SUBMERSIBLE PUMP

Cross-Reference to Related Applications

This application claims the benefit of U.S. Provisional Application No. 61/605,794, filed March 2, 2012, which is incorporated herein by reference.

Field of the Invention

The invention relates to detecting and breaking gas locks in electrical submersible pumps. Background

Electrical submersible pumps are used in deepwater oil and gas production settings to provide artificial lift such that the oil and gas can be raised to the surface for further processing, storage and/or transport. During operation fluid property changes of the oil/gas mixture cause the liquid level inside the casing to drop and consequently gas lock conditions can occur where the pump takes in sufficient quantities of gas to effectively lock up the pump and prevent fluid from flowing through the pump and out the discharge line.

The conventional solution to this problem is to stop the pump to allow the system to stabilize, and then restart the pump once the system returns to normal conditions, usually by the gas flowing up the discharge line as described in US 5,015,151. This solution results in increased downtime, an increased number of pump starts, and consequently decreased pump longevity (due to stresses at start-up). In addition, if gas lock conditions are not detected then pump components can be damaged and/or the pump motor can overheat.

Summary of the Invention

The invention provides a method of breaking a gas lock in an electrical submersible pump, comprising: a) monitoring a derived value related to the electrical current used by a pump motor connected to and providing power to the pump; b) comparing the derived value to a threshold value to detect the occurrence of gas lock conditions; and when those conditions are detected; c) sending an override signal to reduce the speed of the pump for a set period of time; and d) comparing the derived value related to the electrical current to the threshold value at the end of the set period of time, and if gas lock conditions are not detected, then increasing the speed of the pump to return to normal operating conditions. Brief Description of the Drawings

Figure 1 depicts an embodiment of the control system.

Figure 2 depicts an embodiment of the method showing the various steps on a timeline Detailed Description

Gas locking conditions are characterized by a sudden drop in the load on the pump motor when gas enters the pump. Continued operations under gas lock conditions can damage the electrical submersible pump, seal or motor, and it is important to detect gas lock conditions and adjust the conditions as quickly as possible to break the gas lock.

Gas lock conditions can be brought about by any of several reasons including an increased gas volume fraction in the oil/gas mixture, a pump that is operating at too high of a flow rate such that the liquid level is dropped below the pump intake, and rapid fluid property changes due to hot oil reduction, gas/water slugging, etc..

The method to detect and break gas lock conditions comprises an initial step of monitoring the operation of the pump and comparing the operation with one or more predefined limits or alarms that would indicate gas lock conditions. A number of variables may be monitored, and the measurements may be made at the surface, subsea or in the wellbore. A preferred measurement is the running standard deviation of the electrical submersible pump motor amps.

The predefined limit or alarm may be a hard limit that is set based on the design of the system or it may be as in the example of running standard deviation based on the deviation from normal or past operation of the system.

There are a variety of ways to send an override signal to reduce the speed of the pump once gas lock conditions are reached. As shown in Figure 2, an override signal which is normally at maximum frequency may be reduced to a pre-defined minimum frequency value. The override, when activated, may immediately reduce to a value that corresponds to the current frequency of the electric submersible pump. After that, the override may be ramped down gradually to its minimum value, to prevent a too-rapid change in load on the pump. The use of a low signal selector would provide for the reduction in the signal to the pump motor to reduce the flow through the pump. As described above, damage can occur to the pump if action is not taken as soon as possible, so this step of sending the override signal should occur quickly after the gas lock conditions are encountered.

Once the flow is reduced, a timer is set to maintain the low flow for a set period of time. This amount of time is configurable and can be set based on the design of the system and the characteristics of the well. In addition, a manual override of this timer may be present so that an operator can decide to bypass the set time period and attempt to increase the flow rate before the end of the time period.

At the end of the time period or upon manual intervention by an operator, the appropriate variables are again measured and compared to the alarm limit to ensure that the gas lock conditions are no longer present in the system. If the pump is still gas locked then the pump is maintained at low flow for another set period of time. If the pump is not gas locked then the pump flow rate will be increased. This may be accomplished by increasing the frequency of the signal from the pressure controller to the pump and resetting the override signal to a maximum frequency signal.

The flow rate may be increased at a set ramping up rate to prevent the system from ramping up too quickly and going back into gas lock conditions. The ramping up rate is a predetermined rate, but it may be configurable or able to be set by the operator.

This invention will be further described with respect to Figures 1 and 2. Figure 1 shows a standard pressure controller 10 that is used to control the pump operations during normal conditions. The override signal 50 is maintained at an operating frequency during normal operation, but reduced to a minimum frequency when gas lock conditions are encountered. The signals from the pressure controller and the override signal are passed through a low signal selector 20 before being passed to a frequency ramp limiting controller 30. The signal from 30 is passed to the motor controller 40 which controls the motor driving the electrical submersible pump. In certain embodiments, an anti-reset windup signal 60 is included in the system to prevent a bump when the control system switches from the override signal to pressure control.

Figure 2 shows the operation of the method of this invention. During normal operation, the override signal is maintained at an operated frequency. When gas lock conditions are detected, this signal is reduced to a minimum frequency signal to provide minimum flow through the pump. After the timer duration (or earlier if manual intervention is taken) the override is reset and is slowly increased to the maximum frequency. At this point the pressure controller regains control of the pump since both signals pass through the low signal selector and normal, stable operations are continued unless gas lock conditions are encountered again.

This invention results in a controller that is able to deal with gas lock conditions and break that gas lock without repeated shutdown of the electrical submersible pump that can result in decreased pump longevity caused by an increased number of pump starts.