Downhole motor

A mud motor (or drilling motor) is a progressive cavity positive displacement pump (PCPD) placed in the drill string to provide additional power to the bit while drilling. The PCPD pump uses drilling fluid (commonly referred to as drilling mud, or just mud) to create eccentric motion in the power section of the motor which is transferred as concentric power to the drill bit (well). 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.

Based on the principle developed by Rene Moineau, the theory states that a helical rotor with one or more lobes will rotate eccentrically when the stator contains more lobes than the rotor. The flow of the fluid transmits power allowing the assembly to rotate and turn the bit.

Normal mud motor construction consists of a top sub, which connects the mud motor to the drill string; the power section, which consists of the rotor and stator; the transmission section, where the eccentric power from the rotor is transmitted as concentric power to the bit using a constant-velocity joint; the bearing assembly which protects the tool from off bottom and on bottom pressures; and the bottom sub which connects the mud motor to the bit.

When the bit is bottomed and the motor is effectively working, there is a notable increase in the pressure in the fluid system. This is caused by a restriction within the motor and is termed the "differential pressure". If this differential pressure is too high then the motor can stall which means the bit has stopped turning and this can cause severe damage to the internal surface of the stator.

A mud motor is described in terms of its number of stages (usually three), lobe ratio and external diameter. Stages are the number of full twists that the stator makes from one end to the other and the lobe ratio is the number of lobes on the stator, to the number of lobes on the rotor (the stator always has one more lobe than the rotor). A higher number of stages or the number of lobes indicates a more powerful motor, capable of producing more torque. A lower number of lobes indicates a reduction in the torque produced but a faster bit rotation speed.

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Wikipedia