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CROSS-REFERENCE TO RELATED APPLICATION 
     None 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to the control of oil and gas production wells. More particularly, it relates to control of movable elements in well production flow control devices. 
     2. Description of the Related Art 
     The control of oil and gas production wells constitutes an on-going concern of the petroleum industry due, in part, to the enormous monetary expense involved in addition to the risks associated with environmental and safety issues. Production well control has become particularly important and more complex in view of the industry wide recognition that wells having multiple branches (i.e., multilateral wells) will be increasingly important and commonplace. Such multilateral wells include discrete production zones which produce fluid in either common or discrete production tubing. In either case, there is a need for controlling zone production, isolating specific zones and otherwise monitoring each zone in a particular well. Flow control devices such as sliding sleeve valves, downhole safety valves, and downhole chokes are commonly used to control flow between the production tubing and the casing annulus. Such devices are used for zonal isolation, selective production, flow shut-off, commingling production, and transient testing. 
     It is desirable to operate the downhole flow control device with a variable flow control device. The variable control allows the valve to function in a choking mode which is desirable when attempting to commingle multiple producing zones that operate at different reservoir pressures. This choking prevents crossflow, via the wellbore, between downhole producing zones. 
     In the case of a hydraulically powered flow control device such as a sliding sleeve valve, the valve experiences several changes over time. For example, hydraulic fluid ages and exhibits reduced lubricity with exposure to high temperature. Scale and other deposits will occur in the interior of the valve. In addition, seals will degrade and wear with time. For a valve to act effectively as a choke, it needs a reasonably fine level of controllability. One difficulty in the accurate positioning of the moveable element in the flow control device is caused by fluid storage capacity of the hydraulic lines. Another difficulty arises from the fact that the pressure needed to initiate motion of the moveable element is different from the pressure needed to sustain motion, which is caused by the difference between static and dynamic friction coefficients, with the static coefficient being larger than the dynamic coefficient. When pressure is continuously applied through the hydraulic line, the elastic nature of the lines allows some expansion that, in effect, causes the line to act as a fluid accumulator. The longer the line the larger this effect. In operation, the combinations of these effects can cause substantial overshoot in the positioning of the moveable element. For example, if the hydraulic line pressure is raised to overcome the static friction, the sleeve starts to move. A known amount of fluid is commonly pumped into the system to move the element a known distance. However, because of the fluid storage effect of the hydraulic line and the lower force required to continue motion, the element continues to move past the desired position. This can result in undesirable flow restrictions. 
     The present invention overcomes the foregoing disadvantages of the prior art by providing a system and method for overcoming the static friction while substantially reducing the overshoot effect. Still other advantages over the prior art will be apparent to one skilled in the art. 
     SUMMARY OF THE INVENTION 
     In one aspect, the present invention provides a system for controlling a downhole flow control device that includes a flow control device at a downhole location in a well wherein the flow control device has a movable element for controlling a downhole formation flow. The movable element has a hydraulic seal associated therewith. The seal is constructed such that a maximum pressure of an applied pressure pulse is sufficient to overcome a static friction force associated with the seal, and wherein a minimum pressure of an applied pressure pulse is insufficient to overcome a dynamic friction force associated with the seal. 
     In another aspect, a method for controlling a flow control device includes transmitting a pressure pulse from a surface located hydraulic source to the flow control device at a downhole location. A characteristic of the pressure pulse is controlled to incrementally move a moveable element in the flow control device to a desired position. Exemplary controlled characteristic of the pressure pulse comprises pulse magnitude and pulse duration. 
     While the foregoing disclosure is directed to the preferred embodiments of the invention, various modifications will be apparent to those skilled in the art. It is intended that all variations within the scope of the appended claims be embraced disclosure. It will be apparent, however, to one skilled in the art that many modifications and changes to the embodiment set for the above are possible without departing from the scope and the spirit of the invention. It is intended that the following claims be interpreted to embrace all such modifications and changes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For detailed understanding of the present invention, reference should be made to the following detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, in which like elements have been given like numerals, wherein: 
         FIG. 1  is a schematic of a production well flow control system according to one embodiment of the present invention; 
         FIG. 2  is a graph showing continued motion of a moveable element in a flow control device due to the effects of static and dynamic friction; and, 
         FIG. 3  is a schematic of pulsed hydraulic pressure in relation to the pressure required to overcome static and dynamic friction and the related movement of a moveable element in a flow control device. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As is known, a given well may be divided into a plurality of separate zones which are required to isolate specific areas of a well for purposes including, but not limited to, producing selected fluids, preventing blowouts, and preventing water intake. 
     With reference to  FIG. 1 , well  1  includes two exemplary zones, namely zone A and zone B, where the zones are separated by an impermeable barrier. Each of zones A and B have been completed in a known manner.  FIG. 1  shows the completion of zone A using packers  15  and sliding sleeve valve  20  supported on tubing string  10  in wellbore  5 . The packers  15  seal off the annulus between the wellbore and a flow control device, such as sliding sleeve valve  20 , thereby constraining formation fluid to flow only through open sliding sleeve valve  20 . Alternatively, the flow control device may be any flow control device having at least one moveable element for controlling flow, including, but not limited to, a downhole choke and a downhole safety valve. As is known in the art, a common sliding sleeve valve employs an outer housing with slots, also called openings, and an inner spool with slots. The slots are alignable and misalignable with axial movement of the inner spool relative to the outer housing. Such devices are commercially available. Tubing string  10  is connected at the surface to wellhead  35 . 
     In one embodiment, sliding sleeve valve  20  is controlled from the surface by two hydraulic control lines, opening line  25  and closing line  30 , that operate a balanced, dual acting, hydraulic piston (not shown) in the sliding sleeve  20 . The hydraulic piston shifts a moveable element, such as inner spool  22 , also called a sleeve, to align or misalign flow slots, or openings, allowing formation fluid to flow through sliding sleeve valve  20 . Multiple configurations of the moveable element are known in the art, and are not discussed in detail herein. Such a device is commercially available as HCM Hydraulic Sliding Sleeve from Baker Oil Tools, Houston, Texas. In operation, line  25  is pressurized to open the sliding sleeve valve  20 , and line  30  is pressurized to close the sliding sleeve valve  20 . During a pressurization of either line  25  or  30  , the opposite line may be controllably vented by valve manifold  65  to the surface reservoir tank  45 . The line  25  and  30  are connected to pump  40  and the return reservoir  45  through valve manifold  65  which is controlled by processor  60 . The pump  40  takes hydraulic fluid from reservoir  45  and supplies it under pressure to line  41 . Pressure sensor  50  monitors the pressure in pump discharge line  41  and provides a signal to processor  60  related to the detected pressure. The cycle rate or speed of pump  40  is monitored by pump cycle sensor  55  which sends an electrical signal to processor  60  related to the number pump cycles. The signals from sensors  55  and  50  may be any suitable type of signal, including, but not limited to, optical, electrical, pneumatic, and acoustic. By its design, a positive displacement pump discharges a determinable fluid volume for each pump cycle. By determining the number of pump cycles, the volume of fluid pumped can be determined and tracked. Valve manifold  65  acts to direct the pump output flow to the appropriate hydraulic line  25  or  30  to move spool  22  in valve  20  in an opening or closing direction, respectively, as directed by processor  60 . Processor  60  contains suitable interface circuits and processors, acting under programmed instructions, to provide power to and receive output signals from pressure sensor  50  and pump cycle sensor  55 ; to interface with and to control the actuation of manifold  65  and the cycle rate of pump  40 ; and to analyze the signals from the pump cycle sensor  55  and the pressure sensor  50 ,  80 ,  81 , and to issue commands to the pump  40  and the manifold  65  to control the position of the spool  22  in the sliding sleeve valve  20  between an open position and a closed position. The processor provides additional functions as described below. 
     In operation, sliding sleeve valve  20  is commonly operated so that the valve openings are placed in a fully open or fully closed condition. As previously noted, however, it is desirable to be able to proportionally actuate such a device to provide intermediate flow conditions that can be used to choke the flow of the reservoir fluid. Ideally, the pump could be operated to supply a known volume of fluid which would move spool  22  a determinable distance. However, the effects of static and dynamic friction associated with movable elements in the flow control device, such as the spool  22 , when combined with the fluid storage capacity of hydraulic lines  25  and  30  can cause significant overshoot in positioning of spool  22 . These effects can be seen in  FIG. 2 , which shows the movement  103  of spool  22  as fluid is pumped to move spool  22 . Pump pressure builds up along curve  100 . In one embodiment, any pulsations caused by pump  40  are damped out by transmission through the supply line. Pressure is built up to pressure  101  to overcome the static friction of seals (not shown) in sliding sleeve valve  20 . In an ideal hydraulic system, once the spool  22  begins to move, the supply line pressure reduces to line  102  and additional fluid can be supplied at the lower pressure to move spool  22  to a desired position  108 . However, the entire hydraulic supply line  25 ,  30  is pressured to the higher pressure  101 , and expansion of supply line  25 ,  30  results in a significant volume of fluid at pressure  101 . Instead of the fluid pressure being at level  102 , it gradually is reduced along line  107 , forcing spool  22  to position  109 , and overshooting the desired position  108 . 
     To reduce the overshoot issue, see  FIG. 3 , the present invention in one embodiment provides pressure pulses  203  that move spool  22  in incremental steps to the desired position. By using pulses  203 , the effects of supply line expansion are significantly reduced. Each pulse  203  is generated such that pulse peak pressure  207  exceeds the pressure  201  needed to overcome the static friction force resisting motion of spool  22 , and the pulse minimum pressure  208  is less than the pressure  202  required to overcome the force required to overcome the dynamic friction force resisting motion. In one embodiment, pressure pulses  203  are superimposed on a base pressure  205 . The motion  206  of spool  22  is essentially a stair step motion to reach the desired position  210 . While the spool  22  has been discussed, it should be understood that the spool  22  in only one illustrative movable element. Other movable elements and their associated static and dynamic frictions can also be utilized in the above-described manner. 
     As shown in  FIG. 1 , in one embodiment, a pressure source  70 , which may be a hydraulic cylinder, is hydraulically coupled to line  49  via line  75 . Piston  71  is actuated by a hydraulic system  72  through line  73  that moves piston  71  in a predetermined manner to impress pulses  203  on line  41 . Such pulses are transmitted down supply lines  25 ,  30  and cause incremental motion of spool  22 . Hydraulic system  72  may be controlled by processor  60  to alter maximum and minimum pulse pressure and pulse width W, also called pulse duration, to provide additional control of the incremental motion of spool  22 . Alternatively, pump  40  may be a positive displacement pump having sufficient capabilities to generate pulses  203 . 
     In one embodiment, the effects of the compliant supply lines  25 ,  30  are accounted for by comparing signals form pressure sensor  50 , at the surface, to signals from pressure sensors  80  and  81 , located at the downhole location on supply lines  25  and  30 , respectively. Signals from sensors  80  and  81  are transmitted along signal lines (not shown) to processor  60 . The comparisons of such signals can be used to determine a transfer function F that relates the transmitted pressure pulse to the received pulse. Transfer function F may be programmed into processor  60  to control one or more characteristics of the generated pressure pulse, such as for example, pulse magnitude and pulse duration, such that the received pressure pulse is of a selected magnitude and duration to accurately position spool  22  at the desired position. As used herein, pulse magnitude is the difference between the maximum pulse pressure  207  and the minimum pulse pressure  208 . As used herein, pulse duration is the time in which the pressure pulse is able to actually move spool  22 . 
     In another embodiment, position sensor  83  is disposed in sliding sleeve valve  20  to determine the position of spool  22  within sliding sleeve valve  20 . Here, transfer function F′ may be determined by comparing the generated pulse to the actual motion of spool  22 . Position sensor  83  may be any suitable position sensing technique, such as, for example, the position sensing system described in U.S. patent application Ser. No. 10/289,714, filed on Nov. 7, 2002, and assigned to the assignee of the present application, and which is incorporated herein by reference for all purposes. 
     While the systems and methods are described above in reference to production wells, one skilled in the art will realize that the system and methods as described herein are equally applicable to the control of flow in injection wells. In addition, one skilled in the art will realize that the system and methods as described herein are equally applicable to land and seafloor wellhead locations. 
     The foregoing description is directed to particular embodiments of the present invention for the purpose of illustration and explanation. It will be apparent, however, to one skilled in the art that many modifications and changes to the embodiment set forth above are possible. It is intended that the following claims be interpreted to embrace all such modifications and changes.

Summary:
A system for controlling flow in a wellbore uses a downhole flow control device positioned at a downhole location in the wellbore. The flow control device has a movable element for controlling a downhole fluid flow. In response to an applied pressure pulse, the movable element moves in finite increments from one position to another. In one embodiment, a hydraulic source generates a transmitted pressure pulse to the flow control device wherein the maximum pressure of a received pressure pulse downhole is sufficient to overcome a static friction force associated with the movable element, and wherein a minimum pressure of the received pressure pulse downhole is insufficient to overcome a dynamic friction force associated with the movable element.