STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

886 Drilling and Well Completions
The main disadvantages of the downhole positive displacement motors are:
1. When the rotor shaft of the positive displacement motor is not rotating,
the surface pump pressure will rise sharply and little fluid will pass through
the motor.
2. The elastomer of the stator can be damaged by high temperatures and
some hydrocarbons.
Operations
Figure 4-204 gives the typical performance characteristics of a positive
displacement motor. The example in this figure is a 6+-in. outside diameter
positive displacement motor having five chambers activated by a 400-lb/gal
flowrate of drilling mud.
For this example, a pressure of about 100 psi is required to start the rotor
shaft against the internal friction of the rotor moving in the elastomer stator
(and the bearings). With constant flowrate, the positive displacement motor will
run at or near constant speed. Thus, this 1:2 lobe profile example motor has an
Circulation Rate 400 gpm
130
120
110
100
90
80
9
K
I-
40
30
20
10
0 100 200 300 400 500 800
Differential Pressure (psi)
Figure 4-204. Positive displacement motor, 63/44. outside diameter, 1 :2 lobe
profile, 400 gal/min, differential pressure limit 580 psi. (Courtesy Smith
International, Inc.)