Simply wanting a hole in the ground just isn’t enough – you need the right tools for the job. In this Drilling Minute, we’ll introduce the major drill string components and what they do.
Failing the Rock
Every tool used in drilling is there to make the hole, but only drill bits and reamers, or hole-openers, actually fail the rock. The drill bit, which always goes on the very end of the drill string, fails formation ahead of the drill string. Reamers reach out to enlarge the hole made by the bit and can go anywhere below the neutral point to ensure they have some weight to drill with.
Speaking of weight, it has to come from somewhere. Drill collars are pipe joints with extremely thick walls. They are designed to be as heavy as possible, but are also very rigid. Heavy-weight drill pipe has thick, weighted sections like collars but also thin sections for flexibility. Collars are usually used very near the bit, while heavy-weight is used farther up the string.
Most drilling energy comes from string weight and rig torque. But in many cases drive tools like motors or turbines are used to convert hydraulic energy into additional torque, changing fluid flow into RPMs.
Reaching Target
A well always has a directional goal – even if it is straight. Stabilizers work to centralize the BHA as it rotates by contacting the hole-wall on all sides. This can help keep the well straight, but is mainly used to manage vibration. In contrast, using a bias or bend can help steer the bit. Most often the bend is built into a bent, or steerable, motor. Rotating the bend makes it impossible to steer consistently, but rotary steerable tools have been developed to push or point the bit in a controlled direction even while the BHA is rotating. These tools, like all drive tools, are run directly behind the bit.
Steering is only helpful if you can see where you are going, which is the purpose of MWD and LWD, or Measuring and Logging While Drilling tools. These tools send measurements about the formation, drilling parameters and directional orientation back to the surface, and are usually assembled behind any drive tools.
Crossover adapters are used to link together tool joints that don’t have matching connections. Another class of tools exists to aid in axial movement of the drill string, such as jars, shock tools, and drag tools, which either attenuate or amplify the push or pull on the drill string.
There are literally hundreds of different tools which aren’t used alone, but assembled into a built-for-purpose BHA, which is the topic of the next Drilling Minute.
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