Abstract:
A sand control screen assembly ( 200 ) positionable within a production interval of a wellbore that traverses a subterranean hydrocarbon bearing formation comprises a base pipe ( 202 ) having openings ( 204 ) in a sidewall section thereof that allow fluid flow therethrough. A filter medium ( 210 ) is positioned about the exterior of at least a portion of the base pipe ( 202 ). The filter medium ( 210 ) selectively allows fluid flow therethrough but prevents the flow of particulate of a predetermined size therethrough. A seal member ( 218, 220, 222 ) is operably associated with the base pipe ( 202 ). The seal member ( 218, 220, 222 ) has a one-way valve configuration and a valve open configuration such that the seal member ( 218, 220, 222 ) controls fluid flow through the openings ( 204 ) of the base pipe ( 202 ).

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS  
       [0001]    This application is a continuation-in-part application of co-pending application Ser. No. 10/057,042 filed Jan. 25, 2002 entitled Sand Control Screen Assembly and Treatment Method Using the Same and a continuation-in-part application of co-pending application Ser. No. 10/293,721 filed Nov. 13, 2002 entitled Sand Control Screen Assembly and Treatment Method Using the Same. 
     
    
     
       TECHNICAL FIELD OF THE INVENTION  
         [0002]    This invention relates, in general, to sand control and fluid loss prevention and, in particular, to a sand control screen assembly having a seal member that prevents fluid loss from the interior to the exterior of the sand control screen assembly following a treatment process performed within a production interval.  
         BACKGROUND OF THE INVENTION  
         [0003]    It is well known in the subterranean well drilling and completion art that relatively fine particulate materials may be produced during the production of hydrocarbons from a well that traverses an unconsolidated or loosely consolidated formation. Numerous problems may occur as a result of the production of such particulate. For example, the particulate causes abrasive wear to components within the well, such as tubing, pumps and valves. In addition, the particulate may partially or fully clog the well creating the need for an expensive workover. Also, if the particulate matter is produced to the surface, it must be removed from the hydrocarbon fluids using surface processing equipment.  
           [0004]    One method for preventing the production of such particulate material is to gravel pack the well adjacent to the unconsolidated or loosely consolidated production interval. In a typical gravel pack completion, a sand control screen is lowered into the wellbore on a work string to a position proximate the desired production interval. A fluid slurry including a liquid carrier and a relatively coarse particulate material, such as sand, gravel or proppants which are typically sized and graded and which are typically referred to herein as gravel, is then pumped down the work string and into the well annulus formed between the sand control screen and the perforated well casing or open hole production zone.  
           [0005]    The liquid carrier either flows into the formation or returns to the surface by flowing through a wash pipe or both. In either case, the gravel is deposited around the sand control screen to form the gravel pack, which is highly permeable to the flow of hydrocarbon fluids but blocks the flow of the fine particulate materials carried in the hydrocarbon fluids. As such, gravel packs can successfully prevent the problems associated with the production of these particulate materials from the formation.  
           [0006]    It has been found, however, that following a gravel packing operation, the fluid inside the sand control screen tends to leak off into the adjacent formation. This leak off not only results in the loss of the relatively expensive fluid into the formation, but may also result in damage to the gravel pack around the sand control screen and the formation by, for example, fracturing a formation when it is not desirable to fracture that formation. This fluid leak off is particularly problematic in cases where multiple production intervals within a single wellbore require gravel packing as the fluid remains in communication with the various formations for an extended period of time.  
           [0007]    In other cases, it may be desirable to perform a formation fracturing and propping operation prior to or simultaneously with the gravel packing operation. Hydraulic fracturing of a hydrocarbon formation is sometimes necessary to increase the permeability of the formation adjacent the wellbore. According to conventional practice, a fracture fluid such as water, oil, oil/water emulsion, gelled water or gelled oil is pumped down the work string with sufficient volume and pressure to open multiple fractures in the production interval. The fracture fluid may carry a suitable propping agent, such as sand, gravel or proppants, which are typically referred to herein as proppants, into the fractures for the purpose of holding the fractures open following the fracturing operation.  
           [0008]    The fracture fluid must be forced into the formation at a flow rate great enough to fracture the formation allowing the entrained proppants to enter the fractures and prop the formation structures apart, producing channels which will create highly conductive paths reaching out into the production interval, and thereby increasing the reservoir permeability in the fracture region. As such, the success of the fracture operation is dependent upon the ability to inject large volumes of hydraulic fracture fluid along the entire length of the formation at a high pressure and at a high flow rate.  
           [0009]    It has been found, however, that it is difficult to fracture multiple formations traversed by the wellbore that are within a relatively close proximity of one another. This difficulty is the result of the complexity and length of the permanent downhole tools and the associated service tools used to perform the fracture operation. Accordingly, if formations are closer together than the axial length required for the permanent downhole tools and service tool, then certain of the formations cannot be isolated for individual treatment processes.  
           [0010]    Therefore, a need has arisen for an apparatus and a treatment method that provide for the treatment of multiple formations that are located relatively close to one another by allowing the use of relatively simple and compact permanent downhole tools and service tools. A need has also arisen for an apparatus and a treatment method that allow for the gravel packing of one or more production intervals while preventing fluid loss into adjacent formations.  
         SUMMARY OF THE INVENTION  
         [0011]    The present invention disclosed herein comprises a sand control screen assembly and method for treating multiple formations traversed by a wellbore. The sand control screen assembly of the present invention provides for the treatment of relatively closely spaced formations by allowing the use of relatively simple and compact permanent downhole tools and service tools. In addition, the sand control screen assembly of the present invention prevents undesirable fluid loss from the interior thereof to an adjacent formation.  
           [0012]    The sand control screen assembly comprises a base pipe having a plurality of openings that allow fluid flow therethrough. A filter medium is positioned about the exterior of at least a portion of the base pipe. The filter medium selectively allows fluid flow therethrough and prevents particulate flow of a predetermined size therethrough. A seal member is operably associated with the base pipe. The seal member has a one-way valve configuration and a valve open configuration, thereby controlling the fluid flow through the openings of the base pipe. In the one-way valve configuration, the seal member prevents fluid loss from the interior to the exterior of the sand control screen assembly and allows fluid flow from the exterior to the interior of the sand control screen assembly when the differential pressure between the exterior and the interior of the sand control screen assembly exceeds a predetermined threshold. In the valve open configuration, the seal member allows fluid flow from the interior to the exterior of the sand control screen assembly and from the exterior to the interior of the sand control screen assembly.  
           [0013]    In one embodiment, the seal member includes a spring retainer, a biasing member and a shuttle valve. In this embodiment, when the seal member is in the one-way valve configuration, the spring retainer is in a first position relative to the base pipe such that the biasing member urges the shuttle valve into a sealing position. In the first position, the spring retainer may be releasably secured to the base pipe with a plurality of shear pins. When the seal member is in the valve open configuration, the spring retainer is in a second position relative to the base pipe such that the biasing member does not urge the shuttle valve into the sealing position. In the second position, the spring retainer may be secured to the base pipe with a plurality of collet fingers. The spring retainer may be operated from the first position to the second position by the application of a tubing pressure within the base pipe.  
           [0014]    When the seal member is in the one-way valve configuration, the shuttle valve has a sealing position and a non sealing position. When the seal member is in the valve open configuration, the shuttle valve has a disabled position. When the shuttle valve is in the disabled position, the shuttle valve may be secured to the base pipe with a keeper ring. The shuttle valve may be operated to the disabled position in response to a differential pressure above a predetermined threshold between the exterior and the interior of the sand control screen assembly. Alternatively, the shuttle valve may be operated to the disabled position by mechanically shifting the shuttle valve relative to the base pipe.  
           [0015]    In another aspect of the present invention, a downhole treatment method comprises locating a sand control screen assembly within a production interval of a wellbore, pumping a treatment fluid into the production interval, allowing fluid returns to enter the interior of the sand control screen assembly with a seal member of the sand control screen assembly in a one-way valve configuration, preventing fluid loss from the interior to the exterior of the sand control screen assembly with the seal member in the one-way valve configuration, operating the seal member from the one-way valve configuration to a valve open configuration and allowing production fluids to enter the interior of the sand control screen assembly.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    For a more complete understanding of the features and advantages of the present invention, reference is now made to the detailed description of the invention along with the accompanying figures in which corresponding numerals in the different figures refer to corresponding parts and in which:  
         [0017]    [0017]FIG. 1 is a schematic illustration of an offshore oil and gas platform operating a pair of sand control screen assemblies of the present invention;  
         [0018]    [0018]FIG. 2 is a partial cut away view of a sand control screen assembly of the present invention having a seal member disposed within a base pipe;  
         [0019]    FIGS.  3 A- 3 D are cross sectional views of a sand control screen assembly of the present invention having a seal member comprising a plurality of one-way valves;  
         [0020]    [0020]FIG. 4 is a cross sectional view of an alternate embodiment of the sand control screen assembly of the present invention wherein the seal member comprises a plurality of plugs;  
         [0021]    [0021]FIG. 5 is a cross sectional view of an alternate embodiment of a sand control screen assembly of the present invention wherein the seal member comprises a sliding sleeve;  
         [0022]    FIGS.  6 A- 6 B are cross sectional views of an alternate embodiment of a sand control screen assembly of the present invention wherein the seal member comprises a sliding sleeve;  
         [0023]    FIGS.  7 A- 7 B are cross sectional views of an alternate embodiment of a sand control screen assembly of the present invention wherein the seal member comprises a sliding sleeve;  
         [0024]    [0024]FIG. 8 is a front plan view of the internal structure of an alternate embodiment of a sand control screen assembly of the present invention wherein the seal member comprises a sliding sleeve;  
         [0025]    FIGS.  9 A- 9 D are cross sectional views of the embodiment of the sand control screen assembly of FIG. 8 in various positions;  
         [0026]    [0026]FIG. 10 is a half sectional view of a downhole production environment including a pair of sand control screen assemblies of the present invention before a downhole treatment process;  
         [0027]    [0027]FIG. 11 is a half sectional view of a downhole production environment including a pair of sand control screen assemblies of the present invention during a first phase of a downhole treatment process;  
         [0028]    [0028]FIG. 12 is a half sectional view of a downhole production environment including a pair of sand control screen assemblies of the present invention during a second phase of a downhole treatment process;  
         [0029]    [0029]FIG. 13 is a half sectional view of a downhole production environment including a pair of sand control screen assemblies of the present invention during a third phase of a downhole treatment process;  
         [0030]    [0030]FIG. 14 is a half sectional view of a downhole production environment including a pair of sand control screen assemblies of the present invention during a fourth phase of a downhole treatment process;  
         [0031]    [0031]FIG. 15 is a half sectional view of a downhole production environment including a pair of sand control screen assemblies of the present invention during a fifth phase of a downhole treatment process;  
         [0032]    [0032]FIG. 16 is a half sectional view of a downhole production environment including a pair of sand control screen assemblies of the present invention during a sixth phase of a downhole treatment process;  
         [0033]    [0033]FIG. 17 is a half sectional view of a downhole production environment including a pair of sand control screen assemblies of the present invention during an seventh phase of a downhole treatment process;  
         [0034]    [0034]FIG. 18 is a half sectional view of a downhole production environment including a pair of sand control screen assemblies of the present invention during a eighth phase of a downhole treatment process;  
         [0035]    [0035]FIG. 19 is a half sectional view of a downhole production environment including a pair of sand control screen assemblies of the present invention before a downhole treatment process;  
         [0036]    [0036]FIG. 20 is a half sectional view of a downhole production environment including a pair of sand control screen assemblies of the present invention during a first phase of a downhole treatment process;  
         [0037]    [0037]FIG. 21 is a half sectional view of a downhole production environment including a pair of sand control screen assemblies of the present invention during a second phase of a downhole treatment process; and  
         [0038]    [0038]FIG. 22 is a half sectional view of a downhole production environment including a pair of sand control screen assemblies of the present invention during a third phase of a downhole treatment process.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0039]    While the making and using of various embodiments of the present invention are discussed in detail below, it should be appreciated that the present invention provides many applicable inventive concepts which can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the invention, and do not delimit the scope of the present invention.  
         [0040]    Referring initially to FIG. 1, a pair of sand control screen assemblies used during the treatment of multiple intervals of a wellbore in a single trip and operating from an offshore oil and gas platform is schematically illustrated and generally designated  10 . A semi-submersible platform  12  is centered over a pair of submerged oil and gas formations  14 ,  16  located below a sea floor  18 . A subsea conduit  20  extends from a deck  22  of the platform  12  to a wellhead installation  24  including blowout preventers  26 . Platform  12  has a hoisting apparatus  28  and a derrick  30  for raising and lowering pipe strings such as a work string  32 .  
         [0041]    A wellbore  34  extends through the various earth strata including formations  14 ,  16 . A casing  36  is cemented within wellbore  34  by cement  38 . Work string  32  includes various tools such as a sand control screen  40  which is positioned within production interval  44  between packers  46 ,  48  and adjacent to formation  14  and sand control screen  42  which is positioned within production interval  50  between packers  52 ,  54  and adjacent to formation  16 . Thereafter, a treatment fluid containing sand, gravel, proppants or the like is pumped down work string  32  such that formations  14 ,  16  may be sequentially treated.  
         [0042]    Even though FIG. 1 depicts a vertical well, it should be noted by one skilled in the art that the sand control screen assemblies of the present invention are equally well-suited for use in wells having other directional orientations such as deviated wells, inclined wells or horizontal wells. Also, even though FIG. 1 depicts an offshore operation, it should be noted by one skilled in the art that the sand control screen assemblies of the present invention are equally well-suited for use in onshore operations. Also, even though FIG. 1 depicts two formations, it should be understood by one skilled in the art that the treatment processes of the present invention are equally well-suited for use with any number of formations.  
         [0043]    Referring now to FIG. 2 therein is depicted a more detailed illustration of a sand control screen assembly of the present invention, such as, for example, sand control screen assembly  40  of FIG. 1. Sand control screen assembly  40  includes a base pipe  56  that has a plurality of openings  58  which allow the flow of production fluids into sand control screen assembly  40 . The exact number, size and shape of openings  58  are not critical to the present invention, so long as sufficient area is provided for fluid production and the integrity of base pipe  56  is maintained.  
         [0044]    Spaced around base pipe  56  is a plurality of ribs  60 . Ribs  60  are generally symmetrically distributed about the axis of base pipe  56 . Ribs  60  are depicted as having a cylindrical cross section, however, it should be understood by one skilled in the art that ribs  60  may alternatively have a rectangular or triangular cross section or other suitable geometry. Additionally, it should be understood by one skilled in the art that the exact number of ribs  60  will be dependant upon the diameter of base pipe  56  as well as other design characteristics that are well known in the art. Wrapped around ribs  60  is a screen wire  62 . Screen wire  62  forms a plurality of turns, such as turn  64  and turn  66 . Between each of the turns is a gap through which formation fluids flow. The number of turns and the gap between the turns are determined based upon the characteristics of the formation from which fluid is being produced and the size of the gravel to be used during the gravel packing operation. Together, ribs  60  and screen wire  62  may form a sand control screen jacket which is attached to base pipe  56  by welding or other suitable techniques.  
         [0045]    A one-way valve  70  is disposed within each opening  58  of base pipe  56  to prevent fluid flow from the interior to the exterior of the sand control screen assembly  40 . One-way valves  70  may be referred to collectively as a seal member  68 . Preferably, one-way valves  70  are mounted within openings  58  by threading, stamping or other suitable technique. Ball and seat type one-way valves have been found to be suitable, however, other types of one-way valves may also be used including poppet valves, sleeve valves and the like. One-way valves  70  prevent fluid flow from the interior to the exterior of sand control screen assembly  40  and are actuatable to allow fluid flow from the exterior to the interior of sand control screen assembly  40 . Accordingly, when one-way valves  70  are used within base pipe  56  of sand control screen assembly  40  during production, production fluids are allowed to flow through sand control screen assembly  40  through one-way valves  70 .  
         [0046]    Referring now to FIG. 3A, therein is depicted a sand control screen assembly that is generally designated  40 A. Sand control screen assembly  40 A is substantially identical to sand control screen assembly  40  described above as sand control screen assembly  40 A includes base pipe  56  that has a plurality of openings  58 , a plurality of ribs (not pictured) and a screen wire  62 . Together, the ribs and screen wire  62  form a sand control screen jacket that is attached using connectors  69  to base pipe  56  by welding or other suitable techniques.  
         [0047]    One-way valves  70 A are disposed within each opening  58  of base pipe  56  to prevent fluid flow from the interior to the exterior of the sand control screen assembly  40 A. One-way valves  70 A may be referred to collectively as a seal member  68 . Preferably, one-way valves  70 A are flush mounted within openings  58  by threading, stamping or other suitable technique. One-way valves  70 A prevent fluid flow from the interior to the exterior of sand control screen assembly  40 A and are actuatable to allow fluid flow from the exterior to the interior of sand control screen assembly  40 A. Accordingly, when one-way valves  70 A are used within base pipe  56  of sand control screen assembly  40 A during production, production fluids are allowed to flow through sand control screen assembly  40 A through one-way valves  70 A.  
         [0048]    Following the downhole treatment precesses discussed in detail below wherein fluid flow from the interior to the exterior of sand control screen assembly  40 A is prevented, the ability to flow fluids from the interior to the exterior of sand control screen assembly  40 A may be desirable, for example, to perform an acid treatment. Accordingly, one-way valves  70 A may be designed to lock out or be rendered inoperable under certain conditions such that one-way valves  70 A no longer prevent fluid flow from the interior to the exterior of sand control screen assembly  40 A. In such cases, after one-way valves  70 A have been operated into the lock out position, fluid flow is allowed from the exterior to the interior and from the interior to the exterior of sand control screen assembly  40 A. One method of locking out one-way valves  70 A is to expose one-way valves  70 A to a differential pressure above a predetermined threshold.  
         [0049]    Referring now to FIG. 3B, therein is depicted a sand control screen assembly that is generally designated  40 B. Sand control screen assembly  40 B is substantially similar to sand control screen assembly  40 A described above as sand control screen assembly  40 B includes base pipe  56  that has a plurality of openings  58 , a plurality of ribs (not pictured) and a screen wire  62 . Together, the ribs and screen wire  62  form a sand control screen jacket that is attached using connectors  69  to base pipe  56  by welding or other suitable techniques.  
         [0050]    One-way valves  70 B are disposed within each opening  58  of base pipe  56  to prevent fluid flow from the interior to the exterior of the sand control screen assembly  40 B. One-way valves  70 B may be referred to collectively as a seal member  68 . Preferably, one-way valves  70 B are mounted within openings  58  by threading, stamping or other suitable technique. In the illustrated embodiment, one-way valves  70 B extend from openings  58  into base pipe  56 . Due to the thickness of the wall of base pipe  56 , it may be desirable to use one-way valves  70 B that are thicker than the wall of base pipe  56 . In this case, it has been found that one-way valves  70 B may extend into base pipe  56  and may reduce the inner diameter of base pipe  56  up to thirty percent without having a detrimental impact on the installation or operation of sand control screen assembly  40 B during treatment or production. Preferably, one-way valves  70 B may reduce the inner diameter of base pipe  56  between about ten and thirty percent.  
         [0051]    As an alternative and as depicted in FIG. 3C, one-way valves  70 C may be disposed within each opening  58  of base pipe  56  to prevent fluid flow from the interior to the exterior of the sand control screen assembly  40 C. One-way valves  70 C may be referred to collectively as a seal member  68 . Preferably, one-way valves  70 C are mounted within openings  58  by threading, stamping or other suitable technique. In the illustrated embodiment, one-way valves  70 C extend from openings  58  outwardly from base pipe  56  toward screen wire  62 . In his embodiment, the ribs (not pictured) must be positioned around base pipe  56  such that openings  58  may receive one-way valves  70 C that are thicker than the wall of base pipe  56 . In this configuration, base pipe  56  retains its full bore capabilities. Preferably, one-way valves  70 C may increase the outer diameter of base pipe  56  between about ten and thirty percent.  
         [0052]    As yet an alternative and as depicted in FIG. 3D, one-way valves  70 D may be disposed within each opening  58  of base pipe  56  to prevent fluid flow from the interior to the exterior of the sand control screen assembly  40 D. One-way valves  70 D may be referred to collectively as a seal member  68 . Preferably, one-way valves  70 D are mounted within openings  58  by threading, stamping or other suitable technique. In the illustrated embodiment, one-way valves  70 D extend inwardly and outwardly from openings  58  of base pipe  56 . In his embodiment, the ribs (not pictured) must be positioned around base pipe  56  such that openings  58  may receive one-way valves  70 D that are thicker than the wall of base pipe  56 . Preferably, one-way valves  70 D may increase the outer diameter of base pipe  56  between about ten and thirty percent and may reduce the inner diameter of base pipe  56  between about ten and thirty percent.  
         [0053]    Referring now to FIG. 4, therein is depicted an alternative embodiment of a sand control screen assembly that is generally designated  71 . Sand control screen assembly  71  includes base pipe  56  having a plurality of openings  58  with screen wire  62  wrapped therearound and attached to base pipe  56  with connectors  69 . Disposed within openings  58  of base pipe  56  are a plurality of plugs  72  that prevent fluid flow through openings  58  and serve as seal member  68  in this embodiment. Following the downhole treatment processes discussed in more detail below, plugs  72  are removed from openings  58  such that production fluids may flow to the interior of sand control screen assembly  71 .  
         [0054]    Plugs  72  may be any conventional plugs known or unknown in the art, including metal plugs, such as aluminum plugs, ceramic plugs or the like. The techniques used to remove plugs  72  will depend upon the construction of plugs  72 . If plugs  72  are formed from an acid reactive material such as aluminum, an acid treatment may be used to remove plugs  72 . The acid may be pumped into the interior of sand control screen assembly  71  where it will react with the reactive plugs, thereby chemically removing plugs  72 .  
         [0055]    Alternatively, regardless of the type of plug, plugs  72  may be mechanically removed. For example, a scraping mechanism may be used to physically contact plugs  72  and remove plugs  72  from the openings  58 . As another alternative, if plugs  72  are constructed from propellants, a combustion process may be used to remove plugs  72 . Likewise, if plugs  72  are constructed from friable materials such as ceramics, a vibration process, such as sonic vibrations may be used to remove plugs  72 . As a further alternative, plugs  72  may be removed by applying a preselected amount of differential pressure across plugs  72 .  
         [0056]    Referring now to FIG. 5, an alternative embodiment of a sand control screen assembly is illustrated and generally designated  73 . Sand control screen assembly  73  includes base pipe  56  having a plurality of openings  58  with screen wire  62  wrapped therearound. Disposed within base pipe  56  is a sleeve  74  having multiple ports  76  that serves as seal member  68  in this embodiment. When in a first position, ports  76  of sleeve  74  do not align with openings  58  of the base pipe  56 . When in a second position, ports  76  of sleeve  74  align with openings  58  of base pipe  56 . When sleeve  74  is in the first position, fluid flow from the exterior of sand control screen assembly  73  to the interior of sand control screen assembly  73  is prevented, as is fluid flow from the interior to the exterior of sand control screen assembly  73 . When sleeve  74  is in the second position, fluid flow from the exterior of sand control screen assembly  73  to the interior of the sand control screen assembly  73  is allowed, as is fluid flow from the interior to the exterior of sand control screen assembly  73 . Sleeve  74  can be displaced between the first position and second position by any conventional means such as axial displacement or rotational displacement. In an alternative embodiment, sleeve  74  can be a removable sleeve in which case ports  76  are not required.  
         [0057]    Referring now to FIGS.  6 A- 6 B, therein is depicted another embodiment of a sand control screen assembly of the present invention that is generally designated  132 . Sand control screen assembly  132  includes a base pipe  134  that has a non perforated section and a perforated section that includes a series of openings  136  that are circumferentially spaced therearound. Sand control screen assembly  132  has a pair of screen connectors  138 ,  140  that securably and sealingly attach a sand control screen  142  to base pipe  134 . Screen connectors  138 ,  140  may be attached to base pipe  134  by welding or other suitable technique. Sand control screen  142  may comprise a screen wire wrapped around a plurality of ribs as described above. Sand control screen  142  is disposed around the section of base pipe  134  that is not perforated.  
         [0058]    Screen connectors  138 ,  140  attach sand control screen  142  to base pipe  134  such that an annulus  144  is formed between sand control screen  142  and base pipe  134 . It should be noted that centralizers or other support members may be disposed within annulus  144  to support sand control screen  142  and maintain the standoff between sand control screen  142  and base pipe  134 . Screen connector  140  includes one or more fluid passageways  146 . Screen connector  140  also has an upper sealing surface  148 . Securably and sealingly coupled to the upper end of screen connector  140  is a housing member  150 . Housing member  150  forms an annulus  152  with base pipe  134  adjacent to openings  136  and is sealingly coupled to base pipe  134  at its upper end. Disposed within annulus  152  is an annular sliding sleeve  154  having a sealing surface  156  which is preferably made from a resilient material such as an elastomer or polymer. Also disposed within annulus  152  is a spiral wound compression spring  158  that downwardly biases sliding sleeve  154 .  
         [0059]    Together, spring  158 , sliding sleeve  154  and screen connector  140  form an annular one-way valve  160  that may be referred to as a seal member. One-way valve  160  prevents fluid flow from the interior to the exterior of sand control screen assembly  132 , as best seen in FIG. 6A, and is actuatable to allow fluid flow from the exterior to the interior of sand control screen assembly  132 , as best seen in FIG. 6B. For example, during a treatment process as described below wherein a treatment fluid is pumped into the interior of sand control screen assembly  132  and is discharged into the wellbore annulus above sand control screen assembly  132 , fluid flow from the interior to the exterior of sand control screen assembly  132  is prevented. Specifically, the bias force of spring  158  and the force created by differential pressure across sliding sleeve  154  between the interior and the exterior of sand control screen assembly  132  both act downwardly on sliding sleeve  154  such that sealing surface  156  sealingly engages sealing surface  148  of screen connector  140 , thereby preventing fluid flow from the interior to the exterior of sand control screen assembly  132 .  
         [0060]    During production, production fluids are allowed to flow from the exterior to the interior of sand control screen assembly  132  through a fluid flow path within sand control screen assembly  132 . Specifically, the fluid flows through sand control screen  142 , travels along base pipe  134  in annulus  144 , passes through fluid passageways  146  in screen connector  140  to unseat sliding sleeve  154  from sealing surface  148  of screen connector  140  by compressing spring  158 , then travels around sliding sleeve  154 , which may include a fluid bypass (not pictured), in annulus  152  and through openings  136 .  
         [0061]    Following the downhole treatment precesses discussed below wherein fluid flow from the interior to the exterior of sand control screen assembly  132  is prevented, the ability to flow fluids from the interior to the exterior of sand control screen assembly  132  may be desirable, for example, to perform an acid treatment. Accordingly, one-way valve  160  may be designed to lock out or be rendered inoperable under certain conditions such that one-way valve  160  no longer prevents fluid flow from the interior to the exterior of sand control screen assembly  132 . For example, in the illustrated embodiment, when a sufficient differential pressure is placed across sliding sleeve  154  between the interior and the exterior of sand control screen assembly  132 , a ceramic disk  161  in bypass passageway  159  may rupture to permanently open bypass passageway  159 . In such cases, after one-way valve  160  has been rendered inoperable, fluid flow is allowed from the exterior to the interior and from the interior to the exterior of sand control screen assembly  132 .  
         [0062]    Referring now to FIGS.  7 A- 7 B, therein is depicted another embodiment of a sand control screen assembly of the present invention that is generally designated  162 . Sand control screen assembly  162  includes a base pipe  164  that has a non perforated section and a perforated section that includes a series of openings  166  that are circumferentially spaced therearound. Sand control screen assembly  162  has a pair of screen connectors  168 ,  170  that securably and sealingly attach a sand control screen  172  to base pipe  164 . Screen connectors  168 ,  170  may be attached to base pipe  164  by welding or other suitable technique. Sand control screen  172  may comprise a screen wire wrapped around a plurality of ribs as described above. Sand control screen  172  is disposed around the section of base pipe  164  that is not perforated.  
         [0063]    Screen connectors  168 ,  170  attach sand control screen  172  to base pipe  164  such that an annulus  174  is formed between sand control screen  172  and base pipe  164 . Screen connector  170  includes one or more fluid passageways  176 . Securably and sealingly coupled to the upper end of screen connector  170  is a housing member  180 . Housing member  180  forms an annulus  182  with base pipe  164  adjacent to openings  166  and is sealingly coupled to base pipe  164  at its upper end. Disposed within annulus  182  is an annular sliding sleeve  184 . A seal  185  is positioned exteriorly of sliding sleeve  184  to provide a seal against the interior surface of housing member  180 . Likewise, a seal  186  is positioned interiorly of sliding sleeve  184  to provide a seal against the exterior surface of base pipe  164 . Preferably seals  185 ,  186  are made from a resilient material such as an elastomer or polymer. Also disposed within annulus  182  is a spiral wound compression spring  188  that downwardly biases sliding sleeve  184 .  
         [0064]    Together, spring  188 , sliding sleeve  184 , housing member  180  and base pipe  164  form an annular one-way valve  190  that may be referred to as a seal member. One-way valve  190  prevents fluid flow from the interior to the exterior of sand control screen assembly  162 , as best seen in FIG. 7A, and is actuatable to allow fluid flow from the exterior to the interior of sand control screen assembly  162 , as best seen in FIG. 7B. Specifically, during a treatment process as described below, a differential pressure force and spring  188  downwardly bias sliding sleeve  184  such that seal  185  is in sealing engagement with the interior surface of housing member  180  and seal  186  is in sealing engagement with the exterior surface of base pipe  164  which prevents fluid flow from the interior to the exterior of sand control screen assembly  162 . During production, production fluids are allowed to flow from the exterior to the interior of sand control screen assembly  182  by passing through sand control screen  172 , traveling along base pipe  164  in annulus  174 , passing through fluid passageways  176  in screen connector  170  to shift sliding sleeve  184  such that seal  186  is out of sealing engagement with base pipe  164  by compressing spring  188 , then traveling around sliding sleeve  184  in the radially reduced section of base pipe  164  and through openings  166 .  
         [0065]    Even though FIGS.  6 A- 7 B have been described as including annular sliding sleeves  154 ,  184 , it should be understood by those skilled in the art that the illustrated sliding sleeves  154 ,  184  could alternatively represent one or more pistons. For example, sliding sleeves  154 ,  184  could alternatively be one or more semi-annular pistons that are acted upon simultaneously by a single spiral wound compression spring. As a further example, sliding sleeves  154 ,  184  could alternatively be one or more rod type pistons each of which could be acted upon by a corresponding spring.  
         [0066]    Referring next to FIGS.  8 - 9 D in combination, various positions of another embodiment of a sand control screen assembly of the present invention are depicted with the positioned depicted in FIG. 8 corresponding to the position depicted in FIG. 9D. Sand control screen assembly  200  includes a base pipe  202  that has a series of openings  204  that are depicted as slots that are circumferentially spaced around base pipe  202 . Sand control screen assembly  200  has a pair of screen connectors  206 ,  208  that attach sand control screen  210  to base pipe  202 . Screen connectors  206 ,  208  may be attached to base pipe  202  by welding or other suitable technique. Sand control screen  210  may comprise any type of filter medium such as the depicted wire wrapped screen which allows the flow of formation fluids therethrough but which blocks the flow of particulate matter therethrough.  
         [0067]    Screen connectors  206 ,  208  attach sand control screen  210  to base pipe  202  such that an annulus  212  is formed between sand control screen  210  and base pipe  202 . Coupled to screen connector  206  is a housing member  214 . Housing member  214  forms an annulus  216  with base pipe  202  adjacent to openings  204 . Disposed within annulus  216  is an annular sleeve referred to as shuttle valve  218 , a biasing member  220  depicted as a spiral would compression spring and a spring retainer  222  having collet fingers  224 . Shuttle valve  218  has a pair of seals  226 ,  228  positioned on the interior thereof that provide a seal against sealing surface  230  of base pipe  202 . Shuttle valve  218  also has a seal  232  positioned on the exterior thereof that provides a seal against the interior of housing member  214 .  
         [0068]    Positioned between shuttle valve  218  and base pipe  202  is a keeper ring  234 . A plurality of pins  236  extend through openings  238  of shuttle valve  218  into slots  204 . Spring retainer  222  has a seal  240  positioned on the interior thereof that provide a seal against base pipe  202 . Spring retainer  222  also has a seal  242  positioned on the exterior thereof that provides a seal against the interior of housing member  214 . A plurality of shear pins  244  extend through openings  246  of spring retainer  222  and initially into a shear pin receiving groove  248  in the exterior surface of base pipe  202 . Base pipe  202  also has a mating profile  250  and a collet finger receiving groove  252 .  
         [0069]    The operation of sand control screen assembly  200  will now be described. FIG. 9A depicts sand control screen assembly  200  in its run-in position. Specifically, spring retainer  222  is secured to base pipe  202  with shear pins  244 . This causes spring  220  to downwardly bias shuttle valve  218  against screen connector  206 . In this position, a seal is created between shuttle valve  218  and sealing surface  230  of base pipe  202  by seals  226 ,  228 . In addition, a seal is created between shuttle valve  218  and the interior of housing member  214  by seal  232 . Once sand control screen assembly  200  is properly positioned downhole adjacent to a production interval, a treatment process such as a gravel pack, frac pack, fracture operation or the like may then take place.  
         [0070]    During the treatment operation, returns may be taken through sand control screen assembly  200 , as best seen in FIG. 9B. Specifically, spring retainer  222  remains secured to base pipe  202  with shear pins  244  allowing spring  220  to continue to downwardly bias shuttle valve  218 . The fluid pressure created by the returns that pass through sand control screen  210 , annulus  212  and axially oriented passageways  254  in screen connector  206 , however, upwardly biases shuttle valve  218  to unseat shuttle valve  218  allowing the returns to flow through annulus  216  and slots  204  into the interior of base pipe  202  for return to the surface. Once the treatment process is complete, the bias force of spring  220  will return shuttle valve  218  to the sealing position depicted in FIG. 9A. In this position, fluid loss from the interior to the exterior of sand control screen assembly  200  is prevented as a seal is created between shuttle valve  218  and sealing surface  230  of base pipe  202  by seals  226 ,  228  and a seal is created between shuttle valve  218  and the interior of housing member  214  by seal  232 . Accordingly, spring retainer  222 , spring  220 , shuttle valve  218 , housing member  214  and base pipe  202  form an annular one-way valve that may be referred to as a seal member.  
         [0071]    When it is desirable to commence production from the interval adjacent to sand control screen assembly  200 , sand control screen assembly  200  is operated to its production configuration, as best seen in FIG. 9C. First, a tubing pressure is applied within base pipe  202 . This pressure enters annulus  216  via slots  204  to act between spring retainer  222  and shuttle valve  218 . When the upwardly acting force on spring retainer  72  is sufficient, shear pins  244  will break which allows spring retainer  222  and spring  220  to move upwardly relative to base pipe  202  until collet fingers  224  engage collet finger receiving groove  252 . In this configuration, spring retainer  222  is prevented from further axial movement relative to base pipe  202 . In addition, spring  220  no longer applies a downward bias force against shuttle valve  218 .  
         [0072]    As best seen in FIG. 9D, once the tubing pressure is released, formation pressure acting on shuttle valve  218  will shift shuttle valve  218  axially upward until shuttle valve  218  contacts spring  220  which prevent further upward movement of shuttle valve  218 . In addition, as keeper ring  234  has engaged mating profile  250  of base pipe  202 , downward movement of shuttle valve  218  is also prevented. In this configuration, production fluid may flow into base pipe  202  through slots  204  uninhibited by shuttle valve  218 .  
         [0073]    To verify that shuttle valve  218  has moved sufficiently upwardly to allow the free flow of production fluids into base pipe  202  or to overcome any malfunctions of spring retainer  222  or shuttle valve  218 , sand control screen assembly  200  is equipped with pins  236  that extend from shuttle valve  218  into the interior of base pipe  202  through slots  214 . Pins  236  allow for a redundant mechanical lock out procedure of shuttle valve  218  using a tool that is run downhole on a conveyance such as a wireline. For example, a scraper tool may be run downhole such that it engages pins  236 . The scraper tool is then pulled back uphole to operate shuttle valve  218  to the position depicted in FIG. 9D. Alternatively, a sleeve having a profile could be positioned within base pipe  202  and coupled to shuttle valve  218  through slots  214 . A tool having the matching profile could then be run downhole to engage the sleeve and operate shuttle valve  218  to the position depicted in FIG. 9D.  
         [0074]    It should be understood by those skilled in the art that while FIGS.  2 - 9 D have depicted a wire wrapped sand control screen, other types of filter media could alternatively be used in conjunction with the apparatus of the present invention, including, but not limited to, a fluid-porous, particulate restricting material such as a plurality of layers of a wire mesh that are diffusion bonded or sintered together to form a porous wire mesh screen designed to allow fluid flow therethrough but prevent the flow of particulate materials of a predetermined size from passing therethrough.  
         [0075]    Referring now to FIG. 10, therein is schematically depicted an embodiment of the present invention that is used during fracturing and frac packing treatments. It should be clearly understood by those skilled in the art that any of the above-described sand control screen assemblies could be used during the treatment processes described below and the use of the particular embodiment depicted in the following figures is for convenience of illustration. As illustrated, sand control screen assembly  40  including one-way valves  70 , is positioned within casing  36  and is adjacent to formation  14 . Likewise, sand control screen assembly  42  including one-way valves  70 , is positioned within casing  36  and is adjacent to formation  16 . A service tool  78  is positioned within the work string  32 . As illustrated by the break between service tool  78  and sand control screen assemblies  40 , service tool  78  may be operably positioned several feet to several hundred feet uphole of sand control screen assembly  40 .  
         [0076]    To begin the completion process, production interval  44  adjacent to formation  14  is isolated. Packer  46  seals the near end of production interval  44  and packer  48  seals the far end of production interval  44 . Likewise, production interval  50  adjacent to formation  16  is isolated. Packer  52  seals the near end of production interval  50  and packer  54  seals the far end of production interval  50 . Additionally, seal element  88  is coupled to service tool  78 . Seal element  88  contacts the interior of work string  32  forming a seal, thereby preventing fluid flow into the annulus between work string  32  and service tool  78 . Work string  32  includes cross-over ports  90 ,  92  that provide a fluid communication path from the interior of work string  32  to production intervals  44 ,  50 , respectively. Preferably, fluid flow through cross-over ports  90 ,  92  is controlled by suitable valves that are opened and closed by conventional means.  
         [0077]    Referring now to FIG. 11, when the treatment operation is a frac pack, the objective is to enhance the permeability of the treated formation by delivering a fluid slurry containing proppants  96  at a high flow rate and in a large volume above the fracture gradient of the formation such that fractures may be formed within the formation  14  and held open by proppants  96 . In addition, a frac pack also has the objective of preventing the production of fines by packing production interval  44  with proppants  96 .  
         [0078]    In the initial phase of the treatment process of the present invention, the interior of sand control screen assemblies  40  is filled with a sand plug  96 A. This is achieved by pumping treatment fluid downhole such as a relatively low viscosity oil or water based liquid including a high concentration of solid agents such as sand, gravel or proppants, that will fall out of the slurry relatively easily to form sand plug  96 A. Sand plug  96 A improves the ability of one-way valves  70  of sand control screen assembly  40  to prevent fluid flow from the interior to the exterior of sand control screen assembly  40 . In addition, sand plug  96 A prevents sand control screen assembly  40  from seeing the pressure spike that typically occurs at the end of a fracture operation. Accordingly, it is preferred that sand plug  96 A extend past the near end of sand control screen assembly  40  as illustrated. It should be noted that this initial phase of the treatment process may not be necessary if sufficient solid agents fall out of the treatment fluids during the fracture or frac packing operations.  
         [0079]    Referring now to FIG. 12, once sand plug  96 A is deposited in sand control screen assembly  40 , the second phase of the treatment process may begin. The treatment fluid used during the second phase of the treatment process, which is the fracture operation, may be any appropriate fracturing fluid such as oil, water, an oil/water emulsion, gelled water or gelled oil based fracture fluid having a relatively high viscosity to enhance the fracturing process. This treatment fluid may or may not include solid agents such as sand, gravel or proppants but will usually have a lower concentration of solid agents than the treatment fluid of the first phase of the treatment process.  
         [0080]    In the illustrated embodiment, the treatment fluid of the second phase of the treatment process includes a low concentration of proppants indicated by reference character  96 B. The treatment fluid is pumped through service tool  78  and enters the near end of production interval  44  via cross-over ports  90 . As the treatment fluid is being continuously pumped at a high flow rate and in a large volume above the fracture gradient of formation  14  and as no returns are being taken, the treatment fluid fractures formation  14  as indicated by reference character  98 .  
         [0081]    Referring now to FIG. 13, prior to the point at which fractures  98  no longer propagate into formation  14 , the third phase of the treatment process begins. The treatment fluid used during this phase may be any suitable fluid such as oil, water, an oil/water emulsion, gelled water or gelled oil based fluid including a suitable solid agent such as gravel, sand or proppants. In this phase of the treatment process, the solid agents travel into the newly created fractures to prop the fractures open and create a path of high permeability back to wellbore  34 . In addition, the solid agents fill production interval  44  between sand control screen assembly  40  and casing  36  to form a gravel pack  96 C therein which filters particulate matter out of production fluids once production begins. Upon completion of the frac packing of production interval  44 , the valves associated with cross-over ports  90  are closed by conventional means.  
         [0082]    Referring now to FIG. 14, following completion of the first frac packing operation, service tool  78  is operably repositioned to frac pack formation  16 . As illustrated by the break between service tool  78  and sand control screen assembly  42 , the service tool  78  may be several feet to several hundred feet uphole of sand control screen assembly  42 . Once service tool  78  is positioned, a three-phase treatment process similar to that described above may begin.  
         [0083]    Referring now to FIG. 15, the low viscosity treatment fluid with a high concentration of solid agents is pumped into sand control screen assembly  42  to form sand plug  96 D. Fracture treatment fluid is then pumped through service tool  78 , as best seen in FIG. 16. The treatment fluid enters the near end of production interval  50  via cross-over ports  92 . In the illustrated embodiment the fracture fluid contains a low concentration of proppants indicated by  96 E. As the fracture fluid is being delivered at a high flow rate and in a large volume above the fracture gradient of formation  16  and as no returns are being taken, the fracture fluids fracture formation  16  as indicated by fractures  100 .  
         [0084]    Referring now to FIG. 17, toward the end of the fracture operation, the composition of the treatment fluid is changed to include a higher concentration of solid agents. These solid agents are used to prop fractures  100  in formation  16  and to form a gravel pack  96 F in production interval  50  between sand control screen assembly  42  and casing  32 . This three-phase treatment process can be repeated for any number of formations by repositioning service tool  78  sequentially uphole relative to each of the formations requiring treatment. Once all of the formations are treated and prior to beginning production, sand plugs  96 A,  96 D must be washed out of sand control screen assemblies  40 ,  42 . As seen in FIG. 18, service tool  78  may be used to wash out the sand control screen assemblies  40 ,  42  and work string  32 .  
         [0085]    To wash out sand control screen assemblies  40 ,  42 , liquid is delivered through service tool  78  to mix with the solid agents forming sand plugs  96 A,  96 D. The mixture is allowed to reverse out of work string  32  via the annulus between service tool  78  and work string  32  as indicated by arrows  105 . This process of circulating the solid agents to the surface and lowering service tool  78  farther into work string  32  continues until substantially all the solid agents in work string  32  have been removed.  
         [0086]    As explained above, different compositions of treatment fluids are used in the above described method during the different phases of the treatment process. Preferably, the first treatment fluid has a higher concentration of solid agents than the second treatment fluid. The first treatment fluid requires a higher concentration of solid agents as it is intended to place a sand plug in the sand control screen assemblies. The second treatment fluid does not require such solid agents as it is intended to fracture the formations. Additionally, the first treatment fluid preferably has a lower density and lower viscosity than the second treatment fluid. The lower density and lower viscosity in the first treatment fluid allow the solid agents to fall out of the slurry easily. The higher density and higher viscosity of the second treatment fluid allows the second treatment fluid to effectively fracture the formation.  
         [0087]    The third treatment fluid preferably has a higher concentration of solid agents than the second treatment fluid. The third treatment fluid props the fractures and gravel packs the production intervals surrounding the sand control screen assemblies. Therefore, a higher concentration of solid agents is desirable in the third treatment fluid. Additionally, the third treatment fluid may have a lower density and lower viscosity than the second treatment fluid. The lower density and lower viscosity in the third treatment fluid allow the solid agents to fall out of the slurry more readily.  
         [0088]    As should be apparent to those skilled in the art, the above described method allows the use of a relatively simple service tool  78  that allows for the treatment of multiple formations that are relatively close together. This is achieved by using sand control screen assemblies  40 ,  42  that include one-way valves  70  that prevent the flow of fluids from the interior to the exterior of sand control screen assemblies  40 ,  42 . Accordingly, fewer tools are required between sand control screen assemblies  40 ,  42 , thereby the distance between sand control screen assemblies  40 ,  42  may be reduced. This reduced distance and the simplicity of service tool  78  allow relatively narrow and relatively closely spaced formations to be treated according to the present invention.  
         [0089]    Referring now to FIG. 19, therein is schematically depicted an embodiment of the present invention that is used during a gravel packing treatment. As illustrated, sand control screen assembly  40  having one-way valves  70  is positioned within casing  36  and is adjacent to formation  14 . Similarly, sand control screen assembly  42  having one-way valve  70  is positioned within casing  36  and is adjacent to formation  16 . A wash pipe  104  extends through work string  32  traversing cross-over assembly  106 . Cross-over assembly  106  is positioned within work string  32  adjacent to cross-over ports  90  that include valves therein as explained above.  
         [0090]    Sand control screen assemblies  40 ,  42  each have a filter medium associated therewith that is designed to allow fluid to flow therethrough but prevent particulate matter of sufficient size from flowing therethrough. The exact design of the filter medium of sand control screen assemblies  40 ,  42  is not critical to the present invention as long as it is suitably designed for the characteristics of the formation fluids and the treatment fluids. One-way valves  70  of sand control screen assemblies  40 ,  42  may be of any suitable type so long as they prevent fluid flow from the interior to the exterior of sand control screens  40 ,  42 .  
         [0091]    To begin the gravel packing completion process, production interval  44  proximate formation  14  and production interval  50  proximate second formation  16  are isolated. Packer  46  seals the near end of production interval  44  and packer  48  seals the far end of production interval  44 . Similarly, packer  52  seals the near end of production interval  50  and packer  54  seals the far end of production interval  50 . Initially, as illustrated, the cross-over assembly  106  is located proximate to sand control screen assembly  40  and aligned with cross-over ports  90 .  
         [0092]    Referring to FIG. 20, when the treatment operation is a gravel pack, the objective is to uniformly and completely fill production interval  44  between sand control screen assembly  40  and casing  36  with gravel. To help achieve this result, return fluid is taken through sand control screen assembly  40 , indicated by arrows  108 , and travels through wash pipe  104 , as indicated by arrows  110 , for return to the surface.  
         [0093]    More specifically, a treatment fluid, in this case a fluid slurry containing gravel  112  is pumped downhole in work string  32 , as indicated by arrows  114 , and into production interval  44  via cross-over assembly  106 , as indicated by arrows  116 . As the fluid slurry containing gravel  112  travels to the far end of production interval  44 , gravel  112  drops out of the slurry and builds up from formation  14 , filling the perforations and production interval  44  around sand control screen assembly  40  forming gravel pack  112 A. While some of the carrier fluid in the slurry may leak off into formation  14 , the remainder of the carrier fluid passes through sand control screen assembly  40  through one-way valves  70 , as indicated by arrows  108 . The fluid flowing back through sand control screen assembly  40 , as explained above, follows the paths indicated by arrows  110  back to the surface.  
         [0094]    After the gravel packing operation of production interval  44  is complete, cross-over assembly  106  and wash pipe  104  may be moved uphole such that other production intervals may be gravel packed, such as production interval  50 , as best seen in FIG. 21. As the distance between formation  14  and formation  16  may be hundreds or even thousands of feet and as there may be any number of production intervals that require gravel packing, there may be a considerable amount of time between the gravel packing of production interval  44  and eventual production from formation  14 .  
         [0095]    It has been found that in conventional completions, considerable fluid loss may occur from the interior of sand control screen assembly  40  through gravel pack  112 A and into formation  14 . This fluid loss is not only costly but may also damage gravel pack  112 A, formation  14  or both. Using the sand control screen assemblies of the present invention, however, prevents such fluid loss using a seal member, in this case, one-way valves  70 , positioned within sand control screen assembly  40 . Accordingly, one-way valves  70  not only save the expense associated with fluid loss but also protect gravel pack  112 A and formation  14  from the damage caused by fluid loss.  
         [0096]    Referring to FIG. 22, the process of gravel packing production interval  50  is depicted. Wash pipe  104  is now disposed within sand control screen assembly  42 . Wash pipe  104  extends through cross-over assembly  106  such that return fluid passing through sand control screen assemblies  42 , indicated by arrows  118 , and travels through wash pipe  104 , as indicated by arrows  120 , for return to the surface.  
         [0097]    The fluid slurry containing gravel  112  is pumped downhole through work string  32 , as indicated by arrows  122 , and into production interval  50  via cross-over assembly  106  and cross-over ports  92 , as indicated by arrows  124 . As the fluid slurry containing gravel  112  travels to the far end of production interval  50 , the gravel  112  drops out of the slurry and builds up from formation  16 , filling the perforations and production interval  50  around sand control screen assemblies  42  forming gravel pack  112 B.  
         [0098]    While some of the carrier fluid in the slurry may leak off into formation  16 , the remainder of the carrier fluid passes through sand control screen assemblies  42  through one-way valves  70 , as indicated by arrows  118 . The fluid flowing back through sand control screen assembly  42 , as explained above, follows the paths indicated by arrows  120  back to the surface. Once gravel pack  112 B is complete, cross-over assembly  106  may again be repositioned uphole to gravel pack additional production intervals. As explained above, using sand control screen assembly  42  prevents fluid loss from the interior of sand control screen assembly  42  to formation  16  during such subsequent operations.  
         [0099]    As should be apparent to those skilled in the art, even though FIGS.  10 - 22  present the treatment of multiple intervals of a wellbore in a vertical orientation with packers at the top and bottom of the production interval, these figures are intended to also represent wellbores that have alternate directional orientations such as inclined wellbores and horizontal wellbores. In the horizontal orientation, for example, packer  46  is at the heel of production interval  44  and packer  48  is at the toe of production interval  44 . Likewise, while multiple production intervals have been described as being treated during a single trip, the methods described above are also suitable for treating a single production interval traversed by a wellbore or may be accomplished in multiple trips into a wellbore.  
         [0100]    While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is, therefore, intended that the appended claims encompass any such modifications or embodiments.