When permanently abandoning a well, each seal above a Distinct Permeable Zone (DPZ) needs to be restored with a cross-sectional barrier that will withstand the test of time. The Perforate, Wash and Cement (PWC) technology was developed as an alternative to section milling in situations where the wellbore barrier needs to be placed across a section of uncemented casing. Section milling is time consuming and results in exposing the BOP to swarf. The PWC method, on the other hand, is a more time-efficient process that consists of perforating the casing, washing the annulus behind it, and placing cement across the whole cross-section of the wellbore in a single run. However, some of the efficiency gain of the PWC method over section milling is lost when it comes to the barrier verification process. For section milled plugs, the verification process is usually the same as the one used for conventional wellbore cement plugs, since all the casing has been removed across the section milled window: the plug needs to be tagged/weight tested and pressure tested (except for section milled plugs where the top of the plug is left in open hole). On the other hand, as over 90% of the casing remains when installing a cross sectional barrier with PWC, the verification process typically involves two steps: 1) verify the annular component of the barrier, and 2) verify its internal one. In order to verify the annular part of the barrier, the internal barrier (cement plug) needs to be drilled out, and the annulus logged with a cement bond logging tool. A new internal cement plug then needs to be re-set and verified using the same verification method as for section milled plugs.
During the first phase of the Valhall DP Plug & Abandonment (P&A) campaign, the PWC technique was used repeatedly to create primary and secondary barriers in the 9 5/8″ casing and 9 5/8″ casing × 12 1/4″ hole annulus, each time using the same design and execution parameters. After building a successful track record of PWC jobs in 9 5/8″ casing, a process was initiated to develop a non-destructive and more efficient alternative verification process, specific to PWC operations that would not require to systematically having to drill out the wellbore cement to log the annular barrier. The intention was to base this alternative verification method on a consistent track record, a strict design and operational parameter set (Qualification Matrix), and to include this alternative verification method in a proposed NORSOK Element Acceptance Criteria (EAC) Table specific to barriers installed using the PWC technique. This PWC EAC Table is intended to be used as a best practice for designing, executing and verifying PWC jobs, and is recommended for implementation into the next revision of the NORSOK D-010 Guideline.