A gas-handling valve for use in the application where adjustable maximum critical flow rate is required. The valve is a venturi valve and has an annular flow path formed between a housing and a plug. The plug and housing are movable relative to each other to form at least two different throat dimensions in the valve. In one aspect, the throat diversion of the valve is self adjusting. In another aspect, the valve is usable in a gas lift or steam injection well.

BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to valves for use in oil and gas wells as well as other surface flow controls. More particularly, the invention relates to a critical flow valve that provides an annular flow path for gas or other compressible fluid medium and has adjustable throat area to increase or decrease the critical volume rate of gas flowing through the valve due to relative changes of the throat area. The throat area may either be adjustable automatically or manually depending on the purpose of utilization of the valve. 
     2. Description of the Related Art 
     The area of application of the present invention is not limited to the recovery of hydrocarbon fluids, but some typical embodiments are chosen in terms of oilfield application for example. 
     To obtain hydrocarbon fluids from an earth formation, a wellbore is drilled into the earth to intersect an area of interest within a formation. The wellbore may then be “completed” by inserting casing within the wellbore and setting the casing therein using cement. In the alternative, the wellbore may remain uncased (an “open hole” wellbore), or may be only partially cased. Regardless of the form of the wellbore, production tubing is typically run into the wellbore primarily to convey production fluid (e.g., hydrocarbon fluid, which may also include water and gas) from the area of interest within the wellbore to the surface of the wellbore. In an alternative, the annulus formed by tubing and casing may be used as the production stream. 
     Often, pressure within the wellbore is insufficient to cause the production fluid to naturally rise through the production string to the surface of the wellbore. Thus, to carry the production fluid from the area of interest within the wellbore to the surface of the wellbore, artificial lift means is sometimes necessary. Gas lift and steam injection are examples of artificial lift means for increasing production of oil and gas from a wellbore. 
     Gas lift systems are often the preferred artificial lifting systems because fewer moving parts exist during the operation of the gas lift systems than during the operation of other types of artificial lift systems, like sucker rod lift systems. Moreover, because no sucker rod is required to operate the gas lift system, gas lift systems are usable in offshore wells having subsurface safety valves that would interfere with a sucker rod. 
       FIG. 1  is illustrative of a gas lift operation producing production fluid through tubing while the injection gas flows through an annulus formed between tubing and casing. In other applications, the gas may be injected through tubing while the production stream flows through the annulus. Production fluid P flows from formation  55  into wellbore  30  through perforations  60  formed in casing  25 . From the wellbore, the production fluid P flows into the production tubing  20  and to a wellhead  35  for collection. When it is desired to lift the production fluid P with gas G, compressed gas G is introduced into the annulus  15 . Any of the gas lift valves  45  which are in the open position allow the gas G to flow into the production tubing  20  through an opening in the gas lift mandrel  40  to lift the production fluid P to the surface of the wellbore  30 . 
     Gas lift valves typically include a restriction that is intended to control the flow of gas entering the production string. Choked flow or “critical” flow relates to a condition in which the flow of gas through a nozzle reaches its maximum flow rate with the local flow velocity equivalent to the speed of sound (sonic velocity) at the throat. At that point and over some range of pressure differentials between the near upstream and downstream of the valve, the flow rate of gas will remain stable and unchanged in spite of variances in the relative pressure. For example, for the simple orifice nozzle valve, the flow rate will be stable and limited once an absolute pressure ratio is less than 0.528. In more practical terms, a volume of gas passing through a valve at critical flow will not be so affected by pressure variations between the upstream and downstream sides of the valve. The disadvantage of using a simple orifice nozzle valve is that the pressure ratio across the valve is well above the required critical pressure ratio, and therefore the critical flow condition is unlikely to occur for usual gas lift operation. 
     More recently, gas lift valves have included a venturi in place of a simple orifice. A typical venturi includes an inlet portion, a throat portion and a diffuser portion. With a venturi, losses of energy in injection gas flow are significantly smaller and a significant pressure recovery occurs along the diffuser of the venturi. As a consequence, the critical flow condition is easily achievable at the pressure ratio of 0.9 or lower keeping a constant flow rate through the valve for fluctuating pressure environment. 
     U.S. Pat. No. 6,568,473 teaches a venturi with an annular flow area formed between the interior of a valve housing and an exterior of a plug. The '473 patent is incorporated by reference in its entirety herein. Like a more typical venturi, the annular flow path includes an inlet, throat and diffuser portion. In the '473 patent, the plug portion is movable relative to the housing portion in order to close the valve in the event the flow of injection gas does not have enough pressure to overcome the combined force from valve spring and production fluid pressure. 
     While the '473 patent provides an effective venturi design, it falls short of solving some of the continuing problems associated with gas lift valves. One of the problems is “heading”. Heading is a periodic and unstable flow phenomenon in both the production stream and gas injection sides leading to a dramatic reduction of fluid production and excessive injection gas consumption. Heading occurs when a pressure differential between the injection gas and the production fluid changes due to a transient fluctuation in well conditions. This temporary change of production pressure may come about as a result of an increase in a production gas/oil ratio in the well. For example, as the percentage of gas entering the production string from the well increases, the hydrostatic pressure in the production stream decreases. This temporary pressure fluctuation can create an unstable heading phenomena. For most cases, the un-choked simple orifice nozzles respond in an adverse manner by injecting more gas. On the other hand, the choked venturi nozzles of fixed geometry may inject the unchanged gas volume so long as the pressure differential is within a choked flow regime. However, to most effectively operate a gas lift system, the volume of gas injected from the injection side should be decreased in an equal amount to compensate for the increment of production gas in the production string. However, with a venturi of fixed geometry, the only efficient way to decrease the volume of gas entering the (production string) from the injection side is to reduce the throat dimensions of the valve. In current designs, changing the throat dimensions would mean removing the valve from the well and replacing it. 
     The opposite situation can also occur wherein the flow of gas into the tubing from the well deceases. To most efficiently operate the well in this instance, the volume of gas injected should be increased. However, as with the opposite scenario, increasing gas flow across a critical flow valve is impractical without changing the valve for one with a larger throat dimension. 
     Like gas lift, steam injection methods are known to increase the natural flow of production from a wellbore. In a steam injection scenario, wellbore(s) are created adjacent to or near a producing wellbore and steam is injected into these wellbores and permitted to exit and flow into a surrounding formation to heat hydrocarbons and urge them towards a nearby, producing wellbore. Venturi valves are commonly used in injection wells to maximize and stabilize the amount of steam injected from a tubing string into a formation. However, rather than controlling the volume of gas flowing from the annulus to the tubing string, the venturi valves in injection wells are used to control flow from the tubing to the annulus. Methods and apparatus for operating injection wells are taught in U.S. Pat. No. 6,708,763 owned by the assignee of the present application and that patent is incorporated herein in its entirety. Steam injection wells using venturi valves have the same limitations as gas lift wells in that the venturi is sized to permit a given volume of steam at a critical flow rate and increasing or decreasing that volume is impractical without changing the entire valve. There are times when an operator would like to increase or decrease the volume of steam delivered from a wellbore to a formation depending upon changes in natural conditions. The present arrangements make that impractical without changing equipment in the wellbore. 
     A need exists therefore for a critical flow valve which provides a simple way to increase or decrease gas or steam volume through the valve while maintaining a critical flow rate. A further need exists for a critical flow valve which is adjustable in order to adjust the size of the throat area of the valve. A further need exists for a venturi valve which includes an annular flow area defined by portions of plug and housing which are movable relative to each other between at least two positions, either position providing a different throat geometry. 
     SUMMARY OF THE INVENTION 
     The present invention generally relates to a gas-handling valve for use in the application where adjustable maximum critical flow rate is required. The valve is a venturi valve and has an annular flow path formed between a housing and a plug. The plug and housing are movable relative to each other to form at least two different throat dimensions in the valve. In one aspect, the throat diversion of the valve is self adjusting. In another aspect, the valve is usable in a gas lift or steam injection well. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
         FIG. 1  is a section view of a gas injection wellbore. 
         FIGS. 2A-C  illustrate one embodiment of the invention, primarily for a gas lift application, wherein a housing portion is movable relative to a plug portion of a venturi valve. 
         FIGS. 3A-C  illustrate an alternative arrangement of the valve of  FIGS. 2A-C , wherein the plug portion of the valve is movable relative to a housing portion. 
         FIGS. 4A-C  illustrate another embodiment of the invention, primarily for injection wells, wherein a housing portion is movable relative to a plug portion of a venturi valve. 
         FIGS. 5A-C  illustrate an alternative arrangement of the valve of  FIGS. 4A-C , wherein the plug portion of the valve is movable relative to a housing portion. 
         FIG. 6  is a section view of a venturi valve of the present invention installed in a gas lift mandrel. 
         FIGS. 7A-C  are graphs plotting the change of critical flow rate for varying throat areas in gas lift valves. 
         FIGS. 8A-C  are section views of a venturi valve of the present invention adjustable by manual setting of knob. 
         FIG. 9  is a graph plotting the change of critical flow rate for selection of a fixed throat area in the manual change of prescribed setting. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The area of application of the present invention is not limited to the recovery of hydrocarbon fluids, but some typical embodiments are chosen in terms of oilfield application, especially for gas lift and steam injection, for illustrative purposes.  FIGS. 2A-C  illustrate one embodiment of the invention primarily for a gas lift application wherein a housing portion is movable relative to a plug portion of a venturi valve. The valve  100  provides a venturi having a housing  105  with an inner, tapered surface  110  formed in an inner bore. Within the housing is a plug portion  115  and an annular flow path  120  is formed between the plug  115  and housing  105 . The annular path forms an inlet  122 , a throat  123  and a diffuser portion  124 . In the embodiment of  FIGS. 2A-C  the inside surface of the housing tapers  110  from an initially small inner diameter to a relatively larger inner diameter. In the Figures, a side of the valve exposed to the gas injection is the left side and the production side is the right side. Injected gas therefore moves through the valve from left to right. As is evident from a comparison of  FIGS. 2A  through C, the housing  105  is spring mounted relative to the plug  115  with a spring  125  separating the housing  105  from a base portion  106  of the housing. While a spring is used in the preferred embodiment of  FIGS. 2A-C , the spring is only one type of biasing device usable to change the position of one component relative to another. A bellows device, for example, could perform the same function.  FIG. 2A  illustrates the valve in a “neutral” position. The valve is intended to remain in the neutral position under ordinary operating circumstances. In a gas lift valve, ordinary circumstances would mean a somewhat higher pressure on the gas injection (left) side of the valve as injected gas from the surface of the well is forced through the valve and into the production side. Therefore, in the neutral position, the spring  125  and fluid dynamic forces are in balance at a particular design condition. 
       FIG. 2B  shows an automatic valve operation that is intended to compensate for a decrease in pressure on the production side (the right side) of the valve  100 . The decrease typically comes about due to an increase in gas produced by the well. For example, if the gas component of an oil/gas mixture increases, the pressure in the production side will be reduced due to the smaller percentage of fluid adding to a hydrostatic head in the production string. In this instance, the valve of the invention is designed to reconfigure itself to reduce the volume of gas injected at a critical flow rate into the well. The valve  100 , with its spring mounted housing  105  is designed at a particular pressure differential to reconfigure itself to the position seen in  FIG. 2B . The result is a valve with a smaller throat area  123 . The smaller throat permits the valve to pass a reduced volume of gas, a decrease that would typically not be possible with a fixed throat while maintaining a critical flow across the valve. 
     The embodiment shown in  FIG. 2C  is primarily designed for a reverse situation from the one discussed with respect to  FIG. 2B . The embodiment of  FIG. 2C  is for a situation where the production pressure has increased, indicating a decrease in gas production from the well. To address this situation the valve reconfigures itself to permit an increased volume of injection gas to enter the production string. In  FIG. 2C , the housing of the valve has moved to extend the spring and in doing so has increased the throat  123  of the valve. The movement of the housing against the spring is brought about by a pressure differential across the valve, the pressure being increased on the production side of the valve due to the increased liquid portion creating an increased hydrostatic head. 
     In a preferred embodiment shown in the Figures, the inner surface of the housing is tapered in a linear fashion. However, the surface could be non-linear or even undulating to provide a particular throat dimension as the housing moves relative to the plug. Also, while the plug is circumferential and the flow path symmetrical in the embodiments shown, the flow path could be a variety of non-circumferential shapes depending upon the shape of the plug and housing. In one possible design, for example, a “slot-shaped” annular path is formed due to an elongation of the plug in one dimension with a corresponding and matching shape of the housing. 
       FIGS. 3A-C  illustrate a different embodiment of the invention, but one that is intended to provide the same result as the configurations of  FIGS. 2A-C . In  FIGS. 3A-C , the plug portion  105  of the valve is mounted on a spring  126  and is movable relative to the housing  105 . The inner wall of the housing is tapered in an opposing manner to the inner wall of the valve of  FIGS. 2A-C  in order to accomplish the same result using a spring loaded plug.  FIG. 3A  shows the valve in its neutral position where it will remain as injected gas from the surface of the well is forced through the valve and into the production (right) side.  FIGS. 3B and 3C  show the valve in an arrangement with a smaller and larger throat portion  123  respectively, due to a shifting of the plug portion  115  relative to the housing  105 . For example, in  FIG. 3B  the plug portion  115  has moved inwards into the tapered housing  105  to reduce the effective throat  123  dimension and in  FIG. 3C  the plug has been urged outwards relative to the housing to increase the throat. 
       FIGS. 4A-5C  show an alternative embodiment of the invention designed primarily for use in steam injection wells. The design of the steam injection valve is in some respects reversed from the design used in connection with the embodiments of  FIGS. 2A-3C . For example, in  FIGS. 4A-5C , the injection side is the left side and the lower pressure wellbore side is the right side. Like the previous embodiments, the valve  100  shown in  FIG. 4A  includes a housing portion  105  mounted on a spring  125  and having a tapered inner surface  110  movable relative to a plug portion  115 . An annular flow path  122  is formed between the outer surface of the plug  115  and the inner surface  110  of the housing  105 . More specifically, a throat portion  123  of the valve  100  is formed between the plug  115  and an adjacent inner diameter of the housing  105 . In  FIG. 4A , the force of spring  125  is in balance with fluid dynamic forces and the valve is in a neutral position. As illustrated in  FIG. 4B , the housing  105  is movable to a second position when the spring  125  is overcome by increased pressure force. The second position provides a throat  123  dimension which is larger in area than the throat  123  of the valve in the first position of  FIG. 4A .  FIG. 4B  is a position assumed by the valve in the event the supply pressure of steam in the injection side is increased. If an operator chooses to increase the supply pressure of steam, the flow rate of steam through the valve and ultimately, into the area surrounding the wellbore will be increased.  FIG. 4C  shows a position taken by the valve in the event steam pressure from the injection side is decreased. The pressure differential across the valve will cause the spring loaded housing  105  to move towards the injection side and the throat  123  of the valve  100  will be decreased. 
     The embodiment of  FIGS. 5A-C  are designs intended to accomplish the same results as the embodiments of  FIGS. 4A-C . However, in  5 A-C the plug portion  115  of the valve is spring loaded and the plug portion moves relative to the tapered inner housing wall  110  to increase or increase the size of the throat  123  of the valve  100 . 
       FIG. 6  shows one embodiment of the invention installed in a gas lift well with production tubing  200  and casing  210 . In the example of  FIG. 6 , the valve  100  is installed in a side pocket mandrel  215  which extends into an annulus  220  formed between the outer surface of the tubing  200  and the inner surface of the casing  210 . As shown by the arrows  225 , pressurized gas enters a port  230  in the side pocket mandrel and flows through the valve  100 , exiting the valve and entering the tubing  200  to mix with production fluid and help raise it to the surface for collection. In  FIG. 6 , the valve is shown in its neutral position corresponding to  FIG. 2A . From this position, a throat  123  of the valve can either be increased or decreased depending upon the location of the spring loaded housing relative to the plug. 
       FIG. 7A  shows critical flow characteristics of the present venturi nozzle valve at a specified throat area. In the Figure, flow rate is shown on the Y axis and pressure differential is shown on the X axis. As illustrated, the critical flow condition can be achieved at 10% or higher of pressure differential (shown by line  300 ). 
       FIG. 7B  shows how the critical flow rate typically varies as the throat area changes with the relative position of the plug and housing. The horizontal axis shows movement of the housing relative to the plug with “0” being the neutral position described herein. The vertical axis shows the corresponding changes in flow rate. 
       FIG. 7C  is the combination of  FIG. 7A  and  FIG. 7B  representing the typical behavior of the gas lift valve shown in  FIG. 6 . As production gas/oil ratio increases from the design production gas/oil ratio and the hydrostatic pressure in production string decreases, the spring mechanism automatically compresses, moves the housing downwards in relation to the plug, thereby creating a smaller throat area.  FIG. 7C  shows an operating point moving from a neutral point A toward point B, thereby compensating for a pressure fluctuation by decreasing the injection gas rate. The inverse movement occurs toward point C when the production gas/oil ratio decreases to increase the gas injection rate. This self-adjusting gas injection rate of the present invention is not achievable from the earlier venturi having a fixed geometry. 
     The embodiments of the invention illustrated and discussed above teach a venturi valve with a self adjusting feature to change the volume of gas or steam passing through a valve at a critical flow rate. In some instances, the valve self adjusts due to natural conditions and in other instances an operation can cause the valve to shift effecting a change in pressure differential across the valve. 
     In other occasions, a manually adjustable embodiment may be achieved as illustrated in  FIGS. 8A-C . By adjusting the knob  400 , the relative position of plug  125  and housing  105  may be set, allowing the appropriate variation of throat area  123 . As a result, as demonstrated in  FIG. 9 , a single venturi valve may operate at different choked critical flow regimes depending on the prescribed throat setting as shown by the letters A, B, and C for settings represented by  FIGS. 8A-C , respectively. 
     Likewise, an adjustment can selectively be made from a remote location with the use of control devices like fluid or electrical power. For instance, a servo could be used to operate and shift the components or a hydraulic means could be incorporated to move one component relative to another. 
     While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention and utilization of invention in fields other than wellbores may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.