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REFERENCE TO PRIOR APPLICATION 
   This application is a continuation of application Ser. No. 10/095,182, filed Mar. 11, 2002, now U.S. Pat. No. 7,017,664, issued on Mar. 28, 2006, which claims the benefit of U.S. Provisional Application No. 60/314,689, filed Aug. 24, 2001. 

   TECHNICAL FIELD 
   This invention relates in general to the field of gravel packing and stimulation systems for mineral production wells, and more particularly, to an improved method and system for performing gravel packing and stimulation operations. 
   BACKGROUND 
   In an effort to extract natural resources such as oil and gas, it is becoming increasingly common to drill a vertical well, and to subsequently branch off that well and continue to drill horizontally for hundreds or even thousands of feet. The common method for drilling horizontally will be described more fully below, but generally includes the steps of forming a fluid impermeable filter cake surrounding the natural well bore while drilling at the production zone, removing drilling fluid from the downhole service tools (washdown), performing gravel packing operations, and then removing the downhole service tools from the well bore. A stimulation tool is then run back into the well, and the well stimulated with the appropriate chemicals to remove the filter cake so that production may begin. The above-described method requires two “trips” down into the well bore with different tools to accomplish gravel packing and well stimulation. Each trip into the well can take as much as a day, with the cost of a rig running anywhere from $50,000.00 to $250,000.00 per day. Accordingly, achieving both gravel packing and stimulation in a single trip can be substantially beneficial. Further, each additional trip into the well also increases the risk of fluid loss from the formation. Fluid loss in some cases may substantially reduce the ability of the well to effectively produce hydrocarbons. Therefore, there is a need for a system and method that simply and reliably performs gravel packing and stimulation operations in a single trip into the well. 
   SUMMARY 
   In accordance with the present disclosure, there is a system which enable gravel packing and stimulating a horizontal well on a single trip into the well. Where a horizontal well is packed with a filter cake during a drilling operation, the present invention is used to gravel pack proximate to the production zone and stimulate the production zone by removing the filter cake, all in a single trip. 
   According to one aspect of the invention, there is provided a method for completing a well comprising the steps of: inserting a completion tool assembly into the well, the completion tool assembly having a gravel packing assembly and a service tool assembly slidably positioned substantially within an interior cavity in the gravel packing assembly; removably coupling the service tool assembly and the gravel packing assembly; inserting a first plugging device into an interior channel within the service tool assembly to substantially block fluid from flowing through the interior channel past the first plugging device; diverting the fluid blocked by the first plugging device through a first fluid flow path to an exterior of the completion tool assembly; gravel packing the well with the completion tool assembly; inserting a second plugging device into the interior channel of the service tool assembly to substantially block fluid from flowing through the interior channel past the second plugging device; diverting the fluid blocked by the second plugging device through a second flow path that reenters the interior channel at a location distal of the first and second plugging devices; and stimulating the well with the well completion assembly. 
   According to a further aspect of the invention, there is provided a well completion tool assembly for gravel packing and stimulating a well comprising: a gravel packing assembly including a gravel packer; a service tool assembly slidably positioned substantially within an interior channel of the gravel packing assembly and capable of being removably coupled thereto, the service tool assembly including a cross-over tool having a cross-over tool aperture therein, an interior conduit between an annular bypass port into the interior channel located distal of the cross-over tool aperture and a exterior port to an exterior of the service tool assembly located proximal of the cross-over tool aperture, and an annular bypass closing mechanism for selectively opening and closing the annular bypass port. 
   According to still another aspect of the invention, there is provided a method for completing a well comprising the steps of: inserting into the well a completion tool assembly having a gravel packing assembly having a gravel packer, and a service tool assembly slidably positioned substantially within an interior cavity of the gravel packing assembly and having an interior channel therein; removably coupling the service tool assembly to the gravel packing assembly; setting the gravel packer; obstructing the interior channel with a first obstruction device; opening a first fluid flow path between the interior channel at a location proximal of the first obstruction device and an exterior of the well completion assembly at a location distal of the gravel packer; gravel packing the well with the completion tool assembly by pumping a slurry fluid into a proximal end of the interior channel and through the first fluid flow path; obstructing the first fluid flow path with a second obstruction device to prevent fluid flowing into the proximal end of the interior channel from flowing through the first fluid flow path; opening a second fluid flow path between the interior channel at a location proximal of the second obstruction device and the interior channel at a location distal of the first obstruction device, and stimulating the well with the completion tool assembly by pumping a stimulating fluid through into the proximal end of the interior channel and through the second fluid flow path. 
   According to another aspect of the invention, there is provided a method for completing a well in a single trip, the method comprising the steps of: inserting a completion tool assembly into the well, the completion tool assembly having a gravel packing assembly and a service tool assembly slidably positioned substantially within an interior cavity in the gravel packing assembly; removably coupling the service tool assembly and the gravel packing assembly; plugging at a first location, whereby fluid is blocked from flowing through the interior channel; diverting fluid blocked by the plugging at the first location through a first fluid flow path to an exterior of the completion tool assembly; circulating a gravel pack slurry through the completion tool assembly; plugging at a second location, whereby fluid is blocked from flowing through the interior channel; diverting fluid blocked by the plugging at the second location through a second flow path that reenters the interior channel at a location distal of the first and second plugging locations; and circulating a filter cake timulating fluid through the well completion assembly. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete understanding of the present invention and advantages thereof may be acquired by referring to the following description taken in conjunction with the accompanying drawings, in which like reference numbers indicate like features, and wherein: 
       FIG. 1  illustrates a typical horizontal well having a filter cake covering a portion of the wellbore wall; (Prior Art). 
       FIG. 2  is a flow chart illustrating steps for completing a well according to the present disclosure; 
       FIG. 3  illustrates a well completion tool assembly according to the present disclosure during washdown; 
       FIG. 4  illustrates a well completion tool assembly according to the present disclosure during setting of the gravel packer; 
       FIG. 5  illustrates a well completion tool assembly according to the present disclosure during testing of the gravel packer; 
       FIG. 6  illustrates a well completion tool assembly according to the present disclosure during reversing of the gravel packer; 
       FIG. 7  illustrates a well completion tool assembly according to the present disclosure during gravel packing; and 
       FIG. 8  illustrates a well completion tool assembly according to the present disclosure during stimulation of the well. 
   

   DETAILED DESCRIPTION 
   Preferred embodiments of the present invention are illustrated in the Figures, like numeral being used to refer to like and corresponding parts of the various drawings. 
   Referring now to  FIG. 1 , in horizontal wells  101  it is common practice not to form a casing in the well bore  100  along the portion of the horizontal wellbore through which oil or gas  102  is to be extracted. Instead, during drilling operations a “filter cake”  104  is deposited on an inner surface  105  of the wellbore. This filter cake is typically a calcium carbonate or some other saturated salt solution that is relatively fluid impermeable, and therefore, impermeable to the oil or gas in the surrounding formation. The filter cake is formed during drilling by pumping a slurry having particles suspended therein into the wellbore. The particles are deposited on the wellbore surface, eventually forming a barrier that is sufficiently impermeable to liquid. Systems and methods for depositing such a filter cake are well known in the art. 
   With the filter cake in place, the drilling equipment is removed from the well, and other tools are inserted into the well to pack the well with gravel. Once gravel packing is complete, the filter cake must be “stimulated” with the proper chemical solution to dissolve it to maximize production flow into the well. As indicated above, prior art systems and methods require removal of gravel packing tools and subsequent insertion of stimulation tools. According to the present disclosure, however, a single tool assembly can be lowered into the well to perform both gravel packing and stimulation in one trip. 
   A system and method for gravel packing and stimulating a well bore will now be described in greater detail with reference to  FIGS. 1–8 . According to one embodiment of the present disclosure, a completion tool assembly  301  including a gravel packing assembly  300  and a service tool assembly  330  is run into the well  101 . The gravel packing assembly has an interior cavity  345  extending substantially along its entire length, and a substantial portion of the length of the service tool assembly is slidably positioned within the interior cavity of the gravel packing assembly. The service tool assembly can be retracted relative to the gravel packing assembly as is illustrated in  FIGS. 3–8  and as will be described further below Although not explicitly shown in  FIGS. 3–8 , it is to be understood that a filter cake has already been deposited along the appropriate portion of the wellbore  101  (step  202  of  FIG. 2 ). 
   The gravel packing assembly includes at a distal end  343  a production screen  306 . The production screen may be a single screen, or preferably multiple production screen sections  306   a  interconnected by a suitable sealed joint  380 , such as an inverted seal subassembly. When production begins, the production screen filters out sand and other elements of the formation from the oil or gas. The service tool assembly  330  includes a service string  332  coupled to a cross-over tool  334 . A proximal end  336  of the service tool assembly includes a setting tool  382  that removably couples the service tool assembly to the gravel packer  320  of the gravel packing assembly at the proximal end  346  of the completion tool assembly. The proximal end of the service tool assembly is also coupled to a pipe string (not shown) that extends to the surface of the well for manipulating the service tool assembly. 
   Cross-over tool  334  is of a type also well known in the art. Cross-over tool  334  includes at least one cross-over tool aperture  350  providing a fluid flow path between the interior channel  338  and an exterior of the cross-over tool. It also includes a separate internal conduits  349  that form a fluid flow path between an annular bypass port  386  that opens into the interior channel at a location distal of the cross-over tool apertures, and an exterior port  399  that opens to the exterior of the cross-over tool at a location proximal of the cross-over tool apertures. With the gravel packing assembly and service tool assembly in position within the wellbore as shown in  FIG. 3 , washdown operations ( FIG. 2 , step  204 ) are performed to remove any remaining drilling fluid or debris from the service tool assembly by pumping clean fluid therethrough. The fluid flow path during washdown is illustrated by the arrows in  FIG. 3 . 
   As shown, fluid flows in a substantially unobstructed path through an interior channel  338  in the service tool assembly. The fluid flows out into the well area through a distal aperture(s)  340  at the distal end  341  of the service tool assembly and a distal aperture(s)  342  at the distal end  343  of the gravel packing assembly and well completion tool, and back in the annular space between the completion tool assembly and the wellbore that, before setting of the gravel packer, is present along the entire length of the completion tool assembly. In this manner, the service string assembly and the outer annular area between the gravel pack and screen assembly and the casing/formation are flushed clean of any remaining drilling fluid or debris. 
   After washdown is complete, gravel packing operations begin, and the completion tool assembly described herein can simply and readily perform both operations. As indicated above, during washdown the interior channel  338  of the service tool assembly is substantially unobstructed. According to the present system and method, a first plugging device  322  is inserted into the interior channel  338  (step  206 ) to form an obstruction and divert the fluid path to enable setting of the gravel packer. The first plugging device may be made of any suitable material and of any suitable configuration such that it will substantially prevent fluid from flowing through the interior channel past the first plugging device. According to one embodiment, the first plugging device is a spherical steel ball. It is inserted into place by dropping it into the annulus of the tool string at the surface of the well, and will travel into the proper position within the service tool assembly by means of gravity and fluid flow. A primary ball seat  398  may also be positioned within the interior channel of the service tool assembly to help retain the first plugging device in the proper position. 
   As shown in  FIG. 4 , the gravel packing assembly has at least one gravel packing aperture therein that, when the service tool assembly is removably coupled to the gravel packing assembly, is aligned with the cross-over tool aperture such that fluid may flow from the interior channel and through both apertures when unobstructed. A temporary closing sleeve  368 , however, controls fluid flow through the gravel packing assembly apertures, and is in the closed position during setting of the gravel packer as shown in  FIG. 4  (step  208 ). Thus, during setting, the first plugging device  322  obstructs fluid flow through the interior channel  338 , and because the temporary closing sleeve is also closed, fluid pressure within the interior channel  338  of the service tool assembly builds up in the vicinity of the gravel packer sufficiently to force the gravel packer outwards against the wellbore, thereby setting the gravel packer in place against the wellbore. These techniques are well known in the art, as are standard cross-over tools. 
   The completion tool assembly of the present invention, however, is also able to maintain annular pressure on the well formation during setting of the gravel packer. The well completion tool assembly includes an annular bypass closing mechanism for selectively opening and closing the annular bypass port. According to one embodiment, this annular bypass closing mechanism includes a device positioned within the interior channel that is slidable relative to the interior channel between open and closed positions. The device is configured so that when in the closed position, it obstructs the annular bypass port, and when slid into the open position it is configured so as not to obstruct the annular bypass port. According to one embodiment, the device is also the primary ball seat. Seating of the first plugging device within the primary ball seat causes the primary ball seat to slide sufficiently so that an opening therein becomes substantially aligned with the annular bypass port  386  so as not to obstruct it. Thus, fluid may freely flow from a first annular space  347  proximal of the gravel packer through the internal cross-over tool channels and into the interior channel at a location distal of the first plugging device. Thus, annular pressure is maintained on the formation to help maintain its integrity prior to gravel pack operations. 
   Once set, the gravel packer must be tested (step  210 ), and to test the packer the annular bypass port must once again be closed to isolate the annular fluid above the packer. As shown in  FIG. 5 , the proximal end  336  of the service tool assembly is uncoupled from the gravel packer  320 , and the service tool assembly is partially retracted from within the gravel packing assembly. This movement of the service tool assembly relative to the gravel packing assembly opens the temporary closing sleeve  368 , thereby allowing fluid flow between the interior channel  338  and the exterior of the gravel packing assembly. Further, this movement also causes a temporary interference collar  390  of the gravel packer assembly to engage a service tool isolation valve  388  that forms part of the service tool assembly. On further retraction of the service tool assembly, the service tool isolation valve stays substantially stationary relative to the gravel packing assembly, causing the annular bypass to once again be obstructed as shown in  FIG. 5  by an interference member  400 . 
   Following testing, the service tool is moved back downward removing the temporary interference collar to once again open the annular bypass  386  as shown in  FIG. 6 . Once this is accomplished, the service tool assembly is retracted relative to the gravel packing assembly to a point at which the cross-over tool apertures are positioned proximal of the gravel packer and form a flow path between the interior channel  338  and the first annular space. In this position fluid can be circulated at a point above the packer to avoid unnecessary exposure of the formation to such fluids. Thus, the well completion tool assembly according to the present disclosure is capable of selectively opening and closing the annular bypass port to advantageously maintain annular pressure on the formation and also to prevent pressure surges on the formation prior to and during gravel packing operations. 
   Subsequently, gravel packing is performed (step  212 ). As shown in  FIG. 7 , the service tool assembly is once again removable coupled to the gravel packing assembly by the setting tool  382 . In this position, the cross-over tool apertures  350  again substantially line up with the now open gravel packing apertures  384 . Thus, the fluid slurry used for gravel packing is pumped in through annular channel  338 , and is diverted by the first plugging device  322  through the cross-over tool apertures  350  and gravel packing apertures  384 , and out into the second annular space between the completion tool assembly and the wellbore, where it deposits sand in the production zone. Sand free fluid returns into the lower portion of the interior channel  338  through production screen  306 , passes through the annular bypass port  386 , internal conduit, and exterior port  399 , and into the first annular space. 
   Once gravel packing is complete, the filter cake must be removed before oil or gas can be extracted from the surrounding formation. According to the present disclosure, the above-described completion tool assembly can also simply and easily perform well stimulation to remove the filter cake while remaining in the well. 
   As shown in  FIG. 8 , a second plugging device  800  is inserted into the interior channel  338  of the service tool assembly to once again divert fluid flow (step  214 ). This second plugging device can be made of any suitable material, i.e., steel, and can be inserted into the service tool assembly in the same manner as described above for the first plugging device. The second plugging device, however, is of a diameter and configuration such that it forms a seal in a section of the interior channel of the service tool assembly that is above or proximal of the cross-over tool apertures  350 , thereby isolating the cross-over tool apertures with plugging devices both above and below. 
   The interior conduit of the cross-over tool also extends between the annular bypass port and an interior port  349  into the interior channel at a location proximal of the cross-over tool aperture. This interior port is opened by a sleeve which is shifted downward by the second plugging device. This sleeve closes the annular bypass port and opens the interior port. Fluid pumped into the interior channel above the second plugging device is now diverted through the interior port  349 , the interior conduit within the cross-over tool, the annular bypass port, and back into the interior channel  338  at a point below the first plugging device. Thus, fluid will once again flow into the interior channel at a point below or distal of the first plugging device, and the completion tool assembly can now be used to stimulate the well. 
   Stimulating fluid such as acids or solvents are pumped into the distal end of the interior chamber through the fluid path described above, where it exits the completion tool assembly through the distal apertures  340  in the service tool assembly and the production screen  306  of the gravel packing assembly. The stimulation fluid is diverted through the production screen by slick joints  355  that now seal off flow above and below the production screen. The stimulation fluid reacts with the filter cake on the surrounding wellbore to dissolve it. According to the present embodiment, the filter cake in the proximity of each screen element  306   a , is dissolved one section at a time, optimally starting with the most distal screen section. This is done both to ensure that there is adequate pressure to force the stimulation fluid out into the filter cake, and also to ensure that the filter cake is dissolved in a controlled fashion to prevent leakage before production is ready to begin. The service tool assembly is simply retracted from within the gravel packing assembly to move from one section to the next. 
   Subsequently, the service tool assembly is removed from the well. As it is removed, flapper valve  310  closes behind it to prevent loss of oil or gas before the production tubing is in place and production is ready to begin. 
   Although the present invention has been described in detail, it should be understood that various changes, substitutions and alterations can be made hereto without departing from the spirit and scope of the invention as defined by the claims.

Summary:
A method for completing a well in a single trip, including: inserting a completion tool assembly into the well, the completion tool assembly having a gravel packing assembly and a service tool assembly slidably positioned substantially within an interior cavity in the gravel packing assembly; removably coupling the service tool assembly and the gravel packing assembly; plugging at a first location, whereby fluid is blocked from flowing through the interior channel; diverting fluid blocked by the plugging at the first location through a first fluid flow path to an exterior of the completion tool assembly; circulating a gravel pack slurry through the completion tool assembly; plugging at a second location, whereby fluid is blocked from flowing through the interior channel; diverting fluid blocked by the plugging at the second location through a second flow path that reenters the interior channel at a location distal of the first and second plugging locations; and circulating a filter cake stimulating fluid through the well completion assembly.