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CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a U.S. National Phase Application under 35 U.S.C. §371 and claims the benefit of priority to International Application Serial No. PCT/US2013/034843, filed on Apr. 1, 2013. 
     
    
     BACKGROUND 
       [0002]    In a well system, well screen assemblies are used to filter against passage of particulate from the wellbore into the production string. The wellbore around the screens is often packed with gravel to assist in stabilizing the formation and to pre-filter against particulate before the particulate reaches the screens. A uniform gravel packing can, however, be difficult to achieve due to formation of sand bridges and other complications experienced when pumping the gravel slurry into the region around the screens. Therefore, sometimes expandable screens that expand into contact with the wellbore are used in place of gravel packing 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0003]      FIG. 1  is a side view of an example well system incorporating a plurality of radially extending well screen assemblies. 
           [0004]      FIGS. 2A-2F  are example well screen assemblies having a base pipe encircled by an extendable screen that can be extended radially, and in certain instances, into contact with the wall of the wellbore. FIGS.  2 A and  2 C-D are end cross-sectional views of example well screen assemblies shown in a radially compact, unextended, run-in state, with  FIG. 2A  having a corrugated screen,  FIG. 2C  having a flat screen with overlapping ends, and  FIG. 2D  having squat, T-shaped crimps defining corrugations.  FIG. 2B  shows these example screens in a radially extended state.  FIG. 2E  is a side, half cross-sectional view of an example well screen assembly with one or more bladders and the well screen assembly shown in a radially compact, unextended run-in state.  FIG. 2F  shows this screen in a radially extended state. 
           [0005]      FIGS. 3A-3G  are example louver type well screen assemblies having extendable filtration louvers that can be extended radially, and in certain instances, into contact with the wall of the wellbore.  FIGS. 3A and 3B  are a side, half cross-sectional view and an end cross-sectional view, respectively, of an example louver type well screen assembly having a circumferentially corrugated bladder with the well screen assembly shown in a radially compact, unextended, run-in state. The cross section of  FIG. 3B  is taken through the telescoping passageways.  FIG. 3C  shows this example well screen assembly in a radially extended state.  FIGS. 3F and 3G  are end cross-sectional views of an example louver type well screen assembly having an axially corrugated bladder shown in a radially compact, unextended, run-in state and radially extended state, respectively. The cross sections of  FIGS. 3F and 3G  are taken between the telescoping passageways.  FIG. 3D  shows a ratchet mechanism, in side half cross-sectional view, that can be used with any of the example louver type well screen assemblies mentioned here.  FIG. 3E  shows an example hydraulic injection tool used in inflating the bladders. 
           [0006]    Like reference symbols in the various drawings indicate like elements. 
       
    
    
     DETAILED DESCRIPTION 
       [0007]    Referring first to  FIG. 1 , an example well system  10  is shown to illustrate an example application of well screen assemblies  24 . The well system  10  includes a subterranean wellbore  12  extending from the terranean surface through one or more subterranean zones of interest  20 . The subterranean zones  20  can correspond to all or a portion of a subterranean formation (e.g., hydrocarbon bearing formation) and/or multiple formations. The well bore  12  shown in  FIG. 1  is a “horizontal” well bore, and has a substantially vertical section  14  and a substantially horizontal section  18 . The concepts herein, however, are applicable to many other configurations of well bores, such as vertical wells, slanted wells, other deviated wells, multi-laterals, and/or other configurations. The wellbore  12  can be cased or partially cased. For example, in  FIG. 1 , the vertical section  14  includes a casing  16  cemented at an upper portion thereof, and the horizontal section  18  is open hole through the subterranean zone  20 . 
         [0008]    A tubing string  22 , for example, a production and/or injection string, resides in the well bore  12  and extends from the terranean surface. The tubing string  22  can communicate fluids between the subterranean zone  20  and the surface. The screen assemblies  24  are distributed along the tubing string  22  proximate the subterranean zone  20 . The screen assemblies  24  are sand control screen assemblies that can filter out particulate materials from well fluids, direct the well fluids to a center bore of the tubing string  22 , and stabilize the formation. As is discussed in more detail below, the screen assemblies  24  are of a type that radially extend into contact with an interior wall of wellbore  12  and are shown in an operating or a radially extended configuration. Three screen assemblies  24  are shown. In other instances, fewer or more screen assemblies  24  can be used. The screen assemblies may be all of one type or some or all of the screen assemblies  24  can be of a different configuration. In certain instances the screen assemblies  24  are of a type that can be run into the well in a radially compact, unextended run-in state, and subsequently extended using pressure from a fluid supplied into the interior of the screen assembly  200 . The fluid can be supplied from the surface via a tubing string of jointed and/or coiled tubing and/or through the wellbore  12  (apart from a tubing). The fluid can alternately or additionally be supplied from a downhole location (e.g., with a pump and/or other). 
         [0009]      FIGS. 2A-2F  are example well screen assemblies having a base pipe encircled by an extendable screen that can be extended radially, and in certain instances, into contact with the wall of the wellbore.  FIG. 2A  is an end cross-sectional view of an example well screen assembly  200  that can be used as well screen assembly  24 . The well screen assembly  200  includes a base pipe  202  encircled by a screen  204 , and the screen is sealed to the base pipe  202  at its ends by end rings (like end rings  206  shown in  FIG. 2E ) affixed to the base pipe  202 . The base pipe  202  can be of a type having a plurality of apertures distributed along its length, beneath the screen  204 , to allow communication of fluid between an exterior of the screen assembly  200  and the center bore of the base pipe  202  via the screen  204 . Alternately, the base pipe  202  can be unaperatured between the end rings, and the end rings can collect and allow flow between the exterior of the well screen assembly  200  and the center bore of the base pipe  202 . In certain instances, the end rings can contain an inflow control device, such as valve responsive to a signal or to conditions in the well, a flow orifice of specified flow area, and/or other inflow control devices. 
         [0010]    In well screen assembly  200 , the screen  204  is corrugated, having been compressed from a fully extended state by providing partial folds in the screen material around the circumference of the base pipe  202 . The well screen  204  is shown with folds extending axially.  FIG. 2B  shows the screen  204  in the fully extended state. In certain instances, the screen  204  in its extended state has a diameter equal to the diameter of the well bore  14 . 
         [0011]    In certain instances, the screen  204  has a degradable material embedded in its openings, the degradable material sealing against flow through the screen  204 . The degradable material is a material that structurally degrades to allow flow through the screen  204  in response to a specified stimulus. The degradable material can be selected to degrade in response to certain fluids (e.g., the actuating fluid and/or another fluid) and/or when exposed to certain conditions, such as a specified temperature and/or pressure (e.g., high temperatures associated with steam injection). The degradable material can degrade by dissolving, corrosion, hydrolytic cleavage, galvanic reactions, melting and/or in another manner. In certain instances, the degradable material can be a plasticized acid coating such as polylactic acid (PLA), polylactic-co-glycolic acid (PLGA), or similar. Other examples of degradable material exist. The degradable materials can be coated, injected, and/or pressed into the screen  204  before assembly or installation in the well, forming a filled non-porous surface. Therefore, with the screen assembly  200  in the wellbore  14  in location, the screen  204  is extended by supplying fluid through the interior center bore of the base pipe  202  at pressure. The screen  204 , sealed by the degradable material, defines an inflatable (hydraulically, pneumatically and/or otherwise) fluid cavity between the screen  204  and the base pipe  202  that is filled by the fluid. The fluid acts on the screen  204  extending it into contact with the wall of the wellbore  14 . In extending the screen  204 , the pressure of the fluid at least partially straightens the corrugations of the screen  204 , and may elastically and/or plastically deform the screen  204 . In certain instances, as in  FIG. 2B , the pressure extends the screen  204  to contact the entire or substantially the entire inner perimeter of the wellbore  14 . The base pipe  202 , however, is radially unextended, staying generally the same dimensions (save some nominal elastic deformation that may occur when pressure is applied to the screen  204 ) that it was when it was run into the wellbore  14 . The base pipe  202  is not substantially plastically deformed by the fluid. 
         [0012]    Alternatively, the openings of the screen  204  can be open (without a degradable material) when the well screen assembly  200  is run into the wellbore  14 , and a degradable material pumped into the center bore of the base pipe  202 . The degradable material lodges in the openings of the screen  204  sealing against flow through the screen  204  and enables (by defining an inflatable fluid cavity) the fluid pressure to act on and extend the screen  204 . In certain instances, the degradable material can be pumped into the well screen assembly  200  concurrently with extending the screen  204  or prior to extending the screen  204 , in connection with another operation or apart from another operation. 
         [0013]    With the screen  204  extended, the degradable material can be removed. Thus, after the screen  204  has been extended to the wall of the wellbore  14 , a fluid that creates the degrading conditions is pumped through the center of the base pipe  202  into the interior of the screen  204  and/or down the annulus between the screen assembly  200  and the wellbore  14  to contact the screen  204 . The fluid structurally degrades the degradable material in the openings of the screen  204  and opens the screen to allow flow. After installation of the screen into the wellbore, the degrading fluid or fluid that creates the degrading conditions can be filled into the base pipe and/or into the wellbore around the screen to degrade the degradable material and open the screen assembly  200  to flow. 
         [0014]    The degradable materials provide a multitude of functions. For example, the degradable material can eliminate the need to treat the drilling mud prior to running the screen assembly by completely protecting the screens from contamination and clogging. In addition, in instances where the degradable material is or contains an acid, it can also eliminate the need to pump an acid treatment to degrade the filtercake, because the acid of the degradable material can degrade the filtercake. Furthermore, the coating can eliminate the need to run a wash pipe by creating a low pressure barrier/conduit through the screen assembly, and enabling the screen assembly to be used as a wash pipe prior to degrading the degradable material. 
         [0015]    Although  FIG. 2A  shows the well screen  204  having axial corrugations, the screen  204  can be compacted in other manners. For example, the screen  204  could be corrugated in another manner (e.g., with circumferential or other folds).  FIG. 2C  shows a well screen assembly  200 ′ like well screen assembly  200  except that the screen  204 ′ is flat (rather than corrugated) encircling the base pipe  202  and the ends of the screen overlap. Enough screen  204 ′ can be provided that when the screen  204 ′ is extended to circumscribe the perimeter of the wellbore  14  the ends of the screen  204 ′ still overlap.  FIG. 2D  shows another well screen assembly  200 ″ like well screen assembly  200  except that the screen  204 ″ has squat, T-shaped crimps defining the corrugations of the screen  204 ″. As above, when extended the screen  204 ″ can include enough screen material to circumscribe the perimeter of the well bore  14 . 
         [0016]    In yet another configuration  FIG. 2E , the well screen assembly  200 ″′ (shown in a quarter, side cross-sectional view) is like well screen  200  except that it includes one or more bladders  218  between the base pipe  202  and screen  204 ′ that can be used to extend the screen  204 ′. The bladders  218  internally define an inflatable fluid cavity. The screen  204 ″′ can be corrugated as described above, flat and overlapping as described above and/or can otherwise be configured to extend. The bladder  218  can encircle the entire circumference of the base pipe  202  or can be one or more separate elongate bladders arranged on the exterior of the base pipe  202 . The base pipe  202  has one or more apertures  210  that communicate the center bore of the base pipe  202  and the interior of the bladder  218 . With the well screen assembly  200 ″′ in position in the wellbore  14 , fluid is supplied through the center bore the base pipe  202  into the bladder  218  via the aperture  210  to supply fluid into the bladder  218  to extend the screen  204 ′. The apertures communicating with the interior of the screen  204 ″′ (and not the bladders  218 ) can be initially plugged (e.g., by a plug, rupture disk, valve or otherwise) when the well screen assembly  200 ″′ is run into the well to focus all fluid flow to the interior of the bladders  218 . Thereafter, the plugs can be opened. 
         [0017]    In certain instances, the apertures  210  can include a valve (e.g., a check valve) that retains the fluid in the bladder  218  and maintains the bladder  218  and screen  204 ′ extended. In certain instances, the fluid used in extending the bladder  218  can be a solidifying material that is injected into the bladder  218  as a liquid and solidifies (entirely or substantially, e.g. thicken) and remains in the bladder  218 , maintaining the bladder  218  extended. When solidified, it cannot pass back through the apertures  210  into the center bore of the base pipe  202 ″′. Some examples of solidifying materials include extending foam, resin, gravel slurry, cement, gels, hydrating materials, swellable materials, crosslinking materials and/or other material. The material can be selected to solidify after a specified time, in response to temperature, in response to an activating fluid and/or in another manner. In certain instances, the bladder  218  can be constructed of a material that, when extended, maintains its extended state after the pressure is removed and supports the screen  204 ″′ extended. For example, the bladder  218  can be metal, polymer, a metal reinforced polymer, a fiber reinforced polymer and/or another material. In certain instances, the material can plastically deform when expanded to maintain the screen  204 ″′ extended. In certain instances, the bladder  218  can be a memory material (e.g., memory metal) deformed from an initial state into a radially compact state (e.g., as in  FIG. 2E ) and configured to return to the initial state in response to a stimulus (e.g., heat). In certain instances, the initial state can be a diameter equal or slightly smaller or larger than the diameter of the wellbore as in  FIG. 2F , so that when the memory metal is returned to its initial state, it maintains the screen  204 ″ extended. 
         [0018]    In certain instances, the bladder  218  can be constructed of rubber and/or a degradable material. The degradable material is a material that degrades in response to a specified stimulus and may or may not be the same as or related to the degradable material discussed above. The degradable material can be selected to degrade in response to certain fluids (e.g., the actuating fluid and/or another fluid) and/or when exposed to certain conditions, such as a specified temperature and/or pressure (e.g., high temperatures associated with steam injection). For example, the bladder  218  can be degraded in response to an acid and/or other fluid. Using a bladder  218  made of a degradable material allows the bladder  218  to be degraded after the screen has been extended by fluid, so as not to obscure flow between the screen and the base pipe. 
         [0019]    In each of these embodiments,  FIG. 2A-2F , the well screen can be configured to be rigid enough, that once extended by fluid, it maintains the screen in contact with the wall of the wellbore  14  even after the fluid pressure has been removed. For example, the screen can include one or more layers of pre-manufactured filtration mesh (e.g., woven, square and/or other) selected to filter against passage of particulate larger than a specified size and affixed to one or more layers of support material, such as another mesh, extendable tubing, or other support layer selected to provide rigidity to the remainder of the screen. The support material can be provided as a shroud around the exterior of the filtration mesh and/or another layer between or radially beneath the filtration mesh. In certain instances, the support material can be a memory material (e.g., memory metal) deformed from an initial state into a radially compact state (e.g., as in  FIGS. 2A ,  2 C- 2 E) and configured to return to the initial state in response to a stimulus (e.g., heat). In certain instances, the initial state can be a diameter equal or slightly smaller or larger than the diameter of the wellbore as in  FIG. 2B  so that when the memory metal is returned to its initial state, it maintains the screen extended. In certain instances, the screen can include a fluid transport layer, such as a layer of mesh or wire that define unobstructed or relatively unobstructed axial, circumferential and/or other direction passages through the screen and facilitate transport of fluid in the plane of the screen. 
         [0020]      FIGS. 3A-3G  are example louver type well screen assemblies having extendable filtration louvers that can be extended radially, and in certain instances, into contact with the wall of the wellbore.  FIG. 3A  shows another configuration well screen assembly  300  that can be used as well screen assembly  100 . Well screen assembly  300  is a louvered type well screen configuration. An example louver type well screen assembly configured to extend in response to contact with a specified fluid is disclosed in U.S. Patent Publication No. US 2011/0036565, entitled “Control Screen Assembly,” filed Aug. 12 2009, the entirety of which is incorporated herein by reference. The well screen assembly  300  has a similar construction, but includes adaptations to extend in response to fluid pressure. 
         [0021]    To this end,  FIG. 3A  is a half cross-sectional view of the well screen assembly  300  and  FIG. 3B  is an end cross-sectional view of the well screen assembly  300 . Well screen assembly  300  has a plurality of elongate, tubular filtration mediums, i.e., filtration louvers  304 , extending substantially the length of the base pipe  302 . The filtration louvers  304  are defined by a filtration screen folded to define an interior longitudinal passageway through the louver. As above, the screen can include one or more layers of pre-manufactured filtration mesh (e.g., woven, square and/or other) affixed to one or more layers of support material, such as another mesh, tubing, or other support layer selected to provide rigidity to the screen. The louvers  304  can also include a fluid transport layer. 
         [0022]    The louvers  304  are supported relative to the base pipe  302  by a plurality of telescoping passageways formed by a tubular, upper telescoping piston piece  308  affixed to the louver  304  and in fluid communication with its interior longitudinal passageway and a lower cylinder piece  306  affixed to the base pipe  302  and in fluid communication with its center bore.  FIG. 3A  shows the upper telescoping piece  308  as a male portion, extending into an interior, female portion of the lower telescoping piece  306 . In other instances, the male and female portions could be reversed (e.g., upper piece  308  being female and lower piece  306  being male). When the well screen assembly  300  is extended, and the louvers  304  are fully extended into contact with the wall of the wellbore  14  (see  FIG. 3C ), the telescoping joint defines a sealed flow passage between the interior longitudinal passageway of the louver  304  and the interior of the center bore of the base pipe  302  through an aperture  310  in the base pipe  302 . In certain instances, the telescoping joints can be provided near or at the ends of the louver  304  and/or at an intermediate location. 
         [0023]    In certain instances, the screen of the louvers  304  has a degradable material, similar to that described above, embedded in its openings when the screen assembly  300  is run into the wellbore  14 . Also, as above, the degradable material seals against flow through the screen and out of the louvers  304 , but can later be degraded to regain flow. Alternatively, the openings of the screen can be open (without a degradable material) when the well screen assembly  300  is run into the wellbore  14 , and then a degradable material is pumped into the center bore of the base pipe  302 . The degradable material lodges in the openings of the screen sealing against flow through the screen of the louvers  304 . In either instance, the louver  304  sealed by the degradable material defines an inflatable fluid cavity between the louver  304  and the base pipe  302  that allows fluid to act on and extend the louvers  304 . Also, in lieu of the degradable material or in combination with the degradable material, a plug can be provided in the upper telescoping piece  308  to define an inflatable fluid cavity. 
         [0024]    Thus, to extend the louvers  304  into contact with the wall of the wellbore  14 , fluid pressure is supplied down the center bore of the base pipe  302  to push the louvers  304  radially outward (by filling the fluid cavity). Then, the louvers  304  are changed to allow flow therethrough by degrading the degradable material or by raising the pressure in the center bore of the base pipe  302  high enough to dislodge the plug. 
         [0025]    In certain instances, one or more bladders  318  are provided between the base pipe  302  and the louvers  304 . As above, the bladders  318  define an inflatable fluid cavity between the louver  304  and the base pipe  302  that fluid can be supplied into to extend the louvers  304 . The bladder  318  can encircle the base pipe  302  between the telescoping passageways and/or can be one or more separate elongate bladders arranged axially on the exterior of the base pipe  302 . The bladder  318  can be corrugated having undulations that extend axially, circumferentially or otherwise. In this instance, the base pipe  302  includes a plurality of apertures  316  radially beneath and in communication with the interior of the bladders  318 . In certain instances, multiple apertures  316  are arranged circumferentially spaced apart around the circumference of the base pipe  302 . The apertures  310  (communicating with the telescoping passageways) and the apertures  316  (communicating with the interior of the bladder  318 ) are initially sealed with plugs  312 ,  316 . The plug  312  in the aperture  310  is configured to hold a higher pressure than the plugs  314  in the apertures  316 . In certain instances, the plugs  312 ,  316  can be rupture disks selected to rupture at a specified pressure. 
         [0026]    To extend the louvers  304  into contact with the wall of the wellbore  14 , fluid pressure is supplied down the center bore of the base pipe  302  to rupture the plugs  314  to open flow to the interior of the bladders  318 . In certain instances, the pressure is less than the rupture pressure of the plugs  312  sealing the telescoping passageway, so that the louvers  304  stay sealed and no fluid is lost to the annulus via the louvers  304 . The fluid fills the bladder  318  and lifts the louvers  304  radially into contact with the wall of the wellbore  14 . 
         [0027]    In each of the embodiments,  FIGS. 3A-3G , the well screen can be configured to, once extended by fluid, maintain the louvers  304  in contact with the wall of the wellbore  14  even after the fluid pressure has been removed. For example, in certain instances, the apertures  316  can include a valve (e.g., a check valve  320 ) as the plug  314  that allows fluid to flow into the bladder  318 , but retains the fluid in the bladder  318 , and thus maintain the bladder  318  extended. In certain instances, the fluid used in extending the bladder  318  (provided with or without valves in the apertures  316 ) can be a solidifying material that is injected into the bladder  318  as a liquid and solidifies (entirely or substantially). When solidified, it cannot pass back through the apertures  316  into the center bore of the base pipe  302  and remains in the bladder  318 , maintaining the bladder  318  extended. The material can be selected to solidify after a specified time, in response to temperature, in response to an activating fluid and/or in another manner. In certain instances, the bladder  318  can be constructed of a material that, when extended, maintains its extended state after the pressure is removed and supports the louvers  304  in contact with the wall of the wellbore  14 . For example, the bladder  318  can be metal, polymer, a metal reinforced polymer, a fiber reinforced polymer and/or another material. In certain instances, the bladder  218  can be a memory material (e.g., memory metal) deformed from an initial state into a radially compact state (e.g., as in  FIG. 3A ) and configured to return to the initial state in response to a stimulus (e.g., heat). In certain instances, the initial state can be sized to hold the louvers at a diameter equal or slightly smaller or larger than the diameter of the wellbore as in  FIG. 3C  so that when the memory metal is returned to its initial state, it maintains the louvers extended. In certain instances, as shown in  FIG. 3D , the upper telescoping piston piece  308  and lower cylinder piece  306  of the telescoping passageways can include a ratchet mechanism  328  configured to allow the telescopic passageway to telescope radially outward but prevent the telescopic passageway from collapsing back inward after the fluid pressure is removed (and the bladders deflated). In certain instances, the ratchet mechanism  328  includes one or more laterally extendable and retractable teeth on the piston piece  308  and/or the cylinder piece  306  that spring open and grip the other piece as the telescoping passageway is extended. In certain instances, the ratchet mechanism may be provided apart from the telescoping passageways, for example, in a separate structure. In certain instances, another mechanism can be provided to allow the telescoping passageways to telescope radially outward but not collapse back inward. For example, a swellable rubber can be provided between the piston piece  208  and cylinder piece  306 . The swellable rubber is configured to swell in response to contact with certain fluids (e.g., the actuating fluid and/or another fluid) and/or when exposed to certain conditions, such as a specified temperature and/or pressure (e.g., high temperatures associated with steam injection). Thus, after the telescoping passageways radially extend, the swellable rubber is caused to swell and grip the cylinder piece  306  to the piston piece  208  and prevent the telescoping passageways from collapsing back radially inward. 
         [0028]    In certain instances, the bladder  318  can be constructed of rubber and/or a degradable material. The degradable material can be selected to degrade in response to certain fluids (e.g., the actuating fluid and/or another fluid) and/or when exposed to certain conditions, such as a specified temperature and/or pressure (e.g., high temperatures associated with steam injection). For example, the bladder  318  can be degraded in response to an acid and/or other fluid. Using a bladder  318  made of a degradable material allows the bladder  318  to be degraded after the screen has been extended by fluid, so as not to obscure flow between the screen and the base pipe. 
         [0029]    In certain instances, the fluid pressure can be applied to the well screen assembly  300  by pressurizing the center bore of the base pipe  302 . In certain instances, shown in  FIG. 3E , a tubing string of jointed and/or coiled tubing with a hydraulic injection tool  322  run in center bore of the base pipe  302  can be used to apply the fluid. The hydraulic injection tool  322  is tubular and has one or more seals  326  around an aperture  324  (e.g., a seal encircling the aperture  324 , a pair of seals—one uphole and one downhole—encircling the body of the injection tool  322 , and/or another configuration) that communicates between the interior center bore of the injection tool  322  and the exterior of the tool  322 . The seals  326  seal against the interior wall of the base pipe  302 . 
         [0030]    The injection tool  322  can be run into the interior of the well screen assembly  300  and positioned with the seals  326  spanning and sealing the aperture  316  to aperture  324 . Then, hydraulic fluid is supplied down the interior of the injection tool  322  into the bladder  318 . In instances having more than one bladder  318 , the injection tool  322  can be configured to supply fluid to a specific one or more or all of the bladders  318  concurrently. If multiple well screen assemblies  300  are provided in the well, the hydraulic injection tool  322  can be configured to supply fluid to one or more well screen assemblies  300  at a time. Thus, the injection tool  322  can enable actuation of specific well screen assemblies  300  and not others when multiple well screen assemblies  300  are provided. Notably, an injection tool  322  can be used with any of the configurations of well screen assembly described herein. 
         [0031]    A number of implementations have been described. Nevertheless, it will be understood that various modifications may be made. Accordingly, other implementations are within the scope of the following claims.

Summary:
A well screen assembly residing in a well bore has a base pipe and a filtration screen carried on the base pipe. The screen is radially extended with force from fluid while maintaining the base pipe radially unextended.