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Downhole Motor Assembly

There are 2 major types of Downhole Motor’s powered by mudflow.

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1) The turbine, which is basically a centrifugal or axial pump.
2) The positive displacement mud motor (PDM). 
Turbines were in wide use a number of years ago and are seeing some increased use lately but the PDM is the main workhorse for directional drilling.

Four configurations of drilling motors provide the broad range of bit speeds and torque outputs required satisfying a multitude of drilling applications. These configurations include:

High Speed / Low Torque 

Medium Speed / Medium Torque 

Low Speed / High Torque 

Low Speed / High Torque -Gear Reduced

The high speed drilling motor utilizes a 1:2 lobe power section to produce high speeds and low torque outputs. They are popular choices when drilling with a diamond bit, tri-cone bit drilling in soft formations and directional applications where single shot orientations are being used. 

The medium speed drilling motor typically utilizes a 4:5 lobe power section to produce medium speeds and medium torque outputs. They are commonly used in most conventional directional and horizontal wells, in diamond bit and coring applications, as well as sidetracking. 

The low speed drilling motor typically utilizes a 7:8 lobe power section to produce low speeds and high torque outputs. They are used in directional and horizontal wells, medium to hard formation drilling, and PDC bit drilling applications. 

The gear reduced drilling motor combines a patented gear reduction system with a 1:2 lobe high speed power section. This system reduces the output speed of the 1:2 lobe power section by a factor of three, and increases the output torque by a factor of three. The result is a drilling motor with similar performance outputs as a low speed drilling motor, but with some significant benefits. The 1:2 lobe power section is more efficient at converting hydraulic power to mechanical power than a multi-lobe power section and also maintains more consistent bit speed as weight on bit is applied. This motor can be used in directional and horizontal wells, hard formation drilling, and PDC bit drilling applications. 

Some other motor selections are also available including a tandem and modified motor. These variations are described below. 

Tandem Drilling Motor – The drilling motor utilizes two linked power sections for increased torque capacity. 

Modified Drilling Motor – The bearing section of the drilling motor has been modified to provide different drilling characteristics (ie. change bit to bend distance, etc.)

Components

All drilling motors consist of five major assemblies:

  1. Dump Sub Assembly 
  2. Power Section     
  3. Drive Assembly 
  4. Adjustable Assembly 
  5. Sealed or Mud Lubricated Bearing Section.

Tripping In Hole

Generally, a drill string with a drilling motor can be run into the hole like a standard bottom hole assembly. The drilling motor is rugged, but care should be taken to control travel speed while tripping into the hole. The drill string should be tripped with the blocks unlocked and special care must be taken when passing the B.O.P., casing shoe, liner hanger, bridges and nearing bottom. Tight spots should be traversed by starting the pumps and slowly reaming the drilling motor through. When reaming, the drill string should be periodically rotated to prevent sidetracking. Great care should be taken during these operations. 

When tripping to extreme depths, or when hole temperatures are high, periodic stops are recommended to break circulation. This prevents bit plugging and aids in cooling the drilling motor, preventing high temperature damage. 

Fluid should not be circulated through a drilling motor inside casing if a PDC or diamond bit is being used, as this may damage the bit cutters. 

If a dump sub assembly is not used and the pipe is not being filled while tripping in, the back pressure on the power section will cause the rotor to turn in reverse. This could cause internal connections of the drilling motor to unscrew. Stop and break circulation before putting drilling motor on-bottom. Failure to do so could plug jets and/or damage the drilling motor.

Drilling

After the assembly has been tripped to the bottom of the hole, drilling motors should be operated in the following manner:

  1. With the bit 1-2 meters (3-6 feet) off bottom, start the pumps and slowly increase the flow rate to that desired for drilling. Do not exceed the maximum rated flow rate for the drilling motor. 
  2. Make a note of the flow rate and the total pump pressure. Note that the pressure may exceed the calculated off bottom pressure due to some side load effects between the bit and the hole diameter. 
  3. After a short cleaning interval, lower the bit carefully to bottom and slowly increase the weight. Torque can be affected by a dirty, uncirculated hole and the hole should be adequately cleaned prior to orienting the tool. Fill maybe cleaned out of the wellbore by slowly rotating the drilling motor or by staging the drilling motor full circle 30o to 45o at a time. This prevents ledge buildup and side tracking. 
  4. Orient the drill string as desired and slowly apply further weight onto the bit. Pump pressure will rise as the weight on bit is increased. Record the change in system pressure between the off bottom and on bottom values. This will be the differential pressure. Try to drill with steady pump pressure by keeping a steady flow rate and constant weight on bit. 
  5. Adding weight on bit will cause both the differential pressure and torque to increase. Similarly, reducing weight on bit will reduce both the differential pressure and the torque. Therefore, the rig pressure gauge enables the operator to monitor how the drilling motor is performing, as well as a weight on bit indicator. 
  6. Applying excessive weight on bit may cause damage to the on-bottom thrust bearings. Similarly, applying excessive tension while stuck may cause damage to the off-bottom thrust bearings. Refer to the manufacturer specifications for the recommended maximum loads for these conditions. 
  7. Optimum differential pressure can be determined by monitoring motor performance, penetration rate, and drilling requirements. Also, maintaining a constant weight on bit and differential pressure assists in controlling orientation of the drill string.