Wedges are used in conventional or wireline drilling. A wedge is set in a borehole so that it deflects the drill string in the desired direction when drilling recommences.
Roller jet bits are used in oil drilling. A roller bit with a large jet on one side is often used to change direction. The drill rods are held in one position and the face of the borehole often erodes in one direction.
Deflecting and jetting bits are used in soils to push through the formation. These bits incorporate a shoe with a cutting edge, and they are usually set at an angle to the drill string. When pushed into the soil without rotation they tend to change the angle of the hole. Some bits can be rotated to drill ahead in a straight line, while others are fitted with teeth to cut rock if needed. This action is often augmented by water jets orientated out of the line of the borehole. Pushed rods follow the eroded path of the borehole.
Rotating nozzle water jet drilling is used in coal mineral exploration, coal seam methane gas drilling, civil engineering, and trenchless technology sectors. Rotating nozzles are driven by jet reaction and spread across the area over which the jet acts, thus opening up a borehole, as opposed to a discrete hole, in front of the jet. Directional control of such systems can be achieved by redirecting the orientation of the jets.
Downhole motors with bent sub or deflection pads are probably the most widely used systems for controlling the direction of drilling in mineral and coal seam methane gas exploration and trenchless technology sectors. They are mainly positive displacement progressive helical cavity types, though turbines and vane motors have also been used. These are powered by drilling fluid to change direction, hold the drill rods in a fixed orientation, and pump fluid through the motor that rotates the drill bit while advancing the drill rods. Because the motor is set at an angle to the line of the borehole, it preferentially drills in that direction.
Stabiliser load combinations are used in the mineral exploration and water well sectors. They deviate the borehole from the vertical but generally in an uncontrolled direction. Once it starts to deviate, it tends to maintain that direction. The borehole is deviated by applying substantial load to the bottom hole assembly (BHA), thus causing the drill rods to buckle in the borehole and drill of vertical. Usually, the BHA is fitted with one or two stabilisers to improve control and prevent deflection.
Directional coring is commonly used in Australia to drill a core hole for such applications as a raise bore pilot hole, in mineral exploration, and in mineral production drilling. If the hole gets out of line, a correction is made by replacing the core barrel with a downhole motor and bent sub, to drill the correction. This procedure is slow and means that sections of core are missed. An alternative is the Vic drill head. It is equally slow, but it avoids missing core sections. It uses a wide kerf core bit and small diameter barrel deviated by eccentric bushes that are locked in a particular orientation by a packer.
Directional hammers enhance penetration rates, especially in hard rock. They have a similar form to a conventional mud motor and bent sub combination, except that the mud motor is replaced by a downhole hammer with a mechanism next to the bit.
