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A downhole motor is used in the oil field to drive the drill bit or other downhole
tools during directional drilling or performance drilling applications. As drilling
fluid (also called drilling mud) is pumped through the positive displacement
motor, it converts the hydraulic power of the fluid into mechanical power to
cause the bit to rotate.
In other words, a mud motor uses drilling fluid to create eccentric motion in the
power section of the motor which is transferred as concentric power to the drill
bit. The mud motor uses different rotor and stator configurations to provide
optimum performance for the desired drilling operation, typically increasing the
number of lobes and length of power assembly for greater horsepower. In
certain applications, compressed air, or other gas, can be used for mud motor
input power. Normal rotation of the bit while using a mud motor can be from 60
rpm, to over 100 rpm.
During directional drilling, this capability is used while drilling in sliding mode,
when the drillstring is not rotated from the surface. Positive displacement motors
can also be used for performance drilling, straight hole drilling, coring,
underreaming, and milling operations. In straight hole drilling, the motor
functions as a drilling performance tool to increase the rate of penetration and
reduce casing wear by minimizing drillstring rotation.