Challenging environments such as unstable clay formations represent high operational risk when running tubulars after drilling is completed. Use of steerable-drilling-liner (SDL) systems combines drilling and casing of the hole, thereby mitigating the risk of not being able to run the liner because of time-dependent formation collapse. This case study presents the difficulties encountered with conventional approaches with a semisubmersible rig in the North Sea during 7-in.-liner-running operations in an unstable clay formation and details a subsequent SDL operation.
Midgard, a subsea field, is part of the Åsgard Licence and is located in the North Sea. Midgard consists of three templates with four slots each. Production from the field goes to the production platform Åsgard B. The primary objective of this well was to increase reserves, accelerate production, and add robustness. The secondary objective was to evaluate the potential of this segment. The target in the reservoir was approximately 4 km from the template at a depth of 2500-m true vertical depth (TVD), making this well almost an extended-reach well. This paper will only focus on the 8½-in. section.
The 8½-in. section was planned to be drilled to approximately 20-m TVD above the reservoir for a length of approximately 1000 m. The drilling operation was performed without any major problems; however, it was necessary to change the drilling bottomhole assembly (BHA) because of a tool failure. There were also challenges to keep the oil-based-mud system stable enough to drill the section. When running in with the 7-in. liner, obstruction was met approximately 500 m out in the open hole. It was decided to pull out with the liner and plan for a wiper trip. Problems were encountered already in the 9⅝-in.-casing shoe and rathole area during this wiper-trip operation. After 650 m of working and attempting to get the wiper-trip string down, it was decided to pull out with the BHA and cement back the hole and prepare for a sidetrack. A considerable amount of cuttings and 10% mechanical cavings were circulated out during this wiper trip.
Steerable-Drilling-Liner Technology in Unstable Shale
15 May 2016