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CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to provisional application Ser. No. 60/917,749 filed May 14, 2007 (Attorney&#39;s Docket No. 68.0693). 
     
    
     BACKGROUND 
       [0002]    A variety of well screens are used in many well related applications. In fluid production applications, for example, well screens are used to filter particulates that would otherwise damage production pumps and related equipment. Generally, the filter medium is disposed around a base pipe having openings through which the desired production fluid is introduced into a fluid flow path within the base pipe. The filter medium may comprise a one or more mesh layers sized to filter the unwanted particulates. 
         [0003]    During production, hydrocarbons passing through the filter medium require an open, non-restrictive flow path between the filter medium and the base pipe to facilitate fluid movement to the base pipe perforations. Such an open, non-restrictive flow path is provided by a spacer layer, sometimes referred to as a drainage layer. In various production operations, the production drawdown can be sufficient to collapse the filter medium onto the drainage layer. The collapsed filter medium is extruded into the drainage layer, thus closing off the drainage flow path and creating “hot spots” directly above base pipe perforations. 
         [0004]    Additionally, the drainage layers, whether interior or exterior of the filter medium, require clearances to facilitate assembly and this compromises the performance of the filter medium under mechanical loads such as burst or collapse loads. Existing drainage layers also have posed other significant problems whether deployed along interior or exterior regions of the filter medium. For example, drainage layers typically are made from heavier gauge wire that can create many handling problems during installation of the drainage layer into the well screen. Additionally, the available drainage layers provide little protection for the filter medium and can even cause damage to the filter medium if not properly constructed and handled relative to the filter medium. Current drainage layers also fail to provide sufficient collapse and burst resistance. 
       SUMMARY 
       [0005]    In general, the present invention provides a system and method for creating a strong, easy-to-handle well screen in which a diffusion layer is affixed to a filter medium to create a coherent structure. In one example, the filtering medium is created from a wire mesh selected to filter particulates of a specific size. The corresponding diffusion layer is formed as a wire structure with its individual wires having a cross-section substantially larger than that of any wire contained within the filtering medium. The diffusion layer is affixed to the filtering medium along a filtering medium surface to create a coherent structure having great collapse and burst resistance. One method of affixing comprises bonding the diffusion layer to the filtering medium via a sintering process. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    Certain embodiments of the invention will hereafter be described with reference to the accompanying drawings, wherein like reference numerals denote like elements, and: 
           [0007]      FIG. 1  is a cross-sectional view of a section of a well screen, according to an embodiment of the present invention; 
           [0008]      FIG. 2  is a cross-sectional view of another embodiment of the well screen, according to an alternate embodiment of the present invention; 
           [0009]      FIG. 3  is a front elevation view of a well screen positioned in a wellbore and showing partially broken away sections of well screen, according to an embodiment of the present invention; 
           [0010]      FIG. 4  is an illustration of a portion of a diffusion layer used in the well screen, according to an embodiment of the present invention; 
           [0011]      FIG. 5  is an illustration of a portion of another embodiment of the diffusion layer used in the well screen, according to an alternate embodiment of the present invention; 
           [0012]      FIG. 6  is an illustration of a portion of another embodiment of the diffusion layer used in the well screen, according to an alternate embodiment of the present invention; 
           [0013]      FIG. 7  is an illustration of a portion of another embodiment of the diffusion layer used in the well screen, according to an alternate embodiment of the present invention; 
           [0014]      FIG. 8  is a cross-sectional view of the diffusion layer bonded to a filtering medium to create a coherent structure, according to an embodiment of the present invention; 
           [0015]      FIG. 9  is a cross-sectional view similar to  FIG. 8  but showing diffusion layer wire having a different cross-section, according to an embodiment of the present invention; 
           [0016]      FIG. 10  is a cross-sectional view similar to  FIG. 8  but showing diffusion layer wire having a different cross-section, according to an embodiment of the present invention; and 
           [0017]      FIG. 11  is a cross-sectional view similar to  FIG. 8  but showing diffusion layer wire having a different cross-section, according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    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. 
         [0019]    The present invention generally relates to a well screen system utilized in a wellbore. The well screen system comprises a filtering medium to filter particulates and one or more diffusion layers providing a lateral flow pore geometry that reduces pressure drop when deployed along a surface of the filtering medium. The diffusion layer is able to diffuse, i.e. suppress, the fluid velocity entering the filtering medium by maintaining pore geometry and open area under mechanical loads. Lower velocities reduce the potential for erosion. 
         [0020]    The diffusion layer is bonded to the filtering medium to create a coherent structure that provides strong structural integrity and great collapse and burst resistance. The diffusion layer may be bonded to the filtering medium along its surface by, for example, sintering. In addition to improving the structural integrity and collapse/burst resistance of the filtering medium, the coherent structure also provides for easier handling and assembly into the overall well screen system. The coherent structure does not allow the filtering medium to be punctured, for example, which could cause premature failure. The coherent structure removes the need to design clearances into the well screen for assembly purposes which, in turn, minimizes or eliminates the occurrence of ridgelines or crimps in the filtering medium under collapse conditions. Additionally, the attached diffusion layer or layers can facilitate insertion of the coherent structure along adjacent tubulars, such as internal base pipes or external shrouds. 
         [0021]    Referring generally to  FIG. 1 , a well screen  30  is illustrated as comprising a filtering medium  32  constructed to filter particulates of a selected size. The filtering medium  32  may be formed as a mesh having one or more layers  34  formed of wire  36 . Well screen  30  also comprises a diffusion layer  38  positioned along a surface of filtering medium  32 . In the embodiment illustrated in  FIG. 1 , diffusion layer  38  is positioned along an interior surface  40  of filtering medium  32 . Furthermore, diffusion layer  38  is affixed to filtering medium  32  to create a coherent structure  42 . For example, diffusion layer  38  may comprise wire  44  that is sintered or otherwise bonded to filtering medium  32  along interior surface  40  throughout all or a substantial portion of the filtering medium. 
         [0022]    The coherent structure  42  has great strength, and the bonding of diffusion layer  38  to filtering medium  32  along all or a substantial portion of filtering medium  32  greatly increases both the collapse and burst resistance of the filtering medium. The diffusion layer  38  basically provides a space between the filtering medium and an adjacent tubular member. In the embodiment of  FIG. 1 , for example, the diffusion layer  38  is disposed between filtering medium  32  and an internal base pipe  46 . Base pipe  46  has a plurality of openings  48  which may be positioned to extend generally radially through a tubular wall  50  that defines the base pipe. The openings  48  may be, for example, directly below filtering medium  32  or spaced from the filtering medium. Diffusion layer  38  and filtering medium  32  also are tubular in shape and circumferentially disposed about base pipe  46 . The space created by diffusion layer  38  between filtering medium  32  and base pipe  46  accommodates fluid flow in a variety of directions along the filtering medium and the base pipe. Accordingly, if regions of the filtering medium  32  become plugged or blocked, the fluid flowing through other parts of the filtering medium can flow along diffusion layer  38  and enter an interior  52  of base pipe  46  through openings  48  positioned radially below the blocked portion of the filtering medium. The use of diffusion layer  38  is thus able to facilitate flow into base pipe  46  even if a region or regions of the filtering medium  32  become plugged with sand or other particulates. 
         [0023]    Referring generally to  FIG. 2 , another embodiment of well screen  30  is illustrated. In this embodiment, a second diffusion layer  54  is positioned adjacent an exterior surface  56  of filtering medium  32  such that second diffusion layer  54  is circumferentially disposed about filtering medium  32 . Second diffusion layer  54  also may be affixed to filtering medium  32  to create a coherent structure  42  having the beneficial properties described above. Diffusion layer  38  and the second diffusion layer  54  can both be bonded to filtering medium  32 , or coherent structure  42  may be formed with only filtering medium  32  and the second or external diffusion layer  54 . In the embodiment illustrated, second diffusion layer  54  is positioned between filtering medium  32  and an outlying tubular member  58 , such as a shroud. Diffusion layer  38  and/or second diffusion layer  54  are bonded, e.g. sintered, to filtering medium  32  at multiple contact regions  60  across filtering medium surfaces  40  and/or  56 , respectively. 
         [0024]    The formation of the filtering medium and the one or more diffusion layers into coherent structure  42  facilitates the construction and handling of the filtering medium and diffusion layer or layers. However, affixing the diffusion layer to the filtering medium also reduces or illuminates friction and/or snagging of the diffusion layer with respect to adjacent tubular members, such as external shroud  58 . Formation of coherent structure  42  also can minimize the outside diameter of the overall well screen product. These characteristics further enhance the ability to easily construct a variety of well screens  30 . 
         [0025]    The exact structure of filtering medium  32  and diffusion layers  38 ,  54  can vary from one application to another. In  FIG. 3 , for example, one embodiment of well screen assembly  30  is illustrated as deployed in a wellbore  62  as part of an overall completion assembly  64 . Wellbore  62  is drilled into a geological formation  66  that contains, for example, desirable production fluids, such as petroleum or other fluids. A portion of the well screen assembly  30  is illustrated as broken away to expose diffusion layer  38  deployed between base pipe  46  and filtering medium  32 . In this embodiment, diffusion layer  38  is formed by wire  44  arranged in a crisscross pattern  68 . The crisscross pattern  68  may have wire  44  woven in warp and weft directions with the wire running in the warp and weft directions having either the same or dissimilar diameters, respectively. Generally, the warp direction or warp wire is the continuous wire dispersed from a spool, and the weft direction or weft wire is the shoot wire or cross wire that extends across the warp wire. The number of wires can differ in warp as compared to weft directions. More wires in the circumferential orientation than the axial orientation increases hoop strength and/or burst resistance in, for example, the outer diffusion layer. The use of larger circumferential wires relative to axial wires also increases hoop strength and/or burst resistance. 
         [0026]    Referring to  FIG. 3 , the embodiment of filtering medium  32  is illustrated as having a plurality of layers  34  formed as a wire mesh. By way of example, each mesh layer may be formed of wire having similar diameters. However, the mesh layers  34  also can be formed of wire having dissimilar diameters and dissimilarly sized openings, such as small openings  70  of one mesh layer and larger openings  72  of another mesh layer of filtering medium  32 . The one or more layers of filtering medium  32  cooperate to filter particulates of a desired size before those particulates can move into the interior of well screen  30 . 
         [0027]    The diffusion layer  38  is a non-filtering layer designed to provide structural support while allowing the free flow of fluid. The crisscross pattern  68  of either diffusion layer has substantially larger openings  74  formed by the crisscrossing wire  44 . Additionally, the wire  44  is a structural wire that supports filtering medium  32  when the diffusion layer  38  is affixed to the filtering medium by, for example, sintering. Generally, the smallest wire utilized in forming the diffusion layers is at least two times larger in cross-section than the largest wire used in forming the mesh layers of filtering medium  32 . By way of example, the diffusion layer wires have a diameter two to four times greater than the diameter of the largest wire diameter found in the filtering medium  32 . 
         [0028]    Diffusion layer  38  and/or diffusion layer  54  can be constructed in a variety of configurations. One configuration that works well is a twill herringbone configuration or pattern. Many types of applications can utilize a coarse woven configuration; however other wire patterns can be used. Additionally, structural materials other than wire also can be used in constructing each diffusion layer. Examples of diffusion layers having crisscross pattern  68  formed into a woven structure are illustrated in  FIGS. 4-7 . 
         [0029]    In  FIG. 4 , for example, diffusion layer  38 ,  54  has wire  44  formed into a square, plain weave pattern. One specific example of a generally square weave pattern is illustrated in  FIG. 5  as a single crimp weave pattern. Another alternate woven form is a double crimp weave in which the warp and weft wire sections have intermediate crimps  76  disposed between pairs of cross wires, as illustrated in  FIG. 6 . Another drainage layer embodiment utilizes a lock crimp weave pattern, as illustrated in  FIG. 7 . In this latter embodiment, the crisscrossed wire is pre-crimped in both the warp and weft directions to securely lock the wires together. A variety of other woven and non-woven patterns can be used in forming the structural diffusion layers  38  and/or  54 . Regardless, the configuration of the diffusion layer enables bonding, e.g. sintering, of the diffusion layer to the filtering medium  32  at the multiple contact regions  60  along the filtering medium. 
         [0030]    The structural integrity of the coherent structure  42  can be further enhanced by creating greater surface area at the contact regions  60  to enhance the bonding between the diffusion layer  38 ,  54  and the filtering medium  32 . For example, greater surface area enables the creation of a stronger bond when the filtering medium and the diffusion layer are sintered together. One way of creating greater surface area is to form flat surface areas at contact regions  60 . For example, the wire  44  used to create the diffusion layer, e.g. diffusion layer  38 , can be formed with a flat surface or flat side  78 , as illustrated in  FIG. 8 . The flat surface  78  is formed such that it is oriented toward filtering medium  32  when the diffusion layer and filtering medium are joined. 
         [0031]    The flat surface  78  can naturally be created by selecting wire  44  having a cross-section with at least one flat surface oriented in the desired direction. For example, the diffusion layer  38 ,  54  can be formed with wire  44  having a generally rectangular, e.g. square, cross-section  80 , as illustrated in  FIG. 9 . The wire  44  also can be selected with other cross-sectional configurations. In  FIG. 10 , for example, wire  44  is illustrated as having a triangular cross-section  82 . The wire  44  also may have a hexagonal cross-section  84 , as illustrated in  FIG. 11 , as well as a variety of other cross-sectional configurations that provide a flat side having greater surface area for bonding. The filtering medium  32  also may use wires of one shape running in a first direction and wires of a different shape running in a cross direction. The flat side of the wires can be used not only to facilitate bonding but also to affect flow area, flow characteristics, and mechanical characteristics. 
         [0032]    The structure of the filtering medium as well as the diffusion layer or layers can be adjusted according to the desired production parameters and/or wellbore environment. The filtering medium and diffusion layer are readily formed as a coherent structure through sintering, however other techniques can be utilized in affixing the filtering medium and the one or more diffusion layers. Additionally, the coherent structure  42  can be used in a variety of well screens and with a variety of completion assemblies in fluid production and other types of well related operations. 
         [0033]    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. Such modifications are intended to be included within the scope of this invention as defined in the claims.

Summary:
A technique is provided for creating a well screen having a diffusion layer affixed to a filter medium to create a coherent structure. The diffusion layer is formed as a structure that freely allows movement of fluid, while the filtering medium is designed to filter particulates of a specific size. The diffusion layer is affixed to the filtering medium along a filtering medium surface to greatly improve collapse and burst resistance of the filtering medium. One method of affixing comprises bonding the diffusion layer to the filtering medium via a sintering process.