Abstract:
This invention relates to drilling a sidetrack wellbore out of the side of an existing wellbore utilizing a drillable billet having a path formed in the billet to lead a bi-centered drillbit to kick-out from the wellbore and form a sidetrack wellbore. The bi-centered bit is arranged for the lands of the pilot section to follow the path of the billet while the wings of the ream out a portion of the billet and also ream out the earth to form the full dimension sidetrack wellbore. The bi-centered bit is used to drill the sidetrack wellbore to the target zone so that the entire drilling process extends from the existing wellbore to the target zone without a mandatory withdrawal of the drillstring from the wellbore. Tripping time is therefore less and drill time and rig time would inherently be less resulting in lower costs and increased profitability.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a non-provisional application which claims benefit under 35 USC §119(e) to U.S. Provisional Application Ser. No. 61/156,171 filed Feb. 27, 2009, entitled “Directional Sidetrack Drilling System,” which is incorporated herein in its entirety. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    None 
       FIELD OF THE INVENTION 
       [0003]    This invention relates to drilling wells for producing fluids such as oil and gas. 
       BACKGROUND OF THE INVENTION 
       [0004]    In the process of drilling oil and gas wells, it is common to want to steer the drillbit to get the well to a desired location that is likely to have hydrocarbons trapped. While many technologies have been developed to steer the drillbit, there are many forces that resist or prevent steering. For example, when it is desired to sidetrack from an existing wellbore at some distance from the bottom of the existing wellbore, the least resistant path for the drillbit to follow is along the existing wellbore. It is a challenge to get a drillbit to bite into the side of an existing wellbore without something to push against. 
         [0005]    To overcome the tendency of drillbits to follow existing wellbores, the conventional technique is to create a cement plug in the location where it is desired to side track out from the existing wellbore. However, the cement plug closes the existing wellbore and further production in the original wellbore below the location of the sidetrack well is blocked. 
         [0006]    Other efforts to provide some resistance to use for creating a sidetrack wellbore include inserting an aluminum plug. The aluminum plug is more easily installed or at least less time consuming than a cement plug, but the aluminum plug tends to be kind of sloppy in the wellbore in that when the side track goes down from a somewhat horizontal well, the plug is likely to fall down into the hole when the drill string is pulled out of the side track and therefore block the side track. 
         [0007]    Also, it should be recognized that the relative density or resistance of the formation to drilling is likely to be different than the density or resistance to the drillbit of either the cement or aluminum plug. As such, there is still some concern about the drillbit getting out of the original wellbore and being able to drill to the target formation without having too much curvature or “dogleg”. For example, it might be attractive to use a drill motor having an aggressive angle to make sure that the drillbit fully exits the existing wellbore before the end of the plug is reached. However, the aggressive angle may be less preferred if the target formation is a considerable distance from the original wellbore. A wellbore that snakes and twists too much will create too much friction when liner or casing is inserted into the side track wellbore. So, the sidetrack might be created initially with a higher angle motor, recognizing that the most severe angles for well drilling are very small (less than four degrees off center, for example) but replaced with a motor that has a much smaller angle that will reach the target with a straighter wellbore. This strategy requires two “trips” where the drillstring is withdrawn from the wellbore and re-inserted. Trips are noteworthy from a financial standpoint as it sometimes takes quite a bit of time to pull out thousands of feet of a drillstring and then re-insert the drillstring to the same point and time is money on a drilling rig. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention relates to a process for drilling a sidetrack wellbore out of the side of an existing wellbore above the bottom of the existing wellbore where a drillable billet is installed into an existing wellbore at a desired location for stepping out of the wellbore and beginning the sidetrack wellbore. The drillable billet is made of a drillable material and has a long, generally cylindrical body that is suited and sized for insertion into the existing wellbore and block the wellbore below the location of the billet. A path is formed in the billet and is arranged to direct anything descending down the wellbore to a preferred side of the wellbore. A drillstring with a bi-centered drillbit is lowered into the existing wellbore down to the location of the billet where the bi-centered drillbit includes a pilot drill portion suited for drilling into the earth and for following the path in the billet. The bi-centered drillbit also includes at least one wing portion extending radially outwardly from the pilot drill portion suited for reaming out a hole larger than the pilot drill portion. The bi-centered drillbit is rotated so as to direct the drillbit so that the pilot drill portion follows the path and the wing portion reams out at least part of the billet and also into the earth to form a sidetrack well through the side wall of the existing wellbore into the earth. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The invention, together with further advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings in which: 
           [0010]      FIG. 1  is a vertical and fragmentary cross sectional view of a not to scale wellbore used for the production of hydrocarbons from an earthen formation; 
           [0011]      FIG. 2  is a close up vertical and fragmentary cross sectional view of a not to scale drillstring and billet in a wellbore showing the inventive process of drilling a sidetrack wellbore; 
           [0012]      FIG. 3  is an end view of a bi-centered drillbit for use in drilling boreholes and that is suitable for use in the inventive procedure of the present invention; 
           [0013]      FIG. 4  is a close up vertical and fragmentary cross sectional view of a not to scale drillstring and billet in a wellbore similar to  FIG. 2  with the drillbit having progressed further into the sidetrack wellbore; 
           [0014]      FIG. 5  is a perspective view of a billet that may be used in the present invention; 
           [0015]      FIG. 6  is a perspective view of a billet that may be used in the present invention; and 
           [0016]      FIG. 7  is a perspective view of a billet after it has been used to drill a sidetrack wellbore. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]    Turning now to the preferred arrangement for the present invention, reference is made to the drawings to enable a more clear understanding of the invention. However, it is to be understood that the inventive features and concept may be manifested in other arrangements and that the scope of the invention is not limited to the embodiments described or illustrated. The scope of the invention is intended only to be limited by the scope of the claims that follow. 
         [0018]    Turning to  FIG. 1 , a wellbore  10  is shown to be formed deep into the earth  11  from surface  12 . Within the earth  11  are layers of various materials. Some of the layers are porous and other layers are less porous or impermeable. Oil and gas tend to migrate through porous layers until they are trapped by impermeable or much less permeable formations. These traps may be targets for finding oil and gas in economic quantities. 
         [0019]    It is not uncommon for a number of potential traps to be within a few hundred yards of other traps. It is very desirable to access these traps from a common wellbore or access a new trap through an existing wellbore that is thousands of feet deep but only a few hundred of feet away from the untapped deposit. It is very desirable to be able to produce fluids from several of these traps as the same time through a common wellbore. As shown in  FIG. 1 , wellbore  10  is shown to have tapped target formation  15 . In this illustration, target formation is a trap created by a low permeability layer  17  and fault line  18  at which the layer  17  has been split and shifted or offset to create trap  15  where hydrocarbons may accumulate in a porous hydrocarbon bearing zone below layer  17 . An untapped second target formation  20  is similarly shown to be created by the same layer  17  that is split and shifted along second fault line  19 . The second target formation  20  should also be seen to be closer to wellbore  15  than it is to surface  12 . One should understand that  FIG. 1  is not to scale as the target formations are often many thousands of feet below the surface and can be offset above, below and to either side of another target by a broad range of distances. What a reader should understand is that when it is practical to use an already drilled borehole such as wellbore  10 , it may be very efficient to drill a sidetrack wellbore such as  21  that uses a significant portion of the existing borehole, but kicks out from a location substantially above the bottom end of wellbore  10 . Specifically, the sidetrack wellbore  21  is intended to begin at about location  22 . It should also be understood that this illustration is not intended to describe in detail the geology of hydrocarbon traps, but only to set forth a simple and understandable explanation as to why one would have drilled an existing well and then modify that well to go after a separate but nearby zone. 
         [0020]    So, referring now to  FIG. 2 , a billet  31  is installed in the wellbore  10  at the location  22 . The billet  31  may be installed along with a liner or casing string extending down to the first target zone  15  for use in producing fluids from the first target zone  15 . The billet  31  is a relatively long cylindrically shaped piece of aluminum or similar material that is substantial enough to be fixed in place and hold up to rough treatment including rotating drillbits while at the same time being soft enough to accommodate portions of the billet being drilled away. As such, the billet  31  is sometimes referred to as drillable. Preferably, the billet  31  includes an axial port  36  to allow fluids to flow past the billet and be produced to the surface. Other openings and channels in the exterior walls may be created to facilitate the installation process of the billet  31  in the wellbore and the passage of desired fluids to the surface  12 . 
         [0021]    The billet  31 , also includes a path  32  that is preferably formed at the center or at the axis of the top end of the billet  31 . It should be understood that there may be circumstances where the path may start at a location that is off center from the axis of the billet. For simplicity, a path  32  that begins near the center of the top end of the billet or near the axis of the billet will be assumed and described. The path  32  is oriented to extend in a direction that deviates from the axis and leads to the periphery of the billet  31  whether by a curved path or by a straight path that is at an angle relative to the axis of the billet  31 . With the billet  31  installed in the wellbore  10 , a drillstring, generally indicated by the number  40  is inserted into the wellbore to engage billet  31 . At the bottom end of drillstring  40  is a bi-centered drillbit  42 . Referring to now to  FIGS. 2 ,  3  and  4 , a bi-centered drillbit is a known type of drillbit that is designed to drill boreholes larger in diameter than the actual lateral dimension of the drillbit. Focusing on  FIG. 2 , a bi-centered drillbit includes a pilot portion at the tip which includes cutting surfaces at the tip end and several lands  45  that are designed to follow the path  32  in the billet  31 . The diameter of the path  32  is preferably about the same diameter of the pilot portion of the drillbit  42  to provide stability to the drillstring as the drillbit  42  rotates with the pilot portion in the path  32 . The pilot portion comprises about half the length of the drillbit  42 . Wings  46 , which three are shown in  FIG. 3 , is a portion of the drillbit  42  which is arranged to extend radially beyond the diameter formed by the lands  45  and ream out a larger diameter borehole. Since the wings  46  are positioned along a segment comprising about one quarter of the circumference of the drillbit  42 , the drillbit  42  actually drills a hole indicated by dashed line  47  in  FIG. 3  which should be appreciated as being much larger than the actual lateral dimension of the drillbit  42 . Indeed, it is practical with a bi-centered drillbit to lower it through a borehole of a smaller diameter than will be cut when the drillbit is cutting when rotated and boring into the earth or other material. 
         [0022]    Utilizing the special geometry of a bi-centered drillbit  42 , the billet  31  guides or captures the pilot portion along the path  32  while the wings  46  ream out the path as shown at  34  in  FIG. 4  nearer the upper end of the billet  31 . The wings  46  also drill a larger hole  21  than is drilled by end of the pilot portion  45 . With the billet being drilled out, it is preferred that the billet  31  is formed of aluminum, plastic or other polymer or concrete or malleable or drillable iron or some other soft metal. Hardened steel would almost certainly tear up drillbit  42 . 
         [0023]    Referring now to  FIG. 5 , a billet  31  is shown prior to being inserted into wellbore  10 . Path  32  is shown at the end and at the peripheral side. After the bi-centered drillbit  42  has bored through the billet, a substantial amount of the billet will be cut away.  FIG. 7  provides a sample of what might be expected to be left of the billet  31  after the drillbit  42  has removed a substantial portion. The upper end may be fully or nearly fully removed down to a slivered edge where the wings  46  cut into the billet while beginning to cut into the formation opposite from the remaining portion of the billet  31 . As the pilot portion of the drillbit  42  followed the path  32  the entire side of the billet is removed from around the location of the peripheral exit of the path  32 . Surface  49  in  FIG. 7  is essentially all that remains of the billet  31  while axial channel  36  is now revealed. 
         [0024]    In  FIG. 6 , billet  31 A is shown as a second embodiment of the present invention where rather than a path  32  beginning as a generally circular hole at the end, the path  32 A is formed in the shape of an angled trough that extends like a ramp along the surface of the billet  31 A at an angle to the axis thereof. The angled trough is deepest at the end nearest the top of the wellbore and is progressively shallower away from the end. The angled trough shaped path  32 A is designed to capture the pilot portion of the bi-centered drillbit  42  and guide it while the wings  46  enlarge the path and cut substantially into the billet  31 A even though the path  32 A will be open at one side. Billet  31 A also includes an axial channel  36  as shown. The billet  31 A would likely have an appearance similar to that shown in  FIG. 7  after the drillbit  42  has reamed out the angled trough path  32 A. 
         [0025]    Turning back to  FIG. 2 , the two lines  48  comprising short dash segments indicate the dimension of the hole that will be initially drilled by the pilot portion of drillbit  42 . The two lines  47  comprising long dash segments indicate the dimension of borehole after the wings  46  have reamed out and enlarged the hole right behind the pilot portion. It should also be seen that dashed line  49  indicates the extent the wings  46  will cut into the billet  31 . 
         [0026]    It should also be recognized that the path may also be a blind pilot hole which captures the pilot portion of the drillbit so that the wings are restrained from bouncing around the borehole while the drill motor or drillstring rotates. If the motor is steerable such as by a rib steer motor, the billet may provide enough stabilizing resistance to allow the operators to direct the drillbit and drillstring in the preferred direction. 
         [0027]    In another aspect of the invention, while fluid may pass the billet  31  through the axial channel  36 , the billet  31  may be attached to liner pipe or casing at its base or be otherwise locked into place while also allowing fluids to pass through an annular space at the periphery of the billet. While in the preferred embodiment, the billet is not intended to plug the original borehole, there are circumstances where the original borehole may have entered a water zone, gas zone or unstable lost circulation zone where the portion below the billet is not intended to be further accessed Thus, having the lower portion sealed may be preferred so a billet without an axial channel or other passages would be used. 
         [0028]    It should now be seen that a sidetrack well may be drilled out of the side of an original wellbore at a distance from the bottom thereof using a single drillstring utilizing only one trip into and out of the hole. Except for unusual circumstances such as breakdowns or other problems, the technique set forth above is designed to eliminate trips. Considerable time and costs may be saved by initiating and completing the sidetrack in one trip. 
         [0029]    Finally, the scope of protection for this invention is not limited by the description set out above, but is only limited by the claims which follow. That scope of the invention is intended to include all equivalents of the subject matter of the claims. Each and every claim is incorporated into the specification as an embodiment of the present invention. Thus, the claims are part of the description and are a further description and are in addition to the preferred embodiments of the present invention. The discussion of any reference is not an admission that it is prior art to the present invention, especially any reference that may have a publication date after the priority date of this application.