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Hydraulic pumping systems for single wells
Most hydraulic pumping systems operate in centralized field facilities (tank batteries, other lease-level facilities). Sometimes, however, only a few wells in a field are suitable for hydraulic pumping, or spacing considerations make the use of centralized facilities impractical. To address the limitations of the central battery system, single-well systems have been designed, . These have many of the same components as centralized facilities, but have been designed for efficient use by one, or sometimes two to three, wells.
Single well systems
Several of the manufacturers of hydraulic pumping units offer packaged single-well systems that include all the control, metering, and pumping equipment necessary. All components are skid mounted on one or two skids to facilitate installation at the well and to make the systems easily portable if the unit is to be moved to a different well. Usually, the only plumbing required at the wellsite is the power fluid and return-line hookup at the wellhead and the connection of the vessel outlet to the flowline.
An example of a typical single-well power unit is illustrated in Fig. 1. All units of this type share certain design concepts, with small variations depending on the manufacturer. Either one or two pressure vessels are located at the wellsite. The size of the main reservoir vessel depends on the nature of the well and the tubular completion. The reservoir vessel should ensure that, if the wellhead partially empties the return conduit to the flowline, adequate capacity remains to operate the downhole unit until production returns re-enter the vessel. Even if the well does not head, extra capacity is needed. When the unit is shut down for maintenance or pump changes, that portion of the return conduit occupied by gas needs to be filled from the vessel to unseat the pump and to circulate it to the surface. The vessel sizes normally used range from 42 × 120 in. to 60 × 240 in. In some wells, even the largest vessel may not be able to compensate fully for heading, in which case it is common to use backpressure to stabilize the heading. The vessels themselves are normally in the 175- to 240-psi working pressure range, with higher ratings available for special applications. Coal-tar-epoxy internal coatings are common, with special coatings for extreme cases.
Power fluid system
The return power fluid and production for the well enter the vessel system where basic separation of water, oil, and gas phases takes place. Free gas at the vessel pressure is discharged to the flowline with a vent system that ensures a gas cap in the vessel at all times, while the oil and water separate in the vessel, and the desired fluid (oil or water) is withdrawn for use as power fluid. The power fluid passes through one or more cyclone desanders to remove solids before entering the multiplex suction, where it is pressurized for reinjection down the power-fluid tubing. Any excess multiplex output that is bypassed for downhole pump control is returned to the vessel. The underflow from the bottom of the cyclone desanders contains a high-solids concentration and is discharged either into the flowline or back into the vessel system. Once the system is stabilized on the selected power fluid, the well production of oil, water, and gas is discharged into the flowline from the vessel, which is maintained at a pressure above the flowline. Because the flowline is carrying only what the well makes, additional treating and separating facilities are not needed, as they are in the central battery system that handles mixed well production and power fluid. This feature also facilitates individual well testing.
Overall, simple gravity dump piping, which consists of a riser on the outside of the vessel, controls the fluid level in the vessel system. To prevent siphoning of the vessel, the gas-vent line is tied in the top of the riser as a siphon breaker. The choice of oil or water power fluid is made by selection of the appropriate take-off points on the vessel so that the production goes to the flowline and the power fluid goes to the multiplex pump. If the multiplex suction is low in the vessel and the flowline is high in the vessel, water will tend to accumulate in the vessel and will be the power fluid. If the multiplex suction is high in the vessel and the flowline is low, oil will tend to accumulate in the vessel and will be the power fluid. Opening and closing appropriate valves sets the system up for the chosen power fluid. The multiplex suction outlets are positioned with respect to the overall fluid level in the vessel to avoid drawing power fluid from the emulsion layer between the oil and water because this layer generally contains a significantly higher concentration of solids and is not easily cleaned in the cyclones.
Assuring power fluid quality
The power-fluid cleaning is accomplished with cyclone desanders that require a pressure differential across them. In the two-vessel system, a differential pressure valve between the two vessels that stages the pressure drop from the wellhead accomplishes this. The energy to maintain this staged pressure is supplied by the multiplex pump through the downhole pump.
The flow path through a cyclone cleaner is shown in Fig. 2. Fluid enters the top of the cone tangentially through the feed nozzle and spirals downward toward the apex of the cone. Conservation of angular momentum dictates that the rotational speed of the fluid increases as the radius of curvature decreases, and it is the high rotational speed that cleans the fluid by centrifugal force. The clean fluid, called the overflow, spirals back upward through the vortex core to the vortex finder, while the dirty fluid exits downward at the apex through the underflow nozzle. The cones are usually constructed of cast iron with an elastomer interior. Different feed-nozzle and vortex-finder sizes and shapes are available to alter the performance characteristics of the cyclone. Different sizes of cyclones are available, with the smaller sizes having lower flow-rate capacities but somewhat higher cleaning efficiencies.
Maintaining the proper flow through the cyclone to ensure good cleaning depends on correctly adjusting the pressures at the feed nozzle, overflow, and underflow. At the design flow rates, a 30- to 50-psi drop normally occurs from the feed nozzle to the overflow. In the single-vessel system, a charged pump supplies the pressure, while in a dual-vessel system, the pressure is supplied by a higher backpressure on the returns from the well. Because of the centrifugal head, the cyclone overflow pressure is generally 5 to 15 psi higher than the underflow pressure. An underflow restrictor is commonly used to adjust the amount of underflow to between 5 and 10% of the overflow. This ensures good cleaning without circulation of excessive fluid volumes. It should be noted that the volume flow rates through a cyclone vary inversely with the specific gravity of the fluid, and that within the range of normal power fluids, increased viscosity leads to increased flow rates. The viscosity that suppresses the internal vortex action causes this latter effect. Therefore, proper cyclone sizing to match the charge and multiplex pump characteristics must be done carefully and with detailed knowledge of the fluid to be processed. The manufacturers of the packaged systems supply appropriate cyclones for the installation, but it should be noted that moving the portable unit to another well might require resizing of the cyclone system.
The routing of the dirty underflow varies with different systems, and may be an adjustable option in some systems. Two basic choices are available:
- Return of underflow to the vessel
- Routing of the underflow to the flowline
In a dual-vessel system, the underflow must be returned to the flowline downstream of the backpressure valve to provide sufficient pressure differential to ensure underflow. Discharging the solids to the flowline is attractive because they are disposed of immediately and are excluded from possible entry into the power fluid. Under some conditions, however, continuous operation may not be possible. If the net well production is less than the underflow from the cyclone for any length of time, the level of fluid in the vessel will drop, and over an extended period of time, this can result in a shutdown of the system. Shutting off the cyclone underflow during these periods stops the loss of fluid, but apex plugging occurs during the shutoff period. Returning the underflow to the vessel eliminates the problem of running the vessel dry but does potentially reintroduce solids into the power fluid. In single-vessel units, the underflow is generally plumbed back to the vessel in a baffled section adjacent to the flowline outlet. This provides for the maximum conservation but requires a differential pressure valve, between the cyclone underflow and the vessel, which is normally set at about 20 psi to ensure a positive pressure to the underflow fluid.
The vessel pressure is held above the flowline pressure to ensure flow into the flowline and a backpressure control valve is sometimes used for this purpose. This keeps the vessel pressure, which is backpressure on the well, at a minimum for any flowline pressure that may occur during normal field operation. When water is the power fluid, "riding" the flowline in this manner is acceptable. However, when oil is the power fluid, changing vessel pressure causes flashing of gas in the power oil and adversely affects the multiplex suction. When oil is used as power fluid, it is recommended that a pressure control valve be used to keep the vessel at a steady pressure some 10 to 15 psi above the highest expected flowline pressure.
While the single-vessel system was developed for applications involving widely spaced wells, two or three well installations have been successfully operated from a single-well system. This installation is very attractive on offshore platforms. With a large number of highly deviated wells, offshore production is well suited to hydraulic pumping with free pumps, but the extra fluid treating facilities with an open power-fluid system is a drawback when severe weight and space limitations exist. The closed power-fluid system answers this problem, but the extra tubulars in deviated holes create their own set of problems and expense. Furthermore, the use of jet pumps, which is quite attractive offshore, is not possible with the closed power-fluid system. For safety and environmental reasons, water is almost always the power fluid of choice offshore. A large single-well system can receive the returns from all the wells and separate the power water necessary for reinjection to power downhole units. Full 100% separation of the oil from the power water is not necessary, and, in fact, some minor oil carryover will contribute to the power-fluid lubricity. The platform separation facilities then have to handle only the actual production from the wells. A compact bank of cyclone cleaners completes the power-fluid separation and cleaning unit.
- Palmour, H.H. 1971. Produced Water Power Fluid Conditioning Unit. Paper presented at the 1971 Southwestern Petroleum Short Course, Texas Tech. U., Lubbock, Texas, 15–16 April.
- Feldman, H.W. and Kelley, H.L. 1972. A Unitized, One-Well Hydraulic Pumping System. Paper presented at the 1972 Southwestern Petroleum Short Course, Texas Tech. U., Lubbock, Texas, 20–21 April.
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