chimneys. Very strict interpretation of regulatory requirements may impose very high costs or limit the number and size of storage site unnecessarily, thus potentially reducing the regional and global storage resources severely.

Given the important role that carbon capture and storage (CCS) plays, as a part of negative emission technologies (NETs) and emission reduction technologies, in most climate scenarios (IPCC, IEA and others), a balance between the merits of CCS on a global scale and the potential risks at a local scale, needs to be evaluated. A growing body of knowledge, gleaned over the past two decades into the environmental impact of leaked CO2 have included studying the impact of CO2 release in: natural seings, potable aquifers, via laboratory and controlled release experiments and modelling. It was desirable, therefore, that these learnings were summarised and evaluated in a clear and accessible document that would be of value to policymakers, project developers and regulators.

Summary

• Climate change is a global phenomenon and a threat. The benefits of the removal of significant volumes of greenhouse gas emissions to the atmosphere via carbon capture and storage (CCS) far outweigh the very low likelihood of material local environmental impacts associated with any leakage at a local scale.
• The probability of leakage via geological features when storing CO2 at well-characterised sites is negligible for storage in depleted hydrocarbon fields and extremely low for saline aquifer storage. However, it is possible for compromised well or surface integrity issues to facilitate migration or release of CO2 out of its storage reservoir and this could be material in individual cases. Well leakage can be easily detected, and mitigation and remediation are possible. Multiple barriers and partial migration into various intervening aquifers, buffering, dissolution and residual saturation will reduce the total volume ending up in groundwater, soils, the ocean or the atmosphere.
• Extremely high concentrations of CO2 in the ocean, groundwater or the atmosphere are demonstrated in exceptional cases of natural CO2 leakage and can cause material harm to the environment, but are improbable in the case of CO2 geological storage.
• Multiple reviews of laboratory and field experiments on the remobilisation of other harmful substances as a result of CO2 leakage concludes that material impacts are unlikely under CO2 leakage conditions from a well-characterised and monitored geological storage project.
• In the absence of actual CO2 leakage examples from CCS projects, leakage rates and risks are limited to synoptic studies and simulations often based on experience and data from the petroleum industry. Rates based on analogues can vary substantially and the probability of leakage also has a wide range.
• In the event of a material leak, consequences to the local environment could have direct environmental and health implications that should be used in environmental impact assessments using local knowledge and jurisdiction-specific regulatory thresholds. This study provides a non-site-specific and general framework.