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CROSS-REFERENCE TO RELATED APPLICATION 
   This claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Application Ser. No. 60/826,191, entitled “Sand Control Completion with Interval Isolation,” filed Sep. 19, 2006, which is hereby incorporated by reference. 

   TECHNICAL FIELD 
   The invention relates generally to a gravel pack apparatus and method that includes a swellable element that swells in response to an input stimulus to seal against a wellbore. 
   BACKGROUND 
   To complete a well, one or more formation zones adjacent the wellbore are perforated to allow fluid from the formation zones to flow into the well for production to the surface. Perforations are typically created by perforating gun strings that are lowered to desired intervals in the wellbore. When fired, perforating guns extend perforations into the surrounding formation. 
   In producing fluids from a reservoir in a formation, particulates such as sand may be produced with reservoir fluids. Such particulates may damage the well and significantly reduce production and life of the well. Formation fluids containing particulates may act as an abrasive that wears and erodes downhole components, such as tubing. In addition, production of particulates such as sand may create voids in the formation behind the casing which may result in buckling of or other damage to the casing. Moreover, particulates produced to the surface are waste products requiring disposal, which may be costly. 
   Various methods and devices for reducing or eliminating sand and other particulate production have been developed. Gravel packing of the formation is a popular technique for controlling sand production. Although there are variations, gravel packing essentially involves placing a sand screen around the section of the production string containing the production inlets. This section of the production string is aligned with the perforations. A slurry of gravel in a viscous transport fluid is pumped into the annulus between the sand screen and the casing. The deposited gravel blocks the formation particulates, such as sand, from flowing into the production tubing. However, formation fluids are allowed to enter the production string for flow to the well surface. 
   In some scenarios, such as when relatively long formations are being gravel packed, it may be desirable to employ zonal isolation to define multiple zones that are isolated from each other. Conventionally, the isolation used with sand control equipment includes cup packers in cased hole applications. However, use of cup packers reduces flexibility in how zones can be isolated. 
   SUMMARY 
   In general, according to one embodiment, a gravel pack apparatus for use in a wellbore includes a screen assembly to filter particulates, and at least one shunt conduit to carry gravel slurry. A swellable element around a portion of the at least one shunt conduit swells in response to an input stimulus to seal against the wellbore, where the swellable element when swelled expands radially. 
   Other or alternative features will become apparent from the following description, from the drawings, and from the claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a completion string having screen assemblies and swellable elements according to some embodiments. 
       FIGS. 2A and 2B  illustrate a portion of the completion string of  FIG. 1 , with  FIG. 2A  showing a swellable element prior to swelling, and  FIG. 2B  showing the swellable element after swelling. 
       FIG. 3  is a partial longitudinal sectional view of a section of the completion string portion of  FIGS. 2A-2B . 
       FIG. 4  is a cross-sectional view of a section of the completion string of  FIG. 3 . 
       FIG. 5  is a partial longitudinal sectional view of a section of another embodiment of the completion string portion of  FIGS. 2A-2B . 
       FIG. 6  is a cross-sectional view of a section of the completion string of  FIG. 5 . 
       FIG. 7  illustrates a shunt tube with a valve therein, where the shunt tube is useable in the completion string of  FIG. 1 . 
   

   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 skilled in the art that the present invention may be practiced without these details and that numerous variations or modifications from the described embodiments are possible. 
     FIG. 1  illustrates a completion string positioned in a wellbore  100 , where the completion string includes screen assemblies  102  and swellable elements  104 . The screen assemblies  102  include screens (or other types of filtering structures) to perform filtering of particulates such that particulates are not produced into the completion string. In a different implementation, instead of production, the completion string can be used for injecting fluids into the surrounding reservoir. The swellable elements (also referred to as swellable packers)  104  are provided to swell (from a first diameter to a second, larger diameter) in response to some type of input stimulus such that the swellable elements  104  expand to sealably engage an inner surface  106  of the wellbore  100 . 
   The input stimulus that causes swelling of the swellable elements  104  can include stimulus due to exposure to a downhole environment (e.g., well fluids, elevated temperature, and/or elevated pressure). Exposure to the downhole environment causes expansion of the swellable elements  104 . In some implementations, the swellable elements  104  are formed of elastomers that expand upon exposure to well fluids at elevated temperatures or pressures. The swelling of the swellable elements  104  is a chemical swelling process which can cause radial expansion of the swellable elements  104  to exert radial forces on the inner surface  106  of the wellbore  100  such that a sealing barrier is provided to isolate different zones of the wellbore  100 . Upon swelling of the swellable elements  104 , three zones  108 ,  110 , and  112  are defined. 
   Note that if a different number (one or more than two) of swellable elements  104  are used, then a different number of zones are defined. 
   In a different implementation, chemical swelling of the swellable elements  104  can be in response to release of an activating agent. For example, the activating agent can be stored in some container that is sealed prior to activation. Upon activation, the container is opened to allow the activating agent to communicate with the swellable elements  104  such that the swellable elements  104  are caused to chemically swell. For example, a shifting tool in the completion string can be used to open the container to release the activating agent. 
   In yet another implementation, the swellable elements  104  can be inflatable bladders that can be filled with a fluid (e.g., gas or liquid) to cause the swellable elements  104  to expand to engage the inner surface  106  of the wellbore  100 . 
   The benefit of using the swellable elements  104  is that during run-in of the completion string, the swellable elements  104  have an outer diameter that is less than an inner diameter of the wellbore  100 . The annular clearance around the swellable elements  104  allows fluid displacement around the swellable elements  104  during run-in. Also, each swellable element  104  can have a relatively long sealing length, such as on the order of several feet. In permeable formations, the swellable elements  104  can provide reasonable isolation because pressure drop is length dependent. Moreover, swelling of each swellable element  104  provides for relatively good conformity with the inner surface  106  of the wellbore  100  (and with any gravel material in the region to be sealed) such that a good seal is provided. Also, because the swellable elements  104  are able to expand beyond the run-in outer diameter, the swellable elements can seal in a larger range of wellbore sizes. In one example, the swellable elements can be used in an under-reamed open hole. Moreover, the swellable elements  104  provide for greater flexibility in that the swellable elements  104  can be used in either a cased wellbore or in an open hole (un-cased and un-lined wellbore). 
     FIGS. 2A and 2B  illustrate a portion (one swellable element  104  and two screen assemblies  102 A and  102 B on the two sides of the swellable element  104 ) of the completion string depicted in  FIG. 1 .  FIG. 2A  shows the swellable element  104  prior to swelling, whereas  FIG. 2B  shows the swellable element  104  after swelling. The swellable element  104  is mounted on a connection sub  202  that connects the first screen assembly  102 A on one side of the connection sub  202 , and the second screen assembly  102 B on the other side of the connection sub  202 . The connection sub  202  interconnects the screen assemblies  102 A,  102 B. 
   The screen assembly  102 A includes a screen  204 A and an outer shroud  205 B that surrounds the screen  204 A. The shroud  205 B has multiple perforations to allow for communication of fluids. The screen  204 A is used for filtering particulates such that such particulates are not produced into an inner bore of the completion string. 
   Also depicted in  FIG. 2A  are shunt conduits  206  and  208 , where the shunt conduits can be shunt tubes in some embodiments. The shunt tubes  206 ,  208  are positioned between the outer shroud  205 A and screen  204 A. The shunt tubes  206 ,  208  are used to carry gravel slurry to provide for better gravel packing. Although not depicted in  FIGS. 2A and 2B , the shunt tubes  206  and  208  have side ports that allow for gravel slurry to exit the shunt tubes at discrete locations along the shunt conduits  206 ,  208 . In different implementations, different numbers of shunt tubes (one or more than two) can be used. 
   The shunt tubes  206 ,  208  are used to address the gravel bridging problem, in which gravel bridges are formed in an annulus region (between the completion string and wellbore surface) during a gravel packing operation. These gravel bridges block further flow of gravel slurry through the annulus region to prevent or reduce distribution of gravel past the bridge. Shunt conduits can be used to carry gravel slurry to bypass gravel bridges such that a good gravel fill can be provided throughout a wellbore interval. 
   As further depicted in  FIG. 2A , the shunt tubes  206 ,  208  pass through the connection sub  202  (inside the swellable element  104 ), such that the swellable element  104  extends around the shunt tubes  206 ,  208 . 
   The screen assembly  102 B includes similar components as the screen assembly  102 A, including outer shroud  205 B and screen  204 B. The shunt tubes  206 ,  208  extend through a region between the outer shroud  205 B and screen  204 B. 
     FIG. 2B  shows a state after gravel packing has been performed such that a target annulus region between the completion string and the inner surface of the wellbore is filled with a gravel pack. Also,  FIG. 2B  shows the swellable element  104  in its swelled state to provide zonal isolation between different zones. 
     FIG. 3  provides a partial longitudinal sectional view of a section of the completion string portion depicted in  FIGS. 2A-2B .  FIG. 4  is a cross-sectional view of a section of the completion string that includes the swellable element  104 . As depicted in  FIGS. 3 and 4 , the connection sub  202  includes an inner pipe portion  302  (or inner mandrel) that defines an inner axial bore  304  through the connection sub  202 . The connection sub  202  also has an outer shell or sleeve  306  that surrounds the pipe portion  302 . The swellable element  104  is mounted on the outer surface of the outer shell  306 . The shunt tubes  206 ,  208  are positioned between the outer shell  306  and the pipe portion  302 . The tubing portion  302  and the outer shell  306  define an annular path  308  through the connection sub  202  to allow for the shunt tubes  206 ,  208  to pass through the connection sub  202 . 
   The connection sub  202  has a first connector  310  to connect the connection sub  202  to the first screen assembly  102 A, and a second connector  312  to connect the connection sub  202  to the second screen assembly  102 B. 
   The pipe portion  302  of the connection sub  202  is connected (such as threadably connected) to pipe portions  320 A and  320 B of the screen assemblies  102 A and  102 B, respectively. The inner bores of the pipe portions  302 ,  320 A, and  320 B are axially aligned to permit a continuous axial flow of fluid through the completion string. 
   A variant of the connection sub ( 202 A) is depicted in  FIGS. 5 and 6 .  FIG. 5  is a partial longitudinal sectional view of a section of the completion string portion depicted in  FIG. 2A , and  FIG. 6  is a cross-sectional view of a portion of the connection sub  202 A. The connection sub  202 A does not include an outer sleeve or shell, as in the  FIG. 4  embodiment. Instead, the swellable element  104 A in  FIG. 5  is attached to the outer surface of the tubing portion (or inner mandrel)  302 . The swellable element  104 A defines axial paths  402  through which shunt tubes  206 ,  208  can extend. 
   In another implementation, instead of running the shunt tubes  206 ,  208  through the swellable element  104 A, it is noted that the axial paths  402  through the swellable element  104 A can form part of the shunt conduit; in other words, the axial paths  402  in the swellable element  104 A are in fluid communication with the inner bores of the shunt tubes  206 ,  208  so that the axial paths and shunt tubes collectively form the shunt conduits. In such an implementation, the shunt tubes  206 ,  208  are inserted partially into the axial paths  402  of the swellable element  104 A. 
   In some embodiments, as depicted in  FIG. 7 , a valve  502  can be provided in the shunt tube  206 ,  208 . The valve  502  when opened allows for gravel slurry to flow through an inner bore  504  of the shunt tube  206 ,  208 . When closed, the valve  502  blocks the communication of fluid through the bore  504  of the shunt tube  206 ,  208 . The valve  502  can be closed after the gravel packing operation to prevent fluid communication between different zones of the well. Actuation of the valve  502  can be accomplished by moving a shifting tool  506  inside the completion string, where the shifting tool mechanically interacts with the valve  502  to open or close the valve  502 . 
   In operation, a completion string including the components depicted in  FIG. 1  is run into the wellbore  100 , with the swellable elements  104  in a retracted position such that a radial clearance is provided between the swellable elements  104  and the inner surface  106  of the wellbore  100 . When the position of the completion string is set, the gravel packing operation can proceed. Gravel slurry is pumped from the earth surface, either down the inner bore of the completion string or through an upper annulus region between the completion string and the wellbore  100 . The gravel slurry flows through a cross-over device (not shown) to allow for the gravel slurry to enter a target annulus region  114  ( FIG. 1 ) that is to be gravel packed. If gravel bridges were to form, gravel slurry can flow inside the shunt tubes  206 ,  208  to fill voids in the target annulus region  114  caused by the gravel bridges. Once the gravel packing operation is complete, the swellable elements  104  are allowed to swell using a chemical swelling process. The swelling can take a relatively long time, such as on the order of hours, days, or even weeks. In a different implementation, the swelling can be performed quickly. Once the swellable elements  104  engage the inner surface  106  of the wellbore  100 , zonal isolation is accomplished. 
   A benefit of using the swellable elements  104  in the completion string is that swelling of the swellable elements  104  can be accomplished without using mechanical actuation elements. The presence of mechanical actuation elements is undesirable due to the presence of the shunt tubes. 
   Since the swellable elements  104  are in their retracted state during the gravel packing operation, the multiple zones of the target annulus region  114  can be gravel packed with the same gravel packing treatment; in other words, multiple treatments of multiple corresponding zones can be avoided. Also, there is no leak-off facility along the length of each sealing element  104  so that the gravel slurry is not dehydrated in the annulus segment  105  ( FIG. 1 ) between the sealing element  104  and the inner surface  106  of the wellbore  100 . This provides for clear segments (clear of gravel material) between the zones to be gravel packed so that the sealing elements  104  can expand in such segments  105  to seal against the inner surface  106  of the wellbore  100 . 
   Moreover, the outer diameter of each swellable element  104  can be increased to slightly larger than the surrounding screen assemblies during the gravel pack operation. The enlarged outer diameter of the sealing elements  104  allows for an increase in the local velocity of the gravel slurry around each swellable element to prevent gravel from dropping out of the carrier fluid in the corresponding annular segment  105  between the swellable element  104  and the wellbore surface  106 . 
   Note that optionally, a diverter (which can be in the form of a cup packer, for example) can be added to the top of (or otherwise proximate) the swellable packer nearest the toe of the well (the part of the well farthest away from the earth surface) to divert gravel slurry into the shunts and to avoid or reduce the chance of flowing slurry past or around the swellable packer nearest the toe of the well. 
   While the invention has been disclosed with respect to a limited number of embodiments, those skilled in the art, having the benefit of this disclosure, will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover such modifications and variations as fall within the true spirit and scope of the invention.

Summary:
A gravel pack apparatus for use in a wellbore includes a screen assembly to filter particulates, at least one shunt conduit to carry gravel slurry, and a swellable element around a portion of the at least one shunt conduit. The swellable element swells in response to an input stimulus and expands radially outwardly to seal against the wellbore.