Deep geological storage of CO2, offshore UK – Supplementary note A: Breakdown of combined well and geological storage risks for typical storage sites

This note is supplementary to the report “Deep Geological Storage of CO2 on the UK Continental Shelf: Containment Certainty”, and other associated supplementary notes. It is aimed at a technically experienced scientific audience.

The purpose of this Supplementary Note is to provide further detailed tables showing how the leakage risks have been combined for both mechanical and geological pathways, to derive indicative overall risks for ‘typical’ permitted CCS storage sites that could be commissioned for the UK Continental Shelf (UKCS).

The different leakage pathways are defined and further described in the main report and other Supplementary Notes. Tables in this note combine the probabilities of occurrence, reasonable worst case leakage rates and durations per feature, and an estimate of typical numbers of features for typical sites, to derive a risked estimate of the minimum contained amount of CO2. These numbers underpin the values given in Table 20 in the main report. For discussion of the significance of these tables please see either the corresponding supplementary note or the main report.

The probability of any leakage event occurring from a UKCS storage site that has been granted a storage permit is inherently low. To understand the combined total significance of these different potential leakage events for a store, it is necessary to combine probability of occurrence for the different leakage pathways. To provide an illustration of how geological and well leaks could combine to produce an overall view on the total amount and probability of leaks during the operational and post-closure period, two ‘typical’ storage complexes have been conceived and overall containment probabilities derived. Parameters were chosen to reflect realistic values for ‘typical’ depleted oil and gas fields and confined saline aquifer sites, and to allow evaluations of risk to be scaled-up easily for future sites that contain varying numbers of specific leakage pathways. For ease of comparison, the typical depleted fields and confined aquifers are defined to have the same overall parameters where reasonable, except where characteristic differences between the two are likely to exist for permitted sites (e.g. size of monitoring area, the number of decommissioned wells and faults, brine production wells).

Parameters used to estimate leakage risk for typical depleted field and fully and partially confined saline aquifer CO2 stores on the UK Continental Shelf (UKCS).

Notes:
Well leakage rates are more accurately measured than geological leaks. Many sources on leakage rates from wells are quoted in tonnes per day, whereas geological leakage rates are less accurately measured and stated in tonnes per year. Numbers in the tables that are derived by converting between rates per year and per day may be reported with multiple decimal places; this is to make the derivation of the numbers clear, and does not imply either correspondingly high precision or accuracy in the stated values.