Solving Dual Trip Zonal Isolation Issue: Single Trip Plug & Abandonment System Saves Operator Six Days Rig Time, UK North Sea
Authors: Hogg Hamish , Lees Graham , Fearn Murray , Khan Zafar , Adetona Gbade , Strachan Ryan 
Published: October 2014

Conventional techniques for Plug and Abandonment (P&A) operations usually involve multiple trips downhole to mill the casing and underream the section. This paper describes new tools and methodologies developed to reduce rig time in Plug and Abandonment operations, specifically section milling and underreaming.

The service provider has been working with an operator in its on-going Brent Field abandonment campaign, during which over 140 wells will be abandoned. The scope of work requires section milling a window in the 9-5/8" casing, and then underreaming the open hole to a half inch diameter over the original wellbore size. This is to allow placement of an abandonment cement plug against the original formation. The complete operation requires displacing the wellbore to milling fluid, section milling and then underreaming, all of which have traditionally been performed in separate runs.

In response to these operational/economic challenges, a P&A system was developed using an innovative Ball Activated Drilling Valve (BADV) and a unique High Ratio Underreamer (HRU). This combination of tools made it possible to execute all operations in a single trip. The system allows the operator to mill and underream a window in a single trip; this will allow placing a rock-to-rock abandonment barrier plug in the required position.

While tripping in hole, the BADV is open to the annulus which allows the string to self-fill, so no extra time is required top filling the pipe. Once the Bottom-hole Assembly (BHA) is at the section milling depth, the wellbore is displaced to milling fluid through the open BADV. Once displacement is complete and fluid rheology is suitable to ensure good hole cleaning of metal swarf during milling, a ball is dropped to shut the valve. Cut-out and section milling are then performed to achieve the required window length, once this has been competed the HRU tool is activated by drop ball and the open-hole section underreamed.

A field test plan was presented and discussed with the offshore supervisors during pre-job briefing and they were introduced to the tool's functionality in the workshop. The first run with the combined section mill, HRU and BADV assembly took place on Brent and saved over 24 hours rig time. The system was then successfully trialed on seven challenging 9-5/8″ casing milling jobs which resulted in total savings of over six days rig time. In addition, there was a reduction in the safety exposure to the crew by handling only a single BHA.

Over the course of the campaign, the P&A team made several changes to the abandonment process enabling the operator to eliminate dedicated cleanout trips between section milling and underreaming runs. This streamlining process successfully combined operations that previously required four trips to complete down to one. Additional design changes/modifications were made during field testing and performance continually monitored to ensure the field test objectives were met and improvements to the tool/system reliability were being made. Product Engineers and Field Service Managers performed post-job briefings with offshore supervisors to ensure lessons learned and suggestions for improvement were documented. The system is now complete and ready for use in other suitable P&A applications.