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TECHNICAL FIELD 
     The present inventions relate to sand-control apparatus for use in subterranean wells, and in particular contemplate improved configurations for radially expandable sand-control screen jacket apparatus and methods of using the same. 
     BACKGROUND OF THE INVENTION 
     The control of the movement of sand and gravel into a well bore has been the subject of much importance in the oil production industry. The introduction of sand or gravel into the well bore commonly occurs under certain well conditions. The introduction of these materials into the well commonly causes problems including plugged formations or well tubing, and erosion of tubing and equipment. There have therefore been numerous attempts to prevent the introduction of sand and gravel into the production stream. 
     A common method to prevent the introduction of sand and gravel into the production stream has been a procedure known as gravel packing. In general, this involves placing selected sand or gravel into the annular space between the well bore and a base pipe introduced into the wellbore for that purpose. The base pipe contains perforations designed to allow well fluids to flow into the base pipe while excluding other material. A sand-control screen is commonly used in conjunction with a base pipe. An appropriately sized screen is commonly formed into a jacket and placed concentrically around the outside of the base pipe to prevent the entry of sand into the base pipe. Exemplary apparatus and methods of attaching a sand-control screen jacket to a base pipe are disclosed in U.S. Pat. No. 5,931,232, and in application Ser. No. 09/602,387, which are both assigned to this assignee and is incorporated herein for all purposes by this reference thereto. 
     One method of enhancing production in a well using a sand-control screen jacket includes causing the radial expansion of the base pipe and surrounding screen jacket by forcing a mechanical expansion tool through the base pipe. Examples of deformable well screens are disclosed in U.S. Pat. Nos. 5,901,789 and 6,012,522, to Donnelly. These methods use screens and methods to control the screen mesh opening size. 
     It is important that an expandable sand-control screen maintain its integrity throughout its entire length. Any voids in the screen that permit the entry of sand generally lead to erosion, which in turn allows the entry of more sand into the production stream. Wellbores with irregular sides or a particularly tortuous path, in order to allow sufficient clearance for insertion, generally require sand-control screen jackets having a diameter much smaller than the diameter of the wellbore. The radial expansion of a small diameter sand-control screen jacket into a much larger diameter wellbore can be particularly problematic. Extreme levels of expansion can lead to loss of integrity of the screen jacket connections with the base pipe. The result is the introduction of sand into the production stream and all of the attendant problems associated therewith. 
     Due to the aforementioned problems with the introduction of sand and gravel into the production stream, a need exists for apparatus and methods ensuring a radially expandable sand-control screen capable of withstanding a high degree of expansion while retaining sand-controlling connections with the base pipe. 
     SUMMARY OF THE INVENTION 
     The invention provides apparatus and related methods for constructing and using a radially expandable sand-control screen jacket assembly for use in a subterranean well. The screen-jacket assembly has at least two overlapping screen elements surrounding a base pipe, with a longitudinal sand-control seal between screen elements. The assembly maintains sand control after being radially expanded. 
     According to different aspects of the invention, the screen jacket assembly has two, three, four, or more overlapping screen elements. 
     According to another aspect of the invention, the assembly is encased in a screen shroud. 
     According to still another aspect of the invention, the inner screen element edges and outer screen element edges are substantially helical with respect to the to axis of the base pipe. 
     According to another aspect of the invention, the outer screen element is biased toward the inner screen element. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings are incorporated into and form a part of the specification to illustrate several examples of the present inventions. These drawings together with the description serve to explain the principals of the inventions. The drawings are only for the purpose of illustrating preferred and alternative examples of how the inventions can be made and used and are not to be construed as limiting the inventions to only the illustrated and described examples. The various advantages and features of the present inventions will be apparent from a consideration of the drawings in which: 
     FIG. 1 is a top perspective view of a radially expandable sand-control screen jacket; 
     FIG. 2 is a transverse cross-sectional view of the radially expandable sand-control screen jacket taken along line  2 — 2  of FIG. 1; 
     FIG. 3 is a transverse cross-sectional view of the radially expandable sand-control screen jacket of FIG. 2 in an expanded state; 
     FIG. 4 is a top perspective view of an alternative embodiment of a radially expandable sand-control screen jacket in an unexpanded state; and 
     FIG. 5 is a transverse cross-sectional view of an alternative embodiment of the inventions. 
    
    
     DETAILED DESCRIPTION 
     The present inventions are described by reference to drawings showing one or more examples of how the inventions can be made and used. In these drawings, reference characters are used throughout the several views to indicate like or corresponding parts. 
     In the description which follows, like or corresponding parts are marked throughout the specification and drawings with the same reference numerals, respectively. The drawings are not necessarily to scale and the proportions of certain parts have been exaggerated to better illustrate details and features of the invention. In the following description, the terms “upper,” “upward,” “lower,” “below,” “downhole”, “longitudinally” and the like, as used herein, shall mean in relation to the bottom, or furthest extent of, the surrounding wellbore even though the wellbore or portions of it may be deviated or horizontal. Correspondingly, the “transverse” orientation shall mean the orientation perpendicular to the longitudinal orientation. “Longitudinally moveable”, in particular, means movement with a longitudinal component, although a transverse component may be present as well. In the discussion which follows generally cylindrical well, pipe and tube components are assumed unless expressed otherwise. The term “sand-control” used herein means the exclusion of particles larger in cross section than a chosen size, whether sand, gravel, mineral, soil, organic matter, or a combination thereof. 
     Referring now to FIGS. 1-5, the general structure of a radially expandable sand-control apparatus  10  utilizing the present inventive concepts is shown. A base pipe  12  is connected to a pipe string (not shown) used in a subterranean well. The base pipe  12  has a plurality of perforations  14  through which fluids in the well enters the interior of the base pipe  12 . The number and configuration of the perforations  14  is not critical to the invention so long as a balance between fluid production and pipe integrity is maintained. The sand-control screen jacket assembly  16  concentrically surrounds the base pipe  12 . A screen shroud  18  with perforations  19  may surround the screen jacket assembly  16 . It is important to note that the exact ordering and number of components of the screen jacket assembly are not crucial to the invention except as to the features further discussed below. For example, or multiple screens with or without intervening layers of packing medium may be included. The screen shroud  18  may also be omitted or replaced by an additional screen. 
     Now referring primarily to FIG. 2, expandable sand-control apparatus  10  is shown in this cross-section view taken along line  2 - 2  of FIG.  1 . The screen jacket assembly  16  has an inner screen element  22  attached at its longitudinal edges  24  to the base pipe  12 . A base pipe joint  13  is formed at the point of attachment. The joint is preferably spot welded, or may be screwed, pinned, or on slidable channels, and is not necessarily sand-controlling. The inner screen element  22  and outer screen element  26  are preferably made of a screen, but may be made of other relatively low friction material such as “TEFLON”®, a registered trademark, or perforated sheet metal such as nickel or nickel alloy. 
     An outer screen element  26 , substantially encircles the inner screen element  22 . Longitudinal end portions  27  of outer screen element  26  at least partially overlap longitudinal end portions  29  of the inner screen element  22 . The outer screen element may optionally completely encircle the inner screen element. The outer screen element  26  is preferably manufactured with a bias toward closing its edges  28  in the direction of the inner screen  22 . The outer screen element edges  28  each have a seal element  30 . The seal element  30  is preferably made from an elastomeric material but may be made from wire or a hollow wire designed to collapse in operation and seal upon radial expansion of the sand-control apparatus  10  as further discussed below. The contact between the seal elements  30  and the inner screen element  22  forms sand-control joints  32  sufficiently close-fitting after expansion to exclude sand particles of a size to also be excluded by the screen elements  22 ,  26 , but not necessarily fluid tight. The joints  32  may or may not be sand-controlling prior to expansion. As the base pipe  12  and screen jacket assembly  16  are radially expanded, the overlapping portions  27 ,  29  of the joints  32  slide transversely with respect to one another while maintaining sand-controlling fits. 
     Now referring primarily to FIG. 3, in operation, the sand-control apparatus  10  is expanded in the conventional manner, typically with an expansion cone (not shown). The base pipe  12  expands, as do the inner screen element  22 , and the outer screen element  26 , and screen shroud  18 , if included. The components come to rest in the position drawn with seal element  30  in contact with inner screen element  22 . If a hollow wire seal element  30  is used, the hollow wire collapses or partially collapses and maintains a sand-controlling seal. As the base pipe  12  and screen jacket assembly  16  are radially expanded, the overlapping portions  27 ,  29  of the sand-controlling joints  32  slide transversely with respect to one another, maintaining their sand-controlling fits. It is not critical to the invention for the base joints  13  to maintain their attachment to the expanded base pipe  12 . The expanded sand-control apparatus  10  may be used with or without packing medium (not shown) in the annular space  36  between the expanded screen jacket assembly and the wellbore wall  15 . 
     Referring to FIG. 4, an alternative embodiment of the invention is shown wherein the inner and outer screen elements  22  and  26  are in helical form. In this embodiment, sand-control screen jacket assembly  16  surrounds base pipe  12 . The sand-control screen jacket assembly may include a screen shroud (not shown). Inner screen element  22  is in helical form and is affixed along longitudinal edges  24  to the base pipe  12  forming base pipe joints  13 . Outer screen element  26  substantially encircles the inner screen element  22  and is preferably spring-biased toward the inner element  22 . Outer screen edges  28  each have a seal element  30 . Longitudinal end portions  27  of the outer screen element  26  at least partially overlap the longitudinal end portions  29  of the inner screen element  22 . The contact between the seal elements  30  and the inner screen element  22  forms sand-control joints  32  sufficiently close-fitting after expansion to exclude sand particles of a size to also be excluded by the screen elements  22 ,  26 , but not necessarily fluid tight. The joints  32  may or may not be sand-controlling prior to expansion. As the base pipe  12  and screen jacket assembly  16  are radially expanded, the overlapping portions  27 ,  29  of the joints  32  slide transversely with respect to one another, maintaining their sand-controlling fits. 
     Referring primarily to FIG. 5, an alternative example of the invention is shown wherein the invention is embodied with two inner screens  22   a ,  22   b , and two outer screens  26   a ,  26   b . Seal elements  30  are included where the longitudinal edges  28  of the outer screens  26  contact the inner screens  22 . Also in this embodiment, the inner and outer screen elements  22 ,  26 , may be helical in form. The seal elements  30  are preferably made from an elastomeric material but may be made from a wire or hollow wire designed to collapse and seal upon radial expansion of the sand-control apparatus  10 . The contact between the seal elements  30  and the inner screen element  22  forms a sand-control joint  32  sufficiently close-fitting after expansion to exclude sand particles of a size to also be excluded by the screen elements  22   a,b ,  26   a,b , but not necessarily fluid tight. The joints  32  may or may not be sand-controlling prior to expansion. As the base pipe  12  and screen jacket assembly  16  are radially expanded, the overlapping portions  27 ,  29  of the joints  32  slide transversely with respect to one another, maintaining their sand-controlling fits. The outer screen elements  26   a ,  26   b  are preferably manufactured with a spring-bias toward closing their longitudinal edges  28  in the direction of inner screens  22   a  and  22   b . The screen elements may be made from materials known in the art for downhole screens. 
     The embodiments shown and described above are only exemplary. Many details are often found in the art such as: seal element or screen size, configurations and materials. Therefore, many such details are neither shown nor described. It is not claimed that all of the details, parts, elements, or steps described and shown were invented herein. Even though numerous characteristics and advantages of the present inventions have been set forth in the foregoing description, together with details of the structure and function of the inventions, the disclosure is illustrative only, and changes may be made in the detail, especially in matters of shape, size and arrangement of the parts within the principles of the inventions to the full extent indicated by the broad general meaning of the terms used in the attached claims. 
     The restrictive description and drawings of the specific examples above do not point out what an infringement of this patent would be, but are to provide at least one explanation of how to make and use the inventions. The limits of the inventions and the bounds of the patent protection are measured by and defined in the following claims.

Summary:
Disclosed are apparatus and methods for sand-control in a subterranean well. A radially expandable sand-control screen jacket apparatus and related methods are disclosed. The apparatus has a screen jacket assembly surrounding a base pipe and having at least one inner screen element contacting the base pipe and at least one outer screen element overlapping the adjacent inner screen element. The joints between the inner and outer screen elements comprise sand-controlling joints.