Figure 1: Mud Motor Assembly
The purpose of this post is to provide a short overview of the components of mud motors used in oil and gas well drilling.
Top Sub
The top sub, or saver sub, is simply a crossover sub to
ensure the motor has the correct connection type. The top sub is located at the top of the motor. The saver sub also takes the
wear of makeup and breakout to extend the usable life of the motor. This area
of the motor may also have float subs or dump subs. Float subs prevent
backwards flow of fluid into the drill string. Dump subs allow fluid to bypass
the motor and flow into or out of the drill string while tripping. Both dump
and float subs are actuated by spring force and fluid flow, and will be located
above the saver sub.
Power Section
The power section is below the top sub section. The power section
is designed to convert hydraulic energy to rotary mechanical power. The power
conversion is accomplished using a Moineau pump, or progressive cavity pump, in
reverse. The progressive cavity pump moves fluid by progressively rotating a sealed
constant volume down the length of the pump by. A mud motor operates in a
similar manner but, rather than rotation moving fluid, the moving fluid causes
rotation.
The power section consists of a stator housing, stator with
rubber lining and a rotor. The rotor sits inside of the stator. Both stator and
rotor have a helical design with a lobed profile. The stator will typically be
smooth bored with molded rubber lining or have a lobed bore with rubber lining.
The rotor has one less lobe than the stator and is made of corrosion resistant
steel.
Figure 1: Stator and Rotor Cross Section
The ratio of stator lobes to rotor lobes is a defining characteristic
of mud motors. Figure 1 shows a 5:4 lobe ratio. The stator has 5 lobes and the
rotor has 4 lobes. Increasing the lobe ratio has the same consequence as
increasing the length of that cheater bar you use on the rig that is against
company policy, it increases the torque output of the motor at constant
differential pressure. Increasing lobe ratio also decreases final drive speed.
Decreasing the lobe ratio increases final drive speed and reduces torque.
Another defining characteristics of the mud motor is stage
number. Stage number is the number of full twists that the stator makes from
the top end to the bottom end. The stage number for the rotor is not equal to
the stage number for the stator. The rotor has more stages than the stator.
Increasing the number of stages increases the pressure drop across the power
section. Torque output is directly proportional to the pressure drop. So,
increasing the number of stages increases the torque output.
Keep in mind that changing flow rate will change speed and
horsepower for the motor is a function of both torque and speed.
Transmission Section
The transmission section transmits the torque generated by
the power section to the final drive or output shaft of the mud motor. The
transmission section must be capable of handling the eccentric rotation of the
power section’s rotor. Common methods for handling the eccentric rotation
include the use of CV joints or flex shafts. The transmission section also accommodates
the misalignment caused by the use of a bent housing and supports down thrust
of the power section. The bent housing may be adjustable or have a fixed bend.
If the bent housing is adjustable it should come from the shop with the correct
bend, but at least two qualified field personnel should check the bend angle to
confirm that it is correct. If you’re working with a Baker motor be sure to
double and triple check to make sure that the bend is correct as they are
easier to read incorrectly.
Bearing Section
The bearing section is below the transmission/bent housing
section and consists of a drive shaft, flow divertor/restrictor, and both
thrust and radial bearings. The drive shaft, which transmits power from the
transmission section to the bit, is kept concentric by the bearings in the
bearing assembly. The radial bearings in the assembly allow the bit box to
rotate independently of the drill string. The thrust bearings act to support down
thrust from the rotor, bit pressure loss, and upward thrust due to weight being
applied at the bit. The flow diverter is used to divert drilling mud flow
through the bearings which both lubricates and cools the bearings.
The bearing section may be equipped with a stabilizer or
protective sleeve that can be changed out on the rig floor. Personally, I’ve
only seen this process performed once and it added a significant amount of time
to BHA changeout.
Bit Box
The bit box is a box end threaded connection that the bit
threads into. The bit box will typically be integrated directly into the drive
shaft. The bit box is capable of rotating freely within the bearing section. Recall
that the thrust bearings in the bearing assembly support thrust from applied
WOB during drilling. This means that the bit box will have some amount of axial
movement or play. Excessive axial play in the bit box is an indication of worn bearings
or bearing failure.
For more information on mud motors check out the following resources.
https://petrowiki.org/Directional_deviation_tools
http://www.iadc.org/wp-content/uploads/2015/08/preview-dh.pdf
http://torotools.com/Toro_Drilling_Motor_Handbook.pdf
https://cjenergy.com/wp-content/uploads/2017/05/directional-services-motor-handbook-11-7-14.pdf
http://www.cougards.com/wp-content/uploads/2013/04/Motor-Operations-Handbook-2012.pdf
https://www.slb.com/~/media/Files/drilling/brochures/directional_drilling/powerpak_handbook.pdf
http://www.iadc.org/wp-content/uploads/2015/08/preview-dd.pdf
