Hydraulically releasable connection for downhole tools

Abstract

A downhole tool assembly includes a first downhole tool with internal pressure and a second downhole tool, joined by a releasable connection designed to release under shear force from unaligned forces between the tools. The connection has relatively high shear strength when locked and relatively low shear strength when unlocked. A shiftable pin locks the connection in a first position and unlocks it in a second position. Internal pressure is communicated to a first end of the piston directly and to an activation circuit. The activation circuit communicates internal pressure to a second end of the piston before an activation and, after internal pressure reaches a release pressure and breaks a burst disc, communicates wellbore pressure to the second end of the piston to create a pressure differential to retract the pin.

Description

HYDRAULICALLY RELEASABLE CONNECTION FOR DOWNHOLE TOOLSRELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Application No. 63 / 762,155, filed February 24, 2025, which is incorporated herein by reference for all purposes.BACKGROUND

[0002] This disclosure generally pertains to releasable connections for downhole tool assemblies, particularly downhole assemblies for sidetracking, hydraulically releasing connects in downhole tools, and hydraulic circuits for activating hydraulic actuators in downhole holes.

[0003] One common use of releasable connections in downhole tool assemblies is to attach a whipstock to a mill in a sidetracking assembly for a cased wellbore. "Sidetracking" in the oilfield industry refers to drilling a deviated borehole that branches off an existing subterranean wellbore. The deviated borehole may be used, for example, to bypass an obstruction (fish) in the existing borehole, to explore an adjacent section of an oil and gas field, or to develop more economically an oil and gas field using multiple wellbores drilled from a single, preexisting wellbore. Because a wellbore is typically cased with a string of steel pipe joints, a section of the casing is removed to expose the formation or, more commonly, an opening referred to as a "window," is cut in the casing to allow a drill string to exit the wellbore while maintaining the integrity of the original casing.

[0004] A longstanding preference in the oilfield industry is to perform a downhole operation in the fewest possible "trips" into the wellbore, preferably completing it in a single trip. A downhole tool assembly with the necessary downhole tools, which may be referred to as a sidetracking assembly and by other names, is lowered into the wellbore on the end of a work string to cut a full-size casing window and drill at least a full-size rathole in a single trip. The downhole tool assembly has a milling assembly. The milling assembly includes at least one cutting tool configured for milling a hole in the casing and a whipstock assembly that includes a whipstock. The bottom end of the milling assembly is releasably connected to the top end of a whipstock assembly. Once disconnected and separated, the window is cut by lowering and rotating the milling assembly or entire downhole assembly,the whipstock to deflect it toward the casing wall. The milling assembly is rotated by rotating a work string or, optionally, a motor that is part of the downhole tool assembly.

[0005] The configuration of a milling assembly will depend on the requirements of the job, such as the size of the window that is needed, which in turn depends on the tools that will need to be passed through it, the build angle of the sidetrack, and other considerations and requirements that may be unique to the well. For example, a milling assembly may include a lead mill to initiate window cutting. One or more mills above the lead mill may be used to enlarge the window or finish cutting the window. Because of their shape, these "follow" mills are sometimes called watermelon mills.

[0006] A whipstock assembly may, for example, include a whipstock and an anchor, which is a tool used in anchoring the whipstock to the casing. Once the whipstock is lowered to a planned depth and rotated to face in a planned direction, it is anchored to prevent axial and rotational displacement when used to deflect the downhole tool assembly and work string toward the side of the casing to cut the window and drill a rathole. Different anchoring methods can be used. The selection of the method and anchoring tool depends on the circumstances. A standard method is to use a downhole tool with slips that can be extended hydraulically or mechanically to grip the side of the casing. Tools being used to anchor a whipstock will be generally referred to as anchors, even if they may also perform other functions.

[0007] The downhole tool assembly may include other tools for performing downhole operations before setting the whipstock to separate the mill and begin milling. For example, the inside surfaces of the casing can be prepared for a mechanical anchor by incorporating into the downhole tool assembly a scraper and / or brush assembly to clean the surfaces before the whipstock is placed at the planned depth and rotated to the planned orientation, and the anchor is set. It will typically include a tool for orienting the whipstock. Once the window is milled, the milling assembly is raised by tripping out the work string and then lowering a drill string with a directional drilling assembly, which the whipstock deflects the directional drilling assembly through the window. A "drill-ahead" sidetracking downhole tool assembly avoids having to trip the milling assembly out of the hole by including adirectional drilling unit that allows the assembly to drill a borehole in the adjacent geological formation after the window and using a "mill-drill" bit "drill ahead" as a lead mill, which is configured to mill casing to cut the window and to drill a borehole.

[0008] The milling assembly includes multiple mills for cutting a hole in the casing. Each of these mills may be referred to as a window mill, even if they could be used for other purposes in other types of assemblies. The first mill, which may be referred to using terms such as "lead mill," initiates cutting an opening on the inside surface of the sidewall of the steel casing. If needed to enlarge the opening cut by the lead mill or to dress the opening, one or more "follow mills" are included in the milling assembly above the lead mill. For any given sidetracking operation, the configuration of mills and the milling assembly is determined, in part, by the size of the window that needs to be cut, the type and size of casing, and other requirements. The window's dimensions must be large enough to allow a drill string with a directional drilling downhole tool assembly to exit the existing wellbore and drill a borehole for the planned lateral through the adjacent formation.

[0009] FIG. 1 schematically illustrates a representative, nonlimiting example of a downhole tool assembly for single trip sidetracking with a releasable connection between a whipstock and a mill. Many other downhole tool assemblies capable of cutting a window are possible. A directional drilling unit or assembly may also be incorporated into the downhole tool assembly to provide a "drill ahead" capability to drill a borehole through the formation after cutting the window. Additional downhole tools that might be found in a downhole tool assembly used for cutting a window in wellbore casing have been omitted for clarity. The downhole tool assembly 100 is lowered on a work string. It comprises a whipstock assembly 102 that is releasably connected to a milling assembly 104 at connection point 107. The whipstock assembly includes at least a whipstock 108. A whipstock assembly commonly includes an anchor 110 (which can be a packer, slips, or other tool capable of hole the anchor at a planned kick-off depth and orientation. Optionally, the downhole assembly may include additional tools 112 below or downhole from the whipstock 108. These could include, for example, scrapers, cleaners, and other tools for performing downhole operations before anchoring the whipstock. The downhole tool assembly may include additional tools, represented by block 114, above or uphole from the whipstock. Examplesof such tools or assemblies include but are not limited to measurement-while-drilling (MWD) systems, stabilizers, drill collars, motors, directional-drilling units, scrapers, cleaners, centralizers, packers, and valves. These additional tools and components may be placed between one or more of the tools or components shown in the figure or elsewhere in the downhole tool assembly, including below the whipstock assembly, between mills. An additional tool may also be incorporated into the milling or whipstock assemblies. The representative milling assembly 104 in the figures includes a lead mill 106 and multiple sidecutting mills 118 and 120 for enlarging and dressing the window opening. The lead mill, located at the lower end of the assembly, has a shape and arrangement of cutting elements on its cutting face and sides to mill the casing to cut an initial opening. It may also be capable of drilling a borehole through the adjacent geological formation. To enlarge the initial opening formed by the lead mill 106, the side-cutting mills 118 and 120, which may be referred to as "watermelon" mills because of their shape, grind material from edges around an initial opening in the casing cut by the lead mill. The downhole tool assembly, including the milling assembly 104, may include "flexible" or "bendable" tubing joints 122 between any of the tools in the downhole tool assembly to allow for more bending or flexing of the downhole assembly without damage.

[0010] Figures 2A to 2C illustrate a simplified representative example of cutting a window in wellbore casing using downhole assembly 100 for sidetracking. The downhole tool assembly 100 is being lowered on a work string (not shown) into steel casing 200 of an existing wellbore in FIG. 2A. Although not shown, the casing can be cemented by filling with cement an annulus between the outer diameter of the casing 200 and the side wall of the borehole (not shown.) Once the whipstock assembly 102 is lowered to a kick-off point for the planned lateral, it is rotated to face the direction of the planned lateral by rotating the drill string and then anchored by setting the whipstock anchor 110. Before the whipstock is anchored, other downhole operations can be performed. For example, it is not uncommon to clean the inside surfaces of the casing where an anchor, packer, or similar device that grips or seals against the walls of the casing will be set to help ensure proper gripping or sealing. Once anchored, the milling assembly is disconnected from the whipstock assembly and separated at connection point 107 to begin the milling. FIG 2B illustrates the downhole toolassembly after separation being lowered and deflected laterally toward and engaging the inner surface of the casing 200, where a window is to be cut. FIG. 2C illustrates the milling assembly further enlarging a cut window and drilling a "rat hole" 206 in the formation. The optional side-cutting follow mill 118 is milling the edges of the opening to enlarge it, while the lead mill 106 forms the rat hole. The optional second follow mill or dress mill 120 further enlarges or dresses the opening. The figure illustrates just one possible arrangement of a milling assembly. Many different configurations are possible for the lead mill and follow mills to cut a window in wellbore casing. Once the window is milled, the milling assembly is raised by tripping out the work string. A drill string with a directional drilling unit can be lowered and guided by whipstock 108 through the window to drill a borehole. However, as mentioned, the directional drilling unit can be included in the sidetracking assembly so that drilling a borehole in the adjacent formation can be commenced without having to trip out the sidetracking assembly.

[0011] FIG. 3 illustrates an example of a break or shear bolt used for releasably connecting a whipstock 302 and a window or lead mill 304 in a sidetracking tool assembly 300 (partially shown.) The window mill has a body 306 that has cutting structures on its outer diameter that include blades 308 with discrete cutting elements 310 placed along the leading edges of the blade. The mill receives within its interior bore 312 pressurized fluid supplied to the downhole tool assembly from the surface through a work string (not illustrated) connected to the assembly. Pressurized fluid from interior bore 312 is communicated to downhole tools below the whipstock with a hose (not illustrated.) The window mill and whipstock are releasably connected by a shear bolt 314.

[0012] The shear bolt is supported at both ends by the whipstock and mill by inserting through opening 316 at the top end of the whipstock into cavity 318 formed in the body of the window mill. The head 320 of the shear bolt interferes with the whipstock so that the bolt is not pulled through opening 316. A retaining pin 330 secures the shear bolt to the mill. It is inserted through a hole 332 that extends from an opening on the exterior of the body of the mill to the cavity 318, with its distal end 334 cooperating with a circumferential groove 326 around a distal portion 322 of shear bolt 314 to retain the bolt in the cavity, athreaded connection 334 located near the retaining pin's proximal end secures the retaining pin in the hole.

[0013] Surfaces on the mill and whipstock form a joint where they are connected by the shear bolt. These surfaces include planar surfaces that establish a release plane 315. The shear bolt is designed to fracture generally along shear plane 328, where a circumferential notch 326 is formed, and to have predetermined shear strength along the plane. The shear plane 328 is therefore aligned with release plane 315. Shearing stress in the shear bolt may result from, for example, opposing axial or rotational forces on the assembly that are being transferred by the joint between the whipstock and mill. For purposes of this disclosure, these forces are referred to as shearing forces. The joint is designed to be released when uphole axial forces on the mill and downhole axial forces on the whipstock and mill generate stress in the bolt that exceeds its shear strength. These shearing forces will be referred to as release forces. To mitigate the risk of premature or unintended shearing of the bolt by stress caused by other shearing, the joint between the whipstock 304 and mill 302 includes a lug 338 on the whipstock, to which the shear bolt is connected, and a pocket 340 formed on the outside of the body of the mill 306. The lug and pocket have complementary shapes with side surfaces that interfere with each other to transfer shearing forces between the whipstock and mill, except for release forces.

[0014] There have been many approaches to releasably connecting a mill to a whipstock. Many of them use a shear bolt to connect the lead mill to the whipstock. Running a sidetracking assembly into a wellbore and performing downhole operations before setting the anchor, such as those mentioned above, can result in substantial shearing forces on the shear bolt, especially if the assembly is heavy or the wellbore is not straight. The shear strength of the shear bolt may, of course, be increased to guard against failure of the joint before the whipstock is anchored. However, increasing the shear strength of the bolt makes it more difficult to break it to release the connection, thus making the release less reliable.SUMMARY OF THE DISCLOSURE

[0015] According to one aspect of the following disclosure, a releasable connection between a whipstock and mill in a sidetracking assembly has a relatively high shear strengthwhen running the sidetracking assembly into a wellbore and performing downhole operations before separating the whipstock and mill but has a lower shear strength for releasing the connection and separating the whipstock and mill. The relatively high shear strength of the connection reduces risks of premature release due to high shearing forces (transverse forces on the connection that create shear stress) on the connection without requiring a relatively high shearing force to release it, which would make the release more difficult and less reliable. A relatively low shearing force may be used to release the connection for separation.

[0016] According to a different aspect of the disclosure, a releasable connection between downhole tools is locked for run in and downhole tool operations and unlocked for separation of the tools. When locked, a shiftable pin supported by each of the downhole tools increases the shear strength of the connection and reduces the risk of premature separation. Moving the shiftable pin with a hydraulic actuator unlocks the connection by lowering its shear strength, thus permitting it to be more easily sheared to release the tools for separation.

[0017] According to a different aspect of the disclosure, a hydraulic actuator in one of two downhole tools or components of a downhole tool is activated to displace a shiftable pin extending between the tools or components. Increasing internal pressure to a point above a predetermined pressure threshold activates the hydraulic actuator and supplies hydraulic pressure to generate an axial force to displace the shiftable pin. If activated at relatively high pressure, the relatively high activation pressure can generate a relatively high axial force for displacing the shiftable pin.

[0018] According to a different aspect of the disclosure, a hydraulic circuit in a downhole tool is activated to create a pressure differential to shift a piston in the cylinder of a hydraulic actuator. The circuit communicates internal pressure to opposite faces of the actuator's piston. The internal pressure communicated to one of the two faces is communicated through a flow restrictor that also communicates the fluid pressure to a rupture element, such as a rupture or burst disc, which is designed to burst when subjected to predetermined pressure. Increasing internal pressure to the predetermined pressurebursts the rupture element to vent the fluid to the wellbore and drop the pressure on the face to which it is connected to wellbore pressure, thus generating an axial force on the piston to displace it. The flow restrictor maintains the pressure drop as fluid is vented to the wellbore to maintain the displacement.

[0019] These and other aspects are disclosed below in reference to representative, nonlimiting examples of the aspects and representative, nonlimiting embodiments shown in appended figures.BRIEF DESCRIPTION OF THE FIGURES

[0020] FIG. 1 schematically illustrates a representative, nonlimiting example of a downhole tool assembly for single trip sidetracking with a releasable connection between a whipstock and a mill.

[0021] FIGS. 2A-2C illustrate a simplified representative example of cutting a window in wellbore casing using downhole assembly 100 for sidetracking.

[0022] FIG. 3 illustrates a shear bolt releasably connecting a whipstock and lead mill.

[0023] FIGS. 4 illustrates a representative, nonlimiting example of a whipstock.

[0024] FIG. 5 illustrates a representative non-limiting example of a lead mill for cutting a window inboard casing.

[0025] FIGS 6A and 6B illustrate cross sections of a representative example of an embodiment of a releasable connection between a whipstock and a mill.

[0026] FIGS. 7A and 7B are schematic diagrams of a hydraulic circuit for activating a hydraulic actuator to retract a pin.

[0027] FIGS 8A and 8B illustrate cross sections of a representative example of an embodiment of a releasable connection between a whipstock and a mill.

[0028] FIGS. 9A to 9D illustrate cross sections of a representative example of an embodiment of a releasable connection between a whipstock and a mill.

[0029] FIGS. 10A to 10D illustrate cross sections of a representative example of an embodiment of a releasable connection between a whipstock and a mill.

[0030] FIGS. 11A -11C illustrate cross sections of a representative example of an embodiment of a releasable connection between a whipstock and a mill.

[0031] FIGS. 12A-12C illustrate cross sections of a representative example of an embodiment of a releasable connection between a whipstock and a mill.DETAILED DESCRIPTION

[0032] In the following disclosure, like numbers refer to like, but necessarily identical, elements. For example, the same number and term may be used to refer to multiple elements in an embodiment that are conceptually the same but not necessarily identical in structure. Similarly, the same reference number and term might be used to identify conceptually similar elements in two or more embodiments, even though they are not structurally identical. For example, the same number and term could be used to refer to different examples or implementations of an element that share the same or substantially equivalent purpose or functionality. However, it does not imply that any requirements, limitations, or additional functionality or advantages found in one must be found in the other.

[0033] For purposes of the disclosure, the term "mill" refers to a cutting tool capable of milling the sidewall of wellbore casing to create a window for sidetracking. "Mill" may, for example, refer to a window mill, lead mill, drill bit, mil l / d rill bit, and other tools capable of or configured for milling a hole in the sidewall of wellbore casing. Conversely, references to window mill, lead mill, drill bit, and mil l / d rill bit should be understood as referring to a mill for cutting a hole in casing unless explicitly stated otherwise. To avoid any doubt, the term "mill" used in this disclosure does not refer to a section mill, or similar tools used for cutting a casing around its circumference or removing a section or length of casing from a wellbore.

[0034] FIGS. 4 to 6B and 8A tol2C illustrate representative, nonlimiting embodiments 600, 800, 900, 1000, 1100, and 1200 (collectively, "illustrated embodiments") of a joint between a whipstock 400 and a mill 500 connected by releasable connections in a downhole toolassembly, such as a sidetracking downhole assembly. Other parts of the downhole tool assembly are not illustrated. The releasable connection in any of the illustrated embodiments below can be adapted to connect other types of downhole tools in other types of downhole assemblies.

[0035] Whipstock 400 is only partially illustrated and intended only as a representative, nonlimiting example of a whipstock, diverter, or similar downhole tool that, after being anchored within the casing, is used to deflect the remaining part of the sidetracking downhole tool assembly and work string to which is attached to cut a window in the wellbore casing. Several of the figures illustrate an upper end 404 of the whipstock and part of its ramp 402. Other figures illustrate only a lug portion that is attached to the upper end of the whipstock that joins with the pocket on the mill 500.

[0036] Mill 500 is intended as a representative, nonlimiting example of a lead or window mill and, more generally, of a downhole tool that can be used for milling casing to form an opening in the casing. The mill 500 has a body 502 with a central axis 504, about which the mill is rotated to mill the casing. The central axis of the mill will generally coincide with the central axis of rotation of the downhole tool assembly (not indicated) and work string to which the mill is attached during run-in and downhole operations. However, as previously mentioned, the work string and the downhole assembly may bend or flex when being lowered. The body of the mill is tapered, increasing in diameter from its nose to its gauge. Although a tapered shape can be advantageous for a lead mill, the mill is not required to have this shape or any other shape. Mill 500, in this example, has cutting structures in the form of discrete cutting elements 506 mounted multiple blades 508. Different types of cutting structures can be added to the body or substituted for any one or more or all the blades. The cutting elements could be, for example, tungsten carbide cutters or inserts, polycrystalline diamond compact (PDC) cutters, or other types of cutting elements, as well as a combination of different types of cutting elements. Discrete cutting elements may also be mounted elsewhere on the body 502 of the mill. A matrix containing crushed superhard materials such as metal carbides and / or polycrystalline diamond applied to the body or structures on the body, including blades, may, for example, be used in place of or inaddition to discrete cutting elements on blades, elsewhere on the body, or on other types cutting structures.

[0037] Although a connection to the downhole tool assembly uphole from the mill is not shown, the mill has an internal bore 601 (not visible in FIG. 5 but visible in the cross-sections shown in the other figures) that, when the mill is connected to the downhole tool assembly, receives a supply of pressurized fluid from the downhole hole assembly. The pressurized fluid may be supplied to the downhole assembly by, for example, pumping from the surface pressurized fluid through the work string (not shown) to which the downhole tool assembly is connected and then through the assembly. This allows the pressure of the fluid supplied to the mill to be monitored and controlled from the surface. However, pressurized fluid may be supplied to the mill and / or controlled in other ways, including by other downhole tools in the assembly.

[0038] In each of the illustrated embodiments, 600, 800, 900, 1000, 1100, and 1200 (FIGS.6A, 6B, and 8A to 12C) includes a releasable connection that extends generally along an axis perpendicular to the central axis 504. Surfaces on the mill and whipstock form a bolted joint where the mill and whipstock are joined by the releasable bolt. These surfaces include substantially planar surfaces that establish a release plane 608 between the tools. Opposing normal forces on opposite sides of the release plane will result in a shear force oriented along the release plane that, if carried by the releasable connection, results in shear stress in the releasable connection that causes it to fracture or break. Opposing force on the whipstock and mill resulting in shear force along this plane may be referred to as shearing forces. The shearing forces can be caused by axial or rotational forces on the assembly that must be transferred between the whipstock and mill. When the connection is released, the mill and whipstock will move relative to each other along this plane.

[0039] Forces on the downhole tool assembly while it is being lowered into the wellbore, performing downhole operations before anchoring the whipstock, orienting and anchoring the whipstock, separating the mill from the anchored whipstock, and, if necessary, raising the downhole tool assembly without having released the connection, may result in shearing forces. They may also result in radial forces, which result in opposing normal forces on thejoint that are aligned, and torques, which result in opposing normal forces on the joint that are not aligned.

[0040] For example, axial forces on the downhole tool assembly are transferred by the joint. Opposing axial forces on the mill and joint will not be aligned and thus load the joint on opposite sides of release plane 608. Such axial forces are represented in FIG. 6A and 6B by arrows 604 and are shearing forces. The arrows are oriented on FIG. 6A to indicate opposing shearing forces on the mill and whipstock that result in a shear force on the mill's side of release plane 608 oriented downhole and a shear force on whipstock's side oriented uphole. In FIG. 6B, the direction of axial forces are oriented in the opposite direction, resulting in shear forces oriented in opposite directions. In the illustrated embodiments, FIG. 6B indicates the direction of the axial shearing forces on the downhole tool assembly used for the release and separation of the whipstock 400 and mill 500. Rotational forces on the downhole tool assembly, indicated by arrow 607, may also create shearing forces. Radial forces on the joint are indicated by arrow 609. Torsional forces, represented by arrows 611, for example, bending of the downhole tool imparts radial and shear loads on the releasable connection and may cause the two parts of the joint between the mill and whipstock to separate.

[0041] Shearing, radial, and torsional loads on the joint of the illustrated embodiments 600, 800, 900, 1000, 1100, and 1200 are carried at least in part by the releasable connection and other structural elements that are part of the joint. These elements are, for example, surfaces on the mill and whipstock where they are joined that interfere with the relative movement of the mill and whipstock in one or more directions. Interfering surfaces transfer between the whipstock and mill the forces on the joint that cause the surfaces to interfere. A joint with interfering surfaces, such as those shown in the illustrated embodiment, is optional but may offer additional advantages. For example, the interfering surfaces may carry some of the loads that would otherwise need to be carried or cannot be effectively carried by the releasable connection.

[0042] In the illustrated embodiments, the joint formed where the whipstock and mill are releasably connected includes radially extending lug 406 on the upper end 404 of thewhipstock 400 that fits with a pocket 510 or slot on mill 500. The lug 406 may be integrally formed with the whipstock or formed separately and attached to the whipstock. The lug may, alternatively, be attached to the whipstock by, for example, welding, brazing, or otherwise fastening it. The lug in this example is fastened to the whipstock by inserting it through an opening 405 in the upper end 404 of the whipstock and retaining it within the opening. The lug may, optionally, be made from a millable material. The pocket 510 may, for example, be formed by a recess milled in the outer surface of the body 502. In this example, the pocket is located on a blade 508. It may also be formed in part or in whole in other ways, such as by molding a body with part of the recess, adding material or structure to a preformed body to form part or all the recess, or a combination of such operations.

[0043] When the lug 406 is seated in the pocket 510, a release plane 608 (best seen in FIGS.6A, 6B, and 8A-12C) is defined or established by a bottom wall surface 414 on the lug and a bottom wall surface 516 where they meet along the axis of the releasable connection. The release plane 608 is generally parallel to the central axis 504 of the downhole tool assembly and transverse to the radially oriented releasable connection in each of the illustrated embodiments. Forces on the mill and whipstock tool assembly on opposite sides of the plane create shear stress in the releasably connection oriented along release plane 608.

[0044] Opposing surfaces on the outside of the lug and on the inside of the slot can interfere with the relative movement of the lug and slot and, in doing so, transfer forces on the joint that are pushing together the interfering surfaces. In the illustrated embodiments, rotational shearing forces on the joint may be transferred and carried by the interference between side surfaces 414 of the lug and side surfaces 514 of the pocket. Uphole axial forces on the joint can be transferred and carried by interference between an uphole wall surface 410 of the lug and uphole wall surface 514 of the mill. In addition, the lug and pocket may transfer some of the torsional forces on the joint that may cause the joint to separate.

[0045] Because the pocket 510 does not have a downhole wall surface, forces on the joint resulting from uphole forces on the mill and downhole forces on the whipstock are not transferred. Rather, these forces are carried by the releasable connection between the milland whipstock. The lug and pocket may also help to reduce the risk of certain torsional forces that could contribute to the separation of the joint.

[0046] The uphole side surface 410 of the lug and the uphole side surface 514 of pocket 510 have complementary curvature, which may help decrease the risk of the lug and recess binding under a torsional load. The side wall surfaces 412 of the lug 406, the side wall surfaces 514 of the pocket 510, and the uphole side surface 414 of the lug and bottom wall surface 516 of the pocket are generally flat or planar and generally oriented parallel to the central axis 504. The angles at which the side and top wall surfaces of the lug and the end wall surface of the lug are approximately right angles. Similarly, the angle at which the side and uphole wall surfaces meet the bottom wall surface of the pocket are right angles.

[0047] The lug and pocket joint illustrated in the figures are representative and nonlimiting. Structures other than a lug and pocket could be substituted. The geometries of the lug and pocket may be modified or adapted to reduce risks associated with planned separation or premature separation of the joint. For example, surfaces could have non-planar geometries or have added surface features or structures (splines, for example.) The angles between adjacent surfaces could be made shallower or deeper. Furthermore, the joint between the whipstock and mill joint may be configured to allow, for example, their separation in a different direction or to transfer fewer or more forces, including no forces (other than, for example, a radial force that pushes the mill and whipstock together.) For example, the separation direction can be switched, if desired, to uphole rather than downhole or configured for separation of the mill and whipstock using a rotational force on the mill.

[0048] Each of the illustrated embodiments (600, 800, 900, 1000, 1100, and 1200) has a releasable connection between the mill and whipstock, which extends generally along a radial normal to the central axis. The releasable connections carry loads from shearing forces on the downhole tool assembly in the separation direction. It may, optionally, also carry other shearing loads and, possibly, radial and torsional loads depending on the structure of the joint.

[0049] The releasable connection in each embodiment has at least two states: locked and unlocked. When locked, the releasable connection has a relatively high shear strength andcannot be easily released by shearing forces loading the releasable connection. When unlocked, the connection has a relatively low shear strength, allowing relatively low shearing forces to be used to release the connection and separate the whipstock and mill.

[0050] A hydraulically shiftable lock is used in each of the illustrated embodiments to lock and unlock the releasable connection. The releasable connection is locked prior to running it into the well by placing the lock in a locking position. The lock is shifted to a position in which the connection the connection has a lower shear strength (the "release position") to release the connection. Activating a hydraulic actuator with hydraulic pressure generates an axial force to shift the lock.

[0051] In illustrated embodiments and other embodiments disclosed below, the shear strength of the releasable connection when locked may, for example, have at least 1.5 times the shear strength of the releasable connection when unlocked. It may, alternatively, have at least 2 times the shear strength of the releasable connection when unlocked. It may, alternatively, have at least 2.5 times the shear strength of the releasable connection when unlocked.

[0052] To activate the hydraulic actuator in the illustrated embodiments, fluid pressure within the internal bore 601 of the mill is increased to a pressure above a predetermined threshold pressure, at which the hydraulic actuator is activated. This pressure may be referred to as "activation pressure." Once the activation pressure is achieved, the hydraulic actuator uses the activation pressure to generate a force to retract the lock. The higher the activation pressure, the greater force that can be used to retract the lock. Furthermore, the higher the activation pressure, the less likely it is for unintended or unexpected variations in internal pressure to activate the actuator prematurely.

[0053] Though illustrated embodiments utilize a single lockable, releasable connection, the mill and whipstock may have multiple releasable connections, each a lock, or multiple releasable connections with one lock.

[0054] In the illustrated embodiments 600, 800, 900, 1000, 1100, and 1200, the hydraulically shiftable lock is implemented using at least in part a shiftable pin that istranslatable along a radial normal to the central axis 504 of the downhole tool assembly. Three representative, nonlimiting examples of shiftable pins are disclosed by the illustrated embodiments. Embodiments 600, 800, 900, and 1000 use shiftable pin 602. Embodiment 1100 uses shiftable pin 1102 configured to slide inside a hollow cylinder 1104. Embodiment uses a variable strength pin 1202, which is also configured to slide inside hollow cylinder 1104.

[0055] The disclosed shiftable pins are representative, nonlimiting examples of a fastener or similar element that can be retracted and that is capable of transferring shear loads between the mill and whipstock to increase the shear strength of the releasable connection. A shiftable pin may have a circular or other cross-sectional shape. It may be solid, partially hollow, or mostly hollow. It may have one or multiple components. It may be made of a single or multiple materials. It may, for example, take the form of a bar, rod, shaft, length of tubing, or other type of fastener or elongated component. Although implementing the lock using a shiftable pin allows for certain advantages, the lock may, alternatively, be implemented in other ways.

[0056] Each shiftable pin is connected to a hydraulic actuator. A representative example of a hydraulic actuator, which is used by the illustrated embodiments, is a piston in a cylinder. Hydraulic pressure applied to the piston generates an axial force on the shiftable pin connected to the piston to retract it. The connected piston and shiftable pin can be separate parts or integrated into a single component. The piston will apply the axial force to the shiftable pin and displace it if the axial force is greater than any opposing axial force on the pin. Other types of hydraulic actuators that do not use a piston or are not linear could be substituted and adapted to retract the shiftable pin. However, they might not have or enable some of the benefits and advantages of the disclosed releasable connections, such as simplicity and reliability.

[0057] In the illustrated embodiments, an activation circuit uses the internal pressure of the mill — the fluid pressure in bore 601 — to generate the axial force. The source of the internal pressure is system pressure from the surface in these embodiments, although the use of system pressure is not required. The internal pressure can, therefore, be managedfrom the surface. However, the internal pressure could, alternatively, be sourced and controlled in other ways, such as from other downhole tools in the assembly.

[0058] Referring now primarily to FIGS. 6A, 6B, and 8A-10D, the proximal and distal ends of shiftable pin 602 are supported by mill 500 and whipstock 400, respectively, in each of the embodiments 600, 800, 900, and 1000. When the connection is locked, the retractable pin is supported on its proximal end by opening 518 in the mill and on its distal end by opening 416 of the lug 406. If a lug is not used, the distal end can be supported by another opening or structure on the upper end 404 of the whipstock. In this state, it can be used to transfer shearing forces on the downhole tool assembly between the whipstock 400 and mill 500. However, when using the lug 406 and pocket 510 to form the joint, the shiftable pin 602 will only transfer upward axial shearing forces in the illustrated embodiments. When the hydraulic actuator inside the mill retracts pin 602 into the mill, it pulls the distal end of the shiftable pin 602 from the opening 416 in the lug far enough that the whipstock no longer supports, which is to release plane 608. In the retracted position, the shiftable pin is no longer able to transfer forces on the downhole tool assembly between the whipstock and mill. Retracting the shiftable to the release plane 608 or beyond ensures that it will not interfere with releasing the connection and separating the mill from the whipstock.

[0059] The releasable connection in embodiments 600, 800, 900, and 1000 also include an optional shear bolt (or pin or other fastener) having a relatively low shear strength that connects the whipstock and mill radially to reduce the risk of torsional and radial forces on the joint pulling the distal end of the retractable pin 602 out of the lug 406 or otherwise pulling apart the joint. Although the shear bolt is not illustrated, the lug 406 in FIGS 6A-6B and 8A to 10C includes an opening 610, through which a shear bolt can be inserted. If no lug is used, the opening can be inserted through a hole formed in the upper end 404 of the whipstock. A corresponding opening mill body 502 is not illustrated but would also be present. Once installed, the shear bolt or pin extends generally perpendicularly to the central axis 504 and parallel to retractable pin 602. The shear bolt is retained by a head, nut, or other so that it is pulled through the opening 610 in the lug. The other end extends far enough into the mill body 802 that radial or torsional forces on the joint are unlikely to have sufficient space to pull it apart, given the restricted dimensions of the casing and thestructure of the joint. However, if desired, the shear bolt could be retained in the mill by a fastener after it is inserted.

[0060] Referring now primarily to FIGS. 11A to 12C, each of the illustrated embodiments 1100 and 1200 has a releasable connection that includes hollow cylinder 1104 in which a shiftable pin — shiftable pin 1102 in embodiment 1100 (FIGS. 11A-11C) or shiftable pin 1202 (FIGS. 12A-12C) having a variable shear strength — translates inside the hollow center.

[0061] The hollow cylinder 1104 in each embodiment is connected at its proximal end to the lug 406 in the upper end 404 of the whipstock 400 and supported by the opening 416 (not visible in these views) of the lug 406 to transfer shearing loads between the whipstock and mill between. On the distal end of the hollow cylinder 1104, there is an optional threaded stem 1106 that extends through an opening in the lug and is retained by a nut 1108. The hollow cylinder could, alternatively, be attached to and supported by the upper end 404 of the whipstock 400 in ways. The hollow cylinder extends into and is supported by the opening 518 (not visible in these views) in the body 502 of the mill. It may extend far enough into the mill that radial or torsional forces are unlikely to pull it out of the mill within the confines of the casing.

[0062] The hollow tube can made to have a relatively low shear strength, making it relatively easy to release the connection by selectively applying a shearing force on the downhole tool assembly to release and separate the whipstock and mill after shiftable pin 1102 or 1202 is moved to an unlocked position.

[0063] The hollow tub reduces the risk of radial and torsional forces from disconnecting the lock from the whipstock, especially when used in combination with a lug and pocket joint, as described above. Furthermore, its configuration provides a backup in the event the pin is shifted prematurely by mistake. Pulling up on the mill is still required to shear the cylinder before the mill can be separated from an anchored whipstock.

[0064] Turning now specifically to embodiment 1100 (FIGS 11A-11C), when the releasable connection is locked, shiftable pin 1102 extends from the mill body 502 to the upper end 404 of the whipstock, where it is supported (though contact with hollow cylinder) by the lugopening 416, as shown in FIGS. 11A and 11B. In this extended position, pin 1102 can transfer forces between the mill and whipstock. The combination of a shiftable pin and hollow cylinder has a relatively high shear strength and can carry relatively high shear loads where release plane 608 intersects the connection.

[0065] Retracting shiftable pin 1102 with the connected hydraulic actuator to a point where it is no longer supported by the whipstock unlocks the connection. Therefore, as with shiftable pins 602 in embodiments 600, 800, 900, and 1000, substantially all shiftable pin 1102 in a fully retracted position is on the mill side of release plane 608, as shown in FIG. 11C.

[0066] Shiftable pin 1202 in embodiment 1200 (FIGS. 12A to 12C) has, like shiftable pins 602 and 1102 in the other illustrated embodiments, a proximate end supported by the mill body 502 and connected with a piston of a hydraulic actuator. However, shiftable pin 1202 has its distal end supported (through contact with the hollow cylinder) by the opening 416 in the lug 406 in both locked and unlocked positions. Furthermore, it has at least two sections along its length or central axis with different shear strengths. Therefore, the shear strength of the releasable connection depends on the shear strength of the section spanning the release plane 608 between tools and supported by both the whipstock and mill.

[0067] In the illustrated example, section 1202a of the pin has a relatively high shear strength, section 1202b of the shiftable pin 1202 has a relatively low shear strength, and an optional intermediate section 1202c has a shear strength that continuously decreases from the relatively high shear strength of section 1202a to the relatively low strength of section 1202b. The shear strength of the shiftable pin at any point along its axis is determined by the diameter of a bore 1203 inside the pin. Alternatively, the shear strength of the pin can be varied in different ways along its axis, such as by varying its outer diameter and / or shape. For example, it could be notched to create a section of low shear strength. Section 1202a of the pin is solid or otherwise has a high shear strength. Second section 1202b has a relatively low shear strength due to the wide inner diameter cavity that creates a relatively thin outer wall. The cavity along the intermediate section 1202c has a cone shape, allowing for a range of intermediate shear strengths to be selected for an unlocked connection by retracting thepin to an intermediate position. As an alternative to a continuously varying shear strength in the intermediate section, the section may have one or more steps to enable the selection of one or more intermediate shear strengths. Being able to select an intermediate strength allows for the use of a higher, but still relatively low, shearing force to release the connection as a precaution against premature actuation or another event that unlocks the connection before the connection is planned to be released.

[0068] When the pin is in a fully extended position, as shown in FIG. 12A, the high shear strength section 1202a is supported by the lug 406 and mill body 502 and spans release plane 608. This position locks the releasable connection. FIG. 12C shows it fully retracted, with the low shear strength section 1202b spanning shear release plane 608 and supported by the whipstock and mill. FIG. 12B shows the pin partially retracted, with section 1202c spanning the release plane 608 and supported by the whipstock and mill. Either the intermediate (FIG. 12B) or fully retracted position (FIG. 12C) of the shiftable pin 1202 can be selected to unlock the connection.

[0069] If an intermediate position, such as the one shown in FIG. 12B, is selected, the configuration of the hydraulic actuator can be changed to stop the travel of the shiftable without having to fabricate a new mill to change the length of the cylinder in which the piston of the hydraulic actuator travels. Various methods can be used to stop travel. For example, a ring or other structure (not illustrated) could be inserted between the actuator's piston and the piston's cylinder or placed in the opening in which the connection between shiftable pin 1202 and the piston of its hydraulic actuator travels to prevent further travel of the piston when the intermediate shear section is aligned with the release plane 608.Alternatively, the length of the connection between the pin and its hydraulic actuator could be made configurable so that it can be selectively lengthened.

[0070] Turning now to just FIGS. 6A-6B, the hydraulic actuator in embodiment 600 includes a piston 620 connected to shiftable pin 602. A piston retention pin 624 interferes with the movement of piston 620, thus preventing displacement of the piston and movement of the shiftable pin 602 from a locked position by axial forces that result from internal pressureacting on the piston's face 625. FIGS 6A and 6B illustrate cross sections of embodiment 600 in a locked state (FIG. 6A) and an unlocked state (FIG. 6B).

[0071] The embodiment's hydraulic actuator piston 620 is translatable within cylinder 621. The cylinder is defined by a cavity in the mill body 502. The piston divides the cylinder into two portions, 621a and 621b. The fluid pressure in cylinder portions 621a and 621b act on piston face 626 and piston face 625, respectively. The piston faces are located on opposite ends of the piston. Internal pressure is communicated to the cylinder portion 621a through fluid path 634. However, internal pressure is not communicated to cylinder portion 621b or piston face 625. Fluid path 640 communicates wellbore pressure to piston face 625.

[0072] The piston retention pin 624 is inserted through opening 627 in the mill body and extends to a circumferential groove 628 defined on the outer diameter of piston 620, between piston faces 626 and 625. When inserted prior to running the downhole tool assembly, an upper section 624a of the piston retention pin is held in a fixed position relative to the body by, for example, a threaded connection 632. A lower section 624b of the has a tip 624c that extends into groove 628 to interfere with the movement of piston 620.

[0073] The upper section 624a and lower section 624b of the pin 624 are joined by a connector 630 with a gap between the ends of each section. The connector is configured to hold them apart until a predetermined axial force on the lower section 624b causes the ends of the two sections to be pushed together. The gap is large enough to retract tip 624c from the groove far enough to no longer interfere with the translation of the piston when the lower section 624b is pushed toward the upper section 624a. Internal pressure supplied to cylinder portion 621a by path 634 is also supplied to groove 628, resulting in an axial force on the lower section 624b of the retaining pin. When internal pressure from interior bore 601 reaches activation pressure, the activation pressure creates an axial force on the lower section 624b to shift the lower section 624b relative to the upper section 624a and collapse the gap in the connection, freeing tip 624c from the groove so that it no longer interferes with displacement of the piston. The connection may optionally be configured to hold the upper and lower ends in the collapsed position. A pressure differential betweenthe activation pressure on the piston face 626 and wellbore pressure on face 625 results in an axial force on the piston to retract the shiftable pin 602.

[0074] FIGS. 7A and 7B are schematic diagrams of a hydraulic circuit 700 for activating a hydraulic actuator to retract pin 716. Pin 716 represents the shiftable pins 602, 1102, and 1202 in embodiments 800, 900, 1000, 1100, and 1200. These embodiments implement representative examples of the circuit 700. The following description is, therefore, also made in reference to FIGUS 8A to 12C.

[0075] Piston 702 functions as a linear actuator. The piston is translatable in each of two opposite directions within cylinder 708 using an axial force. The shiftable pin in each of the embodiments (represented by pin 716 in the schematic diagram) is mechanically connected to the piston through a connection or by being integrated with the piston 702 so that they translate together. The relative position of the piston may, therefore, be referenced by the position of the shiftable pin in this disclosure: it is in an extended or locked position when the shiftable pin is an extended or locked position; it retracted or unlocked with the shiftable pin is in a retracted or unlocked position. The axial force translating the piston is transferred to the shiftable pin.

[0076] One end of piston 702 forms the first piston face 704. The opposite end forms a second piston face 706. The piston divides cylinder 708 into two portions, cylinder portion 708a and cylinder portion 708b. The volumes of the respective cylinder portions will vary with the position of the piston. The piston is sealed against the cylinder wall to isolate fluid pressure in each portion. Pressure differentials across the piston can be used an axial force on the cylinder to translate it. The axial forces on the piston from the fluid pressure in each cylinder portion is a function of the effective area of the respective piston faces 704 and 706. The greater the area, the greater the axial force that is generated from the same fluid pressure.

[0077] Internal pressure in bore 601 ("Ps") of the mill body 502 is represented by fluid path 712 in FIGS. 7A and 7B. Before activation of the actuator, the internal pressure is communicated to cylinder portions 708a and 708b through fluid path 712a and fluid path 712b, respectively.

[0078] Fluid path 712a represents a direct connection in the illustrated embodiments connection between the internal bore 601 of the mill and the cylinder portion 708a. It creates a closed system that communicates internal pressure to the piston face 704 with no fluid flow, except possibly an insubstantial amount of fluid flow due to displacement of the piston. Therefore, although the pressure in cylinder portion 708a varies with internal pressure, it is not substantially changed by displacement of the piston.

[0079] However, fluid path 712b connects the internal bore 601 through flow restrictor 710 and then to the cylinder portion 708b and to a rupture disc 714 in parallel. The rupture disc seals a port formed in the mill to the wellbore. Bursting the rupture disc connects fluid path 712b to the wellbore through the flow restrictor. Before activation, internal pressure is communicated "Ps" to cylinder portion 708b, ensuring the pressure in each cylinder portion is the same regardless of internal pressure. No fluid will flow through the flow restrictor 710 and fluid path 712b until the rupture disc bursts or the piston 702 is displaced by an axial force.

[0080] An example of an axial force on the piston unrelated to fluid pressure is external forces on the shiftable pin to which the piston is connected. These forces, if high enough to overcome frictional and other axial forces on the piston, can shift the piston. Another example is an axial result resulting from different surface areas of piston faces 704 and 706. In the referenced embodiments, the effective surface area of piston face 706 is greater than piston face 704. The difference in the effective area results in axial force pushing the piston in the direction of the side with the smaller effective surface area. It is, in effect, a biasing force. Before activation, the biasing force can help to extend the shiftable pin or, if the shiftable pin is fully extended, assist with keeping it engaged with the upper end 404 of the whipstock via lug 406. The biasing force is present regardless of the internal pressure.However, the higher the internal pressure, the higher the net force.

[0081] The hydraulic circuit will activate the actuator when the internal pressure Psis increased to the activation pressure well above expected internal pressure levels, which may mitigate the risk of pressure variations inadvertently activating the actuator.

[0082] When internal pressure reaches the activation pressure, the internal pressure communicated to the rupture disc 714 will cause it to burst and cause fluid in path 712b to exit the mill through port 718. The activation pressure, therefore, depends on the predetermined pressure at which the rupture disc is designed to burst ("rupture pressure") and wellbore pressure. A relatively high activation pressure guards against accidental or premature rupture and, therefore, actuation. It also guards against the risk of the wellbore pressure being higher than expected.

[0083] The rupture of the disc results in an immediate, substantial drop in pressure in the cylinder portion 708b to near wellbore pressure, generating a substantial change in axial force on the piston as the result of the difference between internal pressure Pson piston face 704 and the wellbore pressure Pw, inducing fluid flow through the flow restrictor, which creates a pressure drop. When equilibrium is reached, the flow rate creates a pressure drop approximately equal to the difference between internal pressure and wellbore pressure. Hydraulic circuit 700 is configured when activating the hydraulic actuator to generate force that retracts the piston toward the retracted, unlocked position regardless of its position at the time of activation. Continuing to supply fluid to the mill will maintain the pressure differential and the axial force as the internal pressure is higher than the wellbore pressure. As fluid pressure continues to be supplied to the mill, the pressure differential is maintained. The resulting axial force is used to keep the piston in the retracted position. The flow restrictor and the size of the passages connecting the bore to the external part can be configured to vent a relatively small amount of fluid to maintain the pressure differential. The predetermined threshold pressure may, for example, be selected to be substantially higher than the expected pressure within the wellbore at the planned separation depth to reduce the risk of premature separation while also helping to ensure reliable separation.

[0084] The hydraulic circuit 700 may be used or adapted to actuate a shiftable element of other types of releasable connections for joining a whipstock and a mill. It may also be adapted for releasably connecting other types of downhole tools reliably. Pin 716 represents other types of shiftable elements in downhole tools used for releasable connections or other purposes. Furthermore, circuit 700 may be adapted for supplying different pressures to other types or configurations of actuators for using internal or systempressure to supply multiple fluid pressures to other types of components or two multiple components requiring different fluid pressures, including components downhole tools that are not actuators in response to the internal pressure reaching a predefined pressure.

[0085] A rupture disc may also be referred to as, for example, a pressure safety disc, burst disc, or rupture or burst diaphragm. It has a diaphragm or membrane, usually made of metal. The component is designed, as compared to other pressure-relief options, to reliably seal a port or fluid conduit and rupture almost instantly when subjected to a predetermined pressure. A port may be fitted with a coupling that makes installation and replacement of a rupture disc relatively easy. Although there might be advantages realized from using a rupture disc in the referenced embodiments, other shapes or configurations of a component with a diaphragm or membrane designed for reliable rupture at a predetermined pressure may, optionally, be substituted. A reference to a rupture or burst disc, as well as "rupture element" or "rupture component," should be understood as a reference to a pressure-relief component with a diaphragm or membrane that is round or another shape, which is designed to break at a predetermined pressure, regardless of the configuration of its supporting structure unless accompanied by limiting text. Other types of pressure relief components could, optionally, be substituted, such as pressure relief valves. However, they might not provide the advantages of a rupture disc. The term "pressure relief component" may be used to refer to a rupture component, pressure relief valve, or other similar component.

[0086] As compared to other methods that have been used to hydraulically release connections between a whipstock and a lead mill, the hydraulic circuit 700 leverages the availability of relatively high internal pressures and a relatively simple circuit to hydraulically release a connection between a whipstock while mitigating the risk of pressure variations that might prematurely release the connection. A relatively high internal pressure will generate a relatively high axial force to retract the pin. For example, an internal pressure of 3,500 psi can be used to activate the actuator while creating a force of 2,000 pounds to retract a pin. The high activation pressure and communication of internal pressure to both sides of the actuator's piston before activation may also help to mitigate the risk of pressure fluctuations causing an accidental activation or movement of the pin.

[0087] The piston 702 is, in the illustrated embodiments, capable of translation in either direction. It is extended before running the downhole tool assembly into the wellbore. The biasing force mentioned above can be used to help keep the shiftable pin in an extended position before activation of the hydraulic actuator. However, other forces on the shiftable pin or piston during run in or downhole operations before separation of the mill and whipstock may inadvertently shift the piston away from the extended position.

[0088] To guard against this possibility, the embodiment 1100 (FIGS. 11A-11C) includes an optional screw 1112 that connects the shiftable pin 1102 to the whipstock. The axial force on the piston resulting from activation of the hydraulic actuator imposes a tensile stress greater than the tensile strength of the screw, causing it to break. Instead of the screw 1112, a different type of breakable fastener could be substituted. The strength of the screw and fastener is, preferably, chosen to be substantially less than the axial force on the shiftable pin after activation of the actuator.

[0089] Although embodiments 800 (FIGS. 8A and 8B) and 1200 (FIGS. 12A to 12C) do not illustrate a screw or other fastener connecting the shiftable pin 602 (embodiment 800) or shiftable pin 1202, they can be, if desired, adapted to include a breakable fastener, including, for example, a screw or shear ring.

[0090] The embodiment 900 (FIGS. 9A to 9D) and the embodiment 1000 (FIGS. 10A to 10D) each include a mechanical interlock to prevent piston 702 from being shifted from an extended position before activation.

[0091] In embodiment 900, the interlock is formed by an interlock rod 904 and an extension 914 on the end of actuator piston 702, with piston face 706 defining an open channel 920 that is aligned with the central axis of the interlock rod. The interlock rod is connected to an activation piston 902, which is part of the activation circuit. The interlock rod has an interfering section 916, with an outer diameter larger than the channel's width, and a noninterfering section 918, with an outer diameter smaller than the channel width. When piston 702 is extended before activation, the activation piston 902 is in a retracted, unactuated position that is shown in FIGS. 9A and 9B. In this position, the interfering section 916 aligned with the interlock extension 914, thus preventing the retraction of piston 702.When internal pressure on fluid path 712b reaches activation pressure, an axial force extends activation piston 902 to align the non-interfering section 918 of the interlock rod 904 with the opening in channel 920 and thus permit retraction of the piston 702.

[0092] Internal fluid pressure in fluid path 712b is communicated to cylinder portion 708b through a bypass fluid path 903 in the activation piston, which includes flow restrictor 710. Internal pressure is also communicated in parallel to fluid path 922, which connects to external port 718. A valve 908 connected to the activation piston 902 closes fluid path 922 when the activation piston is retracted, as shown in FIGS. 9A and 9B. Internal pressure acts against it, but there is no flow through fluid path 922.

[0093] The internal pressure in fluid path 712b acts on face 910 of the activation piston 902 and on valve 908, generating an axial force on activation piston 902. The rupture disc 714, however, prevents its extension until internal pressure reaches activation pressure. The internal pressure is, in effect, communicated by the piston to rod 906, the end of which contacts the diaphragm of rupture disc 714. The rod may be shaped to distribute the axial force across the diaphragm. When the internal pressure reaches activation pressure, the resulting axial force on activation piston 902, activation rod 906 breaks the rupture disc 714, causing the activation piston to extend to the position shown in FIGS. 9C and 9D. Extending the activation piston unlocks the interlock, allowing piston 702 to retract and shift the axial position of valve 908 to open the fluid path 922 to the eternal port 718.

[0094] The interlock in embodiment 1000 is similar to the one in embodiment 900, with an activation piston 1002 shifting an interlock rod 1004 that has an interfering section 1006 and a smaller diameter non-interfering section 1008. An open channel 1012 formed by extension 1010 of piston 702 aligns with the central axis of interlock rod 1004. Moving the piston 702 from an extended position (shown in FIGS. 10A and 10B), the interlock rod must fit in the channel. The interfering portion of the interconnect rod has an outer diameter larger than the width of the channel and thus blocks movement of the piston 702.Activation shifts the interlock piston from the locking position shown in FIGS. 10A and 10B to an unlocking position shown in FIGS. 10C and 10D, which aligns the non-interferingsection 1008 with the channel 1012. The smaller diameter of the non-interfering section 1008 fits into the channel to permit retraction of the piston.

[0095] Like embodiment 900, the activation piston 1002 includes a bypass fluid path 1013 to communicate internal pressure on fluid path 712b through flow restrictor 710 to the cylinder portion 708b. However, unlike embodiment 900, the bypass passage also communicates internal pressure directly to rupture disk 714. The rupture disc seals external port 718 until internal pressure reaches activation pressure, causing it to burst. Internal pressure acts on both ends 1016 and 1018 of the activation piston before the rupture disc bursts. Bursting of the rupture disc drops the pressure in cylinder portion 708a and on the end of 1018, causing the activation piston to extend to allow piston 702 to retract and the piston to retract, as shown in FIGS. 10C and 10D, allowing the piston 702 to retract. Axial forces, such as variations in pressure on opposite ends of the interlock piston 1002 or external forces on the mill, might cause the interlock piston to shift before activation. A breakable spacing element 1014 may, optionally, be inserted to prevent movement of activation piston 1002 before activation pressure is reached. When activation pressure is reached, the axial force on the activation piston causes it to break and allows the activation piston to extend. The spacing element could take many different forms, including collapsable, shearable, or otherwise breakable components.

[0096] Other embodiments disclosed herein may be adapted to employ an interlock, including the interlocks embodiments 900 and 1000.

[0097] The preceding description of embodiments, including any that might be "preferred," are representative and nonlimiting examples of possible implementations, embodiments, and uses unless stated otherwise. They are disclosed for the purposes of explaining the claimed subject matter and its principles of operation so that skilled individuals in the field may understand and practice the claimed subject matter, including by adapting or modifying it to meet the requirements or limitations specific to their practice or implementation of it. Each disclosed embodiment or example may include multiple features, such as elements or sub-combinations of elements. Unless explicitly noted, nothing in the disclosure restricts or limits a feature from being practiced independently orin combination with other disclosed features. Modifications and substitutions of disclosed embodiments are, therefore, possible and should be expected when practicing the subject matter of a claim. None of the disclosed features are essential to the practice of claimed subject matter unless otherwise explicitly stated as being essential or required for the subject matter of a claim.

[0098] Although the claims are to be interpreted in view of the specification, terms in claims are intended to have their ordinary and customary meaning to those skilled in the art without being limited to details of any described embodiments, examples, or function unless the term is explicitly defined except to extent reasonably necessary to understanding the claim and its scope The meaning of a term in a claim is not intended to be limited to the details of illustrated or described structures, acts, or processes referenced by the term unless explicitly indicated otherwise in the specification or defined. When a common term for a class of structures or acts, or a type of structure or act, is used to identify a specific element or act in a disclosed embodiment or example, the structure or act should be understood to be a representative, nonlimiting example or embodiment of the type or class of structures or acts understood to be referenced by the term. A statement made in reference to a named element, such as component, structure, act, function, or combination in one embodiment or example, or an illustration of it, does not by itself imply that that the statement or details of the illustration are also applicable to element referenced in to by the same term in a different embodiment or example unless expressly indicated otherwise.

[0099] References to geometries — such as perpendicular, normal, plane, radial, circular, square, and the like — or to other properties, including "predetermined" values, in a description of apparatus, structures, processes, and acts are to be understood to allow for variations or differences that are to be reasonably expected or encountered when making and using claimed subject matter. These variations may, by way of example and not limitation, result from manufacturing tolerances, natural variations in materials or environment, the precision and accuracy to which "predetermined" values can be reasonably determined or estimated prior to use, and variations that might occur during use.

[0100] For the avoidance of doubt, the terms mentioned below should be understood and interpreted to have the given meanings, with the understanding that the meaning given below does not exclude the common and ordinary meanings of the term unless it is explicitly stated otherwise or the given meaning creates a substantial ambiguity, in which case the additional meaning given below take precedence, but only to the extent necessary to resolve the ambiguity.

[0101] The use of the term "may" should be interpreted in its normal sense as expressing a possibility and not a requirement, even when not accompanied by words like "option" or "optionally." The use of words like "optional" and "for example" in connection "may" in some instances does not change the meaning of "may" when it is used without such words. They should be interpreted merely as emphasis.

[0102] Each of the terms "couple," "coupled," "coupling," "connect," "connection," "connected," "in connection with," and "connecting" encompasses a direct connection, an integral connection, and an indirect connection or coupling through one or more intermediate elements or members unless the accompanying text explicitly indicates otherwise.

[0103] The terms "comprise," "have," "include," "contain," and "involve," and variations of them are open-ended linking verbs that signal a nonexclusive listing and thus permit the addition of other elements. When used in claims, the term "comprising" should be interpreted in the manner typically done in patents, which is "including but not limited to." On the other hand, the phrase "consisting of," when used in a claim, implies a closed set of elements.

[0104] When used in a claim, the phrase "consisting essentially of" excludes additional material elements but allows the inclusion of non-material elements. A material element substantively modifies, adds to, or subtracts from the functionality or nature of the subject matter recited in the claim.

[0105] If the specification states a component or feature "may," "can," "could," "should," "would," "preferably," "possibly," "typically," "optionally," "for example," "often," or"might" (or other such language) be included or have a characteristic, that component or feature is not required to be included or to have the mentioned characteristic. Such a component, feature, or characteristic can be included or excluded.

[0106] A singular form of an element or components of an apparatus described herein is understood to include its plural form. Use of the word "a" or "an" when used in conjunction with the term "comprising" in the claims or the specification means one or more than one unless context would otherwise make it indefinite. The term "or" in the claims is used to mean "and / or" unless the accompanying text explicitly indicates that it is referring only to alternatives or if the alternatives are by their nature mutually exclusive.

[0107] Terms like "up," "uphole," "upward," "above," and like terms when used in reference to a well, downhole tool, tool string, or the like should normally be interpreted as meaning towards the surface and away from the end of the well. Similarly, terms like "down," "downward," "downhole," and "below" should be interpreted as meaning away from the surface and towards the bottom or end of a well.

[0108] A "work string," for purposes of the disclosure, refers to tubing that can be used to lower, raise, and rotate downhole tools for downhole operations, including drilling, within a wellbore and to convey pressurized fluid from a pump at the surface to downhole tools. It is typically made of connected tubing joints but may also be coiled tubing.

[0109] The preceding disclosure discloses, by way of example, not limitation, the following aspects.

[0110] Aspect 1. A downhole tool assembly, comprising: a first downhole tool with internal pressure; a second downhole tool; a releasable connection joining the first downhole tool and the second downhole tool and configured for release by a shear force resulting from unaligned forces the first and second downhole tools, the releasable connection configured to have a relatively high shear strength when locked and relatively low shear strength when unlocked; a shiftable pin configured to lock the releasable connection in a first position and to unlock the releasable connection in a second position; and a hydraulic actuator coupled with the shiftable pin and configured to generate an axial force on the pin to retract the pinto the second position when activated by internal pressure at an activation pressure above a predetermined threshold pressure; wherein the hydraulic actuator is not activated when internal pressure is below the predetermined threshold pressure and activated when internal pressure reaches an activation pressure above the threshold pressure; wherein the hydraulic actuator comprises a piston connected to the shiftable pin and translatable in a cylinder; and wherein the hydraulic actuator is configured for internal pressure to be communicated to first end of the piston before and after activation and is further configured for internal pressure to be communicated to a second end of the piston opposite the first end before activation and for wellbore pressure to be communicated to the second end after activation.

[0111] Aspect 2. The downhole tool assembly of aspect 1, wherein the second end is connected to internal pressure through a hydraulic activation circuit; wherein the hydraulic activation circuit is configured to communicate internal pressure to the second end of the piston and to a rupture element, the rupture element configured to be ruptured in response to the internal pressure reaching the activation pressure; and wherein, upon rupture of the rupture element, the hydraulic activation circuit is configured to communicate wellbore pressure to the second end of the piston to create a pressure differential in the cylinder on opposite sides of the piston to retract the piston.

[0112] Aspect 3. The downhole tool assembly of aspect 2, wherein the rupture element seals a fluid passage to an external port and, when ruptured, opens the fluid passage to communicate well pressure to the second end of the piston.

[0113] Aspect 4. The downhole tool assembly of aspect 3, wherein pressure differential is maintained by fluid flow through a flow restrictor connecting internal pressure to the second side of the piston and the external port.

[0114] Aspect 5. The downhole tool of aspect 4, wherein the first end of the piston has a smaller effective surface area than the second end of the piston, thereby resulting in an axial force on the piston that biases it toward the first position.

[0115] Aspect 6. A downhole tool assembly, comprising: a first downhole tool with a central axis, the first downhole tool configured to receive fluid pressure from the downhole tool assembly; a second downhole tool with a central axis joined to the second tool; and a releasable connection joining the first downhole tool and the second downhole tool, the releasable connection having a central axis; and a release plane normal to the central axis of the releasable connection, along which the first downhole and second downhole tool are joined by the releasable connection, the releasable connection being configured to be released by a shear force along the release plane; and wherein the releasable connection comprises, a shiftable pin that translates axially in alignment with the central axis of the releasable, the releasable connection having a relatively high shear strength when the shiftable pin is in a first position and relatively low shear strength when the shiftable pin is in a second position; and a fixed connector extending between the first downhole tool and the second downhole tool and a release plane, the connector having relatively low shear strength where it intersects the release plane.

[0116] Aspect 7. The downhole tool assembly of aspect 6, wherein the shiftable pin has two ends; in the first position, one end of the shiftable pin is supported by the first downhole tool, and an opposite other end is supported by the second downhole; and in the second position, the second end is not supported by the second downhole tool.

[0117] Aspect 8. The downhole tool assembly of aspect 6, wherein the shiftable pin has two ends, one end of the shiftable pin being supported by the first downhole tool and an opposite end being supported by the second downhole in each of the first and second positions; and the shiftable pin has a first shear strength where it intersects the release plane in the first position and a second shear strength where it intersects the release plane in the second position, the first shear strength being relatively higher than the second shear strength.

[0118] Aspect 9. The downhole tool assembly of any one of aspects 6 to 8, wherein the fixed connector has a hollow center, in which the shiftable pin translates.

[0119] Aspect 10. A hydraulic actuator for a downhole tool, the hydraulic actuator comprising: an actuator piston translatable in a cylinder along an axis, the piston having afirst end and a second end opposite the first end; and a hydraulic circuit connected to hydraulic actuator and configured to activate the hydraulic actuator in response to internal tool pressure increasing to an activation pressure beyond a predetermined threshold pressure and not to activate the hydraulic when the internal pressure is below the predetermined threshold pressure, the hydraulic circuit comprising: a first path between configured to communicate internal pressure to the first end of the piston; and a second path configured to communicate through a flow restrictor the internal pressure in parallel to the second end of the piston and a rupture element; wherein the rupture element is configured to rupture when the internal pressure reaches the activation pressure; and rupture of the rupture element connects the second path, through the flow restrictor to the second end of the piston and external port in parallel; and the flow restrictor is configured to maintain any pressure differential between the internal pressure and fluid pressure on the second end of the piston.

[0120] Aspect 11. The hydraulic actuator of aspect 10, wherein the rupture element seals a path from the flow restrictor to the external port and rupture opens the path.

[0121] Aspect 12. The hydraulic actuator of aspect 11, wherein the external port defines an opening, and the rupture element seals the opening before rupture.

[0122] Aspect 13. The hydraulic actuator of aspect 10, wherein the actuator piston is held in a first position, and the axial force on the piston shifts it to a second position.

[0123] Aspect 14. The hydraulic actuator of aspect 13, wherein the actuator piston is held in the first position by a shearable element.

[0124] Aspect 15. The hydraulic actuator of aspect 14, wherein the actuator piston is held in the first position by an interlock.

[0125] Aspect 16. The hydraulic actuator of aspect 15, wherein the interlock has a first part connected to the actuator piston and a second part connected to a shiftable activation piston; the first part and the second part are configured to interfere with each other to prevent movement of actuator piston away from first position, and not to interfere with each other after the shiftable activation piston is extended; and the activation pistoncomprises a first end, a second end opposite the first end, and a bypass passage configured to communicate through a flow restrictor internal pressure on the first end to the second end of the activation piston and the second end of the actuator piston.

[0126] Aspect 17. A downhole tool assembly for sidetracking in cased wellbore, the downhole tool assembly comprising: a mill with fluid at an internal pressure; a whipstock; a releasable connection joining the mill and whipstock, the releasable connection configured for release by a shear force along a release plane normal to the connection where the mill and the whipstock meet, the connection comprising: a hollow fastener connection mill and whipstock that is shearable by a shear force along the release plane; a shiftable a pin extending from the mill along an axis normal the release plane between the mill and whipstock, the releasable connection having a relatively high shear strength along the release plane when the shiftable pin is in a first position relative to the mill and relatively low shear strength along the release plane when the shiftable pin is in a second position relative to the mill; and a hydraulic actuator comprising an actuator piston connected to the shiftable pin and configured to receive internal pressure of the mill when the shiftable pin is in the first position and the second position and to generate retraction force on the shiftable pin when internal pressure reaches an activation pressure above a predetermined threshold pressure.

[0127] Aspect 18. The downhole tool assembly of aspect 17 further comprising an activation circuit configured to communicate internal pressure to a first end of the actuator piston and to a second end of the actuator piston opposite the first end when the internal pressure is below the activation pressure and, in response to internal pressure reaching the activation pressure, communicating low pressure to the second end of the actuator piston to create a pressure differential that results in an axial force to move the shiftable pin to the second position.

[0128] Aspect 19. The downhole tool assembly of aspect 18, wherein the activation circuit is configured to communicate internal pressure to the second end of the actuator piston and a rupture disc, in parallel, through a flow restrictor; the rupture disc is configured to burst when the internal pressure reaches release pressure and communicate wellbore pressure tothe second end of the actuator; and the flow restrictor is configured to create a pressure drop in response to fluid flowing through the restrictor resulting from a difference between internal pressure and wellbore pressure.

[0129] Aspect 20. The downhole tool assembly of aspect 19 wherein the whipstock has a lug extending from an upper end of the whipstock into a pocket formed in the mill, the lug and pocket having complementary shapes with side surfaces that interfere with relative movement of the mill and whipstock to transfer shearing forces on the mill and whipstock but do not interfere to transfer shearing forces that result in the shear force to release the releasable connection.

[0130] Aspect 21. A method of disconnecting downhole tools in a downhole tool assembly, the method comprising: connecting a first downhole to a second downhole tool in a downhole tool assembly with a releasable connection, wherein the releasable connection is released by a shear force along a plane between the first and second downhole tools resulting from shearing forces on the first and second downhole tools and a shiftable pin and a fixed connection between the downhole tools; locking the releasable connection with by placing the shiftable pin in an extended position and unlocking the releasable connection by moving the shiftable pin to a retracted position with a hydraulic actuator, the releasable connection having relatively a high shear strength when locked and a relatively low shear strength when unlocked; running the downhole tool assembly into a wellbore on a work string; supplying internal pressure to the first tool through the work string, wherein the first tool is configured to supply internal pressure to a hydraulic actuator when the shiftable pin is in the extended position and in the retracted position; unlocking the releasable connection with the hydraulic actuator by increasing internal pressure to an activation pressure above a predetermined threshold pressure; and releasing the releasable connection by applying shearing forces to the first and second downhole tools to create along the plane a shear force greater than the relatively low shear strength of the unlocked releasable connection.

[0131] Aspect 22. The method of aspect 21, wherein the shiftable pin extends from and is supported by the first downhole tool, and locking the releasable connection extending ashiftable pin in a first position supported by the first downhole tool and unlocked by retracting the shiftable pin to a second position.

[0132] Aspect 23. The method of aspect 22, wherein the shiftable pin in the second position is not supported by the second downhole tool.

[0133] Aspect 24. The method of aspect 22, wherein the shiftable pin in the second position is supported by the second downhole tool, and wherein, in the first position, the shiftable pin has a first shear strength where it intersects the shear plane in the first position and a second, lower shear strength where it intersects the shear plane in the second position.

[0134] Aspect 25. The method of aspect 21, wherein unlocking the releasable connection comprises retracting the shiftable pin by communicating the activation pressure to a hydraulic actuator to generate an axial force on the shiftable pin to retract it to the second position.

[0135] Aspect 26. A method of disconnecting downhole tools in a downhole tool assembly, the method comprising: connecting a first downhole to a second downhole tool in a downhole tool assembly with a shiftable pin, the first and downhole tools being configured to be separated by a shear force along a plane established the connected first and second downhole tools, the shiftable pin having an axis along which it is moved that interests the plane; running the downhole tool assembly into a wellbore on a work string with the shiftable pin in a first position; supplying internal pressure to the first tool through the work string, wherein the first tool is configured to communicate internal pressure to a hydraulic actuator; activating a hydraulic actuator to move the shiftable pin to a second position by increasing internal pressure to an activation pressure above a predetermined threshold pressure; and releasing the releasable connection by applying shearing forces to the first and second downhole tools to create a shear force along the plane; wherein, the hydraulic actuator comprises an actuator piston connected to the shiftable pin; the first tool is configured to communicate internal pressure to a first end of the actuator piston and to an activating circuit; and the activation circuit is configured to communicate internal pressure through a flow restrictor to a second end of the actuator piston opposite the first end and to a rupture device, the rupture device being configured to burst when internal pressurereaches activation pressure; the activation circuit is further configured communicate wellbore pressure from an external port to the second end of the actuator piston in response to internal pressure reaching the activation; and the flow restrictor is configured to maintain a pressure differential between the internal pressure and the second end of the actuator piston.

[0136] Aspect 29. The method of any one of aspects 26 to 26, wherein the hydraulic circuit further comprises a path configured to communicate through a flow restrictor the internal pressure in parallel to the second end of the piston and the rupture element, and rupture of the rupture element connects the second path, through the flow restrictor to the second end of the piston and external port in parallel; and

[0137] Aspect 30. The hydraulic actuator of aspect 29, wherein the rupture element seals a path from the flow restrictor to the external port and rupture opens the path.

[0138] Aspect 31. The hydraulic actuator of aspect 30, wherein the external port defines an opening, and the rupture element seals the opening before rupture.

[0139] Aspect 32. The hydraulic actuator of any one of aspects 26 to 31, wherein the actuator piston is held in a first position prior to activation.

[0140] Aspect 33. The hydraulic actuator of aspect 32, wherein the actuator piston is held in the first position by a shearable element.

[0141] Aspect 34. The hydraulic actuator of aspect 32, wherein the actuator piston is held in the first position by an interlock.

[0142] Aspect 35. The hydraulic actuator of aspect 34, wherein the interlock comprises a first part connected to the actuator piston and a second part connected to a shiftable activation piston, wherein the first part and the second part are configured to interfere with each other to prevent movement of actuator piston away from first position, and not to interfere with each other after the shiftable activation piston is extended, and he activation piston comprises a first end, a second end opposite the first end, and a bypass passage configured to communicate through a flow restrictor internal pressure on the first end to the second end of the activation piston and the second end of the actuator piston.

[0143] Aspect 36. The method of any one of aspects 26 to 35, wherein the rupture device comprises a rupture disc.

[0144] Aspect 37. The method of any one of aspects 26 to 36, wherein the first downhole tool comprises a mill for cutting a window in wellbore casing, and the second downhole tool comprises a whipstock.

[0145] Aspect 39. A downhole tool assembly, comprising a first downhole tool with internal pressure; a second downhole tool; a releasable connection joining the first downhole tool and the second downhole tool and configured for release by a shear force resulting from unaligned forces the first and second downhole tools, the releasable connection configured to have a relatively high shear strength when locked and relatively low shear strength when unlocked; and means for hydraulically unlocking the connection in response to internal pressure reaching an actuation pressure above a predetermined threshold pressure.

[0146] Aspect 40. A downhole tool assembly, comprising a first downhole tool with internal pressure; a second downhole tool; a releasable connection joining the first downhole tool and the second downhole tool and configured for release by a shear force resulting from unaligned forces the first and second downhole tools; means for changing the shear strength of the connection from a relatively high shear to a relatively low shear strength; means for hydraulically actuating the means for changing the shear strength of the releasable connection from the relatively high shear strength to the relatively low shear strength in response to the internal pressure reaching a release pressure above a predetermined threshold pressure.

[0147] Aspect 41. The downhole tool assembly of aspect 40, wherein the means for hydraulically actuating the means for changing the shear strength comprises a rupture device configured to burst in response to the internal pressure reaching the release pressure.

[0148] Aspect 42. The downhole tool assembly of aspect 40 or 41, wherein the releasable connection comprises the shiftable pin and a non-shiftable shearable means for connecting.

[0149] Aspect 43. The downhole tool assembly of aspect 43, wherein the means for changing the shear strength of the connection from the relatively high shear strength to the relatively low shear strength, moves the shiftable pin to change the shear strength.

[0150] Aspect 44. The downhole tool assembly of any one of aspects 42 to 43 wherein the non-shiftable shearable means for connecting comprises a hollow, shearable element, in which the shiftable pin moves to change the search strength of the releasable connection.

[0151] Aspect 45. The downhole tool assembly of aspects 43 to 44, wherein the means for changing the shear strength of the connection from the relatively high shear strength to the relatively low shear strength moves the shiftable pin from a first position, in which the releasable connection has the relatively high shear strength, to a second position, in which the releasable connection has the relatively low shear strength.

[0152] Aspect 46. The downhole tool assembly of aspect 43 further comprising means for assisting with retaining the shiftable pin in the first position.

[0153] Aspect 47. The downhole tool assembly of any one of aspects 1 to 9 and 39 to 46, wherein the downhole tool assembly comprises a sidetracking downhole assembly; the first downhole tool comprises a mill for cutting a window in wellbore casing, and the second tool comprises a whipstock.

Claims

CLAIMSWhat is claimed is:

1. A downhole tool assembly comprising:a first downhole tool with internal pressure;a second downhole tool;a releasable connection joining the first downhole tool and the second downhole tool and configured for release by a shear force resulting from unaligned forces on the first and second downhole tools, the releasable connection configured to have a relatively high shear strength when locked and relatively low shear strength when unlocked;a shiftable pin configured to lock the releasable connection in a first position and to unlock the releasable connection in a second position; anda hydraulic actuator coupled with the shiftable pin and configured to generate an axial force on the pin to retract the pin to the second position when activated by internal pressure at an activation pressure above a predetermined threshold pressure;whereinthe hydraulic actuator is not activated when internal pressure is below the predetermined threshold pressure and activated when internal pressure reaches an activation pressure above the threshold pressure,the hydraulic actuator comprises a piston connected to the shiftable pin and translatable in a cylinder, andthe hydraulic actuator is configured for internal pressure to be communicated to a first end of the piston before and after activation and is further configured for internal pressure to be communicated to a second end of the piston opposite the first end before activation and for wellbore pressure to be communicated to the second end after activation.

2. The downhole tool assembly of claim 1, wherein,the second end is connected to internal pressure through a hydraulic activation circuit;the hydraulic activation circuit is configured to communicate internal pressure to the second end of the piston and to a rupture element, the rupture element being configured to be ruptured in response to the internal pressure reaching the activation pressure; and the hydraulic activation circuit is configured upon rupture of the rupture element to communicate wellbore pressure to the second end of the piston to create a pressure differential in the cylinder on opposite sides of the piston to retract the piston.

3. The downhole tool assembly of claim 2, wherein the rupture element seals a fluid passage to an external port and, when ruptured, opens the fluid passage to communicate well pressure to the second end of the piston.

4. The downhole tool assembly of claim 3, wherein pressure differential is maintained by fluid flow through a flow restrictor connecting internal pressure to the second side of the piston and the external port.

5. The downhole tool of claim 4, wherein the first end of the piston has a smaller effective surface area than the second end of the piston, thereby resulting in an axial force on the piston that biases it toward the first position.

6. A downhole tool assembly comprising:a first downhole tool with a central axis, the first downhole tool configured to receive fluid pressure from the downhole tool assembly;a second downhole tool with a central axis joined to the second tool; and a releasable connection joining the first downhole tool and the second downhole tool, the releasable connection having a central axis; anda release plane normal to the central axis of the releasable connection, along which the first downhole and second downhole tool are joined by the releasable connection, the releasable connection being configured to be released by a shear force along the release plane; andwherein the releasable connection comprises,a shiftable pin that translates axially in alignment with the central axis of the releasable, the releasable connection having a relatively high shear strength when the shiftable pin is in a first position and relatively low shear strength when the shiftable pin is in a second position; anda fixed connector extends between the first downhole tool and the second downhole tool across a release plane, the connector having relatively low shear strength where it intersects the release plane.

7. The downhole tool assembly of claim 6, wherein,the shiftable pin has two ends;in the first position, one end of the shiftable pin is supported by the first downhole tool, and an opposite other end is supported by the second downhole; andin the second position, the second end is not supported by the second downhole tool.

8. The downhole tool assembly of claim 6, whereinthe shiftable pin has two ends, one end of the shiftable pin being supported by the first downhole tool and an opposite end being supported by the second downhole in each of the first and second positions; andthe shiftable pin has a first shear strength where it intersects the release plane in the first position and a second shear strength where it intersects the release plane in the second position, the first shear strength being relatively higher than the second shear strength.

9. The downhole tool assembly of claim 6, wherein the fixed connector has a hollow center, in which the shiftable pin translates.

10. A hydraulic actuator for a downhole tool, the hydraulic actuator comprising: an actuator piston translatable in a cylinder along an axis, the piston having a first end and a second end opposite the first end; anda hydraulic circuit connected to the actuator piston and configured to activate the actuator piston in response to internal tool pressure increasing to an activation pressure beyond a predetermined threshold pressure and not to activate the hydraulic when the internal pressure is below the predetermined threshold pressure, the hydraulic circuit comprising a first path configured to communicate internal pressure to the first end of the piston and a second path configured to communicate through a flow restrictor the internal pressure in parallel to the second end of the piston and a rupture element, whereinthe rupture element is configured to rupture when the internal pressure reaches the activation pressure;rupture of the rupture element connects the second path through the flow restrictor to the second end of the piston and external port in parallel; andthe flow restrictor is configured to maintain differential pressure between the internal pressure and fluid pressure on the second end of the piston.

11. The hydraulic actuator of claim 10, wherein the rupture element seals a path from the flow restrictor to the external port and rupture opens the path.

12. The hydraulic actuator of claim 11, wherein the external port defines an opening, and the rupture element seals the opening before rupture.

13. The hydraulic actuator of claim 10, wherein the actuator piston is held in a first position, and the axial force on the piston shifts it to a second position.

14. The hydraulic actuator of claim 13, wherein the actuator piston is held in the first position by a shearable element.

15. The hydraulic actuator of claim 14, wherein the actuator piston is held in the first position by an interlock.

16. The hydraulic actuator of claim 15, wherein,the interlock has a first part connected to the actuator piston and a second part connected to a shiftable activation piston;the first part and the second part are configured to interfere with each other to impede movement of the actuator piston away from the first position and not to interfere with each other after the shiftable activation piston is extended; andthe activation piston comprises a first end, a second end opposite the first end, and a bypass passage configured to communicate through a flow restrictor internal pressure on the first end to the second end of the activation piston and the second end of the actuator piston.

17. A downhole tool assembly for sidetracking in cased wellbore, the downhole tool assembly comprising:a mill with fluid at an internal pressure;a whipstock;a releasable connection joining the mill and whipstock, the releasable connection configured for release by a shear force along a release plane normal to the connection where the mill and the whipstock meet, the connection comprising:a hollow fastener connection mill and whipstock that is shearable by a shear force along the release plane;a shiftable pin extending from the mill along an axis normal to the release plane between the mill and whipstock, the releasable connection having a relatively high shear strength along the release plane when the shiftable pin is in a first position relative to the mill and relatively low shear strength along the release plane when the shiftable pin is in a second position relative to the mill; anda hydraulic actuator comprising an actuator piston connected to the shiftable pin and configured to receive internal pressure of the mill when the shiftable pin is in the first position and the second position and to generate retraction force on the shiftable pin when the internal pressure reaches an activation pressure above a predetermined threshold pressure.

18. The downhole tool assembly of claim 17, further comprising an activation circuit configured to communicate internal pressure to a first end of the actuator piston and to a second end of the actuator piston opposite the first end when the internal pressure is below the activation pressure and, in response to internal pressure reaching the activation pressure, communicating low pressure to the second end of the actuator piston to create a pressure differential that results in an axial force to move the shiftable pin to the second position.

19. The downhole tool assembly of claim 18, whereinthe activation circuit is configured to communicate internal pressure to the second end of the actuator piston and a rupture disc, in parallel, through a flow restrictor;the rupture disc is configured to burst when internal pressure reaches release pressure and communicate wellbore pressure to the second end of the actuator; and the flow restrictor is configured to generate a pressure drop in response to fluid flowing through the flow restrictor.

20. The downhole tool assembly of claim 19 wherein the whipstock has a lug extending from an upper end of the whipstock into a pocket formed in the mill, the lug and pocket having complementary shapes with side surfaces that interfere with relative movement of the mill and whipstock to transfer shearing forces on the mill and whipstock but do not interfere to transfer shearing forces that result in the shear force to release the releasable connection.

21. A method of disconnecting downhole tools in a downhole tool assembly, the method comprising:connecting a first downhole to a second downhole tool in a downhole tool assembly with a releasable connection, wherein the releasable connection is released by a shear force along a plane between the first and second downhole tools caused by unaligned shearing forces on the first and second downhole tools and a shiftable pin and a fixed connection between the downhole tools;locking the releasable connection by placing the shiftable pin in an extended position and unlocking the releasable connection by moving the shiftable pin to a retracted position with a hydraulic actuator, the releasable connection having relatively a high shear strength when locked and a relatively low shear strength when unlocked;running the downhole tool assembly into a wellbore on a work string; supplying internal pressure to the first tool through the work string, wherein the first tool is configured to supply internal pressure to a hydraulic actuator when the shiftable pin is in the extended position and in the retracted position;unlocking the releasable connection with the hydraulic actuator by increasing internal pressure to an activation pressure above a predetermined threshold pressure; and releasing the releasable connection by applying shearing forces to the first and second downhole tools to create a shear force along the plane greater than the relatively low shear strength of the unlocked releasable connection.

22. The method of claim 21, wherein,the shiftable pin extends from and is supported by the first downhole tool; locking the releasable connection comprises placing a shiftable pin in a first position, in which it is supported by the first downhole tool; andunlocking locking the releasable connection by retracting the shiftable pin to a second position.

23. The method of claim 22, wherein the shiftable pin in the second position is not supported by the second downhole tool.

24. The method of claim 22, wherein the shiftable pin in the second position is supported by the second downhole tool, and wherein, in the first position, the shiftable pin has a first shear strength where it intersects the shear plane in the first position and a second, lower shear strength where it intersects the shear plane in the second position.

25. The method of claim 21, wherein unlocking the releasable connection comprises retracting the shiftable pin by communicating the activation pressure to a hydraulic actuator to generate an axial force on the shiftable pin to retract it to the second position.

26. A method of disconnecting downhole tools in a downhole tool assembly, the method comprising:connecting a first downhole to a second downhole tool in a downhole tool assembly with a shiftable pin, the first and downhole tools being configured to be separated by a shear force along a plane established the connected first and second downhole tools, the shiftable pin having an axis along which it is moved that interests the plane;running the downhole tool assembly into a wellbore on a work string with the shiftable pin in a first position;supplying internal pressure to the first tool through the work string, wherein the first tool is configured to communicate internal pressure to a hydraulic actuator;activating a hydraulic actuator to move the shiftable pin to a second position by increasing internal pressure to an activation pressure above a predetermined threshold pressure; andreleasing the releasable connection by applying shearing forces to the first and second downhole tools to create a shear force along the plane;wherein,the hydraulic actuator comprises an actuator piston connected to the shiftable pin;the first tool is configured to communicate internal pressure to a first end of the actuator piston and to an activating circuit; andthe activation circuit is configured to communicate internal pressure through a flow restrictor to a second end of the actuator piston opposite the first end and to a rupture device, the rupture device being configured to burst when internal pressure reaches activation pressure;the activation circuit is further configured to communicate wellbore pressure from an external port to the second end of the actuator piston in response to internal pressure reaching the activation; andthe flow restrictor is configured to maintain a pressure differential between the internal pressure and the second end of the actuator piston.

27. The method of claim 26, wherein:the hydraulic circuit further comprises a path configured to communicate through a flow restrictor the internal pressure in parallel to the second end of the piston and the rupture element; andrupture of the rupture element connects the second path, through the flow restrictor to the second end of the piston and external port in parallel; and28. The hydraulic actuator of claim 27, wherein the rupture element seals a path from the flow restrictor to the external port and rupture opens the path.

29. The hydraulic actuator of claim 28, wherein the external port defines an opening, and the rupture element seals the opening before rupture.

30. The hydraulic actuator of any one of claims 29, wherein the actuator piston is held in a first position prior to activation.

31. The hydraulic actuator of claim 30, wherein the actuator piston is held in the first position by a shearable element.

32. The hydraulic actuator of claim 30, wherein the actuator piston is held in the first position by an interlock.

33. The hydraulic actuator of claim 32, wherein the interlock comprises a first part connected to the actuator piston and a second part connected to a shiftable activation piston, wherein,the first part and the second part are configured to interfere with each other to prevent movement of the actuator piston away from the first position and not to interfere with each other after the shiftable activation piston is extended, and the activation piston comprises a first end, a second end opposite the first end, and a bypass passage configured to communicate through a flow restrictor internal pressure on the first end to the second end of the activation piston and the second end of the actuator piston.

34. The method of any one of claims 26, wherein the rupture device comprises a rupture disc.

35. The method of any one of claims 26, wherein the first tool comprises a mill for cutting a window in wellbore casing and the second tool comprises a whipstock.

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