Steerable Drilling Liner (SDL) was originally designed to remove downtime related to drilling in unstable formations and/or into depleted formations as it allows the well to be lined/cased off simultaneously as it is drilled. The SDL system consists of a standard drill pipe as the inner-string, inside a conventional liner. The system uses a conventional rotary steerable Bottom Hole Assembly and has been designed to drill complex 3D well trajectories, up to 2000 m long, with the same directional and logging capabilities as conventional drilling. Two sizes are developed: 7" for 8 ½" hole and 9 5/8" for 12 ¼" hole. Using SDL gives several benefits to the drilling process, including:
Reduced openhole time; the liner is always on bottom.
Reduced pressure fluctuations; tripping of BHA/innerstring is always in cased hole.
Reduced mechanical impact on the formation; stable centralised liner with no lateral movement against formations when drilling.
All the above will contribute greatly in the goal of preventing formation collapse prior to drilling a section to planned total depth (TD).
Based on a request from the Grane field in the Norwegian Continental Shelf (NCS), additional ways of using the SDL technology has now been introduced as an 7" SDL system was drilled fully into open hole and left as an open hole clad (with both the liner top and bottom in a reservoir sand) to prevent unstable intermediate formations to collapse and subsequently prevent drilling the 8 ½" reservoir section to planned TD.
Unstable shale protruding into the reservoir at Grane causes regular adjustment to the wellpath. As the formation instability develops over time, getting the liner/completion through a shale interval has been proven difficult and often results in sidetracks due to wellbore collapse and/or pack-offs. The solution has been to place the well path above the shale intrusions to avoid the stability problems. This drilling practice limits the total reservoir drainage since the gravity based drainage only allows the reservoir above the drilled wellpath to be produced. Using 7" Steerable Drilling Liner (SDL) on a well on the Grane platform allowed drilling through the shale section and seal it off immediately before problems started to develop. This enabled optimisation of the well placement for improved access to reserves that otherwise would be inaccessible due to shale intrusions.
The entire operation was carried out withd great success. SDL not only removed drilling related issues but also contributed to, based on the more optimally placed wellpath, increased reserves. At TD, the liner was released from the inner-string without any issues and left as a horizontal protection through the shale with both ends of the liner in the reservoir sand.