Patent Publication Number: US-6702019-B2

Title: Apparatus and method for progressively treating an interval of a wellbore

Description:
TECHNICAL FIELD OF THE INVENTION 
     This invention relates in general to the treatment of a production interval of a wellbore to stimulate hydrocarbon production and prevent the production of fine particulate materials and, in particular, to an apparatus and method for progressively gravel packing or progressively frac packing the production interval of the wellbore. 
     BACKGROUND OF THE INVENTION 
     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 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 using surface processing equipment. 
     One method for preventing the production of such particulate material is to gravel pack the well adjacent 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. 
     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. 
     It is sometimes 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 production interval 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. 
     The fracture fluid must be forced into the formation at a flow rate great enough to fracture the formation allowing the entrained proppant 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. 
     It has been found that it is difficult to achieve a complete gravel pack of the desired production interval either independent of or as part of a fracturing operation, particularly in long or inclined/horizontal production intervals. These incomplete packs are commonly a result of the liquid carrier entering the permeable portions of the production interval causing the gravel to form a sand bridge in the annulus. Thereafter, the sand bridge prevents the gravel pack slurry from flowing to the remainder of the annulus which, in turn, prevents the placement of sufficient gravel in the remainder of the annulus. 
     Therefore a need has arisen for an apparatus and method that are capable of creating fractures along the entire length of a production interval. A need has also arisen for such an apparatus and method that can produce a complete gravel pack of the wellbore adjacent to the production interval either independent of or as part of the fracturing of the production interval. Further, a need has arisen for an apparatus and method that are capable of stimulating the production interval to enhance production and gravel packing the production interval to prevent the production of fine particulate materials when production commences. 
     SUMMARY OF THE INVENTION 
     The present invention disclosed herein comprises an apparatus and method that is capable of enhancing production from a production interval by creating fractures throughout the entire interval and producing a substantially complete gravel pack of the wellbore adjacent to the production interval to prevent the production of fine particulate materials when production commences. The apparatus and method of the present invention achieves these results by progressively treating the production interval from one end to the other. 
     The apparatus comprises a sand control screen that is positioned within the wellbore and a fluid delivery tubular positioned adjacent to sand control screen in the wellbore. The fluid delivery tubular progressively allows fluid communication from the interior of the fluid delivery tubular to the exterior of fluid delivery tubular from a first end to a second end of the interval, thereby delivering the treatment fluid along the entire length of the interval. 
     The fluid delivery tubular may comprises a plurality of actuatable members. The actuatable devices may be rupture disks, pressure actuated one-way valves or other pressure actuated devices that are positioned along a portion of the length of the fluid delivery tubular such that the pressure required to actuate the actuatable members progressively increases from the first end to the second end of the interval. Alternatively, the actuatable device may be progressively actuated from the first end to the second end of the interval using signals sent from the surface using hard wire connections, fiber optics, hydraulics or wireless telemetry. 
     The fluid delivery tubular may alternatively comprise a perforated pipe having a plurality of removable members positioned on the interior or the exterior thereof. The removable members may be propellants or other combustible material members each having an initiator. The initiators may be activated using signals. Alternatively, the initiators may have pressure activated firing devices that are positioned such that the pressure required to fire the pressure activated firing devices progressively increasing from the first end to the second end of the interval. 
     The removable members may alternatively be friable members that are progressively removable from the first end to the second end of the interval. Each friable member may include a pressure actuated vibration generator. In this case, the pressure actuated vibration generators are positioned such that the pressure required to activate the pressure actuated vibration generators progressively increasing from the first end to the second end of the interval. Alternatively, each of the friable members may have a vibration generator that activated by a signal sent from the surface. 
     The method of the present invention comprises traversing the formation with the wellbore, locating a sand control screen eccentrically within the wellbore proximate the formation, positioning a fluid delivery tubular adjacent to the sand control screen within the wellbore, injecting a treatment fluid into the fluid delivery tubular, progressively establishing fluid communication between the interior of the fluid delivery tubular and the exterior of the fluid delivery tubular from the first end to the second end of the interval and terminating the injecting when the interval is treated. 
    
    
     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 an apparatus for progressively treating an interval of a wellbore of the present invention; 
     FIG. 2 is a half sectional view of an apparatus for progressively treating an interval of a wellbore of the present invention in its initial position; 
     FIG. 3 is a half sectional view of an apparatus for progressively treating an interval of a wellbore of the present invention after the first progression of the apparatus; 
     FIG. 4 is a half sectional view of an apparatus for progressively treating an interval of a wellbore of the present invention after the second progression of the apparatus; 
     FIG. 5 is a half sectional view of an apparatus for progressively treating an interval of a wellbore of the present invention after the third progression of the apparatus; 
     FIG. 6 is a half sectional view of an apparatus for progressively treating an interval of a wellbore of the present invention after the next to last progression of the apparatus; 
     FIG. 7 is a half sectional view of an apparatus for progressively treating an interval of a wellbore of the present invention after the last progression of the apparatus; 
     FIG. 8 is a half sectional view of another embodiment of an apparatus for progressively treating an interval of a wellbore of the present invention after the first progression of the apparatus; and 
     FIG. 9 is a half sectional view of another embodiment of an apparatus for progressively treating an interval of a wellbore of the present invention in its initial position. 
    
    
     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, an apparatus for progressively treating an interval of a wellbore operating from an offshore oil and gas platform is 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 including a sand control screen assembly  38  which is positioned within wellbore  32  adjacent to formation  14 . Also extending from platform  12  through wellbore  32  is a fluid delivery tubular  40  having a fluid discharge section  42  positioned adjacent to formation  14  which is used to frac pack or gravel pack the production interval  48  between packers  44 ,  46 . When it is desired to treat interval  48 , work string  30  and fluid delivery tubular  40  are lowered through casing  34  until sand control screen assembly  38  and fluid discharge section  42  are positioned adjacent to formation  14  including perforations  50 . Thereafter, a treatment fluid containing sand, gravel, proppants or the like is pumped down delivery tubular  40  to progressively treat interval  48 . 
     Even though FIG. 1 depicts a vertical well, it should be noted by one skilled in the art that the apparatus for progressively treating an interval of a wellbore of the present invention is equally well-suited for use in 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 apparatus for progressively treating an interval of a wellbore of the present invention is equally well-suited for use in onshore operations. 
     Referring now to FIG. 2, therein is depicted a more detailed illustration of interval  48 . As illustrated, screen assembly  38  is eccentrically positioned within casing  34  and is adjacent to formation  14 . A wash pipe  52  is positioned within screen assembly  38 . Wash pipe  52  extends into a cross-over assembly  54  which is connected to work string  30  extending from the surface. Screen assembly  38  is designed to allow fluid to flow therethrough but prevent particulate matter of sufficient size from flowing therethrough. The exact design of screen assembly  38  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. For example, as illustrated, screen assembly  38  includes a perforated base pipe  56  having a wire  58  wrapped directly thereon. Alternatively, a plurality of ribs may be placed around the base pipe to provide stand off between the base pipe and the wire wrap. 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 used without departing from the principles of the present invention. For example, 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 could alternatively be used. 
     In the illustrated embodiment, fluid discharge section  42  of fluid delivery tubular  40  includes a plurality of progressively actuatable members  60 A- 60 E. Suitable actuatable members  60 A- 60 E include rupture disks or valves and are preferably one-way valves that selectively allow fluid to flow from the interior of fluid delivery tubular  40  to the exterior of fluid delivery tubular  40 . Actuatable members  60 A- 60 E may be progressively actuated using a variety of known techniques such as sending a signal via a direct electrical connection, fiber optics, hydraulics, wireless telemetry including pressure pulses, electromagnetic waves or acoustic signals and the like. Actuatable members  60 A- 60 E are preferably pressure actuated one-way valves as explained in more detail below. 
     To begin the completion process, interval  48  adjacent to formation  14  is isolated. Packer  44  seals the near end of interval  48  and packer  46  seals the far end of interval  48 . Cross-over assembly  54  is located adjacent to screen assembly  38 , traversing packer  44  with portions of cross-over assembly  54  on either side of packer  44 . As illustrated, when the treatment operation is a gravel pack, the objective is to uniformly and completely fill interval  48  with gravel. To help achieve this result, wash pipe  52  is disposed within screen assembly  38 . Wash pipe  52  extends into cross-over assembly  54  such that return fluid passing through screen assembly  38 , indicated by arrows  62 , may travel through wash pipe  52 , as indicated by arrow  64 , and into annulus  66 , as indicted by arrow  68 , for return to the surface. 
     The fluid slurry containing gravel  70  is pumped down fluid delivery tubular  40 . In the illustrated embodiment, the fluid slurry containing gravel  70  travels to the far end of interval  48  through fluid delivery tubular  40 . As illustrated, a portion of fluid slurry containing gravel  70  exits the open end of fluid delivery tubular  40 . As gravel  70  builds up at the far end of interval  48 , the pressure within fluid delivery tubular  40  will begin to increase. Alternatively, the far end of fluid delivery tubular  40  could be closed in which case the pressure also increases in fluid delivery tubular  40  when the fluid slurry containing gravel travels to the far end. 
     Once the pressure in fluid delivery tubular  40  increases to a sufficient level, the progressive operation of the present invention may begin. Specifically, as best seen in FIG. 3, actuatable member  60 A is actuated which allows the fluid slurry containing gravel  70  to travel from fluid delivery tubular  40  through actuatable member  60 A into interval  48 . As the fluid slurry containing gravel  70  enters interval  48 , the gravel  70  drops out of the slurry and builds up from formation  14 , filling perforation  50 A and interval  48  around the far section of screen assembly  38  forming the initial portion of the gravel pack. Some of the carrier fluid in the slurry may leak off through perforation  50 A into formation  14  while the remainder of the carrier fluid passes through screen assembly  38 , as indicated by arrows  62 , that is sized to prevent gravel  70  from flowing therethrough. The fluid flowing back through screen assembly  38 , as explained above, follows the paths indicated by arrows  64 ,  68  back to the surface. 
     As the initial portion of the gravel pack becomes tightly packed, the pressure in fluid deliver tubular  40  again increases. At this point and as best seen in FIG. 4, actuatable member  60 B is actuated which allows the fluid slurry containing gravel  70  to travel from fluid deliver tubular  40  through actuatable member  60 B. As the fluid slurry containing gravel  70  enters interval  48 , the gravel  70  drops out of the slurry and builds up from formation  14 , filling perforation  50 B and interval  48  around the adjacent section of screen assembly  38  forming the next portion of the gravel pack. While some of the carrier fluid in the slurry may leak off through perforation  50 B into formation  14 , the remainder of the carrier fluid passes through screen assembly  38 , as indicated by arrows  62  and returns to the surface as indicated by arrows  64 ,  68 . 
     As this portion of the gravel pack becomes tightly packed, the pressure in fluid delivery tubular  40  again increases. At this point and as best seen in FIG. 5, actuatable member  60 C is actuated which allows the fluid slurry containing gravel  70  to travel from fluid delivery tubular  40  through actuatable member  60 C. As the fluid slurry containing gravel  70  enters interval  48 , the gravel  70  drops out of the slurry and builds up from formation  14 , filling perforation  50 C and interval  48  around the adjacent section of screen assembly  38  forming the next portion of the gravel pack. While some of the carrier fluid in the slurry may leak off through perforation  50 C into formation  14 , the remainder of the carrier fluid passes through screen assembly  38 , as indicated by arrows  62  and returns to the surface as indicated by arrows  64 ,  68 . 
     This process continues to progress from the far end of interval  48  toward the near end of interval  48 . Specifically, as best seen in FIG. 6, actuatable member  60 D is actuated which allows the fluid slurry containing gravel  70  to travel from fluid delivery tubular  40  through actuatable member  60 D. As the fluid slurry containing gravel  70  enters interval  48 , the gravel  70  drops out of the slurry and builds up from formation  14 , filling perforation  50 D and interval  48  around the adjacent section of screen assembly  38  forming the next portion of the gravel pack. While some of the carrier fluid in the slurry may leak off through perforation  50 D into formation  14 , the remainder of the carrier fluid passes through screen assembly  38 , as indicated by arrows  62  and returns to the surface as indicated by arrows  64 ,  68 . 
     As this portion of the gravel pack becomes tightly packed, the pressure in fluid delivery tubular  40  again increases. At this point and as best seen in FIG. 7, the last actuatable member, actuatable member  60 E, is actuated which allows the fluid slurry containing gravel  70  to travel from fluid delivery tubular  40  through actuatable member  60 E. As the fluid slurry containing gravel  70  enters interval  48 , the gravel  70  drops out of the slurry and builds up from formation  14 , filling perforation  50 E and interval  48  around the adjacent section of screen assembly  38  to packer  44  forming the last portion of the gravel pack. While some of the carrier fluid in the slurry may leak off through perforation  50 E into formation  14 , the remainder of the carrier fluid passes through screen assembly  38 , as indicated by arrows  62  and returns to the surface as indicated by arrows  64 ,  68 . 
     As can be seen, using the present invention for progressively treating an interval of a wellbore, a gravel pack may progress from one end of an interval toward the other end of an interval as fluid communication is progressively established along the entire length of the interval. Also, as should be apparent to those skilled in the art, even though FIGS. 2-7 present the progressive gravel packing of an interval of a wellbore in a vertical orientation with packer  44  at the top of interval  48  and packer  46  at the bottom of interval  48 , 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  44  is at the heel of interval  48  and packer  46  is at the toe of interval  48 . 
     Likewise, even though FIGS. 2-7 present the progressive gravel packing of an interval of a wellbore as being progressively performed from the far end of the interval to the near end of the interval, those skilled in the art will understand that the progressive gravel packing process of the present invention can alternatively be performed from the near end of the interval to the far end of the interval. 
     As stated above, there are numerous ways to progressively actuate actuatable members  60 A- 60 E. In the preferred method described above, the pressure created by the fluid slurry within fluid delivery tubular  40  progressively triggers the actuation of actuatable members  60 A- 60 E. One way to implement this method is to position actuatable members  60 A- 60 E within fluid delivery tubular  40  such that the pressure required to actuate actuatable members  60 A- 60 E progressively increases from the one end of interval  48  to the other end of interval  48 . For example, each adjacent actuatable member may be set to actuate at an incremental pressure above the prior actuatable members such as at increments of between about 50-100 psi. This assures a proper progression of the gravel pack by preventing any out of sequence activations. In addition, this approach is particularly advantageous in that the incremental pressure increase of adjacent actuatable members helps to insure that each section of the gravel pack is tightly packed prior to initiating the gravel packing of subsequent sections. 
     Alternatively, a hard wired or wireless telemetry system may be used to progressively actuate actuatable members  60 A- 60 E. For example, each actuatable member may be actuated by sending a signal addressed to a specific actuatable member. This assures a proper progression of the gravel pack by preventing any out of sequence activations. The signals may be manually or automatically sent based upon time or the pressure response in fluid delivery tubular  40 . For example, the signal to actuate the next actuatable member may be sent each time the pressure within fluid delivery tubular  40  reaches a particular level or each time the pressure within fluid delivery tubular  40  reaches the next preselected pressure increment. As with the direct pressure response method, the particular actuation sequence should insure that each section of the gravel pack is tightly packed prior to initiating the gravel packing of subsequent sections. 
     Referring now to FIG. 8, therein is depicted another embodiment of the present invention that is used for frac packing interval  48 . As illustrated, screen assembly  138  is eccentrically positioned within casing  34  and is adjacent to formation  14 . A wash pipe  152  is positioned within screen assembly  138 . Wash pipe  152  extends into a cross-over assembly  154  which is connected to work string  30  extending from the surface. Cross-over assembly  154  includes a valve  150  that is used to selectively allow and prevent the flow of return fluid to the surface via wash pipe  152 . Alternatively, a surface valve (not pictured) may be used to prevent the flow of return fluid. As illustrated, screen assembly  138  includes a perforated base pipe  156  having a wire  158  wrapped directly thereon, however, other types of filter media may alternatively be used. 
     In the illustrated embodiment, fluid discharge section  142  of fluid delivery tubular  140  includes a plurality of progressively actuatable members  160 A- 160 E which are preferable valves, such as pressure actuated one-way valves that selectively allow fluid to flow from the interior of fluid delivery tubular  140  to the exterior of fluid delivery tubular  140 . Actuatable members  160 A- 160 E may alternatively be progressively actuated using a variety of known techniques such as sending a signal via a hard wire connection, fiber optics, hydraulics, wireless telemetry including pressure pulses, electromagnetic waves or acoustic signals and the like. 
     To begin the completion process, interval  48  adjacent to formation  14  is isolated. Packer  44  seals the near end of interval  48  and packer  46  seals the far end of interval  48 . Cross-over assembly  154  is located adjacent to screen assembly  138 , traversing packer  44  with portions of cross-over assembly  154  on either side of packer  44 . As illustrated, when the treatment operation is a frac pack, the objective is to enhance the permeability of formation  14  by delivering a fluid slurry containing proppants  170  at a high flow rate and in a large volume above the fracture gradient of formation  14  such that fractures may be formed within formation  14  and held open by the proppants  170 . In addition, a frac pack also has the objective of preventing the production of fines by packing interval  48  with the proppants  170 . To help achieve these results, valve  150  of cross-over assembly  154  is initially in the closed position to prevent returns from flowing therethrough. 
     The fluid slurry containing proppants  170  is pumped down-fluid delivery tubular  140 . In the illustrated embodiment, the fluid slurry containing proppants  170  travels to the far end of interval  48  through fluid delivery tubular  140 . At this point, the fluid slurry containing proppants  170  may exit the far end of fluid delivery tubular  140  if it is open or builds up in fluid delivery tubular  140  if it is closed at the far end. In either case, the pressure within fluid delivery tubular  140  will begin to increase. 
     Once the pressure in fluid delivery tubular  140  increases to a sufficient level, the progressive operation of the present invention may begin. Specifically, as best seen in FIG. 8, actuatable member  160 A is actuated which allows the fluid slurry containing proppants  170  to travel from fluid delivery tubular  140  through actuatable member  160 A into interval  48 . As the fluid slurry containing proppants  170  is being delivered at a high flowrate and in a large volume above the fracture gradient of formation  14  and as valve  150  is closed, the fluid slurry fractures formation  14  as indicated by fracture  172 . As this portion of interval  48  begins to screen out, the pressure within fluid delivery tubular  140  will rise causing the progressive actuation of actuatable members  160 B- 160 E in the manner described above with reference to FIGS. 3-7. It should be noted that as the frac pack operation progresses some of the proppants  170  in the fluid slurry will remain in interval  48 , thereby packing interval  48  around screen assembly  138 . This packing process may be enhanced by reducing the flow rate of the fluid slurry toward the end of the treatment process and opening valve  150  to allow some returns to flow to the surface as described above. 
     Referring now to FIG. 9, therein is depicted another embodiment of an apparatus for progressively treating an interval of a wellbore. As illustrated, screen assembly  238  is eccentrically positioned within casing  34  and is adjacent to formation  14 . A wash pipe  252  is positioned within screen assembly  238 . Wash pipe  252  extends into a cross-over assembly  254  which is connected to work string  30  extending from the surface. Screen assembly  238  is designed to allow fluid to flow therethrough but prevent particulate matter of sufficient size from flowing therethrough. The exact design of screen assembly  238  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. For example, as illustrated, screen assembly  238  includes a perforated base pipe  256  having a wire  258  wrapped directly thereon. Other types of screen assemblies having other types of filter media may alternatively be used. 
     In the illustrated embodiment, fluid discharge section  242  of fluid delivery tubular  240  includes a plurality of perforations  244 A- 244 J that are selective blocked by removable members  260 A- 260 E. Removable members  260 A- 260 E may be constructed from a variety of materials such as combustible materials, referred to herein as propellants, that are removable by combustion, friable materials, including ceramics, that are removable by disintegration, or other materials that are removable in a downhole environment. 
     When removable members  260 A- 260 E are constructed from propellants, suitable initiators are attached to each removable member  260 A- 260 E such that the combustion process of each removable member  260 A- 260 E may be triggered independently. The initiators may be operated using a variety of known techniques including pressure actuation, electrical actuation, acoustic actuation or the like. For example, as explained in more detail below, the pressure generated by the treatment fluid can be used to trigger the initiators. Alternatively, a signal may be sent to trigger each of the removeable members  260 A- 260 E via a hard wired connection, fiber optics, hydraulics, a wireless telemetry system utilizing pressure pulses, electromagnetic waves or acoustic signals and the like. 
     When removable members  260 A- 260 E are constructed from friable materials, suitable vibration generators are attached to each removable member  260 A- 260 E such that the disintegration process of each removable member  260 A- 260 E may be triggered independently. The vibration generators may be operated using a variety of known techniques such as those described above. 
     To begin the completion process, interval  48  adjacent to formation  14  is isolated. Packer  44  seals the near end of interval  48  and packer  46  seals the far end of interval  48 . Cross-over assembly  254  is located adjacent to screen assembly  238 , traversing packer  44  with portions of cross-over assembly  254  on either side of packer  44 . As illustrated, when the treatment operation is a gravel pack, the objective is to uniformly and completely fill interval  48  with gravel. To help achieve this result, wash pipe  252  is disposed within screen assembly  238 . Wash pipe  252  extends into cross-over assembly  254  such that return fluid passing through screen assembly  238 , indicated by arrows  262 , may travel through wash pipe  252 , as indicated by arrow  264 , and into annulus  66 , as indicted by arrow  268 , for return to the surface. 
     The fluid slurry containing gravel  70  is pumped down fluid delivery tubular  240 . In the illustrated embodiment, the fluid slurry containing gravel  70  travels to the far end of interval  48  through fluid delivery tubular  240 . At this point, a portion of fluid slurry containing gravel  70  exits the open end of the fluid delivery tubular  240  if this end is open to flow. 
     As the pressure in fluid delivery tubular  240  increases to a sufficient level, the progressive operation of the present invention may begin. Specifically, removable member  260 A is removed which allows the fluid slurry containing gravel  70  to travel from fluid delivery tubular  40  through perforations  244 A- 244 B into interval  48 . As the fluid slurry containing gravel  70  enters interval  48 , the gravel  70  drops out of the slurry and builds up from formation  14 , filling perforation  50 A and interval  48  around the end section of screen assembly  238  forming the initial portion of the gravel pack. Some of the carrier fluid in the slurry may leak off through perforation  50 A into formation  14  while the remainder of the carrier fluid passes through screen assembly  238 , as indicated by arrows  262 , that is sized to prevent gravel  70  from flowing therethrough. The fluid flowing back through screen assembly  238 , as explained above, follows the paths indicated by arrows  264 ,  268  back to the surface. 
     As the pressure within fluid delivery tubular  140  increases, removable member  260 B is removed which allows fluid flow through perforations  244 C- 244 D into interval  48  which packs perforation  50 B and the section of screen assembly  238  adjacent thereto. This process progresses from the far end of interval  48  to the near end of interval  48  by progressively removing removable member  260 C, which exposes perforations  244 E- 244 F, removable member  260 D, which exposes perforations  244 G- 244 H, and removable member  260 E, which exposes perforations  244 I- 244 J. Accordingly, the entire interval  48  is progressively gravel packed. After the treatment process is completed, a valve (not pictured) in fluid delivery tubular  240  may be closed to prevent the flow of fluids, for example production fluids, to the surface therethrough. 
     As stated above, there are numerous ways to remove removable members  260  from fluid discharge section  242  of fluid delivery tubular  240  to progressively establish fluid communication between the interior of fluid delivery tubular  240  and the exterior of fluid delivery tubular  240 . One preferred method allows the pressure created by the treatment fluid within fluid delivery tubular  240  to progressively trigger the removal of removable members  260 . For example, when the removable members  260  are constructed of propellant material, pressure activated firing devices may be attached to initiators that are coupled on each of the removable members  260 . The pressure activated firing devices are then positioned within fluid delivery tubular  240  such that the pressure required to fire the pressure activated firing devices progressively increases from, for example, the far end of interval  48  toward the near end of interval  48 . Each adjacent pressure activated firing device may be set to fire at an incremental pressure above the prior pressure activated firing device such as at increments of between about 50-100 psi. This assures a proper progression of the gravel pack by preventing any out of sequence activations. In addition, this approach is particularly advantageous in that the incremental pressure increase of adjacent pressure activated firing devices helps to insure that each section of the gravel pack is tightly packed prior to initiating the gravel packing of subsequent sections. 
     Alternatively, a signal may be used to progressively trigger the removal of removable member  260 . For example, when the removable members  260  are constructed of a friable material, vibration generators may be coupled on each of the removable members  260 . Each vibration generator may be activated by a particular signal addressed specifically for that vibration generator. This assures a proper progression of the gravel pack by preventing any out of sequence activations. The signals may be manually or automatically sent based upon time or the pressure response in fluid delivery tubular  240 . For example, the signal to remove the next removable member  260  may be sent each time the pressure within fluid delivery tubular  240  reaches a particular level or each time the pressure within fluid delivery tubular  240  reaches the next preselected pressure increment. As with the direct pressure response method, the particular removal sequence should insure that each section of the gravel pack is tightly packed prior to initiating the gravel packing of subsequent sections. 
     Even though FIG. 9 has depicted removable members  260 A- 260 E as being positioned on the interior of fluid delivery tubular  240 , it should be understood by those skilled in the art that removable members could alternatively be positioned on the exterior of fluid delivery tubular  240  without departing from the principles of the present invention. Also, even though the embodiment of the apparatus for progressively treating an interval of a wellbore described in FIG. 9 referred to permanently removing the removable members, other types of removable members that are temporarily removed may alternatively be used, including, but not limited to, sliding sleeves and the like, without departing from the principles of the present invention. 
     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.