STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

866 Drilling and Well Completions
Table 4-1 09
Turbine Motor, 6V4-in. Outside Diameter,
Circulation Rate 400 gpm, Mud Weight 10 Wgal
Number Optimum Differential Thrust
of Torque* Bit Speed Pressure Load
Stages (ft-ibs) (rpm) (Psi) Horsepower' (1000 Ibs)
212 1412 807 1324 217 21
318 2118 807 1924 326 30
'At optimum speed
Courtesy Eastman-Christensen
about center even when a side force on the bit is present during directional
drilling operations.
There are of course variations on the downhole turbine motor design, but
the basic sections discussed above will be common to all designs.
The main advantages of the downhole turbine motor are:
1. Hard to extremely hard competent rock formations can be drilled with
turbine motors using diamond or the new polycrystalline diamond bits.
2. Rather high rates of penetration can be achieved since bit rotation speeds
are high.
3. Will allow circulation of the borehole regardless of motor horsepower or
torque being produced by the motor. Circulation can even take place when
the motor is installed.
The main disadvantages of the downhole turbine motor are:
1. Motor speeds and, therefore, bit speeds are high, which limits the use of
roller rock bits.
2. The required flowrate through the downhole turbine motor and the
resulting pressure drop through the motor require large surface pump
systems, significantly larger pump systems than are normally available for
most land and for some offshore drilling operations.
3. Unless a measure while drilling instrument is used, there is no way to
ascertain whether the turbine motor is operating efficiently since rotation
speed and/or torque cannot be measured using normal surface data (i.e.,
standpipe pressure, weight on bit, etc.).
4. Because of the necessity to use many stages in the turbine motor to obtain
the needed power to drill, the downhole turbine motor is often quite long.
Thus the ability to use these motors for high-angle course corrections can
be limited.
5. Downhole turbine motors are sensitive to fouling agents in the mud;
therefore, when running a turbine motor steps must be taken to provide
particle-free drilling mud.
6. Downhole turbine motors can only be operated with drilling mud.
Operations
Figure 4-1 92 gives the typical performance characteristics of a turbine motor.
The example in this figure is a 6$-in. outside diameter turbine motor having
212 stages and activated by a 10-lb/gal mud flowrate of 400 gal/min.