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Hole deviation is the unintentional departure of the drill bit from a preselected borehole trajectory. Whether it involves drilling a straight or curved-hole section, the tendency of the bit to walk away from the desired path can lead to drilling problems such as higher drilling costs and also lease-boundary legal problems Fig. 1 provides examples of hole deviations.
Causes of hole deviation
It is not exactly known what causes a drill bit to deviate from its intended path. It is, however, generally agreed that one or a combination of several of the following factors may be responsible for the deviation:
- Heterogeneous nature of formation and dip angle
- Drillstring characteristics, specifically the bottomhole assembly (BHA) makeup
- Stabilizers (location, number, and clearances)
- Applied weight on bit (WOB)
- Hole-inclination angle from vertical
- Drill-bit type and its basic mechanical design
- Hydraulics at the bit
- Improper hole cleaning
It is known that some resultant force acting on a drill bit causes hole deviation to occur. The mechanics of this resultant force is complex and is governed mainly by the mechanics of the BHA, rock/bit interaction, bit operating conditions, and, to some lesser extent, by the drilling-fluid hydraulics.
The forces imparted to the drill bit because of the BHA are directly related to the makeup of the BHA, i.e.:
The BHA is a flexible, elastic structural member that can buckle under compressive loads. The buckled shape of a given designed BHA depends on the amount of applied WOB. The significance of the BHA buckling is that it causes the axis of the drill bit to misalign with the axis of the intended hole path, thus causing the deviation. Pipe stiffness and length and the number of stabilizers (their location and clearances from the wall of the wellbore) are two major parameters that govern BHA buckling behavior. Actions that can minimize the buckling tendency of the BHA include reducing WOB and using stabilizers with outside diameters that are almost in gauge with the wall of the borehole.
The contribution of the rock/bit interaction to bit deviating forces is governed by:
- Rock properties
- Cohesive strength
- Bedding or dip angle
- Internal friction angle
- Drill-bit design features
- Tooth angle
- Bit size
- Bit type
- Bit offset in case of roller-cone bits
- Teeth location and number
- Bit profile
- Bit hydraulic features
- Drilling parameters
- Tooth penetration into the rock and its cutting mechanism
The mechanics of rock/bit interaction is a very complex subject and is the least understood in regard to hole-deviation problems. Fortunately, the advent of downhole measurement-while-drilling tools that allow monitoring the advance of the drill bit along the desired path makes our lack of understanding of the mechanics of hole deviation more acceptable.