Patent Publication Number: US-10774627-B1

Title: Adjusting speed during beam pump cycle using variable speed drive

Description:
FIELD OF THE INVENTION 
     The invention relates to a method for smoothing rod loading in a beam pump cycle. More particularly, the invention relates to a method for adjusting the speed of a prime mover during a complete pump cycle of a beam pump using a variable speed drive. 
     BACKGROUND OF THE INVENTION 
     Referring to  FIG. 1 , a beam pump or pumpjack, designated generally as  10 , is the mechanical drive that converts rotary motion of a motor or prime mover  12  to reciprocating motion for reciprocating a downhole piston pump  14  in an oil well. Sucker rods  16  join the surface components of beam pump  10  and the downhole piston pump  14 . Downhole pump  14  typically includes a pump barrel  17 , which contains a plunger  21  carrying a travelling valve  18 , and a standing valve  20 . Beam pump  10  is used to mechanically lift liquid out of a well if there is insufficient bottom hole pressure for liquid to flow all the way to the surface. Beam pump  10  may also be used to increase the current production from a low producing well. Beam pumps, such as beam pump  10  of  FIG. 1 , are commonly used for low producing onshore wells and are common in oil-rich areas. About 60% of all artificially lifted wells in North America are beam or sucker rod pump systems and the figure is closer to 70% worldwide. 
     Depending on the size of beam pump  10 , production may be from 15 to 30 liters of liquid per stroke, depending on design parameters. Often, the produced fluid is a mixture of crude oil and water with the possibility of some gas. Pump unit size, the size of sucker rods  16 , and the horsepower capability of prime mover  12  are selected to accommodate the depth and weight of the oil to be removed. Deeper extraction requires greater power to move the increased weight of the discharge column. The diameter of downhole pump  14  and stroke length of the surface unit of beam pump  10 , along with the pumping speed, i.e., strokes per minute (SPM), determine the producing rate of liquids. Liquids are routed up tubing string  24 . 
     Beam pump  10  converts the rotary motion of a pump motor  12  to a vertical reciprocating motion to drive sucker rods  16 , which are connected to the piston pump or downhole pump  14 . The vertical reciprocating motion of beam pump  10  produces the characteristic nodding motion of the pump, which may be referred to as a walking beam. 
     A surface dynamometer card is the plot of measured or predicted surface loads on rods  16  of the pump shaft, i.e., sucker rods, at various positions throughout a complete stroke of beam pump  10 . Surface loads may be measured via a load cell, e.g., located under a rod clamp resting on a carrier bar. Alternatively, a predicted surface load may be obtained from a predictive wave equation computer program, as is known in the art. For purposes of this application, a load cell, or other load measuring device, as well as computer or software that calculates surface load, shall be referred to as a load calculator. A surface dynamometer card reflects forces at the surface but can also be used to calculate and to plot forces in rods  16  above downhole pump  14  or anywhere in the string of sucker rods  16  as a function of position at the bottom of rod string  16  or anywhere in rod string  16 . The loads on the surface card or loads in the rods  16  at the surface are a result of the fluid load and also are a result of the weight of rods  16  in fluid and dynamic forces. The load is typically displayed in pounds of force (Y scale) and the position (X scale) of a rod is typically displayed in inches. Dynamometer cards are displayed by predictive and diagnostic software for the purposes of design and diagnosis of sucker rod pumping systems to show stroke length, maximum/minimum loads for a cycle and other parameters. 
     Some diagnostics may be conducted by an analysis of surface dynamometer card shapes, since certain downhole problems are typically associated with particular surface dynamometer card shapes. In shallow to medium depth wells, such interpretation of the surface dynamometer card may be reasonably effective in diagnosing pump performance. In deeper wells, however, the complex nature of the lift system means that diagnosing pump performance from surface dynamometer cards can be more problematic due to the dynamics of the long string of sucker rods. 
     A downhole dynamometer card, designated generally  30  ( FIG. 2 ), is a plot of calculated loads at various positions of pump stroke and represents the fluid load that pump  14  applies to the bottom of the rod string  16 . Downhole dynamometer card  30  has four indices, i.e., A, B, C, and D, representing opening and closing events of standing valve  20 , i.e., indices B and C, and opening and closing events of travelling valve  18 , i.e., indices A and D. A schematic of pump  12  is shown adjacent to each labeled corners A-D of card  10  wherein the status of pump  14  at each of points A-D is shown. The maximum plunger travel (MPT) is the maximum length of the movement of plunger  21  with respect to barrel  17  of pump  14  during one complete stroke. Most of the load, presented on the Y-axis of downhole dynamometer card  30 , is a force caused by differential pressure acting on plunger  21  of pump  14  or the fluid load at pump  14 . The differential pressure acts across traveling valve  18  on the upstroke and is transferred to standing valve  20  on the down stroke. The differential pressure is the difference between the pressure due to fluids within tubing  24  and the pressure in the wellbore. The magnitude of the fluid load is equal to the pump discharge pressure minus the pump intake pressure multiplied by the plunger area. Loads are shown on a downhole dynamometer card  30  on the Y scale, i.e., load in rod  16  above pump  14 , and position of rods  16  above the pump  14  (X scale) will be transferred to a surface dynamometer card along with the weight of rods  16  in fluid and dynamic loads. A typical surface dynamometer card  40  is shown in  FIG. 3 . 
     Still referring to  FIG. 2 , the successive steps in the downhole pump operation include the following: At the start of the upstroke (point A), traveling valve  18  and standing valve  20  are both closed. 
     Still referring to  FIG. 2 , from points B to C, rods  16  carry the fluid load when traveling valve  18  is closed. From points D to A, tubing  24  carries the fluid load, when standing valve  20  is closed. The effective plunger travel (EPT) is the length of travel of plunger  21  when the full fluid load is acting on standing valve  20 . In  FIG. 2 , the effective travel of plunger  21  is from B to C and is usually a smaller length than the surface stroke length due to stretch of rods  16 . 
     Referring now to  FIG. 3 , a typical surface dynamometer card is shown. A surface dynamometer card is a plot of measured loads on rods  16  at various positions throughout a complete stroke. The load may be displayed in pounds of force and the position may be displayed in inches. With reference to surface dynamometer card  40 , from point A to point B, the fluid load is fully carried by tubing  24  prior to point A and is gradually transferred rods  16  at point B. The load transfers as rods  16  are loaded and exhibit stretch to pick up the fluid load. If tubing  24  is anchored, plunger  21  and travelling valve  18  do not move relative to tubing  24 . Pressure in pump  14  decreases and any free gas in the clearance space between valves  18  and  20  expands from the static tubing pressure (P t ) to the pump intake pressure (P int ). 
     Standing valve  20  begins to open at A, allowing fluid to enter pump  14  when the pressure in pump  14  drops below the intake pressure (P int ). 
     Still referring to  FIG. 3 , with reference to surface dynamometer card  40 , from point B to C, the fluid load is carried by rods  16  as well fluids are drawn into pump  14 . At C, standing valve  20  closes as plunger  21  starts down, and traveling valve  18  remains closed until the pressure inside pump  14  is slightly greater than the pump discharge pressure (P d ). From C to D, gas in pump  14  (if present) is compressed as plunger  21  moves down to increase pressure on the fluid from the intake pressure (P int ) to the static pressure in tubing  24 . However, plunger  21  does not move if pump barrel  17  is full of an incompressible fluid. As fluid in pump barrel  17  is compressed, then the fluid load is gradually transferred from rods  16  to the tubing  24 . 
     At D, the pump discharge pressure (P d ) equals the static tubing pressure (P t ), and traveling valve  18  opens. From D to A, fluid in pump  14  is displaced through traveling valve  18  into tubing  24  and the fluid load is held by tubing  24 . 
     SUMMARY OF THE INVENTION 
     In one embodiment, the method of the invention relates to a method and apparatus for adjusting inner cycle speed control of a pump motor to smooth rod loading and to possibly reduce rod loading, to reduce energy consumption and reduce gearbox loading. 
     The method for smoothing rod load in a beam pump cycle includes the steps of monitoring a surface linear speed of rods and controlling the rod speed by adjusting the motor speed to slow when peaks in rod loading are present and to accelerate when valleys in rod loading are present. In one embodiment, an average loading of the rods in the upstroke portion of the pump cycle is determined. In another embodiment, the step of varying the surface linear speed is used to smooth loading of said rods in a down stroke portion of the pump cycle. 
     In one embodiment, the motor is used to vary the linear speed of the rods such that the linear speed of the rods is adjusted by a percentage amount that is inversely proportional to percentage variations in load of said rods about the selected average of loading. The step of varying the linear speed of the rods is preferably accomplished by an instantaneous speed variation of a driver responding to instructions programmed into a variable speed controller. 
     In one embodiment, the step of varying the surface linear speed inversely proportional to rod loading changes in an upstroke and/or in a down stroke are made proportionally larger or smaller related to the changes in said rod loading during a pump cycle to obtain best results in smoothing a dynamometer card. 
     Variations in rod loading may be used to implement instantaneous variations in said surface linear speed of the rods, wherein the surface linear speed of the rods are controlled by a variable speed drive and wherein the variations are controlled within a cycle rather than effecting an overall speed change for an entire cycle. 
     Rod loads may be averaged across an upstroke portion of a dynamometer card or over a portion of the dynamometer card where loading of the rods varies and is close to peak loading and the surface linear speed is controlled with a variable speed drive, e.g., by a variable speed controller in operative communication with the prime mover, to control minimum loads, i.e., if minimum loads spike downward, then at this location a reduction in downstroke linear speed could be implemented using the method of the invention. 
     The minimum load on the rods during the down stroke may be represented by “MPRL”. The peak load on the rods during upstroke may be represented by PPRL. If MPRL/PPRL is greater than a desired value, e.g., is greater than 0.2 or 20%, or another selected threshold value, a design or application can be expected to have an increased run life. In particular, the run life of the sucker rods can be expected to increase. The method of the invention may be used to ensure that the MPRL/PPRL ratio is approaching desired values. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic of a beam pump deployed in a well; 
         FIG. 2  shows a typical downhole dynamometer card and associated events of a pump cycle for a downhole pump that is completely fluid filled; 
         FIG. 3  shows a typical surface dynamometer card from a beam pump installation showing loads on a fluid plunger of a beam pump at positions of a beam pump stroke; 
         FIG. 4  shows a percentage change of loads deviating from a selected average for the portion of the stroke between two points selected on the top of the card corresponding to a portion of the upstroke; 
         FIG. 5  shows a suggested change in motor speed corresponding to the portion of the upstroke selected in  FIG. 4 ; 
         FIG. 6  shows an alternate suggested change in motor speed corresponding to the portion of the upstroke selected; 
         FIG. 7  shows a second alternate suggested change in motor speed corresponding to the portion of the upstroke selected. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The method of the invention includes the steps of developing a fluid load target in the cycle of a beam pump operation. The pump motor speed is varied to smooth erratic loads, thereby possibly reducing rod loading, gearbox loading, and energy consumption. The peaks in the surface card are considered to be from dynamic forces and it is thought that slower vertical speed in the areas of initial peak loading should reduce dynamics in the rod string and reduce the peaks in the loading. 
     Referring now to  FIG. 3 , a typical surface dynamometer card  40  is shown. An ideal surface card would show a parallelogram. However, dynamic forces in the long string of rods  16  create loads shown at the top and bottom of card  40  that result in the peaks and valleys that can be seen on the top of card  40  (the upstroke portion designated by segment B-C) and on the bottom of the card (the downstroke portion of the pump cycle designated by segment D-A). 
     To smooth loading of pump  14 , a rough average of the surface up/down loads across the top of the card, i.e., across segment B-C, is determined, e.g., an approximately 10,000 pound load in  FIG. 3 . In one embodiment, the load average is estimated visually by a user. The load average could additionally be established via a computer analysis or by other methods. The selection of 10,000 pounds in the example of  FIG. 3  is achieved by drawing a horizontal line across the top of the card. However, loads may be also determined by selecting an inclined line with load variations above and below the line as opposed to the horizontal line that is more suited for the example of  FIG. 3 . 
     Next, a percentage of change of the loads from the selected average is calculated. As shown in  FIG. 4 , the percentage load change about an average selected load may be plotted. 
     One object of the invention is to slow the speed of prime mover  12  when load peaks occur and to increase the speed of prime mover  12  during period of low load, i.e., when there are load valleys. The speed of prime mover  12  may be varied by percentages in an inverse relationship to the percent load changes, which results in making suggested speed changes of the same percentage as the load variations but of an opposite value to the load changes.  FIG. 5  shows a plot of suggested speed variations of prime mover  12 . 
     A greater change in speed of prime mover  12  than the 1:1 percentage variation discussed above may be required to smooth the top of the dyno card  40 , i.e., to smooth segment B-C of card  40 . Therefore, motor speed variance may be multiplied by a factor, e.g., by  2 . A plotted example result is shown in  FIG. 6 . Motor speed variance may be multiplied by another selected factor as well, e.g., 2.5, 3, or another factor. 
     A lower percent change in speed may also be desired. Therefore, the 1:1 percentage speed variation discussed above may be multiplied by a factor of less than one, e.g., by 0.5, as shown in  FIG. 7 , or by another factor such as 0.25, 0.33 or another factor. 
     The same technique can be used to determine a target for the varying loads and speeds across the bottom, or the down stroke portion, i.e. segment D-A, of dynamometer card  40 . 
     Note that the above description is of a surface card, which could be more accurately developed by measurements of load and position at the surface from a “predictive” card at the surface. However, if a surface measured card is available, the surface measured card can be input into a “diagnostic” program and a dynamometer card can be calculated down rod string  16  to obtain a card for the loads/positions in rods  16  just above pump  14 . As this is being done, intermediate dynamometer cards in rod string  16  may be calculated by the “diagnostic” card. Typically, intermediate dynamometer cards are not displayed but easily can be. Therefore, since the intermediate dynamometer cards are available, then the techniques described above for guiding a user with regard to changes in the speed of the unit could be applied to intermediate cards and not just to the surface card. For example, the techniques could be applied to determine speed control for a card calculated by a diagnostic program, e.g., at a location half way down the rod string. Therefore, the technique is not limited to being applied to only the surface card. In an alternate embodiment, intermediate cards can be extracted from a “predictive” design program and, as such, the same techniques could be applied to an intermediate card obtained from a “predictive” program. 
     By using the method of the invention, a user is provided with a target that may be implemented in a program for directing a variable speed drive, e.g., via a variable speed controller  42  in operative communication with prime mover  12 , for inner cycle speed control. Alternatively, variable speed controller  42  may be integral with prime mover  12 . Variable speed controller  42  receives load information from load cell  44  ( FIG. 1 ), from computer  46  running a predictive wave equation computer program, or from other sources. The magnitude of the percent changes in speed can be adjusted for best smoothing of the surface dyno card and for possible reductions in rod loading, energy consumption, and gear box loading. 
     * * * * 
     Thus, the present invention is well adapted to carry out the objectives and attain the ends and advantages mentioned above as well as those inherent therein. While presently preferred embodiments have been described for purposes of this disclosure, numerous changes and modifications will be apparent to those of ordinary skill in the art. Such changes and modifications are encompassed within the spirit of this invention as defined by the claims.