Coiled tubing drilling cost-efficient in chasing Alwyn's small oil pockets

April 1, 2000
Using stuck coil as production liner
Uplift of 40T coiled tubing reel.
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The Alwyn North field is operated by Total Oil Marine p.l.c., with a 33% stake, partnered by Elf Exploration UK PLC with a 66% stake. The Alwyn North field contains small pockets of oil that are uneconomic to develop using conventional sidetrack techniques. The sidetrack of Alwyn North well 3/9a-N30 utilized coiled tubing drilling to reduce well costs, compared with a conventional sidetrack.

A coiled tubing drilling team was established to prepare the well design and work program. The team was composed of personnel from Total Oil Marine together with team leaders from the key contactors.

  • BJ Services for coiled tubing, pumping, and solids control
  • Baker Hughes Inteq for the provision of the bottom-hole assembly (BHA)
  • Baker Oil Tools for the provision of whipstock, fishing and milling services
  • Baroid for fluids

Each phase of the well was reviewed and potential problem areas identified, which helped to focus attention on the critical issues. These included:

  • Platform crane limitations: The feasibility study identified a requirement to upgrade the crane to accommodate a 40-ton lift.
  • Coiled tubing footprint: It was essential to minimize the footprint as the South Rig was drilling an extended reach conventional well concurrently with the coiled tubing drilling.
  • Isolation from motherbore: The existing perforations would be isolated.
  • Whipstock: A retrievable whipstock was required to allow future access to the motherbore.
  • Differential sticking: A high over-balance would be present due to drilling both depleted and virgin pressure fault blocks.
  • Open hole logs: The BHA would provide a gamma ray and resistivity. Wireline conveyed pressure measurements were required.
  • 2-7/8-in. slotted liner: The liner was required to cover the non-reservoir shales; the reservoir section would be left barefoot.
N30 Well Architecture.
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The trial simulated the available footprint on Alwyn North and the well architecture of 3/9a-N30. The objectives of the trials were to run and recover the whipstock, run the BHA, run the milling assembly, run logging tools, commission a solids control system, and develop the project team.

The learning points from the trials were then incorporated into the drilling program.

3/9a-N30 well objectives

To drill a through-tubing coiled tubing sidetrack as an independent operation to access unswept hydrocarbon reserves of the Brent reservoir. The deck management and logistics were complicated but benefited from the pre-planning of the work by the team.

The drilling programme is summarized as follows:

  • Set through-tubing retrievable whipstock at 3,830 meters
  • Mill and dress off the window in 7-in. liner
  • Drill 600 meters of 3 3/4-in. hole
  • Drill a complicated 3-D build and turn well trajectory (turning in azimuth through 180°)
  • Run open-hole wireline pressure measurements
  • Run the liner
  • Recover the whipstock.

The whipstock was set and oriented in one run at 3,830 meters, and the window was then milled in four runs.

3 3/4-in. hole section

Alwyn N30 Coiled Tubing Drilling actual wellpath.
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The drilling assemblies used for the build and turn performed closely to the planned rates, achieving a dog leg of 25°/30 meters. It was necessary to achieve the planned trajectory due to the very tight geological target.

The initial phase of the trajectory was drilled utilizing a gamma ray for geological control; once the reservoir was penetrated, a resistivity tool was added to the assembly to ensure the trajectory did not penetrate the water leg. The BHA encountered problems on the trip back into the hole. An extensive reaming program was carried out, however, as a result of a deterioration of hole conditions it was decided an openhole sidetrack would be the optimal method of achieving the target.

The openhole sidetrack was successfully initiated. Over 50 meters of the sidetrack was drilled before pulling the assembly for a wiper trip. The wiper trip was performed without incident until indications of hole instability were encountered at the junction between the two drains. A second sidetrack was attempted without success. The BHA was recovered.

The BHA was then run back with the objective of attempting to re-enter the sidetrack and continue to drill to total depth. The BHA became irretrievably stuck at the junction between the two drains. Several attempts were made to free the BHA without any degree of success.

Alternative program

N30 Well Status.
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he decision was made to proceed with an alternative program of using the stuck coil as a production liner. The wireline was pulled and recovered from the coil and the coil perforated. The coil was then cut outside the window, and the remaining coil was pulled to surface.

The whipstock retrieval BHA was run to recover the whipstock. The whipstock was tagged on several occasions without any success. At the surface, marks on the overshot indicated the fishneck had entered the overshot without being fully engaged.

A second run was then attempted. On this occasion, the whipstock was again tagged without success. At the surface, it was found that the whipstock fishing neck had sheared off. Owing to the impending platform summer shutdown, no further attempts were made to recover the whipstock. The whipstock will be recovered in the future.

The sidetrack is producing at a stabilized rate of 1,100 b/d and is now in the position of having paid for the operation. Future candidates for the procedure have been identified on Alwyn North. Additional potential also exists in the future for the Dunabar platform, which does not have any drilling facilities on board.


The authors wish to thank Total Oil Marine and Elf Exploration UK PLC for their support and permission to publish this work.


Patrick Bouroumeau-Fuseau is head of Alwyn drilling and head of well services, Total Oil Marine. He joined TotalFina in 1977 and has held various positions in drilling and well servicing in TotalFina subsidiaires throughout his career. He has a BSc. in petroleum engineering from the Institute Fran

Stuart Mackay is Senior Drilling Engineer and Coiled Tubing Project Leader for Total Oil Marine. Since joining the company in 1996 he has been responsible for drilling operations on Alwyn North with a focus on innovative techniques.

Armel Simondin is a Drilling Engineer for Total Oil Marine's Alwyn drilling team responsible for rig upgrades and well engineering. Armel joined TotalFina in 1997 and has held held various positions in exploration and development drilling. He is a graduate of the Ecole Centrale de Lyon and holds a post graduate degree from ENSPM.