Planning, execution and Review of Brent's First Coil Tubing Drilled Well
Authors: Donald D. , Appleby C. , Davies J.B. , Brakel J.D. 
Published: December 2007

Coiled Tubing Drilling (CTD) offers an attractive alternative to conventional rigs for certain categories of Brent field wells. Some 40 wells could be drilled with through tubing technology in the next 5 years. Typical candidates are wells targeted at reserves that would otherwise be marginal if drilled conventionally (0.5 – 1.0 MMstb), and water injector sidetracks to back produce the aquifer during depressurisation.

To establish whether the concept was viable for Brent, a well was sidetracked from the Delta platform during Q3 '95. The well, BD38, was drilled to TD and completed, despite the challenges presented by the complex well trajectory. The success of this well can largely be attributed to meticulous planning and co-ordination by the multi-disciplinary team which comprised Well Engineers, PEs, the selected contractor and alliance Partners.

Background

A significant proportion of the remaining reserves of the Brent field, the so called Locate The Remaining Oil (LTRO) portfolio, possibly some 10 MMstb, is unlikely to be recoverable economically with conventional wells. These smaller prospects, with reserves typically between 0.5 and 1.0 MMstb, will be sacrificed unless a viable alternative to conventional drilling and completion technology can be developed, and within a relatively short time frame. Currently, coiled tubing drilling appears to offer that alternative; a number of wells having already been drilled successfully utilising this technology, both by the Shell Group and within the industry at large. In addition to the LTRO wells, a further application would be mechanical sidetracks, and sidetracks of the former water injectors, the (BPW) wells that will be used to back produce the aquifer during the latter stages of Brent de-pressurisation. In the base case, up to some 30 of these wells are anticipated to require sidetracking from 1998, both for mechanical reasons, to by pass selective completion components over the reservoir section, (Multi Straddle Assemblies) and to install sand control to protect the ESPs and facilities. If coiled tubing technology can be utilised for this application, not only would substantial CAPEX savings be realised, but all operations could be carried out underbalance.

The time remaining to develop the oil reserves associated with the LTRO prospects is limited due to the imminent depressurisation and consequential lift loss. These prospects should be drilled by end 1998 or reserves could be sacrificed.

PROJECT OBJECTIVES

In view of the perceived long term strategic benefits of coiled tubing drilling, and the limited time frame, a well was selected from the LTRO inventory to trial the technology. The objectives of this well were twofold. The first was economic, to access and safeguard reserves. The second was strategic, i.e. to prove that the application of coiled drilling technology is viable in the Brent field. More specifically: Reserves. To provide a Statfjord (sandstone,) reservoir offtake point in an area of sub-optimal well spacing.; developing reserves anticipated to be in the order of 0.8 MMstb.

Strategic. To establish:the real economic benefit of drilling, logging and completing a well with coiled tubing.the practicality of running and cementing long, small diameter liners on CTthe practicalities of logging (open/cased hole) coiled tubing drilled wells.the (life cycle) practicalities of maintaining and surveying a slim hole horizontal well.that (heavy) Coiled Tubing equipment can be located on the Brent platforms.the feasibility of undertaking CTD concurrently with main rig activity.

If the concept was to be proven for Brent, all of these strategic questions had to be answered. A stepped development approach was considered but it was felt that, if correctly managed and risk assessed, all of the strategic questions could be addressed simultaneously. In any event, the tight depressurisation time frame precluded protracted development schedules.

CANDIDATE SELECTION

Candidate selection was both time consuming and involved. Typical of the constraints that had to be considered included: P. 663