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OTHER ARTIFICIAL LIFT METHODS 14/209
the ESP, sensing such conditions as overload, well pump-
off, short in cable, and so on. It also shuts down or starts
up in response to down-hole pressure switches, tank levels,
or remote commands. These controllers are available in
conventional electromechanical or solid-state devices.
Diffuser
Conventional electromechanical controllers give a fixed-
speed, fixed flow rate pumping. To overcome this limita-
tion, the variable speed controller has been developed
(solid state). These controllers allow the frequency of the
electric current to vary. This results in a variation in speed
Impeller
(rpm) and, thus, flow rate. Such a device allows changes to
be made (on the fly) whenever a well changes volume
(static level), pressure, GLR, or WOR. It also allows flexi-
bility for operations in wells where the PI is not well
known. The transformer (at surface) changes the voltage
of the distribution system to a voltage required by the ESP
system.
Unlike positive-displacement pumps, centrifugal pumps
do not displace a fixed amount of fluid but create a rela-
tively constant amount of pressure increase to the flow
Figure 14.2 An internal schematic of centrifugal pump. system. The output flow rate depends on backpressure.
The pressure increase is usually expressed as pumping
head, the equivalent height of freshwater that the pressure
differential can support (pumps are tested with freshwater
Like most down-hole tools in the oil field, ESPs are clas-
sified by their outside diameter (from 3.5 to 10.0 in.). The by the manufacturer). In U.S. field units, the pumping
number of stages to be used in a particular outside diam- head is expressed as
eter sized pump is determined by the volumetric flow rate Dp
and the lift (height) required. Thus, the length of a pump h ¼ 0:433 , (14:1)
module can be 40–344 in. in length. Electric motors are where
three-phase (AC), squirrel cage, induction type. They can h ¼ pumping head, ft
vary from 10 to 750 hp at 60 Hz or 50 Hz (and range from Dp ¼ pump pressure differential, psi.
3
1
3 ⁄ 4 to 7 ⁄ 4 in. in diameter). Their voltage requirements vary
from 420–4,200 V. As the volumetric throughput increases, the pumping
The seal system (the protector) separates the well fluids head of a centrifugal pump decreases and power slightly
from the electric motor lubrication fluids and the electrical increases. However, there exists an optimal range of flow
wiring. The electric controller (surface) serves to energize rate where the pump efficiency is maximal. A typical ESP
characteristic chart is shown in Fig. 14.4.
ESPs can operate over a wide range of parameters
(depths and volumes), to depths over 12,000 ft and volu-
metric flow rates of up to 45,000 bbl/day. Certain operat-
ing variables can severely limit ESP applications, including
the following:
. Free gas in oil
. Temperature at depth
. Viscosity of oil
. Sand content fluid
. Paraffin content of fluid
Excessive free gas results in pump cavitation that
leads to motor fluctuations that ultimately reduces run
life and reliability. High temperature at depth will
limit the life of the thrust bearing, the epoxy encapsula-
tions (of electronics, etc.), insulation, and elastomers.
Increased viscosity of the fluid to be pumped reduces
the total head that the pump system can generate, which
leads to an increased number of pump stages and increased
horsepower requirements. Sand and paraffin content in
the fluid will lead to wear and choking conditions inside
the pump.

14.2.2 ESP Applications
The following factors are important in designing ESP
applications:
. PI of the well
. Casing and tubing sizes
. Static liquid level
ESPs are usually for high PI wells. More and more ESP
applications are found in offshore wells. The outside di-
ameter of the ESP down-hole equipment is determined by
Figure 14.3 A sketch of a multistage centrifugal pump. the inside diameter (ID) of the borehole. There must be