Deep geological storage of CO2, offshore UK: containment certainty – main report

The UK Government has committed to reach Net Zero by 2050. Carbon Capture and Storage (CCS) is expected to play a critical role in delivering the UK’s Net Zero Strategy, reducing carbon dioxide (CO2) emissions, and ensuring that the UK meets this target.

The effects of climate change are already evident, both globally and here in the UK. The UK Climate Change Committee (the UK’s independent climate advisory body) has described CCS as ‘a necessity, not an option’ for the transition to Net Zero. CCS offers a way to mitigate climate change whilst progressing towards the decarbonisation of energy, transport, and industry, and is critical to achieving the UK’s 2050 Net Zero target. The government aims to establish four CCS industrial clusters by 2030, capturing 20-30Mt CO2 across the economy (equivalent to annual emissions from 4.2 to 6.3 million British households). Two clusters have already been selected for ’Track-1’ negotiations, with a view to becoming operational by the mid-2020s (HyNet and the East Coast Cluster).

Deep geological storage of CO2 is the secure containment of CO2 in CCS systems. The 2019 Global CCS Institute (GCCSI) Report states that 260Mt CO2 has already been safely stored (this includes in storage sites within saline aquifers and depleted fields, and Enhanced Oil Recovery), and estimates indicate a current storage rate of around 40Mt per year globally. However, there is not yet sufficient direct experience to extract reliable statistics on long-term containment certainty. This report provides an up-to-date synthesis and estimation of the containment certainty of CO2 in deep geological storage sites on the UK Continental Shelf (UKCS). Relatively few studies have estimated leak rates, probabilities and durations as has been done here, although many studies address part of the picture.

Estimates of containment probabilities for two example sites modelled over 25 years of injection operations and 100 years of post-injection monitoring indicate that more than 99.9% of the injected CO2 will be retained within the storage complex. The two modelled sites are designed to reflect the features of ‘typical’ UK offshore sites, for depleted fields and saline aquifer stores within permitted storage complexes. While the risks will vary on a site-specific basis, the results (see summary table of results below) indicate a very high level of confidence in the long-term security of CO2 containment in typical CCS storage complexes on the UKCS.

Containment risks are divided into geological and well leakage pathways. A containment issue via a well leakage pathway is likely to be faster to remedy than via a geological leakage pathway. The type of storage site also has an impact on the containment risk. Typical sites can be within depleted oil or gas fields or saline aquifers (sites that contain saline water instead of oil or gas). Whilst containment risks for both well and geological pathways are extremely low, generally, depleted field storage sites have a higher likelihood of a well containment issue occurring than a geological containment issue. A saline aquifer store may have higher geological risk than a depleted oil or gas field store because there is less familiarity with the storage site and its geological sealing characteristics. However, irrespective of CO2 store type, the well containment risk may be lower if fewer wells have been drilled in the past at the storage site. Wells that were decommissioned prior to CCS being planned for the site are likely to have a higher risk. It is also possible for leakage paths to be a combination of well and geological leakage paths.

The leaks that are more likely to occur, for both geological and well leakage pathways, are at lower leak rates. The authors interpret the possibility of major or moderate leakage rates from geological features on sites that have been awarded a storage permit in the UK to be improbable from either a depleted field or a saline aquifer store. Significant loss of containment events (at higher leakage rates) via well pathways are very improbable and unlikely to happen, particularly where the injection wells are designed and constructed to modern standards and operated within the requirements of the NSTA (North Sea Transition Authority) storage permit. Thus, the risks associated with containment are not significant for a site that has a storage permit, compared to the net benefit of industrial scale deep geological storage of CO2, which is critical to deliver on the UK 2050 Net Zero Strategy.

The combined risks for each individual site will be specific to the geology and well history at that site. The UKCS is a well-regulated environment, and a CO2 storage site will only be granted a CO2 storage permit if the NSTA is satisfied that under the proposed conditions of use of the storage site, there is no significant risk of leakage or harm to the environment or human health. This further reinforces the degree of confidence in CO2 containment that may be placed in a storage site that has received a permit.

To calculate the statistical probable ‘worst-case’ leakage from an example storage complex over its injection life and post closure period, the maximum probability of occurrence and maximum leak rate have been used. This provides a conservative calculation of the risked estimate of overall leakage. The overall leakage calculation also assumes the maximum leak rate continues for the full duration until remediation is accomplished (recognising that some geological leaks may be of longer duration, albeit at a low rate). This is a conservative approach as the majority of well leaks can be shut in (i.e. mechanically isolated from the external environment) relatively quickly using other valves or installing temporary plugs. This approach therefore gives confidence in the security of CO2 containment in deep geological storage sites in the UK continental shelf, and that in reality the degree of containment may be better than the levels stated in this study.

Statistical estimates of reasonable worst-case leakage amount from two ‘typical’ CO2 storage complexes that have been awarded a storage permit in the well-regulated UKCS regime.

This report was commissioned by the Department for Business, Energy & Industrial Strategy (BEIS) from a group of independent expert advisors under contract to BEIS as part of its CCUS delivery programme (through the WSP CCUS Technical Framework contract).

The main report is written for a non-technical audience, to inform on the containment certainty of CCS. The supplementary notes provide the technical syntheses, analyses and assessments underpinning the summarised results within the main report and are written for a technical readership.