Abstract:
A short radius exit from a window milled in casing is possible using a whipstock with a sloping surface in excess of 3.5° and a window mill whose diameter is reduced to a percentage generally below about 95% of the casing inside diameter in a mono-bore or non-through tubing application. The system provides a greater flexibility in choosing the window location and eliminates having to penetrate adjacent formations as compared to previous techniques using a longer exit radius. The decrease in mill diameter, as compared to previous techniques, limits stresses on the milling equipment to minimize equipment failures during window milling and subsequent drilling of the lateral.

Description:
PRIORITY INFORMATION 
   This application claims the benefit of U.S. Provisional Application No. 60/440,268 on Jan. 15, 2003. 

   FIELD OF THE INVENTION 
   The field of this invention is whipstock design and the associated milling systems that are used with whipstocks particularly in application where short exit radius is necessary or desired. 
   BACKGROUND OF THE INVENTION 
   Typically, whipstocks are used to create laterals from an existing bore to reach an as yet untapped formation. Whipstocks have traditionally been fairly lengthy and have incorporated a sloping surface to direct a milling assembly through a casing wall to form an opening in the casing wall known as a window. After the window is fully formed, the milling assembly is removed and the whipstock guides a drilling assembly through the window to drill the lateral. Casings have what is known in the industry as a drift diameter. The drift diameter is the largest dimension a tool can be and still fit through the inside diameter of the casing. Typically, milling assemblies that are frequently delivered with a whipstock have had external diameters at or near the drift diameter or approximately 97% of the casing inside diameter. The angle of inclination on the whipstock face has typically been less than 3.5°. This small angle creates limitations depending the location of available exit points for laterals and location and composition of adjacent formations. The slight angle on the whipstock requires an exit point from the casing and an exit trajectory of the drill bit that undesirably penetrates an adjacent formation that might produce water or sand or it could be highly unconsolidated and difficult to drill or complete. 
   The apparatus and method of the present invention allows for shorter radius exits from a window than had been accomplished in the past. It employs whipstock face inclinations of greater than about 3.5° and a window mill diameter of less than 95% of the casing inside diameter. This combination allows for short radius exits and avoids overstressing the milling equipment that forms the window. Those skilled in the art will better appreciate the features of the claimed invention from a review of the description of the preferred embodiment and the claims, which appear below. 
   SUMMARY OF THE INVENTION 
   A short radius exit from a window milled in casing is possible using a whipstock with a sloping surface in excess of 3.5° and a window mill whose diameter is reduced to a percentage generally below about 95% of the casing inside diameter in a mono-bore or non-through tubing application. The system provides a greater flexibility in choosing the window location and eliminates having to penetrate adjacent formations as compared to previous techniques using a longer exit radius. The decrease in mill diameter, as compared to previous techniques, limits stresses on the milling equipment to minimize equipment failures during window milling and subsequent drilling of the lateral. 

   
     DETAILED DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a prior art window milling system starting to form the window; 
       FIG. 2  is the system of  FIG. 1  showing the window nearly fully formed; 
       FIG. 3  shows the present invention initiating the window; 
       FIG. 4  is the view of  FIG. 3  with the window nearly fully formed; 
       FIG. 5  is a section view with dimensions of a prior art system used in 9.63 inch casing that weighs 40 pounds per foot; and 
       FIG. 6  is the present invention that is used to create an ultra short radius in the same casing as the example of  FIG. 5  showing dimensions to allow comparison. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   In the past, a whipstock  10  had a lug  12 , which was generally secured at the lower end of a lower string mill  14 . A window mill  16  starts the window  18  in the casing  20 . A flexible joint  22  is mounted above the lower string mill  14 . An upper string mill  24  (see  FIG. 5 ) is mounted above the flexible joint  22 . As shown in  FIG. 5 , the assembly of such equipment when used in a 9.63 inch casing weighing 40 pounds per foot would typically be used with a whipstock  10  having a sloping surface  26  oriented at about 2.3° from the longitudinal axis. This made for a whipstock length of about 247 inches. The window mill  16  had an outside diameter of about 8.195 inches and the mill assembly was about another 260 inches. The window mill  16  being up against the upper end  28  of the whipstock  10  together created a profile close to the drift diameter of the casing  20 . It should be noted that the lower string mill  14  had a maximum diameter of about 8.125 inches, which is larger than the window mill  16  outside diameter but still less than the drift diameter of casing  20 . The upper string mill  24  had a maximum diameter of 8.675 inches, which is even larger than the lower string mill  16  diameter but at the same time still smaller than the drift diameter of the casing  20 , but not by much. The upper string mill was generally larger because by the time it reached the window that had already been milled by the window mill  16  and the lower string mill  14 , the upper end  28  of the whipstock  10  would have been somewhat worn down so as not to allow the upper string mill  24  to get jammed. The objective of prior designs was to get the mills as close as possible to the drift diameter of the casing  20 . As long as the combined diameter of the upper end  28  of the whipstock  10  and the window mill  16  was less than the drift diameter, the assembly would pass quickly to the desired kick-off point for the lateral without serious concerns of getting it stuck. Additionally, the bigger the window mill  16  diameter the bigger the window  18  and the easier it was for a drill to make an exit for the drilling of the lateral. The downside of this arrangement using a downhole assembly having a maximum outside diameter of 97% or greater of the casing drift diameter is that it would need to exit in a fairly long radius to avoid failure from lateral overstressing. The use of a long exit radius also required a very small angle on surface  26 . The overall whipstock length would grow as the angle became smaller to accommodate the expected stresses from forming the window with an outside diameter of the milling assembly closely approximating the drift diameter. The long exit radius also required the lateral to penetrate adjacent formations before reaching the zone of interest. This could result in completion problems if the zone adjacent the window produced sand or water or was very unconsolidated. The use of a whipstock  10  with face angles on surface  26  of 3.5° or less often required the positioning of the window well above the target zone and, at times, in an inconvenient location in the casing for milling to begin. 
   To resolve these shortcomings of the prior designs, the present invention has been developed. It features a window mill  30  and a lower string mill  32 . The whipstock  34  has a lug  35  that allows connection to the string mill  32  for the trip downhole. The whipstock face  36  is at an angle greater than 3.5° with the preferred range at 4.5+/−0.5°. As seen by comparing  FIGS. 5 and 6  the length of the whipstock  34  having an angle of 4.5° and a length of 97.50 inches, is less than half the 247 inch length of the whipstock  27  that has a slope of 2.3° on surface  26 . What makes the higher slope angle on surface  36  possible without overstressing the milling assembly is that its outside diameter is less than 95% of the drift diameter with a preferred range in the order of 70–75% of the casing drift diameter. As seen in comparing  FIGS. 5 and 6 , the outside diameter of the window mill and whipstock top has been decreased from 8.195 inches to 6.25 inches for the same casing size. The string mill diameter has been reduced from 8.215 inches to 6.25 inches and the assembly in  FIG. 6  omits the flexible joint  22  and the upper string mill  24 . As a result, the assembly in  FIG. 6  is about a third the length of the  FIG. 5  assembly. It exits at a far larger radius due to the higher slope of the whipstock face. Overstresses are avoided by a decrease in diameter of the bottom hole assembly. The base of the whipstock in  FIGS. 5 and 6  remains the same to facilitate the anchoring process. Trimming the diameter of the assembly relocates the maximum stress region from the previous design. As seen in  FIG. 1 , the maximum stress region, when starting the window, is in the area of the connection between the window mill  16  and lower string mill  16 . By comparison,  FIG. 3  illustrates the present invention with the higher slope angle on the whipstock  34  and the smaller diameter on the window mill  30  and string mill  32 . As a result of the changed parameters, the maximum stressed region has been relocated upwardly to the upper end of the string mill  32 . Similarly, when finishing the window, the maximum stressed region has been moved down from the upper string mill  24  to the connection between the window mill  30  and the string mill  32 . This is schematically illustrated by comparing  FIGS. 2 and 4 . 
   Those skilled in the art will appreciate that the present invention allows for shorter bottom hole assemblies and lateral exits at far shorter radii than had been possible with previous designs. The angle on the whipstock face has been altered to a range of greater than 3.5° with the preferred range of 4° to 5°. At the same time the maximum dimension of the assembly where the whipstock is connected to the window mill has been reduced to less than 95% of the casing drift diameter, with the preferred range being 70–75% of the drift diameter. Preferably, as shown in  FIG. 6 , the string mill  32  can have the same outside diameter as around the whipstock  34  and the window mill  30 . Alternatively, the string mill can be somewhat larger. Optionally, there can be only one string mill  32  but use of more than one string mill is within the scope of the invention. Ultimately, after the window  38  is milled with the assembly shown in  FIG. 6 , a drill bit (not shown) is inserted through the window  38  in casing  40  for the far shorted exit radius for the new lateral. The combination of the higher slope on the whipstock to enable the shorter radius and the smaller diameter of the window mill  30  and the string mill  32  prevents overstress from reducing the exit radius of the milling equipment in making the window. Optionally, the size of the subsequent drill bit can be chosen to pass through the window previously made or to be somewhat larger, thereby enlarging the window and then exiting to drill the lateral. Reducing the size of the drill bit as a percentage of the drift diameter, along the same lines as the window milling assembly dimensions of the present invention, further aids in the drilling of a short radius lateral. The ability to exit with such a short radius, avoids the problems previously described when using the prior designs and having to penetrate adjacent formations or being faced with having to locate a window at an undesirable location in the casing in order to wind up in the desired formation while using a large exit radius. 
   The above description of the preferred embodiment is merely illustrative of the optimal way of practicing the invention and various modifications in form, size, material or placement of the components can be made within the scope of the invention defined by the claims below.