Drilling on the Troll Oil field has been challenged by the need for long and extensive horizontal hole sections while strictly controlling true vertical depth. The horizontal reach has been extended from initially about 800 meters to more than 4,400 m in a single well bore using the latest generation of Rotary Steerable Systems. At the same time, multilateral drilling technology has become a standardized application on Troll with up to four reservoir sections completed per well.
The application of front-end drilling technology has been instrumental to successfully developing the challenging Troll Oil reservoir. Throughout more than 10 years of production drilling on Troll, Hydro has constantly pursued advancements in drilling and formation technology to extend the reservoir section lengths and improved well placement accuracy. Optimal well placement and lengths have resulted in lower lifting costs and higher production rates. The wells are accurately positioned with tight tolerances in the thin reservoir oil column. With the introduction of multilateral systems, total reservoir section length has been extended considerably. Today, a total of 106 wells have been drilled and completed on the Troll Oil field, some 34 of which are multilaterals ranging from two to four branches. Since 2001, all wells drilled on Troll Oil have been multilateral wells. The wells have up to 14,162 m of reservoir completed with sand screens. Improved bit design, use of mud friction reducers and low-weight mud systems has also contributed to extended section lengths, ref Figs. 1 and 2. The introduction and development of integrated, automated drilling systems have been essential to meeting Troll Oil development goals. Detailed understanding of the drilling challenges has resulted in improved bottom hole assemblies and drilling practices. The introduction of rotary steerable systems, drilling dynamics and logging while drilling tools has contributed significantly to developments seen in the Troll Oil field. How this technology has been developed and applied over time to overcome field-specific challenges will be further addressed in this paper.
During the first years of drilling on the Troll West Oil Province (TWOP), conventional drilling assemblies comprising Positive Displacement Mud motors (PDM) and Measurement While Drilling (MWD) tools, were used to drill the horizontal reservoir sections. These bottom hole assemblies (BHA) had a 15–20 m distance between the bit and the logging while drilling (LWD) sensors. The long sensor-to-bit distance made it difficult to achieve an effective true vertical depth (TVD) control. The desired well placement of 3 m above the Oil Water Contact (OWC) was not possible to attain over long horizontal sections. In addition, these assemblies had limitations in orienting and steering the well bore, limiting the horizontal section lengths to about 800 m.
In the early 1990s, an instrumented PDM was developed, featuring a reservoir navigation tool with integrated inclination, gamma ray and resistivity sensors. This "sub" was placed directly above the mud motor bearing assembly to reduce the sensor-to-bit distance.