ESTSP Description
An ESTSP is a multistage positive displacement pump. Twin screw technology has been around for many years. However, a new design and materials allow the technology to be fitted into oil and gas wells.
It is installed similarly to an ESP with a submersible motor. The ESTSP replaces the ESP, gas intake and standard seal section. All the other parts of the pump set (motors, MLE/pothead, main power cable, variable speed drive, and transformers) are similar for both pumps.
Each stage (module) of an ESTSP carries a short section of screws (rotors) that interlock at extremely close tolerances but do not touch. Because the screws do not touch, less friction and less torque result in lower power consumption. The counter rotating screws in each stage enclose a constant fluid volume of gas, oil, water or all three. Although a twin screw pump accelerates liquid to the extremities of each screw, the liquid then forms a viscous seal with the other screw or housing, allowing gas to be efficiently transported to the discharge. Oil, gas and water are moved through the pump, in discrete volumes, along the screws, similar to a conveyor belt. This movement has a very low shear rate on the fluid.
The modules may be configured to increase the pressure capacity (more stages) or the volume capacity (larger pump diameter and more speed). Up to 19 stages can be added to increase efficiency and reduce slippage.
Because it is a positive displacement pump, a twin screw pump is always volumetric. Pressures remain nearly the same with varying speed. The pump does not distinguish between gas and liquid because it interprets volume only.
Unlike a centrifugal pump, the twin screw unit has no BEP/efficiency range, and it can draw down to vacuum conditions. Gas content can be as high as 97 percent with 3 percent liquid, which is required for motor cooling and lubrication.
The ESTSP works well for ashphaltene and waxy wells because it has low shear, which minimizes breakout. In any case, residual ashphaltene improves efficiency because it increases the viscosity, which decreases backflow past the pump screws. Scale deposition within the screws will also increase efficiency to a certain extent and makes up for any pump wear.
The ESTSP is not a sand pump, but it can handle an average of 2 percent by volume of sand in high viscosity fluids. It can handle more sand than a conventional ESP because it does not have a centrifugal component to the fluid movement.
The use of a positive displacement pump such as an ESTP can allow a subsurface safety valve (SSSV) to be omitted. The clearance between the screws is minimal and acts as a closed chamber if the fluid is 100 percent liquid. However, if an automatic diverter valve (ADV) is used with the pump a SSSV will be required.
Comparison of ESP and ESTSP Technology
ESPs have been used in the petroleum industry for about 60 years. The ESTSP is relatively new to the oil business. However, the twin screw technology has been used for surface pumping for many years.
- ESP susceptibility to changing well conditions
- ESP susceptibility to scale and ashphaltenes
- ESP low ability to handle free gas
- ESP motor susceptibility to high temperatures
- ESP motors inability to operate at low speeds
- ESP susceptibility to
operator error
High horsepower motors will be required (greater than 1,600 horsepower) for the high PI wells.
While an ESP is ideal for moving large amounts of liquids from wells as long as there is no change in reservoir pressure or well fluids, high well temperature is a major challenge. However, new technology developed for steam assisted gravity drainage (SAGD) applications and geothermal wells is improving run life.
Upthrust & Downthrust
The twin screw pump does not experience the upthrust or downthrust that a centrifugal pump does. The centrifugal pump works best near the middle of its pump curve, the best efficiency point (BEP). If the pump is operated to the left or right of the BEP (outside recommended range), the pump life is immediately compromised because of shaft deflection and/or increased cavitation and diffuser/impeller wear. In a centrifugal pump when the fluid moves up, the thrust is also applied downward, but it must run within the BEP range. If not, upthrust or downthrust can occur and prematurely wear the pump, depending on which side of the curve the pump is running.
In a twin screw pump, no BEP range must be considered. The twin screw pump can operate at any point on its pump curve. When fluid moves up through a twin screw pump, downward thrust is applied to the bearings only, regardless of the pump speed (1 rpm to full-design rpm). Both pump designs use hydrodynamic bearings. However, the twin screw pump uses a stacked bearing design that better distributes the pump loads. In an ESTSP, the load is distributed by two shafts. This reduces shaft buckling under severe thrust conditions. An example of axial loading and radial loading, as they are applied in a twin screw pump, are shown in Figure 3. Fax is the axial load being applied by the fluid on the screws acting downward from the discharge to the intake. The fin is how the radial load is applied on a twin screw pump at the intake of the screw stage. This wrap of the screw determines exactly where the resultant radial load is applied.
Conclusion
The ESTSP could be a superior pump system. However, the application of high power electrical pump systems in high temperature/high pressure wells is new for both ESPs and ESTSPs. The ESTSP is poised to replace the ESP in complicated deep wells. ESTSPs’ new technology will undergo variations to adapt for use in simple wells over a period of time. However, the real advantage in using ESTSP will be in complicated wells with unknown PI. This pump technology will eliminate upthrust and downthrust conditions and provide complete artificial lift in remote areas with little operator involvement.
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