Drill holes can be very unpredictable in the path that they take, so drillers often have to deal with hole deviation. Drill hole deviation is when the hole ends up going in a direction other than the chosen trajectory [1]. Let’s explore the various factors inducting hole deviation and provide several solutions.
What causes drillhole deviation?
Improper setup
Every mining operation has had to deal with damaged drill rods or stalled operations due to hole deviation. While your choice of equipment can help, good operating practice is the most effective way to mitigate the problem [2]. Inaccurate initial rig and bit alignment, poor leveling, incorrect feed pressure, and inadequate string stabilization in the setup phase initiate and contribute significantly to hole deviation in drilling operations.
Collaring errors
collaring errors create the highest risk of early misalignment in the drilling cycle, and since the hole trajectory depends heavily on the correctness of the initial collar position and angle, these errors translate directly into hole deviation [2].
Excessive feed pressure
Excessive feed pressure causes the string to bend, thus tilting the bit off-center. The downward force of the string weight and feed pressure cause the bit to pivot during rotation. This motion continuously forces the bit to one side of the hole, resulting in deviation [2].
Misalignment problems
Misalignment in rig setup, feed equipment, collaring, and drill string components causes early bit off-centering and continuous lateral forces, which result in significant hole deviation during drilling.
Ground conditions
In banded ground (successive hard and soft layers), the bit tends to turn into the softer layers. In fractured ground, the bit tends to follow the path of least resistance and follow fissures [2].
How to minimize drillhole deviation?
Strategies to reduce drillhole deviation include:
- Using steerable motor systems in directional drilling to control the drill bit path.
- Maintaining proper weight on the drill bit to keep it aligned.
- Proper planning with the right drilling techniques, fluid management, and drill bit selection.
- Managing drilling fluids to reduce friction and keep the wellbore stable.
- Applying real-time monitoring techniques such as logging while drilling (LWD) and measurement while drilling (MWD) to adjust operations as needed.
- Choosing the appropriate drill bits for the rock formation to avoid bit wandering.
- Practicing good drilling operations like careful hole collaring to avoid bit misalignment.
- Avoiding excessive feed pressure to prevent bending of the drill string and off-center bit rotation.
- Using stabilizers or guide tubes near the drill bit to reduce lateral deviation forces.
- Regular equipment maintenance to avoid worn threads and dull bits which increase deviation.
- Considering geological conditions like banded or fractured ground and adjusting feed pressure accordingly.
To conclude, proper setup, collaring errors, ground conditions and alignment issues are various factors that have a significative impact on drillhole direction. Improper setup can lead to worst results in drilling. Nevertheless, choosing the right tool and using stabilizers and guide tubes are strategies that help minimizing hole deviation.
Reference
[1] Epiroc, “Hole Deviation,” Epiroc. Accessed: Aug. 25, 2025. [Online]. Available: https://www.epiroc.com/en-ca/applications/mining/exploration-and-geoscience/resources/solutions/hole-deviation
[2] Marketing, “How to reduce drill hole deviation,” Boart Longyear. Accessed: Aug. 25, 2025. [Online]. Available: https://www.boartlongyear.com/insite/how-to-reduce-drill-hole-deviation/
