Well drilling (oil and gas).

Andrew hit the nail right on the head. As an MWD field engineer for a directional drill team, I will add some details to the physical process of directional drilling.

A well can be "steered" by moving the drill bit in the desired direction to increase its angle. A well's deviation can vary depending on the type of drilling assembly used. In extreme cases, it may be as high as 30-35 degrees per 100 feet. A lot of companies will deviate 90 degrees in order to create horizontal wells that can extend from 1500 to 20,000 feet.

There are two main ways to divert a well from the desired direction.

1. To drill "rotary" and slide sequences, use a bent-sub motor mud motor.

A bent-submotor is similar to a normal motor (a stator assembly that converts mud flow into clockwise torque in order to turn a drill bits at the bottom), except it has a slight bend towards the bottom.

This bend serves to facilitate "sliding". The entire drill string, which includes all segments of drill pipe that run down to the bit, is rotated. Because the drill string is evenly distributed, the bend does not affect the direction of rotation.

Slide refers to when the rig stops rotating the dill string, but continues to circulate fluid through it to turn the bit. The motor bend is directed in a specific direction so the bit tends not to drill in the same direction as the existing wellbore.

It is important to determine which direction the bend is facing. This is where measurement-while-drilling (MWD) tools come into play. MWD instruments are small computers that are housed in cylindrical metal tubes and are placed within special pipes approximately 30 feet above the motor or behind it. Or 60-70ft behind the drill bit. They have magnetometers and accelerometers that determine the orientation of the motor and wellbore.

MWD instruments have one function: to determine the orientation or position of the toolface, which is the position of the motor bend relative to the drill string axis. This information can be used for pointing the bend in the desired direction. The directional driller can keep that position, while sliding for as much time as they feel necessary to achieve the desired angle.

As the well is being drilled, the MWD instruments measure the inclination and azimuth at regular intervals. These measurements are known as "deviation surveys." These measurements are called "deviation surveys" by directional drillers. They use this information, along with the measured well depth (based on the total length of the pipe in the ground), to triangulate both the position and the entire path of a well in 3-D space.

MWD tools need to transmit data to the surface in order to be used for drilling wells. This can be done by encode binary data in one of many physical medias.

Another method is to send electrical current through the earth formation to engineer's surface equipment. A special valve can be used to temporarily restrict the flow of mud along the inside of the drill pipe. This creates small pressure spikes that can be detected at surface.

When there is a lot of sliding to create high angles, the slide/rotate sequence may be slow. Drill string rotation has a greater rate of penetration into earth formations than drill string rotation, so rotation must be stopped in order to slide.

Although the slide/rotate process can be complicated and tedious at times, many companies see huge benefits from increased oil recovery through optimal well placement.

2. To rotate the well, use a rotary steering motor.

The rotary steerable motors are higher-tech than the bent-sub motors and I don't really know much about them. However, I will share the things I know.

They have no bend in their rotary steerable motors. They are instead shaped with 3 metal pads that run around the motor's circumference and push against the wall.

The pads are usually motionless when the motor is set to drill straight. When the motor is set to drill in a straight line, the pads remain motionless. Instead, the pads are extended in rapid succession one at a while so that each pad forces a particular side of the motor while it rotates.

The motor houses a computer that measures the rotation rate, inclination, and azimuth. It can also perform MWD instrumentation functions, as it should.

The control of the rotary steerable motor uses the same process MWD tools use for communicating with surface equipment. However, it works in reverse. The motor will be controlled by the directional driller, which rotates at different speeds and at certain times. The signal is decoded by the computer and the instruction is executed (e.g. "Seek 90.5 deg. inclination and 346.8deg. azimuth").

A rotary steerable motor has a greater drilling speed than a bent-sub motor. This is due to its constant rotation. Their high cost is the main problem. Rotary steerable motors may not be practical for small businesses. Larger companies might benefit from faster drilling due to higher drilling costs per hour overall.

There are other methods of well deviation, but they are rare and only used in certain situations.