Deep geological storage of CO2, offshore UK – Supplementary note C: Well analysis using Peloton WellMaster database

The purpose of this Supplementary Note is to assess the risks of loss of containment from inside the well to outside of the well at any point between seabed (subsea wells)/surface (platform wells) and storage site, for a CO2 Injection (Active) well and a Decommissioned (Inactive) well. This Supplementary Note C has been written for a technical audience to support the probabilities and leak rates used in the main report Deep Geological Storage of CO2 on the UK Continental Shelf: Containment Certainty.

Assuring well integrity over the life cycle of the well is fully defined in documents such as ISO 16530-1 Well Integrity or NORSOK D-010 Well Integrity in Drilling and Well Operations or OEUK Well Life Cycle Integrity Guidelines. Well integrity refers to maintaining full control of fluids within a well at all times by employing and maintaining one or more well barriers to prevent unintended fluid movement between geological formations with different pressure regimes or loss of containment to the environment.

In practice this means that the well is designed and operated with two independent barriers, each barrier consisting of several elements. There are established performance standards for each barrier and barrier element, to support monitoring, maintenance, and testing to verify the condition of the barriers. Should one barrier fail then the second barrier will prevent any loss of containment or fluid movement while the failed barrier is repaired.

Whilst the barriers materially reduce the leakage risk for wells there is still a residual potential for this to occur. To assess the potential frequency of such events for a CO2 Injection Well the following basis and assumptions are used. These reflect the good industry practice embedded in the storage permitting regime.

• Well operator has a clear policy on how well integrity is established and preserved. Performance standards, monitoring requirements, maintenance routines, testing frequencies are defined documented and reported.
• The well design has two independent well barriers.
• The well equipment has been designed, tested and certified for use.
• On handover from the drilling and completion phase to the operating injection phase the two independent well barriers have been installed and tested against the performance standard(s).
• Responsible follow-up of the wells performance and integrity is performed, with regulatory and operator prioritisation of well anomalies and leaks, in order to maintain well integrity. Wells are managed according to Performance Standards and closed in, if required.