System, method and apparatus for controlling the flow rate of an electrical submersible pump based on fluid density

An electrical submersible pump that regulates pump flow rate based on sensor measurements of the fluid is disclosed. The sensor measures a property of the fluid being processed. The sensor may be located at the intake, discharge or other area of the pump. The sensor measures the relative proportion of gas in the pumped liquid. The pump flow rate is adjusted to maintain a desired level for the gas in a production environment. The pump may be used to operate and control a seabed gas-liquid separation and centrifugal pump system.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates in general to electrical submersible pump assemblies and, in particular, to an improved system, method, and apparatus for controlling the flow rate of an electrical submersible pump based on measurements of at least one physical property of the fluid being produced.

2. Description of the Related Art

The separation of gases and liquids carried out in a well bore is common. In addition, separation of gasses and liquids at the seabed as part of a subsea oilfield exploitation is becoming increasingly common. Separating the gas and using a high head centrifugal pump to pump the liquids vastly improves the project economics (e.g., asset net present value and recovery factor). The separation of the gas from the liquid also results in improved flow assurance. Moreover, pumping fluids that contain excessive amounts of gas can cause gas lock in a pump or can cause a pump to overheat and fail prematurely.

Currently, in a well bore, the accepted method of controlling the gas-liquid interface level is to manually control the amount of fluid produced by a down hole electric submersible pump (ESP). Generally, the ESP is installed and the production rate is set. If the pump encounters a gas lock condition, it is shut down to allow the well to recover, restarted and a new lower production rate is manually set. This is continued until the ESP is operating in a continuous and stable manner. Conversely, if the pump does not gas lock when the ESP is first installed and is operating in a stable manner, the production rate is manually increased in steps until a gas lock condition occurs. After recovery, the production rate is then reduced to the point of the last stable operation. The object is to produce the maximum fluid available from the well with the pumping equipment.

In surface or subsea canned boosters, the methods for measuring and controlling the gas-liquid interface level is insufficient. In one type of installation, pressure transducers are used to infer, rather than measure, the interface level based on the pressures of the fluids at given elevations in the can or vessel. This method requires a significant difference in height between the transducers to achieve the required resolution in a high pressure vessel. Consequently, oversized and more expensive pressure vessels are used to enable this method. While this solution is satisfactory for some applications, an improved method to monitor fluid parameters and optimize pump performance would be desirable.

SUMMARY OF THE INVENTION

Embodiments of a system, method, and apparatus for regulating the flow rate of a pump in an electrical submersible pump assembly according to sensor measurements are disclosed. The sensor may comprise a fluid property measurement device that detects a property, such as density or capacitance, of the fluid being produced. The sensor may be located at the intake, discharge or other areas of the assembly. The sensor measures the relative proportion of gas in the liquid by the change in the property being measured. The pump flow rate, such as pump speed, may be adjusted to maintain a constant level for the gas in a production environment.

The invention is particularly well suited for operation and control of a seabed gas-liquid separation and centrifugal pump system. This design actually controls the flow rate of the pump rather than the trial and error method of merely monitoring the fluid levels inside a production vessel. Detecting the discharge fluid property that is affected by gas content enables the well or production vessel to be operated for more efficient production. One of the primary concerns for such operations is to maintain liquid-free gas. Maximizing the free volume inside the vessel maximizes the gas quality. Control of the pump flow rate according to a known level of entrained gas maintains the gas-liquid level at its lowest possible level, thereby maximizing the gas separation volume.

For example, an intake flow path to the pump may be provided by a motor shroud with the pump located below perforations in the well. The pump receives the fluid intake from the lower open end of the shroud which allows the gas to move up the well between the casing and the discharge tube and out the casing vent at the surface. Alternatively, the intake flow path may use an inverted shroud that directs the flow up past the ESP. The fluid is then directed down inside the inverted shroud while the gas moves up the well between the casing and discharges out of the casing vent at the surface.

In another embodiment, the intake flow path is in a canned pump where the flow of both oil and gas enters at the top of the can. The ESP is in the can with a shroud to direct the fluid down and past the motor. The gas separates out of the fluid, travels to the top of the can where it is routed to surface processing facilities. The flow rate of the ESP is regulated using a controller and a sensor that measures the amount of gas going to the intake of the pump.

The sensor measures the fluid density or other fluid properties that are related to the amount of gas in the fluid. When the fluid contains more gas than desired by the controller set point, the pump flow rate is reduced to allow more time for the gas to separate and bypass the pump intake. If there is less gas in the fluid than desired, the pump flow rate is increased to allow less time for separation. One object of the invention is to produce the maximum amount of fluid without gas locking the pump.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIGS. l-6. embodiments of a system, method and apparatus for regulating the fluid flow rate of a pump according to sensor measurements are disclosed. The pump may comprise a centrifugal pump in an electrical submersible pump (ESP) assembly, a sucker rod pump, a hydraulic pump, or any kind of pump as well as an ESP. The ESP pumps a gassy fluid in a well or production vessel with the intake flow to the pump routed in such a way that the gas substantially separates from the oil and is not drawn into the pump. Means are provided to remove the gas to gas processing facilities located at the surface.

In a basic embodiment (FIG. 1), the invention comprises a system1for controlling a pump11in a well or other type of gas-oil separation and production environment, such as a production vessel23(e.g., a caisson, canned pump assembly, booster pump assembly, etc.). The production vessel23is the sealed vessel that contains the oil to be pumped to the surface. The system1uses the pump11to retrieve fluid15from the production vessel23.

One or more instruments or sensors17a-17gare located adjacent the pump11to obtain physical property measurements of the fluid15. Physical properties such as density, capacitance, etc., that are influenced by the presence of a gas are suitable for these applications. For example, the rotational speed of a turbine flow meter is directly proportional to the gas content in fluid. Although the sensor17bis shown located at the fluid discharge area201(i.e., after gas separation) of the pump, it also may be located at a fluid intake area202relative to the pump, or at any position along the assembly. In addition, the sensor17may comprise a plurality of sensors17a-17glocated at different positions along the fluid flow path203relative to the assembly.

In one embodiment, density measurements may be used as an indicator of the relative proportion of gas19in the fluid15. A controller21coupled to the sensor17controls the flow rate of the pump11. The flow rate of the pump11is modified responsive to the density measurements in order to maintain a desired constant or set point level of gas within the production vessel23. The desired level of gas within the vessel may be selected based on many criteria and depends on the application. For example, in one embodiment, the set point level may be established at or near the pump intake to provide the maximum gas volume and the maximum gas liquid separation prior to producing the fluid to the surface.

As shown in the embodiment ofFIG. 1, the invention is employed in an oil and gas production system comprising a plurality of wells31for producing oil and gas. The production vessel23may be provided with an inlet pipe32for fluid communication with the plurality of wells31. The production vessel23contains a volume of oil15and a volume of gas19produced by the plurality of wells. The production vessel23has a gas port33for releasing the gas19.

An ESP assembly35is located in a shroud37and installed in the production vessel23for pumping oil15out of the production vessel23. The shroud37has an opening39on a lower end thereof that is submerged beneath an interface41between the volumes of oil15and gas19. Shroud37serves as a means for separating gas from well fluid flowing to ESP assembly35. The ESP assembly35comprises a motor43, a seal section45and the pump11, and may include a gas separator. The sensor17measures a property (e.g., density) of the fluid processed by the ESP assembly35. The controller21controls the flow rate of pump11in response to the sensor17.

As described herein, the flow rate of the pump11may be modified responsive to the fluid density measurements to maintain a desired level41of gas within the production vessel23. The fluid density indicates a relative proportion of gas in the oil. The sensor17may be located at the fluid discharge or fluid intake areas relative to the pump. In alternate embodiments, the sensor17may comprise multiple sensors located at different positions along a fluid flow path relative to the ESP assembly35. Such sensors may sense or measure more than one property of the fluid.

The automated flow rate control of the pump may be manipulated by, e.g., modifying the speed of the pump. Alternatively, a choke205(e.g., discharge choke valve) may be provided in the fluid flow path downstream from the pump to regulate the flow rate of fluid through the pump. See system2and other embodiments having a choke205inFIGS. 4-6.

The invention also comprises a method for controlling a pump. In one embodiment, the method comprises the steps of installing a pump in a production vessel, the pump having a fluid intake located in a shroud; receiving fluid comprising oil and gas into the production vessel and pumping the fluid out of the production vessel with the pump; sensing a property of the fluid being pumped, the property being a measurement of a relative proportion of gas in the oil; and modifying a flow rate of the pump in response to the property measurements to maintain a desired level of gas within the production vessel.

In still other embodiments, the pump flow rate is controlled based on the density of the gas outlet stream (i.e., inversed) as measured by a gas density sensor suitable for the environment. A liquid density sensor for in-well use also may be used.

Referring now toFIG. 2, another embodiment of an electrical submersible pump assembly for a production environment is shown. In this embodiment, a well having well casing51with perforations53permits a liquid flow55and gas flow57. The ESP assembly includes a motor59, seal61and pump63, which are mounted to a discharge tube or outlet pipe65, and power is provided via power cable67. The ESP assembly is located below perforations53.

In addition, at least a portion of the ESP assembly is located in a motor jacket or shroud69. The intake flow path71of liquids55to the pump63is defined by the shroud69. The pump63receives the fluid intake71from the lower open end of the shroud69. In the embodiment shown, the shroud69is sealed and mounted to the pump63, it extends downward past the pump, and it is open at a lower end thereof. This configuration allows the gas57to move up the well between the casing51and the outlet pipe65and out the casing vent at the surface.

The shroud69also directs flow past the motor59for cooling purposes. In another embodiment, some of the fluid being produced by the pump63is directed down the well so that it flows past the motor59for cooling purposes. In embodiments where the pump intake is located below the fluid inlet to the production vessel (e.g., such as inlet pipe32inFIG. 1, and perforations53inFIG. 2), the shroud69is not necessarily required but additionally provides the cooling advantage for the motor. Alternatively, a recirculating pump may be provided in the ESP to direct a portion of the fluid flow past the motor to provide additional cooling capacity.

Referring now toFIG. 3, a third embodiment of an electrical submersible pump assembly for a production environment is shown. Like the preceding embodiment, the well has well casing81with perforations83that permit a liquid flow85and a gas flow87. However, the ESP assembly is located above perforations83. The ESP assembly includes motor59, seal91and pump93, which are mounted to outlet pipe95with power provided by power cable97.

At least a portion of the ESP assembly is located in an inverted shroud99(i.e., the open end is at the top of the shroud). Thus, in the embodiment shown, the shroud99is mounted below the pump93, it extends upward past the pump, and it is open at an upper end thereof. The intake flow path101of liquids85to the pump83is defined by the shroud99, which also directs the flow of gas87up past the ESP assembly. The liquids85are directed down inside the inverted shroud99while the gas87moves Up the well between the casing81and pipe95and discharges out of the casing vent at the surface.

The invention has numerous advantages. The pump intake is located in a production vessel such that the fluid flow comes to the pump intake from above so that there is a natural tendency for gas separation. The automated property measurement and control also permits the use of shorter and wider vessels compared to prior art monitoring methods. The prior art pressure transducer method currently used requires some difference in height between the various transducers in order to get the required resolution in a high pressure vessel. In contrast, the invention permits the pump to operate with a minimum fluid level over pump (FLOP). This design maximizes the gas slug capacity which further enables the capacity or size of the vessel to be reduced. The low FLOP allows the gas to be the highest quality possible with minimum entrained liquids.