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This application claims the benefit of U.S. provisional application No. 60/081,351, filed Apr. 10, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to plunger lift technology and, more particular, is concerned with a casing differential pressure based control method for gas-producing wells. 
     2. Description of the Prior Art 
     In a typical plunger lift system, such as seen in FIG. 1, a gas-producing well W employs a freely movable plunger P disposed within a tubing T in the well that is capable of traveling vertically in the tubing T as the well W is cycled between shut-in and open conditions. The well W is shut-in for an interval during which the pressure of gas G gradually elevates within the well casing C. When the pressure of gas G reaches a desired level, a master gas flow control valve A, commonly referred to in the industry as the A valve, is opened causing the plunger P to be propelled by the accumulated gas pressure from a lower initial position, at a bottom bumper B, upward in the tubing T toward an upper terminal position adjacent to a plunger arrival sensor S at the wellhead. Fluid and gas above the plunger P in the tubing T discharges from the wellhead through a horizontal conduit H into a flow line L, called a gas sales line, leading to a separator (not shown). At the separator, gas and water separate from one another and are routed to separate storage vessels. The plunger P is held at the upper terminal position until the gas pressure diminishes to an extent permitting the plunger P to fall under gravity to its lower initial position. 
     Many plunger lift systems, in addition to the master flow control or A valve, will typically utilize a second flow control valve, commonly referred to in the industry as the B valve (FIG.  2 ), with an electronic controller E to control cycling of the well between shut-in and open times and thereby the production of gas from the well. As mentioned above, the A valve is interposed in the gas sales line L whereas the B valve is interposed in a vent line (not shown) that leads to a containment tank or pit or sometimes directly to atmosphere. The gas sales line L is under a higher pressure than the vent line. The shut-in and open times of the cycles providing optimum well production will vary from well to well due to the differing conditions of the wells. 
     The electronic controller E is programmed by an operator to set close, open, delay and shut-in times for the A and B valves so as to control the times of opening and closing of the A and B valves as well as other functions to provide for desired production and sales of gas from a given well. Also, the plunger lift system typically employs the arrival sensor S at the wellhead to sense the arrival of the plunger P at the upper terminal position. The arrival sensor S sends an electrical signal to the controller E in response to the arrival of the plunger P. 
     The employment of the B valve is necessary on many wells due to pressure fluctuations experienced in the high pressure gas sales line L of such wells which can impede efficient production of gas from the well W. There are various causes of pressure variation, the main ones being conditions created by mechanical equipment attached to the gas sales line L or the weather. When gas sales line pressure fluctuates enough that it becomes too great for the well casing pressure to exceed it and drive the plunger P to the upper terminal position of the wellhead, the plunger P may stall before reaching the surface or not arrive at the upper terminal position within the preset open time of the A valve. The controller E is programmed to then close the A valve and open the B valve to vent the well casing C to atmosphere or a low pressure tank or pit and thereby permit the plunger P to reach the upper terminal position and blow out the fluid that has accumulated above the plunger P. After the plunger P arrives and blows out the fluid, the controller E will shut the B valve and open the A valve and thus commence sale of gas from the well W through the A valve and the gas sales line L. 
     The key to efficient gas production is to prevent a head of fluid from building in the tubing T above the plunger P that will exceed the gas pressure in the casing C and prevent lifting the plunger P and fluid to the wellhead. To keep the well casing C and tubing T relatively free of fluid, the plunger P must be cycled at a rate generally matched to the rate that fluid comes into the well casing C from the production formation through perforations in the casing D so as to allow gas to come into the well casing C through the same perforations. The function times programmed in the electronic controller E by the operator are selected based on the particular condition of the well. As the well ages there is typically less gas pressure and more fluid flowing into the well casing C. An operator, therefore, needs to periodically monitor the operation of the well and change the programmed function times as the condition of the well changes. 
     Electronic controllers have been devised in the past to relieve an operator of this task by automatically counting the number and times of past plunger trip times, comparing them with target numbers and times and changing the programmed times using an algorithm stored in the memory of the electronic controller. While automatic controllers have accomplished this task in a generally satisfactory manner, still they are complicated and expensive and generally fail to optimize the A valve open time when gas produced by a well is being sold. Typically, these controllers will be programmed to close the A valve and terminate gas sales much earlier than needed resulting in a substantial reduction in the level of sales. 
     Consequently, a need exists for improvement of control of A valve open time to improve the cycling of a gas-producing well between shut-in and open times and thereby improve the efficiency of gas production and sales from the well. 
     SUMMARY OF THE INVENTION 
     The present invention provides a casing differential pressure based control method for gas-producing wells designed to satisfy the aforementioned need. The casing differential pressure based control method of the present invention involves monitoring the sales line pressure, casing pressure and tubing pressure and changing or switching between opening and closing the A and B valves based on these pressures relative to preset minimum differentials between selected pairs of these pressures. 
     As known heretofore, an Open Differential Pressure is used to determine when to open the A valve. The Open Differential Pressure is a preset minimum pressure difference by which the casing pressure needs to exceed the sales line pressure for opening of the A valve to occur. The major improvement fostered by the present invention, however, is a more precise way to determine when to close the A valve and terminate sales so that sales will be allowed to continue for as long as possible with each trip of the plunger. Basically, due to the present invention once sales are occurring they are allowed to continue as long as the current casing pressure is dropping and until the casing pressure reverses. When the current casing pressure reverses and rises by a preset minimum pressure, the A valve is then switched to close state. This preset minimum pressure used to determine when to close the A valve is termed the Close Pressure. 
     Thus, the approach of the present invention delays closing the A valve so that gas sales will continue as long as the current casing pressure has not decreased to the minimum pressure which is the level when the casing pressure reverses and starts to rise again. After the A valve is closed, the plunger is allowed to drop to the lower initial position. Thereafter, the controller monitors the various current pressures for determining when Open Differential Pressure is reached again to cause the A valve to switch to open state and initiate gas sales. 
     Accordingly, the present invention is directed to a casing differential pressure based control method used in conjunction with a gas-producing well, a casing within the well for receiving a flow of gas under pressure from a production formation, a tubing extending downward within the casing to a lower portion being in communication with the casing for receiving the flow of gas under pressure therefrom, a sales line located outside of the well and connected in flow communication with the tubing for routing the flow of gas under pressure away from the well, an A valve interposed in the sales line and being convertable between open and close states in which flow of gas is correspondingly allowed and blocked from the tubing to the sales line, and an electronic controller connected to the A valve for controlling the cycling of the A valve between open and close states and thereby the well between the open and shut-in conditions in which the gas under pressure flows correspondingly from the tubing and elevates in pressure in the casing. 
     The casing differential pressure based control method comprises the steps of: (a) sensing the current casing pressure, current tubing pressure and current sales line pressures; (b) switching the A valve to open state such that gas sales are initiated in response to sensing when the current casing pressure exceeds the current sales line pressure; and (c) switching the A valve to close state such that gas sales are terminated in response to sensing when the current casing pressure has decreased, reversed and then risen by a preset minimum pressure. More particularly, the control method also comprises presetting an Open Differential Pressure equal to a preset minimum pressure difference by which the casing pressure exceeds the sales line pressure, and calculating the difference between the current casing pressure and current sales line pressure. The switching of the A valve to open state is in response to sensing when the difference between the current casing pressure and current sales line pressure reaches the preset Open Differential Pressure. The control method further comprises presetting a Close Pressure equal to a preset minimum pressure of the casing, and sensing when the preset minimum pressure of the casing has been reached. The switching of the A valve to close state is in response to sensing when the current casing pressure has decreased, reversed and then risen by the preset Close Pressure. 
     These and other features and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described an illustrative embodiment of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the following detailed description, reference will be made to the attached drawings in which: 
     FIG. 1 is a diagrammatic view of a prior art plunger lift system which can employ the casing differential pressure based control method of the present invention. 
     FIG. 2 is a block diagram of an electronic controller programmed to operate in accordance with the method of the present invention. 
     FIG. 3 is a plan diagram of a keypad on the controller of FIG.  2 . 
     FIGS. 4A and 4B are plan diagrams of two different states of a display on the controller of FIG.  2 . 
     FIGS. 5 to  16  taken together are a flow diagram representing the steps of a software program run by the electronic controller of FIG. 2 which includes the steps performed in carrying out the method of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawings and particularly to FIG. 2, there is illustrated in block diagram form the components of an electronic controller  10  for practicing the casing differential pressure based control method of the present invention. The electronic controller  10  is connected to A and B valves  12 ,  14  and casing, sales line and tubing pressure-sensing transducers  24 ,  26 ,  28 . The electronic controller  10  is programmed to operate in accordance with the casing differential pressure based control method of the present invention to control open, or flow, close and shut-in times of the well W so as to maximumize the efficiency of gas production from the well. FIG. 3 depicts a keypad  16  on the controller  10  of FIG. 2 having sixteen keyswitches  18  that are assigned numbers  0 ,  1 ,  2 ,  3 ,  4 ,  5 ,  6 ,  7 ,  8  and  9  and parameters ON, OFF, READ, SET, CE and B. FIGS. 4A and 4B depict two different states of a display  20  provided on the controller  10 . 
     The controller  10  includes a micro controller  22  interfaced with the A and B valves  12 ,  14 , the keypad  16 , the display  20  and the casing, sales line and tubing pressure-sensing transducers  24 ,  26 ,  28 . The micro controller  22  has an internal program memory for receiving and executing instructions and outputting commands and values, and an external user program memory  30 , such as a ROM or PROM, is interfaced with the micro controller  22 . A software program which functions in accordance with the present invention resides in the external user memory  30  that is executed by the micro controller  22  in accordance with instructions and values inputted or programmed into the internal program memory of the micro controller  22  by an operator using the keypad  16  for efficiently operating the well to achieve maximized gas production. 
     FIG. 4A shows the state of the display  20  during normal operation the controller  10  when all three pressure transducers  24 ,  26 ,  28  are active. “MODE” represents the current program mode of the controller, “00:00:00” in the time left in the current mode, “Ccccc” is a reading of casing pressure from the casing pressure transducer  24 , “Illl” is a reading of sales line pressure from the sales line transducer  26 , and “Ttttt” is a reading of tubing pressure from the tubing transducer  28 . FIG. 4B shows the state of the display  20  when the tubing transducer  28  has been deactivated. The different controller program MODEs are CLOSE, A OPEN, B OPEN, A DELAY, B DELAY, MANDATORY SHUT-IN, OPEN DELAY and CLOSE DELAY. 
     FIGS. 5 to  16  together depict a flow diagram representing the steps of the software program run by the electronic controller  10 . The program includes the steps performed during the various program modes set forth above. 
     FIG. 5 depicts the CLOSE mode of the controller program in which the Close Time programmed for the A valve is monitored and once the Close Time expires, that is, equals zero, the program goes to the A OPEN mode (FIG.  6 ). The A Delay timer also must have expired to ensure that the plunger will have time to fall to the lower starting position in the tubing before the A OPEN mode is initiated. The controller program will temporary switch to the OPEN DELAY mode of FIG. 11 before going to the A OPEN mode. 
     FIG. 6 depicts the A OPEN mode of the controller program in which the A valve is switched from close to open condition and the program loops and awaits the arrival of the plunger to the “up” or upper terminal position in the wellhead. If the plunger is sensed by the arrival sensor as being “up” before A Open Time expires or equals zero, then the program goes to the A DELAY mode of FIG.  9 . If the plunger is not sensed as being “up” when A Open Time expires or equals zero, then the program goes to the B OPEN mode of FIG.  7 . (The A and B Open Times are initially set at various values to accommodate different well conditions.) 
     FIG. 7 depicts the B OPEN mode wherein initially the A valve  12  is closed and the B valve  14  is opened. If the plunger is sensed as being “up” before the B Open Time expires or equals zero, then the program goes to the B DELAY mode of FIG.  8 . If the plunger is not sensed as being “up” when B Open Time expires or equals zero, then the program goes to the Mand SHUT-IN mode of FIG.  10 . 
     FIG. 8 depicts the B DELAY mode in which the B valve  14  is maintained open for the programmed B Delay Time. Before the B Delay time expires or equals zero, the program branches and loops through the CLOSE DELAY mode  1  of FIG. 12 before returning to the B DELAY mode of FIG.  8 . Once the B Delay Time expires or equals zero, the program goes to the A DELAY mode of FIG.  9 . 
     FIG. 9 depicts the A DELAY mode in which the B valve  14  is closed and the A valve  12  is maintained open and the plunger is maintained up for the programmed A Delay Time to prolong sale of gas. Before the A Delay time expires or equals zero, the program branches and loops through the CLOSE DELAY mode  2  of FIG. 13 before returning to the A DELAY mode of FIG.  9 . Once the A Delay Time expires or equals zero the program returns to the CLOSE mode of FIG.  5 . 
     FIG. 10 depicts the MANDATORY SHUT-IN (Mand SI) mode in which both A and B valves  12 ,  14  are closed for a programmed mandatory shut-in time in response to the plunger not arriving at the surface within both A and B Open Times. Once the mandatory shut-in time expires or equals zero the program returns to the A OPEN mode of FIG.  6 . 
     FIG. 14 depicts the OPEN SWITCH operation. The open switch input condition can become true, that is, the answer is “yes”, in either one of three different ways. First, an input is received from the aux switch-gauge that is connected to the G-Open input of the micro controller  22 . Second, when the differential pressure between the casing and sales line exceeds the programmed open pressure, the controller  10  treats it like an open switch input. Third, when the casing pressure has peaked and quits rising for a programmed amount of time, controller  10  treats it like an open switch input. 
     FIG. 15 depicts the CLOSE SWITCH operation. The close switch input condition can become true, that is, the answer is “yes”, in either one of three different ways. First, an input is received from the aux switch-gauge that is connected to the G-Open input of the micro controller  22 . Second, when the differential pressure between the casing and tubing falls below the programmed close pressure, the controller  10  treats it like a close switch input. Third, when the casing pressure has dipped to a minimum value and the pressure begins rising again, the controller  10  watches for the casing pressure to rise above the preset minimum pressure by the programmed value. The controller  10  treats it like a close switch input. 
     FIG. 16 depicts the ARRIVAL SWITCH operation. The arrival switch input condition can become true, that is, the answer is “yes”, in either one of two different ways. First, the plunger P can come up the tubing and trip the arrival sensor S on the lubricator at the wellhead. The arrival sensor S is connected to the sensor input of the micro controller  22 . Second, when the well W is first opened to flow, the controller  10  makes note of the casing pressure. If the casing pressure falls to a programmed value below this pressure during the open period, the controller  10  treats it like a sensor switch input. 
     Tables I and II list the various menu selections that can be made by the operator for keying instructions and values into and reading values from the controller  10 . Table I lists in the first column the menu selections for displaying the current settings correspondingly listed in the second column. 
     
       
         
               
               
               
             
           
               
                   
                 TABLE I 
               
               
                   
                   
               
             
             
               
                   
                 READ 00 
                 Display Battery Status 
               
               
                   
                 READ 01 
                 Display Current Status 
               
               
                   
                 READ 02 
                 Display A Delay Time 
               
               
                   
                 READ 03 
                 Display Mandatory Shut-In Time 
               
               
                   
                 READ 04 
                 Display A Valve &amp; Plunger Counts 
               
               
                   
                 READ 05 
                 Display Open Delay Time 
               
               
                   
                 READ 06 
                 Display History 
               
               
                   
                 READ 07 
                 Display A Valve Total Open Time and 
               
               
                   
                   
                 Total Close Time 
               
               
                   
                 READ 08 
                 Display Close Delay Time 
               
               
                   
                 READ 09 
                 Display Sensor Status 
               
               
                   
                 READ 10 
                 Display Accumulated Times and Counts 
               
               
                   
                 READ 11 
                 Display Controller Mode 
               
               
                   
                 READ 12 
                 Display Open Differential Pressure 
               
               
                   
                 READ 13 
                 Display Close Differential Pressure 
               
               
                   
                 READ 14 
                 Display Sales Line Low Limit Pressure 
               
               
                   
                 READ 15 
                 Display Sales Line High Limit Pressure 
               
               
                   
                 READ 19 
                 Display Last Open and Close times 
               
               
                   
                 READ 20 
                 Display Casing Drop for Delay Time 
               
               
                   
                 READ ON 
                 Display A Open Time 
               
               
                   
                 READ OFF 
                 Display Close Time 
               
               
                   
                 READ B0 
                 Display Valve Mode A/B Open/Close 
               
               
                   
                 READ B2 
                 Display B Delay Time 
               
               
                   
                 READ B4 
                 Display B Valve &amp; Plunger Counts 
               
               
                   
                 READ B7 
                 Display B Total Open Time 
               
               
                   
                 READ B ON 
                 Display B Open Time 
               
               
                   
                 READ B OFF 
                 Display Current State of Trip Count 
               
               
                   
                   
                 (only in Time Mode) 
               
               
                   
                   
               
             
          
         
       
     
     Table II lists in the first column the menu selections for modifying the current settings correspondingly listed in the second column. 
     
       
         
               
               
               
             
           
               
                   
                 TABLE II 
               
               
                   
                   
               
             
             
               
                   
                 SET 00 
                 Not Used 
               
               
                   
                 SET 01 
                 Not Used 
               
               
                   
                 SET 02 
                 Program A Delay Time 
               
               
                   
                 SET 03 
                 Program Mandatory Shut-In Time 
               
               
                   
                 SET 04 
                 Program (clear) A Valve &amp; Plunger Counts 
               
               
                   
                 SET 05 
                 Program Open Delay Time 
               
               
                   
                 SET 06 
                 Not Used 
               
               
                   
                 SET 07 
                 Clear A Total Open Time 
               
               
                   
                 SET 08 
                 Program Close Delay Time 
               
               
                   
                 SET 09 
                 Enable/Disable the arrival sensor 
               
               
                   
                 SET 10 
                 Clear Accumulated Times and Counts 
               
               
                   
                 SET 11 
                 Select Controller Operational Mode: 
               
               
                   
                   
                 (1) Timed 
               
               
                   
                   
                 (2) Differential Pressure 
               
               
                   
                   
                 (3) Absolute Pressure 
               
               
                   
                   
                 (4) Differential Pressure with Casing Minimum 
               
               
                   
                 SET 12 
                 Program Open Differential Pressure 
               
               
                   
                 SET 13 
                 Program Close Differential Pressure 
               
               
                   
                 SET 14 
                 Program Low Close Pressure for Sales Line 
               
               
                   
                 SET 15 
                 Program High Close Pressure for Sales Line 
               
               
                   
                 SET 16 
                 Set Transducer #1 Activation 
               
               
                   
                   
                 (Casing Transducer) 
               
               
                   
                 SET 17 
                 Set Transducer #2 Activation 
               
               
                   
                   
                 (Tubing Transducer) 
               
               
                   
                 SET 18 
                 Set Transducer #3 Activation 
               
               
                   
                   
                 (Line Transducer) 
               
               
                   
                 SET 20 
                 Set Casing Drop for Delay Time 
               
               
                   
                 SET B0 
                 Select A Valve Mode when B Valve is Open 
               
               
                   
                   
                 ON - A Valve Open when B Valve is Open 
               
               
                   
                   
                 OFF - A Valve Closed when B Valve is Open 
               
               
                   
                 SET B2 
                 Program B Delay Time 
               
               
                   
                 SET B4 
                 Program (clear) B Valve &amp; Plunger Counts 
               
               
                   
                 SET B7 
                 Clear B Total Open Time 
               
               
                   
                 SET B OFF 
                 Program Trip Counter 
               
               
                   
                   
               
             
          
         
       
     
     With respect to SET  11  involving selection of the controller operational modes, in the Timed operational mode the controller  10  uses only the programmed times and switch inputs. In the Differential Pressure operational mode, the controller  10  uses the programmed times, switch inputs, and both the programmed Close Differential Pressure value (which is the amount by which the casing pressure exceeds the tubing pressure) and the sales line pressure. In the Absolute Pressure operational mode, the controller  10  uses the programmed times, switch inputs, and the sales line pressure. In the Differential Pressure with Casing Minimum operational mode, the controller  10  uses the programmed times, switch inputs, and the programmed Open Differential Pressure value (which is the amount by which the casing pressure exceeds the sales line pressure) to open. In the DELAY program modes (FIGS.  8  and  9 ), the controller watches for a minimum casing pressure and goes to the CLOSE program mode (FIG. 5) when the Casing pressure has dipped and gone back up greater than the programmed Close Pressure value. 
     With respect to SET  12  involving programming the Open Differential Pressure value, when the sales line pressure plus the programmed Open Differential Pressure value is less than the casing pressure and the controller  10  is in the CLOSE program mode (FIG.  5 ), the controller  10  will check the OPEN DELAY program mode (FIG.  11 ). If the controller  10  has been closed longer than the programmed Open Delay Time, it will go to the A OPEN program mode (FIG.  6 ). If the controller  10  has been closed shorter than the programmed Open Delay Time, it will time out in the OPEN DELAY program mode (FIG. 11) before going to the A OPEN program mode (FIG.  6 ). The Open Delay Time should be set at some minimum that is long enough for the plunger to fall to the lower starting position in the tubing before the well opens up again. In other words, the Open Delay Time should not be set to a value less than the fall time of the plunger. 
     With respect to SET  13  involving programming the Close Differential Pressure value, when the casing pressure minus the tubing pressure is greater than the programmed Close Differential Pressure value and the controller  10  is in either the A DELAY or B DELAY program mode (FIGS.  8  and  9 ), the controller  10  will go to the CLOSE DELAY program mode (FIGS.  12  and  13 ). If zero time is programmed in the CLOSE DELAY program mode, the controller  10  goes to the CLOSE mode (FIG.  5 ). If there is a non-zero time in the CLOSE DELAY program mode, the controller  10  waits for it to time out before checking the pressure again. If the close condition is still valid, the controller  10  will go to the CLOSE program mode, otherwise it will return to either the A DELAY or B DELAY program mode and continue timing down from the point it was interrupted. The CLOSE DELAY program mode prevents spikes in the casing pressure from shutting in the well prematurely. 
     If the controller  10  is in the Differential Pressure with Casing Minimum operational mode and in the A DELAY or B DELAY program mode, the controller  10  will monitor the casing pressure for a programmed minimum value. When the controller  10  senses that the casing pressure has reached some selected minimum and started to rise above this minimum by the amount of the programmed Close Differential Pressure value, the controller  10  will either go to the CLOSE program mode or the CLOSE DELAY program mode as described above. The CLOSE DELAY program mode can be used to ensure the well is not shut in by pressure spikes. When the latter mode is used, the controller  10  does not need a tubing pressure transducer. 
     With respect to SET  14  involving programming the Low Close Pressure for Sales Line value, if the controller  10  is in any OPEN program mode and the sales line pressure falls below the programmed Low Close Pressure for Sales Line value, the controller  10  will go to the CLOSE program mode (FIG.  5 ). If the sales line pressure rises above the programmed Low Close Pressure for Sales Line value and is still below the programmed High Close Pressure for Sales Line value, the controller  10  will open and close as a function of the current program mode, the countdown clock, and the differential pressure. 
     With respect to SET  15  involving programming the High Close Pressure for Sales Line value, if the controller  10  is in any OPEN program mode and the sales line pressure rises above the programmed High Close Pressure for Sales Line value, the controller  10  will go to the CLOSE program mode (FIG.  5 ). If the sales line pressure falls below the programmed High Close Pressure for Sales Line value, the controller  10  will open and close as a function of the current mode, the countdown clock, and the differential pressure. 
     With respect to SET  16 , setting activation of the casing transducer  24  is accomplished by: (1) pressing ON or OFF to activate the casing transducer  24 ; and (2) with the casing transducer  24  open to atmospheric pressure, pressing OFF to calibrate the low end. The display  20  will prompt the user for zero pressure. To set the high end calibration, apply a known pressure near the high rating for the casing transducer  24  and repeat step (1) from above. Then press ON in step (2). The display  20  will prompt the user for the applied pressure. 
     To disable the high/low control of the sales line pressure, set the low pressure to 0 psi and the high line pressure to some arbitrarily high value. The sales line pressure transducer will be disabled for high/low operation, but will still function with the casing transducer to open the well. The Open and Close switch inputs can also be used with a Murphy gauge to monitor any other pressure. A close input from any source always takes precedence over any open inputs and close in the well. 
     With respect to SET  17 , setting activation of the tubing transducer  28  is accomplished by: (1) pressing ON or OFF to activate the tubing transducer  28 ; and (2) with the tubing transducer  28  open to atmospheric pressure, pressing OFF to calibrate the low end. The display  20  will prompt the user for zero pressure. To set the high end calibration, apply a known pressure near the high rating for the tubing transducer  28  and repeat step (1) from above. Then press ON in step (2). The display  20  will prompt the user for the applied pressure. 
     With respect to SET  18 , setting activation of the sales line transducer  26  is accomplished by: (1) pressing ON or OFF to activate the sales line transducer  26 ; and (2) with the sales line transducer  26  open to atmospheric pressure, pressing OFF to calibrate the low end. The display  20  will prompt the user for zero pressure. To set the high end calibration, apply a known pressure near the high rating for the sales line transducer  26  and repeat step (1) from above. Then press ON in step (2). The display  20  will prompt the user for the applied pressure. 
     With respect to the Open Differential Pressure the controller  10  always monitors the casing/tubing differential pressure. With respect to the Close Differential Pressure the controller  10  always monitors the casing/tubing differential pressure. The programmed times will continue to count down to zero and switch the controller  10  to the next appropriate mode if the differential pressures do not change enough to cause the mode switch. 
     The controller  10  only monitors for a casing pressure minimum for a casing-tubing pressure differential when the controller  10  is in the DELAY program mode. On wells running without a plunger, the previous solution to this limitation was to place a short across the sensor input so that the controller  10  went straight from the CLOSE program mode to the DELAY program mode. On flowing wells that need to be vented, this option will allow B valve operation when the casing pressure hasn&#39;t dropped by the programmed limit by the end of the A Open Time. 
     When the controller  10  begins the A OPEN program mode, the casing pressure is read. The controller  10  then monitors the casing pressure for a drop equal to or greater than the programmed minimum value. When this drop is observed in either the A OPEN OR B OPEN program mode, it is treated the same as a plunger arrival and sends the controller  10  to either the A DELAY or B DELAY program mode. This arrival will be listed in the travel time histories stored in the controller memory as a plunger arrival. If the programmed Casing Drop for Delay Time value is set to 0 psi, the controller  10  will not monitor the casing pressure for occurrence of the aforementioned drop. 
     To recapitulate, the major improvement fostered by the present invention is a more accurate way to determine when to close the A valve and terminate gas sales so that gas sales are allowed to continue for as long as possible with each trip of the plunger. Due to approach of the present invention once gas sales are occurring they will be allowed to continue as long as the casing pressure is dropping and until the casing pressure reverses and starts to rise. When the casing pressure rises by a chosen minimum pressure, such as one to two pounds, the A valve will be closed. This minimum pressure used to determine when to close the A valve is termed the Close Pressure. The casing pressure-sensing transducer connected to the controller senses when the current pressure of the casing reaches a minimum, reverses and starts to rise. The controller will close the A valve terminating gas sales when the rise in casing pressure exceeds the programmed Close Pressure. This approach of the present invention delays closing the A valve so that gas sales will continue as long as the casing pressure has not decreased to the minimum pressure which is the level when the casing pressure reverses and starts to rise again. 
     It is thought that the present invention and its advantages will be understood from the foregoing description and it will be apparent that various changes may be made thereto without departing from the spirit and scope of the invention or sacrificing all of its material advantages, the form hereinbefore described being merely preferred or exemplary embodiment thereof.

Summary:
A casing differential pressure based control method used in conjunction with a gas-producing well includes the steps of sensing current sales line pressure and well casing and tubing pressures and changing or switching an A valve between open and close states based on differences between selected pairs of these current pressures relative to preset minimum differentials between the same pairs of these pressures. Open Differential Pressure determines when to open the A valve and initiate gas sales. Open Differential Pressure is the preset minimum pressure difference by which the casing pressure exceeds the sales line pressure. Close Pressure determines when to close the A valve and terminate gas sales. Close Pressure is the preset minimum pressure of the casing. Once Open Differential Pressure has been reached and the A valve switched to the open state such that gas sales are occurring, the gas sales are allowed to continue as long as the current casing pressure is dropping and until the current casing pressure reverses. When the current casing pressure reverses and rises by the Close Pressure, the A valve is switched to the close state.