The Troll Oil subsea development involves innovative completion technology such as sand control screens with inflow control devices, hydraulically controlled gas cap gas-lift and multilateral wells with up to three junctions stacked on top of each other. Wireline-less completions contribute to time and cost savings.
The Troll field, initially planned as a gas field, has with its thin oil zone become one of the largest oil producing fields offshore Norway. The thin oil column, 4-27 m thick, has required the development of a number of novel technologies within completion. The main challenge has been to install more than 4,000 m of sand screens in the 8 1/2" reservoir sections with a challenging 1,580 m TVD RKB to reservoir. Well-bore friction, running string weight and the safety aspects of using drill collars as a running string are challenges that have been overcome. The introduction of roller centralizers and fluid friction reducers has also been essential. Sand screen completion for the horizontal oil producers has evolved to become an efficient design with stand-alone mesh-type screens that allow for drilling mud to be produced back through the screen filter media. To enable this, a novel screen selection guide was developed that considers parameters other than simply sand exclusion. To improve oil recovery and prevent gas-coning, Inflow Control Devices (ICD) have been utilized. Each screen joint is fitted with this choking device. The wireline-less completion installation was at the time regarded as a breakthrough for wells using horizontal X-mas trees and has contributed to great time and cost-savings throughout the completion phase. Gas lift is needed as the water cut increases, and the wells are equipped with hydraulically controlled down-hole gas lift valves producing gas from the gas cap. Multilateral technology is needed to increase the drainage area cost efficiently. Some wells have up to five branches, open hole and stand-alone screen completions. Coupled with remote operated valves to control the different laterals, these enhance the flexibility of the design.
Long horizontal reservoir sections are essential to the Troll Oil development. The open hole sections were originally intended to be 800 m long when production drilling commenced in 1994. During the past 10 years, this has been extended up to 4,400 m of horizontal reservoir sections in a single well bore. Multilateral technology has enabled stacking of well junctions, and there are now 34 multilateral wells on the field, two of which are quad-lateral and seven trilateral. Many of these wells also have additional barefoot lateral bores, resulting in up to 14,200 m of reservoir section per well. Screen design. The Troll reservoir is relatively shallow at 1,240 m below the seabed. Heavy landing strings are required to obtain sufficient weight to push the screen sections into the reservoir at total depths up to 6,500 m. The screen landing string typically consists of 5 ½" HWDP, 8" drill collars and 9 ½" drill collars.