Geological Storage of CO₂: Seal Integrity Review

Overview

CCS is a well proven technology and can demonstrably safely store carbon dioxide CO₂. The success of geological sequestration of CO₂ depends on the permanent containment of the injected CO₂ in the storage formation at depth. Prior to CO₂ injection commencing, the storage formation and the sealing capacity of the caprock overlying the formation are exhaustively studied to confirm suitability. This includes establishing the boundaries of the reservoir, which provide an effective seal to prevent leaking to either the surface or to adjacent geological formations.

This comprehensive seal integrity review, undertaken by CO₂CRC on behalf of IEAGHG, provides a detailed, updated exploration of the critical aspects of seal potential in the context of the geological storage of CO₂. The review focuses on developments in this field since 2011. It highlights the importance of seals in ensuring the containment of CO₂ and the considerations involved in predicting the long-term impact of CO₂ interactions with seal formations.

Summary

The study reviewed the regulatory frameworks in four of the most mature jurisdictions. As demonstrated in these examples, jurisdictions should include a robust legislative framework to govern reservoir seals and permanent CO₂ storage. These should be regularly updated as technological advances occur. This will also help project developers/operators, who must comply with the requirements, to stay up-to-date with best practices.

CO₂ may be contained in a variety of geological settings ‘plays’, including a conventional reservoir/seal pair, composite seals or utilising residual or mineral trapping.

In the rare instance of leakage, shear failure at the caprock/storage unit interface or fault reactivation poses the primary risk for leakage (not accounting for leakage risk along legacy wells).

Many methods to characterise and evaluate seal integrity rely on access to core samples and well testing, which can be site-specific and localised. Best practice assessments will consider a wide range of sources and use other datasets for a regional approach.

The long-term impacts of CO₂ on seal formations are evaluated and show largely positive results indicating that re-precipitation of minerals may enhance seal potential. In some cases, alteration of cements may impact geomechanical properties.

The impact of pressure on seals at a local scale needs to be paired with basin-scale understanding of the regional stress field, the extent of the seal and potential interaction with adjacent geological CO₂ storage projects. This is an area that requires further research and understanding.

CO₂ controlled release experiments require representative test data and are thus much slower to undertake than running numerical models on faults, however, they offer valuable learnings. There are still scale-up challenges in transferring such learnings to commercial projects at greater depths.