Abstract:
A downhole drilling apparatus for interconnection in a casing or liner string having a drill bit disposed thereon for enabling the drilling of intersecting wellbores without removal of the drill bit is disclosed. In a disclosed embodiment, the apparatus comprises a housing having a window. A whipstock is disposed within the housing. Between the window and the whipstock is a filler. The whipstock and the filler define a central bore providing a fluid path through the apparatus. A back pressure valve may be disposed within the central bore to prevent back flow of fluids through the apparatus. Once the total depth of an initial wellbore is reached, the casing or liner string, including the apparatus, may be cemented in place. Thereafter, an intersecting wellbore may be drilled by laterally deflecting a second drill bit with the whipstock through the window of the housing.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates in general to drilling a wellbore and, in particular, to drilling an intersecting wellbore through a drill string including well casing or liner and a downhole drilling apparatus interconnected therein. 
     Without limiting the scope of the invention, its background is described in connection with drilling a wellbore for hydrocarbon production, as an example. 
     Heretofore, in this field, a typical drilling operation has involved attaching a drill bit on the lower end of a drill string and rotating the drill bit along with the drill string to create a wellbore through which subsurface formation fluids may be produced. As the drill bit penetrates the various earth strata to form the wellbore, additional joints of drill pipe are coupled to the drill string. During drilling, drilling fluid is circulated through the drill string and the drill bit to force cuttings out of the wellbore to the surface, and to cool the drill bit. 
     Periodically as the drilling of the wellbore progresses, the drill bit and drill string are removed from the wellbore and tubular steel casing is inserted into the wellbore to prevent the wall of the wellbore from caving in during subsequent drilling. Typically, after casing is inserted into the wellbore, the annulus between the casing and wellbore is filled with a cement slurry that hardens to support the casing in the wellbore. Thereafter, deeper sections of wellbore with progressively smaller diameters than the previously installed casing may be drilled. 
     Once a predetermined depth is reached for each subsequent section of wellbore, the drill bit and drill string are again removed from the wellbore and that section of the wellbore may be cased. Alternatively, however, a liner may be used to case an open section of wellbore instead of a full casing string. The liner, which is a string of connected lengths of tubular steel pipe joints, is lowered through the casing and into the open wellbore. At its upper end, the liner is attached to a setting tool and liner hanger. The liner hanger attaches the liner to the previous casing such that the casing will support the weight of the liner. 
     The length of the liner is predetermined such that its lower end will be proximate the bottom of the open wellbore, with its upper end, including the liner hanger, overlapping the lower end of the casing above. As with the casing, after the liner is inserted into the wellbore, the annulus between the liner and the wellbore may be filled with a cement slurry that hardens to support the liner in the wellbore. 
     It has been found, however, that in many well drilling operations it is desirable to minimize rig time by utilizing the casing or liner string as the drill string for rotating a drill bit, which may be left in the wellbore upon the completion of drilling a section of the wellbore. As such, this procedure does not require the use of a separate liner or casing upon the withdrawal of the drill bit and drill string as in conventional drilling operations, and thereby reduces the time needed to drill, case and cement a section of wellbore. 
     For example, attempts have been made to utilize the casing or liner string as the drill string along with a drill bit that is rotatable relative to the casing or liner string. The drill bit is rotated by a downhole drill motor that is driven by drilling fluid. Upon completion of drilling operations, the motor and the retrievable portions of the drill bit may be removed from the wellbore so that further wellbore operations, such as cementing, may be carried out and further wellbore extending or drilling operations may be conducted. This system, however, requires the use of expensive and sometimes unreliable downhole drill motors and a specially designed drill bit. 
     Alternatively, other attempts have been made to utilize the casing or liner string as the drill string using conventional rotary techniques wherein the drill bit is rotated by rotating the entire casing or liner string. This approach, however, requires the use of a drill bit with minimal cutting structure, since a drill out could not be performed through a typical drill bit having a full cutting structure, such as a tricone bit. 
     Therefore, a need has arisen for a drill string which may be used as a well casing or liner, which includes a drill bit on its lower end, and which, upon completion of drilling operations, may be retained within the wellbore without the need to retrieve the drill bit or the drill string. A need has also arisen for such a well casing or liner string that may be left in the wellbore along with a drill bit, and which does not require the use of expensive, unreliable or specialty equipment. Further, a need has arisen for such a well casing or liner string which may be cemented in place along with a drill bit having a full cutting structure. 
     SUMMARY OF THE INVENTION 
     The present invention, as exemplified by an embodiment disclosed herein, comprises a downhole drilling apparatus that is interconnectable in a casing or liner drill string and includes a drill bit connected thereto which, upon completion of drilling operations, may be retained within the wellbore without the need to retrieve the drill bit or the drill string. The apparatus allows the well casing or liner to be left in the wellbore along with the drill bit and does not require the use of expensive, unreliable or specialty equipment. The apparatus also allows for the well casing or liner to be cemented in place along with a drill bit having a full cutting structure. 
     The downhole drilling apparatus includes a housing that is interconnectable in a casing string. The housing has a window cut therein to allow a subsequent drill bit and pipe string to pass therethrough during a drill out operation. To facilitate the deflection of the drill bit and pipe string through the window, a whipstock is disposed within the housing. A filler material is also disposed within the housing between the whipstock and the window to prevent the flow of drilling fluids or cement through the window prior to the drill out. The filler and the whipstock have a central bore that permits the passage of fluids through the center of the downhole drilling apparatus. One or more valves may be disposed within the central bore to control the flow of fluids therethrough. The valves may be, for example, back pressure or float valves that allow one-way flow of fluids downwardly through the apparatus. 
     A drill bit having a full cutting structure, such as a tricone bit, may be operably coupled to the downhole drilling apparatus. The casing or liner string may be used to rotate the drill bit. Alternatively, a downhole motor may be coupled between the downhole drilling apparatus and the drill bit to facilitate rotation of the drill bit, without the need for rotating the casing string. 
     In another embodiment, a downhole drilling apparatus includes a housing having a window, an alignment member disposed within the housing and a back pressure valve assembly. The back pressure valve assembly includes a central bore that permits the passage of fluids therethrough. Once downhole, a whipstock may be run into the apparatus such that the whipstock operably engages the alignment member. The alignment member orients the whipstock within the housing relative to the window, so that the drill bit may subsequently be deflected through the window. 
     In operation, either embodiment of the downhole drilling apparatus may be interconnected in a casing or liner string having a drill bit disposed on its lower end. A first wellbore is drilled. Following the drilling of the first wellbore, the casing or liner string may be cemented within the wellbore. A pipe string having another drill bit on its lower end is passed through the casing or liner string, such that a drill out through the downhole drilling apparatus is performed to drill a second wellbore. The pipe string and drill bit that are used to create the second wellbore are deflected through the window in the housing of the downhole drilling apparatus by the whipstock disposed within the apparatus. 
     Thus, with the use of the downhole drilling apparatus, a casing or liner string including a drill bit having a full cutting structure may be used as a drill string to create a wellbore. The drill string may be cemented in place within the wellbore, and thereafter have a drill out performed therethrough to create an intersecting wellbore. 
     These and other features, advantages, benefits and objects of the present invention will become apparent to one of ordinary skill in the art upon careful consideration of the detailed description of representative embodiments of the invention hereinbelow and the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, including its features and advantages, reference is now made to the detailed description of the invention, taken in conjunction with the accompanying drawings of which: 
     FIG. 1 is a schematic illustration of an offshore oil and gas platform during a drilling operating wherein a downhole drilling apparatus embodying principles of the present invention is utilized; 
     FIG. 2 is a schematic illustration of a first downhole drilling apparatus embodying principles of the present invention; 
     FIG. 3 is a cross sectional view of the downhole drilling apparatus of FIG. 2, taken along line  3 — 3 ; 
     FIG. 4 is a cross sectional view of the downhole drilling apparatus of FIG. 2, taken along line  4 — 4 ; 
     FIG. 5 is a schematic illustration of an offshore oil and gas platform during a drilling operating wherein a downhole drilling apparatus embodying principles of the present invention is being utilized in conjunction with a downhole motor; 
     FIG. 6 is a cross sectional view of a second downhole drilling apparatus embodying principles of the present invention before insertion of a whipstock therein; and 
     FIG. 7 is a cross sectional view of the second downhole drilling apparatus after insertion of a whipstock therein. 
    
    
     DETAILED DESCRIPTION 
     While the making and using of various embodiments of the present invention are discussed in detail below, it should be appreciated that the present invention provides many applicable inventive concepts which can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention. 
     Referring to FIG. 1, an offshore oil and gas platform is schematically illustrated and generally designated  10 . A semi-submersible platform  12  is centered over a subterranean oil and gas formation  14  located below sea floor  16 . A well  18  extends through the sea  20 , penetrating sea floor  16  to form wellbore  22 , which traverses various earth strata. A wellbore extension is formed by wellbore  24 , which extends from wellbore  22  through additional earth strata, including formation  14 . 
     Platform  12  has a hoisting apparatus  26  and a derrick  28  for raising and lowering pipe strings, such as drill string  30 , including drill bit  32  located in wellbore  24 , and casing string  34 , including drill bit  36 , crossover subassembly  38  and downhole drilling apparatus  40  located in wellbore  22 . As used herein, the term “casing string” is used to refer to a tubular string which includes sections of casing or liner. 
     As in a typical drilling operation, wellbore  22  is formed by rotating drill bit  36  while adding additional sections of pipe to casing string  34 . When drill bit  36  reaches total depth, however, casing string  34  and drill bit  36  are not retrieved from wellbore  22 . Rather, casing string  34  and drill bit  36  are cemented in place by cement  42  which fills the annular area between casing string  34  and wellbore  22 . 
     Cementing casing string  34  and drill bit  36  in place within wellbore  22  is a cost effective alternative to conventional drilling, in that significant rig time is saved by minimizing the number of trips into and out of wellbore  22 . At least one trip out of wellbore  22  and one trip into wellbore  22  are saved by using downhole drilling apparatus  40 . Additionally, the use of downhole drilling apparatus  40  avoids the possibility of collapse of wellbore  22 , particularly in unconsolidated or weakly consolidated formations. 
     Alternatively, downhole drilling apparatus  40  may be used in conjunction with conventional drilling operations once a conventional drill string and bit have been tripped out of wellbore  22 . For example, if wellbore  22  has traversed an unconsolidated or weakly consolidated formation and it is likely that a collapse has occurred within wellbore  22 , it may be necessary to reopen that portion of wellbore  22 . In this case, wellbore  22  may be reopened using casing string  34  with downhole drilling apparatus  40  and drill bit  36 . 
     Once cementing of wellbore  22  has been completed, wellbore  24  may be drilled. Drill bit  32  creates wellbore  24  by traveling through window  44  of downhole drilling apparatus  40 , as will be more fully discussed with reference to FIGS. 2-4 below. As drill bit  32  and drill string  30  continue to form wellbore  24 , formation  14  is traversed. Note that the drill string  30  may include another apparatus  40 , if desired. 
     Even though FIG. 1 depicts wellbore  22  as a vertical wellbore, it should be understood by those skilled in the art that wellbore  22  may be vertical, substantially vertical, inclined or even horizontal. It should also be understood by those skilled in the art that wellbore  22  may include multilateral completions wherein wellbore  22  may be the primary wellbore having one or more branch wellbore extending laterally therefrom, or wellbore  22  may be a branch wellbore. Additionally, while FIG. 1 depicts an offshore environment, it should be understood by one skilled in the art that the use of downhole drilling apparatus  40  is equally well suited for operation in an onshore environment. 
     Schematically illustrated in FIG. 2 is a downhole drilling apparatus  50  embodying principles of the present invention. Apparatus  50  has a pin end  52 , so that the apparatus  50  is interconnectable in a drill string, such as casing string  34  of FIG.  1 . Downhole drilling apparatus  50  also has a box end  54  that may be threadedly connected to crossover subassembly  38  as depicted in FIG.  1 . 
     Apparatus  50  has a generally tubular housing  56  with a window  58  cut through a sidewall thereof. Window  58  is generally elliptically shaped and is sized such that a drill bit, such as drill bit  32  of FIG. 1, may pass therethrough during a drill out operation. 
     Now referring to FIG. 3, a cross sectional view of downhole drilling apparatus  50  taken along line  3 — 3  of FIG. 2 is depicted. Disposed within housing  56  of apparatus  50  is a whipstock  60 . A central bore  62  extends through whipstock  60  to provide fluid passage for drilling mud and cement through apparatus  50  during drilling and cementing operations. Valves  64 ,  66  are disposed within central bore  62  of the downhole drilling apparatus  50 . Valves  64 ,  66  may be back pressure or float valves that allow one-way flow of drilling mud or cement through the apparatus  50 . As an example, valves  64 ,  66  may be SuperSeal II back pressure valves, available from Halliburton Energy Services, Inc. of Duncan, Okla. 
     Whipstock  60  has an inclined upper surface, so that it directs a drill bit, such as drill bit  32  of FIG. 1, through window  58  of downhole drilling apparatus  50 . Whipstock  60  may be constructed of any material, such as steel, having sufficient strength to deflect a drill bit through window  58 . Whipstock  60  may also provide additional torsional strength to the downhole drilling apparatus  50 . 
     A filler  68  occupies the volume between whipstock  60  and window  58  of downhole drilling apparatus  50 . Filler  68  prevents the flow of drilling mud or cement through window  58  of apparatus  50 . Filler  68  may be, for example, concrete that has been poured into downhole drilling apparatus  50 . Window  58  may also be filled with filler  68  to provide protection to window  58 . Other suitable solid materials, such as resins, may be used for filler  68 , so long as they set sufficiently and permit the directional passage of a drill bit through window  58  of apparatus  50 . 
     In operation, when a drill bit, such as drill bit  32  of FIG. 1, encounters whipstock  60 , the drill bit cuts through filler  68  and is deflected laterally by whipstock  60  toward window  58  in housing  56 . Window  58  is wider that the outer diameter of the drill bit, permitting the drill bit to laterally exit the apparatus  50 . 
     Referring now to FIG. 4, a cross sectional view of downhole drilling apparatus  50  is depicted that is taken along line  4 — 4  of FIG.  2 . Apparatus  50  includes housing  56 , whipstock  60 , filler  68  and window  58 . As with typical drill down shoes, downhole drilling apparatus  50  may have sufficient torsional strength to rotate a drill bit, such as drill bit  36  of FIG.  1 . The wall thickness of housing  56  and the size of window  58  will affect the torsional strength of downhole drilling apparatus  50 . Of course, the window  58  should be dimensioned to permit a drill bit to pass therethrough. 
     The shape of whipstock  60  can be varied to maximize its deflecting capability. For example, whipstock  60  may be made concave or convex to direct a drill bit, such as drill bit  32 , through window  58  of downhole drilling apparatus  50 . If whipstock  60  is made concave, drill bit  32  will encounter window  58  at a position slightly below that where a straight whipstock  60  would direct the bit. Conversely, a convex whipstock  60  will force the encounter of drill bit  32  with window  58  at a position above that of the flat-surfaced whipstock  60 . 
     Referring now to FIG. 5, an offshore oil and gas platform is schematically illustrated and generally designated  70 . A semi-submersible platform  72  is centered over a subterranean oil and gas formation  74  located below sea floor  76 . A well  78  extends through the sea  80 , penetrating sea floor  76  to form wellbore  82 , which traverses various earth strata. Wellbore  82  has a wellbore extension that is formed by wellbore  84 , which extends from wellbore  82  through additional earth strata, including formation  74 . 
     Platform  72  has a hoisting apparatus  86  and a derrick  88  for raising and lowering pipe strings, such as drill string  90 , including drill bit  92  located in wellbore  84 , and casing string  94 , including drill bit  96 , downhole motor  98 , crossover subassembly  100  and downhole drilling apparatus  102  located in wellbore  82 . Using downhole motor  98 , it is not necessary to rotate casing string  94 , including downhole drilling apparatus  102 , in order to rotate drill bit  96 . 
     Drilling mud, used to cool drill bit  96  and carry cuttings to the surface, also provides the power to operate downhole motor  98 . As the drilling mud travels through downhole motor  98 , downhole motor  98  imparts rotation to drill bit  96 , so that wellbore  82  is drilled. Using downhole motor  98  in conjunction with downhole drilling apparatus  102  reduces the torsional stress typically encountered by downhole drilling apparatus  102  when casing string  94  is used to rotate drill bit  96 . This reduction in torsional stress allows for the use of a maximum width window  106  in downhole drilling apparatus  102 . 
     When drill bit  96  reaches total depth, casing string  94 , including drill bit  96 , downhole motor  98 , crossover subassembly  100  and downhole drilling apparatus  102 , is not retrieved from wellbore  82 . Rather, casing string  94  is cemented in place by cement  104 , which fills the annular area between casing string  94  and wellbore  82 . 
     Once cementing of wellbore  82  has been completed, wellbore  84  may be drilled using downhole drilling apparatus  102 . Drill bit  92  creates wellbore  84  by traveling through window  106  of downhole drilling apparatus  102  in the manner discussed above with reference to FIGS. 2-4. 
     Referring next to FIG. 6, a cross sectional view of another downhole drilling apparatus  120  embodying principles of the present invention is depicted. Downhole drilling apparatus  120  has a pin end  122 , so that downhole drilling apparatus  12  is interconnectable in a drill string, such as casing string  94  of FIG. 5, or to other downhole tools. Downhole drilling apparatus  120  also has a box end  123  which may be threadedly connected to crossover subassembly  100  as depicted in FIG.  5 . 
     Apparatus  120  has a generally tubular housing  124  with a window  126  cut through a sidewall thereof. Window  126  is generally elliptically shaped and is sized such that a drill bit, such as drill bit  92  of FIG. 5, may pass therethrough during a drill out operation. Surrounding window  126  is a cover or shield  128  that prevents the flow of drilling mud or cement through window  126 . Apparatus  120  also has at least one alignment member  130 , such as a track, within housing  124 . 
     Disposed within housing  124  is a back pressure valve assembly  132 . A central bore  134  extends through back pressure valve assembly  132  to provide fluid passage for drilling mud and cement used during drilling and cementing operations. Valves  136 ,  138  are disposed within central bore  134  of back pressure valve assembly  132 . Valves  136 ,  138  may be back pressure valves or float valves that allow one-way flow of drilling mud or cement therethrough. 
     As best seen in FIG. 7, a whipstock  140  may be run into downhole drilling apparatus  120  to direct a drill bit, such as drill bit  92  of FIG. 5, through window  126  of apparatus  120 . Whipstock  140  may be installed within downhole drilling apparatus  120  following a cementing operation and subsequent use of a conventional cementing plug  142 . Whipstock  140  includes one or more alignment lugs  144  that cooperate with track  130  of downhole drilling apparatus  120  to radially orient whipstock  140  with respect to window  126 . 
     After cementing the casing string  94  within wellbore  82 , including installing the plug  142  in the drilling apparatus  120 , the whipstock  140  is conveyed into the drilling apparatus. The alignment track  130  and lugs  144  cooperatively engage and thereby radially orient the whipstock  140  to face toward the window  126 . A drill bit may then be deflected off of the whipstock  140  to cut through the shield  128 , or the shield may be previously displaced to open the window  126 , for example, by using a conventional shifting tool. 
     In the embodiments described above, the present invention provides the ability to drill a wellbore using a well casing or liner string as the drill string, and using a drill bit having a full cutting structure. The use of a downhole drilling apparatus embodying principles of the present invention as part of the drill string allows a well extension to be drilled from the existing wellbore, without having to bore through a drill bit on the end of the casing or liner string. Thus, trips into and out of the wellbore may be eliminated and a drill bit having a full cutting structure may be used. 
     While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is, therefore, intended that the appended claims encompass any such modifications or embodiments.