A State-of-the-Art Modeling Technique for Thrust Prediction in Bottom Hole Electrical Submersible Pumps

SPE-173945-MS
A State-of-the-Art Modeling Technique for Thrust Prediction in Bottom
Hole Electrical Submersible Pumps
Emanuel Marsis, PhD., Abhay Patil, PhD., David Baillargeon, David McManus, Steven Gary II, Brett Williams,
Dario Lana, Jonathan Nichols, Cristian Von Zedtwitz, and Jason Ives, Baker Hughes Inc.
Copyright 2015, Society of Petroleum Engineers
This paper was prepared for presentation at the SPE Artificial Lift Conference — Latin America and Caribbean held in Salvador, Bahia, Brazil, 27–28 May 2015.
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Abstract
Thrust in Electrical Submersible Pumps (ESPs) is a very important factor that affects pumps’ performance. Thrust forces in pumps can increase the friction losses and reduce the overall pump efficiency and
lifetime. Pump designers have to design pumps to handle thrust generated in the operating range whether
by adding up-thrust protection, down-thrust protection, or sometimes both kinds of protections if the
operating range includes both up and down thrust.
Hydraulic thrust mainly depends on the hydrodynamic forces generated inside ESPs. These hydrodynamic forces depend on many factors like blade loading, seal geometries, seal diameters, seal engagement,
and balance holes location and sizes. Not only is the thrust magnitude important to predict, but also the
shape of the thrust curve plays an important role in defining the operating range. If the thrust curve is flat,
then this means a wider operating range for the pump.
Currently, no model is available to predict or design for thrust curve. Engineers use an iterative process
of manufacturing and testing trying to reach the optimum seal configuration and balance holes geometry
for better thrust and pump performance.
In this paper, a detailed CFD model of a mixed flow multistage ESP is presented including all seal
elements and balance holes to be able to predict hydraulic thrust. Thrust curve is predicted for the first
time using CFD analysis. Different seal geometries and balance holes configuration is also modeled to
study the effect of geometry change on thrust curve. The model is validated using experimental test
results. Also, other design parameters like the shape of the head curve, pump efficiency, and gas handling
capability were controlled early in the design phase using CFD analysis.
Introduction
Electrical submersible pump systems are being implemented extensively in many oil and gas wells for
their capability of pumping hydrocarbons from low pressure or depleted reservoirs to the surface. ESP
systems are composed of five main items; the bottom hole motor, bottom hole pump, main seal, cable, and
a variable frequency drive (VFD). The motor is connected to the VFD through the cable that goes up to
the surface and the motor drives the centrifugal pump to pump the hydrocarbons downstream. The ESP
is a multistage pump that can be either radial or mixed flow pump. Each stage contains of a rotating
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impeller and a stationary diffuser that redirects the flow from the previous impeller outlet to the next
impeller inlet. Although sand screens are used to prevent sand from going into production, fine sand
particles can still make its way through the sand screens and go through the pump causing high erosion
rates and leading to degradation in pump performance. Most failures in ESPs are due to sand erosion
caused by sand production. Many areas inside the pump are affected by sand erosion, like the diffuser
inner wall, impeller and diffuser blades. However, sand erosion at the sealing clearances of the skirt and
balance ring seals is the main reason behind pump failures and loss of performance. Small sealing
clearances would improve the pump performance by reducing leakage, but if sand particles are trapped
in these small clearances, there is a high probability of widening the clearances and losing performance.
CFD simulation has been used extensively by many researchers and engineers trying to predict the pump
performance. However, due to the complexity of the geometry, the full pump model including the sealing
diameters, sealing clearances, bearing clearances, and balance holes was not investigated before. Modeling and simulating the pump and predicting thrust under different seal clearances, and locations can be
of great value to design more reliable pumps.
Research on Pumps Modeling and Testing
Modeling and understanding the behavior of fluids inside centrifugal pumps have been of great interest
to many scientists and engineers starting the twentieth century. Researchers started investigating the
centrifugal pump performance and came up with semi empirical models that can predict the pump
performance such as the work done by Mikielewicz et al. (Mikielewicz 1978). In 1980, Zakem (Zakem
1980) presented a semi empirical formula that predicts a centrifugal pump performance. He based his
model on both an analytical formulation coupled with experimental testing. Later in the 1990’s with the
evolution of computers, many researchers started modeling pumps using computational fluid dynamics.
Pak and Lee (Pak 1998) used CFD modeling to model bubbly flow inside centrifugal pump. It used one
set of momentum, continuity and energy equations and used the gas volume fraction as a variable to model
the mixture as one flow. Pirouzpanah et al. studied the ESP behavior with sand and two phase flow
(Pirouzpanah 2013) & (Pirouzpanah 2014). Morrison et al. studied multiphase flow in twin screw pump
both experimentally and theroeticall (Morrison 2012), (Morrison 2013), & (Morrison 2014). In a recent
paper by Marsis et al. (Marsis 2013), the authors used CFD to model the single and two phase flow inside
a multi-vane ESP. The CFD analysis was used to improve the original pump design and help achieving
better gas handling and higher efficiency by changing the diffuser design. In their work, Marsis and
Russell (Marsis 2013), presented the first erosion model for ESPs showing the erosion rate prediction
inside the hydraulic path. This model utilized the CFD and modeled sand particles using both discrete
phase model and Eulerian granular multiphase model to predict erosion rates inside ESPs.
Seals Modeling and Testing
Annular seals have been extensively used in turbmachineries to minimize gas and liquid leakage from
high pressure zones to low pressure zones. Minimizing the leakage across annular seals improves the
turbomachinery efficiency. There are mainly two types of seals; contacting and non-contacting seals. In
ESPs, most of the annular seals used are non-contacting. Different geometries have been studied and
compared to minimize the leakage across annular seals. Many experimental testing and computational
fluid dynamics simulations were done to understand the fluid flow across the annular seals. Nelson and
Nguyen (Nelson 1987) used a bulk flow model to analyze annular seals performance. Different rotordynamics investigations were performed to better understand the seal performance and stability like the
studies shown by (Lucas 1994). Morrison et al. (Morrison 1992) studied the effect of whirling on the flow
field inside a labyrinth seal and focused his study on the secondary recirculation zone using Laser Doppler
Anemometry. Ustun et al. (Ustun 2013) performed a numerical analysis to understand the effect of surface
roughness on convergent and smooth annular seals. Marsis and Morrison (Marsis 2013) performed a
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numerical study to compare leakage and rotordynamics coefficients of rectangular grooved, circular
grooved, and smooth annular seals. Modeling seals together with the whole ESP is crucial as the
performance of both the pump and the seal is interdependent.
Thrust Effect on ESPs
Unconventional wells are highly unpredictable, mainly characterized by gas slugs, frequent changes in
flow rate and production of abrasives along with frac sand. These wells require very wide operating range
for their ESPs due to the frequent change of production flow rates. An ESP performs optimally within a
range from BEP. Any deviation outside this range changes up-thrust/down-thrust condition. Up-thrust or
down-thrust is the force exerted by the pressure across impeller in axial direction. At low flow conditions,
due to high down-thrust, the pump impeller may rub against the thrust bearing causing undesirable
consequences to pump efficiency and reliability. Other causes of down-thrust conditions are wearing out
of sealing surfaces and excessive gas in the well fluid. There are different design parameters that affect
thrust loading. Thrust balancing is usually achieved by changing skirt diameter (front clearance) and
balance ring (back clearance), changing the size, position and number of balance holes, and optimizing
the hydraulic design. Thrust is very important parameter that affects pump reliability in production wells
(Ye 2015).
Hydrodynamic Modeling and Validation
The ESP stage is simulated using a 3D model. The model includes the skirt seal which provides a seal
between high pressure zone at the impeller exit and low pressure zone at the impeller inlet. The model also
includes the balance ring seal which is the sealing element between the high pressure zone at the impeller
exit and the balance chamber between the diffuser and the impeller hubs. The balance holes are included
in the model. Hybrid mesh was used including mapped hexahedral elements and unstructured polyhedral
elements. No tetrahedral elements are used as they are not good for convergence and accuracy criteria.
Mesh independence study was performed. A single stage model has 3.5 to 4 million nodes. Standard
k-epsilon model was used. The y⫹ value was kept within the acceptable range. Transient analysis was
implemented and the impeller motion was simulated as well. The K-epsilon model is an eddy viscosity
model that is based on Reynolds Averaged Navier-Stokes equation (RANS). The continuity and momentum equations are given as follows:
(1)
(2)
The working fluid used in both the simulation and testing was water. The model used is the one
presented by Marsis et al. (Marsis 2013). The head curve of the pump was generated using the CFD model
and was compared with the actual test of the stages that were simulated in these paper. All the cases
showed very good agreement between the simulation and the test with 3% maximum error. Two full
rotations of the pump were simulated transiently to ensure the full development of the flow inside the
pump.
Fig. 1 shows a 2D cross-section of the 3D model showing all clearances as well as the main hydraulic
path. A 3D model is shown in Fig. 2 showing the impeller, diffuser, and the fluid domain behind the
shroud of the impeller.
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Figure 1—Cross-section of a 3D ESP model showing all clearances and the main hydraulic path
Figure 2—3D model showing the diffuser and impeller including the shroud on the back of the impeller
Thrust model validation
In this paper, thrust of ESP stages is being modeled for the first time. The model was first applied on small
338 series pump stage that is designed and manufactured by Baker Hughes Inc. This stage has a BEP at
610 BPD. In order for this stage to perform as a Flex stage, it had to be designed for a wide operating range
to meet the challenging requirements for unconventional wells. In order to have a wide operating range,
the head curve has to be continuously rising within the operating range with no dips, flat spots, or droop.
Also, thrust must not exceed a certain limit in order to be handled by thrust bearings. Also the minimum
the thrust the better as it affects the reliability, efficiency, and the run life of the ESP. Thrust was modeled
early in the design phase and the size of the balance ring seal, skirt seal, and balance holes location and
size were optimized.
Fig. 3 shows the thrust prediction from the CFD model compared to the actual thrust tested for the 338
series pump stage at different balance holes numbers. Results show very good agreement between the
actual test and the CFD predictions. In some specific ESP stages, since the first stage performance is
significantly different from the second stage performance in order to achieve constant rising head curve,
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two stages had to be simulated to accurately predict head and thrust for the second stage as seen in
Fig. 4.
Figure 3—338 series stage thrust prediction vs. test results
Figure 4 —2D view from the 3D model showing 2 stages simulation for 338 series pump for thrust prediction
Simulation-Based Design
Different geometric configurations were studied extensively to understand the thrust behavior and design
for a flat low thrust curve throughout the operating range of ESPs. These simulations showed that the
diameters of the balance ring and skirt seals affect thrust curve significantly by shifting the whole curve
up or down. Also widening the balance holes in an impeller shift the thrust curve down and cause
hydraulic efficiency to go down as well due to fluid circulation. However, changing the blade loading on
an impeller was used to change the shape of the thrust curve and was very effective to make the thrust
curve flat for wider operating range.
A 400 series stage was designed for optimum hydraulic performance using CFD-based design
techniques. The test matched the predicted performance for head, thrust and efficiency. Fig. 5 shows the
predicted thrust curves early in the design phase for this stage with different balance holes sizes and
balance ring diameters in order to obtain a flat and low thrust curve.
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Figure 5—CFD-Based thrust design for 1900 BPD 400 series stage
Computational Fluid Dynamics are used not only to predict and control thrust, but also to predict and
control other factors that affect the ESP performance. The shape of the head curve is crucial in
determining the pump operating range. Having a constant rising head curve for an ESP stage within the
operating range is a great advantage as it allows the operator to accurately control the flow rate of the
pump and avoid running in unstable conditions. The 338 series-610 BPD pump stage was designed to have
high head at shutoff, and yet maintain a constant rising head from 0 BPD up to the maximum flow rate
as seen in Fig. 6. Fig. 7 shows another 538 series pump that is designed using CFD simulations and it
shows a high head per stage and yet a constant rising head with no dips or flat spots. At 11 KBPD, it is
hard to design a stage with no dips as this has been a challenging flow condition in the industry. However,
by controlling the blade loading on the impeller and diffuser, a constant rising head was achieved.
Figure 6 —338 series tested pump curve
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Figure 7—538 series pump performance test matching CFD prediction showing constant rising high head
Two phase flow simulation was also used as part of simulation-based design in order to ensure the best
performance of ESP stages in gassy wells. Two phase flow simulation model presented in Marsis et al.
(Marsis 2013), was used to visualize the gas pockets and gas accumulation inside impellers. Blade loading
of the impeller was changed and optimized in order to minimize the gas pocket accumulation between the
impeller blades as seen in Fig. 8.
Figure 8 —Gas volume fraction in two different impellers; (a) Before changing blade loading, (b) After changing blade loading
Conclusion
This study shows that CFD modeling was used to accurately predict and control thrust of ESPs for the first
time. This was based on transient CFD analysis with high quality mesh. This technique is developed to
minimize testing time and cost. This model allows pump companies to design their stages, optimize the
blade loading, define the right dimensions of seals and balance holes early in the design phase so the pump
would meet the expected performance from the first iteration.
CFD based design was implemented not only on thrust, but also on other factors that affect the pump
performance like shape of the head curve, and gas handling. CFD based design is very useful in predicting
and controlling the shape of the head curve to ensure a wide operating range for the ESP stages. Also gas
pockets were visualized and eliminated early in the design phase of a new stage development. This new
design technique was developed to meet the challenging requirements in many unconventional wells and
improve the run life of the ESP systems.
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Therefore CFD-based design takes pump design to a new level. It allows pump manufacturers to
control all aspects of performance early in the design phase. Stage thrust, the shape of the head curve, and
gas handling are no more unpredictable or uncontrollable. This technique allows engineers to design
stages that will meet the challenging market need and challenging applications in artificial lift industry.
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