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TECHNICAL FIELD 
       [0001]    This description relates to well screen assemblies for use in subterranean wellbores. 
       BACKGROUND 
       [0002]    For centuries, wells have been drilled to extract oil, natural gas, water, and other fluids from subterranean formations. In extracting the fluids, a production string is provided in a wellbore, both reinforcing the structural integrity of the wellbore, as well as assisting in extraction of fluids from the well. To allow fluids to flow into production string, apertures are often provided in the tubing string in the section of the string corresponding with production zones of the well. Although perforations allow for ingress of the desired fluids from the formation, these perforations can also allow unwanted materials to flow into the well from the surrounding foundations during production. Debris, such as formation sand and other particulate, can fall or be swept into the tubing together with formation fluid, contaminating the recovered fluid. Not only do sand and other particulates contaminate the recovered fluid, this particulate can cause many additional problems for the well operator. For example, as the particulate flows through production equipment, it gradually erodes the equipment. Unwanted particulate can block flow passages, accumulate in chambers, and abrade components. Repairing and replacing production equipment damaged by particulate in-flow can be exceedingly costly and time-consuming, particularly for downhole equipment sometimes located several thousand feet below the earth&#39;s surface. Consequently, to guard against particulate from entering production equipment, while at the same time preserving sufficient fluid flow pathways, various production filters and filtration methods have been developed and employed including gravel packs and well screen assemblies. 
         [0003]    A number of well screen filtration designs have been employed. A well screen assembly is a screen of one or more layers installed in the well, capable of filtering against passage of particulate of a specified size and larger, such as sand, rock fragments and gravel from surrounding gravel packing. The specific design of the well screen can take into account the type of subterranean formation likely to be encountered, as well as the well-type, well screen. 
       SUMMARY 
       [0004]    An aspect encompasses a well screen assembly having an elongate base pipe and a shroud layer about the base pipe. A mesh layer resides between the shroud layer and the base pipe. A portion of the mesh layer overlaps another portion of the mesh layer to form all area of overlap. A spine resides proximate substantially an entire length of the area of overlap and transmitting a force from the shroud layer to the mesh layer that compresses and seals the area of overlap against passage of particulate. 
         [0005]    An aspect encompasses a well screen assembly having a base pipe and an inner filtration layer with all overlap formed by overlapping ends of the filtration layer. An over layer is wrapped on top of the filtration layer and has a rib substantially aligned with and compressing the overlap against the base pipe along the length of the overlap. 
         [0006]    An aspect encompasses a method for sealing a mesh layer carried on a base pipe. A portion of the mesh layer overlaps another portion of the mesh layer to form an area of overlap. In the method a force is applied to a rib aligned with at least a portion of the area of overlap and the area of overlap is sealed against passage of particulate with the rib. 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0007]      FIG. 1A  is a side cross-sectional view of an example well system including well screen assemblies. 
           [0008]      FIG. 1B  is a side cross-sectional view of an example well screen assembly. 
           [0009]      FIG. 2A  is an axial cross-sectional view of one implementation of a well screen assembly taken intermediate the ends of the well screen assembly. 
           [0010]      FIG. 2B  is a perspective view of the well screen assembly of  FIG. 2A  employing an axial spine and shown without a shroud layer. 
           [0011]      FIG. 2C  is a perspective view of an alternate implementation of the well screen assembly employing a non-axial spine shown without a shroud layer. 
           [0012]      FIG. 3  is an axial cross-sectional view of a second implementation of a well screen assembly taken intermediate the ends of the well screen assembly. 
           [0013]      FIGS. 4A-4C  illustrate the assembly of an example well screen. 
           [0014]      FIGS. 5A-5B  illustrate an example spine in uncompressed ( FIG. 5A ) and compressed ( FIG. 5B ) states. 
           [0015]      FIGS. 5C-5D  illustrate another example, C-shaped spine in uncompressed ( FIG. 5C ) and compressed ( FIG. 5D ) states. 
       
    
    
       [0016]    Like reference symbols in the various drawings indicate like elements. 
       DETAILED DESCRIPTION 
       [0017]    Various implementations of a well screen assembly are provided for filtering sediment and other particulates from entering tubing in a subterranean well. Some well screen implementations have a rigid outer shroud positioned over other filtration layers and components in the well screen. In addition to providing a protective layer over the more vulnerable filtration screen layers, the outer shroud can be used, in connection with a spine, to secure the filtration layers within the well screen assembly. The spine can be aligned with overlapping edges of a filtration layer, and is placed between the filtration layer and either the shroud layer or the base pipe of the well screen assembly. When the shroud layer is wrapped, or otherwise tightly placed around the filtration layer, spine, and base pipe, the spine compresses the overlap of the filtration layer pinching the overlap between the spine and either the inside of the shroud layer or outside of the base pipe. Compressing the overlap of the filtration layer secures the filtration layer within the well screen assembly and seals the overlap, so that particulates, otherwise filtered by the filtration layer, cannot enter the base pipe through the overlap. Using the spine to seal a filtration layer can simplify the well screen production process, among other benefits, while allowing a standoff to exist between the filter layer and the production tube, promoting axial flow paths within the assembly for more efficient fluid extraction in the base pipe. 
         [0018]      FIG. 1A  illustrates an example well system  10  including a plurality of well screen assemblies  12 . The well system  10  is shown as being a horizontal well, having a wellbore  14  that deviates to horizontal or substantially horizontal in the subterranean zone of interest  24 . A casing  16  is cemented in the vertical portion of the wellbore and coupled to a wellhead  18  at the surface  20 . The remainder of the wellbore  14  is completed open hole (i.e., without casing). A production string  22  extends from wellhead  18 , through the wellbore  14  and into the subterranean zone of interest  24 . A production packer  26  seals the annulus between the production string  22  and the casing  16 . The production string  22  operates in producing fluids (e.g., oil, gas, and/or other fluids) from the subterranean zone  24  to the surface  20 . The production string  22  includes one or more well screen assemblies  12  (two shown). In some instances, the annulus between the production string  22  and the open hole portion of the wellbore  14  may be packed with gravel and/or sand (hereinafter referred to as gravel packing  26  for convenience). The well screen assemblies  12  and gravel packing  26  allow communication of fluids between the production string  22  and subterranean zone  24 . The gravel packing  26  provides a first stage of filtration against passage of particulate and larger fragments of the formation to the production string  22 . The well screen assemblies provide a second stage of filtration, and are configured to filter against passage of particulate of a specified size and larger into the production string  22 . 
         [0019]    Although shown in the context of a horizontal well system  10 , well screen assemblies  12  call be provided in other well configurations, including vertical well systems having a vertical or substantial vertical wellbore, multi-lateral well systems having multiple wellbores deviating from a common wellbore and/or other well systems. Also, although described in a production context, well screen assemblies  12  can be used in other contexts, including injection, well treatment and/or other applications. 
         [0020]    As shown in the half side cross-sectional view of  FIG. 1B , a well screen assembly  12  includes a base pipe  100  that carries a layer  105  of one or more screens and a rigid outer shroud  110 . The outer shroud  110  protects the inner screen layers. 
         [0021]    An outer shroud layer  110  can include apertures  120  allowing fluid to flow to screen layers  105  and the base pipe  100 . The screen layers  105  can include at least one filtration layer  125  to filter against entry of particulate into the base pipe  100 . The base pipe  100  may also include apertures  130  allowing, fluids, filtered by filtration layer  125 , to enter the interior  135  of the base pipe  100 . 
         [0022]      FIG. 2A  is an axial cross-sectional view taken intermediate the ends of one implementation of a well screen assembly  200  that could be used as screen assembly  12  of  FIG. 1 . As shown in  FIG. 2A , well screen assembly  200  can include a rigid, tubular outer shroud layer  205  around a base pipe  210 . Between shroud layer  205  and base pipe  210  is at least one filtration layer  215 . Additional layers can be included. The filtration layer  215  is wrapped around the outside of base pipe  210 . Filtration layer  215  may be a filtration screen sheet, such as a sheet of wire mesh, composite mesh, plastic mesh, micro-perforated or sintered sheet metal or plastic sheeting, and/or any other sheet material capable of being used to form a tubular covering over a base pipe  210  and filter against passage of particulate larger than a specified size. A spine  220  can also be disposed between the filtration layer  215  and another layer. For example, the spine  220  can be disposed between the filtration layer  215  and the outer shroud  205 , between the filtration layer  215  and base pipe  210  as shown in  FIG. 2A , between the filtration layer  215  and another layer, and/or multiple spines  220  can be provided, each positioned between different layers. The spine  220  can traverse the entire axial length of the filtration layer  215 , and, in some cases, also the shroud  205 , well screen assembly  200 , and/or base pipe  210 . The spine  220  is positioned to correspond with an area of the filtration layer  215  where first  225  and second  230  ends of the filtration layer  215  overlap. The spine  220  is positioned at and along this overlap interface  235 , across the axial length of the filtration layer  215 . In some instances, the area of overlap  235 , as well as the spine  220 , will be purely longitudinal (or axial), in that it runs parallel to a central axis of the tubular well screen assembly  200 , such as illustrated in  FIG. 2B . 
         [0023]      FIGS. 2B and 2C  illustrate portions of example implementations of well screen assembly  200 , with spine  220 .  FIGS. 2B and 2C  provide views of well screen assembly  200  elements positioned inside the shroud layer  205 . In each instance, spine  220  is clamped between the tightly-wrapped shroud layer  205  and base pipe  210 , and applies force to overlapping edges of the filtration layer  215  to close and seal the overlapping edges together against passage of particulate. Additionally, a tightly clamped spine  220  may also serve to secure the filtration layer  215  within the well screen assembly  200 , between the shroud  205  and base pipe  210 .  FIG. 2B  illustrates a filtration layer  215  with an axial area of overlap  235 . The axial spine member  220  is positioned on top of, and aligned with area of overlap  235 .  FIG. 2C  illustrates an example implementation of well screen assembly  200  also with a spine  220  aligned with an area of overlap  235 . However, in  FIG. 2C , the area of overlap  235 , and consequently, the spine  220 , are non-axial. In this particular example, the area of overlap  235  and spine  220  exhibit a somewhat helical shape. Other filtration layer  215  products and designs, as well as wrapping methods, may result in other, non-axial overlap area  235  formations not illustrated, requiring coordinating, non-axial spines  220 . Accordingly, in other configurations, the spine  220  can be positioned at an acute angle, transverse and/or in another relationship to the axis of the well screen assembly  200 . Additionally, while the examples illustrated in  FIGS. 2B and 2C  show spine members  220  as a single piece, other implementations may provide for spines constructed of multiple pieces. Some or all of a multi-piece spine may be positioned with spine pieces end-to-end to effectively form a continuous spine, with spine pieces having overlapping areas to form a continuous spine, and/or with spine pieces in a non-continuous configuration. 
         [0024]    Spines  220 , used in connection with well screen assembly  200 , can take a wide variety of shapes, sizes, and material compositions. For instance, spine  220  can be relatively rigid member, such that the spine  220  is not deformed or insubstantially deformed when clamped between the tightly-wrapped shroud layer  205  and base pipe  210 . In other instances, spine  220  can be made to substantially elastically and/or plastically deform when clamped between the shroud layer  205  and base pipe  210 . Some example materials for spine  220  include a polymer (e.g., plastic, rubber and/or other polymers), metal, fiber reinforced composite and/or other materials. 
         [0025]    Returning to  FIG. 2A , an offset h call be provided, by virtue of the spine  220 , between the filtration layer  215  and another layer.  FIG. 2A  illustrates an offset h between the filtration layer  215  and the base pipe  210 . Providing an offset h can serve to form axial flow paths, allowing fluid filtered by filtration layer  215  to flow axially along the outside of base pipe  210  to any one of a plurality of apertures provided on the base pipe  210 . Providing axial flow paths within a well screen assembly  200  can provide better distribution of flow into the base pipe  210 . 
         [0026]    A spine  220  aligned with the overlap area  235  of a filtration layer  215  can be bonded to the filtration layer, for example at one of the ends  225 ,  230  of the filtration layer  215 , the exterior surface of the base pipe  210 , the interior surface of the shroud  205 , and/or another well screen assembly component to ease working with, aligning, and installing the spine  220 . For example, the spine  220  may be braised, welded, adhered with an adhesive and/or otherwise bonded to a component of the screen assembly. In other examples, the spine  220  may be a free member, unsecured to other well screen assembly components until the spine  220  is securely compressed between the shroud  205  and base pipe  210 . 
         [0027]    In still other examples, spine  220  may be integrated, built into or formed in another component, such as the base pipe  210 , shroud  205  and/or another layer.  FIG. 3  illustrates such an example.  FIG. 3  is an axial cross-sectional view of an alternate implementation of a well screen assembly  300  that could be used as screen assembly  12  of  FIG. 1 . The cross-section is taken intermediate the ends of the well screen assembly  300  and shows an integrated spine  305  formed in shroud  310  as a dimple running the axial length of at least a filtration layer  215  disposed within the assembly  300 . In this particular implementation, the spine  305  is formed by plastically deforming or molding the shroud  310  to form a spine  305  that can correlate with an overlap area of a filtration layer  215  included in the well screen assembly  300 . As in  FIGS. 2B and 2C , an integrated spine  305  can be purely longitudinal or axial in shape and orientation, be non-axial, helical, or any other configuration. Additionally, while spine  300  is shown as a longitudinal dimple in a shroud layer  305  in  FIG. 3 , the spine  305  may instead be a solid, protruding rib formed on the interior surface of the shroud  310  (or even the outer surface of the base pipe  210 ). In certain instances, the spine  220  may be a welded or brazed bead deposited on the surface of a component of the screen assembly. 
         [0028]    In certain instances, dimple  305  can be formed in the shroud layer  310  after the shroud layer has been placed around other well screen assembly components, such as a filtration layer  215  with an area of overlap. Accordingly, in some examples, the dimple  305  can be formed with the shroud  310 , filtration layer  215 , and base pipe  210  in place in the assembly  300 . Forming the spine  305  in this manner call allow the spine to be specifically formed to accord with how and where the overlap area  235  has ended up after overlapping filtration layer ends  225 ,  230 , including requisite depth of the dimple, given placement of the base pipe  210 , relative the shroud  305 . 
         [0029]      FIGS. 4A-4C  illustrate a sequence for constructing a well screen assembly  400  employing a spine  405 . As illustrated in  FIG. 4A , a filtration layer  410  can be cut to desired dimensions from one or more sheets of mesh material, such that the sheet can be formed into a tubular screen capable of covering the exterior surface  415  of base pipe  420 . If the design calls for standoff between the base pipe  420  and screen layer  410 , the sheet  410  can be similarly trimmed so as to provide for a tubular filtration screen with a larger diameter. 
         [0030]    Turning to  FIG. 4B , with filtration screen sheet  410  cut to proper dimensions, the sheet  410  can be wrapped around the exterior surface  415  of the base pipe  420 . Sheet ends  420 ,  425  overlap to form a strip of overlapping area  435  running the axial length of the sheet. The sheet so wrapped forms a tubular filtration layer  410 . With the overlapping area  435  in place, it may be desirable to temporarily bind the ends  425 ,  430  so as to easily align spine  440  with the determined area of overlap  430 . Additionally, as described above, spine  440  may also first be bonded to the surface of filtration layer  410 , for example at one of ends  425 ,  430 . In some examples, assembly may include bonding spine  440  instead to an interior surface of a shroud layer or other layer placed around filtration layer  410 , or the outside surface  415  of base pipe  420 . In any event, spine  440  is to be aligned with area of overlap  435 . 
         [0031]      FIG. 4C  illustrates the placement of an outer shroud  445 , around the filtration layer  410  and spine  440 . In one instance, the outer shroud may be formed from a sheet and wrapped tightly around the filtration layer and spine, then welded to enclose the sheet into a tubular shroud  445 . In other examples, base pipe  420 , carrying filtration layer  410  and spine  440 , can be passed into a pre-fabricated, tubular shroud  445  to complete installation of the well screen assembly  400 . To complete assembly, the axial ends of the well screen assembly, including both the shroud  445  and filtration layer  410 , may need to be sealed or capped, so as to prevent sediment or fluid from leaking to or from the axial ends of the assembly  400 . In certain instances, the axial ends of the shroud  445  are crimped and welded to the base pipe  420 . 
         [0032]    In some instances, compression of the spine can result in deformation of the spine.  FIG. 5A  illustrates a detailed front view of a spine  500 , positioned between overlapping layer ends  505 ,  510  of a filtration screen layer  515  and base pipe  520 . Prior to placement of an outer shroud layer, the cross section of the spine  500 , can be circular, as illustrated in this example.  FIG. 5B  illustrates the effect of tightly wrapping an outer shroud layer  525  around the spine  500 , filtration layer  515 , and base pipe  520 . As illustrated, spine  500  is compressed, so that the circular cross-section of the spine  500  appears oval-shaped. In its compressed state, a wider area of spine  500  is in contact with screen layer  515 . This contact and resulting radial force, translated to the overlapping layer ends  505 ,  510  through spine  500 , creates a seal  530  along the longitudinal length of the spine  500 . Such a seal blocks particulate from entering the seam of the overlapping ends that would otherwise be blocked by the filtration screen&#39;s apertures. 
         [0033]    While the example of  FIGS. 5A and 5B  illustrated a spine  500  with a circular cross section, other spine cross-sections can be employed to enhance or otherwise customize performance of the seal  530  created by spine  500 . One such example, as illustrated in  FIG. 5C , can include a spine  500  with a C-shaped cross-section, shown prior to compression. Upon being compressed, as shown in  FIG. 5D , C-shaped spine  500  can elastically collapse to securely press the filtration layer ends  505 ,  510  against the inner surface  535  of a shroud layer  525  to form seal  530 . Other spine cross-sectional geometries are also within the scope of the present description, including a hollow circular or O-shaped cross section, triangular cross-sections, flat or rectangular cross-sections and/or other geometries. 
         [0034]    A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.

Summary:
A well screen assembly includes an elongate base pipe, a shroud layer about the base pipe, and a mesh layer between the shroud layer and the base pipe. A portion of the mesh layer overlaps another position of the mesh layer to form an area of overlap. A spine is positioned proximate substantially an entire length of lie area of overlap, and transmits a force from the shroud layer to the mesh layer that compresses and seals the area of overlap against passage of particulate.