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2.4b Progressing Cavity Pumping

This section presents the operating principals, operating limits, and systemrequirements for progressing cavity pump systems. Recommendedpractices, operating considerations, and costs are discussed. This

discussion will be limited to conventional and insertable PC pumps used forgas well deliquification. The emphasis will be on CBMC!" wells andshallow gas applications since production from PC pump systems becomeslimited for deeper applications.

2.4b.1 System Description

The surface and subsurface equipment for a typical electric drive system areshown below#

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Selection of Artificial Lift Systems for Deliquifying Gas ells Page 2

$%llustrations courtesy of &eatherford %nternational'

!urface drives are typically electrically driven. "as engine driven generatorscan be used to supply electricity where line power is not available. (ydraulic

transmissions consisting of an engine driven pump driving a hydraulic motoron the surface drive are also common.

Most surface drives have belt and sheave reductions to provide additionalspeed ad)ustment. "eared systems and inline electric or hydraulic systemsare also available. *ll surface drive systems must have adequate bac+spincontrol and the ability to absorb the stored energy of the rod string torsionplus the full column of fluid.

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Selection of Artificial Lift Systems for Deliquifying Gas ells Page !

The stuffing bo includes a seal to +eep pressuri-ed well fluids fromescaping. !tuffing boes for PC pump systems are specifically designed toseal against rotating polished rods. Stuffing boxes designed forreciprocating rod applications should not be used for PC pump systems

unless they are qualified by the manufacturer for use with rotating rods or PCpump systems.

Below the stuffing bo the pumping tee provides a flow connection to theproduction tubing. !pecial composite/ pumping tees for PC pump systemsclamp on the polished rod to support the rod string and seal against thepolished rod to isolate well fluids. This allows the stuffing bo and surfacedrive to be serviced or removed safely without pulling the rod string.

The stators for conventional PC pumps are run as part of the productiontubing string. The rotor is run into the well on the end of the rod string which

consists of either )ointed suc+er rod or continuous rod.

The downhole assembly includes a tagbar to allow the rotor to be positivelylocated relative to the stator. %nstallation procedures include running therotor to the tagbar and then spacing bac+ to align the rotor into the stator.The rotor space0out is unique to the pump geometry, well operatingconditions, and rod string configuration.

Tubing insertable systems are available in which the stator and rotor are runas one assembly with the rod string inside of the tubing. This simplifiesrunning and retrieval, so insertable pumps are usually preferred over

conventional pumps. The use of insertable pumps is limited by the tubingsi-e.

1arge systems may require a no0turn device to prevent the torque in thepump from loosening the tubing connections. !maller systems with properlytorqued tubing typically do not require no0turn devices although no0turndevices are often included as a precaution.

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Tubing

Pump Seating Nipple

Extension Tube

PC Pump

Seating Mandrel

Pull Rod

Tag Bar

No-Turn Tool

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%nsertable PC Pump 2ownhole *ssembly$%llustration courtesy of &eatherford %nternational'

2.4b.2 "perating Principals

PC pumps consist of a rotor turning inside of a stator whereby the rotor is theonly moving component. The rotor is helical and typically has a round crosssection $single lobe'. The stator cavity is also helical, but the stator pitch istwice the pitch of the rotor. The stator cavity cross0sectional shape has onemore lobe than the rotor. 3or a single lobe rotor, the stator cavity cross0section is li+e a rectangle with rounded ends $4 lobes' similar to a race trac+.

The resulting assembly creates sealed cavities between the rotor and statorwhich progress/ from the pump inlet to the outlet as the rotor turns $aprogressingcavity pump'. The cavities are sealed so the pump is a positivedisplacement device. Therefore a PC pump will hold a column of fluid whenthe pump rotation stops.

Seal line

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Selection of Artificial Lift Systems for Deliquifying Gas ells Page #

$3igures courtesy of &eatherford %nternational'

The lift capacity $depth rating' of the pump is dependent on the number ofstages and the fit of the rotor to the stator. The volume capacity $productionrate' of the pump is dependent upon the cavity si-e and the pump rate ofrotation. The cavity si-e is determined by the rotor eccentricity and pitch.1ong pitches and high eccentricity result in high displacement $highproduction volume' per rotor revolution. !hort pitch pumps reduce the fluidvelocity through the pump which reduces abrasive wear on the pump fromfluids that contain particulate matter. Relatively long pitches relative toeccentricity are used for less viscous liquids such as water, while relativelyshort pitches relative to eccentricity are used for more viscous liquids suchas heavy oil.

Multi-lobeSingle Lobe

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Selection of Artificial Lift Systems for Deliquifying Gas ells Page %

PC pumps have no valves or centrifugal stages so they will not gas loc+although they will have reduced efficiency in the presence of gas. ;*P% $limited for aromatic B

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Selection of Artificial Lift Systems for Deliquifying Gas ells Page *

depth, fluid properties, production rates, etc. of individual wells. !ystemsshould be designed by eperienced PC pump applications personnel.Trained PC pump service personnel are required to install, commission, andmaintain PC pump systems.

!ystem monitoring is required in order to optimi-e pump operation relative tochanging well conditions and to prevent pump0off conditions. 9ariable speedcontrollers with automatic PC pump pump0off controls are important.Telemetry and remote monitoring is recommended.

2.4b.# Life +)pectancy

1ife epectancy in CBMC!" wells has averaged around :? months globallyand typically ranges from :4 to A6 months. *lthough solids and coal finescan be detrimental to any pump, PC pump system failures in CBMC!"have more often been related to rod0tubing wear and continuous gas

production through the pump. The rod0tubing wear is related to the wellgeometry and the resulting water0wet side loads between the suc+er rodcouplings and the tubing %2. &ater is not a good lubricant and can beespecially abrasive when solids are present. %n some areas highconcentration of C=4 in the produced water can cause

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Selection of Artificial Lift Systems for Deliquifying Gas ells Page ,

2.4b.$ Costs

&hile costs will be dependent upon system features, compleity and localoperational costs some general rules0of0thumb apply# %n low to mediumvolume CBMC!" applications procurement costs $C*P

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Selection of Artificial Lift Systems for Deliquifying Gas ells Page 1/

$%llustration courtesy of &eatherford %nternational'

Design for 0igly Deviate- ells

Continuous suc+er rod is recommended for any PC pump system installed indeviated wells. The continuous rod will greatly reduce side loads and tubingwear without the reliability issues associated with suc+er rod guides.

Biased inta+e separators are available to reduce gas ingestion in deviatedwells. The separators draw in liquids from the low side of the tubing whileallowing gas to pass along the high side of the tubing.

Design for Particulate (atter

(igh concentrations of particulate matter can be produced with PC pumpsystems if attention is given to not allow the particulate matter to settle in thetubing or rat0hole.

Production tubing should be si-ed so that produced fluid velocities are

adequate to lift particulate matter to the surface. !maller tubing willincrease fluid velocities. The following chart illustrates the critical

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Selection of Artificial Lift Systems for Deliquifying Gas ells Page 11

velocities required to lift a coal particle to surface inside 4.?@;/ tubing

with 7.?@;/ suc+er rods. Based on the chart a production rate of :7G

bblsday would be required to lift a F! !tandard !ieve Humber 47particle to surface.

$Chart courtesy of &eatherford %nternational'

3or high concentrations of particulate matter a recirculation pump

arrangement can be used to +eep particulate matter in suspensionprior to pumping. This is similar to the charge pump arrangementdescribed above ecept a perforated nipple is located between thetandem pumps to allow fluid to recirculate from the discharge of thelower pump bac+ to its inta+e.

Design for ater an- &ermal +lastomer Sell

The amount of fluid and thermal elastomer swell is unique to the elastomercompound and fluid conditions. *t elevated temperatures 567IC the amountof fluid swell is significant with some elastomers. Therefore it is common for

the manufacturer to perform a fluid compatibility test using the produced fluidunder simulated operating conditions. The fluid compatibility testingaccomplishes two purposes.

:. %dentifies the best elastomer for the application.

4. Provides the ability to model and select rotor dimensions that willprovide an optimi-ed and balanced rotorstator interference fit across

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Selection of Artificial Lift Systems for Deliquifying Gas ells Page 12

the stator cavity profile. %ncorrect rotorstator fit can result in poorperformance and run life.

2.4b.* &rouble3sooting

Possible causes

Observed problems Suggeste- solutions

Hoproduction

Productiondropsoff

%ntermittentproduction

Pumpwillnotstart

Motorstallsatpump0up

Motoroverheats