ESP WORKSHOPGulf Coast Section, 1998Paper #18

High Volume Down-Hole Progressing Cavity Pumps with ElectricSubmersible MotorsR & M Energy Systems, a Unit of Robbins and Myers Inc.

AbstractThe submersible motor driven progressingcavity (PC) pumps are becoming more popularin horizontal and deviated wells. In theseapplications, bottom-driven pumps eliminate rodbreak problems and significantly reduce tubingwear. In addition, the ability to handle abrasivesand viscous fluids gives a special niche to theelectric submersible PC pumps.

The existing PC pumps are designed to bedriven by sucker rods. These pumps are limitedin their flow and depth capabilities due to torquelimitations of the sucker rods. The Electricsubmersible motors can provide higher torqueand speed to the PC pumps. Therefore, there isa need to design new PC pumps with higherflow rate and speed capabilities.

This paper presents an overview of issuesrelated to using progressing cavity pumps forpumping high speed and high fluid volume usingelectric submersible motors.

BackgroundA PC pump consists of a helical steel rotorwhich turns within a stationary tube with a helicalelastomeric lining (stator). As the rotor turnsinside the stator, fluid moves through the pumpfrom cavity to cavity. As one cavity diminishes,the opposing cavity increases at exactly thesame rate which results in a pulsationlesspositive displacement flow through the pump.The cavities are separated from each other by aseries of seal lines which are created betweenthe rotor and stator.

Figure 1 shows the cross section of a PCpump with a single lobe rotor. Other rotor/statorlobe configurations (multi-lobes) can also beused as a PC pump. Figure 2 shows differentmulti-lobe cross sections. Detail discussions ofthe comparison between the single lobe andmulti-lobe pump elements are beyond the scopeof this paper. The topics discussed in this paperare generally applicable to both single lobe andmulti-lobe pumps.Definitions:Ps = Stator Pitch

D = Rotor Minor DiameterEcc = Pump EccentricityPump flow rate can be calculated as follows:

Pressure. Pressure capability of a PC

pump is a function of the number of times theprogressing seal lines are repeated. If cavitypressure increases beyond the seal limits, theseal lines will open, and fluid will slip from onecavity to the other at a very high speed. The PCpump slippage is generally a function ofpressure differential across the pump, fluidviscosity, and the compression fit between therotor and stator.

Viscosity and Cavitation. The fluid viscosity

affect pump performance by increasing fluidlosses through the pump and by changing the

Equation 1. Q = K* Ps*4*Ecc*D*Nwhere:Q = flow rateN = number of revolutions per unit timek = conversion factor

Figure 1 Single Lobe PC Pump Cross Section

D

ECC

Ps

Figure 2 Various Rotor (solid) and Stator LobeConfigurations

High Volume Down-Hole Progressing Cavity Pumps with Electric Submersible MotorsESP Workshop 1998, Paper #18R & M Energy Systems, a Unit of Robbins and Myers Inc.

pump slippage. In most cases, production fluidexhibit non-Newtonian behavior and theirviscosity changes as a function of pump speedand shear rate. The viscosity at the pump speedmust be calculated (apparent viscosity) toestimate the impact of viscosity on the pumpperformance.

High fluid viscosity may result in the loss ofpump efficiency due to cavitation. In a PC pump,until the rotor closes behind the fluid and appliespositive pressure to it, the pump can only createa void. The amount of fluid to flow into the voidwill depend on the fluid viscosity, entrancelosses, and pressure at the pump intake. If thefluid losses are greater than the available head,the void will not fill and pumps efficiency isreduced. This problem rarely occurs in thedown-hole applications where several feet offluid column exists above the pump intake.However, the cavitation problem can be moreserious when high volume pumps are laid in thehorizontal sections with only a few feet of headavailable at the pump intake.

Flow Rate and Pump Speed. Theoreticalpump flow rate is a function of pump speed anddesign parameters such as stator lead, rotordiameter, and pump eccentricity. PC pump flowrate increases linearly with the pump speed.Parameters such as fluid viscosity, as discussedearlier, introduce non-linearity to the flow curves.Figure x shows the impact of the viscosity onthe PC pump theoretical. displacement.

Abrasives and Pump Wear. PC pumps areknown for their ability to handle abrasiveapplications. The feature that gives the PCpumps its advantage for handling abrasives isthe use of elastomers in the pump stators. Theparameters that are most relevant to wear in PCpumps are the pump speed, amount and size ofparticles, and pump internal velocity. Selecting aPC pump for an abrasive application involves

selection of a pump which can produce thedesired volume at lowest fluid internal velocityand pump rotational speed. Once these criteriaare considered, the materials of constructionmust be selected for maximum wear life. Designof the PC pumps for abrasive applications isdiscussed in detail in Reference 1.

High Volume ESPCPThe ESPs are traditionally used in high volume,deep, deviated, or horizontal wells. However,the conventional centrifugal ESPs are not verypopular in sandy and high viscous applications.This is due to centrifugal pumps short wear lifeand high shear rate which is not efficient inviscous applications. The impact of sandcontribution to ESP failures are documented inmany technical papers such as the one listed inReference 2. It is in viscous and sandyapplications where PC pumps are more superiorthan the centrifugal ESP pumps.

The surface driven PC pumps have depthand volume flow rate limitations. Theselimitations are primarily due to torquecapabilities of the rod string and flow line lossesdue to rod, couplings, and centralizers. On theother hand, the conventional ESPs can producemore than 35000 BFPD with pressurecapabilities in excess of 8000 psi. The electricsubmersible motors can produce much highertorque compared to what rod strings can handle.Therefore, the marriage between theprogressing cavity pumps and electricsubmersible motors can provide a bettersolution to high volume and viscous applicationsthan either the conventional ESPs or rod-drivenPC pumps.Tubing Losses. In a rod driven PC pump, theproduced fluid travels through an annulusformed between the tubing and the rod string.In an electric submersible PC pump, flow lossesare smaller due to elimination of the couplings,centralizers, and rod strings. Flow losscalculations are very difficult due to non-Newtonian behavior of most production fluids.

Figure 3 Deviation From Theoretical Displacementat a Given Pump Speed Due to Cavitation

High Volume Down-Hole Progressing Cavity Pumps with Electric Submersible MotorsESP Workshop 1998, Paper #18R & M Energy Systems, a Unit of Robbins and Myers Inc.

Fundamental fluid mechanics equations for pipefluid losses are well documented in manysources and can be used to estimate tubing flowlosses. Figure 4 shows a comparison betweenthe tubing flow losses for an electricsubmersible pump compared to a rod drivenpump.High Volume PC PumpsThere are several methods of increasing PCpump volumetric flow rate. From equation 1,pump flow rate can be increased by increasingpump diameter or stator pitch length.Increasing the stator pitch length will increasethe pumps internal velocity as well as reduce itspressure per length capabilities. A special highvolume PC pump is designed and optimized.Figure 5 compares this new pump with otherstandard pumps. Figure 5 shows that in this newdesign, while the volumetric flow rate hasincreased, the pump internal velocity and shearrate is reduced. Lower internal velocities andshear rates will reduce pump flow losses inviscous applications and will minimize pumpwear due to abrasion.

Models Max Lift (ft) Max Flow (BFPD)Outside

Diameter (in)

Pump Length (in)

Internal Velocity (ft/sec)

Shear Rate (1/sec)

ESPCP 2000 2050 5.375 205 5.8 168

20-H-685 2000 1750 4.5 393 11.2 506

20-H-950 2000 2400 4.5 416 12.5 458

* Theoritica data based on 300 rpm pump speed

Figure 5 High Volume PC Pump Design for ESPApplications

Case Analysis

Figure 4 Tubing Losses Comparison

010

0020

00

0 500 1000 1500 2000Q (BBL/DAY)

Tubi

ng P

ress

ure

Loss

(PSI

)

With Sucker RodWithout Sucker Rod

Assmuptions:Viscosity:1000 cpTubing: 3 1/2"Length: 2000 ft.