New Multiphase Pump System Can Handle Up to 100% Gas Volume Fraction
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After production startup of lean and rich gas fields in Algeria, some reservoirs experienced a continuous pressure decline that affects the real production potential of many wells. To have the flexibility to treat both rich and lean gas wells [up to 100% gas volume fraction (GVF)], a new multiphase pump system has been developed by comparing the different boosting systems performance and analyzing well-by-well production scenarios. With two systems installed successfully in different configurations, all involved wells have produced with an optimized bottomhole drawdown to extend productive life.
The gas fields are situated on the Berkine Basin in Algeria and are part of a joint commercial project. The facilities gather and process produced fluids from lean gas wells from Field B, along with rich and lean gas wells from Field A in order to produce gas, oil, condensate and liquefied-petroleum-gas products. The development concept consists of a gathering system, a central processing facility (CPF), infrastructure, and export pipelines.
The gathering system consists of manifolds that are similar functionally. Each site typically gathers four or five wells. The combined well flow is then sent to the CPF by the main trunk line. Some of the field manifolds also gather flows from other manifolds as they do from wells. In Field A, well configuration and surface facilities are similar to those in Field B, but many independent wells exist with a few functional manifolds.
After a complete flow-assurance analysis, production performance calculated on the basis of a noninterference strategy could not guarantee the original production target and might not even guarantee the minimum production for CPF continuous operation.
New local boosting systems have been positioned strategically to affect as many wells as possible and to increase the total production. In Field A, well distribution suggested an installation close to the CPF to produce almost all wells from that field. The optimal location for a multiphase pump in Field B was at the connected manifold sites in order to boost production from several wells.
Twin-Screw Pump Description and Function
Twin-screw multiphase pumps are used in the oil and gas industry to transfer a volume of liquid (oil and water) and gas from inlet to discharge. This is achieved by two rotors, each carrying a set of two screws. The screws are intermeshed and thereby form locks or chambers. The fluids are pushed out against the backpressure of the downstream system by counter-rotation of the rotors, moving axially from inlet to discharge. Fig. 1 shows the flow paths of the fluid on a rotor assembly. A liner is inserted in the casing which, among other features, may be regarded as a wear element.
The screws have a special flank profile, which is vital to minimize the backflow through the clearances. The lathing process offers the possibility of decreasing pitch toward the discharge end of the screws, creating internal compression for a higher efficiency in multiphase flow. The pitch size determines the pump operating parameters, so the smaller the pitch, the higher the differential pressure the pump can provide (at a lower rate). In contrast, a greater pitch will allow a higher rate (at a lower differential pressure).
There is no metal-to-metal contact between the rotors and the liner, which allows very small gaps. This space must be sealed by liquid to enable an efficient pumping process. Local internal overheating must be avoided under all circumstances because it will cause a subsequent shutdown of the pump.
Pumps mainly are designed to produce liquid, or liquid and gas, but not dry gas or 100% GVF, which would result in overheating and a potential no-flow scenario because of system shutdown. However, compressors will not be able to manage an increased amount of liquid when wells must be produced with a high water cut. In this situation, pumps will provide an advantage over compressors.
Multiphase Twin-Screw Pump System Design For Up to 100% GVF
The most-suitable multiphase-pump system identified for this study are twin-screw pumps, positive displacement machines that can work independently of density and inlet pressure as well as backpressure and changes in GVF assisted by inlet separators and an internal recirculating system.
The system has been proved ideal for multiphase production (rich and lean gas) to handle up to 100% GVF. During the life of the field, the multiphase flow can be managed by controlling the speed of the pumps to navigate a wide range of production scenarios.
The pump systems have been designed to cope with slug flow with no effect on the operations. High GVF and wet gas are handled with an integrated recirculation of liquids. The compression heat is removed with liquid recirculation. However, for the lean-gas-flow (100% GVF) process, the coolers have been included as well.
The twin-screw pumps’ essential components, such as the cooler, lubrication oil, and all safety devices, are complemented with secondary equipment to create a complex system that has ensured continuous optimal performance of the installed pumps at the time of writing. Two different scenarios have been considered. Both single- and multiple-pump installations have been designed and installed successfully at the manifold level and close to the CPF. The designs for both configurations are concentrated to properly address all fluid-flow constraints.
The pumps installed in parallel have required special attention to split the upstream flow. These include a general inlet separator to manage any potential slugs arriving from wells far away, while providing more liquid to the recirculating system in addition to the internal separator vessel on every pump.
For the single-pump installation, the pump does not have the internal separator, which has been included externally downstream of the pump. The separator provides the necessary liquid to keep the system within the temperature range. Components of the systems are detailed in the complete paper.
Field A and B Multiphase Flow Characteristics and Production Results
In Field A, the pump configuration was selected after considering as-low-as-possible inlet pressures based on the expected treated volume (gas, condensate, and water) and the discharge pressure to the CPF slug catcher. The expected volume from the wells has been calculated on the basis of well production history.
The calculations showed that three 600-kW pumps were required to manage the expected production from many Field A wells by applying a 30-bar differential pressure on surface and allowing a significant production increase. After installation, the pumps not only were able to stop the dramatic and sharp production decline but also increased production by more than 50% at startup. By following optimization of the well parameters and imposing 25 bar of differential pressure, the system has operated successfully with a GVF of 99.9%.
In Field B, the pump configuration was selected after considering the lowest inlet pressures, the expected treated volume (gas, condensate, and water), and the discharge pressure through the trunk line to the CPF. The expected volume from the wells has been calculated on the basis of well production history.
The calculations predicted that at least one 2.2-MW pump was required to manage the expected production from two manifolds by applying 30-bar differential pressure at surface. This pump would also allow a significant production increase. After installation, the pump not only was able to stop the sharp production decline but also increased production by more than 50% at startup. By following optimization of the well parameters and imposing 20 bar of differential pressure, this system, as in Field A, has operated successfully with a GVF of 99.9%.
All multiphase pumps installed so far with different configurations and capacities have proved very effective in managing wellhead pressure decline and flow stream, with almost 100% GVF achieved while allowing a sustained significant production increase. The performance continues to improve with time, although it is affected by required programmed maintenance after a preset duration of continuous service.
The downtime for all installed pumps has been minimized because of a continuously active satellite monitoring system that shows all critical parameters in real time while allowing data storage for trend analytics and system optimization. This provides the response-maintenance team with fundamental data either to predict problems in the system or to assess potential problems quickly in case of unscheduled shutdowns.
New Multiphase Pump System Can Handle Up to 100% Gas Volume Fraction
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