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The field of this invention is whipstock design for making windows in tubulars downhole for the purpose of extending a lateral bore from a main bore.
Whipstocks are devices that have long been in use to deflect a mill system through casing to create an opening known as a window. Whipstocks tend to be very long so that an inclined face on them can gradually nudge a mill system into the casing wall to start the window and to further guide the milling system until it makes an exit though the window. As a result the window shape is long and thin and narrows at the upper and lower ends. The slope on the whipstock guide surface is generally a small angle or series of angles.
Window milling with whipstocks has several unique issues that can affect the performance of the milling equipment. One concern has been when the center of the mill comes even with the casing wall and there is a tendency for the mill to bog down in that position so some designs have featured a steeply inclined surface on the whipstock guide face to engage the mill just as its center is in line with the casing wall. This kick-out surface occurs part way down the guide surface to address one specific problem.
Other approaches to whipstock design concern themselves with avoiding the milling of the whipstock at the onset of milling and configuring the contact angle between the mill and the whipstock guide surface at its upper end to be closer to parallel to decrease contact stress of the mill on the whipstock ramp at the top. The idea was to increase contact stress against the casing so that the casing would be milled rather than the top of the whipstock as the window is initiated.
There have been other approaches to insure getting a long enough window which have mostly been dealt with by making the ramp on the whipstock longer than the needed window length. The thinking here was that the longer ramp was insurance that the window length would not fall below a minimum desired dimension. In these designs, it was anticipated that the window mill will completely leave the ramp well before its lower end and this would insure the window was long enough and that the mill would safely enter the lateral. Even if this concept worked to give a window of a desired length, it created subsequent problems when downhole equipment needed to get through the window. FIG. 1 illustrates the problem. The whipstock face 10 extends downhole below the bottom of the window 12 in wellbore 14. In subsequent trips to get into lateral 16 and complete drilling and completion of it, a pocket trap 18 is created. If the later runs had mills or bits with blades, the blades would hang in the pocket 18 and the equipment would stall. The rig crew would sense this and have to stop rotation and pick up and then resume rotation and hope that on a subsequent attempt they would not direct a blade into the very same pocket trap. On the other hand if the equipment run subsequently had peripheral soft components like rubber seals or packer seals, the edge 20 of casing 22 acted as a razor sharp surface that could easily shred the softer components and render them unserviceable.
FIG. 2 shows another problem with whipstocks in the past. Here the mill fails to make an exit at the ramp end and simply continues to mill the whipstock base 24 and an anchor 26 below as indicated by the dashed line 28.
Over the years different ideas have been tried in whipstock designs but these problems have persisted. Several designs have tried gradual slopes and arcuate guide surfaces over the length of the whipstock all in the name of better mill guidance but none of these designs have eliminated the conditions depicted in FIGS. 1 and 2. Some examples of such designs are U.S. Pat. Nos. 4,420,049; 3,116,799; 6,401,821; 2,699,920; 6,105,675; and 6,209, 645. The last patent actually puts a radial surface 300 for the mill to catch on to hopefully define the lower end of the window.
One attempt to solve the above described problems has been to include an end surface on the guide face of the whipstock that is a sharper angle than the long gradual guide surface that is normally in the range of about 3 degrees. In soft formations the increase in angle at the ramp bottom puts a greater lateral load on the mill to help keep it against the whipstock surface as opposed to kicking out too early. In the hard formations the steeper end face directs the mill away from the whipstock to try to avoid having it continue down misdirecting the lateral and potentially damaging the whipstock or underlying anchor. While this approach it was hoped to eliminate the problems described above and left unresolved by the prior art but such was not the case.
Accordingly what is provided by this invention is recognition of the solution to these problems in the form of a gradual transition at the lower end of a guide ramp from a low degree taper angle to a larger exit angle along a preferably arcuate surface. In another aspect of the invention the final surface is positioned in the vicinity of the expected window lower end to eliminate the effect of pocket 18, described above. These and other features of the present invention will be more readily appreciated by those skilled in the art from a review of the description of the preferred embodiment and the claims that appear below.
An improved whipstock design features a ramp having a lower terminus at the location of the expected window bottom. The lower end of the whipstock ramp transitions gradually from a preferably flat guide surface to a more drastic exit orientation that is preferably obtained by going to an arcuate exit of a rather large radius to present a smooth transition from the flat guide surface to an orientation that allows kick-out to occur by the end of the ramp. This in turn allows the ramp end to closely approximate the window bottom to reduce the risk of creation of a pocket near the window bottom that could trap or damage equipment delivered in subsequent runs.
FIG. 1 is section view of a prior art design with the ramp longer than the intended window and the formation of a pocket as a result;
FIG. 2 is a section view of a prior art design that shows milling the whipstock and the anchor instead of licking out when needed to make the window bottom;
FIG. 3 is a section view of the present invention show after the window is milled and the mills are removed.
Referring to FIG. 3, the casing 10 is cemented at 12 and has the whipstock 14 in position for milling the window 16, in a known manner. The whipstock 14 is supported by a known anchor 18 and the orientation of the ramp 20 has been previously accomplished by known orientation equipment. Ramp 20 has a top end 22 and a bottom end 24 and in the preferred embodiment has a transition location 26. Segment 28 lies between top end 22 and transition 26. It can be a single sloping surface at about 3 degrees to longitudinal axis 30. It can be a single flat sloping surface or it can have multiple faces at different or similar slopes. There can be an alternative sloping surface next to a non sloping surface followed by another sloping surface at the same or a different slope as had been located higher up the ramp 20. Preferably segment 28 regardless of its configuration should be about 80% of the length of ramp 20 measured along its faces 28 and 32.
Preferably, face 32 should be arcuate with a preferably gradual transition from the adjacent flat face 28 and gradually going into an arcuate turn using a fairly large radius. This can start as a surface continuation in essentially the same plane or it can be within about 5 degrees of face 28 at the transition. The radius to the lower end 24 need not be constant and it is preferred to decrease the radius on the way to lower end 24. It can also be possible to have segment 32 reach outer cylindrical surface 34 at close to a 60 degree angle, with an angle of at least 10 degrees preferred. The objective is to slowly transition the window mill (not shown) into the formation in the lower 20% of the ramp 20 so as to avoid the problem in prior designs described above.
While a kick out surface near the lower end of the ramp has been tried as a flat surface at a more drastic angle to the longitudinal axis that the adjacent ramp surface, the problem has been that the severe angle transition accounted for an early exit of the window mill right at the transition of the two angles so as to make the window to short or to direct the direction of the subsequent lateral at a steeper angle than was intended. The gradual transition from segment 28 to the lowest point 24 insures that the mill go down along the ramp 20 to as close as possible to the depth of point 24 if not right to it. Thus the present invention allows the ramp 20 to terminate within inches or right at the intended bottom of window 16 and early termination of the window 16 or having a lateral come out at an unintended angle is a possibility that is reduced if not eliminated.
Segment 32 may have a flat section or sections along with the arcuate guide surface.
It is to be understood that this disclosure is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended other than as described in the appended claims.