Oil & Gas News
Friday, 29 May 2009
ESP Pumps: The Operators Options for Successful Installation and Run Time
By: J F Lea, PLTech LLC, David L. Divine, P.E. Wood Group ESP, & Lynn Rowlan, Echometer Co.
The electrical submersible pump system has been developed over the years by Engineers and scientists involved in metallurgy, hydraulics, electronics, heat transfer, plastics, many aspects of mechanical engineering, and other disciplines. It is not practical to outline all of the many aspects of the system in the short introduction section. Instead, the major components are introduced.
The pump assembly is hung on the tubing with the electric cable banded to the outside of the tubing from surface to pump. The equipment is arranged from top to bottom with the pump first, with the gas separator below, then the seal section, followed by the motor. If a downhole pressure sensor is used, it is hung at the bottom of the motor. ESP's are thought of as high volume lift perhaps producing -20,000 bpd at 4000' down to -5000 bpd at 10,000' depending on many factors, but low volume (-100 bpd) stages exist.
The electric submersible motor is a two-pole, three-phase, squirrel cage induction type. The motor runs at a nominal speed of 3500 rpm on 60 Hz frequency and 2900 rpm on 50 Hz. The motor is filled with a refined mineral oil to provide dielectric strength, lubrication of bearings and thermal conductivity. The thrust bearing of the motor carries the load of the rotors. The electrically nonconductive mineral oil lubricates the motor bearings and transfers heat in the motor to the motor housing. Heat from the motor housing is in turn carried away by the well fluids moving past the exterior surface of the motor. For this reason, the motor should not be set below the point of fluid entry unless some means of directing the fluid by the motor is utilized. Typical nominal motor diameters of equipment may be: (a) 3.75", (b) 4.56", (c) 5.402, 5.44", 5.62", and (d) 7.38" for various casing sizes. Some motors are offered with somewhat different diameters and some manufacturers do not carry some of the diameters indicated. Some Motor construction may be a single housing or several "tandems" bolted together to reach a desired horsepower rating. Motors range in horsepower from 5 to 1000 hp and larger.
The electric submersible pump is a multistage centrifugal type. The type of stage used determines the approximate design volume rate of fluid produced but as the fluid compresses, each stage will have progressively less volume to handle. The number of stages determines the total head designed for and the motor horsepower required.
The usual materials used in manufacturing an impeller are Ni-Resist with some options for sand handling. Diffusers are typically manufactured of Ni-Resist. The standard shaft material is K-monel. Optional, high-strength shaft materials are Inconel and Hastalloy. Bolt-on heads and bases make it possible to vary the capacity and total head of a pump by using more than one pump section. However, large capacity pumps typically will have integral heads and bases. The nominal outside diameter of a pump will range from 3.38" to 11.25" but 7.62" to 8.38" could be largest oil well applications.
Seal Section. Protector, Equalizer:
The motor protector's primary purpose is to isolate the motor from the well fluid. There are, in general, two types of industry protector or seal section designs although there are specific differences from one brand to another. One type uses a positive bag seal and the other type uses a labyrinth or tortuous path. The "positive seal" design incorporates a fluid barrier bag to allow for thermal expansion of the motor fluid yet still provided isolation of motor fluids from wellbore fluids. The "labyrinth path" utilizes differential fluid specific gravity to prevent well fluid from entering the motor. This is accomplished by paths where the motor fluid is allowed to expand to displace more or less of the wellbore fluid as it expands through a tortuous path at an interface near the top of the protector. There are usually several "labyrinth paths" in one protector and more could be added by placing protectors in series. Normally the bag type positive seal protector is backed up with "labyrinth paths" so that bag failure is not necessarily catastrophic.
The protector or seal section performs four basic functions. These are: (1) It connects the pump to the motor by connecting the housing and drive shaft; (2) Houses a thrust bearing to absorb pump shaft thrust (if present); (3) Isolates the well fluid from the motor while still allowing pressure equalization between the wellbore and the oil-filled motor; and (4) provides for thermal expansion of the motor oil due to heat generated by the motor during operation and thermal contraction of the motor oil following pump shutdown/startup.
The gas separator is installed between the protector or seal section and the pump. Its purpose is to separate a significant portion of any free gas in the produced fluid and provide a fluid intake section for the pump.
There are two major types of gas separator designs - the static type and the rotary type. The static type reverses the fluid flow direction within the housing but the use is not as frequent now. At this point of low pressure there is gas separation. Any gas remaining in the fluid is separated by the pickup impeller which causes a vortex. The vortex allows the gas and fluid to separate. The separated gas is vented to the annulus and the higher density fluid flows into the first stage of the pump.
The rotary type design utilizes a rotary inducer/centrifuge to centrifugally separate the gas and produced liquids. The gas/fluid mixture initially enters the intake ports and moves into the inducer. This increases the pressure of the fluid and moves it through the transition section into the centrifuge. In the centrifuge the fluid is forced to the outside and gas rises through the centrifuge and flow divider into the crossover section. Here, the gas vented into the annulus and fluid is directed into the first pump stage. At present three (four in the near future) manufacturers are producing this type of separator. A "Vortex" separator may have a smaller paddle wheel at the bottom of a chamber where gas and fluids can swirl before exiting the separator.
Special stages are offered by some manufacturers when there is no path for separated gas. The special stages mix the gas and fluids and some are more proficient in producing head in the presence of high gas content.
Pressure Sensing Instrument:
The instrument has two major components - a surface readout unit and a downhole pressure and temperature sensing instrument. The downhole sensor is bolted to the base of the motor and sends a "ghost" signal to the surface unit through the motor windings and power cable as opposed to older designs requiring an extra "I" wire. One readout instrument alternates pressure and temperature readings on a 20-second interval. Other downhole instruments including intake and motor winding temperature. Other types of instrumentation are available.
There are many factors involved in operating ESP systems to lift a field. Below is an outline covering many of the aspects to be aware of when operating ESP's.
Outline of Factors for Good ESP Operations:
1) Well Data for Design and Operation:
i) Well tests
ii) IPR data
iii) Temperature and fluid properties
iv) Harsh conditions present?
(c) H2S, CO2
(d) Viscosity, emulsion
(e) High Temperature
(f) High gas production with the liquids
v) Well Profile
viii) HZ of power supply available
ix) VSD part of installation?
2) Select Target Production:
i) AOF of well
ii) Bubble point
iii) Produce above or below bubble point
iv) Target production
3) Equipment Design:
i) Determine TDH
ii) Select type of pump and calculate number of stages
iii) Intake: Standard or gas separator
(b) Labyrinth sections (*)
(c) Tandem protectors?
v) Motor, type, HP
vi) Downhole instrumentation
vii) Cable: round / flat, size
Bands or cross coupling protectors
viii) Well head feed through type
ix) Control panel: Standard or VSD
x) See API RP 11S4 Recommended Practice for Sizing & Selection of ESP Installations
Example Simple Conceptual Design:
Consider the following data for design purposes. More detailed data would be required for actual application design:
SIBHP: 2900 psi
Test Rate: 4000 bpd
Test Pressure on Perforations: 400 psi
Perforations Depth: 6500 ft
Pump Depth 6000 ft
Casing: 5.5 inch
Tubing (to be determined but for 4000 bpd should be 3 ½, 4 or 4 ½ inch approximately)
WHP: 100 psi
Consider combination of water and oil such that the combined SpGr is 0.9. Approximate using volume of liquids do not change with down hole pressure and temperature. This is not true of course but approximately true if high water cut and little gas. This assumption allows a simple design example. For more and more gas and oil with water, this would be less and less true.
Power supply is 60 HZ. Use the above pump performance curve for this example.
Target rate: 4000 bpd
The pressure at the perforations is 400 psi. Consider the casing flow to the pump intake has little friction.
The pump intake pressure, PIP, is 400 psi – 500 ft ( .9*.433 psi/ft) = 205.15 psi.
For tubing flow to calculate the discharge pressure, consider tubing is selected such that friction pressure is 2-5% of the tubing pressure drop. This is typical for design of ESP. For this design use 3% for friction pressure drop.
Discharge pressure = WHP + .433(.9)(Depth)(1.+ % Friction) =
= 100 + .433(.9)(6000)(1. + .03) = 2508.3 psi
Then the TDH or total dynamic head is : TDH = (Pd – PIP)/( (.433)(.9))
= (2508.3-205.15) / ( (.433)(.9)) = 5901 ft
From the above performance curve read about 43.5 ft / stage.
Then the number of stages required is:
* Stages = TDH/ (head/stage) = 5901/43.5 = 136 stages
The HP required from the motor would be:
(* Stages) ( HP/Stage) (SpGr) = 136(1.95)(.9) = 238.7 HP
A larger somewhat de-rated motor would normally be selected for application
To complete the design, a cable would be selected (normally with no more that 30 V/1000 ft voltage drop), a switch board or VSD would be selected, and use of tubing for this design should be such that the pressure drop due to friction would be about 3% of the total tubing pressure drop. Other hardware would be ordered.
For heavy oil viscosity correction factors would come into play. For free gas at the pump intake, the gas would become part of the volume digested by the pump and the gas would also reduce the effective SpGr of the mixture. For more than 10-15% at the pump intake, we would become more concerned with the need for gas separation.
VFD or Variable Drives:
For critical installations, many times the data is such that the design may not fit the well conditions as the operator would prefer. Also changing well conditions may require changes in the ESP operation before the unit is pulled. If sufficient motor capacity is available, then a VSD can help achieve optimum operating conditions before the unit is pulled.
Variable frequency drive (VFD) controllers are solid state electronic power conversion devices. AC input power is first converted to DC intermediate power using a diode rectifier and/or thyristor (SCR) bridge. The DC intermediate power is then converted to quasi-sinusoidal AC power using an inverter switching circuit.  Figure 1 is a basic block diagram of a VFD connected to a motor.
For the electrical submersible pump (ESP) application there is a step up transformer and a length of cable between the output of the VFD and the motor.
VFD's for ESP oil well applications are divided into two major categories. They are either variable voltage inverters (VVI) or constant voltage inverters (CVI).
AC motor characteristics require the applied voltage to be proportionally adjusted whenever the frequency is changed in order to deliver the rated torque. For example, if a motor is designed to operate at 460 volts at 60 Hz, the applied voltage must be reduced to 230 volts when the frequency is reduced to 30 Hz. Thus the ratio of volts per hertz must be regulated to a constant value (460/60 = 7.67 V/Hz in this case). For optimum performance, some further voltage adjustment may be necessary, but nominally constant volts per hertz is the general rule. This ratio can be changed in order to change the torque delivered by the motor. The VVI VFD controls the output voltage by controlling the DC voltage level with SCRs. The output of this type of drive is a quasi-sinusoidal wave called a 6 step shown below in Figure 2.
The vertical distance from the top of the top step to the bottom of the bottom step equals the DC bus voltage. As the frequency increases the SCRs on the input will cause the bus voltage increase and conversely when the frequency decreases the SCRs will reduce the bus voltage.
VVI VFDs with 6 step outputs have been applied to ESP oil well applications for over 30 years. There is some additional motor heating associated with the use of 6 step because on the harmonic content of the quasi-sinusoidal wave shape. This additional heating as been compensated for by using motors that have be re-rated for the application of 6-step VFDs.
The CVI VFD controls the output voltage and frequency with a pulse width modulated (PWM) output shown in figure 3 below.
The peak between the top of the positive pulses and the bottom of the negative pulses always stays the same (or constant voltage). The width (or duty cycle) of each individual pulse increases with increasing frequency therefore increasing the average applied voltage. This voltage and frequency control is shown in Figure 4 below. The average voltage over the low frequency period will be lower than the average voltage over the higher frequency period.
When the CVI VFDs are applied to the ESP oil well application, the rapid switching of the PWM output causes reflections to occur over the long lengths of power cable. This can cause voltage spikes up twice the peak system voltage to appear at the output of the step up transformer and the ESP motor terminals. Figure 5 shows the ringing that occurs at the end of the voltage transitions during the PWM switching.
To reduce the risk of insulation failure and to reduce motor heating due to harmonics the manufactures of these drives have included low pass filters on the output of their CVI VFDs. This is filtered PWM (FPWM3) or variable sine wave generation PWM (VSG PWM4). A typical voltage output waveform of a filtered CVI VSD is shown in figure 6 below.
Variable frequency drives for ESP oil well applications range in size from 25 KVA to 2000 KVA at 480 volts to 2400/4160 volts. They can be designed for stand alone applications in the field in NEMA 3 or 4 enclosures or they can be in NEMA 1 enclosures for motor control room applications. When purchased from an ESP vendor they will come with the necessary controls for motor and VFD protection and control.
a) There are many factors to be considered to prepare for installation, install the cable and unit components and start up and monitor the unit. See API RP 11 S3, Recommended Practice for ESP Installations. See API RP11S5 Recommended Practice for Application of ESP Cable. See APIRP 11S6 Recommended Practice for Testing ESP Cable Systems.
5) Operation / Monitoring:
i) Monitor: Amps, surface voltage, downhole temperature and pressure starts/stops, power supply frequency
(a) Motor winding and well temperature
(b) Motor fluid dielectric strength
(d) Discharge pressure
(e) See API RP 11S Operation, Maintenance & Toubleshooting of ESP Installations
6) Removal from Well/ Inspection;
i) Remove with care
ii) Inspect as removed: Sample fluids , solids etc
iii) Collect fluid and solids samples
iv) Observe color indicating exposure to excessive heat
v) Note Vibration marks if any
vi) Any evidence of cable or pothead burns
vii) Mechanical damage if evident
viii) Package including pothead and instrumentation (without removal) to shop for teardown
7) Shop Teardown:
i) Have available historical run data and documentation
ii) Sample internal materials and fluids
iii) Search for primary cause of failure and other conditions:
(b) Foreign materials
(c) Electrical transients or electrical burns
(d) Water in motor?
(e) Seal function or failure of:
1. Shaft seals
2. Bag preventer
3. Contamination of labyrinth sections
4. Wear or failure of thrust bearing
(f) Motor: Burned or contaminated
(g) See API RP 11S Recommended Practice for ESP Teardown Report
iv) Determine possible reuse of pump and motor if reconditioned and tested. See APIRP11S2 Recommended Practice for ESP Testing. See API R P11S8 Recommended Practice on ESP Vibrations. See API RP 11S7 RP on Application and Testing of ESP Seal Chamber Sections
8) Determination of failure:
i) Examine removal and teardown data and assess cause/s of failure
9) Continuous Improvement:
i) Indicate equipment that could extend run life such as sand resistant
(1) Stages/ impellers or high temperature trim or need for better checks at installation etc. Note that these recommendations my not be implemented on the new equipment going in but possibly on the following run/pull/installation.
10) Maintenance of Failure Data Base:
a) In order to show improvements with time in run life, it is necessary to have a good record of past failures and the cause of each. Only then can attention be focused on the most critical areas and only then can improvements in run life be achieved.
For additional information on a failure tracking project details see: Industry
Reliability and Failure Tracking Joint Industry Projects seek to increase ESP and PCP Run-Life By Jesus Chacin, Paul Skoczylas and Darren Worth, Rogtec, Issue 7.11:48