Abstract:
A hydraulic oil and gas well downhole packer apparatus for use in a well casing below a wellhead and in combination with a coil tubing unit provides a tool body having a longitudinally extending tool bore and an upper end portion that connects to the lower free end of the coiled tubing unit during use. The tool body includes an inner elongated hollow mandrel with a hydraulic piston movably disposed upon the external surface of the mandrel. The piston is movable between an initial “running” position and a final “setting” position. An external sleeve is engaged by the piston when it moves between the running and setting positions, the external sleeve engaging slips that expand to anchor the tool body to the well casing. An annular packer member is expandable responsive to sliding movement of the external sleeve and is positioned below the slips for forming a seal between the tool body and the casing at a position near the lower end portion of the tool body.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not applicable 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable 
     REFERENCE TO A “MICROFICHE APPENDIX” 
     Not applicable 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The apparatus of the present invention relates to downhole oil well tools, and more particularly relates to an improved method and apparatus for setting a gravel pack in a downhole oil and gas well environment. 
     2. General Background of the Invention 
     There are a number of applications in the oil and gas well drilling industry where it is desirable to install a packer in an oil and gas well whose “annulus” or internal diameter is restricted by existing equipment. One downhole oil and gas well delivery system is known in the industry as a “coil tubing” unit. By using a coil tubing unit, it is possible to run a tool in a well that is very restricted in diameter because of existing equipment. However, there are many oil and gas well drilling operations that are not feasible heretofore with the small diameter coil tubing units. 
     Gravel packing is a mechanical means of preventing sand flow from unconsolidated formations in a producing well. If the sand flow is not controlled, serious and costly problems, such as, loss of production due to sand bridging, failure of casing or liners from removal of surrounding formation, compaction, erosion and disposal of produced materials. U.S. Patents that relate to gravel packs include U.S. Pat. Nos. 5,620,050 and 5,377,749 issued to Phil Barbee, applicant herein, each hereby incorporated herein by reference. 
     The purpose of a screen in gravel packs is to hold the gravel in place. The slot width or wire spacing should be smaller than the smallest gravel used. The outside diameter of the screen should provide maximum radial clearance of the casing wall while maintaining an adequate internal diameter for anticipated production rates. Screen sections should provide five feet of minimum overlap above and below the perforated interval to compensate for depth measurement inaccuracies. If the gravel is to be circulated into place, the screen may extend further above the perforated interval to develop a higher column of compacted gravel above the completion interval. 
     Two commonly used techniques for the placement of gravel are the “squeeze technique” and the “one trip circulating technique”. 
     The squeeze technique is primarily used for gravel packing short intervals. Gravel is squeezed through the perforations to pack outside the casing and in the screen annulus without circulation. If the squeeze technique is used in longer intervals, variations of the formation permeability may cause all the slurry to go into the highest permeable section of the interval. Although longer intervals have been successfully squeezed, it is recommended that this technique be limited to shorter intervals. 
     A squeeze packer with a crossover tool is used to place the gravel pack. The screen and the blank pipe are run in the hole and positioned across the productive interval. The packer is set and the crossover opened. The slurry is then “bullheaded” down a workstring, through crossover tool, into the screen-casing annulus, and through the perforations in the casing. Pumping is continued until a pre-determined pressure increase or “sandout” pressure occurs, indicating that no more gravel can be “squeezed” outside of the casing or into the annulus. Once a “sandout” is achieved, pumping is discontinued and treatment pressures are vented before physically pulling the crossover tool to the “upper” circulating position. After the upper circulating position has been accomplished, pumping is resumed to circulate any excess gravel remaining in the workstring to surface. 
     The one trip circulating technique is typically better suited for longer intervals than the squeeze technique, but can be used for any length interval. A washpipe is positioned inside and extending through the screen to accommodate the circulation of fluids and gravel to the bottom of the screen. A gravel slurry is circulated down the tubing, through a crossover tool, down the screen-casing annulus, through the screen, up the washpipe, through the crossover tool and returns up the workstring-casing annulus. Gravel contained in the slurry is separated out of the circulating fluid as it passes through the screen. 
     As the screen is covered with gravel, the circulation pressure increases, forcing gravel into the perforations. Pumping is continued until a pre-determined pressure increase or “sandout” occurs indicating that no more gravel can be circulated outside of the casing or into the annulus. Once a “sandout” is achieved, pumping is discontinued and treatment pressures are vented before pulling the crossover tool to the “upper” circulating position. After the upper circulating position is accomplished, pumping is resumed to circulate out any excess gravel remaining in the workstring. 
     Slurry rates may vary as low as 0.25 bbl/min to in excess of 16.0 bbl/min depending on workstring or tubular diameters and the well configuration. Higher rates result in higher treating pressures which generally promote tighter packing of gravel. At higher placement rates, friction pressure is incurred due to pumping the slurry down relatively long lengths of tubing. These friction pressures tend to “mask” the actual down-hole differential pressure from the screen exterior to the screen interior during a “sandout”. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention features a sliding sleeve, type pressure activated bypass valve. Once in the “lower” circulating position, the hydraulic pressure, incurred as a result of achieving a predetermined “sandout” pressure, shifts the sleeve-type valve to an open position. Once the valve is open, a flow path or by-pass for the excess slurry is exposed allowing the gravel pack media to be circulated back to surface. 
     A pre-determined “sandout” pressure can be accurately controlled by the adjustable shear value for activation (opening) of the crossover tool by-pass valve. The crossover tool by-pass valve allows for a non-stop pumping operation. The continuous pumping eliminates the opportunity for gravel to settle out of static fluid that may inhibit crossover tool movement or removal from the packer bore. The excess gravel is totally evacuated from the packer/crossover tool vicinity prior to repositioning or removing the crossover tool from the packer bore. 
     The present invention provides an improved oil and gas well downhole packer apparatus for use in well casing below the wellhead, and can be used in combination with a coil tubing unit having an elongated coil tubing portion, a reel portion for coiling the tubing thereupon, and a free end portion of the tubing that can be transmitted into the well casing below the wellhead area. 
     In the preferred embodiment, the apparatus includes a tool body having a central, longitudinally extending hollow tool body bore, an upper end portion and a lower end portion. Threads at the top end portion of the tool body assembly are provided for forming a connection between the tool body assembly and the lowermost free end portion of the coil tubing. In this fashion, as coil tubing is unwound from the reel, the coil tubing pays out and the free end portion of the coil tubing lowers into the well with the tool body attached. 
     The coil tubing provides a bore that can be used to transmit pressurized fluid to the tool body during use. The hydraulic pressure transmitted to the tool body via the coil tubing unit is used to activate the tool body such as, for example, in setting of the packer. Further, the bore of the coil tubing unit is used to transmit coarse sand or gravel from the wellhead area to the tool body for use in gravel packing operation. 
     The tool body includes an elongated tubular inner mandrel having a polished inner bore, a hydraulic piston that is movably disposed upon the mandrel between a first running position and a second setting position. An external sleeve portion of the tool body surrounds the mandrel and the piston and can be in several parts connected end to end. The external sleeve defines a sliding portion that connects for movement with the hydraulic piston when the hydraulic piston moves from the initial running position to the second setting position. 
     Slips on the lower end portion of the tool body are annularly spaced around the mandrel for engaging the well casing to anchor the tool body to the casing at a selected position. Means are provided for forming a connection between the piston and the slips for activating the slips to grip the well casing. 
     An expandable annular packer is provided for forming a seal with the well casing and between the well casing and the inner mandrel. The packer is expandable responsive to movement of a sliding portion of a tool so that the packer expands when the piston moves downwardly from the initial running position to the final setting position. 
     In the preferred embodiment, the packer is a resilient member such as, for example, of a rubber or polymeric construction. In the preferred embodiment, the coil tubing and tool body are sized to enter a very restricted well bore such as, for example, an internal diameter of about two inches or less. 
     During use, the tool body assembly comprises in part an uppermost running tool portion that includes means for connecting the running tool portion to the coil tubing. 
     The method of the present invention provides a method for gravel packing an oil and gas well having a wellhead at the earth&#39;s surface and a well annulus defined by the well casing. The method includes the initial step of lowering a packer having a valving member into the well casing on the coil tubing string, and attached to the straight, free end portion of the coil tubing. 
     The packer is placed in the well annulus and at a selected elevational position of the well casing to be packed with coarse sand or gravel. 
     The packer is activated to form an annular seal against the casing by elevating pressure in the coil tubing. 
     The valve is opened at a selected position below the seal element. After opening the valve, gravel or coarse sand (as selected) can be transmitted via the coil tubing unit bore and into the tool body bore with a carrying fluid. The coarse sand or gravel and carrying fluid enters the well annulus below the seal element. 
     In the method of the present invention, the valve member includes a sliding sleeve valve that opens responsive to an increase in pressure within the tool body bore. 
     In the preferred method, the tool body supports a screening member at the lower end portion of the tool body so that the carrying fluid that enters the well annulus can be returned to the surface via the screen and the bore of the tool body so that the screen prevents return flow of coarse sand and gravel that is used for the gravel pack. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a further understanding of the nature, objects, and advantages of the present invention, reference should be had to the following detailed description, read in conjunction with the following drawings, wherein like reference numerals denote like elements and wherein: 
     FIGS. 1A,  1 B and  1 C are upper, middle and lower respective portions of a sectional view of the preferred embodiment of the apparatus of the present invention shown in a preliminary “running in” position wherein  1 A,  1 B and  1 C match together end to end; 
     FIGS. 2A,  2 B, and  2 C are upper, middle and lower respective portions of a sectional view of the preferred embodiment of the apparatus of the present invention showing the apparatus set and with the coil tubing unit in tension but  2 A,  2 B and  2 C match together end to end; 
     FIGS. 3A,  3 B, and  3 C are upper, middle and lower respective portions of a sectional view of the preferred embodiment of the apparatus of the present invention showing the tool body set with the coil tubing unit in compression and circulating a slurry through the tool body wherein  3 A,  3 B, and  3 C match together end to end; 
     FIGS. 4A,  4 B and  4 C are upper, middle and lower respective portions of a sectional view of the preferred embodiment of the apparatus of the present invention showing the circulating slurry during the building of sand height on the screen when setting the gravel pack wherein  4 A,  4 B and  4 C match together end to end; 
     FIGS. 5A,  5 B and  5 C are upper, middle and lower respective portions of a sectional view of the preferred embodiment of the apparatus of the present invention showing the differential valve opened, formation isolated, and circulating out through the bypass channel wherein  5 A,  5 B and  5 C match together end to end; and 
     FIGS. 6A,  6 B and  6 C are upper, middle and lower respective portions of a sectional view of the preferred embodiment of the apparatus of the present invention showing the well producing and sleeve latched across the gravel ports wherein  6 A,  6 B,  6 C match together end to end. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1 and 2 show the preferred embodiment of the apparatus of the present invention designated generally by the numeral  10 . Downhole well tool apparatus  10  is shown in FIGS. 1 and 2 in a downhole position inside casing  11 . The casing  11  is generally cylindrically shaped, comprising a casing wall  12  having an inside surface  13  and an outside surface  14 . 
     FIG. 1 shows a position of the tool apparatus  10  as it is being lowered into the well, known in the industry as a “running in” position. In FIG. 1, the tool apparatus has not been deployed, and is free to move up and down in the well casing  11 , being lowered on preferably a coil tubing unit. Coil tubing units are well known in the art for lowering elongated downhole well tools into an oil and gas well. A coil tubing unit provides an elongated length of continuous tubing with an internal flow bore that can flow pressurized fluid to the tool apparatus  10  for activating its slips  40  and for expanding its annular elastomeric seal member  39  into engagement with the inside surface  13  of the casing  11 . 
     The slips  40  and annular elastomeric seal member  39  are activated as the first step of the method of the present invention as shown in FIG.  2 . When the well tool apparatus  10  has been lowered to a desired elevational position, the slips  40  and elastomeric seal member  39  are activated so that they both grip the inside surface  13  of casing  11 . With the method and apparatus of the present invention, the tool apparatus  10  is lowered to a desired elevational position that is next to a perforated zone  71 . The perforated zone  71 , as is known in the art, is a portion of the casing  11  that has been perforated so that oil and gas can flow from the surrounding formation through the perforations in the casing  11  and into the well annulus  15 . 
     The well  15 , as is known in the art, is that portion of the well inside the casing  11  surrounded by inside surface  13  of casing wall  12 . With the method of the present invention, a gravel pack is placed to form an interface in between the surrounding formation and a flow bore of the tool apparatus  10  through which oil and gas will flow to the surface, as indicated by the arrows  75  in FIG.  6 . 
     In FIG. 2, the slips  40  and elastomeric seal member  39  have been activated by pumping pressurized fluid through the coil tubing unit to the bore  22  of the tool body  10 . The pressurized fluid enters bore  22  of tool body  10  and then flows through circulating channel  23  of crossover tool  20  to port  28 . 
     The tool body  10  includes a crossover tool  20  and a packer body  21 . The packer body  21  includes a fixed section  34  and moving portions as will be described more fully hereinafter. In FIGS. 1 and 2, port  28  receives pressurized fluid that is pumped via the coil tubing unit to bore  22  and circulation channel  23 . Pressurized fluid flows through port  28  into annular space  29  so that it acts upon piston  27 . 
     The piston  27  is forced downwardly as shown in a comparison of FIGS. 1 and 2. When the piston  27  moves downwardly with respect to fixed section  34  it pushes upon connector sub  35 , ratchet mechanism  36 , release sleeve  58 , cones  41 ,  42 , gauge sub  44 , and sub  45 . This downward movement of the aforementioned parts causes the cones  41 ,  42  to push slips  40  outwardly so that they engage the inside surface  13  of casing  11  as shown in FIGS. 2-6. At the same time, the gauge sub  44  and sub  45  move together squeezing the annular elastomeric seal member  39  outwardly so that it engages the inside surface  13  of casing  11  as shown in FIGS. 2-6. These parts are held in this position by the ratchet mechanism  36 . The ratchet mechanism  36  moves downwardly, engaging toothed section  37  as shown in FIGS. 1 and 2. The ratchet mechanism includes segment retainer  59  and body lock ring  60 . 
     The pressurized fluid that is used to activate the tool apparatus  10  is attached to tool body  16  at its upper end portion  16  at a suitable connection such as, for example, a connector at the lower end portion of the coil tubing unit that engages internal threads  18  of tool body  16 . 
     In order to properly register the tool apparatus  10  at a desired position in the well, one technique is to clean the well to a desired depth so as to create a bottom  25  of the well that is engaged by lower end portion  19 . Lower end  19  provides a preferably hemispherically shaped tip  26  as shown in FIGS. 1-6. Once the tool apparatus  10  has been set, slips  40  and annular elastomeric seal member  39  grip the inside surface  13  of casing  11 . A slurry that includes gravel and/or coarse sand can be pumped downhole through the coil tubing unit to the bore  22  of the tool body  16  and then into circulating channel  23 . In FIG. 2, a tension test can be used to assure that the slips  40  are properly gripping the casing  11  wall  12 . By pulling on the coil tubing unit, tension is applied to the tool apparatus  10  to test the grip of the slips  40  against the casing  11 . 
     After the pull test of FIG. 2, the tool apparatus is then set by applying compression with the coil tubing unit thus forcing a portion of the tool apparatus  10  downwardly as shown in FIG.  3 . In FIG. 3, compression has been applied by the coil tubing unit to the upper end portion  17  of the tool body  16 . A shear pin  30  is used to prevent inadvertent preliminary shifting of the tool apparatus  10  between FIGS. 1 and 2. In FIG. 1, the shear pin  30  has not yet been cut. In FIG. 2, the shear pin  30  has been sheared so that the piston  27  can move downwardly. 
     Downward movement of the cross over tool  20  relative to the fixed section  34  of tool body  16  is limited by engagement of annular surface  76  on cross over tool  20  with annular shoulder  77  on packer body  21 . This engagement of annular surfaces  76 ,  77  can be seen in FIG. 3. A comparison of FIGS. 2 and 3 shows movement of the cross over tool downwardly relative to the packer body  21 . 
     In FIG. 3, downward movement of the cross over tool  20  opens gravel port  68  as shown in FIG.  3 . In FIG. 2, an expanded portion  78  of cross over tool  20  engages middle seal  46 . When the cross over tool  20  shifts downwardly in FIG. 3, the expanded portion  78  moves away from middle seal  46  so that port  68  is opened. In this position (FIG. 3) a slurry of fluid and gravel and/or coarse sand can be pumped from the coil tubing unit to the tool body bore  22  to the circulating channel  23  and then to the port  68 . This gravel slurry is indicated by the number  69  in FIGS. 3 and 4. 
     After the slurry  69  passes from circulating channel  23  through port  68  to annulus  15 , it flows downwardly in the annulus  15  past a number of portions of the apparatus  10  until it reaches well screen  57 . In FIGS. 1-4, a portion of the tool body  16  below gravel port  68  includes sub  45 , middle seals  46 , lower seal  47 , annular section  48 , annular section  49 , connector  50 , sleeve  51 , shear pin  52 , sleeve  53 , connector  54 , sleeve  55 , connector  56 , and well screen  57 . In FIG. 4, the slurry  69  flows down until it reaches the bottom  25  of the well and the area in between perforated section  71  and screen  57 . 
     One of the features of the present invention is that the desired pressure across the screen after the gravel pack is in place (sand out pressure) can be set to a very specific pressure value. This is accomplished by first measuring circulating pressure before any sand or gravel is pumped down hole into the bore  22  of the tool body  16 . This circulating pressure of fluid only can be, for example, 2,000 p.s.i. If it is desired to have a pressure of, for example, 3,000 p.s.i. across the gravel pack and screen, the present invention will automatically set that pressure value at 3,000 lbs. by opening bypass valve  62  as soon as the downhole fluid pressure reaches 3,000 p.s.i. Thus, with the present invention, the circulating pressure rises as more and more sand and/or gravel is pumped with the gravel slurry  69  to the area in between well screen  57  and perforated zone  71 . As more and more sand and/or gravel is pumped to this area as shown in FIGS. 2,  3  and  4 , resistance to the fluid being pumped and the slurry being pumped increases. Petroleum engineers can calculate a desired sand out pressure knowing the formation that they are dealing with. For example, if the sand out pressure is set at 3,000 p.s.i., the engineer knows that when 3,000 p.s.i. has been reached by measuring the pump pressure, sufficient gravel and/or sand has been packed in between the perforated zone  71  and the well screen  57 . 
     The present invention provides a valving mechanism that automatically stops the flow of circulating gravel slurry  69  to the area in between the perforated zone  71  and the well screen  57  by opening a bypass port  64 . In FIG. 4, the bypass port  64  is closed with bypass valve  62 . Shear pin  63  holds the bypass valve  62  in the closed position. As the gravel pack area  72  receives more and more sand and/or gravel, the circulating pressure of the pump at the well head rises. When the selected pressure value (for example 3,000 psi) is reached, that pressure value of 3,000 psi is acting upon the valving member  62 . The shear pin  63  is sized and of a selected material such that it shears at exactly the desired downhole well pressure of, for example, 3,000 psi. When the shear pin  63  shears, the valving member  62  moves downwardly to the position shown in FIG.  5 . This causes fluid to take the path of least resistance as shown by arrows  79  in FIG.  5 . As fluid flows through bypass port  64 , it enters the return channel  24 . This creates pressure that pushes ball valving member  66  down so that it seals upon beveled annular seat  65 . 
     A fluted section  67  of cross over tool  20  enables fluid to flow upwardly in return channel  24  as indicated by arrows  70  during the formation of the gravel pack. The ball valving member  66  may move upwardly and engage fluted section  67  during such return flow as indicated by the arrows  70  in FIG.  3 . In such a situation, circulation can take place by simply flowing through the fluted section and around the ball valving member. 
     One of the advantages of the apparatus  10  of the present invention is that circulating slurry  69  will automatically divert through the bypass port  64  into return channel  24  as soon as the desired circulating pressure value is reached. An additional benefit of the bypass port  64  and valve  62  construction is that any sand and gravel that is flowing in circulating channel  23  when the valving member  62  is activated to move to the position of FIG. 5 will either fall harmlessly into the well annulus  15  below gravel port  68  or will travel back to the well head area via return channel  24  and the well annulus  15  above tool body  16 . 
     After the gravel pack has been completed, closure sleeve  73  can be used to close gravel port  68 . The tool body  16  is lifted upwardly and the tool body  16  and cross over tool  20  separated from packer body  21 . In such a situation, closure sleeve  73  shifts upwardly to engage middle seal  46  and lower seal  47 . Now, the well can produce oil and gas as it flows from the surrounding formation through the perforated section  71  of well casing  11 , and through well screen  57  into the tool body bore  22  as shown by arrows  75  in FIG.  6 . 
     Shear pin  52  enables the majority of the packer body  21  to be removed from the well bore by applying tension in case the bottom of the tool body is stuck. This shear pin  80  thus provides a safety feature so that the top of the tool body can be pulled out if the well screen  57  is stuck. 
     The sleeve sections  51 ,  53 ,  55 , can be blank tubing that are very long in length such as for example, any distance of 10-2,000 ft. Similarly, the well screen  57  can be very long such as for example, 10-2,000 ft. 
     PARTS LIST 
     The following is a list of suitable parts and materials for the various elements of the preferred embodiment of the present invention. 
     
       
         
               
             
               
               
             
           
               
                   
               
               
                 PARTS LIST 
               
             
          
           
               
                 NUMBER 
                 PART 
               
               
                   
               
               
                 10 
                 downhole well tool 
               
               
                 11 
                 casing 
               
               
                 12 
                 casing wall 
               
               
                 13 
                 inside surface 
               
               
                 14 
                 outside surface 
               
               
                 15 
                 annulus 
               
               
                 16 
                 tool body 
               
               
                 17 
                 upper end portion 
               
               
                 18 
                 internal threads 
               
               
                 19 
                 lower end portion 
               
               
                 20 
                 cross over tool 
               
               
                 21 
                 packer body 
               
               
                 22 
                 bore 
               
               
                 23 
                 circulating channel 
               
               
                 24 
                 return channel 
               
               
                 25 
                 bottom of well 
               
               
                 26 
                 hemispherical tip 
               
               
                 27 
                 piston 
               
               
                 28 
                 port 
               
               
                 29 
                 annular space 
               
               
                 30 
                 shear screw 
               
               
                 31 
                 moving annular set sleeve 
               
               
                 32 
                 relief surface 
               
               
                 33 
                 running segment 
               
               
                 34 
                 fixed section tool body 
               
               
                 35 
                 connector sub 
               
               
                 36 
                 ratchet mechanism 
               
               
                 37 
                 toothed section 
               
               
                 38 
                 seal 
               
               
                 39 
                 annular elastomeric seal member 
               
               
                 40 
                 slip 
               
               
                 41 
                 cone 
               
               
                 42 
                 cone 
               
               
                 43 
                 slip holder 
               
               
                 44 
                 gauge sub 
               
               
                 45 
                 sub 
               
               
                 46 
                 middle seal 
               
               
                 47 
                 lower seal 
               
               
                 48 
                 annular section 
               
               
                 49 
                 annular section 
               
               
                 50 
                 connector 
               
               
                 51 
                 sleeve 
               
               
                 52 
                 shear pin 
               
               
                 53 
                 sleeve 
               
               
                 54 
                 connector 
               
               
                 55 
                 sleeve 
               
               
                 56 
                 connector 
               
               
                 57 
                 well screen 
               
               
                 58 
                 release sleeve 
               
               
                 59 
                 retainer segment 
               
               
                 60 
                 body lock ring 
               
               
                 61 
                 seal 
               
               
                 62 
                 bypass valve 
               
               
                 63 
                 shear pin 
               
               
                 64 
                 bypass port 
               
               
                 65 
                 beveled annular seat 
               
               
                 66 
                 ball valve member 
               
               
                 67 
                 fluted section 
               
               
                 68 
                 gravel port 
               
               
                 69 
                 gravel slurry 
               
               
                 70 
                 arrow 
               
               
                 71 
                 perforated area 
               
               
                 72 
                 gravel pack area 
               
               
                 73 
                 closure sleeve 
               
               
                 74 
                 thickened section 
               
               
                 75 
                 arrow 
               
               
                 76 
                 annular shoulder 
               
               
                 77 
                 annular shoulder 
               
               
                 78 
                 expanded portion 
               
               
                 79 
                 arrow 
               
               
                   
               
             
          
         
       
     
     The foregoing embodiments are presented by way of example only; the scope of the present invention is to be limited only by the following claims.