Abstract:
A screen assembly ( 60 ) comprises a base pipe ( 62 ) having perforated and nonperforated sections ( 66, 68 ), ribs ( 70 ) circumferentially spaced therearound and a filter medium ( 84 ) positioned around the ribs ( 70 ) having voids ( 92, 94 ) therethrough. The screen assembly ( 60 ) includes a slurry passageway ( 98 ) defined by the nonperforated section ( 68 ) of the base pipe ( 62 ), two of the ribs ( 70 ) and the portion ( 100 ) of the filter medium ( 84 ) that is circumferentially aligned with the nonperforated section ( 68 ). This portion ( 100 ) of the filter medium ( 84 ) has a filler material ( 96 ) disposed within the voids ( 92, 94 ) to create a fluid tight seal for a fluid slurry. The fluid slurry is discharged from the screen assembly ( 60 ) to a plurality of levels of the interval through exit ports ( 106 ) in a plurality of manifolds ( 102 ) when the screen assembly ( 60 ) is in an operable position.

Description:
TECHNICAL FIELD OF THE INVENTION 
     This invention relates in general to preventing the production of particulate materials through a wellbore traversing an unconsolidated or loosely consolidated subterranean formation and, in particular, to a screen assembly and method for obtaining a substantially complete gravel pack within an interval of the wellbore. 
     BACKGROUND OF THE INVENTION 
     Without limiting the scope of the present invention, its background is described with reference to the production of hydrocarbon fluids through a wellbore traversing an unconsolidated or loosely consolidated formation, as an example. 
     It is well known in the subterranean well drilling and completion art that particulate materials such as sand may be produced during the production of hydrocarbons from a well traversing an unconsolidated or loosely consolidated subterranean formation. Numerous problems may occur as a result of the production of such particulate. For example, the particulate cause 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 by processing equipment at the surface. 
     One method for preventing the production of such particulate material to the surface is gravel packing 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 particulate material known 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. 
     The liquid carrier either flows into the formation or returns to the surface by flowing through the sand control screen or both. In either case, the gravel is deposited around the sand control screen to form a gravel pack, which is highly permeable to the flow of hydrocarbon fluids but blocks the flow of the particulate carried in the hydrocarbon fluids. As such, gravel packs can successfully prevent the problems associated with the production of particulate materials from the formation. 
     It has been found, however, that a complete gravel pack of the desired production interval is difficult to achieve particularly in long or inclined/horizontal production intervals. These incomplete packs are commonly a result of the liquid carrier entering a permeable portion of the production interval causing the gravel to form a sand bridge in the annulus. Thereafter, the sand bridge prevents the slurry from flowing to the remainder of the annulus which, in turn, prevents the placement of sufficient gravel in the remainder of the annulus. 
     Prior art devices and methods have been developed which attempt to overcome this sand bridge problem. For example, attempts have been made to use devices having perforated shunt tubes or bypass conduits that extend along the length of the sand control screen to provide an alternate path for the fluid slurry around the sand bridge. 
     It has been found, however, that shunt tubes installed on the exterior of sand control screens are susceptible to damage during installation and may fail during a gravel packing operation. In addition, it has been found that on site assembly of a shunt tube system around a sand control screen is difficult and time consuming due to the large number of fluid connections required for typical production intervals. Further, it has been found that the effective screen area available for filtering out particulate from the production fluids is reduced when shunt tubes are installed on the exterior of a sand control screen. 
     Therefore a need has arisen for an apparatus and method for gravel packing a production interval traversed by a wellbore that overcomes the problems created by sand bridges. A need has also arisen for such an apparatus that is not susceptible to damage during installation and will not fail during a gravel packing operation. Additionally, a need has arisen for such an apparatus that is cost effective and does not require difficult or time consuming on site assembly. Further, a need has arisen for such an apparatus that does not require a reduction in the effective screen area available for filtering out particulate from the production fluids. 
     SUMMARY OF THE INVENTION 
     The present invention disclosed herein comprises a screen assembly and method for gravel packing a production interval of a wellbore that traverses an unconsolidated or loosely consolidated formation that overcomes the problems created by the development of a sand bridge between a sand control screen and the wellbore. Importantly, the screen assembly of the present invention is not susceptible to damage during installation or failure during the gravel packing operation, is cost effective to manufacture and does not require difficult or time consuming on site assembly. In addition, the screen assembly of the present invention allows for a relatively large effective screen area for filtering out particulate from the production fluids. 
     The sand control screen assembly of the present invention comprises a base pipe that has one or more perforated sections and one or more nonperforated sections. A plurality of ribs are circumferentially spaced around and axially extending along the exterior surface of the base pipe. Two of the ribs are positioned within each of the nonperforated sections of the base pipe. A screen wire is wrapped around the plurality of ribs forming a plurality of turns having gaps therebetween. A filler material is disposed within the portions of the gaps that are circumferentially aligned with the nonperforated sections of the base pipe. 
     The screen assembly includes one or more slurry passageways each of which are defined by one of the nonperforated section of the base pipe, the two ribs positioned within that nonperforated section of the base pipe and the portion of the wire and the filler material in the gaps that are circumferentially aligned with that nonperforated section of the base pipe. The slurry passageways are used to carry a fluid slurry containing gravel past any sand bridges that may form in the annulus surrounding the screen assembly. The fluid slurry is discharged from the screen assembly via a plurality of manifolds that are in fluid communication with the slurry passageways. The manifolds selectively discharge the fluid slurry to a plurality of levels of the interval through exit ports formed therein when the screen assembly is in an operable position. The exit ports may be either circumferentially aligned with the slurry passageways, circumferentially misaligned with the slurry passageways or both. The fluid communication between the manifolds and the slurry passageways may be established using tubes that extend from the manifolds into each adjacent sections of the slurry passageways. 
     In embodiments of the present invention wherein the screen assembly includes more than one section of sand control screen, each including a portion of the slurry passageway, the screen assembly includes a manifold between each of the sand control screen sections. These manifolds provide fluid communication between the portions of the slurry passageways of the adjacent sand control screen sections and deliver the fluid slurry into the interval surrounding the screen assembly. 
     In one embodiment of the present invention, the exit ports are created directly through the wire and the filler material in the gaps that are circumferentially aligned with the nonperforated sections of the base pipe instead of in manifolds. In this embodiment, tube segments may be disposed within the slurry passageways at the locations where the exit ports are created to provide support to the screen wire at these locations. 
     The method of the present invention includes traversing a formation with the wellbore, positioning a sand control screen assembly having one or more slurry passageways as described above, within the wellbore, injecting a fluid slurry containing gravel through the slurry passageways such that the fluid slurry exits the screen assembly through exit ports in manifolds or through the screen wire at a plurality of levels of the interval and terminating the injecting when the interval is substantially completely packed with the gravel. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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: 
     FIG. 1 is a schematic illustration of an offshore oil and gas platform operating a sand control screen assembly for gravel packing an interval of a wellbore of the present invention; 
     FIG. 2 is partial cut away view of a sand control screen assembly for gravel packing an interval of a wellbore of the present invention; 
     FIG. 3 is cross sectional view of the sand control screen assembly for gravel packing an interval of a wellbore of FIG. 2 taken along line  3 — 3 ; 
     FIG. 4 is cross sectional view of the sand control screen assembly for gravel packing an interval of a wellbore of FIG. 2 taken along line  4 — 4 ; 
     FIG. 5 is cross sectional view of the sand control screen assembly for gravel packing an interval of a wellbore of FIG. 2 taken along line  5 — 5 ; 
     FIG. 6 is a side view of two adjacent sand control screens of a sand control screen assembly for gravel packing an interval of a wellbore of the present invention; 
     FIG. 7 is side view of a sand control screen assembly for gravel packing an interval of a wellbore of the present invention; 
     FIG. 8 is a cross sectional view of the sand control screen assembly for gravel packing an interval of a wellbore of FIG. 7 taken along line  8 — 8 ; and 
     FIG. 9 is a half sectional view depicting the operation of a sand control screen assembly for gravel packing an interval of a wellbore of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     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. 
     Referring initially to FIG. 1, a sand control screen assembly for gravel packing an interval of a wellbore operating from an offshore oil and gas platform are schematically illustrated and generally designated  10 . A semi-submersible platform  12  is centered over a submerged oil and gas formation  14  located below sea floor  16 . A subsea conduit  18  extends from deck  20  of platform  12  to wellhead installation  22  including blowout preventers  24 . Platform  12  has a hoisting apparatus  26  and a derrick  28  for raising and lowering pipe strings such as work string  30 . 
     A wellbore  32  extends through the various earth strata including formation  14 . A casing  34  is cemented within wellbore  32  by cement  36 . Work string  30  includes various tools for completing the well. On the lower end of work string  30  is a sand control screen assembly  38  for gravel packing an interval of wellbore  32  made up of a plurality of sections of sand control screens  40 , three of which are depicted in FIG.  1 . Sand control screen assembly  38  is positioned adjacent to formation  14  between packers  44 ,  46  in annular region or interval  48  including perforations  50 . When it is desired to gravel pack annular interval  48 , a fluid slurry including a liquid carrier and a particulate material such as gravel is pumped down work string  30 . 
     As explained in more detail below, the fluid slurry will generally be injected into annular interval  48  between screen assembly  38  and wellbore  32  in a known manner such as through a cross-over tool (not pictured) which allows the slurry to travel from the interior of work string  30  to the exterior of work string  30 . Once the fluid slurry is in annular interval  48 , a portion of the gravel in the fluid slurry is deposited in annular interval  48 . Some of the liquid carrier may enter formation  14  through perforation  50  while the remainder of the fluid carrier entering sand control screen assembly  38 . More specifically, sand control screen assembly  38  disallows further migration of the gravel in the fluid slurry but allows the liquid carrier to travel therethrough and up to the surface in a known manner, such as through a wash pipe and into the annulus  52  above packer  44 . 
     If a sand bridge forms during the injection of the fluid slurry into annular region  48 , the fluid slurry will be diverted into one or more slurry passageways in sand control screen assembly  38  to bypass this sand bridge. In this case, the fluid slurry will be discharged from sand control screen assembly  38  through exit port at various levels within interval  48 . Again, once in annular interval  48 , the gravel in the fluid slurry is deposited therein. Some of the liquid carrier may enter formation  14  through perforation  50  while the remainder of the fluid carrier enters sand control screen assembly  38 , as described above, and returns to the surface. The operator continues to pump the fluid slurry down work string  30  into annular interval  48  and through the slurry passageways of sand control screen assembly  38 , as necessary, until annular interval  48  surrounding sand control screen assembly  38  is filled with gravel, thereby achieving a complete pack of interval  48 . Alternatively, it should be noted by those skilled in the art, that the fluid slurry may be injected entirely into the slurry passageways of sand control screen assembly  38  without first injecting the fluid slurry directly into annular interval  48 . 
     Even though FIG. 1 depicts a vertical well, it should be noted by one skilled in the art that the screen assembly for gravel packing an interval of a wellbore of the present invention is equally well-suited for use in deviated wells, inclined wells or horizontal wells. In addition, it should be apparent to those skilled in the art that the use of directional terms such as above, below, upper, lower, upward, downward and the like are used in relation to the illustrative embodiments as they are depicted in the figures, the upward direction being toward the top of the corresponding figure and the downward direction being toward the bottom of the corresponding figure. 
     Also, even though FIG. 1 depicts an offshore operation, it should be noted by one skilled in the art that the screen assembly for gravel packing an interval of a wellbore of the present invention is equally well-suited for use in onshore operations. Further, even though FIG. 1 has been described with regard to a gravel packing operation, it should be noted by one skilled in the art that the screen assembly of the present invention is equally well-suited for fracture operations and frac pack operations wherein a fluid slurry containing propping agents is delivered at a high flow rate and at a pressure above the fracture pressure of formation  14  such that fractures may be formed within formation  14  and held open by the propping agents and such that annular interval  48  is packed with the propping agents or other suitable particulate materials to prevent the production of fines from formation  14 . 
     Referring now to FIG. 2, therein is depicted a partial cut away view of a sand control screen assembly for gravel packing an interval of a wellbore of the present invention that is generally designated  60 . Screen assembly  60  has a base pipe  62  that has a plurality of perforated sections and a plurality of nonperforated sections. In the illustrated embodiment and as best seen in FIG. 3, screen assembly  60  has three perforated sections  64  each of which include a plurality of openings  66 . The exact number, size and shape of openings  66  are not critical to the present invention, so long as sufficient area is provided for fluid production and the integrity of base pipe  62  is maintained. Screen assembly  60  also has three nonperforated sections  68  which are positioned at approximately 120 degree intervals from one another. 
     Circumferentially distributed around and axially extending along the outer surface of base pipe  62  is a plurality of ribs  70 . In the illustrated embodiment, ribs  70  are generally symmetrically distributed about the axis of base pipe  62 . Preferably, ribs  70  have a generally triangular cross section wherein the base portion of ribs  70  that contacts base pipe  62  has an arcuate shape that substantially matches the curvature of base pipe  62 . Alternatively, the base portion of ribs  70  may be shaped such that ribs  70  contact base pipe  62  only proximate the apexes of the base portion of ribs  70 . In either case, once screen assembly  60  is fully assembled, the base portion of ribs  70  should securely contact base pipe  62  and provide the necessary fluid seal at the locations where the base portion of ribs  70  contact base pipe  62 . Importantly, two of the ribs  70  are positioned against each of the nonperforated sections  68  of base pipe  62 . Specifically, ribs  72 ,  74 , ribs  76 ,  78  and ribs  80 ,  82  are respectively positioned against nonperforated sections  68 . 
     Even though ribs  70  have been described as having a generally triangular cross section, it should be understood by one skilled in the art that ribs  70  may alternatively have other cross sectional geometries including, but not limited to, rectangular and circular cross sections so long as a proper seal between the ribs and the base pipe is established. Additionally, it should be understood by one skilled in the art that the exact number of ribs  70  will be dependent upon factors such as the diameter of base pipe  62 , the width of nonperforated sections  68 , as well as other design characteristics that are well known in the art. 
     Wrapped around and welded to ribs  70  is a screen wire  84 . Screen wire  84  forms a plurality of turns, such as turn  86 , turn  88  and turn  90 . Between each of the turns is a void or gap through which formation fluids flow such as gap  92  between turns  86 ,  88  and gap  94  between turns  88 ,  90 . The number of turns and the gap between the turns are determined based upon factors such as 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. As illustrated, the gaps in the sections of screen wire  84  that are circumferentially aligned with nonperforated sections  68  of base pipe  62  are sealed with a filler material  96  such as an epoxy resin. Filler material  96  is selectively placed in the gaps between the turns of screen wire  84  such that fluid sealed slurry passageways  98  are created between respective nonperforated sections  68 , ribs  70  and sealed sections  100  of screen wire  84 . 
     Together, ribs  70  and screen wire  84  may form a sand control screen jacket that is attached to base pipe  62  by welding or other suitable technique forming each screen section of screen assembly  60 . Alternatively, screen wire  84  may be wrapped around and welded to ribs  70  in place against base pipe  62 . It should be noted by those skilled in the art that even though FIG. 2 has depicted a wire wrapped screen, other types of filter media could alternatively be placed over ribs  70  without departing from the principles of the present invention including, but not limited to, a fluid-porous, particulate restricting, sintered metal material such as a plurality of layers of a wire mesh that are sintered together to form a porous sintered wire mesh screen that is seam welded or spiral welded over ribs  70 . 
     Positioned at selected intervals, such as every five to ten feet, along each screen section of sand control screen assembly  60  is a manifold  102 . Manifold  102  is in fluid communication with slurry passageways  98  via tubes  104  which extend partially into slurry passageways  98 , as best seen in FIG.  4 . In the illustrated embodiment, tubes  104  are welded within slurry passageways  98 . Tubes  104  deliver the fluid slurry carried in slurry passageways  98  into manifold  102 . A portion of the fluid slurry in manifold  102  will enter the annular interval surrounding screen assembly  60  via exit ports  106 . The remainder of the fluid slurry passes through annular area  108  of manifold  102  and enters the next section of slurry passageways  98 , as best seen in FIG.  5 . This process continues through the various levels of screen assembly  60  along the entire length of the interval to be gravel packed such that a complete gravel pack of the interval can be achieved. 
     In the illustrated embodiment, exit ports  106  of manifold  102  are not circumferentially aligned with slurry passageways  98  of screen assembly  60 . This configuration helps to minimize liquid leak off after the area adjacent to a particular manifold has been packed with the gravel. Specifically, even after an area surrounding one of the manifolds has been packed with the gravel, it has been found that liquid from the fluid slurry may nonetheless leak off into this porous region causing not only a reduction in the velocity of the fluid slurry in slurry passageways  98 , but also, an increase in the effective density of particles in the fluid slurry, each of which is a hindrance to particle transport to locations further along screen assembly  60 . Positioning exit ports  106  out of phase with slurry passageways  98  reduces the liquid leak off by increasing the pressure required to push the liquid through the porous matrix and reduces the velocity of the liquid near exit ports  106 , thereby reducing the rate of liquid leak off. This rate of liquid leak off is further reduced by using a liquid in the fluid slurry that is thixotropic such that its viscosity increases with reduced velocity through the porous matrix. 
     Even though FIG. 2 has depicted exit ports  106  as being circular, it should be understood by those skilled in the art that exit ports  106  could alternatively have other shapes without departing from the principles of the present invention, those shapes being considered within the scope of the present invention. Also, it should be noted by those skilled in the art that even though FIGS. 2-4 have depicted three slurry passageways  98  at 120 degree intervals around screen assembly  60 , other numbers of slurry passageways, either greater or fewer, and other intervals between such slurry passageways may be used without departing from the principles of the present invention and are considered within the scope of the present invention. Likewise, even though FIGS. 2 and 5 have depicted three exit ports  106  at 120 degree intervals around manifold  102 , other numbers of exit port, either greater or fewer, and other intervals between such exit ports may be used without departing from the principles of the present invention and are considered within the scope of the present invention. 
     Referring now to FIG. 6, therein is depicted a screen assembly for gravel packing an interval of a wellbore at the point where two sand control screens are joined together, that is generally designated  120 . As illustrated, screen assembly  120  includes sand control screen  122  and sand control screen  124  each of which have the substantially identical construction as that described above with reference with reference to FIGS. 2-5. Screens  122 ,  124  are coupled together in a known manner such as via a threaded coupling (not pictured). Between screens  122 ,  124 , screen assembly  120  includes a tube and manifold system  126 . Tube and manifold system  126  includes three tubes  128 , only two of which are pictured, that deliver the fluid slurry from slurry passageways  98  of screen  122  to manifold  130 . A portion of the fluid slurry in manifold  130  will enter the annular interval surrounding screen assembly  120  via three exit ports  132 , only one of which is shown. The remainder of the fluid slurry enters three tubes  134 , only two of which are pictured, and is delivered to slurry passageways  98  of screen  124 . 
     Even though FIG. 6 depicts tubes  128  that deliver the fluid slurry to manifold  130  as being circumferentially aligned with tubes  134  that transport the fluid slurry from manifold  130 , it is likely that tubes  128 ,  134  will not be circumferentially aligned as the adjoining sections of tube and manifold system  126  are threadably coupled when screen sections  122 ,  124  of screen assembly  120  are threaded together. Accordingly, it is likely that tubes  128  and tubes  134  on opposite sides of manifold  130  will not be circumferentially aligned with one another. 
     As should be apparent to those skilled in the art, even when tubes  128  and tubes  134  are positioned with a circumferential phase shift relative to one another, this does not affect the operation of the present invention as manifold  130  has a substantially annular region, such as annular region  108  depicted in FIG. 5, through which the fluid slurry travels allowing for such misalignment. As such, the mating of adjoining sections of the screen assembly for gravel packing an interval of a wellbore of the present invention is substantially similar to mating typical joints of pipe to form a pipe string requiring no special coupling tools or techniques. 
     Referring now to FIGS. 7 and 8, therein is depicted another embodiment of a screen assembly for gravel packing an interval of a wellbore that is generally designated  140 . Screen assembly  140  includes a base pipe  62  that has three perforated sections  64  having openings  66  and three nonperforated sections  68 . Circumferentially distributed around and axially extending along the outer surface of base pipe  62  is a plurality of ribs  70  having a generally triangular cross section. Importantly, two of the ribs  70  are positioned against each of the nonperforated sections  68  of screen assembly  60 . Specifically, ribs  72 ,  74 , ribs  76 ,  78  and ribs  80 ,  82  are respectively positioned against nonperforated sections  68 . Wrapped around and welded to ribs  70  is a screen wire  84 . Screen wire  84  forms a plurality of turns, such as turn  86 , turn  88  and turn  90 . Between each of the turns is a gap through which formation fluids flow such as gap  92  between turns  86 ,  88  and gap  94  between turns  88 ,  90 . The gaps in the sections of screen wire  84  that are circumferentially aligned with nonperforated sections  68  of base pipe  62  are sealed with a filler material  96 . Filler material  96  is selectively placed in the gaps between the turns of screen wire  84  such that fluid sealed slurry passageways  98  are created between respective nonperforated sections  68 , ribs  70  and sealed sections  100  of screen wire  84 . 
     Positioned at selected intervals, such as every five to ten feet, along each screen section of sand control screen assembly  140  and within slurry passageways  98  are tube segments  142 , as best seen in FIG.  8 . In the illustrated embodiment, tube segments  142  are welded within slurry passageways  98 . Tube segments  142 , which may be several inches to a foot long, are used to support screen wire  84  such that exit ports  144  may be drilled therethrough. A portion of the fluid slurry traveling through tube segments  142  will enter the annular interval surrounding screen assembly  140  via exit ports  144 . The remainder of the fluid slurry passes through tube segments  142  and enters the next section of slurry passageways  98 . This process continues through the various levels of screen assembly  140  along the entire length of the interval to be gravel packed such that a complete gravel pack of the interval can be achieved. 
     Referring now to FIG. 9, a typical completion process using screen assembly  60  for gravel packing an interval of a wellbore of the present invention will be described. First, screen assembly  60  is positioned within wellbore  32  proximate formation  14  and interval  48  adjacent to formation  14  is isolated. Packer  44  seals the upper end of annular interval  48  and packer  46  seals the lower end of annular interval  48 . Cross-over assembly  150  is located adjacent to screen assembly  60 , traversing packer  44  with portions of cross-over assembly  150  on either side of packer  44 . When the gravel packing operation commences, the objective is to uniformly and completely fill interval  48  with gravel. To help achieve this result, wash pipe  154  is disposed within screen assembly  60 . Wash pipe  154  extends into cross-over assembly  150  such that return fluid passing through screen assembly  60 , indicated by arrows  156 , may travel through wash pipe  154 , as indicated by arrow  158 , and into annulus  52 , as indicted by arrow  160 , for return to the surface. 
     The fluid slurry containing gravel is pumped down work string  30  into cross-over assembly  150  along the path indicated by arrows  162 . The fluid slurry containing gravel exits cross-over assembly  150  through cross-over ports  164  and is discharged into annular interval  48  as indicated by arrows  166 . This is the primary path as the fluid slurry seeks the path of least resistance. Under ideal conditions, the fluid slurry travels throughout the entire interval  48  until interval  48  is completely packed with gravel. If, however, a sand bridge forms in annular interval  48  before the gravel packing operation is complete, the fluid slurry containing gravel will enter slurry passageways  98  of screen assembly  60  to bypass the sand bridge as indicated by arrow  168 . The fluid slurry then travels within slurry passageways  98  with some of the fluid slurry exiting screen assembly  60  at each of the manifolds  102  through exit ports  106 , as indicated by arrows  170 . 
     As the fluid slurry containing gravel enters annular interval  48 , the gravel drops out of the slurry and builds up from formation  14 , filling perforations  50  and annular interval  48  around screen assembly  60  forming the gravel pack. Some of the carrier fluid in the slurry may leak off through perforations  50  into formation  14  while the remainder of the carrier fluid passes through screen assembly  60 , as indicated by arrows  156 , that is sized to prevent gravel from flowing therethrough. The fluid flowing back through screen assembly  60 , as explained above, follows the paths indicated by arrows  158 ,  160  back to the surface. 
     In operation, the screen assembly for gravel packing an interval of a wellbore of the present invention is used to distribute the fluid slurry to various locations within the interval to be gravel packed by injecting the fluid slurry into the slurry passageways of the screen assembly when sand bridge formation occurs. The fluid slurry exits through the various exit ports in the manifolds along the length of the screen assembly into the annulus between the screen assembly and the wellbore which may be cased or uncased. Once in this annulus, a portion of the gravel in the fluid slurry is deposited around the screen assembly in the annulus such that the gravel migrates both circumferentially and axially from the exit ports. This process progresses along the entire length of the screen assembly such that the annular area becomes completely packed with the gravel. Once the annulus is completely packed with gravel, the gravel pack operation may cease. 
     Alternatively, it should be noted by those skilled in the art that instead of first injecting the fluid slurry directly into annular interval  48  until a sand bridge forms, the fluid slurry may initially be injected directly into the slurry passageways of the screen assembly for gravel packing an interval of a wellbore of the present invention. In either embodiment, once the gravel pack is completed and the well is brought on line, formation fluids that are produced into the gravel packed interval must travel through the gravel pack in the annulus prior to entering the sand control screen assembly. As such, the screen assembly for gravel packing an interval of a wellbore of the present invention allows for a complete gravel pack of an interval so that particulate materials in the formation fluid are filtered out. 
     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.