Directional Drilling Fundamentals Introduction to Directional Drilling

When preparing to drill a vertical or directional well, all operational components of the process are reviewed, optimized and included in a drilling program. The surface location is scouted to determine the best site that will allow for any natural drift, provide suitable access for drilling rig, service rig, production facilities and can be constructed for a reasonable cost. A casing program is prepared to provide 1) adequate well control, 2) prevent water table contamination, 3) maintain wellbore integrity, 4) plan for varying formation fracture gradients, and 5) provide hydraulic isolation of various producing zones.

Mud programs are developed to provide good wellbore cleaning, reasonable filter cake development and minimal formation damage. Cement programs are required to provide good hydraulic isolation and casing support given the bottom hole temperature and pressure.

Since the highest possible rate of penetration possible is desired, considerable time is spent preparing an effective bit program to optimally drill the well. Previous wells drilled in the area are reviewed, to determine any potential drilling problems. Finally a proper BHA and drill string design is prepared to provide sufficient design safety parameters.

A directional drilling company will review most of these same components or ask the operator what he is selecting and apply it to the well profile and equipment limitations. For example the drilling fluid needs to be compatible with the Measurement While Drilling (MWD) equipment and motors. With their area knowledge it will also be reviewed for hole cleaning capability for high inclination wells. A drilling motor is selected that will provide optimum performance for the planned hydraulics or modifications are recommended.

A bottom hole assembly (BHA) and drill string design is suggested that will allow the best ROP for the different drilling conditions (rotating versus orient or slide drilling). In some cases the desired well path cannot be optimally drilled with the drill string currently available on the rig and changes are recommended.

Bit selection for a standard vertical well may not be suitable for the planned directional well path. Although a particular PDC bit provides the best ROP for the area, it may not provide the directional control needed. Also special drilling motors may be required to provide sufficient horsepower. If the project involves sidetracking of the horizontal legs special diamond sidetrack bits may be required.

Area formation integrity knowledge while it is being directional drilled through (sloughing, loss of inclination, inability to control direction, potential for differential sticking to name a few) is extremely important to minimize drilling time or potential problems. Let's assume a directional plan with a very tight target size is prepared that kicks off very low in a formation that has a history of erratic build rates. Several things could happen in this scenario:

  • Planned build rates are attained and target reached
  • Very aggressive oriented drilling operations are required (full single slides) and the ROP is half of normal
  • Trips required to change the motor setting (increase or decrease the adjustable housing setting)
  • Erratic doglegs are created that may cause problems later when running casing
  • Target is missed and well must be plugged back and sidetracked

When permitted, a directional company also reviews the well pad layout and provides their recommendations to reduce directional costs for multiple well pads. When they are involved with a project from the start and are aware of future re-entries, multi-laterals, sidetracks and production requirements a more optimally tuned well path can be designed.

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