In the field of oil and gas exploration and production, various tools are used to provide barriers in the wellbore which prevent or restrict the fluid flow.  A wellbore packer provides a seal in the annular space between two tubing strings, or between an outer casing and an open hole.  A packer may be run with a completion string to a downhole location, and may be inflated or expanded into contact with the outer casing or open hole.  The packer may be designed to create a complete fluid seal capable of withholding a differential pressure on either side of the packer, thereby isolating one portion of the annulus from another.  Alternatively, the packer may simply provide an annular barrier, to prevent or restrict flow of fluids and/or solid particles in the annulus.  Packers may for example be run on completion strings, specialised mandrels, coiled tubing, wireline and slickline tools. 
Conventional packers are activated by mechanical or hydraulic systems.  More recently, packers have been developed which include a mantle of swellable elastomeric material formed around a tubular body.  The swellable elastomer is selected to increase in volume on exposure to a triggering fluid, which may be a hydrocarbon fluid or an aqueous fluid or brine.  Alternatively, the elastomer may be selected to increase in volume on exposure to another triggering mechanism, such as heat or pressure.  The packer is run to a downhole location in its unexpanded state, where it is exposed to a triggering fluid and caused to expand.  The design, dimensions and swelling characteristics are chosen such that the swellable mantle increases in volume to create an annular barrier and/or a fluid seal in the annulus.  Swellable packers have several advantages over conventional packers including passive actuation, simplicity of construction, and robustness in long term isolation applications.  Examples of swellable packers and suitable materials are described in GB 2411918. 
One application of a wellbore packer is as an isolation device in a multi-zone completion system.  An example of a multi-zone completion system is shown in FIG.  1.  The system, generally shown at 100, includes a production facility at surface, which in this case is a floating production storage and offloading (FPSO) vessel 102, coupled to a well 104 via subsea tree 106.  The wellbore in this case is an inclined wellbore which extends through multiple production intervals 107a, 107b, 107c in the formation 108.  The production tubing 110 provides a continuous flow path which penetrates through the multiple zones.  The production tubing is provided with ports or inflow control devices (not shown) which allow production fluid to flow into the production tubing and out to the subsea tree 106.  However, in order to provide control over the production process, the annulus 112 is sealed by packers 114 between the different production zones 107 to prevent fluid flowing in the annulus between the different zones. 
Depending on the formation, the production tubing may be provided with sand control devices 116, to prevent solid particles from the formation entering the production tubing.  The sand control devices 116 may for example be any suitable sand screen system, including expandable screen systems.  The sand control devices may be used in conjunction with one or more gravel packs 118, which comprise gravel or other particulate matter around the sand control device to improve filtration and to provide additional support to the formation.  Gravel packing requires a good distribution of gravel in the annulus at the sand control device.  To improve the delivery of gravel, sand control devices have been provided with shunt tubes, which create alternate flow paths for the gravel and its carrier fluid.  These alternate flow paths significantly improve the distribution of gravel in the production interval, for example by allowing the carrier fluid and gravel to be delivered through sand bridges that may be formed in the annulus before the gravel pack has been completed. 
FIGS.  2A and 2B are schematic views of examples of sand screens provided with shunt tubes in a completion system 200.  A first sand control device 202a is coupled to a second sand control device 202b, and each comprise base pipes 204 joined to define a production bore 206.  Screens 208 including filter media surround the base pipe 204 and are supported by ribs 210.  The apparatus is provided with shunt tubes 212, which in this example are steel tubes having substantially rectangular cross-section.  The shunt tubes 212 are supported on the exterior of the screen and provide a flow path 213 alternate to the main production bore 206.  Jumper tubes 211 are used to provide fluid communication between shunt tubes of adjacent sand control devices.  The shunt tubes 212 maintain a flow path 213, even if the annular space 214 is bridged, for example by a loss of integrity in a part of the formation 216.  Examples of shunt tube arrangements can be found in U.S. Pat.  No.  4,945,991 and U.S. Pat.  No.  5,113,935.  The shunt tubes may also be internal to the filter media, as described in U.S. Pat.  No.  5,515,915 and U.S. Pat.  No.  6,227,303. 
Use of alternate path screen systems creates difficulties in wellbore isolation.  In particular, alternate paths prevent the use of conventional wellbore packers to isolate multiple production zones.  It is proposed in WO 2007/092082 and WO 2007/092083 to provide packers with alternate path mechanisms which may be used to provide zonal isolation between gravel packs in a well.  The packers described may include individual jumper tubes over a common manifold or manifold region that provides fluid communication through the packer to shunt tubes of sand control devices.  Embodiments described in WO 2007/092082 and WO 2007/092083 include packers with swellable mantles which increase in volume on exposure to a triggering fluid. 
However, WO 2007/092082 and WO 2007/092083 do not fully address the complexities of providing fluid barriers and/or fluid isolation using swellable elastomer systems.  For example, WO 2007/092082 and WO 2007/092083 are concerned with providing a continuous flow path, but do not address the problems of maintaining the required annular barrier or fluid seal functions of the packer with the provision of the secondary flow path through the apparatus.  Such problems may arise due to removal of a volume of elastomer from the isolation device, improper sealing around the conduits, displacement of the conduits due to expansion of the element, and/or coupling of the conduits at opposing ends of the isolation device. 
In particular, the arrangements proposed in these WO 2007/092082 and WO 2007/092083 necessitate a reduction in the overall volume of the expanding element, and in particular a reduction in the volume of the expanding element which is radially outward of the conduit.  An arrangement with individual jumper tubes requires the jumper tubes to be aligned with the shunt tubes of the adjacent sand control devices.  WO 2007/092082 discloses an outer diameter of expanding element which is significantly below the outer diameter of adjacent sand control devices.  This configuration would limit the swelling performance from a swellable mantle as it provides minimal mantle thickness.  It is possible that at its fully swollen state it would not contact the internal diameter of the drilled wellbore.  In addition, configuring a swellable elastomer well packer to achieve a seal at a fully swollen condition may mean extremely long or impractical sealing times and marginal pressure sealing performance if the swellable mantle did manage to contact the wellbore. 
The arrangement which comprises a manifold would also be inefficient in finding a nominal balance of swellable mantle thickness.  The arrangement requires the outer diameter of the sleeve defining the manifold to extend beyond the radial position of the shunt tubes such that the sleeve has an outer diameter equivalent to the outer diameter of adjacent sand control devices.  This has the effect of reducing the volume of the expanding element which may be positioned on the outside of the conduit.  This may compromise the integrity of the seal provided by the expanding element and/or increase the time to seal.  Alternatively, if the volume of the expanding element is to be maintained, the run-in diameter of the expanding element is increased beyond the diameter of the shunt tubes, and the swellable mantle is be the largest tool diametrically within a sand control string.  This limits swelling performance and can impact on the success of deployment operations.  It is desirable for the packer outer diameter to be small during run-in to avoid contact with obstructions, for example ledges or washout zones.  When using swellable elastomer materials, they may begin to expand as they contact drilling or wellbore fluids during run-in to the desired position in the wellbore. 
It is therefore an object of the invention to provide an apparatus in the form of an isolation device, packer and/or annular barrier and method of use which overcome or mitigate at least one drawback or deficiency of previously proposed apparatus and methods. 
It is a further object of the invention to provide a wellbore completion and/or production system or method of use which incorporates such an apparatus or method. 
It is a further object of the invention to provide an apparatus or method which is an alternative to the method or apparatus described in the prior art. 
Further aims and objects of the invention will become apparent from the reading of the following description.