Overview
The requirements for closure of CO2 storage sites and transfer of responsibility are still evolving as the CO2 industry grows internationally. Conformance refers to the degree to which modelled behaviour matches measured data and meets storage permit requirements. Alongside this, effective containment of CO₂ and brine – demonstrated by the absence of leakage – is a critical factor in project success. Not all geological systems have the same risk profile and the nature of the physical forces driving potential migration will not only affect the stored CO2 but will also change over time. The evidence gained from monitoring during injection informs an updated assessment of the remaining storage risks at the end of injection. An important metric for CO₂ storage is the CO₂ fate plot, which displays the partitioning of mobile/capillary trapped/dissolved/mineralised CO2 as a function of time within a geological storage site. Recent regulatory approaches governing the post-closure phase of CO₂ storage sites have, in some cases, shifted away from site-specific assessments of risk, favouring the application of prescriptive and often lengthy post-closure stewardship periods before responsibility can be transferred.
A well-selected, designed, and managed carbon capture and storage (CCS) site has a very high likelihood of securely retaining injected CO₂, with studies indicating that 99% or more will remain stored, although the risk of leakage is not entirely zero. To manage this risk, modern sites implement comprehensive monitoring, reporting, and verification (MRV) systems (required by regulators) to detect any potential leakage and ensure strict environmental criteria are met before site closure and transfer to the state.
This report, by the British Geological Survey, summarises conformance and containment measures at CO₂ storage projects across the world, particularly focusing on characterisation and operational phases. A modelling study was undertaken using CO₂ fate plots produced for different reservoir models to better understand reservoir flow properties and trapping mechanisms. Subsequently, consequences in terms of risk evolution and pathways to efficient post-closure handover are discussed.
