At some point, every driller realizes that despite precautions and preventative measures, the drill bit can deviate from the trajectory chosen. The degree of the deviation depends on various factors such as the size of the hole, the type of ground, the pressure on the bit and the depth of the hole.
Generally speaking, holes drilled horizontally deviate more than vertical or steep angled holes, holes drilled in variable rock deviate more than holes drilled in competent rock. When your drilling program requires changing the directing of a bore hole, there are a few options to consider.
In the past, wedges have been used to correct both minor and major bore hole deviations. They are also used to drill around tools that have become stuck in the hole, such as drill rods and core barrels, and to deflect off a bore hole to make an additional intersection of an ore body. A product like the one-trip wedge can reduce the time required to correct a bore hole deviation by reducing the trip time.
Downhole Motor (DHM)
DHM drilling is an ideal option for steering or deviating a bore hole as it creates a smooth transition in deviation from the old course to a new course. The downhole motor is powered by water circulation so the bit is not rotated by the drill rod. The system uses a spring-loaded deflection shoe that pushes against the drill hole wall to force the bit in the opposite direction. Progress of the hole’s new trajectory is monitored by survey tests and corrected if necessary. Once the target is reached the rods are tripped out of the hole, DHM motor is removed and normal coring operations can continue
Directional Core Drilling (DCD)
Directional core drilling is done with a specialized core barrel that can control borehole deviation and steer the hole towards the target, while collecting core at the same time. This core barrel has a drive shaft running through a bushing offset from the center line of the tool. Expanding pads keep a fixed orientation while drilling in a curve. In a typical operation, when the borehole begins to deviate off of the desired path, the tool is installed and the borehole is steered back on line to remain on target. Core is still collected during the directional cut, however the size of the core is smaller (approx. 1.062” in Diameter)
You must consider all the variables to find the ideal solution for your drilling operation. While DCD allows you to collect core during the directional cut, it is offered as a service. This means it is more expensive as it requires trained technicians. DHM is a less expensive and simpler option that can be performed by most drillers, but you will not be able to recover core. Wedging can be done by most drillers but it is time consuming, costly and less accurate. However, keep in mind that all of the above options can be a better choice than moving a drill rig and starting a new hole which can be more costly and has greater environmental impact.