STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

Downhole Motors 885
stator is made up of ns lodes, which is equal to one lobe more than the rotor.
Typical cross-sections of positive displacement motor lobe profiles are shown in
Figure 4-203. As drilling fluid is pumped through the cavities in each chamber
that lies open between the stator and rotor, the pressure of the flowing fluid
causes the rotor to rotate within the stator. There are several chambers in a
positive displacement motor because the chambers leak fluid. If the first chamber
did not leak when operating, there would be no need for additional chambers.
In general, the larger lobe profile number ratios of a positive displacement
motor, the higher the torque output and the lower the speed (assuming all other
design limitations remain the same).
The rotors are eccentric in their rotation at the bottom of the motor section.
Thus, the connecting rod section provides a flexible coupling between the rotor
and the main drive shaft located in the thrust and radial bearing section. The
main drive shaft has the drill bit connected to its bottom end.
The thrust and radial-bearing section contains the thrust bearings that transfer
the weight-on-bit to the outside wall of the positive displacement motor. The
radial support bearings, usually located above the thrust bearings, ensure that
the main drive shaft rotates about a fixed center. As in most turbine motor
designs, the bearings are cooled by the drilling fluid. There are some recent
positive displacement motor designs that are now using grease-packed, sealed
bearing assemblies. There is usually a smaller upper thrust bearing that allows
rotation of the motor while pulling out of the hole. This upper thrust bearing
is usually at the upper end of thrust and radial bearing section.
There are, of course, variations on the downhole positive displacement motor
design, but the basic sections discussed above will be common to all designs.
The main advantages of the downhole positive displacement motor are:
1. Soft, medium and hard rock formations can be drilled with a positive
displacement motor using nearly any type of rock bit. The positive displacement
motor is especially adaptable to drilling with roller rock bits.
2. Rather moderate flow rates and pressures are required to operate the
positive displacement motor. Thus, most surface pump systems can be used
to operate these downhole motors.
3. Rotary speed of the positive displacement motor is directly proportional
to flowrate. Torque is directly proportional to pressure. Thus, normal surface
instruments can be used to monitor the operation of the motor downhole.
4. High torques and low speeds are obtainable with certain positive displacement
motor designs, particularly, the higher lobe profiles (see Figure 4203).
5. Positive displacement motors can be operated with aerated muds, foam and
air mist.
1.2 3.4 5.6 7.8 9,lO
Figure 4-203. Typical positive displacement motor lobe profiles. (Courtesy
Smith International, Inc.)