Troll West Oil Province - Subsea Horizontal Completion Experience
Authors: Storegjerde D. , Soltveit R. 
Published: December 2007

Oil production from the Troll West Oil Province to the Troll B platform commenced 25 September 1995 and reached the design rate of 180,000 bopd 3 days later, below budget and 3 months ahead of time. This is no mean feat for a development which was not considered technically feasible less than a decade ago. Subsea drilling and completion operations commenced some 1 1/2 year earlier, in March 1994, and have generally been successful, with substantial time and cost savings compared to the original budget. Inevitably, with a new development, there were some initial equipment- and operational problems, but these have now been overcome.

Completion operations include the installation of up to 2,000 m (6,500 ft) pre-packed screens, upper completion and Xmas tree as well as production clean-up flow to a semi-submersible drilling rig. Selected wells have been production logged using coiled tubing, in some cases combined with clean-up and stimulation.

Although not boasting novel completion techniques and equipment, conventional limits have certainly been pushed further. The Troll wells are among the longest single zone horizontal producers in the world and produce up to 50,000 bopd. This paper gives a summary of the experience gained, with emphasis on the downhole completion installation.

Introduction

The Troll West Oil Province is located approximately 100 km North West of Bergen, in the Norwegian Sector of the North Sea, in a water depth of 315-340 m (Fig. 1). The area is part of the giant Troll gas field (Fig. 2), in which Norsk Hydro is responsible for development of the thin oil rim (22-26 m) at the shallow depth of 1529 m (5,000 ft) sub sea. Development of the even thinner oil rim of the Troll West Gas Province (around 13 m) has also recently started. The combined development is estimated to recover a total of 1 billion barrels of oil.

The development of the Troll West Oil Province includes the drilling and completion of 18 subsea horizontal oil producers and 1 deviated gas injector with a semi-submersible drilling rig. The wells are gathered in 4 separate clusters with production to a floating concrete platform, the largest of its kind (Figs. 3 and 4).

The development of the Troll West Gas Province possibly entails another 50-60 wells, depending on early production results. The time factor for drainage from these wells is critical, as reservoir conditions are expected to deteriorate rapidly when gas production to the Troll A platform commences in 1996.

To date (December 1995), 9 wells have been drilled and completed in the Troll West Oil Province, including the gas injector.

In addition, the first 2 horizontal producers in the Troll West Gas Province have been drilled and completed.

Table 1 gives general reservoir and fluid data.

Completion Design

The main factors governing the completion design were:

Reliability. With rig operating cost in the region of 1.3 mill NOK/d ($200,000/d), the cost of a major workover amounts to approximately 40 mill NOK ($6.5 mill). The initial investment cost for completion equipment becomes relatively insignificant in comparison, so every effort was made to reduce the workover and intervention frequency in the completion design.

Sand Control. The unconsolidated sandstone of the Troll field, requires sand control.

Pressure. As the reservoir pressure is very low, pressure drop must be minimized from the sandface to surface. This called for pre-dominantly 7" production tubing.

Vertical Access. Vertical access was limited by the Xmas tree and workover system to 5 1/8" nominal ID.

Well Design. A fairly typical well path is shown in Fig. 5. Some wells turn up to 160-170 degrees in order to place the horizontal section in the optimum location. Fig. 6 shows a cross section of a well through the reservoir. The 9 1/2" open hole section is drilled close to (2-6 m) the oil water contact in order to delay gas breakthrough.

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