Designing High-Angle, Casing-Directionally-Drilled Wells
Authors: Denney Dennis 
Published: February 2015

This article, written by Technology Editor Dennis Denney, contains highlights of paper OTC 18374, "Designing High-Angle, Casing-Directionally-Drilled Wells," by B. Borland, SPE, R. Watts, SPE, ConocoPhillips; B. Lesso, SPE, Schlumberger; and T. Warren, SPE, Tesco, prepared for the 2006 Offshore Technology Conference, Houston, 1–4 May. Copyright 2006 Offshore Technology Conference. Reproduced by permission.
Casing-directional-drilling equipment and procedures for use on a mature North Sea asset were designed and tested. Testing this equipment in commercial North Sea operations is prohibitively expensive. Therefore, tests were conducted at a drilling test facility near Cameron, Texas, where operations were conducted over a wide range of rotating speed, weight, and flow conditions and with inclinations ranging from vertical to horizontal.

Introduction
To reduce implementation risk of drilling directional wells with casing, tests were conducted in a wellbore having a profile similar in complexity (i.e., hole angle, build rate, and depth) to a well to be drilled with 10 3/4- and 7 3/4-in. casing off the Eldfisk B platform offshore Norway. The equipment used for the field trial included the same surface equipment required to modify or adapt the rig in Norway. This procedure mirrored those activities that would be necessary to execute the project on the platform. Safety and training were also integral parts of the project.

Potential
Casing drilling has demonstrated operational time savings of approximately 15% in straight-hole applications compared with normal drillpipe drilling, and such results are expected in casing directional drilling offshore. Eliminating the need to pick up drillpipe, condition the well, and trip out of the hole to run casing can save a substantial amount of time. Routine trips for bits, underreamers, and measurement-while-drilling (MWD) tests also will provide time savings.
However, the major benefit is expected to be minimizing formation-related problem time. Improved hole cleaning in the 10 3/4-in. section should eliminate problems washing casing to bottom and should improve cement jobs. The mud remains well conditioned because the casing is not run in a separate trip. The smear effect should provide significant improvement in dealing with fluid-loss and shale-stability problems in the 7 3/4-in. section. Even in the cases in which losses occur with the casing already on bottom, drilling to the casing point can be achieved and cementing can be performed without having to stabilize the hole. Achievements at Eldfisk similar to those seen in straight-hole casing drilling provide the potential to reduce well times by 5 to 7 days per well.
Additionally, if the smear effect is as effective in chalks as it is in sandstones, then a casing string may be eliminated also. Drilling times will, therefore, be reduced further, allowing one to two more wells to be drilled per year. In the longer term, successful casing directional drilling will allow fit-for-purpose rigs and casing-drilling-enabled rigs to be built for offshore use.