Selection of Artificial Lift Systems for Deliquifying Gas WellsPage 2

Guidelines & Recommended Practices

Selection of Artificial Lift Systems

for Deliquifying Gas Wells

Prepared by Artificial Lift R&D Council

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:

(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.

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.

(TubingPump Seating NippleExtension TubePC PumpSeating MandrelPull RodTag BarNo-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 cross-section 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)

(Multi-lobe) (Single 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.

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.

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

Operating temperature

75 to 185F

Over 300F

Wellbore deviation

185 F

Gas tolerance

Limited for CO2 and aromatic BETEX gasses

Gas handling

Will not gas lock, high GLR reduces efficiency

Solids handling

Excellent

Fluid Gravity

8 to 45 API (limited for aromatic BETEX gasses)

Prime mover type

Gas engine or electric motor

Offshore application

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 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.

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 for the subsurface pump assembly and components, for the rod string, for the surface drive, and 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)

(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 velocities 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 >60C 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 stator cavity profile. Incorrect rotor/stator fit can result in poor performance and run life.

2.4b.8 Trouble-shooting

Possible causes

Observed problems

Suggested solutions

No production

Production drops off

Intermittent production

Pump will not start

Motor stalls at pump-up

Motor overheats

Excessive power

Excessive noise and vibration

Wear on pump components

Excessive packing gland wear

Packing gland leakage

Pump locks up

Percentage abrasion above maximum recommended

X

X

Select correct rotor fit, decrease pump speed

Sucker rods parted

X

X

Fish parted rod and replace

Tubing parted

X

X

X

Tighten new tubing adequately

Inadequate fluid (reservoir or completion related)

X

X

X

X

Reduce pump speed/put well on timer

Hole in tubing or collar

X

X

X

Replace tubing or collar

Motor supply or wiring

X

X

X

Check electrical supply and wiring

Pump intake blocked

X

X

X

X

Pull up rotor, circulate well

Fluid viscosity above design point

X

X

X

X

X

X

X

Decrease pump speed

Fluid temperature above / below design point

X

X

X

Select correct rotor fit

Fluid viscosity below design point

X

Increase pump speed

Discharge pressure above design point

X

X

X

X

X

X

X

Check flow line for blockage/closed valve

Packing gland too tight

X

X

X

X

X

X

Adjust packing gland

Packing gland not tight enough

X

Adjust packing gland

Excessive free gas at pump intake

X

X

X

Install gas anchor, reduce speed or lower pump

Pump speed above design point

X

X

X

X

X

X

Decrease pump speed

Pump speed too slow

X

X

Increase pump speed

Drive belts slipping

X

X

X

X

Check belt tension

Incorrect rotor setting

X

X

X

X

X

Check and adjust rotor spacing

Drive mounting insecure

X

Check/tighten all mounting hardware

Drive head bearing wear / failure

X

X

X

X

X

X

Replace or overhaul surface drive

Worn pump (rotor / stator)

X

Replace worn components

Low voltage

X

X

X

Check voltage / wiring sizes

Abrasives in the packing gland area

X

X

X

Check packing type and condition

Failure of drive arrangement

X

X

X

X

X

Check failed drive components

Incompatible treating chemicals

X

X

X

X

X

X

Re-check chemical compatibility

Pump discharge blocked/valve closed

X

X

X

X

X

X

X

X

X

Relieve pressure. Clear blockages

Stator worn / damaged

X

X

X

X

X

X

Replace worn parts

Packing glands destroy packing

X

X

X

Check polished rod for wear

Motor is too small

X

X

X

Check and re-calculate motor size

Incorrect rotor spacing

X

X

X

X

X

Re-space rotor

Stator elastomer swollen

X

X

X

X

X

Re-evaluate elastomer selection

Pump sanded in

X

X

X

X

Perform flush by or pull pump

(Table courtesy of PCM)

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

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.)

Figure4 Blistered stator, possibly from decomprerssion of absorbed gasses

Figure 5 Burned/overheated stator

Figure 6 Eroded/pressure washed stator

Figure 7 De-bonded stator

Figure 8 Scratched/grooved stator

Figure9 Torn/chunked stator

Figure10 Stators contaminated with foreign material (two examples)

Figure 11 Worn stator

3

rd

cavity2

nd

cavity1

st

cavity

Belts & Sheaves

Surface Drive

Stuffing Box

Pumping Tee

Electric Motor

Polished Rod

Sucker Rods

w/ Couplings

Surface Equipment

Belts & Sheaves

Surface Drive

Stuffing Box

Pumping Tee

Electric Motor

Polished Rod

Sucker Rods

w/ Couplings

Surface Equipment

Downhole Equipment

PC Pump Stator

PC Pump Rotor

No-Turn Tool

Tubing String

Tagbar

Sucker Rods

w/ Couplings