This paper discusses the systematic approach adopted by BP Exploration to achieve its goal of safely abandoning the wells on a major North Sea platform. Having determined the abandonment methodology, an integrated team, comprising oil company engineers and key service company personnel developed processes and procedures to reach the objective. These included advances in coiled tubing rig-up procedures and the use of Technical Limit to capture learning. This approach facilitated the timely and cost effective abandonment of 24 wells in a safe and environmentally friendly manner and in achieving world-class performance.
BP Exploration operates the NW Hutton oil production platform located in Block 211 of the North Sea, 130 kms North East of the Shetland Islands. The field, developed by Amoco, began production from the Brent formation in April 1983, with peak production reaching 86,500 barrels of oil per day (BOPD). Output declined steadily, to less than 4,000 BOPD, until May 2002, when the decision was made to begin operations to decommission the facility.
The platform has 40 well slots, served by two drilling packages. It stands in 473 ft of water and the total structure weight is approximately 40,000 tonnes.
Amoco had abandoned 13 wells in 1993 (1). The current programme for reservoir and well abandonment was planned, in a similar manner, in two distinct phases. The first, using well intervention techniques such as coiled tubing, wireline and cementing service, through the existing completion strings, to set cement plugs in the wellbore. During this phase anticipated problems included leaking completions and casing strings, junk or fill and lost circulation. These operations began in May 2002 and were completed in May 2003 and are the subject of this paper.
The second phase was to recover the tubing, casing and conductors, using the two existing platform drilling rigs and facilities. This phase began in June 2003 and was completed in January 2004.
Production on NW Hutton came from the Brent reservoir. It has a true vertical depth of 11,500 ft. The original reservoir pressure was 7,500 psi but had declined, in areas, to be subhydrostatic, reservoir temperature is 270°- 290°F, and the well depths vary from 12,300 to 20,000 feet.
The principles used to adequately seal and abandon the wellbores on NW Hutton were designed to conform to UK legislation, BP Policies and relevant Industry Standards (2). Typically the requirement is to have two barriers between hydrocarbon zones or over-pressure water zones and the surface and one barrier between normally pressured water zones and the surface.
For NW Hutton this involved placing a permanent cement primary barrier immediately above the reservoir. Typically this barrier would be 200 to 500 ft long and require 30 bbls of cement. A secondary barrier backed up the primary barrier. The secondary barrier was normally 1,200 to 1,500 ft long and required 100 bbls of cement. A third barrier was placed near to surface to isolate some shallow, normally pressured, waterbearing sands and would be 700 ft long, typically using 100 to 120 bbls of cement. Each barrier was verified by pressure testing or tagging. Detailed design depended on actual well conditions. Appendix A gives further detail of the procedure.
The cement placement techniques were either to use coiled tubing or to circulate or bullhead the cement into place. Once the well was sealed the completion tubing was severed in preparation for recovery during the second phase of the project.
This methodology was decided as being most suitable for NW Hutton, based on previous abandonments on the platform (1), experience and lessons learned from the abandonments of the Maureen and Hutton TLP platforms.