THIS INVENTION relates to a whipstock system, and in particular concerns a whipstock system that allows the easy and efficient creation of a sub-surface bore branching away from an existing sub-surface bore.

In the drilling of, for example, oil wells, it is often necessary to form a bore which branches away from an existing bore. Such a branching bore is often known as a lateral or a sidetrack. A branching bore of this type is generally formed by anchoring a whipstock into the existing bore at a pre-determined location. A milling assembly is then driven downwards towards the whipstock, such that the milling assembly is deflected laterally off a tapering face of the whipstock and is turned through an angle with respect to the axis of the existing bore. The milling assembly therefore creates an aperture in the side wall or casing of the existing bore, and begins the creation of a separate, branching bore.

Existing whipstock systems suffer from several disadvantages, however. Firstly, it is often necessary to place an anchor device in the existing bore as a preliminary operation, before the whipstock is introduced into the bore and attached to the anchor device. Separate visits must, therefore, be made to the existing bore to set the anchor device and subsequently to introduce the whipstock.

More recently it has been proposed to provide an expanding anchor or "packer" attached to the lower end of the whipstock, so that both of these components can be then inserted into the existing bore together and, once the desired depth has been reached, hydraulic fluid is supplied to the packer through the drill string, to expand the packer and fix the packer in place in the existing bore, to support the whipstock, which in turn supports the milling assembly during milling and drilling and serves to deflect the milling assembly to cut a window in the casing of the existing bore.

In such arrangements, the milling arrangement is often attached to the upper end of the whipstock, so that the anchor or packer, whipstock and milling assembly are all attached to one another to form an integrated drilling arrangement. In such arrangements, the milling assembly is generally connected at or near its tip to an upper portion of the tapered face of the whipstock. This arrangement generally has the disadvantage, however, that the milling arrangement must therefore be of a significantly lesser diameter than that of the existing bore, and hence the branching bore that is initially formed will be of a lesser diameter than the existing bore. When using such systems, it may be necessary to mill an initial cut-out in the casing of the existing bore during a first milling operation and to mill the final window in the casing during a subsequent operation, before drilling of the branching bore can begin. Such a system is commonly known as a two trip, or multi trip whipstock system.

It is an object of the present invention to seek to provide an improved single-trip whipstock system, which alleviates some or all of the above disadvantages.

Accordingly, one aspect of the present invention provides a whipstock system comprising: a whipstock having a deflection face, a recess being formed in the deflection face; and a milling head of a milling arrangement, the milling head being operable to cut a bore into a subsurface formation and having one or more blades and a locking projection, wherein the locking projection may be received in the recess in the deflection face of the whipstock; wherein the whipstock and milling head are adapted to be connected to one another by a retaining element, such that the milling head may be separated from the whipstock.

Conveniently, the whipstock system further comprises a retaining element connecting the milling arrangement and the whipstock to one another, and being adapted to allow separation of the milling head from the whipstock.

Advantageously, the one or more blades have an outer cutting edge and the locking projection does not project outwardly beyond the outer cutting edge.

Preferably, the retaining element may allow separation of the milling head from the whipstock while leaving the locking projection intact.

Conveniently, when the locking projection is received in the recess in the deflecting face, the at least one blade of the milling head is located adjacent the deflection face.

Advantageously, the retaining element passes through a part of the milling arrangement and a part of the whipstock to hold the milling head and whipstock together.

Preferably, wherein the retaining element passes through the locking projection.

Conveniently, the milling head may be detached from the deflection face of the whipstock such that the locking projection remains part of the milling head during subsequent milling of a bore in a subsurface formation.

Advantageously, the retaining element is adapted to be broken to allow separation of the milling head from the whipstock.

Preferably, the recess comprises a space between two spaced apart tines forming a part of the deflection face.

Conveniently, the at least one blade of the milling head may not be received in the recess.

Advantageously, the milling head comprises a plurality of spaced apart blades, including blades adjacent the locking projection, spaces between the locking projection and each of the blades adjacent to the locking

projection being sufficiently large to accommodate respective ones of the tines and spaces between adjacent blades not being sufficiently large to accommodate either of the tines.

Preferably, the retaining element is elongate and passes through at least one of the tines and through the locking projection.

Conveniently, at least one lug projects outwardly from the deflection face, the retaining element passing through the lug. ,

Advantageously, when the locking projection is received in the recess, side surfaces of the locking projection abut internal surfaces of the recess so that rotation of the milling head in either direction about a longitudinal axis thereof may be transmitted to the whipstock.

Preferably, the greatest diameter of the milling head adjacent the deflection surface is approximately equal to the greatest diameter of the whipstock.

A further aspect of the present invention provides a whipstock having a connection region which is adapted .£$. to be releasably connected to a milling head of a milling arrangement, wherein the connection region comprises a pair of spaced apart tines having a space defined therebetween.

Advantageously, the tines are not connected to one another at distal ends thereof.

Another aspect of the present invention provides a fluid delivery system for a milling system, the fluid delivery system comprising: a fluid inlet vessel; an anchor fluid delivery vessel connected to the inlet vessel and defining an anchor flow path to supply fluid to an anchor; a milling head fluid delivery vessel connected to the inlet vessel and defining a milling head flow path to supply fluid to a milling head; a barrier located between the inlet vessel and the milling head fluid delivery vessel to prevent fluid flowing from the inlet vessel along the milling head flow path, the barrier being adapted to open when exposed to fluid under a predetermined pressure, to allow fluid to flow from the inlet vessel along the milling head flow path.

Conveniently, the barrier comprises a rupture element which is adapted to rupture when exposed to fluid under the predetermined pressure.

Advantageously, the rupture element is provided in a seal located between the fluid inlet vessel and the milling head fluid delivery vessel.

Preferably, the anchor fluid delivery vessel passes through an aperture in the seal.

Alternatively, the barrier is held in place by one or more retention elements which are adapted to break when the barrier is exposed to fluid under the predetermined pressure to allow the barrier to move so that fluid may flow from the inlet vessel along the milling head flow path.

A further aspect of the present invention provides a milling system comprising: an anchor; a milling arrangement having a milling head; and a fluid delivery system according to the above for supplying fluid to the anchor and milling head.

Conveniently, the barrier element is provided within the milling arrangement.

Advantageously, the barrier element is provided in a further component which is located upstream of the milling arrangement.

Another aspect of the present invention provides a milling method comprising the steps of: providing a milling system according to the above; inserting the milling system into an existing bore; supplying fluid under a first pressure to the fluid delivery system to set the anchor; and supplying fluid under a second pressure to the fluid delivery system to open the barrier element and allow the fluid to flow along the milling . ^ head flow path to supply fluid to the milling head.

A further aspect of the present invention provides a milling system for milling an aperture in a side wall of an existing bore, the milling system comprising: a milling arrangement comprising a milling head having at least one blade, at least a forward-facing portion of an outer cutting surface of the at least one blade having a continuously curved outer surface; and a whipstock having a longitudinal axis and a deflection face, the deflection face being inclined at an angle of less than 10° with respect to the longitudinal axis and being absent of any ramp portions disposed at an angle steeper than 10° with respect to the longitudinal axis that would, if the milling arrangement were used conjunction with the whipstock to mill an aperture in a conventional manner in a side wall of an existing bore having a conventional steel casing, have the effect of deflecting the milling head into the casing to initiate cutting of the aperture in the casing.

Preferably, the milling head has. a longitudinal axis and a milling direction which comprises one direction along the longitudinal axis, and wherein the forward facing portion of the outer cutting surface comprises substantially all parts of the outer cutting surface that face in a direction having a component which parallel with the longitudinal axis and in the milling direction.

Conveniently, the deflection face is inclined at an angle of less than 3° with respect to the longitudinal axis of the whipstock.

Advantageously, the deflection face of the whipstock comprises all surfaces of the whipstock with which the milling head would come into contact if the milling arrangement were used conjunction with the whipstock to mill an aperture in a conventional manner in a side wall of an existing bore.

Preferably, at least the forward-facing portion of the outer cutting surface of the at least one blade has substantially the shape of at least part of an ellipse.

Conveniently, the milling arrangement is attached to the whipstock.

In order that the present invention may be more readily understood, embodiments thereof will now be described, by way of example, with reference to the accompanying drawings, in which:

Turning firstly to figure 1 , components of a whipstock system embodying the present invention are shown. The whipstock system is adopted to be inserted into an existing sub-surface bore of a known diameter. An anchor in the form of an expandable packer 1 is generally cylindrical in shape, having an external diameter approximately equal to or slightly less than the internal diameter of the existing bore. In this specification, the term "anchor" refers to any device which may be fixed in place within a bore, whether operated mechanically, hydraulically, or by any other means. An upper surface of the packer 1 is provided with a fluid intake port 2, into which hydraulic fluid may be introduced. When hydraulic fluid is introduced into the fluid intake port 2 under pressure, an expanding portion 3 of the outer surface of the packer 1 is forced outwardly _* M. to increase the diameter thereof. The expanding portion may therefore press tightly against the casing provided on the inner surface of the existing bore, holding the packer 1 firmly in place in the existing bore to act as an anchor for other components.

Connected to an upper surface of the packer 1 is a whipstock 4. Figures 2, 3 and 4 show side, front and rear views respectively of the whipstock 4. The whipstock 4 is elongate and, at a bottom end thereof, the body 5 is substantially cylindrical and has an external diameter which is approximately the same as, or slightly less than, the internal diameter of the existing bore. Towards the top of the whipstock 4, however, the body thereof tapers to provide an inclined deflection face 6, which runs along the majority of the length of the whipstock 4. The resulting shape of the whipstock 4 is as though a fully cylindrical body had originally been provided, but a long, oblique cut had been made across the cylindrical body to leave a substantially elliptical cut surface. At the top end of whipstock 4, the thickness of the body 5 is around a tenth of the thickness at the lower end thereof.

At the lower end of the whipstock 4, a connecting insert 7 is provided, which fits onto a projection on the bottom end of the whipstock and also into the fluid intake port 2 of the packer 1. Side and front views of the connecting insert 7 are shown in figures 5 and 6 respectively. A fluid communication pipe 8 runs along the length of the whipstock 4 and communicates with the projection, as shown in figure 4.

The connecting insert 7 is substantially hollow, the centre thereof forming a fluid delivery channel. Figure 7 shows a close-up view of the connection between the projection on the bottom of the whipstock 4 and the connecting insert 7. A recess in the top part of the connecting insert 7 receives projection on the bottom of the whipstock 4, with a hinge pin 45 being passed through appropriately-sized apertures in the whipstock

and the connecting insert 7 to hold these components together. One or more retaining bolts 46 are driven into lateral bores formed in the whipstock 4 and connecting insert 7, to hold the hinge pin 45 in place.

The fluid communication pipe 8 that passes along the length of the whipstock 4 is connected to the fluid delivery channel via a flexible connector 47. A lower part 48 of the connecting insert 7 is flexible, to allow deflection of the packer 1 with respect to the whipstock 4 It will be understood that fluid passing along the fluid communication pipe 8 may flow through the fluid delivery channel in the connecting insert 7 and hence into the fluid intake port 2 of the packer 1. The fluid delivery pipe 8 may be installed into a slot 9 milled into a rear surface of the whipstock 4, and retained by pins 10 or other means such as clamps. In embodiments of the invention several pipes may be provided to traverse the length of the whipstock 4, and these may be connected to one another by suitable connectors 11. Any other suitable means of forming this hydraulic communication through the whipstock 4 may be used, for example by gun drilling a hole along the length of the body of the whipstock 4.

At the upper end of the whipstock 4, where the body of the whipstock 4 is thinnest, two upward-facing fingers or tines 12 are provided, there being a slot 49 provided between the tines 12. Apertures 39 pass through the tines in a direction substantially perpendicular to the longitudinal axis of the whipstock 4, and ,j*, aligned to the coaxial with one another so that the aperture 39 in each tine 12 faces towards the other tine 12.

Figures 8, 9 and 10 show plan, perspective and elevation views respectively of the top end of the whipstock 4. Each tine 12 has a lug 47 projecting outwardly therefrom, with the apertures 39 being formed through the lugs 47. The tines 12 taper towards the tips of the lugs 47, so that the tip of the lug is narrower than the rear portion of each tine 12. It is envisaged that the lugs 47 will be consumed during the milling process.

Figure 11 shows a side view of an alternative design for the top part of the whipstock 4 in which the tines 12 do not have lugs 47, but are of a sufficient depth that the apertures 39 can be formed directly therethrough.

Figure 12 shows a milling assembly 13 for use with the present invention. The milling assembly 13 comprises a substantially cylindrical body 14, terminating at a lower end thereof in a milling head 15, comprising a plurality of blades, which will be described in greater detail below. The diameter of the body 14 is slightly smaller diameter than that of the milling head 15.

At a lower end of the milling assembly 13 is a provided a milling head 15, comprising a plurality of blades, which will be described in greater detail below.

At an upper end of the milling assembly 13 a fluid intake port 16 is provided, which communicates with an elongate, cylindrical channel 17 which passes down the centre of the body 14 of the milling assembly 13. The diameter of the channel 17 is substantially constant in an upper portion 18 of the channel, with the channel 17 having a wider portion, having a greater diameter than that of the upper portion 18 beneath the

upper portion 18. Beneath the wider portion 19 there is provided a narrow portion 20, in which the diameter of the channel is less than in the upper portion 18.

Following the narrow portion 20, the channel 17 widens out into a lower portion 21 , whose diameter is approximately equal to that in the upper portion 18.

Partway along the lower portion 21 , a seal 22 is provided, blocking the channel 17. Figure 13 shows a close-up view of the seal 22. The seal 22 has a passage 23 therethrough, this passage 23 is blocked by a rupture disc 24, which is impermeable to fluid but which is arranged to rupture at a pre-determined pressure. Once the rupture disc 24 has ruptured, fluid may pass through the passage 23.

Alternatively, a piston may be provided in place of the rupture disc 24, the piston being held in place by one or more shear pins which are adapted to break when the piston is exposed to a predetermined pressure, allowing the piston to move and allow fluid to flow therepast.

Below the seal 22 the channel 17 communicates with a series of circulation ports 25, which in turn exit the milling assembly 13 in the region of the blades of the milling head 15. ^{■ ■ " 1} OF

In addition, a relatively narrow tube 26 is sealed in the seal 22 and passes therethrough down through the remaining length of the milling arrangement 13, exiting the milling arrangement 13 from a lower end of the milling head 15. The tube 26 is connected to the fluid communication pipe 8 of the whipstock 4.

A piston 27 is provided in the upper portion 18 of the channel 17. The piston 27 comprises a block 28 which fits snugly against the interior of the upper portion 18 of the channel 17, so that fluid may not pass around the sides of the block 28. The block 28 is preferably made from a resilient material such as steel. Outward- facing side surfaces of the block 28 are provided with intake holes 29, which communicate with a central chamber 30 of the block 28. The central chamber 30 is further in communication with a downward-facing aperture 31 which, through the central chamber 30, is in communication with the side apertures 29.

A rigid tube 32 extends upwardly from the block 28, the rigid tube 32 being hollow, being in communication with the central chamber 30 of the block 28, and being open at its top end. A cap 33 may be placed on the top of the tube 32, to seal the top end thereof.

Alternatively, a separate running tool 43 may be provided, containing the channel 17 and piston 27 as shown in figure 14. Figure 15 shows an alternative milling arrangement 44, having a channel of a constant width running therealong and into an upper end 45 of which the running tool 43 may be inserted. It will be appreciated that this will fulfil the same function as the milling assembly 13 described above.

Turning to figure 16 the milling head 15 is shown in more detail. The milling head 15 comprises a series of blades 34, which are supported on a central hub 36 provided on the lower end of the body 14 and extend

radially outwardly from the hub 36. Each blade 34 has a curved outer surface which reaches a maximum displacement from the central axis of the milling assembly 13, and then curves inwardly towards a lower end of the milling head 15, the radius of curvature of the portion following the point of maximum displacement from the central axis preferably being larger than the radius of the milling head 15 at the point of maximum displacement The milling head 15 is therefore generally elliptical in shape. At the widest point of the blades 34, the diameter of the milling head 15 is approximately equal to the internal diameter of the existing bore. Located between two normal blades 34 is a locking projection 35, which extends radially outwardly from the central hub 36. The locking projection 35 is generally planar in shape, having first and second substantially flat side surfaces, and preferably does not extend outwardly substantially beyond the outer cutting edge of the remaining blades 34. A location hole 37 passes through the locking projection 35 in a direction which is perpendicular to the longitudinal axis of the milling assembly 13 and also to the direction of displacement of the locking projection 35 from the central axis. Cut-away views of the milling head 15 are shown in figures 17 to 19, a perspective view is shown in figure 20, upward-facing views are shown in figures 21 and 22, and a cut-away upward-facing view is shown in figure 23. In figures 21 and 22, the lower end of the tube 26 which passes through the seal 22 may be seen.

A second milling head 15a may be provided on the body 14 of the milling arrangement 13, behind the milling ^' ,§<* head 15. If necessary, third or further milling heads (not shown) may also be provided.

Referring to figures 24 to 26, the connection between the milling assembly 13 and the whipstock 4- will be explained.

The locking projection 35 of the milling head 15 fits into the slot between the upwardly-projecting tines 12 of the whipstock 4, and the tines 12 themselves are received in recesses on either side of the locking projection 35. The side surfaces of the locking projection 35 abut corresponding inner surfaces of the tines 12.

In preferred embodiments of the invention, the blades 34 extend radially outward from the hub 36 of the milling head 15, there being a pre-set radial spacing between each of the blades 34. Preferably, the locking projection 35 projects outwardly from the hub 36 between two of the blades 34, and the radial spacing between the locking projection 35 and the two adjacent blades 34 is larger than the pre-set spacing between any two of the blades 34 themselves.

The width of the tines 12 is set so that the tines 12 may be received in the larger spaces between the locking projection 35 and the adjacent blades 34, but not between any two adjacent blades 34. Effectively, this means that only the locking projection 35 may be received in the slot 49 between the tines 12, and that none of the blades 34 may be received in this slot 49. An advantage of this is that, if the milling head 15 does accidentally become detached from the whipstock 4 at any stage, it is possible to re-insert the locking projection 35 into the slot 49 between the tines 12. Since only the locking projection 35 and none of the

blades 34 will fit into this slot 49, it is possible to determine accurately the orientation of the whipstock 4 since its orientation relative to the milling head 15 will be known.

Importantly, it will be noted that this arrangement allows the gauge of the milling head 15 to be equal to the internal diameter of the existing bore, i.e. for the milling head 15 to be "full gauge" - since the milling head 15 is not sandwiched between the upper end of the whipstock 4 and the internal casing of the existing bore, it is not necessary to use a milling head 15 having a reduced gauge.

Of course, the milling head 15 need not be full gauge, and a milling head of any appropriate diameter may be used with the present invention. In particular, it will be understood that, if the drilling arrangement must pass through a restriction in a subsurface bore then all of the components will need to be of a reduced gauge.

A shear bolt 38 passes through the apertures 39 provided in the tines 12, and also through the location hole 36 in the locking projection 35. Referring to figure 15, a cross pin 40 may be driven into a radial aperture 41 in the locking projection 35, to hold the shear bolt 38 in place. The arrangement is such that the milling head 15 may pivot around the shear bolt 38 with respect to the upper end of the whipstock 4, as shown in figure _: <ØL 27.

The operation of the drilling arrangement comprising the packer 1 , whipstock 4 and drilling assembly 13 will now be described.

Firstly, a clean setting fluid (for example, water) is introduced into the packer 1 , the fluid communication delivery pipe 8 that passes along the length of the whipstock 4, the tube 26 that communicates with the channel 17 in the milling assembly 14, and the channel 17 itself, The piston 27 is then introduced into the upper portion 18 of the channel 17, with the cap 33 being left off the upward-facing tube 32 of the piston, so that air in the region beneath the piston 27 is driven out through this tube 32. Once any such air has been driven out, the cap 33 is placed on the open upper end of the tube 32 to create a fluid-tight seal.

Next, the drilling arrangement is inserted down an existing bore until a desired depth is reached. If the bore is not perfectly straight, it will be understood that the drilling arrangement may flex to accommodate any turns by flexing about the shear bolt 38 or about the flexible connection between the whipstock 4 and the packer 1.

Should rotation of the whipstock 4 be required, it will be understood that the milling assembly 13 may be rotated, and that rotation may be transmitted from the milling arrangement 13 to the whipstock 4 due to the sandwiching of the robust locking projection 35 between the two tines 12 of the whipstock 4. In embodiments in which the rupture disc 24 is not used, mill fluid may be circulated around the mill head 15 and other components of the drill string to assist in setting the correct orientation. A measurement while drilling (MWD) tool may also be utilised, as will be understood by a skilled reader.

In embodiments where the rupture disc 24 is used, means may be provided to allow circulation for orientation purposes with a MWD tool to the existing bore without setting the packer 1. Typically, a bypass valve or ported sub is required, to allow fluid bypass through the drill string. Actuation of the bypass valve is usually determined by flow rate and subsequent pressure drop through a piston, or piston and nozzle combination, which is used to shear pins or cycle the piston in response to switching the flow on and off until the valve closes, allowing a static pressure to build up in the system to set the packer 1 in the existing bore. If a ported sub is used, which may be in conjunction with a valve, the flow rate is increased dynamically until the pressure drop across the port circulating flow to the annulus is high enough to initiate the setting sequence of the packer 1.

Once the drilling arrangement is at the desired depth, well bore fluid is introduced into the upper portion 18 of the channel 17 in the milling arrangement 30. This well bore fluid will apply pressure to the upper surface of the block 28 of the piston 27, thus driving the block 28 downwardly along the upper portion 18 of the channel 17, and increasing the pressure in the setting fluid which is beneath the block 28.

As the block 28 is driven downwardly, it will arrive in the wide portion 19 of the channel 17, but will be unable røi to pass through the narrow portion 20 thereof, and will come to rest on the "shoulder" provided by the narrowing of the channel 17 at the narrow portion 20. It will be appreciated that, at this stage, the well bore fluid which is introduced from above the piston will be able to flow around the sides of the block 28 (due to the additional diameter of the wide portion 19 of the channel 17) into the central chamber 30 of the block 28 via the side apertures 29 thereof, and continue down the channel 17 by passing out of the lower aperture 31 of the block 28 (this flow is shown in figure 28). Well bore fluid may therefore flow along the length of the channel 17, but before the block 28 reaches the wide portion 19 of the channel 17, the well bore fluid is kept separate from the setting fluid.

The additional pressure in the lower portion of the channel 17 will be communicated through the tube 26 which extends downwardly through the seal 22, through the fluid communication pipe 8 which passes along the length of the whipstock 4, and into the packer 1. As described above, introducing fluid under pressure into the packer 1 allows the packer to be "set" against the inner casing of the existing bore, and the introduction of well bore fluid into the drill string under high pressure will therefore anchor the packer 1 firmly in place at the desired depth. In preferred embodiments of the invention, the packer 1 may be set with a pressure of around 1200 psi (8.3 MPa).

Once the packer 1 has been set, the pressure in the well bore fluid may be increased to around 2000 psi (13.8MPa) or 3000 psi (20.7MPa), so that the rupture disc 24 ruptures and allows fluid to pass therethrough. Fluid will then, instead of passing along the drilling arrangement into the packer 1 , pass into the lower portion 21 of the channel 17 and through the circulation ports portion to be deposited near the blades 34 of the milling head 15.

A relatively large upward or downward force is then applied to the milling arrangement 13, so that the shear bolt 38 is broken and the milling arrangement 13 is released from its connection with the whipstock 4. Milling may now begin and the rotation of the blades 34 is commenced.

The milling arrangement 13 is driven downwards and, as described above, is deflected by the deflection face 6 of the whipstock 4 so that the angle of attack of the milling head 15 is altered and the milling head 15 cuts an aperture in the casing of the existing bore and begins to create a branching bore 50. Typically the casing of the existing bore will be formed from a softer material than the deflecting face 6 of the whipstock 4, or the material of the milling blades 34. For example, the wall thickness of a steel casing for certain drilling operations may be in the region of 0.4 inches (1 cm) and have a hardness of around 207 on the Brinel scale, whereas the material of the deflection face 6 may be in the region of 340 on the Brinel scale.

During this milling process the elliptical shape of the blades 34 of the milling head 15 are of assistance. Figures 29a to 29d show the progress of the beginning of the milling process.

As can be seen in figure 29a, the first point of contact between the milling head 15 and the inner casing 42 of the existing bore is at the widest point of the elliptical shape of the blades 34. This will assist in the ease £$ of breaking through the casing 42. It will be understood that, as milling progresses (as shown in figures 29b to 29d), the point at which the casing 42 meets the milling head 15 will move down towards the bottom tip of the elliptical shape of the blades 34 and, once the centre of the tip of the milling head 15 passes through the casing 42 the point of contact will move to the inner side of the milling head 15 (i.e. the side closest to the whipstock 4). At this point the milling head 15 is becoming fully exposed to the outer sub-surface formation, and ultimately there will be no support from the face of the whipstock 4 and the milling head 15 will be fully surrounded by the surrounding sub-surface formation.

In many conventional whipstock arrangements, the whipstock is provided with a relatively steep ramped portion, set at an angle of around 15° to the longitudinal axis of the existing bore, which "kicks" the milling head relatively sharply towards the casing of the existing bore, to initiate the cutting of a hole in the casing. It will be understood that this exposes the ramped portion of the whipstock to relatively large forces, with the result that the ramped portion of the whipstock is milled significantly by the milling head, with a great deal of the energy input to the system being consumed in this milling of the whipstock face.

In contrast, the present invention combines a milling head in which at least the forward-facing portions of the outer cutting surfaces have a smoothly curved outer surface, in conjunction with a whipstock whose deflection surface is disposed at a relatively shallow angle to the longitudinal axis of the whipstock, of less than 10°, and preferably less than 3°. This allows a greater proportion of the energy input to the system to be expended in milling a hole in the casing of the existing bore.

It is appreciated that, in embodiments of the invention lugs 47 are provided on the whipstock face, with parts of the lugs having an angle of inclination that is greater than 10°. However, the size of these lugs is not

sufficient to "kick" the milling head away from the deflection surface to commence milling of a hole in the casing of the existing bore, and indeed it is envisaged that these lugs 47 will in any event be consumed by the milling process.

Figure 1 also shows a bypass valve 51 and a flex joint 52, which may be located upstream of the milling arrangement 13.

It will be appreciated that the present invention provides an improved whipstock system which may be used to create a branching bore swiftly and efficiently in a single trip.

While the above-described embodiment is described as using a hydraulically-set anchor, it is also envisaged that the invention may be utilised with one or more mechanical anchors. Such mechanical anchors would typically be set or tripped against a bridge plug or similar device in the well bore.

When used in this specification and claims, the terms "comprises" and "comprising" and variations thereof mean that the specified features, steps or integers are included. The terms are not to be interpreted to exclude the presence of other features, steps or components. - . . • . . ^• . . - ¹ V*.

The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.