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TECHNICAL FIELD OF THE INVENTION  
         [0001]    The present invention relates, in general, to enhancing recovery from a hydrocarbon formation and, in particular, to a self-regulatory lift fluid injection tool for controlling the flow rate of a lift fluid injected into the production tubing to enhance the recovery of formation fluids from a hydrocarbon producing well.  
         BACKGROUND OF THE INVENTION  
         [0002]    Efficiently producing hydrocarbon fluids from downhole formations is a challenging process involving a multitude of different types of equipment and techniques for recovering the fluids from the selected formation. Normally, when production from a hydrocarbon reservoir is commenced, the fluid pressure present in the formation is sufficient to force the liquids to the surface for recovery. After a period of time, however, the natural formation pressure may decline to a point where the pressure is not sufficient to lift the formation fluids to the surface at the desired rate of recovery. In these instances, alternative methods of enhancing the extraction of hydrocarbon fluids from the formation may be employed to augment recovery of formation fluids.  
           [0003]    One method of enhancing the recovery of hydrocarbons from a formation is to decrease the hydrostatic head of the column of fluid in the wellbore. Decreasing the hydrostatic head enhances recovery by reducing the amount of pressure required to lift the fluids to the surface. Decreasing the density of the column of fluid extending from the formation to the surface is a technique utilized to reduce the hydrostatic head of the fluid column. For example, mixing a lower density fluid with formation fluids reduces the overall density of the fluid column and consequently decreases the hydrostatic head.  
           [0004]    One way to achieve this is by forcing a lift fluid, typically a gas or hydraulic fluid having low density, down the annulus between the production tubing and the casing of the well. The low density fluid is then injected into the production tubing at one or more predetermined locations where it mixes with formation fluids, lowering the density of the fluid column above the formation. The injection of the low density fluid into the production tubing, however, must be carefully controlled to avoid equipment damage while simultaneously providing for optimal recovery. For example, excessive injection rates can result in pressure surges in the tubing and related equipment. Such pressure surges may produce large and destructive forces within the production equipment.  
           [0005]    Control of the injection rate is typically accomplished using a metering means such as an orifice, the size of which is typically determined using a trial and error procedure. Thus, the operator attempts to achieve optimum performance of the well by regulating the rate of injection of the lift fluids with various size orifice valves. In practice, the well operator will typically try several orifice settings, allowing the well to stabilize after each adjustment. Due to the distances, location of the valves and the fluid volumes involved, the operator may spend a significant amount of time in making the adjustments, stabilizing production after each adjustment and collecting comparative data from the different settings to establish performance trends.  
           [0006]    Therefore, a need has arisen for a lift fluid injection tool that controls the flow of a lift fluid into the production tubing based upon well parameters in an artificial lift well. A need has also arisen for such a tool that does not require the intervention of the well operator to optimize production from the formation. Additionally, a need has arisen for such a tool that periodically monitors and adjusts the injection rate of the lift fluid.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention disclosed herein provides a self-regulating lift fluid injection tool that controls the flow of a lift fluid into the production tubing based upon well parameters in an artificial lift well. The tool of the present invention does not require the intervention of the well operator to optimize production from the formation. The tool of the present invention monitors and adjusts the injection rate of the lift fluid in response to changes in well parameters, prompting by the operator or simply on a periodic basis.  
           [0008]    The tool of the present invention is adapted for placement generally concentrically within production tubing disposed within a well casing. The tool includes a control valve that controls the rate of injection of the lift fluid into the formation fluids being produced through the production tubing. A sensor monitors the flow rate of the formation fluids through the production tubing and providing a signal indicative thereof. An electronics package is communicably coupled to the sensor and generates a control signal in response to the signal received from the sensor. An actuator is communicably coupled to the electronics package and adjusts the position of the control valve to regulate the flow rate of the lift fluid therethrough in response to the control signal.  
           [0009]    The sensor may include an impeller that rotates in response to the flow of the formation fluids through the production tubing. The impeller may also be used to control the flow rate of the formation fluids.  
           [0010]    The tool includes a power source for providing electrical power. The power source may be a battery pack which may be charged using a downhole generator powered by the flow of the lift fluid through the tool or the flow of formation fluids around the tool.  
           [0011]    The tool may be linked to a remote location such as a surface facility using a transmitter either alone or in combination with a receiver, each of which are disposed within the tool.  
           [0012]    The electronics package of the tool includes a set of preprogrammed instructions for controlling the actuator. For example, the actuator may incrementally adjust the position of the control valve to increase the rate of injection of the lift fluid when the sensor indicates that the rate of recovery of the formation fluids increased in response to a prior incremental adjustment of the position of the control valve to increase the rate of injection of the lift fluid. Alternatively, the actuator may incrementally adjust the position of the control valve to decrease the rate of injection of the lift fluid when the sensor indicates that the rate of recovery of the formation fluids decreased in response to a prior incremental adjustment of the position of the control valve to increase the rate of injection of the lift fluid.  
           [0013]    The control valve may include an orifice plate having an orifice and a poppet that is operably connected to the actuator. The poppet may be advanced and retracted relative to the orifice to control the flow of the lift fluid therethrough.  
           [0014]    The self-regulating method for controlling the injection of a lift fluid into formation fluids of the present invention involves disposing a lift fluid injection tool having a control valve and a sensor within the production tubing, monitoring the flow rate of the formation fluids through the production tubing with the sensor and adjusting the position of the control valve in response to the flow rate of the formation fluids, thereby controlling the injection of a lift fluid into formation fluids. The step of monitoring the flow rate of the formation fluids through the production tubing may be accomplished by rotating an impeller in response to the flow of the formation fluids. In the method of the present invention, information may be communicated between the tool and a remote location using transmitter and a receiver disposed within the tool.  
           [0015]    The step of adjusting the position of the control valve in response to the flow rate of the formation fluids may involve incrementally adjusting the position of the control valve to increase the rate of injection of the lift fluid when the sensor indicates that the rate of recovery of the formation fluids increased in response to a prior incremental adjustment of the position of the control valve to increase the rate of injection of the lift fluid. Alternatively, the step of adjusting the position of the control valve in response to the flow rate of the formation fluids may involve incrementally adjusting the position of the control valve to decrease the rate of injection of the lift fluid when the sensor indicates that the rate of recovery of the formation fluids decreased in response to a prior incremental adjustment of the position of the control valve to increase the rate of injection of the lift fluid.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, wherein like elements are numbered alike and wherein:  
         [0017]    [0017]FIG. 1 is a schematic illustration of an offshore production platform operating a self-regulating lift fluid injection tool of the present invention;  
         [0018]    [0018]FIG. 2 is a cross-sectional view of a self-regulating lift fluid injection tool of the present invention;  
         [0019]    [0019]FIG. 3 is a cross-sectional view of a self-regulating lift fluid injection tool of the present invention;  
         [0020]    [0020]FIG. 4 is a schematic illustration of a control valve for use with a self-regulating lift fluid injection tool of the present invention;  
         [0021]    [0021]FIG. 5 is a graphical representation of the relationship between the injection rate of a lift fluid and the flow rate of formation fluids from a well; and  
         [0022]    [0022]FIG. 6 is a block diagram illustrating various steps utilized in the system of the present invention to control the injection rate of a lift fluid into a well.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]    While the making and using of various embodiments of the present invention are discussed in detail below, it should be appreciated that the present invention provides many applicable inventive concepts which can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention.  
         [0024]    Referring to FIG. 1, a self-regulating lift fluid injection tool in use with an offshore oil and gas production platform is schematically illustrated and generally designated  10 . A semi-submersible platform  12  is centered over a submerged oil and gas formation  14  located below sea floor  16 . Wellhead  18  is located on deck  20  of platform  12 . Well  22  extends through the sea  24  and penetrates the various earth strata including formation  14  to form wellbore  26 . Disposed within wellbore  26  is casing  28 . Disposed within casing  28  and extending from wellhead  18  is production tubing  30 . A pair of seal assemblies  32 ,  34  provide a seal between tubing  30  and casing  28  to prevent the flow of production fluids therebetween. During production, formation fluids enter wellbore  26  through perforations  36  of casing  28  and travel into tubing  30  through sand control device  38  to wellhead  18 .  
         [0025]    As explained above, when the formation pressure is not adequate to lift the formation fluids to the surface, artificial lift may be necessary. In the illustrated embodiment, a self-regulating lift fluid injection tool  40  is disposed within tubing  30 . A lift fluid is provided to injection tool  40  from a lift fluid source  42  which may be a compressor, a pump or the like. The lift fluid travels to injection tool  40  through the annulus  44  defined between casing  28  and tubing  30 . The lift fluid enters tubing  30  through injection tool  40  and mixes with formation fluids to lower the density of the formation fluids, which allows the formation fluids to travel up tubing  30  to wellhead  18 . Alternatively, it should be noted that the lift fluid may be provided from a different location in the same well or from another well. It should also be noted by those skilled in the art that even though FIG. 1 depicts an offshore environment, injection tool  40  of the present invention is equally well-suited for onshore service.  
         [0026]    Turning now to FIG. 2, a self-regulating lift fluid injecting tool is schematically depicted and generally designated  100 . Injection tool  100  of the present invention is positioned in tubing string  30  at a preselected depth for injection of lift fluid  102  at the desired location. As will be appreciated by those skilled in the art, lift fluid  102  may be a gas or a liquid utilized to adjust the density of formation fluids represented herein by arrows  104  during the recovery process irrespective of the physical phase of lift fluid  102 .  
         [0027]    As illustrated, injection tool  100  is received within an inner mandrel  106  of tubing  30 . Inner mandrel  106  includes a landing nipple  108  that engages and supports locking device  110  of injection tool  100 . A pair of seal assemblies  112 ,  114  sealing engage radially reduced areas  116 ,  118  of inner mandrel  106 , respectively. Tool  100  may be configured as a tubing retrievable device or as a wireline tool.  
         [0028]    In the practice of the present invention, lift fluid  102  is injected down annulus  44  between tubing  30  and casing  28 . Lift fluid  102  then flows through cross over ports  122  into inlet ports  124 . It should be noted that the use of directional terms such as vertical, horizontal, above, below, upper, lower, upward, downward, etc. are used in relation to the illustrative embodiments as depicted in the figures, the upward direction being toward the top of the corresponding figure and the downward direction being toward the bottom of the corresponding figure. Thus, it is to be understood that tool  100  of the present invention may be operated in vertical, horizontal, inverted or inclined orientations without deviating from the principles of the invention.  
         [0029]    The flow rate of lift fluid  102  through tool  100  is regulated by a control valve  126  within tool  100  as will be more fully described with reference to FIG. 4 below. Generally, control valve  126  is actuated by an actuator. The actuator may be electrical, mechanical or a combination of the foregoing. For example, the actuator may be an electric motor or a solenoid equipped with a mechanical linkage to advance and retract a poppet relative to an orifice. Seal assemblies  112 ,  114  isolate formation fluids  104  from lift fluid  102  as lift fluid  102  passes through crossover ports  122  and inlet ports  124 .  
         [0030]    Lift fluid  102  then flows upwardly through control valve  126  and the remainder of tool  100 . Lift fluid  102  is then injected into tubing  30  at exit port  130  as generally indicated by arrow  132 . Although, as illustrated, lift fluid  102  is injected into tubing string  30  at a single location through port  130 , those skilled in the art will appreciate that lift fluid  102  may be injected into tubing string  30  at multiple locations using multiple ports and further, that lift fluid  102  may be mixed with formation fluids  104  through the use of a stationary mixer such as a fixed vane mixer or through the use of a powered mixer such as a motor driven impeller.  
         [0031]    Tool  100  is equipped with a flow measuring device  134 . In the illustrated embodiment, flow measuring device  134  includes an impeller  136  mounted on impeller shaft  138 . As formation fluids  104  flow through impeller  136  and around tool  100  as generally indicated by arrows  104 , a sheer force is imposed upon impeller  136 , causing impeller  136  to rotate at a rate proportional to the flow rate of formation fluids  104  through tool  100 . As impeller  136  rotates, the rate of rotation is transmitted to a flow sensor  140  via impeller shaft  138 . Flow sensor  140  generates a signal proportional to the rate of rotation of impeller  136  which is subsequently interpreted as a flow rate. The signal from flow sensor  140  is relayed to electronics package  142  and optionally to transmitter  144  for transmission to a surface or remote location for recording and review by the well operator. Transmitter  144  may be equipped to transmit information via electromagnetic waves, acoustic waves, mud pulses or other means of telemetry known to those skilled in the art. Transmitter  144  may also be hard-wired to a surface or remote location for the transmission of information.  
         [0032]    Electronics package  142  and transmitter  144  may be powered by a battery pack  146  which may be charged by one or more power generators  148 ,  150 . The power generator  148  may be a turbine powered by lift fluid  102  as lift fluid  102  travels through tool  100 . Alternatively, power generator  148  may be a thermoelectric device. Alternatively or additionally, power generator  150  may be an electrical generator coupled directly to impeller shaft  138 . The use of an electrical generator coupled to impeller shaft  138  also provides the capability of controlling, to some extent, the velocity of formation fluids  104  flowing past impeller  136  by regulating the speed of impeller  136 .  
         [0033]    As best illustrated in FIG. 2, tool  100  of the present invention is situated in a central location of tubing string  30 , generally referred to hereinafter as “concentric positioning.” Concentrically locating tool  100  provides numerous advantages over side pocket positioning. Since the size and weight of side pocket mounted tools is limited by the ability of kick over devices to position the tools, a concentrically located tool provides the capability of incorporating instrumentation and equipment that cannot be incorporated into a side pocket mounted unit. In addition, tool  100  is not constrained by well deviation angles as are side pocket tools and tool  100  can be run and pulled by coiled tubing as well as wireline. Because the weight and dimensions of the tool  100  are not constrained by the limits imposed by side pocket positioning, tool  100  may include equipment, features and functionalities without regard to the limitations inherent to side pocket mounted tools.  
         [0034]    In the illustrated embodiment, the cross sectional area of tubing  30  above and below tool  100  is generally equivalent to the cross sectional areas of the annulus  152  between tubing  30  and inner mandrel  106 . Consequently, the flow of formation fluids  104  past tool  100  is not impeded by the positioning of tool  100  in tubing string  30 . As illustrated, the flow of formation fluids  104  is along the path generally designated with arrows  104 . Thus, tool  100  of the present invention provides the capability of locating the desired equipment and instrumentation at the desired downhole location to augment the recovery of formation fluids  104  without significantly impeding the flow of formation fluids  104  through tubing string  30 .  
         [0035]    Turning now to FIG. 3, another embodiment of the self-regulating lift fluid tool is depicted and generally designated  160 . Tool  160  is generally identical to tool  100  except tool  106  includes a receiver  162  for reception of signals transmitted from a remote location, for example, from a transmitter located at a surface location (not shown). Receiver  162  is operatively connected to electronics package  142 . This allows an operator at a remote location to override preprogramed instructions resident in electronics package  142  and control the operation of the tool  160 , e.g., the flow rate of lift fluid  102 , from a surface or remote location. While it is anticipated that in most cases it will be desirable to maximize well production, in some instances it may desirable to override the preprogrammed instructions that would normally optimize well production as a function of the flow rate of lift fluid  102  as will be discussed in more detail below. For example, if the available supply of lift fluid  102  in a particular field is limited, it may be advantageous to utilize less than the optimum amount of lift fluid  102  in a particular well in order to increase production from a higher producing well. Receiver  162  may receive information transmitted downhole via electromagnetic waves, acoustic waves, pressure pulses or other suitable telemetry system known to those skilled in the art. Receiver  162  may alternatively be hardwired to the surface or remote location.  
         [0036]    Referring now to FIG. 4, a schematic illustration of a downhole adjustable choke valve  164  for use in control valve  126  of tool  100  of FIG. 2 or tool  160  of Figure is depicted. Choke valve  164  is disposed within inner mandrel  106  of tubing  30 . In the illustrated embodiment, lift fluid  102  flows down between tubing  30  and casing  28  through cross over ports  122  and into choke valve  164  as generally indicated by arrows  102 . The lift fluid  102  travels through central bore  166  of choke valve  164 . Formation fluids  104  are diverted around choke valve  164  as generally indicated by arrows  104 . Lift fluid  102  is injected into the formation fluids  104  above choke valve  164  inside of tubing  30 .  
         [0037]    As illustrated, lift fluid  102  enters choke valve  164  and passes through orifice plate  170  via orifice  172 . The flow of lift fluid  102  through choke valve  164  is controlled with poppet  174  which is positioned relative to the orifice plate  170  by actuator  176 . In order to adjust the flow of lift fluid  102  through choke valve  164 , poppet  174  is advanced or retracted relative to the orifice plate  170 , thereby decreasing or increasing the effective opening of orifice  172 .  
         [0038]    Actuator  176  positions poppet  174  relative to orifice plate  170  in response to a control signal received from electronics package  142 . As noted with reference to FIGS.  2  and  3  above, the control signal may be generated periodically by electronics package  142  based upon the preprogrammed instructions stored therein or in response to a change in the flow rate of formation fluids  104  sensed by flow measuring device  134  and flow sensor  140  as will be more fully explained below. Although choke valve  164  is illustrated as a poppet type valve, other variable position flow control devices may be utilized in the practice of the invention including, but not limited to, annular sleeves, ball valves, labyrinths and the like.  
         [0039]    Referring now to FIG. 5, the flow rate of formation fluids  104  in a well where fluid recovery is enhanced using a self-regulating lift fluid tool of the present invention is depicted as a function of the injection rate of lift fluid  102 . Notably, the recovery as a function of injection rate reaches a maximum where the flow rate curve intersects the axes designated y′ and z′. Injecting additional lift fluid  102  beyond this maximum actually decreases the productivity of the well.  
         [0040]    As best illustrated in FIGS. 5 and 6 in conjunction, the self-regulating lift fluid injection tool of the present invention optimizes well productivity by adjusting the rate of injection of lift fluid  102 . Electronics package  142  includes preprogrammed instructions stored on a conventional memory device that generates a signal at step  200  to initiate or reset the flow rate of lift fluid  102  at predetermined intervals or in response to a change in the flow rate of formation fluids  104 . The flow rate of lift fluid  102  is cut back to a predetermined level in response to the signal and then incrementally increased as best illustrated in FIG. 5. Production flow rate input data is sampled with flow sensor  140  via impeller  136  and compared to a predetermined value which may be preset by the operator, determined as a function of prior production over a given period of time or in response to a change in the flow rate of formation fluids  104 .  
         [0041]    In step  220 , the production flow rate is monitored to determine whether a decrease has occurred as a function of an incremental change in the rate of injection of lift fluid  102 . If the flow rate of formation fluids  104  has not decreased, in step  230 , the injection rate of lift fluid  102  is increased initially in increments  300  and then increments  310  as the flow rate of formation fluids  104  reaches the maximum. Increments  310  allow the self-regulating lift fluid injection tool of the present invention to fine tune the flow rate of lift fluid  102  near the maximum. In addition, it should be noted that a zone of instability  320  may exist as production nears the maximum. Thus, the optimum flow rate of formation fluids  104  may be less than the theoretical maximum as indicated by FIG. 5. Generally, however, as long as the flow rate response in step  240  increases with each incremental increase  300  or  310 , steps  210 ,  220 ,  230  and  240  are repeated.  
         [0042]    If the flow rate of formation fluids  104  begins to decrease as determined in step  220  and as indicated by increments  330 , the flow rate of lift fluid  102  is decreased in step  250  with the flow rate response being monitored in step  260 . This process continues until the maximum flow rate at the location indicated by the intersection of the y′ and z′ axes or the optimum flow rate is reached. The instructions programmed into electronics package  142  may also include commands to adjust or reduce the flow of lift fluid  102  in the event that the flow of formation fluids  104  becomes unstable, e.g., in the event of sudden changes in pressure or flow rate.  
         [0043]    While this invention has been described with a reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is, therefore, intended that the appended claims encompass any such modifications or embodiments.

Summary:
A self-regulating lift fluid injection tool ( 100 ) adapted for placement within production tubing ( 30 ) is disclosed. The tool ( 100 ) has a control valve ( 126 ) that controls the rate of injection of a lift fluid ( 102 ) into the formation fluids( 104 ) being produced through the production tubing ( 30 ). A sensor ( 140 ) monitors the flow rate of the formation fluids ( 104 ) through the production tubing ( 30 ). The sensor ( 140 ) generates a signal indicative the flow rate of the formation fluids ( 104 ) which is sent to an electronics package ( 142 ). The electronics package ( 142 ) generates a control signal in response to the signal received from the sensor ( 140 ) that is received by an actuator ( 176 ). The actuator ( 176 ) adjusts the position of the control valve ( 126 ) to regulate the flow rate of the lift fluid ( 102 ) therethrough in response to the control signal, thereby optimizing the flow rate of the formation fluids ( 104 ).