STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

Downhole Motors 863
In the late 19509, with the growing need in the United States and elsewhere in
the world for directional drilling capabilities, the drilling industry in the United
States and elsewhere began to reconsider the downhole turbine motor technology.
There are presently three service companies that offer downhole turbine motors for
drilling of oil and gas wells. These motors are now used extensively throughout the
world for directional drilling operations and for some straight-hole drilling operations.
The downhole turbine motors that are hydraulically operated have some
fundamental limitations. One of these is high rotary speed of the motor and
drill bit. The high rotary speeds limit the use of downhole turbine motors when
drilling with roller rock bits. The high speed of these direct drive motors
shortens the life of the roller rock bit.
In the 1980s in the United States an effort was initiated to develop a downhole
turbine motor that was activated by compressed air. This motor was provided
with a gear reducer transmission. This downhole pneumatic turbine has been
successfully field tested [82].
The development of positive displacement downhole motors began in the late
1950s. The initial development was the result of a United States patent filed by
W. Clark in 1957. This downhole motor was based on the original work of a
French engineer, RenC Monineau, and is classified as a helimotor. The motor
is actuated by drilling mud pumped from the surface. There are two other types
of positive displacement motors that have been used, or are at present in use
today: the vane motor and the reciprocating motor. However, by far the most
widely used positive displacement motor is the helimotor [ 79,831.
The initial work in the United States led to the highly successful single-lobe
helimotor. From the late 1950s until the late 1980s there have been a number
of other versions of the helimotor developed and fielded. In general, most of
the recent development work in helimotors has centered around multilobe
motors. The higher the lobe system, the lower the speed of these direct drive
motors and the higher the operating torque.
There have been some efforts over the past three decades to develop positive
development vane motors and reciprocating motors for operation with drilling
mud as the actuating fluid. These efforts have not been successful.
In the early 1960s efforts were made in the United States to operate vane
motors and reciprocating motors with compressed air. The vane motors experienced
some limited test success but were not competitive in the market of that
day [84]. Out of these development efforts evolved the reciprocating (compressed)
air hammers that have been quite successful and are operated extensively in the
mining industry and have some limited application in the oil and gas industry
[85]. The air hammer is not a motor in the true sense of rotating equipment.
The reciprocating action of the air hammer provides a percussion effect on the
drill bit, the rotation of the bit to new rock face location is carried out by the
conventional rotation of the drill string.
In this section the design and the operational characteristics and procedures
of the most frequently used downhole motors will be discussed. These are the
downhole turbine motor and the downhole positive displacement motor.
Turbine Motors
Figure 4-190 shows the typical rotor and stator configuration for a single stage
of a multistage downhole turbine motor section. The activating drilling mud or
freshwater is pumped at high velocity through the motor section, which, because
of the vane angle of each rotor and stator (which is a stage), causes the rotor to
rotate the shaft of the motor. The kinetic energy of the flowing drilling mud is
converted through these rotor and stator stages into mechanical rotational energy.