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CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is the National Stage of International Application No. PCT/US03/40349, filed Dec. 18, 2003, which claims the benefit of U.S. Provisional Patent Application No. 60/451,156, filed Feb. 26, 2003. 

   FIELD OF THE INVENTION 
   This invention generally relates to wellbores used for production of formation fluids. More particularly, this invention relates to well completion providing the ability to utilize one fluid for drilling the wellbore, running the gravel packing assembly and sand control screens, and then displacing and gravel packing the completion interval with the same or another fluid. 
   BACKGROUND 
   The proper fluids for drilling, gravel packing and sand screens installation are essential for well completion success. Careful planning, well preparation and completion execution are required to increase completion productivity and longevity. Historically, a minimum of three fluids has been used to drill and complete gravel packed wells. The first fluid is a solids-laden drilling-fluid used to drill the completion interval. The second fluid is a solids-free completion-fluid used to displace the solids-laden drilling-fluid and to run sand-exclusion equipment and gravel packing tools in a generally solids-free environment. The third fluid is a carrier fluid for the gravel during gravel packing of the completion interval. 
   In producing hydrocarbons a wellbore is drilled through a subterranean reservoir. Drilling practices can affect a gravel pack and sand screen the same way they can affect conventionally perforated wells. The well should be drilled to maintain wellbore stability, and drilling fluids should be used that will not damage the formation. 
   The drilling fluid typically contains weighting solids, viscosifying solids, and drilled solids at varying concentrations. Drilling fluid filtrates should be compatible with completion fluids and should not interfere with the completion operations. Preferably, the drilling fluid selected should be dense enough to result in a well that is slightly overbalanced, should have low fluid loss and should be compatible with the clays in the productive formation. 
   The proper preparation of a well for gravel packing can be the key to completion success. Cleanliness is one of the most important considerations in the preparation of gravel packs. The presence of any particulate materials can result in a damaged completion. Currently tanks are often dedicated to gravel pack use to avoid repeated cleaning operations for drilling mud removal. 
   Completion fluids are used to displace the solids-laden drilling fluid and to run sand-exclusion equipment and gravel packing tools in a generally solids-free environment. Completion fluids can be oil- or water-based fluids. The water-based fluids are usually considered to be more flexible. Their densities, viscosities, and formation compatabilities are more easily controlled than those of oil-based fluids. Therefore, water-based fluids are most commonly utilized. 
   Regardless of the source of the completion fluid, the fluid should contain minimum particulate material and its chemistry must be compatible with the rock formation and connate water. Fresh water may cause clays to swell or disperse, while the presence of some ions may cause precipitation when in contact with formation water. The most common sources of completion fluids are field or produced brine, seawater, bay water or fresh water. The density of the completion fluids is often controlled with soluble salts. 
   Gravel placement involves those operations required to transport gravel from the surface to the completion interval to form a downhole filter that will permit the flow of fluids into the well but will prevent the entry of formation sands. Preferably, the gravel placement provides a uniform pack with a porosity of thirty-nine percent or less. 
   The gravel placement requires fluid to transport the gravel slurry to the completion interval. Oil- and water-based fluids and foams are commonly used as the gravel placement fluid. Clean fluids are essential for gravel placement. Depending on well pressures, high-density, solids-free soluble salt solutions may be required to maintain well control. In addition, the gravel placement fluids can be viscified by adding polymers. 
   Poor distribution of the gravel slurry is often caused when carrier fluid from the slurry is lost prematurely into the more permeable portions of the formation and/or into the screen, itself, thereby causing “sand bridge(s)” to form in the well annulus around the screen. These sand bridges effectively block further flow of the gravel slurry through the well annulus thereby preventing delivery of gravel to all levels within the completion interval. 
   To alleviate poor gravel distribution, “alternate-path” well tools or technology have been proposed and are now in use which provide for uniform distribution of gravel throughout the entire completion interval notwithstanding sand bridges formation before completion of gravel distribution. Such devices typically include perforated shunts or by-pass conduits which extend along the length of the device and which are adapted to receive the gravel slurry as it enters the well annulus around the device. If a sand bridge forms before the operation is complete, the gravel slurry can still be delivered through the perforated shunt tubes (such as, “alternate-paths”) to the different levels within the annulus, both above and/or below the bridge. U.S. Pat. Nos. 4,945,994 and 6,220,345 provides descriptions of typical alternate-path well screens and how they operate. 
   To summarize, the current method used to install open-hole gravel packs typically involves drilling the completion interval with water- or oil-based drilling fluid, displacing the fluid in the open-hole to a solids-free completion fluid (typically brine), running the gravel pack assembly and sand screens to depth in the solids-free completion fluid, and gravel packing the interval with a water-based carrier fluid. A common limitation of this method involves the inability to run the gravel pack assembly and sand screens to depth due to wellbore instability (collapse) caused by incompatibility between the water-based completion fluid (brine) and the formation. This method is inefficient since at least three fluids are required (drilling fluid, completion fluid, and gravel carrier fluid). 
   A frequent modification to the method described above involves placing a pre-drilled liner in the completion interval prior to displacing the open-hole to completion fluid and running the gravel pack assembly and sand screens (Murray, G., Morton, K., Blattel, S., Davidson, E., MacMillan, N., Roberts, J., SPE 73727, Feb. 20-21, 2002. Development of the Alba Field—Evolution of Completion Practices, Part 2 Open Hole Completions; Successful Outcome—Drilling with SBM and Gravel Packing with Water Based Carrier Fluid and Gilchrist, J. M., Sutton, Jr., L. W., Elliot, F. J., SPE 48976, Sep. 27-30, 1988. Advancing Horizontal Well Sand Control Technology: An OHGP Using Synthetic OBM.). The pre-drilled liner mitigates wellbore collapse and provides a conduit for running the gravel pack assembly and sand screens. While the pre-drilled liner improves the ability to run the gravel pack assembly and sand screens to depth, it provides an additional resistance to flow and may have a negative impact on productivity. 
   The current practice of using separate fluids for drilling, displacing the solids-laden drilling fluid and running sand-exclusion equipment and gravel packing tool, and gravel placement is both costly and time-consuming. Accordingly, there is a need to reduce operational complexity and time by simplifying the fluid system and eliminating the need for the pre-drilled liner. This invention satisfies that need. 
   SUMMARY 
   In an embodiment, the method comprises drilling a wellbore with a drilling fluid, conditioning the drilling fluid, running the gravel packing assembly tools to depth in a wellbore with the conditioned drilling fluid, and gravel packing an interval of the wellbore using a carrier fluid. The carrier-fluid may be the same as the drilling fluid. This method may be combined with alternate-path sand screen technology to ensure proper distribution of the gravel pack 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a flow chart of an embodiment of the invention; 
       FIG. 2  is an illustration of a wellbore with a gravel pack using a two-fluid system illustrating the installation of an alternate-path sand screen in an oil-based conditioned fluid; 
       FIG. 3  is an illustration of a wellbore with a gravel pack using a two-fluid system illustrating the installation of a GP packer and the introduction of the neat gravel pack with the crossover tool in the reverse position; 
       FIG. 4  is an illustration of a wellbore with a gravel pack using a two-fluid system illustrating the sweeping of mud from the open hole interval adjacent to the screens by the carrier fluid with the crossover tool in the circulating position; 
       FIG. 5  is an illustration of a wellbore with a gravel pack using a two-fluid system illustrating the reverse position of the crossover tool after sweeping of mud from the open hole interval to reverse-out the remaining neat gravel pack fluid and the conditioned oil-based fluid; 
       FIG. 6  is an illustration of a wellbore with a gravel pack using a two-fluid system illustrating the location of the viscous spacer, neat gravel pack fluid and the gravel pack slurry in the drillpipe with the crossover tool in the reverse position and placement of the gravel pack fluid in the annulus; 
       FIG. 7  is an illustration of a wellbore with a gravel pack using a two-fluid system illustrating the crossover tool in the circulating position to gravel pack the open-hole section of the wellbore annulus; 
       FIG. 8  is an illustration of a wellbore with a gravel pack using a two-fluid system illustrating the continued displacement of the neat gravel packing fluid out of the annulus and the diversion of a gravel pack slurry around a sand bridge; 
       FIG. 9  is an illustration of a wellbore with a gravel pack using a two-fluid system illustrating the displacement of the gravel pack slurry with a completion-fluid until screen-out occurs; 
       FIG. 10  is an illustration of a wellbore with a gravel pack using a two-fluid system illustrating the reverse position of the crossover tool with completion fluid pumped into the annulus and a reverse-out of the excess sand and gravel pack fluid from the drill pipe; and 
       FIG. 11  is an illustration of a wellbore with a gravel pack using a two-fluid system illustrating a complete gravel pack of the openhole interval, a well fully displaced to completion fluid and the gravel pack assembly pulled out of the wellbore. 
   

   DETAILED DESCRIPTION 
   The invention described herein provides a method for installing an open-hole gravel pack completion. The installation process involves drilling the completion interval with drilling fluid, conditioning the drilling fluid, running the gravel packing assembly and sand control screens to depth in the conditioned drilling fluid, then displacing and gravel packing the completion interval with the same or another fluid. 
   This new method for installing open-hole gravel pack completions addresses problems that have been experienced while attempting to run sand screens to depth in the wellbore prior to gravel packing. In addition, benefits of the new procedure include reduced completion time due to simplified operational procedures and potential elimination of a slotted liner. 
   As shown in  FIG. 1 , the method has four basic steps. First, a well is drilled in an interval through a subterranean formation with a drilling fluid  1 , which may be referred to as an open-hole interval drilling fluid, non-aqueous fluid (NAF), and/or solids-laden fluid. Second, the drilling fluid is conditioned  2 . Third, the gravel pack assembly tools are run to depth in the wellbore with the conditioned fluid  3 , which may be referred to as conditioned drilling fluid. Fourth, an interval of the wellbore is gravel packed with a carrier fluid  4 . The carrier fluid can be the same as the conditioned fluid or a separate fluid, which may be referred to and include neat fluid, neat gravel pack fluid, displacement fluid and/or solids-free fluid. If the wellbore does not need to be gravel packed a screen can be run to depth in the wellbore with the conditioned fluid with the fourth step no longer necessary. 
   The completion interval is drilled with either water-based or oil-based drilling fluid. After drilling the completion interval, the drilling fluid is circulated through the wellbore and filtered (or conditioned) using equipment on the rig floor. Typically, the drilling fluid contains particles (such as, drill cuttings) that may plug the openings (or slots) in the sand screen and potentially plug the gravel pack if not sufficiently removed. Therefore, the drilling fluid is conditioned (or filtered) before running the sand screens to preferably remove solid particles larger than approximately one-third the slot opening size and/or one-sixth the diameter of the gravel pack particle size. 
   The one-third slot size is based on the general rule of thumb for size of spherical particles required to bridge a given slot size. The one-sixth diameter of the gravel pack particle size is based on the general rule of thumb for the required size of pore throats in a pack of spherical particles at a given diameter to avoid plugging. For example, typical wire-wrapped sand screens have 8.5 gauge slots (approximately 215 microns) and 30/50 proppant (approximately 425 microns) may be used for the gravel packs. The drilling fluid can be conditioned over 310 mesh shaker screens (approximately 50 microns) on the drilling rig, which should sufficiently filter-out the oversized particles. 
   Also, during field operations, a screen tester apparatus may be utilized to check samples of the conditioned fluid to verify whether it freely passes through a screen sample with a specified slot size. Typically, the recommended slot size is 3 to 4 gauge sizes less than nominal screen slots. Once the fluid conditioning process is adequately verified using the screen tester apparatus, the gravel pack assembly and sand screens can be run to depth in the wellbore. 
   Running sand screens in conditioned- 13  fluid for stand-alone screen completions is a frequent operational practice for persons skilled in the art. For example, this practice is often conducted in the North Sea where gravel packing is not necessary due to the high permeability formations that have large sand grains with uniform size distributions. For open-hole completions that must be gravel packed due to heterogeneous formation with non-uniform grain size distributions, prior to this new method sand screens were not run in conditioned fluid. 
   After the gravel pack assembly and sand screens are run to depth, the open-hole interval fluid is typically displaced with a volume of neat fluid. Neat fluid is gravel carrier fluid not laden with gravel pack proppants. The displacement removes conditioned drilling fluid and drill cuttings that remain in the open-hole. The displacement fluid is circulated in a direction that does not direct solids-laden fluid through the screen in an effort to avoid screen plugging. For example, the fluid can be circulated down the annulus, through the crossover to the washpipe, down the washpipe of the screen assembly, and out the screen. Previously, open-hole gravel pack installation methods required the completion of the displacement operation before installing sand screens because previous methods assumed sand screens should be run in solids-free fluid. 
   After the open-hole interval is displaced, the completion interval is gravel packed using standard operational procedures. The pump rate for the gravel pack operation should be slower than the displacement rate to avoid screen plugging. 
   In addition, after the gravel pack assembly has been run and prior to the gravel packing operation, several gravel pack service tool manipulations must be performed, as discussed below in the example. The new method requires that the manipulations be performed in solids-laden fluid which was not done in previous methods (gravel pack assembly previously run in solids-free fluid). 
   In another embodiment, the invention involves drilling a completion interval in a wellbore with an oil-based drilling fluid and gravel packing an interval of the wellbore with a water-based carrier fluid using alternate-path technology. Compared to water-based fluids, oil-based fluid filter cakes have lower lift-off pressures that can be problematic for installing a complete gravel pack. Filter cake is a concentrated layer of solids from the drilling fluid that forms on the borehole wall opposite a permeable formation. Loss of the filtercake during gravel packing may result in the formation of a bridge. As described previously in the background section, alternate path allows transport of sand beyond the bridge. As a result, alternate-path technology is desirable for wells that are to be gravel packed and are drilled with oil-based fluid. 
   The water-based gravel pack carrier fluid should have favorable rheology for effectively displacing the conditioned fluid and favorable rheology and sand carrying capacity for gravel packing using alternate path technology. Examples of the water-based carrier fluid include but are not limited to a fluid viscosified with HEC polymer, xanthan polymer, visco-elastic surfactant (VES) or combinations thereof. Persons skilled in the art will recognize other carrier fluids that may be chosen because of their favorable properties. 
   In another embodiment, the gravel pack carrier fluid is oil-based. The method using the oil-based carrier fluid would be the same as described above with the water-based carrier fluid. 
   EXAMPLE 
   The invention was developed as a result of operational difficulties experienced while attempting to run the gravel pack assembly in a wellbore. The planned procedure for the wellbore was to drill the completion interval, displace to solids-free brine, run the gravel pack assembly and screens, then gravel pack the completion interval using water-based carrier fluid. However, after displacing the open-hole completion interval to completion brine, the gravel pack assembly and sand screens could not be run to depth after several attempts due to wellbore stability problems. Unsuccessful attempts were also made to run a pre-drilled liner. The wellbore was suspended and operations were moved to a nearby wellbore. After the experience at the first failed wellbore, a new completion procedure (the present inventive method) was developed and utilized for the nearby wellbore and subsequent wellbores. The new completion procedure has been successfully employed for multiple wells. Well tests have indicated that the new method provides an efficient, low-skin completion. 
     FIGS. 2 through 11  illustrate the two-fluid system well completion using an alternate path well screen in a field test wherein like elements of  FIGS. 2 through 11  have been given like numerals. First, a well is drilled using a drilling fluid with techniques known to persons skilled in the art. Next, a well screen is installed in a wellbore filled with conditioned drilling fluid, such as non-aqueous fluid (NAF).  FIG. 2  is an illustration of a screen  27  with alternate path technology  21  inside a wellbore  23 , which is part of the gravel pack assembly. The gravel pack assembly consists of a screen  27 , alternate path technology  21 , a GP Packer  29 , and a crossover tool  35  with fluid ports  26  connecting the drillpipe  28 , washpipe  41  and the annulus of the wellbore  23  above and below the GP Packer  29 . This wellbore  23  consists of a cased section having a casing  22  and a lower open-hole section  24 . Typically, the gravel pack assembly is lowered and set in the wellbore  23  on a drillpipe  28 . The NAF  25  in the wellbore  23  had previously been conditioned over 310 mesh shakers (not shown) and passed through a screen sample (not shown) 2-3 gauge sizes smaller than the gravel pack screen  27  in the wellbore  23 . 
   As illustrated in  FIG. 3 , the GP packer  29  is set in the wellbore  23  directly above the interval to be gravel packed. The GP Packer seals the interval from the rest of the wellbore  23 . After the GP Packer  29  is set, the crossover tool  35  is shifted into the reverse position and neat gravel pack fluid  33  is pumped down the drillpipe  28  and placed into the annulus between the casing  22  and the drillpipe  28 , displacing the conditioned oil-based fluid, which is the NAF  25 . The arrows  36  indicate the flowpath of the fluid. 
   Next, as illustrated in  FIG. 4 , the crossover tool  35  is shifted into the circulating position, which may also be referred to as the circulating gravel pack position or gravel pack position. Conditioned NAF  25  is then pumped down the annulus between the casing  22  and the drillpipe  28  pushing the neat gravel pack fluid  33  through the washpipe  41 , out the screen screens  27 , sweeping the open-hole annulus  45  between the alternate path technology  21  and the wellbore wall in open-hole section  24  and through the crossover tool  35  into the drillpipe  28 . The arrows  46  indicate the flowpath through the open-hole section  24  and the alternate path technology  21  in the wellbore  23 . 
   As illustrated in  FIG. 5 , once the open-hole annulus  45  between the alternate path technology  21  and the wellbore wall in open-hole section  24  has been swept with neat gravel pack fluid  33 , the crossover tool  35  is shifted to the reverse position. Conditioned NAF  25  is pumped down the annulus between the casing  22  and the drillpipe  28  causing a reverse-out by pushing NAF  25  and dirty gravel pack fluid  51  out of the drillpipe  28 , as shown by the arrows  56 . 
   Next, as illustrated in  FIG. 6 , while the crossover tool  35  remains in the reverse position, a viscous spacer  61 , neat gravel pack fluid  33  and gravel pack slurry  63  are pumped down the drillpipe  28 . The arrows  66  indicate direction of fluid flow of fluid while the crossover tool  35  is in the reverse position. After the viscous spacer  61  and 50% of the neat gravel pack fluid  33  are in the annulus between the casing  22  and drillpipe  28 , the crossover tool  35  is shifted into the circulating gravel pack position. 
   Next, as illustrated in  FIG. 7 , the appropriate amount of gravel pack slurry  63  to pack the open-hole annulus  45  between the alternate path technology  21  and the wellbore wall of the open-hole section  24  is pumped down the drillpipe  28 , with the crossover tool  35  in the circulating gravel pack position. The arrows  77  indicate direction of fluid flow of fluid while the crossover tool  35  is in the gravel pack position. The pumping of the gravel pack slurry  63  down the drillpipe  28 , forces the neat gravel pack fluid  33  through the screen  27 , up the washpipe  41  and into the annulus between the casing  22  and the drillpipe  28 . Conditioned NAF  25  returns are forced through the annulus between the casing  22  and the drillpipe  28  as the neat gravel pack fluid  33  enters the annulus between the casing  22  and the drillpipe  28 . 
   As illustrated in  FIG. 8 , the gravel pack slurry  63  is then pumped down the drillpipe  28  by introducing a completion fluid  101  into the drillpipe  28 . The gravel pack slurry  63  displaces the conditioned NAF (not shown) out of the annulus between the casing  22  and the drillpipe  28 . Next, gravel is deposited in the open-hole annulus  45  between the alternate path technology  21  and the wellbore walls of the open-hole  24 . If a sand bridge  81  forms as shown in  FIG. 8 , then gravel pack slurry  63  is diverted into the shunt tubes of the alternate-path technology  21  and resumes packing the open-hole annulus  45  between the alternate path technology  21  and the wellbore walls of the open-hole section  24  and below the sand bridge  81 . The arrows  86  illustrate the fluid flow of the gravel pack slurry  63  down the drillpipe  28  through the crossover tool  35  into the annulus of the wellbore below the GP Packer  29  through the alternate-path technology  21  to the open-hole annulus  45  between the alternate path technology  21  and the wellbore walls of the open-hole section  24  and below the sand bridge  81 . The arrows  86  further indicate the fluid flow of the neat gravel pack fluid  33  up the washpipe  41  through the crossover tool  35  in the annulus between the casing  22  and the drillpipe  28 . 
     FIG. 9  illustrates a wellbore  23  immediately after fully packing the annulus between the screen  27  and casing  22  below the GP packer  29 . Once the screen  27  is covered with sand  91  and the shunt tubes of the alternate path technology  21  are full of sand, the drillpipe  28  fluid pressure increases, which is known as a screenout. The arrows  96  illustrate the fluid flowpath as the gravel pack slurry  63  and the neat gravel pack fluid  33  is displaced by completion fluid  101 . 
   As illustrated in  FIG. 10 , after a screenout occurs, the crossover tool  35  is shifted to the reverse position. A viscous spacer  61  is pumped down the annulus between the drillpipe  28  and the casing  22  followed by completion fluid  101  down the annulus between the casing  22  and the drillpipe  28 . Thus, creating a reverse-out by pushing the remaining gravel pack slurry  63  and neat gravel pack fluid  33  out of the drillpipe  28 . 
   Finally, as shown in  FIG. 11 , the fluid in the annulus between the casing  22  and the drillpipe  28  has been displaced with completion fluid  101 , and the crossover tool (not shown) and drillpipe (not shown) are pulled out of the wellbore  23  leaving behind a fully-packed well interval below the GP Packer  29 . 
   Laboratory testing was conducted to qualify the inventive method described above before the method was field-tested. Laboratory testing indicated that the solids contamination of a gravel pack potential result of an inefficient displacement of solids laden drilling fluid) does not impair the pack permeability. The test involved mixing a volume of gravel with a volume of drilling fluid and packing the mixture into a cylindrical flow apparatus. The drilling fluid was displaced from the gravel by flowing another fluid through the pack. Measurements of the permeability of the initial gravel pack not previously mixed with solids-laden drilling fluid and measurements of the gravel pack after the drilling fluid had been displaced from the pack were similar indicating negligible potential for impairment. In addition, to the laboratory test, the successful field trial, described above verified the feasibility of the procedures described above. The procedures include fluid conditioning procedures, field testing apparatus procedures to monitor the conditioning process and the manipulation procedures (reverse and circulating positions) of gravel pack service tools described above. Furthermore, the fluid displacement efficiencies of using solids-laden drilling fluid and gravel carrier fluids with sand screens in the wellbore were also verified.

Summary:
A method for drilling and completing a gravel packed well is disclosed. The method comprises drilling a wellbore with a drilling fluid, conditioning the drilling fluid, running the gravel packing assembly tools to depth in the wellbore with the conditioned drilling-fluid, and gravel packing a wellbore interval with a completion-fluid. The completion fluid may be the same as the drilling-fluid. This method may be combined with alternate-path sand screen technology to ensure proper distribution of the gravel pack.