EP1878868A1

EP1878868A1 - Whipstock-assembly

Info

Publication number: EP1878868A1
Application number: EP06117065A
Authority: EP
Inventors: Darrell Lee Davidson
Original assignee: Catch Fishing Services BV
Current assignee: Catch Fishing Services BV
Priority date: 2006-07-12
Filing date: 2006-07-12
Publication date: 2008-01-16

Abstract

The present invention relates to a whipstock-assembly comprising a milling tool having a mill head, and a whipstock connected or connectable to the mill head with a shear bolt, wherein the shear bolt runs tangentially through the mill head at a predetermined distance from the mill head's centre.

Description

The invention relates to a whipstock-assembly comprising a milling tool having a mill head and a whipstock connected or connectable to the mill head with a shear bolt.
Such a whipstock-assembly is known from the European patent specification EP-B-1 165 930 .
In the known whipstock-assembly the whipstock is connected to the mill head of the milling tool with a shear bolt that runs substantially radially with respect to the mill head.
Such a known whipstock-assembly is used when it is decided to side-track a well. Many issues may warrant the decision to sidetrack a well. Drilling, casing, fishing operations or uneconomical hydrocarbons issues can prompt the decision to sidetrack a well. When faced with the prospect of plugging and abandoning (P&A) a mature well, operators may choose to sidetrack from an existing well bore and search for a new target zone to maintain production. This 'deviated' hole is called the side track hole. Although it is somewhat risky, drilling a side track hole is much more cost effective than starting a new borehole from the surface. This procedure is currently the only alternative to drilling a new well to reach the same or another target. Experience learns that it is often uneconomical to plug and abandon a current well and drill a complete new one, especially in deep water environments. The industry standard is to abandon the existing zone to re-complete or re-drill to the new target by milling a window in the casing, drilling off a cement plug in the open hole or milling off of a whipstock tapping into unrecovered or new reserves.
US-B-6,664,002 discloses a whipstock-assembly for use in a wellbore to form a lateral wellbore therefrom. According to this disclosure a whipstock is attached to a cutting tool by a shear bolt connection whereby the whipstock and cutting tool assembly may be run into the wellbore simultaneously. Upon compressive force from above, the shear bolt connection fails and the cutting action can begin. The shear bolt connection is designed to fail in compression but to withstand forces in tension brought about by the whipstock, and by accessories and extensions required to properly place the whipstock above a preset packer in the wellbore. The shear bolt connection of this known whipstock-assembly runs radially into the cutting tool and practice learns that this construction is prone to premature shearing.
The instant invention aims at improving the whipstock-assembly's resistance to shearing due to forces in tension brought about by the actions of the whipstock, yet be able to still fail due to compressive forces from above.
The whipstock-assembly of the invention is to that end characterized in that the shear bolt runs tangentially through the mill head at a predetermined distance from the mill head's centre.
In the following the invention will be further elucidated with reference to an exemplary preferred embodiment of the whipstock-assembly according to the invention and with reference to the drawing.
In the drawing:

Figure 1 illustrates the general assembly of the milling tool with the whipstock;
Figure 2 and Figure 2a show a detail of the whipstock-assembly near the shearable connection in a longitudinal section and a cross section respectively; and
Figure 3 shows the detail of Figure 2/Figure 2a looked at from the side.

Wherever the same reference numerals are applied in the figures, they refer to the same parts.
Referring first to Figure 1, a window mill 1 is shown that is secured by means of a shear bolt 2 to a proximal end of a whipstock 3.
As the detail of Figure 2 shows the window mill 1 includes a parabolic shaped mill head 4 which corresponds to a raised section 5 at the proximal end of the whipstock's 3 whipface. The said raised section 5 is machined at 15°. This angle has two purposes. First to allow the parabolic design of the head 4 of the window mill 1 to be seated to the top of the whipstock 3, the second one is to enhance rapid engagement of the mill 1 into the casing to promote the initial opening of the casing wall. This raised section of the mill head 4 will be partially or completely milled away during the initial cut out and will most likely not provide much support while milling down the whipface.
The key to the whipstock single trip system of the invention is the mill 1 interface with the whipstock 3. The mill 1 is designed with a profile that contains surfaces that closely match those of the surfaces on the whipface. This means that several different surfaces have to be produced on the mill-head to correspond to the whipstock 3 surfaces. The lower most taper on the mill 1 is aggressive to match the angle at the top of the whipstock 3. This angle is used to quickly force the mill 1 into the wall of the casing and achieve 'initial' cut out.
There is another very important feature that is produced by this unique mill design. The aggressive angle at the very top of the whipstock 3 means that its thickness increases rapidly. This allows for sufficient room to install an attachment shear bolt 2 through the top of the whipstock 3 and the bottom of the mill 1 while maintaining a full gauge lead mill.
After this illustrated casing milling system has been run in a hole a packer 6 or anchor secured to the distal end of the whipstock 3 is set and the window mill 1 is released by shearing the bolt 2 in a upward or downward motion. The drill is then rotated and weight is applied to the window mill 1. The window mill 1 then travels down the raised surface of the whipstock 3 and is thereby forced laterally into the casing opposite the whipstock 3.

Claims

Whipstock-assembly comprising a milling tool having a mill head, and a whipstock connected or connectable to the mill head with a shear bolt, characterized in that the shear bolt runs tangentially through the mill head at a predetermined distance from the mill head's centre.