Patent Application
20090266536
The present invention generally relates to a system and methodology for determining parameters in a wellbore. Specifically, the invention is a device that determines both fluid flow and specific gravity (or density) of the fluid going into an electric submersible pump (ESP) based upon measured pressure increases.
It is very beneficial to be able to independently control production from each one of multiple zones of a well. For example, when water begins to be produced from a particular zone, it may be desired to cease production from that zone, while still producing from other zones of the well. As another example, when gas begins to be produced from a particular zone, it may be desired to decrease production from that zone, while still producing from other zones of the well. As a further example, rates of production from various zones may be independently regulated to maximize overall production from a reservoir.
However, in order to accurately determine the particular zones to regulate production from, and the manner in which production from those zones should be regulated, a well operator needs to be able to determine what fluids, and what quantities of those fluids, are being produced from each zone. Prior methods of making these determinations have relied on use of wireline conveyed tools. However, use of these tools usually requires that the well be shut in and that an intervention be made into the well.
It would be far more convenient and useful to be able to continuously monitor what fluids, and what quantities of those fluids, are being produced from each zone of a well. It is accordingly one of the objects of the present invention to provide fluid property sensors for relatively permanent installation in a well, and methods of using and calibrating those sensors.
An electric submersible pumping system generally is formed as an electric submersible pump string having at least three main component sections. The sections comprise a three-phase motor, pump stages, and a motor protector generally located between the motor and the pump stages. In a typical arrangement, the motor is located below the pump stages within the wellbore. Historically, measurement of parameters within the well was constrained to sensors located below the motor. For example, certain existing electric submersible pump string sensor systems utilize a sensing unit connected at the bottom of the submersible motor.
Attempts have been made to collect data from locations along the electric submersible pump string on various parameters. For example, a complete transducer has been attached to the side of the pump string by clamps or gauge carriers. In other attempts, a pressure line has been routed from a location along the pump string to a pressure sensor in a unit mounted below the motor. Also, sensors have been attached to the outside of the pump string and coupled to a dedicated electrical or fiber optic line run from a surface location. However, none of these approaches has succeeded in providing a rugged system of sensors for integration into an electric submersible pump string, and therefore, they all fail to provide accurate, real time data to the operator at the surface.
The present invention is directed to a process that satisfies at least one of these needs. One embodiment of the present invention provides for a method for determining wellbore parameters of a wellbore fluid flowing into a submersible pump having a plurality of pump stages. The embodiment generally includes measuring a first pressure increase across a first pump stage of the submersible pump, measuring a second pressure increase across a second pump stage of the submersible pump, calculating a pressure increase ratio wherein the pressure increase ratio is a ratio of the first pressure increase over the second pressure increase, determining a flow rate based upon the pressure increase ratio, determining a head of a selected one of the first and second pump stage based upon the flow rate, and finally calculating a specific gravity of the wellbore fluid using the determined head and the pressure increase of the selected one of the first and second pump stage. Preferably, the wellbore fluid is substantially free of free gas.
In one embodiment, the first and second pump stages are centrifugal pump stages. In another embodiment, the respective pump stages are rated for different flow rates. For example, the first pump stage could be rated for a flow rate of 11000 barrels per day, while the second pump stage could be rated for 3000 barrels per day, or vice versa. In a preferred embodiment, the step of determining the flow rate comprises constructing a pump curve of flow rate versus head ratio for a plurality of pump sizes, and using the calculated pressure increase ratio to determine the flow rate of the wellbore fluid based upon the pump curve of flow rate versus head ratio, wherein the pressure increase ratio and head ratio are equivalent.
In another embodiment, a user must first obtain a pump curve of head versus flow rate for an identified pump in order to determine the head of a selected one of the first and second pump stages based upon the flow rate. In a preferred embodiment, the identified pump is identical to the one used in either the first pump stage or the second pump stage. Preferably, the pumps used in each of the stages are strategically selected such that the pressure ratio at various given flow rates yields a pump curve of flow rate versus pressure increase ratio with a sufficiently distinguishing plot line. A sufficiently distinguishing plot line is one that has a relatively high slope such that small changes in the pressure increase ratio yield a larger change in the flow rate. Preferably, the slope is at an angle between about 20 degrees to about 70 degrees from horizontal, and more preferably about 3 5 degrees to about 55 degrees from horizontal, and most preferably about 45 degrees from horizontal.
In one embodiment, the method includes taking pressure measurements at various points within the submersible pump. For example, the pressure measurements can be taken at the inlet and outlet of each of the pump stages, thereby allowing a user to calculate the pressure increase across any given stage simply by finding the pressure difference between the inlet and outlet of each stage.
In another embodiment, the method further includes determining the viscosity of the wellbore fluid. In a further embodiment, the viscosity of the wellbore fluid is determined according to methods known by those skilled in the art. For example, the viscosity may be determined by measuring the viscosity of the fluid at the surface, and then using known fluid characteristics and downhole temperatures, the user may determine what the viscosity would be downhole at the submersible pump.
The present invention is also drawn to a device for measuring parameters within a wellbore comprising an electric submersible pump (ESP). In one embodiment, the ESP comprises a pump member, at least three pressure sensors, a receiver, and a program.
In a further embodiment of the present invention, the pump member has an inlet for receiving fluid and an outlet for discharging fluid, and is disposed within the wellbore. The pump member also has at least two pump stages, wherein each pump stage includes a moveable member for moving said fluids. In a preferred embodiment, the moveable member is a rotatable impeller.
In a preferred embodiment, the at least three pressure sensors are placed such that the three pressure sensors, in combination with each other, are operable to measure the pressure increase before and after each of the two pump stages. In one embodiment of the present invention, a receiver is communicatively coupled to the pressure sensors. In a further embodiment, the receiver is located at the surface of the wellbore. In another embodiment of the present invention, a program is composed of instructions, executable by the receiver, for receiving data from the three pressure sensors and calculating the specific gravity of fluids within the wellbore based upon relationships of the pressure increases across each of the two pump stages. The program can also have access to stored pump-characteristic data such that the program can determine the specific gravity of the wellbore fluid using pump curve data as described above.
In a preferred embodiment, the device further comprises an electrically-powered motor located in a remote downhole location within the wellbore, with the motor being mechanically coupled to the moveable member of each pump stage. In one embodiment, the motor is mechanically coupled to the moveable member by a shaft. Furthermore, the device may include an electrical conductor member extending from a remote surface location to the ESP for providing electrical power to the electrically-powered motor. Additionally, the electrical conductor member can also be used to transmit data from the three pressure sensors by superimposing the signals from the three pressure sensors.
In a preferred embodiment of the present invention, the two pump stages further comprise a diffuser. Furthermore, the device can further comprise three or more spacer sleeves, wherein the three or more spacer sleeves are positioned within the ESP such that the three or more spacer sleeves are, in combination with each other, operable to fixedly attach the three pressure sensors,
So that the manner in which the above-recited features, advantages, and objectives of the invention, as well as others that will become apparent, are attained and can be understood in detail, more particular description of the invention briefly summarized above may be had by reference to the embodiments thereof that are illustrated in the drawings that form a part of this specification. It is to be noted, however, that the appended drawings illustrate only several embodiments of the invention and are, therefore, not to be considered limiting of the invention's scope, for the invention may admit to other equally effective embodiments.
The present invention provides both a method and a device to measure both the flow rate and specific gravity of a wellbore fluid as it enters a submersible pump. Now turning to
Referring to
Special pump housing section [20] is shown connected to a seal section [18] for a three-phase alternating current motor [21], which has a shaft that will drive primary pump stages [17], as well as pump stages within special pump housing section [20]. Seal section [18] is located at the upper end of motor [21] to seal the lubricant within motor [21] and may be considered a part of the electric motor assembly. Seal section [18] also equalizes pressure of motor lubricant with the hydrostatic pressure of the exterior. Seal section [18] may also have a thrust bearing for handling downthrust created by primary pump stages [17]. Power cable [23] extends from the surface to motor [21] for supplying electrical power. The output shaft (not shown) of seal section [18] will drive primary pump stages [17] and the secondary pump stages (not shown) located within special pump housing section [20]. Electrical line [27] connects each pressure sensor to an additional temperature pressure sensor [25] mounted at the bottom of motor [21]. In one embodiment, receiver [19] is located at the surface and is in communication with the pressure sensors located within special pump housing section [20] and the pressure and temperature sensor connected to the bottom of motor [21].
In one embodiment, a program is composed of instructions and is communication with receiver [19], such that receiver [19] is operable to receive data from the three pressure sensors within special pump housing section [20] as well as the temperature and pressure sensor connected to the bottom of motor [21], and to execute the program in order to calculate the specific gravity of the fluids within the wellbore based upon the received pressure increase data. The program preferably has stored pump-characteristic data such that the program can iteratively determine the specific gravity of the wellbore fluid.
Motor [21] typically can be driven by the frequency of the power supplied to rotate in the range from 2,400 to 4,800 rpm. The power supplied can be at a fixed frequency or it can be varied,
Wellbore fluid enters special pump housing section [20] at fluid inlets [22] and travels upwards where the fluid's first pressure is measured by first pressure sensor [32]. The fluid continues past first pressure sensor [32] and into first pump stage [26], where the pressure of the fluid is increased through first rotatable member [30a]. In a preferred embodiment, first rotatable member [30a] is a centrifugal impeller. The fluid, which was traveling substantially vertically prior to entering first pump stage [26], exits first rotatable member [30a] at least partially radially. In the embodiment shown in
Pressure increase across a centrifugal pump stage is determined from Equation 1 below:
ΔP=H·SG·k (1)
wherein ΔP is the pressure increase across a pump stage, H is feet of head developed by the pump stage, SG is the specific gravity of the wellbore fluid, and k is a constant, which in the present case has a value of
Therefore, the pressure increase across the first pump stage and the second pump stage can be expressed as:
ΔP1=H1·SG1·k (2)
ΔP2=H2·SG2·k (3)
respectively. Additionally, the pressure increase across the first and second pump stages can also be calculated according to the following equations:
ΔP1=P2−P1 (4)
ΔP2=P3−P2 (5)
Dividing Equation (4) by Equation (5) would yield:
Consequently, by dividing Equation (2) by Equation (3), and assuming that the specific gravity of the fluid is constant, we can find that the ratio of pressure increase is equivalent to the head ratio of each pump stage, as shown in Equation (7).
Of course one skilled in the art will recognize that it is irrelevant as to whether Eq. (2) was divided by Eq. (3) or vice versa. The only important feature is that a ratio of the pressure increases is calculated, and this ratio corresponds to the same notation used to construct the pump curve used to determine flow rate.
The assumption of a constant specific gravity is accurate as long as the pump stage heads are minimized (possible through design criteria) and there is substantially no free gas in the flow stream (application criteria).
For a given application, pump stages can be designed or selected so that the flow rate (Q) is a function of the head (and vice versa) over a known flow range. Through proper design, the flow rate can also be a function of the ratio of the stage heads, and is shown in the following equation:
Furthermore, substituting Equation (7) into Equation (8) yields the following equation:
Finally, substituting Equation (6) into Equation (9) yields the following equation:
Consequently, knowing a value for the ratio of the pressure increase across the stages yields a flow rate. Once a flow rate is determined, the head for a given pump stage can be determined since head and flow rate are a function of each other. Once the head has been determined, the specific gravity may be calculated using Equation (1), and solving for the only unknown value (SG).
The following is an example of how the flow rate and specific gravity would be determined. Suppose the device is constructed with a pump stage rated for flow at 11,000 barrels per day (B/D) as the first pump stage and a pump stage rated for flow at 3,000 B/D as the second pump stage. Furthermore, the pressures are measured using three pressure sensors, P1, P2, and P3, with P1 at the inlet of the first pump stage, P2 between the first and second pump stages, and P3 being at the discharge of the second pump stage. The three recorded measurements are as follows:
According to Equation (6), the pressure increase ratio would be about 4.07. Using
In another embodiment, receiver [19] (
As will be readily apparent to those skilled in the art, the present invention may easily be produced in other specific forms without departing from its spirit or essential characteristics. The present embodiment is, therefore, to be considered as merely illustrative and not restrictive, the scope of the invention being indicated by the claims rather than the foregoing description, and all changes which come within the meaning and range of equivalence of the claims are therefore intended to be embraced therein.
