Abstract:
A system and method to identify a condition of cavitation or gas lock in a pump configured to convey a liquid to a surface from a subsurface environment via tubing are described. The system includes a tool to create a binary event based on the condition, the binary event representing a change in state of a parameter. The system also includes a sensor to detect the binary event based on the parameter, and a processor to process output from the sensor to identify the condition.

Description:
BACKGROUND 
       [0001]    In subsurface production efforts, a pump (e.g., Electric Submersible Pump or Progressive Cavity Pump) is generally used to bring a liquid (e.g., oil) to the surface. Specifically, a pump in a production well will pull the liquid (in some cases the pump carries mostly water, but the desired “product” can be minerals or gas, and can be produced with other means) into tubing that carries the liquid to the surface. The pump cannot discriminate between the liquid, and other material (e.g., sand, dirt, rocks) that may also be pulled into the tubing. When gas enters the tubing or when liquid level drops in the annulus from which it is being pumped, the lack of fluid in the tubing creates a cavity or void (e.g., cavitation or gas lock or vapor lock in the pump). This condition caused by gas or low fluid level can cause damage to the pump based on the frequency and duration of its occurrence. 
       SUMMARY 
       [0002]    According to one embodiment, a system to identify a condition of cavitation or gas lock in a pump configured to convey a liquid to a surface from a subsurface environment via tubing includes a tool configured to create a binary event based on the condition, the binary event representing a change in state of a parameter; a sensor configured to detect the binary event based on the parameter; and a processor configured to process output from the sensor to identify the condition. 
         [0003]    According to another embodiment, a method of identifying a condition of cavitation or gas lock in a pump configured to convey a liquid to a surface from a subsurface environment via tubing includes creating, using a tool in the subsurface environment, a binary event based on the condition, the binary event representing a change in state of a parameter; detecting, using a sensor, the binary event based on the parameter; and processing, using a processor, an output from the sensor to identify the condition. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]    Referring now to the drawings wherein like elements are numbered alike in the several Figures: 
           [0005]      FIG. 1  is a block diagram of a system to identify a cavitation condition according to embodiments of the invention; 
           [0006]      FIG. 2  is a cross sectional block diagram of a system to identify cavitation in tubing according to an exemplary embodiment; and 
           [0007]      FIG. 3  is a process flow of a method of identifying cavitation in tubing according to an exemplary embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0008]    As noted above, cavitation in the production tubing can result in damage to the pump. Thus, awareness of the condition can aid in extending the useful life of the pump. Currently, flow rate of liquid (e.g., oil) production at the surface is monitored. This monitoring allows an operator to identify when flow rate has dropped and further investigate whether the drop in flow rate is due to cavitation. However, a change in flow rate or a particular value of the flow rate is not dispositive, and the analysis and investigation needed to make a determination may require the pump to be shut off. Embodiments of the systems and methods described herein relate to a sensor identifying cavitation in the tubing based on a dispositive or binary event. 
         [0009]    As used in the present application, “binary event” refers to an event that indicates an objective and discernable switch or change in state of a parameter. The exemplary binary event detailed below is a change from positive to negative pressure (pressure to no pressure) for fluid flow of liquid being pumped to the surface. That is, the exemplary binary event is a switch in state of the exemplary parameter of pressure. The exemplary embodiment detailed herein relates to a diverter whose operation results in a switch in pressure (from positive to negative) when cavitation occurs in the tubing. This binary event or switch in pressure in the particular embodiment can be detected by a sensor. Alternate embodiments contemplate a different downhole tool than the diverter causing a different dispositive or binary event based on cavitation and a different sensor identifying cavitation based on that binary event. 
         [0010]      FIG. 1  is a block diagram of a system to identify a cavitation condition according to embodiments of the invention. Generally a tool  5  is disposed in a downhole environment  2 . The tool  5  creates a binary condition based on cavitation in tubing  20 . Although the tool  5  is shown in the tubing  20 , embodiments of the system may include the tool  5  being disposed on or outside the tubing  20 , as well. A sensor  6  identifies the binary event created by the tool  5 . A processing system  7  coupled to the sensor  6  processes the sensor  6  output to automatically take action or provide information to an operator. 
         [0011]      FIG. 2  is a cross sectional block diagram of a system to identify cavitation in tubing  20  according to an exemplary embodiment. The exemplary embodiment relates to a pressure switch sensor  110 , which is an embodiment of the sensor  6 , identifying cavitation based on a switch in pressure caused by a diverter  120 , which is an embodiment of the tool  5 , during a cavitation condition. A subsurface environment  2  including a borehole  10  is shown below the earth&#39;s surface  1 . The borehole  10  may be cased and has tubing  20  disposed therein that may be production tubing, for example. The tubing  20  is comprised of sections of tubes with interfaces  30  between them. In the embodiment of the cavitation identification system discussed with reference to  FIG. 1 , a diverter  120 , discussed further below, is disposed at an interface  30  of the tube sections, and sensor  110  is disposed in the flow of the tubing  20  at the surface  1 . The sensor  110  is coupled to a surface processing system  130 , which is an embodiment of the processing system  7 . The surface processing system  130  includes one or more processors  132  processing data based on instructions stored in one or more memory devices  134  and outputting the results through an output interface  136 . In addition to identifying cavitation based on data received from the sensor  110 , the surface processing system  130  may perform additional functions related to the production effort and may include additional components involved in that effort. 
         [0012]    According to the embodiment shown in  FIG. 1 , the diverter  120  is designed to divert debris such as rocks, sand, and dirt that are suspended in the fluid out of the (production) tubing  20  and into the annulus  15  between the (cased) borehole  10  and the tubing  20  when the pump  40  is turned off. However, when gas is in the tubing  20  or, for another reason, fluid levels drop in the tubing  20 , the diverter  120  according to one embodiment of the invention operates while the pump  40  is running. Under these conditions (pump  40  is on and diverter  120  is functional), any gas (and fluid) in the tubing  20  will be diverted out of the tubing  20 . When fluid levels are sufficiently low in the tubing  20  during this procedure, the diverter  120  operation causes pressure drop in the fluid flow and a vacuum is created at the diverter  120  causing fluid to flow in the opposite direction (drop toward the pump). At the pressure switch sensor  110 , this change in flow direction of the fluid is seen as a switch from pressure to no pressure (a binary event). As a result, the pressure switch sensor  110  need not be a sophisticated measurement device that measures flow or any particular parameter. The pressure switch sensor  110  may instead be a check valve that switches between on and off or a pressure valve that switches from positive to negative pressure to indicate that the cavitation condition has occurred in the tubing  20 . The surface processing system  130  coupled to the pressure switch sensor  110  may monitor a length of time that the condition lasts or a frequency of the condition over a period of time to take automatic action (e.g., shutoff of the pump  40 ). In alternate embodiments, the surface processing system  130  may provide the information indicated by the pressure switch sensor  110  to an operator through the output interface  136  so that the operator determines the action to take. According to the embodiment discussed with reference to  FIG. 1 , the diverter  120  includes features described in U.S. Pat. No. 6,289,990. In alternate embodiments, the diverter  120  is another diverter that produces the vacuum and subsequent change in fluid flow direction when it operates while the pump is on during a cavitation condition. 
         [0013]      FIG. 3  is a process flow of a method of identifying cavitation in tubing according to an exemplary embodiment of the invention. At block  310 , disposing a tool  5  along the tubing  20  includes disposing the diverter  120  at an interface  30  between tube sections, for example. The diverter  120 , according to the exemplary embodiment described above, diverts gas while the pump is on such that a vacuum is created. At block  320 , positioning a sensor  6  to sense a binary event created by the tool  5  includes positioning the pressure switch sensor  110  at the surface  1  in the flow of the tubing  20 . As noted above, the pressure switch sensor  110  according to the exemplary embodiment described above may be a check valve or pressure valve. Processing the sensor  5  output, at block  330 , includes processing system  7  (e.g., surface processing system  130 ) providing the indication of a cavitation condition to an operator. Alternatively, processing the sensor  5  (pressure switch sensor  110 ) output includes monitoring the frequency or duration or both of the cavitation condition to determine an action such as, for example, shutting down or slowing down the pump  40 . 
         [0014]    While one or more embodiments have been shown and described, modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustrations and not limitation.