2.4b Progressing Cavity Pumping
This section presents the operating principals, operating limits, and system
requirements for progressing cavity pump systems. Recommended practices, operating considerations, and costs are discussed. This discussion will
be limited to conventional and insertable PC pumps used for gas well deliquification. The emphasis will be on CBM/CSG wells and shallow gas applications since production from PC pump systems becomes limited for deeper
applications.
2.4b.1 System Description
The surface and subsurface equipment for a typical electric drive system are
shown below:
Surface Equipment
Belts & Sheaves
Surface Drive
Electric Motor
Stuffing Box
Pumping Tee
Polished Rod
Sucker Rods
w/ Couplings
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Downhole Equipment
Tubing String
Sucker Rods
w/ Couplings
PC Pump Stator
PC Pump Rotor
Tagbar
No-Turn Tool
(Illustrations courtesy of Weatherford International)
Surface drives are typically electrically driven. Gas engine driven generators
can be used to supply electricity where line power is not available. Hydraulic
transmissions consisting of an engine driven pump driving a hydraulic motor
on the surface drive are also common.
Most surface drives have belt and sheave reductions to provide additional
speed adjustment. Geared systems and inline electric or hydraulic systems
are also available. All surface drive systems must have adequate backspin
control and the ability to absorb the stored energy of the rod string torsion
plus the full column of fluid.
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The stuffing box includes a seal to keep pressurized well fluids from escaping. Stuffing boxes for PC pump systems are specifically designed to seal
against rotating polished rods. Stuffing boxes designed for reciprocating rod
applications should not be used for PC pump systems unless they are qualified by the manufacturer for use with rotating rods or PC pump systems.
Below the stuffing box the pumping tee provides a flow connection to the
production tubing. Special “composite” pumping tees for PC pump systems
clamp on the polished rod to support the rod string and seal against the polished rod to isolate well fluids. This allows the stuffing box and surface drive
to be serviced or removed safely without pulling the rod string.
The stators for conventional PC pumps are run as part of the production tubing string. The rotor is run into the well on the end of the rod string which
consists of either jointed sucker rod or continuous rod.
The downhole assembly includes a tagbar to allow the rotor to be positively
located relative to the stator. Installation procedures include running the rotor to the tagbar and then spacing back to align the rotor into the stator. The
rotor space-out is unique to the pump geometry, well operating conditions,
and rod string configuration.
Tubing insertable systems are available in which the stator and rotor are run
as one assembly with the rod string inside of the tubing. This simplifies running and retrieval, so insertable pumps are usually preferred over conventional pumps. The use of insertable pumps is limited by the tubing size.
Large systems may require a no-turn device to prevent the torque in the
pump from loosening the tubing connections. Smaller systems with properly
torqued tubing typically do not require no-turn devices although no-turn devices are often included as a precaution.
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Tubin
g
Pump Seating
Nipple
Seating
Mandrel
Extension
Tube
Pull
R
o
d
PC
P
u
m
p
Tag
B
a
r
No-Turn
Tool
Insertable PC Pump Downhole Assembly
(Illustration courtesy of Weatherford International)
2.4b.2 Operating Principals
PC pumps consist of a rotor turning inside of a stator whereby the rotor is the
only moving component. The rotor is helical and typically has a round cross
section (single lobe). The stator cavity is also helical, but the stator pitch is
twice the pitch of the rotor. The stator cavity cross-sectional shape has one
more lobe than the rotor. For a single lobe rotor, the stator cavity crosssection is like a rectangle with rounded ends (2 lobes) similar to a race track.
The resulting assembly creates sealed cavities between the rotor and stator
which “progress” from the pump inlet to the outlet as the rotor turns (a progressing cavity pump). The cavities are sealed so the pump is a positive
displacement device. Therefore a PC pump will hold a column of fluid when
the pump rotation stops.
Seal line
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3rd cavity
2nd cavity
1st cavity
Single Lobe
Multi-lobe
(Figures courtesy of Weatherford International)
The lift capacity (depth rating) of the pump is dependent on the number of
stages and the fit of the rotor to the stator. The volume capacity (production
rate) of the pump is dependent upon the cavity size and the pump rate of rotation. The cavity size is determined by the rotor eccentricity and pitch.
Long pitches and high eccentricity result in high displacement (high production volume) per rotor revolution. Short pitch pumps reduce the fluid velocity
through the pump which reduces abrasive wear on the pump from fluids that
contain particulate matter. Relatively long pitches relative to eccentricity are
used for less viscous liquids such as water, while relatively short pitches
relative to eccentricity are used for more viscous liquids such as heavy oil.
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Figures courtesy of Weatherford International
2.4b.3 Operating Limits
The PC pump rotary motion and positive displacement combined with direct
mechanical drive from the surface result in the highest system efficiency of
any lift system. The volumetric efficiency of the pump is directly related to
the stator-to-rotor interference fit. Tighter fits allow less slippage and result
in higher volumetric efficiency. Looser fits provide increased cooling and lubrication resulting in longer life for the pump. Therefore, efficiency is often a
trade-off for operating life.
PC pump systems are relatively tolerant of sand and particulate matter compared to most common lift technologies. The elastomer in the stator deforms
to accommodate particulate matter pressed against the stator ID by the rotor
lobes. The particles are then released back into the flow stream after the rotor lobe passes by.
Because of high efficiency and tolerance to particulate matter, PC pump systems are ideal for use in many CBM/CSG wells. In deep CBM/CSG wells
(typically > 6000’ TVD) and where local PC pump service expertise is limited
rod pump systems tend to be more competitive. In shallow CBM/CSG wells
(less than 1500’ TVD) lower cost small ESP systems can be competitive.
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PC pumps have no valves or centrifugal stages so they will not gas lock although they will have reduced efficiency in the presence of gas. Excessive
GLR through the pump for extended periods of time can damage the elastomer due to elastomer hysteresis heating in the absence of liquid cooling.
Typical Range
Maximum
Operating depth TVD
1,000 to 4,500 ft
8,600 ft
Operating volume
5 to 2,500 BPD
5,000 BPD
75 to 185F
Over 300F
Wellbore deviation
<15/100 ft build angle
15/100 ft build angle
Corrosion handling
Good to 185 F
Fair > 185 F
Gas tolerance
Limited for CO2 and aromatic BETEX gasses
Gas handling
Will not gas lock, high GLR reduces efficiency
Operating temperature
Solids handling
Fluid Gravity
Excellent
8 to 45 API (limited for aromatic BETEX gasses)
Prime mover type
Offshore application
Gas engine or electric motor
Good for pump landed above deep-set SSSV
System efficiency
55% to 70%
Figure courtesy of Weatherford International
Typical operating speeds are between 150 and 400 rpm. Speeds slower
than 150 rpm can result in stick-slip behavior due to system fluid properties
and friction elements (rod friction within the tubing, rotor/stator interference
fit). Speeds over 500 rpm can result in excessive rod whirl which can damage the rotor and tubing.
Rotors are sized for a specific interference fit with the stator elastomer in order to allow a designed amount of slippage (fluid leakage) between stages to
provide pump lubrication and cooling. Most systems are designed for volumetric efficiencies of 60% to 85%. Depending on the application conditions
higher efficiencies may compromise pump run life, and lower efficiencies will
increase operating costs.
2.4b.4 Operating Requirements
PC pump systems can run off of the electric grid, or they can use natural gas
powered engines or local power generation.
PC pump technology may be the most efficient lift technology, especially for
abrasive fluids, but it is also the most easily misapplied technology. PC
pump systems should be designed on a well-by-well basis according to the
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depth, fluid properties, production rates, etc. of individual wells. Systems
should be designed by experienced PC pump applications personnel.
Trained PC pump service personnel are required to install, commission, and
maintain PC pump systems.
System monitoring is required in order to optimize pump operation relative to
changing well conditions and to prevent pump-off conditions. Variable speed
controllers with automatic PC pump pump-off controls are important. Telemetry and remote monitoring is recommended.
2.4b.5 Life Expectancy
Life expectancy in CBM/CSG wells has averaged around 18 months globally
and typically ranges from 12 to 36 months. Although solids and coal fines
can be detrimental to any pump, PC pump system failures in CBM/CSG
have more often been related to rod-tubing wear and continuous gas production through the pump. The rod-tubing wear is related to the well geometry
and the resulting water-wet side loads between the sucker rod couplings and
the tubing ID. Water is not a good lubricant and can be especially abrasive
when solids are present. In some areas high concentration of CO2 in the
produced water can cause Explosive Decompression (ED) of the stator elastomer. Special elastomers have been developed to mitigate the damage
caused by ED.
Pump failures can be minimized by close attention to avoiding pump-off conditions and by minimizing the amount of gas through the pump. Landing the
pump below the gas entry interval, the use of subsurface gauges and VFDs
(variable frequency drives) with pump-off logic all help extend pump operating life.
Tubing and rod damage can be minimized by using continuous rod systems
to eliminate the couplings and the corresponding concentrated side contact
forces. Continuous rod will have 75% less side load pressure than conventional sucker rod couplings in PC pump applications because the side load is
spread along the length of the rod. If conventional sucker rod is used, tubing
rotators and repositioning of the rod string can extend the life of tubing.
In Canada and similar areas with significant PC pump manufacturing infrastructure it is common to re-use rotors as the stators wear and are replaced.
Rotors for these applications have extra-chrome or similar hard coatings to
provide extended life. Typically, three stators will be replaced for each rotor
replaced. In locations where PC pump manufacturing infrastructure is less
developed the rotor hard coating thickness may be reduced in order to minimize costs since the rotors will not be reused. In these applications the rotors are designed to last as long as the stators.
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2.4b.6 Costs
While costs will be dependent upon system features, complexity and local
operational costs some general rules-of-thumb apply: In low to medium volume CBM/CSG applications procurement costs (CAPEX) tend to be split ¼
for the subsurface pump assembly and components, ¼ for the rod string, ¼
for the surface drive, and ¼ for the surface VFD controls.
Operating costs (OPEX) vary greatly depending on the local cost of power or
fuel and must include maintenance and service costs since PC pump systems require periodic attention. In general, OPEX will be less than comparable other lift technologies due to high operating efficiency and low failure
rates. Only lift technologies such as plunger lift and gas lift that leverage
formation energy will have lower operating costs per barrel lifted.
Although PC pumps are the most energy efficient form of lift, skilled PC
pump service personnel will be required to keep the systems in optimum operating condition. In the absence of skilled service personnel reliability and
associated costs will be compromised.
2.4b.7 Recommended Practices
Specific installation, operation, and maintenance guidelines are provided by
suppliers for systems and components. The following suggested guidelines
represent best practices for PC pump systems.
Design for Free Gas
Free gas occupies space in the pump cavity. This reduces liquid displacement and reduces pump lubrication and cooling. In gas wells attempts should
be made to reduce the amount of free gas that enters the PCP. The following
practices should be considered when free gas is present:

Land the PCP below the perforations.

If the pump is landed above the perforations a tail-joint assembly
should extend below the pump to effectively place the intake below
the perforations.

Use a downhole gas separator.

Use a charge “tandem” PC pump configuration in which a higher volume capacity lower lift pump compresses well fluids prior to the intake
of the primary lift pump. Tandem pumps must be sized to allow the rotor of the lower pump to pass through the stator of the upper pump
during run-in and retrieval (See illustration, below)
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(Illustration courtesy of Weatherford International)
Design for Highly Deviated Wells
Continuous sucker rod is recommended for any PC pump system installed in
deviated wells. The continuous rod will greatly reduce side loads and tubing
wear without the reliability issues associated with sucker rod guides.
Biased intake separators are available to reduce gas ingestion in deviated
wells. The separators draw in liquids from the low side of the tubing while allowing gas to pass along the high side of the tubing.
Design for Particulate Matter
High concentrations of particulate matter can be produced with PC pump
systems if attention is given to not allow the particulate matter to settle in the
tubing or rat-hole.

Production tubing should be sized so that produced fluid velocities are
adequate to lift particulate matter to the surface. Smaller tubing will
increase fluid velocities. The following chart illustrates the critical ve-
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locities required to lift a coal particle to surface inside 2.875” tubing
with 0.875” sucker rods. Based on the chart a production rate of 109
bbls/day would be required to lift a US Standard Sieve Number 20
particle to surface.
(Chart courtesy of Weatherford International)

For high concentrations of particulate matter a recirculation pump arrangement can be used to keep particulate matter in suspension prior
to pumping. This is similar to the charge pump arrangement described
above except a perforated nipple is located between the tandem
pumps to allow fluid to recirculate from the discharge of the lower
pump back to its intake.
Design for Water and Thermal Elastomer Swell
The amount of fluid and thermal elastomer swell is unique to the elastomer
compound and fluid conditions. At elevated temperatures >60°C the amount
of fluid swell is significant with some elastomers. Therefore it is common for
the manufacturer to perform a fluid compatibility test using the produced fluid
under simulated operating conditions. The fluid compatibility testing accomplishes two purposes.
1. Identifies the best elastomer for the application.
2. Provides the ability to model and select rotor dimensions that will provide an optimized and balanced rotor/stator interference fit across the
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stator cavity profile. Incorrect rotor/stator fit can result in poor performance and run life.
2.4b.8 Trouble-shooting
Tubing parted
Inadequate fluid (reservoir or completion related)
Hole in tubing or collar
X
X X X
X
Tighten new tubing adequately
Reduce pump speed/put well on
timer
Replace tubing or collar
X
Check electrical supply and wiring
X X X
X
X X X
Fluid temperature above / below design point
X
Fluid viscosity below design point
X
Discharge pressure above design point
X
X
X
Pull up rotor, circulate well
X X X X X
Decrease pump speed
X
X Select correct rotor fit
X X X X
X
Packing gland not tight enough
X
Excessive free gas at pump intake
X X
Pump speed above design point
X
X
X X X X X
X X
Drive belts slipping
X
X
X
Check belt tension
X X X X
Check and adjust rotor spacing
X
X X X X X
Worn pump (rotor / stator)
Check/tighten all mounting hardware
X Replace or overhaul surface drive
X
Replace worn components
Low voltage
X X X
Abrasives in the packing gland area
Check voltage / wiring sizes
X
Failure of drive arrangement
X
Incompatible treating chemicals
Pump discharge blocked/valve closed
X X
X X X
Stator worn / damaged
X X X
Packing glands destroy packing
X
X X
X
X
Check packing type and condition
X Check failed drive components
X X X X Re-check chemical compatibility
X X X X X
X Relieve pressure. Clear blockages
X X
X Replace worn parts
X
X X
Motor is too small
X X X
Incorrect rotor spacing
X X
X X
X X
X X
X
X
Adjust packing gland
Install gas anchor, reduce speed or
lower pump
Decrease pump speed
Increase pump speed
X X
Drive mounting insecure
Drive head bearing wear / failure
(Table courtesy of PCM)
Increase pump speed
Check flow line for blockage/closed
X
valve
X Adjust packing gland
X X X X X
Packing gland too tight
Stator elastomer swollen
Pump sanded in
Pump locks up
X
X X
X X
Incorrect rotor setting
Packing gland leakage
Excessive packing gland wear
Wear on pump components
Excessive noise and vibration
Excessive power
Motor overheats
Motor stalls at pump-up
X
Fluid viscosity above design point
Pump speed too slow
Suggested solutions
Select correct rotor fit, decrease
pump speed
Fish parted rod and replace
X X
Motor supply or wiring
Pump intake blocked
Pump will not start
Intermittent production
Production drops off
Possible causes
Percentage abrasion above maximum recommended
Sucker rods parted
No production
Observed problems
Check polished rod for wear
Check and re-calculate motor size
X Re-space rotor
X
X Re-evaluate elastomer selection
X Perform flush by or pull pump
Selection of Artificial Lift Systems for Deliquifying Gas Wells
Examples of damaged rotors
Figures 1 to 3 show examples of different types of rotor damage.
(Photographs provided courtesy of CFER Technologies.)
Figure 1 — Worn rotor
Figure 2 — Rotor cracked from excessive heat
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Figure 3 — Pitted rotor
Examples of damaged stators
Figures 4 to 11 show examples of different types of stator damage.
(Photographs provided courtesy of CFER Technologies.)
Figure 4 — Blistered stator, possibly from decomprerssion of absorbed gasses
Selection of Artificial Lift Systems for Deliquifying Gas Wells
Figure 5 — Burned/overheated stator
Figure 6 — Eroded/pressure washed stator
Figure 7 — De-bonded stator
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Selection of Artificial Lift Systems for Deliquifying Gas Wells
Figure 8 — Scratched/grooved stator
Figure 9 — Torn/chunked stator
Figure 10 — Stators contaminated with foreign material (two examples)
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Selection of Artificial Lift Systems for Deliquifying Gas Wells
Figure 11 — Worn stator
Page 17