Hole deviation in drilling occurs when the drill bit unintentionally strays from its planned path, influenced by a combination of geological, mechanical, and operational factors. These deviations can lead to inefficiencies, increased costs, and safety risks.
Borehole deviation is significant when it’s more than 2⁰/30m. It is influenced by multiple parameters such as Geological factors (structural parameters), hole depth, hole diameter, bit pressure and core barrel and the quality of equipment.
Geological factors
Alternating layers of hard and soft rock cause uneven weight distribution on the bit, leading to lateral forces that steer the bit toward softer layers. The bit follows paths of least resistance, such as fractures or faults. Steeply inclined formations exert directional forces on the bit.
The hole depth
The depth of the borehole influences the deviation. The greater the depth, the greater the likelihood of a borehole deviation.
The hole diameter
The diameter of the borehole is a parameter that should not be overlooked. The larger the diameter of the hole, the less likely it is that a deviation will be observed, which is not the case with smaller diameters where the probability is greater.
The Bit pressure and core barrel
If the bit pressure is too high, it might reach to hole deviation, so it is important to calibrate the pressure to avoid deviation. The core barrel helps to reduce the deviation.
Equipment and bit design
Cross bits generally drill straighter than button bits but are costlier. Damaged drill strings or stabilizers reduce control over bit trajectory. Insufficient stabilization near the bit amplifies asymmetric forces.
Operational and setup issues
Misalignment during initial drilling (“skating”) sets a deviated trajectory. Improper alignment of the drill rig or feed beam introduces early deviations. Cuttings accumulation alters bit dynamics and increases drag.
