Abstract:
A process and assembly for completing and providing sand control in a subterranean well and/or fracturing and preventing proppant flowback in a subterranean formation in a single trip. One or more perforating gun assemblies are juxtaposed and secured to one or more screen assemblies. Once positioned in a well adjacent a subterranean formation of interest, the explosive charges in each perforating gun assembly are detonated so as to penetrate the well and formation thereby initiating fracturing. The penetrations and the annulus defined between the well and screen assembly are then packed with gravel. Well fluid may be pressurized to in excess of the formation pressure prior to detonation of the explosive charges so as to enhance formation fracturing.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a process for completing, providing sand control and/or fracturing a subterranean well in a single trip, and more particularly, to the use of one or more perforating gun assemblies positioned within a screen assembly to permit perforation of a well and formation while fluid in the well bore is pressured to an predetermined condition, such as an overbalanced condition, and proppant is subsequently placed in the well without removal of the assemblies. 
   2. Description of Related Art 
   Production of unconsolidated materials, e.g. sand and other fines, from subterranean formations into wells is problematic. Left unabated, continued production of such unconsolidated materials can result in erosion of production equipment, well plugging, and/or reduced or complete loss of fluid production from a well. Thus, it is conventional practice to control the production of unconsolidated materials into many subterranean wells. Where the subterranean formation is composed of relatively hard, consolidated material and fracturing operations are performed so as to enhance fluid communication with the well, conventional practice is to control the flow of proppant that is utilized in the fracturing operations back into the well thereby ensuring that the fractures remained propped open. 
   In accordance with the most commonly practiced technique, “gravel packing”, a tubular liner is positioned in the well bore and a proppant gravel is placed in the annulus between the liner and the well bore. Gravel is commonly mixed with the fluid, such as a liquid or foam, to form a slurry which is pumped through a work string and a crossover tool into the annulus between the well bore and the liner. The slurry flows down the annulus to the bottom of the well bore or to a sump packer in the well bore. Some of the fluid of the slurry flows through the apertures in the liner into the open bottom end of a wash pipe situated within the liner and returns to the surface through the crossover tool and the annulus between the work string and the well casing. The bulk of the slurry fluid flows into the subterranean zone through perforations in the well bore. Gravel is thus deposited in the annulus and against the subterranean zone. The liner has slots or other apertures in its walls which are smaller in size than the gravel particles, thereby permitting formation fluids to flow through the slots while preventing entry of any unconsolidated materials. Gravel packing operations are typically performed at pressures below the formation fracture gradient, and the primary design considerations are placement of proppant inside perforation tunnels and in the annulus between the well bore and liner. The small apertures may be provided by a screen encircling the outer circumference of the liner tube, in which case the openings in the tube may be larger than the gravel particles. As a result of improved technology, gravel packs have become quite effective in excluding sand from oil and gas production. In addition to this function, the gravel also assists in supporting the walls of uncased wells and preventing caving of loose material against the liner. Despite the effectiveness of gravel packs once they are properly placed and operating, the procedure often results in undesirable completion skins or damage to the walls of the well bore which reduce the flow of formation fluids into a well. 
   In accordance with a relatively recent technique of completing well bores while practicing sand control termed “frac packing”, the unconsolidated formation is fractured and propping material is deposited in the fracture. Typically, a completion fluid of sufficient density for pressure control is first placed in a cased well, the cased well is perforated adjacent the subterranean zone or formation of interest. The perforating equipment is then removed from the well and a separate trip is required to place sand control equipment in the well adjacent the perforations. A fracturing fluid having proppant material incorporated therein is pumped, with the sand control equipment in place, at a sufficiently high pressure to propagate a fracture into the subterranean formation. The proppant materials within the fracturing fluid are deposited in the resulting fracture(s). While several variations of this process are practiced, the steps set forth above are employed to complete a given frac pack operation. However, significant costs are incurred with the material, equipment and time necessary to perform this series of operations. 
   The problems associated with conventional frac packing operations have spawned significant interest in reducing fluid costs, in developing simplified equipment and methods for minimizing the number of trips necessary to deploy equipment in the well and in eliminating the use of a rig at the surface of the earth. Methods and apparatus have been recently developed that allow perforating operations and screen placement to be performed in a single trip. U.S. Pat. No. 5,722,490 discloses a method of completing and hydraulically fracturing a well wherein a tubing conveyed perforating gun assembly is attached below a gravel pack screen. The perforating gun assembly is lowered to a depth opposite a productive zone and activated. The perforating gun assembly may be designed to be released from the tubing and fall to the bottom of the well after firing. The tubing string is then lowered to place the gravel pack screen opposite at least one of the perforations formed. Hydraulic fracturing operations are subsequently performed. However, this method still requires intervention with a rig to perform operations for positioning, perforating, setting of packer(s), etc. that are necessary to accomplish the method. Accordingly, a need still exists for a cost effective method for providing the stimulation benefits of a frac pack method together with sand control without necessarily requiring the use of a rig at the surface of the earth. 
   Methods have also recently been developed for exerting extreme pressures on a subterranean formation instantaneously with perforating the well casing so as to clean the perforation tunnels that are formed and to generate near-wellbore fractures to connect with existing natural fractures in the formation. U.S. Pat. No. 5,131,472 discloses such a method and provides for non-mechanical sand control by use of resin coated sand. However, a need exists for performing an overbalanced perforating operation while utilizing mechanical means and methods to provide for increased sand control, decreased time and costs and increased safety. 
   SUMMARY OF THE INVENTION 
   To achieve the foregoing and other objects, and in accordance with the purposes of the present invention, as embodied and broadly described herein, one characterization of the present invention may comprise a process for completing a well is provided which comprises positioning at least one explosive charge juxtaposed to a screen that is positioned in a well and detonating the at least one explosive charge. 
   In another characterization of the present invention, a process is provided for completing a subterranean well which comprises securing at least one explosive charge radially juxtaposed to a screen, positioning the screen and the at least one explosive charge in a subterranean well adjacent a subterranean formation, and detonating the at least one explosive charge thereby perforating the subterranean formation. 
   In yet another characterization of the present invention, a one trip well process is provided for fracturing a subterranean formation and for completing a well penetrating the formation. The one trip process comprises pressuring fluid present in a subterranean well to an predetermined condition and forming perforations in the well while a screen assembly is present in the well adjacent a subterranean formation, the fluid causing said formation to fracture. 
   In still another characterization of the present invention, a one trip well process for fracturing a subterranean formation and for completing a well penetrating the formation is provided. The process comprises pressuring fluid present in a subterranean well to an overbalanced condition and forming perforations in the well, said fluid causing said formation to fracture. A slurry of gravel is injected into an annulus defined between the well and the screen assembly thereby packing the perforations with the gravel and forming a gravel pack in the annulus. 
   In a still further characterization of the present invention, a one trip process for completing a well is provided which comprises securing at least one perforating gun assembly in a juxtaposed relationship to a screen assembly, positioning the at least one perforating gun assembly and the screen assembly in a well adjacent a subterranean formation, and pressurizing fluid in the well to an overbalanced condition thereby detonating the at least one perforating gun assembly so as to form perforations in the subterranean formation. The pressured fluid fractures the formation via the perforations. 
   In a still further characterization of the present invention, a well completion assembly is provided which comprises a screen assembly having at least one aperture, at least one perforating gun assembly having at least one explosive charge, and a pressure activated firing assembly connected to the at least one perforating gun assembly. The at least one perforating gun assembly is positioned within the aperture and secured to the screen assembly such that each of said at least one explosive charge is aimed through said at least one aperture. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and form a part of the specification, illustrate the embodiments of the present invention and, together with the description, serve to explain the principles of the invention. 
     In the drawings: 
       FIG. 1  is a partial cross sectional view of one embodiment of the assembly of the present invention; 
       FIG. 1A  is a cross sectional view of a pressure firing head assembly that may be utilized in conjunction with the assembly and process of the present invention that is illustrated in  FIG. 1 ; 
       FIG. 2  is a partially cutaway view of one embodiment of a screen assembly used in the assembly and process of the present invention; 
       FIG. 2A  is a partially cutaway view of the embodiment of a screen assembly illustrated in  FIG. 2  which has been rotated to illustrate apertures in the perforating charge carrier; 
       FIG. 2B  is a cross sectional view of the embodiment of a screen assembly taken along line  2 B— 2 B in  FIG. 2 ; 
       FIG. 3  is a partially cutaway view of another embodiment of a screen assembly used in the assembly and process of the present invention; 
       FIG. 3A  is a partially cutaway view of the embodiment of a screen assembly illustrated in  FIG. 3  which has been rotated to illustrate apertures in the perforating charge carrier; 
       FIG. 3B  is a cross sectional view of the embodiment of a screen assembly taken along line  3 B— 3 B in  FIG. 3 ; 
       FIG. 4  is a partial cross sectioned, perspective view of one embodiment of the assembly of the present invention as positioned adjacent a subterranean formation of interest; 
       FIG. 5  is a partial cross sectioned, perspective view of another embodiment of the assembly of the present invention as positioned adjacent a subterranean formation of interest; 
       FIG. 6  is a partial cross sectioned, perspective view of yet another embodiment of the assembly of the present invention as positioned adjacent a subterranean formation of interest; 
       FIG. 7  is a cross sectional view taken through a screen assembly utilized in conjunction with the assembly and process of the present invention wherein two perforating charge carriers are positioned within the screen assembly; 
       FIG. 8  is a cross sectional view taken through a screen assembly utilized in conjunction with the assembly and process of the present invention wherein three perforating charge carriers are positioned within the screen assembly; 
       FIG. 9  is a cross sectional view taken through another embodiment of a screen assembly utilized in conjunction with the assembly and process of the present invention wherein at least one perforating gun assembly is positioned within a housing that is secured to a half pipe configured screen assembly; and 
       FIG. 10  is a cross sectional view taken through a further embodiment of a screen assembly utilized in conjunction with the assembly and process of the present invention wherein at least one perforating gun assembly is positioned outside of and secured to a screen assembly. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   One embodiment of the assembly of the present invention is illustrated generally as  10  in FIG.  1 . Assembly  10  is comprised of a perforating gun assembly  20  and a firing assembly  40  secured to each other and positioned within a screen assembly  50  that is secured to the end of a tubular  90 . Perforating gun assembly, as illustrated, is comprised of a sub  24 , a perforating charge carrier  26  and a bull plug  28 . One end of the perforating charge carrier  26  is attached to sub  24  by any suitable means, such as by screw threads  25 . A pair of O-rings  33  provide a fluid tight seal between carrier  26  and sub  24 . The other end of perforating charge carrier  26  is attached to bull plug  28  by any means, such as screw threads  27  and O-rings  29  which provide a fluid tight seal therebetween. Charge carrier  26  and perforating charge tube  30  are generally tubular. Perforating charge tube  30  is designed to be aligned as positioned inside perforating charge carrier  26  so that the large ends  38  of charges  34  are adjacent scallops  32  formed in the exterior of perforating charge carrier  26 . As illustrated, openings  31  in the wall of charge tube  30  are positioned generally linear along axis of the tube. Although charges  34  are preferably lined charges, any other equivalent charge, explosive or bullet known to those skilled in the art as useful in perforating casing and/or a subterranean formation may be utilized in the assembly and process of the present invention. The charge, explosive or bullet may be designed to produce any suitably configured perforation or hole in the casing and/or subterranean formation, such as round, oblong, linear, etc. A detonating cord  35  is connected to the firing assembly  40  above sub  24 , to the small end  35  of each perforating charge  34 , and to an aluminum or rubber closure  39  in bull plug  28 . Where another gun assembly  20  is threaded onto the perforating charge carrier  26  in lieu of bull plug  28 , reference numeral  39  would refer to a booster transfer as will be evident to a skilled artisan. 
   Any suitable detonating system known to those skilled in the art may be used in the assembly and process of the present invention. The detonating system may be electrical or mechanical, may be used in conjunction with a timer, and may be initiated by fluid pressure (gas or liquid), electrical current, and/or any other suitable means, such as electromagnetic or acoustic signals as will be evident to a skilled artisan. An example of a detonating system suitable for use with the assembly of the present invention is illustrated in  FIG. 1A. A  vent  42  is provided in one end of vent housing  41  while the other end of vent housing is secured to a firing head  70  by any suitable means, such as by screw threads  71  and O-rings  72  to provide a fluid tight seal there between. A piston  43  is positioned within housing  41  and is releasably secured therein by means of shear pins  44 . O-rings  45  provide a fluid tight seal between piston  43  and housing  41 . An annular chamber  46  between piston  43  and the interior wall of housing  41  is filled with air. A firing pin  47  is connected to and extends downward from the bottom of piston  43 . A percussion firing assembly  81  is retained within firing head  70  by the pin end of housing  41 . Sub  24  is attached to firing head  70  by any suitable means, such as by screw threads  84  and O-rings  85  to provide a fluid tight seal there-between. An ignition transfer  83  at the top of sub  24  is in contact with detonating cord  35  passing through the perforating charge carrier  26 , as described above. 
   As illustrated in  FIG. 1 , a screen assembly  50  has one end thereof secured to an end plug  51  while the other end thereof is secured to a length of tubular  90 , such as a section of blank pipe which may be attached to a packer assembly or designed for a screen assembly. Tubular  90  typically has a length of about 60 to 90 feet. In accordance with the present invention, screen assembly  50  is comprised of a pipe  52  having apertures  53  therethrough. A generally tubular screen  56  is positioned about the pipe so as to cover apertures  53  and the ends of screen  56  are secured to pipe  52  by any suitable means, such as by welds. Screen  56  may be any conventional screen employed for sand control, as will be readily evident to a skilled artisan. For example, screen  56  may be a conventional wire wrapped screen such as illustrated in  FIGS. 3 ,  3 A and  3 B and commercially available from the Johnson Screens, a Weatherford Company, under the trademark SuperWeld® or a sintered laminate such as is illustrated in  FIGS. 2 ,  2 A and  2 B and commercially available from Johnson Screens under the trademark Excelflo™. 
   In accordance with the present invention, firing assembly  40  and perforating gun assembly  20  are positioned within and axially offset to one side of the interior of screen assembly  50  and tubular  90  and secured thereto in a manner described below. The screen assembly is provided with an aperture or opening which is configured to encompass scallops  32  that are formed in the exterior of perforating charge carrier  26 . Specifically, as illustrated in  FIGS. 2A and 3A , the aperture in screen assembly  50  is formed of an aperture  54  in perforated pipe  52  and an aperture  57  in screen  56  that is configured substantially similar to but is slightly larger than aperture  54 . As illustrated in  FIGS. 2B and 3B , pipe  52  is secured to perforating charge carrier  26  at or near the edges  55  of aperture  54  in pipe  52  by any suitable means, such as welds  59 , so as to form a seal between pipe  52  and perforating charge carrier  26  that is sufficient to prevent proppant entry. In a similar manner, screen  56  is secured to pipe  52  at or near the edges  58  of aperture  57  in screen  56  by any suitable means, such as welds  60 , so as to form a seal between screen  56  and pipe  52  that is sufficient to prevent proppant entry. Although charge carrier  26  and charge(s)  34  are illustrated in  FIGS. 2B and 3B  as being within screen assembly  50 , it will be evident to a skilled artisan that aperture  54  may be sized such that both charge carrier and charges  34  protrude outwardly from screen assembly. 
   Referring to  FIG. 4 , a subterranean well  100  is illustrated as comprising a well bore  101 , casing  102  and cement sheath  104 . Well bore  101  is drilled from the surface of the earth in a conventional manner so as to penetrate at least one subterranean formation or zone of interest  108 . A generally tubular casing string  102  is then positioned within well bore  101  and secured therein by means of a cement sheath  104  that is placed in the annulus between the casing string and the well bore in accordance with any conventional technique as will be evident to a skilled artisan. To complete well  100  in accordance with one embodiment of the present invention, assembly  10  of the present invention, blank pipe  112 , isolation plug  114 , closing sleeve  116  and packer  118  are assembled on tubing string  90  at desired spacing prior to or while tubing string  90  is run into well  100 . Tubing string  90  is then lowered into well  100  and snapped into a sump packer  119  which may be previously run into well  100  and set to isolate production of fluid from subterranean formation or zone  108  from that portion of the well below sump packer  119 . Other than assembly  10 , the component parts assembled on tubing string  90  are conventionally available. For example, a suitable isolation plug is commercially available from Halliburton Energy Services, Inc. of Dallas, Tex. under the trade name designation PX, RX or NX plug and a suitable closing sleeve is commercially available from Halliburton Energy Services, Inc. under the trade name designation MCS closing sleeve or from Weatherford International, Inc. of Houston, Tex. under the trade name designation Frac Sleeve. The packer utilized in accordance with the present invention will vary with the exact method employed, and as such, may be permanent or retrievable, may be wireline deployed or tubing conveyed, and may have a seal bore or be run with tubing as will be evident to a skilled artisan. Examples of a wireline deployed, retrievable packer is that commercially available from Halliburton Energy Services, Inc. under the trademark Versa-Trieve®, of a tubing deployed, retrievable packer is that commercially available from Halliburton Energy Services, Inc. under the trade name designation Perma Latch, of a high temperature, high pressure version of either a wireline or tubing deployed, retrievable packer is that commercially available from Halliburton Energy Services, Inc. under the trade name designation HTHP, and of a tubing deployed, retrievable packer is that commercially available from Halliburton Energy Services, Inc. under the trade name designation RH. 
   As positioned within well  100 , closing sleeve  116  is preferably placed in the open position. Wireline or coiled tubing may be used to open closing sleeve  116 , if necessary such as in a high angle well. Once tubing string  90  is located at the desired position within well  100 , i.e. such that assembly  10  is adjacent formation or zone  108 , packer  118  is set either by hydraulic or mechanical means depending upon the packer employed as will be evident to a skilled artisan thereby effectively isolating formation or zone  108 . At this point, the rig at the surface can be moved off location or may remain on location if appropriate for the completion operations. A coiled tubing unit and hydraulic fracturing equipment are moved on location. Hydraulic fracturing fluid is then pumped down tubing string  90  and is communicated via the opened sleeve  116  into the annulus defined between tubing string  90  and casing  102  and between packers  118  and  119 . This fracturing fluid may be any fluid deemed to have the proppant carrying properties as dictated by the subterranean formation of interest and completion method employed. Suitable carrier fluids include gels, for example hydroxyethylcellulose or crosslinked polymers. Water will be sufficient for certain applications, such as a high rate water pack in which the primary emphasis is packing perforations and the annulus without fracturing the formation. The fracturing fluid is also communicated via port  42  to piston  43  in firing assembly  40 . Pressure on the fracturing fluid is increased to a pressure that is significantly greater than the formation pressure until pins  44  shear causing firing pin  47  to strike percussion firing assembly  81  in firing head  70 . The ignition of percussion firing assembly  81  causes a secondary detonation in ignition transfer  83  which in turn ignites detonating cord  35 . Ignition of cord  35  detonates each perforating charge  34  which blasts through each adjacent scallop  32  in perforating charge carrier  30  and creates a perforation  122  which extends or penetrates through casing  102  and cement  104  and into subterranean formation or zone  108 . Pins  44  are designed to shear at an predetermined pressure, e.g. a pressure greater than the fracturing pressure of the subterranean formation or zone  108  of interest. In this manner, immediately upon detonation of perforating charge(s)  34 , the formation will be subjected to an condition that is in excess of the formation fracture gradient thereby fracturing the formation. Perforation(s)  122  will be surged with high pressure and fluid present in the annulus  120  will be injected into the formation or zone  108  at a high rate and pressure. Since perforation(s)  122  immediately upon creation thereof, the formation  108  is not allowed sufficient time to heal itself thereby increasing the efficiency and effectiveness of the fracturing process. 
   Once a pressure drop is noted at the surface indicating that the perforating charge(s) have fired and fluid has been injected into the formation, a frac pack operation is then performed via tubing string  90 . Fluid is pumped via string  90  at a pressure in excess of the fracture gradient of formation or zone  108 . Preferably, a “tip screen-out” technique is employed wherein a high concentration of proppant is pumped in the fracturing fluid near the end of the treatment. As proppant may be left in the tubing string  90 , coiled tubing may be run Into the well to wash proppant out of the tubing and casing and to pull the isolation plug  114  from the well. The coil tubing may then be used to close sleeve  116  and the well may be pressure tested, production tested or placed on production. 
   An alternative embodiment of the process of the present invention is illustrated in  FIG. 5  in which a bridge plug  130 , for example a cast iron bridge plug commercially available from Alpha Oil Tools of Fort Worth, Tex. under the trade name designation A-1 Bridge Plug or B-1 Bridge Plug, is set in casing  102  below the subterranean formation or zone of interest  108 . The assembly  10  of the present invention is then lowered into well  100  by any suitable means, such as wireline, slick line or coiled tubing, and placed upon bridge plug  130 . Assembly  10  is secured to a blank pipe  112  and a centralizer  132 , for example a bow type centralizer, is secured to the outer surface of blank pipe  112  by any suitable means, such as by welds. A vent screen  113  is secured to the upper portion of blank pipe  112 . As previously discussed the lower end of the screen assembly  10  is closed to fluid flow while the upper end of blank pipe  112  or vent screen  113  is closed to fluid flow by means of bull plug or retrievable fishing neck  115 . If a retrievable fishing neck is employed, the neck is releasably secured to the upper end of blank pipe  112  or vent screen  113  by any suitable means, such as by shear pins. A tubing string  134  is positioned with well  100  and a packer  135  is hydraulically or mechanically set as will be evident to a skilled artisan to effectively isolating formation or zone  118 . Thereafter, hydraulic fracturing fluid is pumped down tubing string  134  and is communicated via blank pipe  112  and port  42  to piston  43  in firing assembly  40 . Pressure on the fracturing fluid is increased to an predetermined condition until pins  44  shear causing firing pin  47  to strike percussion detonator  81  in firing head  70 . The ignition of percussion detonator  81  causes a secondary detonation in ignition transfer  83  which in turn ignites detonating cord  35 . Ignition of cord  35  detonates each perforating charge  34  which blasts through each adjacent scallop  32  in perforating charge carrier  30  and creates a perforation  122  which extends or penetrates through casing  102  and cement  104  and into subterranean formation or zone  108 . Pins  44  are designed to shear at an predetermined pressure, e.g. a pressure greater than the fracturing pressure of the subterranean formation or zone  118  of interest. in this manner, once perforating charge(s)  34  detonate, the formation will be subjected to an pressure that is in excess of the formation fracture gradient. Perforation(s)  122  will be surged with high pressure and fluid present in the annulus  120  will be injected into the formation or zone  108  at a high rate and pressure. 
   Once a pressure drop is noted at the surface indicating that the perforating charge(s) have fired and fluid has been injected into the formation, a frac pack operation is then performed via tubing string  134 . Fluid is pumped via string  134  at a pressure in excess of the fracture gradient of formation or zone  108 . Preferably, a “tip screen-out” technique is employed wherein a high concentration of proppant is pumped in the fracturing fluid near the end of the treatment. As proppant may be left in the tubing string  134  and in well  100  above the top of the vented screen  113 , coiled tubing may be run into the well to wash proppant out of the tubing string  134  and well  100  to the location of vented screen  113 . The removed proppant is then circulated with the wash fluid to the surface of the earth. The coiled tubing is removed and the well may be pressure tested, production tested or placed on production. As placed on production, fluid flows from formation  108  through the proppant pack present in perforations  108  and annulus  120  and into assembly  10  through screen assembly through screen assembly  50 . Produced fluid then flows through blank pipe  112 , outwardly through vented screen  113  and to the surface through tubing string  134 . Alternatively, where a retrievable fishing neck is employed as  115 , wireline, slick line or coiled tubing may be lowered through tubing string  134  prior to placing the well on production, secured to fishing neck  115  and raised to release fishing neck  115  from vented screen  113  or blank pipe  112 . Once the fishing neck is retrieved from well  100 , the well is placed on production and fluid is produced from the formation into assembly  10  and through the top of vented screen  113  of blank pipe  112  prior to entry into tubing string  134 . As illustrated in  FIG. 6 , assembly  10  may be used in the upper and/or lower zone of a multiple well completion process in a similar manner to that described above with respect to  FIG. 4  as long as the perforating charges in the upper assembly  10 ′ are oriented to fire away from tubing string  140  so as not penetrate such string upon detonation. 
   The embodiments of the assembly and process of the present invention set forth above describe a combined perforating, fracturing and/or sand control tool that can be run into a subterranean well in a single trip and does not require that the tool be moved during operation. In accordance with the present invention, the perforating gun assembly  20  is not “dropped” during operation nor does the screen assembly  50  have to be “spaced out” across the subterranean zone of interest after perforating and prior to pumping fluid containing proppant. In this manner, pumping operations can be commenced immediately after perforating and sand control operations thereby eliminating the need for heavy completion fluid for pressure control in the well. 
   The following examples demonstrate the practice and utility of the present invention, but are not to be construed as limiting the scope thereof. 
   EXAMPLE 1 
   A well is drilled in the Gulf of Mexico, U.S.A. to 15,000 feet and is cased with 7″ OD, 32.0 lb/ft casing. A casing cleanup and fluid displacement is performed to displace the drilling mud and cement from the casing, and to prepare it for completion operations. A bit and scraper/gauge run, with casing brushes, is used to ensure the integrity of the casing, and to clean the casing walls. 
   The formation of interest has an equivalent pore pressure of 16.5 ppg. In this straight hole, that equates to a bottom hole pressure of 12,870 psi. Based upon experience in the field, it is anticipated that the formation fracture gradient is 17.9 ppg, which is equivalent to about 14,000 psi. The mud in the casing is displaced with the relatively inexpensive calcium chloride completion fluid of 11.6 ppg density. This fluid exerts an equivalent pressure on bottom of 9048 psi. 
   The workstring is pulled from the well, and electric line is utilized to run a cast iron bridge plug to the desired depth near the bottom of the well, and within a few feet of the desired location of the bottom perforation. The centralized, dual firing head assembly of the present invention is made up with a bull plug on bottom, 60 feet of blank pipe above the assembly, a frac port within the blank pipe section (run in the open position), and a frac pack packer near the top of the blank pipe. This assembly is then run in the hole via electric line, and lightly tags the bridge plug. The assembly is picked up to get on depth and ready to perforate. Alternatively, a work string could be utilized to run the assembly in the well as will be evident to a skilled artisan. 
   The packer is set and electric line is pulled out of the hole. The production tubing assembly, with the seal assembly, is run and stung into the packer. The tubing is landed in the tubing hanger and the tree is nippled up. A tree saver assembly is utilized to protect the tree during frac packing operations, and the well is prepared for pumping operations. 
   A frac boat is mobilized to pump the frac pack, and upon its arrival on location, a high pressure flexible hose is lifted up to the rig and surface equipment, including a high pressure manifold assembly, is rigged up to the well. The boat is utilized to initiate blending of a gelled carrier fluid, and prepare the equipment for injecting proppant. The boat is set up to circulate the gelled fluid against pressure, and is ready to pump the fracture treatment immediately upon determination that the casing has been perforated. 
   The firing heads are set to fire at a pressure in excess of the fracture gradient of the formation of interest. In this case, with 11.6 ppg fluid in the hole and a 17.9 ppg frac gradient, it is determined that a differential pressure 1000 psi over fracture gradient is satisfactory. Accordingly, the guns are set to fire at a pressure of 15,000 psi. The pressure applied to the 11.6 ppg fluid to exert this pressure on the firing heads is 5914 psi at the surface. 
   While the boat is circulating fluid, a choke is gradually closed on the loop to increase the circulating pressure to greater than 5914 psi. A choke between the loop and the workstring is gradually opened as the pressure on the workstring is raised to 5914 psi. As soon as a pressure drop is observed, indicating that the guns have fired, the choke to the worksting is opened fully, and the fracture treatment is pumped as planned without allowing the pressure to drop below the formation fracture gradient. Additional pumps on the boat are then utilized to bring the injection rate up to the desired rate for the fracture treatment. The injection rate is stabilized by the time the gel pre-pad reached the formation. 
   The fracture treatment is terminated with a pumping schedule intended to induce a screenout via the tip-screenout method. This method results in proppant being left in the wellbore. Pressure is bled off abruptly to allow fractures in the formation to close and flow some of the proppant back to the wellbore in order to assure a good annular pack. Coiled tubing is utilized to wash proppant out of the tubing, and to close the frac sleeve in the blank pipe assembly. Alternately, electric line is used to close the sleeve after the proppant is washed form the well. After the frac sleeve is closed, coiled tubing and electric line are pulled out of the hole and the tree saver is removed from the wellhead. The well is flow tested and then put on production. 
   EXAMPLE 2 
   A well is drilled in a similar fashion to that described in Example at the same locale and to approximately the same depth. In this example, a vented screen is employed in the blank pipe above the assembly and the tubing string is run with a single packer above the screen assembly. The screen assembly is not connected to the tubing string or packer assembly. The tubing below the packer consists of a joint of tubing, a landing nipple, another joint of tubing, and a muleshoe. 
   The well can be completed in accordance with the process set forth in Example 1 and tree nippled up prior to perforating and pumping a gravel pack or frac pack. A tree saver will be used to protect the tree during pumping operations. 
   As in Example 1, pumping operations are configured such that upon determining that the guns have fired, pumping operations are continued until tip screenout. Coiled tubing is mobilized to wash proppant out of the well down to the top of the vented screen. After cleaning out the tubing/wellbore and rigging down the coiled tubing and the tree saver, the well is first tested and then brought online. 
   As will be evident to a skilled artisan, the methods of Examples 1 and 2 can be applied in cases where the tree is not nippled up prior to perforating. In these cases, it will be necessary to provide some mechanism to prevent the well from flowing during completion operations. The options may include running a flapper valve assembly in the packer extension to isolate the lower interval, setting a plug in the blank pipe, or killing the well with heavy weight completion fluid. Since the latter is one of the reasons for the development of this tool and process, it should be used only after it is determined that the other options are not feasible under the completion scenario. 
   Although assembly  10  of the present invention has been illustrated in  FIGS. 1-6  as containing only perforating gun assembly having one set or row of spaced apart perforating charges which are aligned in a generally linear pattern, it is within the scope of the present invention as illustrated in  FIGS. 7 and 8  to use multiple perforating gun assemblies having aligned perforating charges which are arranged in parallel within screen assembly  50 . This parallel gun configuration may be employed where rotationally spaced perforations in a well and surrounding formation are desired for a specific subterranean completion application and where space within screen assembly  50  permits placement of multiple gun assemblies. 
   Further, although the screen assembly  50  that is utilized in the assembly of the present invention has been illustrated in  FIGS. 1-8  as being a pipe which has a generally annular cross sectional configuration, it is within the scope of the present invention to utilized other screen configurations, such as a trough or half pipe as is illustrated in FIG.  9 . In this embodiment, the longitudinal edges of screen assembly  50  are secured to one side of an elongated housing  64  by any suitable means, for example by welds. In this embodiment, one or more perforating gun assemblies  20  are positioned within and secured to the housing  64  by any suitable means. Each charge  34  in said perforating gun assembly  20  is aimed to penetrate through charge carrier  26  and housing  64 . Although illustrated in  FIG. 9  as having a generally triangular cross sectional configuration, housing  64  may have any cross sectional configuration, for example rectangular or oblong, that can be sized to be positioned within a cased or open hole well bore as will be evident to a skilled artisan. Those portion(s) of housing  64  directly in front of perforating charges  34  may be provided with scallops to assist in penetration of housing  64  upon detonation as will be evident to a skilled artisan. In addition, it is within the scope of the present invention that housing  64  could serve as a carrier for the perforating charges  34  of the perforating gun assemblies positioned therein. In this manner, charge carriers  26  may be eliminated. When housing  64  functions as a carrier for perforating charges  34 , housing  64  will have sufficient thickness to provide the structural integrity necessary for operation of the assembly and process of the present invention. 
   It is also within the scope of the present invention to position one or more perforating gun assemblies  20  on the outside of screen assembly  50  as illustrated in FIG.  10  and to secure each perforating gun assembly  20  to the screen  56  by means of at least one spacer or standoff  66 . Each spacer or standoff  66  is secured to screen assembly  50  and perforating gun assembly  20  by any suitable means, for example by welds. In this embodiment, the charges  34  may be assembled with any phasing that does not affect the screen assembly  50 . To ensure that the screen assembly  50  is not damaged upon detonation of the perforating gun assemblies during the process of the present invention, it is within the scope of the present invention to secure a shield (not illustrated) along that portion of screen assembly  50  that is closest to perforating gun assembly  20 . The exact construction, configuration and assembly of a shield will be evident to a skilled artisan. 
   Further, multiple assemblies  10  of the present invention may be employed where the formation or zone of interest is of a sufficient thickness so as to require a larger length of perforations than can be formed using one assembly for proper completion. Where more than one assembly is employed in this embodiment of the present invention, the assemblies are arranged in series with adjacent assemblies mechanically and ballistically connected by means of a sub and booster transfer, respectively, as will be readily apparent to a skilled artisan. 
   The assembly and process of the present invention has been described and illustrated herein as being applied to a well bore having casing positioned therein. It will be evident to a skilled artisan that the assembly and process of the present invention is equally applicable to open hole applications, i.e. in subterranean well bores that are not cased. When utilized in an open hole, the assembly of the present invention is deployed as depicted in  FIGS. 4 and 5  or in  FIG. 6 , detonation of the charges initiates fracturing of the subterranean zone of interest and the screen assembly  50  functions to prevent flow back of proppant into the production string. 
   While the foregoing preferred embodiments of the invention have been described and shown, it is understood that the alternatives and modifications, such as those suggested and others, may be made thereto and fall within the scope of the invention.