A Multiple Well Abandonment Program: Methodology And Techniques
Authors: Matkowski R.S. , Carter J.A. , Townsend S.C. 
Published: October 2008

This paper discusses the systematic approach adopted by Amoco (UK) Exploration Co. Ltd. (AMOEX) in order to achieve its project objectives during a recent platform well abandonment campaign. An integrated team, comprised of key service company personnel working in conjunction with the oil company operations and cementing research staff, developed and implemented innovative procedures and cement slurry designs to support the deployment of coiled tubing. This facilitated the subsequent timely and cost effective field operation to abandon 13 wells in a safe and environmentally sensitive manner.

INTRODUCTION

AMOEX operate an offshore oil production platform located in Block 211 of the United Kingdom Continental Shelf. The field began producing from the Brent formation in April 1983, with peak production reaching 86,500 barrels of oil per day (BOPD). Output has since declined to about 12,000 BOPD, which has been maintained since mid 1992. The arrest in the field's decline is attributed to improved waterflood response and optimized production operations, which have pushed back the anticipated cessation of production till 1995 or beyond.

The disposition of the platform or its conversion to an alternative use is currently being assessed under a separate engineering initiative. When these evaluations are complete, a final plan will be submitted for approval to the Department of Trade and Industry (DTI).

The reservoir top has a true vertical depth of 11,500 ft, with temperatures in excess of 270° F. The program for reservoir and well abandonment has been planned in three distinct operational phases, the first of which utilizes 1-1/2" coiled tubing (CT) through the existing completion string to set cement plugs across the perforation intervals at measured depths from 12000 to 20000 ft. During this phase anticipated problems included instances of leaking completions or casing strings, junk or fill, and high cross flows or loss of circulation associated with the mature multilayered reservoir.

The second phase employs the existing two platform drilling rigs in order to recover a practicable quantity of tubing and casing prior to setting surface cement plugs. The last phase involves the removal of conductors, which will not occur until after production has ceased.

Close liaison between personnel from the operator and the relevant service companies was fostered by regular team meetings, which explored and developed various innovative options.

Two engineers with experience in platform drilling, workover, CT and wireline operations were dedicated to the project. The primary objective of their initial studies was to determine the most cost effective method of abandoning the wells, consistent with Amoco's own standards and satisfying the DTI and Health and Safety Executive (HSE).

Initial tasks included the following:-Review of experience gained in well abandonments from within Amoco Production Co. and other operators.Compilation of data packs for all 40 well slots, collating information on casing, cement tops, completion details, drilling problems, and production wireline history.Identification of major issues pertinent to developing a conductor removal technique.Discussions with potential vendors about the optimization of procedures associated with CT, casing cutting, milling, bridge plugs, wireline tools, etc.