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CONTINUATION STATEMENT 
     This application is a Continuation-in-Part of U.S. patent application Ser. No. 09/467,363, filed Dec. 20, 1999, now U.S. Pat. No. 6,206,100. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to apparatuses and methods for the completion of mineral production wells. In particular, the invention is related to a perforating and gravel packing system and method. 
     Modern oil and gas wells are typically equipped with a protective casing which is run into the wellbore. Production tubing is then run into the casing for producing minerals from the well. Adjacent the production zones, the protective casing is perforated to allow production fluids to enter the casing bore. Since particles of sand are typically carried with the mineral from the production zone into the casing, it is sometimes necessary to install a gravel pack or production screen to filter the particles of sand. Therefore, it is common practice to complete a mineral well in two steps: (1) run-in the well with a perforating gun to perforate the casing; and (2) run-in the well with a gravel pack tool to gravel pack and/or isolate the perforated zone. However, this method is disadvantageous because it requires multiple trips into the well to perforate and gravel pack the zone. 
     To reduce the required number of trips into the wellbore casing, various single trip perforation/gravel packing devices have been developed. For example, as described in U.S. Pat. No. 4,372,384, incorporated herein by reference, a single trip apparatus for completing a formation in a case borehole is disclosed. The patent teaches the use of a tool string which includes a perforating gun, gravel packing tools and a packer means. The casing is perforated by running a gun firing device down through the tubing string. The well is allowed to flow freely to clean up the perforated formation. The system is then moved to position a sandscreen of the gravel packer adjacent the perforations and packers are used at each end of the screen to straddle and pack off the perforated pay zone. With the screen and packers in position, a gravel pack is established in the annulus between the perforated casing and the screen. The tool screen is left downhole in the casing as a permanent completion device. The produced fluid is allowed to flow through the perforations, the gravel, screen, and finally up through the tubing screen to the surface. 
     An alternative well completion system is disclosed in U.S. Pat. No. 5,954,133, incorporated herein by reference. In particular, a method of displacing a perforating gun in a well bore is used to perforate multiple zones without the need to unset or reset a packer. Multiple perforating guns in a positioning device are configured in an axially compressed configuration. The perforating guns are attached to the positioning device and inserted into the wellbore. With a first perforating gun positioned adjacent a first zone, the gun is fired to perforate the casing. The positioning device is then extended to axially displace a second perforating gun within the casing to a position adjacent a second zone. The second gun is then fired to perforate the casing. After a zone(s) has been perforated, the positioning device is further axially extended to displace a production screen and packer. The production screen is positioned adjacent the perforations and the packer is positioned opposite the perforations. 
     U.S. Pat. No. 5,722,490, incorporated herein by reference, discloses a method and system wherein a gravel pack screen is placed in the well along with equipment in the tubing string to control flow from inside to outside the tubing below a production packer. The rig used to place the equipment may then be released from the well. The well is then hydraulically fractured. If the well is producing from a high permeability zone, the hydraulic fracture is preferably formed with the tip screen-out technique. The method can also be used in a well already containing production tubing without moving a rig on the well to remove the tubing from the well and can be used in a well not yet perforated by adding tubing-conveyed perforating apparatus below the screen. 
     As illustrated in some of the above referenced patent documents, in traditional one-trip systems, the perforating gun assembly is mechanically connected to the gravel pack assembly during run-in and perforating operations. A basic problem with traditional one-trip perforation/gravel packing systems is that the gravel packing portions of the system are damaged when the guns of the perforation portion of the system are detonated. In particular, a major factor affecting the reliability of one-trip perforation/gravel packing systems is the effects of gunshock on the gravel pack assembly. This shock loading can be in the form of a mechanical force which is communicated through a pipe string or similar structure connecting the perforating guns to the gravel packing assembly. Alternatively, a pressure wave created during detonation in the fluid column inside the wellbore casing can damage the gravel packing apparatus due to a shock effect It has been very difficult to predict the size of this shock effect and even more difficult to prevent it. 
     Therefore, there is a need for a one-trip perforation/gravel packing system which is more reliable than traditional systems in that the gravel packing portion of the system is protected from shock waves generated by the guns of the perforating portion of the system. 
     SUMMARY OF THE INVENTION 
     The present invention is a system and method of operation which performs both the perforating and gravel packing operations during a single-trip into a wellbore, and which also protects the gravel packing portion of the system from becoming damaged when the guns of the perforating portion of the system are detonated. The process that is described here represents a novel approach which involves a modification to traditional performing/gravel pack systems to eliminate the effects of gun shock on the gravel pack apparatus. 
     The present invention involves running the perforating apparatus into the wellbore on the same pipe string as the gravel pack assembly and anchoring the perforating apparatus to the wellbore. The perforating apparatus is then decoupled from the gravel pack assembly and the gravel pack assembly is picked up above the perforating apparatus. This accomplishes two things. First, mechanical shock is eliminated because the guns are no longer in mechanical contact with the gravel pack assembly. Mechanical shock is further dampened because the perforating apparatus is anchored into the wellbore. Second, the effects of a pressure wave are eliminated due to the dampening effect of the fluid column that exists between the top of the perforating apparatus and the bottom of the gravel pack assembly which is pulled away from and set above the perforating apparatus. Upon detonation, the guns and anchor device of the perforating apparatus are released or unset from the casing and are allowed to free fall or be pushed to the bottom of the wellbore. With the guns released from the wellbore casing, the gravel pack assembly is repositioned across the perforated zone. Sand control and stimulation treatments are then conducted to complete the well. 
     According to one aspect of the invention, there is provided a method of perforating and gravel packing a wellbore casing, the method comprising: making-up to a pipe string, a gravel packer assembly and a perforating apparatus; running-in the pipe string until the perforating apparatus is at a depth of intended perforations; and setting the perforating apparatus in the wellbore casing at a depth of intended perforations; and disconnecting the perforating apparatus from the pipe string. 
     According to a further aspect of the invention, there is provided a system for perforating and gravel packing a wellbore casing in a single trip into the wellbore, the system comprising: a gravel packer assembly having a production screen and at least one packer; a perforating apparatus connected to the gravel packer assembly, wherein the perforating apparatus is detachable from the gravel packer assembly after the system is placed in the wellbore and before a detonation of the perforating apparatus; a tool having at least one casing engaging slip segment, wherein the tool is matable with the perforating apparatus, and wherein the tool is settable in the wellbore casing. 
     According to still another aspect of the invention, there is provided a system for perforating and gravel packing a wellbore casing in a single trip into the wellbore, the system comprising: a gravel packer assembly having a production screen and at least one packer, wherein the gravel packer assembly is connected to a pipe string for running the system into the wellbore; a perforating apparatus connected to the gravel packer assembly, wherein the perforating apparatus is detachable from the gravel packer assembly after the system is placed in the wellbore and before a detonation of the perforating apparatus; a tool having at least one casing engaging slip segment, wherein the tool is matable with the perforating apparatus, and wherein the tool is settable in the wellbore casing; a release mechanism that releases the tool from being set in the wellbore casing; and a tube that extends between the gravel packer assembly and the perforating apparatus, whereby a drop bar is guided from the gravel packer to the perforating apparatus. 
     An aspect of the invention provides a method of perforating and gravel packing a wellbore casing, having the following steps: (1) making-up to a pipe string: a packer, a screen, and a perforating apparatus; (2) running-in the pipe string until the perforating apparatus is at a depth of intended perforations; (3) setting the perforating apparatus in the wellbore casing at a depth of intended perforations; and (4) disconnecting the screen and perforating apparatus from the pipe string. 
     Another aspect provides a system for perforating and gravel packing a wellbore casing, having: a packer which is mechanically communicable with a service string: a screen in mechanical communication with the packer; a perforating apparatus in mechanical communication with the screen, wherein the screen and perforating apparatus are detachable from the packer; and a tool having at least one casing engaging slip segment, wherein the tool is matable with the perforating apparatus, and wherein the tool is settable in the wellbore casing. 
     The invention has a further aspect, including a system for perforating and gravel packing a wellbore casing, having: a packer connectable to a pipe string for running the system into the casing, wherein the packer has a through path extending from a top end to a bottom end of the packer; a screen comprising a production screen and a vent screen, wherein the screen mechanically communicates with the packer; a perforating apparatus in mechanical communication with the packer, wherein the perforating apparatus and the screen are detachable from the packer; and a tool comprising at least one casing engaging slip segment and a release mechanism, wherein the tool is matable with the perforating apparatus, and wherein the tool is settable in the wellbore casing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is better understood by reading the following description of non-limitative embodiments with reference to the attached drawings wherein like parts in each of the several figures are identified by the same reference characters, and which are briefly described as follows. 
     FIG. 1 is a flow chart of a method embodiment of the invention for perforating and gravel packing a wellbore casing. 
     FIG. 2 is a sideview of a wellbore casing and a depth verification tool anchored in the casing. 
     FIG. 3 is a sideview of a wellbore casing and depth verification tool anchored in the casing. Further, a gravel packer assembly and perforating apparatus are shown suspended from a pipe string in the well casing above the depth verification tool. 
     FIG. 4 is a sideview of a wellbore casing with an anchored depth verification tool, perforating apparatus and gravel packer assembly. The perforating apparatus is secured to the depth verification tool and detached from the gravel packer assembly. Further, this figure shows the gravel packer assembly elevated to a position well above the perforating guns and a lower packer is set within the wellbore casing. 
     FIG. 5 is a sideview of a wellbore casing with a depth verification tool, perforating apparatus, and gravel packer assembly. As shown in FIG. 5, the perforating gun has detonated to perforate the wellbore casing and the depth verification tool has released or unset from the casing so that the depth verification tool and perforating apparatus have fallen to a position below the perforations. 
     FIG. 6 is a sideview of a wellbore casing wherein a depth verification tool and perforating apparatus have fallen to a low position in the wellbore casing, and a gravel pack assembly is positioned to straddle perforations in the wellbore casing. 
     FIG. 7 is a flow chart of a method embodiment of the invention for perforating and gravel packing a wellbore casing. 
     FIG. 8 is a sideview of a wellbore casing and a gravel pack/perforation system, wherein a depth verification tool is attached to a perforating apparatus so that a gravel pack assembly, a perforating apparatus and the depth verification tool are all run-in the well on the same pipe string. 
     FIG. 9 is a side view of a wellbore casing and gravel pack/perforation system wherein the system comprises a guide tube between a gravel packer assembly and a perforating apparatus. The guide tube ensures a denotation bar dropped through the gravel packer assembly will squarely contact and detonate the perforating apparatus. 
     FIG. 10 is a side, cross-sectional view of a depth verification tool. 
     FIG. 11A is a side cross-sectional view of a depth verification tool and release mechanism. In this figure, the depth verification tool is shown in a set position. 
     FIG. 11B is a side cross-sectional view of the depth verification tool and release mechanism shown in FIG.  11 A. In this figure, the depth verification tool is shown in a release position. 
     FIG. 12 is a flow chart of a method embodiment of the invention for perforating and gravel packing a wellbore casing. 
     FIG. 13 is a sideview of a wellbore casing and a depth verification tool anchored in a casing having a plug. This is a “Set Depth Verification Tool” configuration. 
     FIG. 14 is a sideview of a wellbore casing and depth verification tool anchored in the casing. Further, a gravel packer assembly and perforating apparatus are shown suspended from a pipe string in the well casing and seated on the top of the depth verification tool. This is a “Running” configuration. 
     FIG. 15 is a sideview of a wellbore casing with an anchored depth verification tool, perforating apparatus and gravel packer assembly. The perforating apparatus has a production screen attached to its top and is secured at its bottom to the depth verification tool. The gravel packer assembly is detached from the production screen and is elevated to a position well above the perforating guns. This is a “Disengage” configuration. 
     FIG. 16 is a sideview of a wellbore casing with a depth verification tool, perforating apparatus, and gravel packer assembly. The perforating gun has detonated to perforate the wellbore casing and the depth verification tool has released or unset from the casing so that the depth verification tool, perforating apparatus and production screen have fallen to rest on the plug. The production zone is gravel packed. This is a “Detonate/Pack” configuration. 
     FIG. 17 is a sideview of a wellbore casing with a depth verification tool, perforating apparatus, and gravel packer assembly. A washpipe extends from the gravel packer assembly to complete the gravel pack around the production screen. This is the “Washout” configuration of the system. 
     FIG. 18 is a sideview of a wellbore casing with a depth verification tool, perforating apparatus, and gravel packer assembly. The washpipe is withdrawn and the production fluids are allowed to flow through the gravel packer assembly. This is the “Production” configuration. 
     FIG. 19 is a sideview of an embodiment of the invention having a packer, screen, perforating apparatus and depth verification tool. 
    
    
     It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and therefore not to be considered limiting of its scope, as the invention may admit to other equally effective embodiments. 
     DETAILED DESCRIPTION OF THE INVENTION 
     According to a first embodiment of the invention, a depth verification tool is anchored in a wellbore casing at a depth adjacent a mineral production zone. A gravel packer assembly and a perforating apparatus are then run-in the casing on a single pipe string. The perforating apparatus is deposited on the depth verification tool and secured thereto. The perforating apparatus is detached from the pipe string and the pipe string is used to reposition the gravel packer assembly to a location separate from and above the perforating apparatus. A perforation packer at a lower end of the gravel packer assembly is then set in the wellbore casing. With the gravel packer assembly secured, perforating guns of the perforating apparatus are detonated to perforate the casing. Upon detonation, the depth verification tool and perforating apparatus are released or unset from the casing and allowed to fall to the bottom of the well. The perforation packer at the lower end of the gravel packer assembly is then released and the gravel packer assembly is repositioned to straddle the perforations in the casing. The packers of the gravel packer assembly are set and complete operations are conducted on the production zone. 
     This method embodiment of the invention is described in greater detail with reference to FIGS. 1 through 6. Referring to FIG. 1, a flowchart of a method for operation of a particular embodiment of the present invention is shown. FIGS. 2 through 6 illustrate cross sectional views of downhole tools in a wellbore casing at various stages of the method described in FIG.  1 . 
     The first step of the process is to anchor  101  a depth verification tool  40  in a wellbore casing  2 . As shown in FIG. 2, the depth verification tool  40  is anchored  101  at a depth and location which is proximate to a production formation  5  outside the casing  2 . The depth verification tool  40  may be lowered to this location by any means known to those of skill in the art. For example, the depth verification tool  40  may be lowered in the well casing  2  by a wireline, coil tubing or a pipe string. According to different embodiments of the invention, the depth verification tool  40  is set above, below, or in the interval of the wellbore casing  2  which spans the production formation  5 . 
     With further reference to FIG. 3, a gravel packer  10 , a perforating apparatus  20 , and a release mechanism  30  are run-in  102  the wellbore casing  2  on a pipe string  3 . The gravel packer  10  is equipped with a perforating packer  11  at its lower end and an upper packer  12  at its upper end. Between the packers  11  and  12 , the gravel packer  10  has a production screen  13 . Finally, the gravel packer  10  has a fracturing sleeve  14  and a cross-over tool  15 . According to various embodiments of the invention, nearly any gravel packer apparatus may be used with the invention. For example, the isolation and gravel packing systems disclosed in U.S. Pat. Nos. 5,609,204 and 5,865,251, incorporated herein by reference, are suitable for use with the present invention. The perforating apparatus  20  comprises a gun cylinder  21  and detonator  22 . The gun cylinder  21  is positioned with its longitudinal axis collinear with the central axis of the wellbore casing  2 . Perforating guns are located about the circumference of the gun cylinder  21  as is known in the perforating gun art. The detonator  22  is located at the top of the perforating apparatus  20  where the perforating apparatus is made-up to the bottom of the gravel packer  10 . The system is further equipped with a release mechanism  30  which is made-up to the bottom of the perforating apparatus  20 . The release mechanism  30  is configured to extend into the depth verification tool  40  and mate therewith. 
     As shown in FIG. 3, system is run-in  102  the wellbore casing  2  until the release mechanism  30  and perforating apparatus  20  are deposited  103  on the depth verification tool  40 . The perforating apparatus  20  is then secured  104  to the depth verification tool  40  by the release mechanism  30 . In an alternative embodiment of the invention, the release mechanism  30  is separate from the latching mechanism that attaches the perforating apparatus  20  to the depth verification tool  40 . The depth verification tool  40  is anchored into the casing  2  and a standard anchor latch assembly (not shown) is used to anchor the perforating apparatus  20  to the depth verification tool  40 . The release mechanism  30  is a separate tool that is threaded to the anchor latch or the perforating apparatus  20  depending on the particular application. 
     With particular reference to FIG. 4, once the perforating apparatus  20  is secured  104  to the depth verification device  40 , the gravel packer  10  is detached  105  from the perforating apparatus  20 . In alternative embodiments, the perforating apparatus  20  is connected to the gravel packer  10  by a “J-coupling” and the perforating apparatus  20  is detached  105  by an “un-J” procedure as is known in the art. The gravel packer  10  is then repositioned  106  to a location separate from and above the perforating apparatus  20  by pulling up on the pipe string  3 . The gravel packer  10  is repositioned  106  to a location between about 100 meters and about 200 meters separate from the perforating apparatus  20 . Once the gravel packer  10  is repositioned  106 , the perforation packer  11  is set  107  in the wellbore casing  2 . By setting the perforation packer  11 , the gravel packer  10  is secured in the wellbore casing  2  to prevent the gravel packer  10  from being damaged during detonation of the perforating apparatus  20 . Also, the perforation packer  11  is used to control the well after perforation to prevent fluids from travelling up through the annulus between the casing and the pipe string. 
     In an alternative embodiment of the invention, the perforation packer  11  is not set  107 . This step in the process is unnecessary where the well is perforated in an overbalanced condition. However, the gravel packer assembly  10  is still protected from the detonation shock effects of the perforating apparatus  20  because it is detached and separated from the perforating apparatus  20 . 
     Referring to FIG. 5, a view of the system is shown immediately after detonation of the perforating apparatus  20 . With the perforation packer  11  set  107 , the perforating apparatus  20  is detonated  108  to perforate the wellbore casing  2 . According to various embodiments of the invention, the detonator  22  is triggered by dropping a detonation bar or ball on the detonator, increasing the hydrostatic pressure in the wellbore, sending and electronic signal, or any other triggering mechanism known to those of skill in the art. In one embodiment, the gravel packer assembly  10  has a through path  16  which is large enough to allow a detonation bar or ball to be dropped from the pipe string  3 , through the through path  16  to the detonator  22 . As the guns of the perforating apparatus  20  are detonated  108 , the depth verification tool  40  is released  109  from the wellbore casing  2  to allow the perforating apparatus  20 , release mechanism  30  and depth verification tool  40  to fall to the bottom of the wellbore. The release mechanism  30  releases  109  or unsets these tools by deactivating the anchoring device of the depth verification tool  40  as described in greater detail below. Once the depth verification tool  40  is released  109  from the wellbore casing  2 , both the perforating apparatus  20  and the depth verification tool  40  are allowed to drop to the bottom of the wellbore. 
     Referring to FIG. 6, the perforation packer  11  is then released  110  from the wellbore casing  2 . The gravel packer  10  is then repositioned  111  to straddle the perforations in the wellbore casing  2 . This repositioning  111  is accomplished by lowering or running the pipe string  3  into the wellbore. The gravel packer  10  is repositioned  111  until the production screen  13  is immediately adjacent the perforations  4 . Once the gravel packer  10  is repositioned  111 , the perforation packer  11  is set to seal the lower end of the gravel packer  10 . The upper packer  12  is also set  112  to seal the upper end of the gravel packer  10 . The system is now properly configured to conduct  113  completion operations on the production zone. In embodiments of the invention having a through path  16  through the gravel packer assembly  10 , a plug is dropped into the through path  19  to close the through path  16  prior to completion operations. 
     Referring to FIGS. 4,  5 ,  6 ,  7  and  8 , an alternative method and apparatus of the invention is described and shown. In this embodiment, the depth verification device  40  is secured to the perforating apparatus  20  before the system is run into the wellbore. Therefore, a gravel packer  10 , perforating apparatus  20  and a depth verification tool  40  are all made up together on the surface before running into the wellbore. 
     As shown in FIGS. 7 and 8, the gravel packer  10 , perforating apparatus  20  and depth verification tool  40  are run-in  701  the wellbore casing  2  on a single pipe string  3 . The system is run-in  701  the wellbore until the perforating apparatus  20  is adjacent a mineral production formation  5  on the outside of the wellbore. Once depth has been achieved, the depth verification tool  40  is anchored  702  in the casing  2 . The perforating apparatus  20  is then detached  703  from the gravel packer  10 . With the perforating apparatus  20  detached  703 , the gravel packer apparatus  10  is repositioned  704  to a location separate and uphole from the perforating apparatus  20 . A perforation packer  11  of the gravel packer assembly  10  is set  705  to secure the gravel packer assembly  10  against the detonation of the perforating apparatus  20 . Next, the guns in the gun cylinder  21  of the perforating apparatus  20  are detonated  706  to perforate the casing. The depth verification device  40  is released  707  or unset from the casing so that the perforating apparatus  20  and depth verification tool  40  will fall to the bottom of the wellbore. The gravel packer assembly  10  is repositioned  708  to straddle the perforations in the casing and the packers  11  and  12  of the gravel packer assembly  10  are set  709  in the casing. The perforation packer  11  and upper packer  12  are set  709  to isolate the annulus between the production screen  13  and casing  2 . Completing operations are finally conducted  710  on the perforated portion of the wellbore casing  2 . 
     An alternative embodiment of the invention is shown in FIG.  9 . This embodiment is equipped with a guide tube  50 . The guide tube  50  ensures that a detonation bar dropped through the gravel packer  10  will travel through the guide tube  50  and squarely contact the detonator  22  of the perforating apparatus  20 . In the embodiment shown, the guide tube  50  is a telescoping mechanism having cylindrical sections which are concentric. Thus, a gravel pack cylinder  51  is attached to the bottom of the gravel packer  20  and a detonation cylinder  52  is attached to the top of the perforating apparatus  20 . The cylindrical sections are allowed to slide freely one within the other after the perforating gun is released or detached from the gravel packer  10 . These cylindrical sections are allowed to freely slide relative to each other to ensure mechanical vibrations are not transferred from the perforating apparatus to the gravel packer  10 . 
     Referring to FIG. 10, a side cross-sectional view of a depth verification tool  40  is shown. The depth verification tool  40  has exterior and interior sleeves which are both comprised of several independent components. The exterior sleeve has a setting sleeve connector  41  at its upper end. The setting sleeve connector  41  is made-up to a setting sleeve  42 . Both of these components make up a portion of the exterior of the depth verification tool  40 . The exterior is further comprised of a locking key mandrel  45  that communicates with the bottom of the setting sleeve  42 . Below the locking key mandrel  45  is an upper retainer  47  that holds a key  46 . The upper retainer  47  is made-up to a slip cage  53 , wherein the slip cage  53  extends below the upper retainer  47 . Finally, the exterior of the depth verification tool  40  comprises a bottom retainer  54 . The interior sleeve has a top coupling  43  near the top of the depth verification tool  40 . A mandrel  49  is made-up to the bottom of the top coupling  43  and extends from the top coupling  43  to approximately the bottom of the depth verification tool  40 . The depth verification tool  40  is made to be in set and release configurations by manipulating the relative positions of the exterior and interior sleeves. 
     Toward the top of the depth verification device  40  there is a shear pin(s)  68  which prevents relative axial movement of the setting sleeve  42  and top coupling  43 . Toward the bottom, the depth verification tool  40  is further comprised of slip segments  60  for engaging wellbore casing. In the embodiment shown, three slip segments  60  are spaced equal distance from each other around the circumference of the slip cage  53 . In alternative embodiments, more or less than three slip segments  60  are used. Slip return springs  61  are placed between the slip segments  60  and the slip cage  53  to bias the slip segments to a non-engaging position. A spacer  48  is positioned between the mandrel  49  and the slip cage  53  above the slip segments  60 . A bottom shoe  62  is positioned between the mandrel  49  and the slip cage  53  below the slip segments  60 . A release seat catcher  57  is made-up to the bottom of the bottom shoe  62 . Dogs  55  are positioned between the release seat catcher  57  and a releasing seat  56 . A shear pin(s)  70  extends between the release seat catcher  57  and the releasing seat  56  to prevent relative movement of these members. 
     The depth verification tool  40  is assembled by sliding the top coupling  43  into the setting sleeve  42  and screwing a shear pin(s)  68  through the setting sleeve  42  into the top coupling  43 . The key  46  and the upper retainer  47  are slipped over the locking key mandrel  45  and the body lock ring  44  is placed within the locking key mandrel  45 . The locking key mandrel  45  is then made-up to the setting sleeve  42 . The mandrel  49  is then made-up to the top coupling  43 . The slip segments  60  and slip return springs  61  are assembled to the slip cage  53  and the spacer  48  is placed inside the top of the slip cage  53 . The slip cage  53  is then made-up to the upper retainer  47 . The bottom shoe  62  is inserted between the slip cage  53  and the mandrel  49 . The dogs  55  are then placed in holes found at the lower end of the mandrel  49  and the releasing seat  56  is inserted into the lower end of the mandrel  49  until the releasing seat  56  is adjacent the dogs  55 . The releasing seat  56  is then held in place by a shear pin(s)  70 . The release seat catcher  57  is made-up to the bottom shoe  62  and shear pin(s)  69  is inserted through the release seat catcher  57  into the mandrel  49 . Finally, the bottom retainer  54  is made-up to the slip cage  53 . 
     According to one embodiment of the invention, the depth verification tool  40  is set in a wellbore casing at a desired depth by a setting tool (not shown). The setting tool has two concentric mechanisms, wherein one engages the setting sleeve connector  41  and the other engages the top coupling  43 . The setting tool sets the depth verification tool  40  in a wellbore casing by sliding the setting sleeve connector  41  and the top coupling  43  axially relative to each other. In particular, as shown in FIG. 10, the setting sleeve connector  41  is moved downward relative to the top coupling  43 . This action shears the shear pin(s)  68 , and moves the locking key mandrel  45  downward relative to the mandrel  49 . Since the dogs  55  are pushed radially outward by the releasing seat  56  through holes in the mandrel  49 , the dogs  55  engage the bottom of the bottom shoe  62  to hold the bottom shoe  62  stationary relative to the mandrel  49 . Similarly, the spacer  48  is pushed by the locking key mandrel  45 . Thus, when the setting sleeve connector  41  is moved downward relative to the top coupling  43 , the spacer  8  and bottom shoe  62  squeeze the slip segments  60 . The slip segments  60  are forced radially outward against the radially inward bias of the slip return springs  61 , so that the slip segments  60  engage a wellbore casing in a set position. The locking key mandrel  45  locks the slip segments  60  in the set position by the body lock ring  44  which engage teeth on the exterior of the mandrel  49 . According to different embodiments of the invention, setting tools (not shown) such as a hydraulic device, electromechanical device or any other device known to those of skill in the art may be used. 
     Referring to FIGS. 11A and 11B, side cross-sectional views of a depth verification tool  40  and release mechanism  30  are shown, wherein FIG. 11A depicts a set position and FIG. 11B depicts a release position. The release mechanism  30  comprises a piston  31  which drives a plunger  32 . The piston  31  slides within a piston cylinder  34 . In one embodiment of the invention, the piston cylinder  34  of the release mechanism  30  is made-up to the bottom of the perforating apparatus  20  (see FIG.  3 ). 
     The release mechanism  30  further comprises a coupling  33  which makes-up to the top coupling  43  of the depth verification device  40 . In particular, according to one embodiment of the invention described above, when the perforating apparatus  20  is deposited  103  on the depth verification tool  40  (see FIGS.  1  and  3 ), the coupling  33  of the release mechanism  30  mates with the top coupling  43  of the depth verification tool  40 . Upon mating, the plunger  32  of the release mechanism  30  extends down through the center of the mandrel  49  of the depth verification tool  40 . 
     According to one embodiment of the invention, when the release mechanism  30  is run-in  102  (see FIG. 1) the wellbore casing  2 , the pressure in the piston cylinder  34  is atmospheric pressure. When the perforating apparatus  20  is detonated  108 , pressure in the piston cylinder  34  increases because the casing is exposed to relatively higher pressure in the production zone  5  through the newly formed perforations  4  (see FIG.  5 ). The relatively higher hydrostatic pressure pushes the piston  31  in the piston cylinder  34  to move the plunger  32  downward (see FIGS.  11 A and  11 B). In an alternative embodiment, the pressure in the piston cylinder is increased by the explosion that occurs upon detonation of perforating guns. In a further embodiment, the pressure is increased by increasing the hydrostatic head of the completion fluid in the annulus of the well. In any case, as the plunger  32  moves downward, the distal end of the plunger  32  contacts the release seat  56  and exerts a downward force on the release seat  56 . This downward force eventually surpasses the shear strength of the shear pin(s)  69  and the shear pin(s)  69  is sheared. The release seat  56  is then pushed downward relative to the mandrel  49  until it falls in the release seat catcher  57 . With the release seat  56  removed from the mandrel  49 , the dogs  55  are free to move radially inward so that the bottom shoe  62  is free to move axially downward. At this point, the bottom shoe  62  may fall downward due to gravity or it may be pushed by further downward movement of the plunger  32 . In any case, the bottom shoe  62  is pulled from its set position behind the slip segments  60 . With nothing to support the slip segments  60 , the slip segments  60  are pushed radially inward by the slip return springs  61  to release the depth verification tool  40  from the wellbore casing  2 . This allows the depth verification tool  40  and the perforating apparatus  20  to fall in the wellbore casing  2  as described above. 
     Another embodiment of the invention is described with reference to FIGS. 12 through 18. FIG. 12 is a flow chart of describing a method for fracturing and packing a well casing, and FIGS. 13 through 18 illustrate cross sectional views of downhole tools in a wellbore casing at various stages of the method described in FIG.  12 . 
     A sufficient rathole is established in the well adequate to house in the well casing a depth verification tool, a perforating gun assembly, a cup tool and a screen overlap. The bottom of the rathole is defined by formation material in the well casing or a bridge plug. In the embodiment shown in FIGS. 13-18, a bridge plug  80  defines the bottom of the rathole. An electric line (not shown) is run into the well casing  2  to anchor  201  the depth verification tool  40  below the perforation depth. After the electric wire line is removed, the service string  3  is picked up and run  202  into the well casing  2  with the perforation/completion system attached. 
     In this embodiment, the perforation/completion system  6  comprises the service tool  17 , a packer  18 , a screen overlap  90 , and a perforating apparatus  20 . These devices are made up to each other and run into the well together on the service string  3 . The service string  3  is made of production pipe as described below. As shown in FIG. 14, the service tool  17  is made up to the lower end of the service string  3 . The packer  18  is made up to the lower end of the service tool  17 . At the lower end of the packer  18 , there is attached the screen overlap  90 . The screen overlap  90  has several components including: a cup tool  95 , a production screen  91 , a blank pipe  92 , a vent screen  93 , a nose plug  94 . Finally, the perforating apparatus  20  is attached to the bottom of the screen overlap  90 . Each of these components made be of any type known to persons of skill in the art. 
     The perforation/completion system  6  is run  202  into the well casing  2  until the perforating apparatus  20  is deposited  203  on and secured to the depth verification tool  40 . The perforating apparatus  20  is secured or snapped  203  to the depth verification tool  40  (see FIG. 14) so that the perforating apparatus  20  is anchored in the well casing  2  adjacent the formation  5  to be produced. The packer  18  is then detached  204  from the screen overlap  90  and the service tool  17  and packer  18  are repositioned  204  up the well casing  2  from the screen overlap  90  and perforating apparatus  20  to a desired depth (see FIG.  15 ). 
     The packer  18  is then set  205  at the desired depth above the perforation depth. In one embodiment, a slickline (not shown) is run down the service string  3  to set a plug in a nipple below the packer  18 . Pressure is then increased within the service string  3  (for example 2,500 psi) to set  207  the packer  18  in the well casing  2  at the desired depth. After the packer  18  is set, the service string  3  internal pressure is released. Pressure is then increased within the annulus between the service string  3  and the well casing  2  (for example 1,500 psi) to release the service tool  17  from the packer. The positive annulus pressure may also be used to test the integrity of the seal of the packer  18 . After the service tool  17  is released from the packer  18 , the annulus pressure is released. In alternative embodiments, any means known to persons of skill is used to set the packer  18 . In any case, the packer  18  is set  207  in the well casing  2  at the desired depth. 
     With the packer  18  set in the well casing  2 , the production tubing and Christmas tree are configured  206  at the well head and the rig is removed from the site. In one embodiment of the invention, the service string  3  (which also serves as the production tubing) is hung  206  from the well head. A nipple-up procedure is implemented to configure the Christmas tree to the top of the well head (not shown) as is known in the art. A tree saver, a stimulation vessel and a stimulation pump are made to communicate with the christmas tree. The rig (not shown) is removed since it is no longer needed at the well site. In this configuration, the annulus between the service string  3  and the well casing  2  is completely sealed by the packer  18  at the bottom and the christmas tree at the top. While this step of the process is herein described, it is to be noted that this step is not required in all embodiments of the invention. In some cases, the situation may demand that the rig remain on site. 
     Next, the perforation guns of the perforating apparatus  20  are detonated  207  to perforate the well casing  2 . In one embodiment of the invention, pressure is built up and bleed off to detonate the guns. Alternatively, a drop ball, electric signal or any means known to persons of skill may be used to fire the guns. The detonation of the gun causes the depth verification tool  40  to release from the well casing  2  and fall in the well to the bridge plug  90 . Of course, perforations  4  are formed in the well casing  2  adjacent the production formation  5  (see FIG.  16 ). The distance between the perforations  4  and the bridge plug  80  is made to correlate with the sizes of the tools so that when the tools fall in the well, the production screen  91  is adjacent the perforations  4 . 
     A gravel pack and fracture procedure is then followed to treat  208  the well. In one embodiment, a gravel slurry is pumped down the service string  3 . The slurry comprising proppant falls around the screen overlap  90  and out into the formation  5  through the perforations  4  in the well casing  2 . The cup tool  95  is positioned below the production screen  91  to substantially prevent the slurry with proppant from flowing down around the perforating apparatus  20  and the depth verification device  40 . Pressure is increased in the service string  2  to fracture the formation  5  and the proppant of the slurry prop open the fractures in the formation  5 . The pressure is released. A sufficient amount of proppant is deposited in the annulus between the screen overlap  90  and the well casing  2  to pack the screen overlap  90 . In an alternative embodiment, a first portion of the proppant is deposited to pack the production screen  91 , a concrete plug is placed on top of the pack adjacent the blank pipe  92 , and a second portion of proppant is deposited to pack the vent screen  93 . 
     Since an excess amount of proppant is typically packed on top of the nose plug  94  of the screen overlap  90 , the pack is washed  209  to remove the excess. For example, a wash pipe  100  comprising coil tubing is run into the service string  3  until the end of the wash pipe  100  is immediately above the top of the nose plug  94 . The excess proppant is then pumped up the wash pipe  100 . Once the excess proppant is removed, the wash pipe  100  is withdrawn from the service string  3 . In alternative embodiments, it is not necessary to wash the excess proppant and/or gravel pack. Rather, the well is simply brought into production and the excess proppant and/or gravel pack will be produced with the initial product from the well. 
     The well is now ready to produce  210  minerals up the service string. The flow path for the production zone  5  is through the perforations  4 , through the production screen  91  and into an interior of the screen overlap  90 , up the interior of the blank pipe, out the vent screen  93  to the interior of the well casing  2 , through the interior of the packer  18 , and up the inside of the service string  3 . While mineral may flow up the gravel packed annulus between the screen overlap  90  and the casing  2 , the mineral will preferentially follow the path of least resistance which is through the interior of the screen overlap  90  as described. As noted above, the service string  3  and well head assemblies are properly configured even before the well casing is perforated. Thus, once the completion processes are finished, the well may be immediately brought into production. 
     This embodiment of the invention provides many benefits, depending on the particular well conditions. First, a gamma ray electric line run is eliminated as compared to other systems where a sump packer is run below the perforation depth, the casing is perforated, and a completion system is stung into the sump packer. Second, the system of the present invention eliminates cycle time because only two trips into the well are required: (1) an electric line run to set the depth verification tool, and (2) service string run to place perforation/completion system. Third, the need for a crossover tool is eliminated because there is no recirculation during the gravel pack operation. Fourth, the Christmas tree is placed at the well head and the rig is removed before the casing is perforated. The christmas tree seals the annulus and the service string. The Christmas tree has a flange that seals off the casing. Fifth, since the Christmas tree and packer are set before perforation, there is no need to fill the well casing with heavier completion fluid. For example, typical completion system require 17 lbs. completion fluid in the well during perforation to prevent blow out in an overbalanced condition. This heavier fluid is very expensive and an isolation system must be rapidly installed to prevent the fluid from flowing out into the formation in an underbalanced condition. In the present invention, regular 11.6 lbs. completion fluid may remain in the well since the Christmas tree and packer are set prior to perforation. Further, even if there is an underbalanced condition, only the 11.6 lbs. completion fluid in the service string will flow to the formation and the completion fluid in the annulus is retained by the packer. Thus, unlike other systems, the present invention does not require a fluid loss device, such as a flapper valve or sliding sleeve to prevent fluid loss while production tubing is tripped into the well. Sixth, the present invention requires a very short rathole, for example, a depth equal to the combined length of the depth verification device and the perforating apparatus. Seventh, for reasons outlined above the present invention is recommendable in both overbalanced and underbalanced operations. 
     In an alternative embodiment, the depth verification device  40  is made up to the bottom of the perforating assembly  20  before the perforation/completion system  6  is run-in the well casing  2 . This eliminates the need for the separate electric line trip into the well to set the depth verification tool  40 . 
     In still another embodiment of the invention, the system comprises a gravel packer  10  having perforating and upper packers  11  and  12  as described above with reference to FIG.  3 . The perforating packer  11  is attached at its bottom to the perforating apparatus  20  as previously described, but a screen overlap  90  is attached to its top. When the system is bottomed on the depth verification device, the upper packer  12  disconnects from the top of the screen overlap  90  for relocation up the well casing. Of course, in this embodiment, the screen overlap  90  does not comprise a nose plug  94  and the crossover tool assembly of the upper packer is stung into the screen overlap  90  and the production packer  11 . 
     Referring to FIG. 19, a sideview of an embodiment of the invention is shown. A packer  18  is shown at the top and is connectable to a service string (not shown). A suitable packer is a Comp-Set  11  “HP” Rotational Lock Packer. Below the packer  18  and by several sections of pipes and connectors, a vent screen  93  is made-up to the packer  18 . The vent screen  93  may be any screen or vent know to persons of skill, but in particular, it may be a wire wrap screen. There is also a production screen  91  and a blank pipe  92  between the two screens. Similarly, the production screen  91  may be any screen known to persons of skill, but in particular, it may be a micro-pack screen. Below the production screen  91 , there is made-up a cup tool  95  which serves to keep particles from falling in the annulus below the cup tool  95 . A second vent screen  93  is made up below the cup tool  95 . At the bottom of the system, there is a perforation apparatus  20  and a depth verification tool  40 . The second vent screen  93  (below the cup tool  95 ) enables the apparatus to fall freely in the casing after release by the depth verification tool  40 . In particular, the second vent screen  93  allows fluid trapped below the cup tool  95  to pass through the interior of the system from below the cup tool  95  to above the cup tool  95 . A bridge plug  80  is shown set in the casing below the system. 
     A further embodiment of the invention comprises a configuration similar to that shown in FIGS. 13-18. While the embodiment has a screen overlap  90  which is attached at its bottom to a perforating apparatus  20 , the screen overlap  90  is not attached directly to the packer  18 . Rather, the screen overlap  90  is connected to the packer  18  by a telescoping joint similar to the guide tube  50  shown in FIG.  9 . There is no nose plug  94  between the screen overlap  90  and the telescoping joint. This telescoping joint has holes above the screen overlap  90  to communicate gravel pack material from the service string to the annulus. In operation, after the system is gravel packed, both the interior of the screen overlap  90  and the annulus will be full of gravel pack material. A washpipe  100  is then extended into the interior of the screen overlap  90  to wash the interior. The system is then ready for production. 
     While the particular embodiments for single-trip perforating/gravel packing systems and methods as herein shown and disclosed in detail are fully capable of obtaining the objects and advantages hereinbefore stated, it is to be understood that they are merely illustrative of the preferred embodiments of the invention and that no limitations are intended by the details of construction or design herein shown other than as described in appended claims. 
     
       
         
               
             
               
               
             
           
               
                   
               
               
                 PARTS LIST 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 2 
                 Wellbore casing 
               
               
                 3 
                 Pipe string 
               
               
                 4 
                 Perforations 
               
               
                 6 
                 perforation/completion system 
               
               
                 5 
                 Production formation 
               
               
                 10 
                 Gravel packer 
               
               
                 11 
                 Perforation packer 
               
               
                 12 
                 Upper packer 
               
               
                 13 
                 Production screen 
               
               
                 14 
                 Fracturing sleeve 
               
               
                 15 
                 Cross-over tool 
               
               
                 16 
                 Through path 
               
               
                 17 
                 Service Tool 
               
               
                 18 
                 Packer 
               
               
                 20 
                 Perforating apparatus 
               
               
                 21 
                 Gun cylinder 
               
               
                 22 
                 Detonator 
               
               
                 30 
                 Release mechanism 
               
               
                 31 
                 Piston 
               
               
                 32 
                 Plunger 
               
               
                 33 
                 Coupling 
               
               
                 34 
                 Piston cylinder 
               
               
                 40 
                 Depth verification tool 
               
               
                 41 
                 Setting sleeve connector 
               
               
                 42 
                 Setting sleeve 
               
               
                 43 
                 Top coupling 
               
               
                 44 
                 Body lock ring 
               
               
                 45 
                 Locking key mandrel 
               
               
                 46 
                 Key 
               
               
                 47 
                 Upper retainer 
               
               
                 48 
                 Spacer 
               
               
                 49 
                 Mandrel 
               
               
                 50 
                 Guide tube 
               
               
                 51 
                 Gravel pack cylinder 
               
               
                 52 
                 Detonation cylinder 
               
               
                 53 
                 Slip cage 
               
               
                 54 
                 Bottom retainer 
               
               
                 55 
                 Dogs 
               
               
                 56 
                 Releasing seat 
               
               
                 57 
                 Release seat catcher 
               
               
                 60 
                 Slip segments 
               
               
                 61 
                 Slip return springs 
               
               
                 62 
                 Bottom shoe 
               
               
                 68 
                 Shear pin(s) 
               
               
                 69 
                 Shear pin(s) 
               
               
                 70 
                 Shear pin(s) 
               
               
                 80 
                 Bridge Plug 
               
               
                 90 
                 Screen Overlap 
               
               
                 91 
                 Production Screen 
               
               
                 92 
                 Blank Pipe 
               
               
                 93 
                 Vent Screen 
               
               
                 94 
                 Nose Plug 
               
               
                 95 
                 Cup Tool 
               
               
                 100 
                 Wash Pipe

Summary:
A method of perforating and gravel packing a wellbore casing, having the following steps: (1) making-up to a pipe string: a packer, a screen, and a perforating apparatus; (2) running-in the pipe string until the perforating apparatus is at a depth of intended perforations; (3) setting the perforating apparatus in the wellbore casing at a depth of intended perforations; and (4) disconnecting the screen and perforating apparatus from the pipe string. A system for perforating and gravel packing a wellbore casing, having: a packer which is mechanically communicable with a service string: a screen in mechanical communication with the packer; a perforating apparatus in mechanical communication with the screen, wherein the screen and perforating apparatus are detachable from the packer; and a tool having at least one casing engaging slip segment, wherein the tool is matable with the perforating apparatus, and wherein the tool is settable in the wellbore casing.