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This application is based on U.S. Provisional Patent Application No. 60/315,609, which was filed on Aug. 29, 2001, and U.S. Provisional Patent Application No. 60/317,749, which was filed on Sep. 6, 2001. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention is directed to a system for completing multiple hydrocarbon wells. More particularly, the invention is directed to a system and method for guiding a drill string during the drilling of multiple well bores in a side-by-side wellhead system. 
     In the oil and gas industry, side-by-side (“SXS”) wellhead systems, or multiple completion systems, are commonly used to drill and complete a number of hydrocarbon wells from within a single conductor housing. In such systems, multiple casing strings are run through the same conductor housing in order to reduce cost and space requirements. As shown in FIG. 1, a prior art SXS wellhead system may comprise a drive pipe housing A which is welded to the top of a conductor housing B, two or more casing strings C which are each suspended from the drive pipe housing, and a base plate D which is bolted to the top of the drive pipe housing. In this example, the well on the right is covered by an abandonment cap E, and a wellhead F is installed in the well on the left. In the drilling mode of operation of the SXS wellhead system, a drilling riser G may be bolted to the top of the wellhead F or connected directly to the base plate D. 
     One difficulty with SXS wellhead systems arises in the need to align and guide the drill string while drilling the well bores for the individual casing strings. If the first well bore is drilled too close to the center of the conductor housing, insufficient space will exist for the remaining well bores. One solution to this problem is disclosed in U.S. Pat. No. 6,142,235 to Monjure et al., which teaches using a permanently installed drilling guidance device in the conductor housing. However, this guidance device must be specially fabricated for each specific application, and this can be both costly and time consuming. Moreover, the guidance device must be permanently installed in the conductor housing, which requires that the cost for each guidance device be born for each SXS completion. 
     SUMMARY OF THE INVENTION 
     These and other disadvantages in the prior art are addressed by providing a system and method for drilling a plurality of well bores through a single conductor housing. The method comprises providing a base plate which includes at least first and second well slots that extend therethrough, supporting the base plate on the conductor housing, suspending a temporary casing string from the first well slot, drilling a first well bore through the second well slot, removing the temporary casing string from the first well slot, and then drilling a second well bore through the first well slot. 
     In accordance with one embodiment of the invention the diameter of the temporary casing string is larger than the diameter of a casing string which is subsequently suspended in the first well slot during completion of a well through the second well bore. In accordance with another embodiment of the invention, the temporary casing string is suspended from an offset casing hanger. Furthermore, individual lengths of the temporary casing string may be connected together by connector collars which have a larger diameter than the diameter of the temporary casing string. 
     Thus, the present invention comprises an economical method for guiding a drill string during the drilling of individual well bores in an SXS or multiple completion wellhead system. Since the diameter of the temporary casing string is larger than that of a subsequently installed casing string, sufficient room will exist after drilling the first well bore to drill the remaining well bores. In addition, because the temporary casing string is suspended from an offset casing hanger, the diameter of the temporary casing string can be significantly larger than the diameter of the subsequently installed casing string. Furthermore, after all the well bores have been drilled, the offset casing hanger and temporary casing string can be removed from the well. Consequently, these components can be reused on other multiple well completions. 
     These and other objects and advantages of the present invention will be made apparent from the following detailed description, with reference to the accompanying drawings. In the drawings, the same reference numbers are used to denote similar components in the various embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross sectional view of a prior art SXS wellhead system; 
     FIGS. 2 and 3 are a cross sectional view and a top view, respectively, of the drilling alignment system of the present invention with an offset casing hanger and a placeholder casing string installed in the right-hand well slot; 
     FIGS. 4 and 5 are a cross sectional view and a top view, respectively, of the drilling alignment system of FIG. 2 with a drilling riser installed over the left-hand well slot; 
     FIGS. 6 and 7 are a cross sectional view and a top view, respectively, of the drilling alignment system of FIG. 2 with a wellhead and a mandrel casing hanger installed in the left-hand well slot and a drilling riser installed over the right-hand well slot; 
     FIGS. 8 and 9 are a cross sectional view and a top view, respectively, of the drilling alignment system of FIG. 2 showing a mandrel casing hanger being installed in the right-hand well slot through the drilling riser; 
     FIGS. 10 and 11 are a cross sectional view and a top view, respectively, of the drilling alignment system of FIG. 2 with a wellhead and a mandrel tubing hanger installed in each well slot; 
     FIGS. 12 and 13 are a cross sectional view and a top view, respectively, of a second embodiment of a drilling alignment system of the present invention with a base plate installed in the drive pipe housing component of the invention; 
     FIG. 14 is a cross sectional view of the drilling alignment system of FIG. 12 showing an end cap guide being installed through the right-hand well slot; 
     FIG. 15 is a cross sectional view of the drilling alignment system of FIG. 12 showing a first connector collar being installed through the right-hand well slot; 
     FIG. 16 is a cross sectional view of the drilling alignment system of FIG. 12 showing an offset casing hanger being installed in the right-hand well slot; 
     FIGS. 17 and 18 are a cross sectional view and a top view, respectively, of the drilling alignment system of FIG. 12 with a drilling riser installed over the left-hand well slot; 
     FIG. 19 is a cross sectional view of the drilling alignment system of FIG. 12 showing a mandrel casing hanger being installed in the left-hand well slot through the drilling riser; 
     FIG. 20 is a cross sectional view of the drilling alignment system of FIG. 12 with a wellhead installed over the left-hand well slot, a mandrel casing hanger installed in the right-hand well slot and a drilling riser installed over the right-hand weld slot; and 
     FIGS. 21 and 22 are a cross sectional view and a top view, respectively, of the drilling alignment system of the present invention with a wellhead installed over both well slots. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIGS. 2 and 3, the drilling alignment system of the present invention may be used in conjunction with an exemplary SXS wellhead system  10  which comprises a drive pipe housing  12  that is attached to a conductor housing or pipe  14  by any suitable means, such as welding. In accordance with the present invention, a base plate  16  is attached to the drive pipe housing  12  with a number of studs and nuts  18 . The base plate  16  preferably comprises a radial lip  20  by which the base plate may be suspended in the drive pipe housing  12  to ensure that the weight of any subsequently installed wellheads is transferred directly to the conductor pipe  14 . A number of preferably elastomeric seals  22  are ideally provided to seal between the base plate  16  and an internal seal bore which is formed on the inner diameter of the drive pipe housing  12 . In the illustrated embodiment of the invention, the base plate  16  is shown to comprise two annular bowl profiles  24  and  26 , each of which is formed by a bore that extends through the base plate and defines a corresponding well slot therein. In addition, for reasons which will be made apparent below, the base plate  16  preferably includes a plurality of tapped holes  28  that extend into its top surface around each of the bowl profiles  24 ,  26 . 
     In accordance with the present invention, an offset casing hanger  30  suspending a string of oversize casing  32  is run through the well slot  24  and landed on the base plate  16 . If desired or required, an initial bore may drilled through the well slot  24  prior to running the oversize casing  32 . The oversize casing  32  is connected to the offset casing hanger  30  via a conventional threaded connection  34 . The offset casing hanger  30  is machined off-center and is oriented in the bowl profile  24  using a conventional key and slot arrangement (not shown). Thus, as shown in FIG. 2, the centerline of the casing hanger  30  is offset toward the centerline of the conductor pipe  14 . The offset casing hanger  30  is preferably sealed to the bowl profile  24  with a suitable seal, such as an elastomeric belt type packing  36 , and is provided with a hole  38  which is adapted to accept any straight thread or pipe thread as may be necessary. 
     The offset casing hanger  30  and the oversize casing  32  serve to ensure that sufficient space exists to drill out the final well bore and run and set a final casing string through the well slot  24  after a first well bore has been drilled through the well slot  26 . The diameter of the oversize casing  32  is selected so that it will occupy at least as much space in the conductor housing as may be required to drill out the well bore for the final casing string at a later time. In addition, since the centerline of the offset casing hanger  30  is offset toward the centerline of the conductor pipe  14 , the offset casing hanger is capable of suspending casing which is larger in diameter than the final casing string. The oversize casing  32  and the offset casing hanger  30  are ideally left in place in the SXS wellhead system  10  until such time as the well is reclaimed for completion through the well slot  24 . 
     Referring to FIGS. 4 and 5, a drilling riser  40  is lowered over the empty bowl profile  26  in preparation for drilling out a well bore and running a final casing string through this well slot. The drilling riser  40  comprises a generally D-shaped flange  42  which includes a flat or truncated portion  44 . The flange  42  is aligned with the holes  28  in the base plate  16  and is secured thereto with a number of bolts  46 . A preferably elastomeric face seal  48  is provided to seal between the drilling riser  40  and the base plate  16  during drilling of a well bore through the well slot  26 . 
     As shown in FIGS. 6 and 7, after a well bore is drilled through the well slot  26 , a mandrel casing hanger  50  is landed in the bowl profile through the drilling riser  40 . The mandrel casing hanger  50  is machined concentric about its centerline and suspends a final casing string  52  within the conductor pipe  14 . Preferably two ideally elastomeric seals  54  are provided to seal between the mandrel casing hanger  50  and the bowl profile  26 . 
     After the mandrel casing hanger  50  is installed, the drilling riser  40  is removed and a wellhead  56  is landed over the mandrel casing hanger. The wellhead  56  comprises an axially depending nose  58  and a generally D-shaped bottom flange  60 . The nose  58  engages the top of the mandrel casing hanger  50  to prevent vertical movement of the hanger due to thermal expansion. A number of seals  62  are ideally provided on the outer diameter of the nose  58  to seal the wellhead  56  to the bowl profile  26 . The bottom flange  60  of the wellhead  56  is aligned with the bolt holes  28  in the base plate  16  and is secured thereto with bolts  64 . If required, suitable seals  66  may be provided to seal between the wellhead  56  and the mandrel casing hanger  50 . 
     Once the final casing string  52  is installed through the bowl profile  26 , the offset casing hanger  30  and oversize casing  32  may be removed and a drilling riser  68  installed over the bowl profile  24 . The drilling riser  68  may be the same as the drilling riser  40  used in the previous steps and is accordingly secured to the base plate  16  in a similar fashion. A well bore is then drilled through the well slot  24  for the final casing string. As shown most clearly in FIG. 7, the truncated portions of the flanges of the drilling riser  68  and the wellhead  56  cooperate to allow very close center-to-center spacing between these two components. The truncated portions of the flanges preferably comprise opposing milled faces, and a constant gap between the flanges may be established using close tolerance pins (not shown). 
     Referring to FIGS. 8 and 9, after the well bore is drilled through the well slot  24 , a second mandrel casing hanger  70  suspending a second final casing string  72  is landed in the bowl profile through the drilling riser  68 . As shown in FIGS. 10 and 11, the drilling riser  68  is then retrieved and a second wellhead  74  is installed over the mandrel casing hanger  70  in a manner similar to that described above for wellhead  56 . 
     A second embodiment of the invention will now be described with reference to FIGS. 12 through 22. Referring first to FIGS. 12 and 13, the drilling alignment system according to this embodiment may be used in conjunction with an SXS wellhead system  76  which comprises a drive pipe housing  12  that is attached to a conductor housing or pipe  16  by suitable means, such as welding. A base plate  16  is connected to the drive pipe housing  12 , preferably with a number of studs and nuts  18 . As in the previous embodiment, the base plate  16  includes an annular lip  20  by which it may be suspended in the drive pipe housing  12  and two annular bowl profiles  24  and  26 , each of which is formed by a bore that extends through the base plate and defines a corresponding well slot therein. The base plate  16  is ideally sealed to the drive pipe housing  12  by a number of preferably elastomeric seals  22 , and a number of tapped holes  28  are formed in the top of the base plate for reasons which will be made apparent below. 
     Referring to FIG. 14, after an initial bore is optionally drilled through the well slot  24 , an oversized end cap guide  78  is threaded onto a length of oversize casing string  80  generally at  82  and lowered into the conductor pipe  14  through the well slot. The end cap guide  78  is ideally provided with a chamfer  84  at its lower end to help prevent the end cap guide from becoming snagged or hung up as it progresses downhole. As shown in FIG. 15, as the oversize casing string  80  is lowered into the conductor pipe  14 , the individual lengths of casing are connected together with oversized connector collars  86 , each of which is threadedly connected to the length of casing below generally at  88  and to the length of casing above generally at  90 . 
     Referring to FIG. 16, once the desired length of oversize casing  80  has been run, an offset casing hanger  92  is threadedly connected to the uppermost length of casing generally at  94  and then landed in the bowl profile  24  in the base plate  16 . The offset casing hanger  92  is machined off-center and is oriented in the bowl profile  24  via a conventional key and slot arrangement (not shown). In addition, the offset casing hanger  92  is ideally sealed to the bowl profile  24  with a suitable seal, such as a preferably elastomeric belt type packing  96 . 
     As in the previous embodiment, the offset casing hanger  92  and the oversized casing  80  ensure that sufficient space exists to drill out, run and set a final casing string in the well slot  24  after a first well bore has been drilled through the well slot  26 . Moreover, the end cap guide  78  and the connector collars  86  also serve to occupy the space that would be normally required for drilling the bore for the final casing string. The offset casing hanger  92  and its associated components preferably remain suspended in the well slot  24  until such time the well is reclaimed for completion. 
     Referring now to FIGS. 17 and 18, after the offset casing hanger  92  has been installed in the well slot  24 , a drilling riser  40  is lowered over the empty bowl profile  26 . The drilling riser comprises a generally D-shaped bottom flange  42  which is aligned with the bolt holes  28  on the base plate  16  and secured thereto with a number of bolts  46 . The drilling riser  40  permits the drilling out of a well bore and the running of the final casing through the well slot  26 . An elastomeric face seal  48  is preferably provided to seal between the drilling riser  40  and the base plate  16  during drilling of the well bore. 
     As shown in FIG. 19, a mandrel casing hanger  50  is subsequently landed in the bowl profile  26  through the drilling riser  40 . The mandrel casing hanger  50  is machined concentric about its centerline and suspends a final casing string  52  within the conductor pipe  14 . In addition, a number of preferably elastomeric seals  54  are ideally provided to seal between the mandrel casing hanger  52  and the bowl profile  26 . 
     Referring now to FIG. 20, after the drilling riser  40  is retrieved, a wellhead  98  is landed over the mandrel casing hanger  50 . The wellhead  98  comprises a bottom flange  100  which is aligned with the bolt holes  28  in the base plate  16  and secured thereto with a number of bolts  102 . In addition, the wellhead  98  includes a depending nose portion  104  that is sealed to the bowl profile  26  by a number of suitable seals  106 . A number of suitable seals  108  may also be provided to seal the wellhead  98  to the mandrel casing hanger  50 . 
     Once the final casing  52  is installed through the well slot  26 , the offset casing hanger  92 , including the oversized casing string  80 , the oversized end cap guide  78  and the oversized connector collars  86 , can be removed from the well slot  24 . A drilling riser  68  is then installed over the well slot  24 . The drilling riser  68  may be the same as the drilling riser  40  used in the previous steps and is accordingly secured to the base plate  16  in a similar fashion. A well bore is then drilled through the well slot  24  for the final casing string. As in the previous embodiments, the bottom flanges of the drilling riser  68  and the wellhead  98  may comprise truncated portions which cooperate to allow very close center-to-center spacing between these two components. These truncated portions preferably comprise opposing milled faces, and a constant gap between the flanges may be established using close tolerance pins (not shown). 
     Referring still to FIG. 20, after the well bore is drilled through the well slot  24 , a second mandrel casing hanger  70  suspending a second final casing string  72  is landed into the well slot through the drilling riser  68 . As shown in FIGS. 21 and 22, after the drilling riser  68  is retrieved, a second wellhead  110  is installed over the well slot  24  in a manner similar to that described above for the wellhead  98 . 
     In alternative embodiments, the method and apparatus of the present invention can be used for multiple completion wells having any number of casing strings suspended in a single conductor pipe. It is contemplated that while each well bore is being drilled out, the device of the present invention would be installed in each well bore which has not yet been drilled or completed. This would ensure that each successively completed casing string will not interfere with completion of the remaining strings. 
     It should be recognized that, while the present invention has been described in relation to the preferred embodiments thereof, those skilled in the art may develop a wide variation of structural and operational details without departing from the principles of the invention. Therefore, the appended claims are to be construed to cover all equivalents falling within the true scope and spirit of the invention.

Summary:
A method for drilling a plurality of well bores through a single conductor housing which comprises providing a base plate which includes at least first and second well slots that extend therethrough, supporting the base plate on the conductor housing, suspending a temporary casing string from the first well slot, drilling a first well bore through the second well slot, removing the temporary casing string from the first well slot, and then drilling a second well bore through the first well slot.