The world largest subsea development Statoil Troll field consists of 115 subsea wells and more than one million drilled reservoir meters. A large number of existing wells, thinning oil column, and a highly varying oil-water contact (OWC) due to production challenge the optimal placement of new oil producers in terms of planning and execution.

To contribute to production, production wells must penetrate micaceous M-Sand and clean C-Sand. Due to an initially thin oil column (11-26m), the horizontal drainage from each well (branch) is limited; the most effective way to increase the recovery from the reservoir is to drill infill wells. This method has led to a high number of wells on Troll and has created a high number of well crossings in the reservoir section; which is why, landing in the reservoir section has become a challenge. True vertical depth (TVD) control is critical to success.

Dual Inclination (DI) and Continuous Inclination (CI) are survey methods that can be used to increase inclination accuracy and thereby reduce TVD uncertainty. Both DI and CI are applied on Troll wells. Although DI and CI are often used to hit and then stay within thin horizontal targets, these techniques are equally useful for reducing the probability of collision in environments like the Troll field, in which existing wells must be crossed within a few meters, at near horizontal. The increased survey frequency provided by CI is critical for TVD accuracy whenever drilling conditions introduce significant inclination variations over short distances. Forty eight Troll wells were reprocessed using CI. Fifty-six percent of the wells shifted in TVD by more than 0.5m; 17% shifted by more than 1.0m. These relatively small shifts can be significant in the Troll environment, especially as some wells are corrected up and some down. The errors can be attributed to a variety of drilling conditions common in the Troll field, such as drilling in hold mode with rotary steerable systems (RSS), continuous geosteering, and drilling hard stringers, all of which can cause inclination changes that may be missed by surveys taken at standard length intervals.

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