With the advent of steerable systems, some directional wells are planned and drilled with complex profile types involving 3-dimensional turns. Generally, this happens mainly in the case of re-drills, where old wells are sidetracked and drilled to entirely new targets.
However, these complex well paths are harder to drill, and the old adage that “the simplest method is usually the best” holds true. Therefore, most directional wells are still planned using the traditional patterns which have been in use for many years.
Build and Hold Type of Directional Wells
The features of the type of this directional drilling well are:
- Shallow kick-off point (KOP)
- Build-up section (which may have more than one build-up rate).
- Tangent section to TD
- Deep wells with large horizontal displacements.
- Moderately deep wells with moderate horizontal displacement, where there is no requirement for intermediate casing type.
S Type Well
The features of this trajectory are:
- Shallow KOP
- Build-up section
- Tangent section
- Drop-off section
The drilling of such type will be with the following sequence:
- First, the wells can be vertically drilled to the kick-off point (shallow depth).
- Then, the well is deflected smoothly until a maximum inclination angle with the desired azimuth.
- Then, it is preferred to run casing if this is applicable.
- After that, keep holding both inclination and azimuth (HOLD).
- Further, Start dropping smoothly till well became vertical again.
- Finally, keep holding the angle till you reach well target.
In addition, there are several variations for these directional wells type:
- Build, hold, and drop back to vertical
- Build, hold, drop, and hold (illustrated here)
- Build, hold, and continuously drop through the reservoir
S Type Wells Profile Applications
- Multiple pay zones.
- To reduce the final angle in the reservoir for easier completions.
- Lease or target limitations.
- For well spacing requirements on multi-well fields.
- Deep wells with a small horizontal displacement.
Disadvantages of Directional Wells S Type
- Increased torque and drag.
- Increased risk of key-seating.
- It may cause logging problems due to increased maximum inclination.
Deep Kick-off and Build
Generally, the features of this trajectory are:
- Deep KOP
- Build-up section
- Short tangent section (optional)
Its applications are:
- Firstly, Appraisal wells to assess the extent of a newly discovered reservoir.
- Secondly, Repositioning of the bottom part of the hole or re-drilling.
- Finally, Salt dome drilling.
The Disadvantages of the Directional Deep Kick-off and Build-Type well
- Formations are more complex, so the initial deflection may be more challenging.
- In addition, it is harder to achieve desired tool face orientation with downhole motor deflection assemblies – more reactive torque.
- Longer tripping pipe time for any bottom hole assembly changes required.
- On multi-well platforms, only a few wells may be given deep kick-off points because of the slight separation of the slots and the difficulty of keeping wells vertical in firmer formations. Most wells must be given shallow kick-off points to reduce congestion below the platform and minimize the risk of collisions.