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BACKGROUND 
     Many subterranean formations contain hydrocarbon based fluids, e.g. oil or gas, that can be produced to a surface location for collection. Generally, a wellbore is drilled, and a completion is moved downhole to facilitate production of desired fluids from the surrounding formation. In many applications, the wellbore completion includes one or more well tools, such as packers, valves or other tools useful in a given application, that are selectively actuated once the completion is deployed in the wellbore. 
     Actuation of many well devices is accomplished by physically moving a mechanical actuating member that changes the tool from one state to another. Examples include moving a valve from a closed position to an open position, setting a packer, or actuating a wide variety of other well tool types. The force to actuate such well tools can be provided by, for example, hydraulic pressure, solenoid actuators or combinations of electric motors, gear boxes and ball screw actuators. 
     Actuation of a well device typically occurs during movement of the completion downhole or after the completion has been fully deployed at the downhole location. Often, the downhole environment in which such tools are operated is a relatively harsh environment, susceptible to relatively high temperatures, pressures and deleterious substances. Accordingly, actuators having a high degree of complexity in construction or operation can have an increased susceptibility to malfunction due to the adverse conditions. 
     SUMMARY 
     In general, the present invention provides a system and method for dependable actuation of well devices, e.g. well tools, used in a wellbore environment. An actuator is positioned to move or actuate a specific downhole device from one state to another by physical movement of an actuator member of the downhole device. The actuator utilizes a phase change material to provide the motive force to move the actuator member. Upon providing an appropriate input, the phase change material can be caused to undergo a selective phase change, thus providing power for actuation of the well device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Certain embodiments of the invention will hereafter be described with reference to the accompanying drawings, wherein like reference numerals denote like elements, and: 
         FIG. 1  is a front elevation view of a completion deployed in wellbore, according to an embodiment of the present invention; 
         FIG. 2  is a schematic illustration of an actuator system coupled to a downhole well device for actuation of the well device, according to an embodiment of the present invention; 
         FIG. 3  is a schematic illustration of another embodiment of the actuator system illustrated in  FIG. 2 ; 
         FIG. 4  is a graphical representation of pressure that can be applied by a phase change material utilized with the actuator system illustrated in  FIG. 3 ; 
         FIG. 5  is a schematic illustration of another embodiment of the actuator system illustrated in  FIG. 2 ; 
         FIG. 6  is a schematic illustration of another embodiment of the actuator system illustrated in  FIG. 2 , showing a valve in a closed position; and 
         FIG. 7  is a schematic illustration similar to that of  FIG. 6 , but showing the valve in an open position. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous details are set forth to provide an understanding of the present invention. However, it will be understood by those of ordinary skill in the art that the present invention may be practiced without these details and that numerous variations or modifications from the described embodiments may be possible. 
     The present invention relates to well systems comprising one or more wellbore completions having devices that are mechanically actuated from one state of operation to another. Generally, a completion is deployed within a wellbore drilled in a formation containing desirable production fluids. The completion may be used, for example, in the production of hydrocarbon based fluids, e.g. oil or gas, in well treatment applications or in other well related applications. In many applications, the wellbore completion incorporates a plurality of devices, e.g. well tools, that may be individually actuated at desired times. 
     Referring generally to  FIG. 1 , a well system  20  is illustrated as comprising a completion  22  deployed for use in a well  24  having a wellbore  26  that may be lined with a wellbore casing  28 . Completion  22  extends downwardly from a wellhead  30  disposed at a surface location  32 , such as the surface of the Earth or a seabed floor. Wellbore  26  is formed, e.g. drilled, in a formation  34  that may contain, for example, desirable fluids, such as oil or gas. Completion  22  is located within the interior of casing  28  and comprises a tubing  36  and at least one device  38 , e.g. well tool, mechanically actuated by a corresponding actuator  40 . By way of example, completion  22  may comprise two devices  38 , as illustrated. However, a variety of numbers and types of mechanically actuated devices  38  can be used in the completion, depending on the overall design of well system  20 . 
     In the embodiment illustrated, actuators  40  are phase change actuators able to apply directed forces upon undergoing a phase change, such as a transition from a solid state to a liquid state. Upon appropriate input to each actuator  40 , the phase change is initiated and a change in volume of a given phase change material occurs. This volumetric change, e.g. a volumetric expansion as the material transitions from a solid to a liquid, can be used to physically move components which, in turn, actuate the corresponding wellbore device  38 . The volumetric change can be initiated by, for example, an electrical input provided to each actuator by an appropriate electrical line or lines  42 . The ability to provide signals to each actuator enables the well operator to selectively actuate each individual device  38  when desired. 
     Referring now to  FIG. 2 , an embodiment of a phase change actuator  40  is illustrated as positioned in a wellbore device  38 . In this embodiment, a phase change material  44  is deployed in a chamber or cavity  46  and trapped within the cavity  46  by a movable component  48 . Movable component  48  may comprise a dynamic seal, such as a piston  50  having one or more sealing rings  52 . In this embodiment, piston  50  is deployed within a cylinder  54  along which the piston moves when phase change material  44  undergoes a phase change. For example, the phase change material  44  may undergo volumetric expansion as it transitions from a solid state to liquid state. This transition from a solid to liquid state can be initiated by a thermal unit  56  powered by electricity supplied via electrical line  42 . In the embodiment illustrated, thermal unit  56  comprises an electrical heater element  58  for selectively heating phase change material  44  to cause the phase change from solid state to liquid state. However, thermal unit  56  also may comprise an electric cooling element  60 , such as a thermo-electric cooling (TEC) unit, for selectively cooling phase change material  44  and thus causing a reverse transition, e.g. from liquid state to solid state. Additionally, chamber  46  may be insulated to facilitate the heating and/or cooling of phase change material  44 . 
     Movable component  48  is coupled to an actuating member  62  of wellbore device  38  by an appropriate linking element  64 . Accordingly, when phase change material  44  undergoes volumetric expansion due to phase change, movable component  48  is forced along cylinder  54 . The movement of component  48  forces the movement of actuating member  62 , via linkage  64 , for mechanical actuation of wellbore device  38 . By way of example, wellbore device  38  may comprise a packer actuated, at least in part, by physical movement of actuating member  62 . In another embodiment, wellbore device  38  may comprise a valve actuated, at least in part, by physical movement of valve actuating member  62 . 
     In this embodiment, actuator  40  operates the wellbore device  38 , e.g. a valve, a packer or another well device, when power is connected or disconnected from thermal unit  56 . Insulation of chamber  46  enables the use of a relatively small amount of electrical power to be transmitted downhole to thermal unit  56  to melt or solidify phase change material  44 . Alternatively, the electrical power can be generated downhole by, for example, a battery coupled to thermal unit  56 . When the electrical power is supplied to thermal unit  56 , phase change material  44  undergoes a change in volume which changes the pressure acting against movable component  48 , e.g. dynamic piston  50 . If the pressure opposing movement of piston  50  is less than the pressure applied by phase change material  44 , the piston moves and performs useful work, such as actuating wellbore device  38 . 
     The phase change material  44  may be selected such that the actuating forces are derived by a phase change from solid state to liquid state or vice versa. However, in other applications, phase change material  44  may be selected to exert the requisite forces during changes between gas, liquid and/or solid states. In the embodiment described, the actuating work can be accomplished by a phase change material formed of a polymer material, however other types of phase change materials can be utilized. 
     A specific example of a well device  38  is illustrated in  FIG. 3 . In this embodiment, well device  38  comprises a flow control valve  66  having a generally tubular outer housing  68  with radial ports  70  formed therethrough. Flow control valve  66  further includes an internal flow passage  72  that may be selectively placed in communication with ports  74  to enable flow of fluid through ports  70  and internal flow passage  72 . This flow, however, is controlled by an adjustable choke  74  slidingly mounted within outer housing  68  for engagement with a sealing surface  76 . When adjustable choke  74  is sealed against sealing surface  76 , fluid does not flow between ports  70  and internal flow passage  72 . However, upon displacement of adjustable choke  74  from sealing surface  76 , fluid flow is enabled. 
     The adjustable choke  74  is actuated by movable component  48 , e.g. a piston, that forms a dynamic seal via a seal ring  78 . Chamber  46  is disposed at an opposite end of movable member  48  from adjustable choke  74  and is filled with volumetric phase change material  44 . Thermal unit  56  is deployed within outer housing  68  adjacent cavity  46  to selectively heat and/or cool phase change material  44 . Electrical power is supplied to thermal unit  56  via an electrical input  80 . In this embodiment, an insulating material  82  surrounds chamber  46  and may be deployed either along the exterior of tubular outer housing  68  or within the outer housing. Additionally, a position sensor  84  may be deployed along movable component  48  to determine the position of component  48  and thus the position of adjustable choke  74  and the degree to which fluid flow is enabled. Position sensor  84  can be used to output a position signal, thereby creating a closed loop system able to provide feedback as to the actuation of device  38  relative to the electrical power input to thermal unit  56 . 
     In many operating conditions, e.g. in gas production wells, an advantage of phase change actuator  40  is that the differential pressure across a dynamic seal is less than the absolute pressure applied upstream of the valve, as illustrated in  FIG. 4 .  FIG. 4  simply provides one graphical example of upstream pressure relative to choke diameter and the differential pressure across the dynamic seal of such a valve with a given amount of back pressure. By properly defining the operational specifications of actuator  40 , the pressure ratings of the phase change actuator can be relatively high. 
     Another example of valve  66  is illustrated in  FIG. 5 . This valve embodiment can be used in high-temperature gas lift applications where the geothermal temperature exceeds the melting point of phase change material  44 . An annular volume of the phase change material  44  is confined between dynamic seals  86  and  88  which have different diameters. A choke  90  is positioned by regulating the temperature of phase change material  44  between dynamic seals  86  and  88  via thermal unit  56 . For example, choke  90  can be positioned in sealing engagement with a flow control seal surface  91  by initiating a phase change to increase the volume of phase change material  44 , thereby completely blocking fluid flow through ports  70 . By then decreasing the volume of phase change material  44 , via thermal unit  56 , choke  90  can be moved away from flow control seal surface  91  to enable gas flow through valve  66 . In the embodiment illustrated, a thermal insulator  92  is deployed along an exterior surface of tubular outer housing  68 . Some heat transfer, however, is allowed between the inner surface of a venturi  94  and the sealed chamber  46 . The cooling effect of throttling gases through valve  66  is utilized to decrease the power required to electrically cool the phase change material via, for example, a TEC contained in thermal unit  56 . 
     Referring to  FIGS. 6 and 7 , another embodiment of wellbore device  38  is illustrated in which actuator  40  comprises a puller-type actuator. The actuator uses a movable component  48  in the form of a dynamically sealed movable piston  96  coupled to actuating member  62  by linkage  64  and an indexer  98 . In the specific embodiment illustrated, device  38  is a valve and actuating member  62  comprises a variable choke  100  used to control the flow of fluid between ports  102  and a venturi  104 . The position of variable choke  100  can be set by reciprocating indexer  98  via linkage  64 , as accomplished with conventional indexing mechanisms. The reciprocating movement of linkage  64  and indexer  98  is accomplished by sequential phase changes of the phase change material  44  which is trapped in chamber  46 . Chamber  46  is positioned generally between movable piston  96  and indexer  98  such that piston  96  pulls on linkage  64  and indexer  98  when phase change material  44  undergoes volumetric expansion. Accordingly, the actuating member  62 , e.g. variable choke  100 , can be moved in gradations from a first state, as illustrated in  FIG. 6  to a second state, as illustrated in  FIG. 7 . In the specific example illustrated, the variable choke  100  is moved between a closed position and a fully open position in increments established by indexer  98 . 
     With further reference to the embodiment of  FIGS. 6 and 7 , chamber  46  is formed by an interior housing  106  disposed within an outer device housing  108 . Outer housing  108  includes an electrical feed-through  110  by which electrical input can be provided to thermal unit  56  to heat and/or cool elements deployed between interior housing  106  and outer housing  108 . The heating and cooling of phase change material  44  creates reciprocating motion of movable piston  96  and the indexing of actuating member  62  to a desired position. In this specific embodiment, the valve further comprises a compensation bellows  112  disposed on an opposite end of movable piston  96  from chamber  46 . The embodiment further comprises a seal bellows  114  deployed between variable choke  100  and indexer  98 . Compensation bellows  112  and seal bellows  114  provide isolation from wellbore fluids and can be filled with a liquid, such as an oil, that is communicated between the seal bellows  114  and the compensation bellows  112  via a liquid flow path  116 . Accordingly, the internal liquid can move from one bellows to the other as the volume of each individual bellows is changed during actuation of the choke. 
     The examples of wellbore devices illustrated and described herein are just a few examples of the many types of wellbore devices that can be actuated with a phase change actuator. Many other low-power, high work actuator applications are amenable to implementation of phase change actuators. For example, phase change actuators can be used for actuation of a flow tube in a subsurface safety valve, actuation of a flapper valve, actuation of a ball valve, actuation of a variety of packer components, and for actuating many other downhole devices. Additionally, initiation of phase change in the phase change material can be provided by input other than electrical input. In one example, a chemical reaction, e.g. an exothermic chemical reaction, can be initiated to create heat that causes the phase change material  44  to undergo a change of phase sufficient to actuate a given wellbore device  38 . 
     Accordingly, although only a few embodiments of the present invention have been described in detail above, those of ordinary skill in the art will readily appreciate that many modifications are possible without materially departing from the teachings of this invention. Accordingly, such modifications are intended to be included within the scope of this invention as defined in the claims.

Summary:
A technique is provided for actuating devices deployed in a wellbore. The technique utilizes an actuator that cooperates with a downhole device, such as a well tool. The actuator has a phase change material that can be caused to undergo a phase change upon an appropriate input. The phase change of the material is used to provide the force necessary for actuation of the downhole device.