Staged Actuation Shear Sub for Use Downhole

Abstract

A shear assembly used downhole to actuate a work string function includes a dampening subassembly to dampen the momentum of moving parts following shearing to avoid excessive acceleration and resulting jarring forces within the work string. A dual shear assembly is provided, in which the dampening subassembly prevents inertial force resulting from a first shear from exceeding the threshold for actuating the second, higher threshold, shear mechanism thereby allowing more precise control of the forces applied to the work string to prevent premature shearing of the second shear mechanism. In a specific embodiment, the dampening subassembly is a hydraulic dampening subassembly. The dual shear assembly may be used, for example, to first open a wash port above a tool lodged within the wellbore to loosen debris above the lodged tool, and then to shear the tool from the work string if necessary.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

The present invention claims the benefit of U.S. Provisional Patent Application Ser. No. 61/012,457 entitled “Staged Actuation Shear Sub for Use Downhole” filed Dec. 9, 2007.

FIELD OF THE INVENTION

The present invention relates generally to tools for use within a wellbore. More particularly, the present invention relates to a shear assembly for integration within a work string.

BACKGROUND OF THE INVENTION

During use of downhole equipment within a wellbore, sand and other debris may enter the wellbore and accumulate above the bottom hole assembly. From time to time, the tool may become lodged within the wellbore due to this accumulation of debris. Various means of attempting to loosen lodged equipment are known, which generally include pulling (such as the controlled pulling taught by U.S. Pat. No. 7,249,633), vibrating, or reciprocating the bottom hole assembly within the wellbore until same is freed of debris. Unfortunately, these loosening methods often fail, requiring the bottom hole assembly to be sheared from the tubing string and fished out of the well. Such fishing operations are expensive and time consuming, as an independent contractor must transport specialized equipment to the site in order to fish the bottomhole assembly from the wellbore.

Once the tubing string has been sheared from the fishneck, the fishneck may remain inaccessible due to further accumulation of debris during shearing and removal of the tubing string. Accordingly, fishing/pulling equipment may incorporate a wash feature in order to loosen debris from about the fishing neck of a lodged downhole tool (see for example U.S. Pat. No. 3,020,957; U.S. Pat. No. 4,749,044; and U.S. Pat. No. 5,887,925).

Similarly, U.S. Pat. No. 6,352,113 describes a submersible pump assembly for attachment within a tubing string, the pump assembly having a fluid tube extending from surface down to the bottom hole assembly to loosen debris from above the upper packer prior to pulling the bottom hole assembly from the wellbore.

To date, a shear-actuated wash mechanism has not been incorporated within a work string above a bottom hole assembly for use in freeing the bottom hole assembly should it become lodged within the wellbore. Moreover, it has not to date been possible to include two tension shear-actuated devices within a work string in close proximity, or with similar shear thresholds.

It is, therefore, desirable to provide a simple, staged device for washing the area above a bottom hole assembly to free lodged equipment, and to shear the bottomhole assembly from the coiled tubing should the wash be ineffective in releasing the tool from the wellbore.

In addition to the desire for a tool able to clear debris from above lodged downhole equipment, there is a more general need to provide a staged means of actuating downhole equipment or processes. Although tension shear devices are among the most simple means of tool actuation, the shearing of pins to actuate, open, or slideably operate an otherwise fixed element is generally avoided due to the unpredictable inertial forces that result from the application and release of tension within the coiled tubing. In other words, application of pulling force to the tubing string over a great vertical distance causes the coiled tubing to stretch. Thus, when the shearable element is released upon application of an appropriate threshold force at the shear location, the sheared element will generally slide forcibly into a corresponding shoulder. This inertial force may be magnified by the simultaneous release of tension within the extended or stretched coiled tubing. Should a second tension-shearable element be present in close proximity within the work string, the inertial force of the moving parts immediately subsequent to the first shearing event may be great enough to exceed the second shear threshold unintentionally.

Due in part to the above-noted limitations associated with current shear devices, typically only one tension shear-actuated function is useful within a tubing string. As such, a shear sub is usually limited to use as a last resort mechanism by which the operator may separate the tubing string from the equipment in an emergency or otherwise problematic situation.

It is therefore further desirable to provide a shear assembly in which recoil force may be minimized or dampened, as this may facilitate development of a dual or multiple shear device for controlled, staged actuation of downhole equipment or processes.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the invention, there is provided a shear assembly for use downhole, the shear assembly comprising: a mandrel for integration within a work string; a shear sleeve slideable with respect to the mandrel to actuate a work string function; setting means for setting the shear sleeve in a fixed position with respect to the mandrel, the setting means shearable by application of a threshold force to the work string, thereby actuating the work string function; and a dampening subassembly operatively associated with the mandrel and with the shear sleeve for slowing the sliding momentum of the shear sleeve with respect to the mandrel following shearing of the setting means.

In accordance with a second aspect of the invention, there is provided a dual shear assembly for use downhole, the dual shear assembly comprising: a mandrel for integration within a work string; a first shear sleeve slideable with respect to the mandrel to actuate a first work string function; first setting means for setting the first shear sleeve in a fixed position with respect to the mandrel, the first setting means shearable by application of a first threshold force to the work string, thereby actuating the first work string function; a second shear sleeve slideable with respect to the mandrel to actuate a second work string function; second shearable setting means for setting the second shear sleeve in a fixed position with respect to the mandrel, the second setting means shearable by application of a second threshold force to the work string, thereby actuating the second work string function; and a dampening subassembly operatively associated with the mandrel and with the first shear sleeve for slowing the sliding momentum of the mandrel with respect to the first shear sleeve following shearing of the first setting means. It will be apparent that a multiple shear assembly, that is one with three or more shearing sub-assemblies, is possible where n is the number of shearing functions and n-1 is the number of associated momentum-damping assemblies.

In an embodiment, the first work string function is actuated upon abutment of a mandrel surface with a shear sleeve surface, and wherein the dampening subassembly slows the sliding momentum of the mandrel to limit the force of said abutment.

In a suitable embodiment, the force of abutment is limited to a value less than the second threshold force.

In an embodiment, the dampening subassembly is a hydraulic dampening subassembly. The hydraulic dampening subassembly may comprise one or more annular pistons disposed within hydraulic chambers operatively attached to the mandrel. The chambers may be filled with hydraulic fluid of suitable viscosity to provide the desired degree of dampening for a specific application.

In an embodiment, the dampening subassembly reduces the abutment force by at least 10,000 pounds force.

In an embodiment, the second threshold force exceeds the first threshold force by more than 15%.

In an embodiment in which the work string includes a frac packer, the first threshold force may be 40,000 pounds force or greater. Further, the second threshold force may be between 56,000 and 96,000 pounds.

In an embodiment, the first work string shear-initiated function is opening of a wash port within the work string.

In an embodiment, the second work string shear-initiated function is shearing of the mandrel from the work string, which may expose a fishneck for use in recovery of the mandrel from the wellbore.

In accordance with a third aspect of the invention, there is provided a method for returning a work string to surface when a downhole tool within the work string has become lodged within the wellbore, the method comprising the steps of: opening a wash port within the work string above the lodged tool; circulating fluid within the work string and the wellbore above the lodged tool to loosen debris from above the lodged tool; and pulling the work string and tool to surface.

In an embodiment, the method further comprises the step of shearing a first shearable setting means within the work string to open the wash port.

In a further embodiment, the method further comprises the step of shearing the lodged tool from the work string.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way of example only, with reference to the attached Figures, wherein:

FIG. 1 is a longitudinal cross sectional view of a dual shear assembly with first and second sets of shear pins;

FIG. 2 is a longitudinal cross-sectional view of the assembly shown in FIG. 1, with the lower set of shear pins sheared and the mandrel shifted to actuate wash ports;

FIG. 3 is a longitudinal cross sectional view of the assembly shown in FIGS. 1 and 2, with the upper set of shear pins sheared and the upper shear collar separated from the mandrel to reveal the mandrel fishneck.

DETAILED DESCRIPTION

Generally, the present invention provides a downhole shear assembly for actuating a work string tool or process. Specifically, the device incorporates dampening means for minimizing post-shear inertial force within the work string and related equipment. Minimizing the post-shear inertial force prevents damage or premature actuation of sensitive components within the work string, and permits more precise control of forces applied to work string tubing and equipment. This additional degree of precision in force application permits an additional shear point to be incorporated within the assembly for actuation of a second process or function, the second shear mechanism having a higher actuation force threshold. In such dual shear devices, a first shear sleeve may be designated for initiation of a downhole process or to actuate a downhole tool, while the second shear sleeve (set to a higher force threshold) may be assigned to initiate a second possibly independent, process. For example, the first shear collar may hold a wash port closed, while the second, higher threshold shear collar may secure a bottomhole assembly to the tubing string. With reference to the Figures, such an assembly is described below by way of example.

The following definitions are provided for the purpose of understanding specific terms within the present specification.

“Work string” as mentioned herein refers to any means of conveying equipment downhole, such as coiled tubing, jointed pipe, wireline, slickline, etc. that is lowered into a well to deliver downhole equipment (known as a bottom hole assembly) to an appropriate depth. For clarity, work string refers to such lowering means with or without attached downhole equipment.

“Bottom hole assembly” refers to any downhole tool, equipment, or equipment assembly lowered downhole by or as part of a work string, for example a fracturing bottom hole assembly or completion assembly, including packers, tubing anchors, or a variety of plugs.

Overview

In FIG. 1, a dual shear device 10 is shown for use within a work string. An upper shear sleeve 30 may be connected to a work string such as a tubing string, while lower shear sleeve 40 may be connected to a bottom hole assembly or other downhole equipment. Both shear sleeves 30, 40 are slideably disposed over an inner mandrel 20. The basic operation of the device can be readily appreciated when viewed as three slideably associated functional portions (as referenced in FIG. 1):

A) inner mandrel 20, with fishneck 23, mandrel extension 21 and pistons 22;

B) upper shear sleeve 30, which telescopes over fishneck 23 of the inner mandrel 20, and is temporarily held in fixed position against the inner mandrel 20 by upper shear pins 31; and

C) lower shear sleeve 40, with connected hydraulic chamber subs 41 and hydraulic cylinder cap 42, which together slide back and forth a given distance over the remainder of the inner mandrel 20. As shown in FIG. 1, lower shear pins 43 extend through the lower shear sleeve 40 and mandrel extension 21 to fix the position of the lower shear sleeve and related components with respect to the inner mandrel 20.

Notably, in this position the pistons 22 are nearest a first end (the lower end) of hydraulic chambers 44; the hydraulic cylinder cap 42 abuts the lower facing surface of the upper shear sleeve 30; and mandrel extension 21 closes wash ports 45 within the lower shear sleeve. It should also be noted that in the present example, the lower shear pins 43 are fewer in number than the upper shear pins 31, and are intended to shear at a lower force threshold than upper shear pins 31.

Application of a first threshold pulling force to the work string from surface will cause lower shear pins 43 to break, releasing the mandrel and related components from the aforementioned fixed position.

However, the available sliding distance of the mandrel within the shear sleeve is limited by contact of the abutment portion 21a of the mandrel extension 21 with the facing edge 41a of the lowermost hydraulic chamber sub 41, as shown in FIG. 2. Such contact could potentially cause a great jarring force within the work string as these components accelerate towards each other following shear of the lower shear pins 43. The resulting force of abutment would, in some circumstances, prematurely shear the upper shear pins 31. A hydraulic dampening subassembly C, particularly element 22, 42 and 44 is therefore present to reduce this acceleration and resulting inertial force, limiting the force of abutment to an acceptable amount (less than the threshold to shear the upper shear pins 31). Thus, the first and second shear mechanisms may be in close proximity along the work string, and the differential between the two shear thresholds can be minimized such that both thresholds are greater than the normal downhole operating range, while still being within the range of available pulling force from surface.

Lower shear pins 43 may therefore be sheared when necessary by the operator to actuate a first work string function or process. In the presently described embodiment, this first work string function is the opening of wash ports 45, as shown in FIG. 2.

Application of a further and higher threshold pulling force to the work string will cause upper shear pins 31 to be broken, actuating a second work string function or process. In the presently described embodiment, this second work string function is the separation of the downhole equipment from the work string to expose the fishneck 23 of the inner mandrel 20, as shown in FIG. 3.

Inner Mandrel and Related Components

The inner mandrel 20 shown in the Figures is of the type typically used in the field. That is, the inner mandrel is generally a steel cylinder with a fishneck 23 formed at one end to facilitate fishing of the downhole equipment from the wellbore should this become necessary. Recesses are formed within the mandrel at appropriate locations to receive the shear pins.

A mandrel extension 21 is threaded to the opposing end of the inner mandrel. An abutment portion 21a of the mandrel extension is of greater outer diameter so as to slide within a corresponding mandrel guide 49 formed within the lower shear sleeve. The mandrel guide 49 and abutment portion 21a therefore maintain the inner mandrel 20 and related components (A) in association with the lower shear sleeve and related components (C) even when lower shear pins 43 have been broken. Accordingly, should the upper shear sleeve 30 become disengaged from the inner mandrel 20 by shearing of the upper shear pins 31, the downhole equipment attached to the lower shear sleeve 40 may be removed from downhole by pulling the fishneck 23 to surface using known retrieval equipment and methods.

The length of the mandrel extension is appropriate to cover wash ports 45 within the lower shear sleeve when the abutment portion 21a is at the lowermost end of the mandrel guide 49, and to reveal wash ports 45 when the abutment portion 21a is at the uppermost end of the mandrel guide 49.

With reference to the embodiment shown in the Figures, three annular pistons 22 may be fastened to the inner mandrel 20 at spaced apart locations along its length.

The mandrel and mandrel extension include recesses for alignment with corresponding apertures within the upper and lower shear sleeves, respectively. Once aligned, shear pins 31, 43, may be inserted there within to fix the shear assembly to both mandrel and mandrel extension in appropriate configuration for use within a work string.

Upper Shear Sleeve

The upper shear sleeve 30 is machined from a steel cylinder, which telescopes or slides onto and over the fishneck 23 of the inner mandrel 20 and includes shear pin apertures for alignment with corresponding apertures/recesses in the mandrel 20. Depth guide 32 assists in this alignment, permitting the appropriate degree of telescoping over the mandrel 20.

Lower Shear Sleeve

The lower shear sleeve 40 is machined from a steel cylinder so as to be threaded at one end for attachment to a hydraulic chamber housing 41, while the opposing end is adapted to connect to downhole equipment, for example a bottom hole assembly. The lower shear sleeve further provides a mandrel guide 49 to accommodate abutment portion 21a of the mandrel extension 21 to limit sliding movement and maintain the attachment of these components.

Wash ports 45 are formed within the lower portion of the sleeve 30 as shown, and shear pin apertures are formed to align with corresponding recesses/apertures in the mandrel extension. The lower shear sleeve further includes a depth guide 46 to assist in this alignment.

Dampening Subassembly

The damping subassembly reduces the post-shear acceleration of the mandrel due to release of tension within the coiled tubing. In embodiments having more than one set of shear pins, the damping subassembly limits the amount of inertial force generated following shearing a first set of shear pins, to a value that is less than the force required to shear a second set of shear pins.

In a suitable embodiment, a hydraulic dampening subassembly provides the appropriate degree of dampening as above described. In the specific embodiment shown in the Figures, the subassembly includes three hydraulic chamber housings 44 filled with hydraulic fluid, and a piston 22 for movement within each chamber 44 to control the acceleration of the mandrel with respect to the lower shear sleeve 40.

Hydraulic Chamber Housing—In the embodiment shown in the figures, the hydraulic chamber housings 41 are made as a cylinder of appropriate inner and outer diameter so as to provide a continuous outer diameter to the shear assembly and to telescope over the inner mandrel 20. The hydraulic chamber housings 41 are formed with threadable ends for connection in series above the lower shear collar 40. Each housing 41 provides one hydraulic chamber 44 about one piston 22 that has previously been fixed to the mandrel 20. A hydraulic chamber cap 42 closes the uppermost hydraulic chamber. Once assembled, the hydraulic chamber housings 41 slide as a unit (C) with the lower shear sleeve 40 over the pistons 22 and inner mandrel 20 (A).

The size of each hydraulic chamber 44 is sufficient to allow movement of the pistons there within as well as to house hydraulic fluid, which will be displaced from one side of each piston to the other within the housing upon shearing of the first set of shear pins 43. Seals are placed at appropriate locations to seal the hydraulic chambers against fluid leakage.

The number of hydraulic chambers included in the shear assembly is dependent upon the burst pressure of each chamber, and upon the shear threshold. That is, when the threshold force is applied to the work string at the shear assembly, the shear pins will break, causing pressure to build within each hydraulic cylinder as the pistons are stroked through the hydraulic fluid. An appropriate number of hydraulic cylinders should be present to avoid exceeding the burst pressure of the walls of each individual hydraulic cylinder. Accordingly, the configuration of the piston and chamber, as well as the selection of an appropriate hydraulic fluid should be designed based on the shear threshold and the desired degree of force dissipation, while the resulting burst pressure of each chamber will determine the number of chambers required.

Pistons—The piston rings are attached to the mandrel, for example by threads or using snap rings. Fluid flow is permitted past the piston by an aperture through the piston from its upper surface to its lower surface. A seal is provided between the inner surface of the piston and the mandrel on one side of the aperture, and between the outer surface of the piston and the inner surface of the hydraulic chamber housing on the opposing side of the aperture.

In the embodiment shown, one aperture is present in each piston. In other embodiments, it may be desirable to add further similar apertures about the circumference of the piston ring or to otherwise change the size or configuration of the flowpath to adjust the available flow area past the piston.

The aperture may include a check valve and/or choke in suitable embodiments for greater control over the flow within the hydraulic chamber.

The speed at which the pistons are able to move through the hydraulic fluid in response to a given degree of axial force will determine the degree of dampening. When considering the degree of dampening, it should be noted that upon application of a threshold axial force to the work string, the sheared element will accelerate from the remainder of the assembly. While the tension in the coiled tubing will accelerate the mandrel upwards, the lower shear sleeve 40, is fixed downhole. Mandrel extension 21a forcibly contacts the lower edge 41a of the lowermost hydraulic chamber housing 41, and the degree to which this multiplied force will be dampened or absorbed by the dampening subassembly prior to this forcible contact is determined by the equipment's ability to slow movement of the mandrel, i.e. by slowing the movement of the pistons through the hydraulic chambers. Accordingly, the available flow area cross section of the aperture of hydraulic fluid past the piston and the composition of the hydraulic fluid within the chambers will determine the inertial force dissipated and that ultimately imparted to the work string.

Hydraulic Fluid—A suitable hydraulic fluid is introduced into each hydraulic cross section of each piston aperture (not shown) and each chamber 44, having a viscosity appropriate to provide the desired dampening effect. Such considerations will be known to those skilled in the art.

Parameters

In shallow gas well fracturing operations, for example, typical shear subs for use in disengaging downhole equipment from the work string are set to shear at axial threshold forces less than 50,000 pounds force. However it is generally not practical to reduce this initial shear threshold substantially, in this example as normal operating work string tensions may be within the range of 30-40,000 pounds, and may incidentally exceed the shear force when the string is in use, which may cause premature shearing. In different formations or in different operational situations these axial forces may be different. In accordance with the described embodiment, it is similarly advisable that threshold shear forces be set well above the range of normal operating work string tensions.

Conversely, the maximum force that can be exerted upon a work string is limited by the equipment at surface, depending also upon the depth and deviation of the wellbore. For example, in a deviated well, drag of the work string against the deviated walls will significantly reduce the degree of force that is ultimately applied at the bottomhole assembly, as the top-hole equipment and the work string each have force limits by virtue of their design and materials. Reductions in force transmittal are also expected with increasing well depths. Accordingly, the shear thresholds should be set with these upper limiting factors in mind. Such considerations may be readily made by calculations known to those skilled in the art.

In certain embodiments, it may be possible to incorporate three or more independently actuated tension shear mechanisms within one work string, with appropriate nested or sequential dampening subassemblies to prevent premature initiation of successive shear-actuable functions.

Operation

When operating a fracturing bottom hole assembly, the assembly may become lodged within the wellbore due to a build-up of debris above the upper packer. When the dual shear assembly shown in the Figures is integrated within a work string above the bottom hole assembly, a pulling force may be applied to the work string from surface to generate a first threshold axial force at the shear assembly. This will cause the lower set of shear pins 43 to break, and the mandrel 20 will slide upward with respect to the lower shear sleeve 40 until the mandrel extension 21a contacts the lowermost hydraulic chamber housing 41. The force of this contact is dampened by the hydraulic dampening subassembly (including pistons 22 and hydraulic chamber housings 41), which slows the momentum of the mandrel with respect to the lower shear sleeve and bottom hole assembly prior to impact.

Once the sliding movement has been completed, wash ports 45 will be open, and fluid may be pumped from surface through the work string to the wash ports 45. Fluid exiting the wash ports 45 can then circulate in the annulus between the work string and the wellbore above the wash points and the bottom hole assembly. Should this circulation be sufficient to dislodge the bottomhole assembly (which includes the upper packers in the frac operation), the work string (including bottom hole assembly) may be pulled to surface. Conversely, should the circulation be insufficient to release the bottomhole assembly, application of further pulling force to the work string from surface will generate a second above-threshold pulling force at the top or second shear assembly. Thus, the upper shear pins 31 will be broken and the inner mandrel 20 will be released from the upper shear collar, exposing the fishneck 23 and releasing the work string from the bottom hole assembly (at and below the fishneck). The work string (without bottom hole assembly) is pulled to surface and the bottom hole assembly may be retrieved in the known manner by engaging and pulling on the fishneck 23.

Other Embodiments

As shown in the Figures, the multiple shear sub system is provided as a unit. It is contemplated that various components of the system may be provided as modules. For example, the mandrel may be segmented, allowing extension as necessary. Further, the mandrel and hydraulic chamber units may be provided together as a module, with one or more units threaded together in series. In further embodiments, the function and orientation of the shear assemblies may be reversed or substituted; or downward force may be applied to the work string to shear the shear pins, rather than a pulling force from surface, with appropriate configuration changes to the dampers and pins, etc.

While the above-described specific embodiments are provided to illustrate the invention, the present shear assembly and dampening system may be applied to various fields and functions. Notably, it has been stated above that the shear assembly may be used to actuate various downhole tools or processes. Such tools and processes may include opening or closing of wash ports, tool release, releasing a ball or other actuating device downhole, opening or closing of frac ports, detonation of a perforating charge, applying a cleaning, fracturing, acidizing or other treatment fluid, or setting of packers, among other possibilities.

It is further noted that shear pins provide only one suitable setting and release means in accordance with an embodiment of the invention. However, any other suitable setting and release means may be used to set the fixed position of the sliding components about a downhole mandrel, and to subsequently effect a dampened release and a second or subsequent release.

As the desired threshold longitudinal force for shearing any set of shear pins depends on the specific downhole application and configuration, the degree of dampening required will also vary. Given the disclosure provided, it is assumed to be within the ability of a person having ordinary skill in the art to adapt the dampening system of the presently described embodiment as necessary for use in a variety of downhole applications without further innovation or under experimentation. Further, with respect to dampening subassemblies, alternate dampening subassemblies may be used and may be preferred in certain embodiments. For example, the recoil force may be absorbed by a series of rubber or otherwise cushioning elements, foam or otherwise compressible material, tapered deformable channels, detent mechanisms, pressurized air chambers, burst discs, springs, or different fluids, etc.

The above-described embodiments of the present invention are intended to be examples only. Alterations, modifications and variations may be effected to the particular embodiments by those of skill in the art without departing from the scope of the invention, which is defined by the claims appended hereto.

Claims

1. A shear assembly for use downhole, the shear assembly comprising:

a. a mandrel for integration within a work string;

b. a shear sleeve slideable with respect to the mandrel to actuate a work string function;

c. setting means for setting the shear sleeve in a fixed position with respect to the mandrel, the setting means shearable by application of a threshold force to the work string, thereby actuating the work string function; and

d. a dampening subassembly operatively associated with the mandrel and with the shear sleeve for slowing the sliding momentum of the shear sleeve with respect to the mandrel following shearing of the setting means.

2. A multiple shear assembly for use downhole, the shear assembly comprising:

a. a mandrel for integration within a work string;

b. a first shear sleeve slideable with respect to the mandrel to actuate a first work string function;

c. first setting means for setting the first shear sleeve in a fixed position with respect to the mandrel, the first setting means shearable by application of a first threshold force to the work string, thereby actuating the first work string function;

d. a second shear sleeve slideable with respect to the mandrel to actuate a second work string function;

e. second shearable setting means for setting the second shear sleeve in a fixed position with respect to the mandrel, the second setting means shearable by application of a second threshold force to the work string, thereby actuating the second work string function; and

f. a dampening subassembly operatively associated with the mandrel and with the first shear sleeve for slowing the sliding momentum of the mandrel with respect to the first shear sleeve following shearing of the first setting means.

3. The assembly as in claim 2 wherein the first work string function is actuated upon abutment of a mandrel surface with a shear sleeve surface, and wherein the dampening subassembly slows the sliding momentum of the mandrel to limit the force of said abutment.

4. The assembly as in claim 3 wherein the force of abutment is limited to a value less than the second threshold force.

5. The assembly as in claim 2, wherein the dampening subassembly is a hydraulic dampening subassembly.

6. The assembly as in claim 5, wherein the hydraulic dampening subassembly comprises one or more annular pistons disposed within hydraulic chambers operatively attached to the mandrel.

7. The assembly as in claim 6 wherein the hydraulic chambers are filled with a hydraulic fluid of suitable viscosity to provide the desired degree of dampening.

8. The assembly as in claim 3 wherein the dampening subassembly reduces the abutment force by at least, 10,000 pounds force.

9. The assembly as in claim 2 wherein the second threshold force exceeds the first threshold force by more than 15%.

10. The assembly as in claim 2, wherein the work string includes a frac packer, and wherein the first threshold force is at least 40,000 pounds force.

11. The assembly as in claim 10 wherein the second threshold force is less than 96000 pounds.

12. The assembly as in claim 10 wherein the second threshold force is greater than 56000 pounds.

13. The assembly as in claim 2 wherein the first work string function is opening of a wash port within the work string.

14. The assembly as in claim 2 wherein the second work string function is shearing of the mandrel from the work string.

15. The assembly as in claim 14, wherein said shearing of the mandrel from the work string exposes a fishneck for use in recovery of the mandrel from the wellbore.

16. A method for returning a work string to surface when a downhole tool within the work string has become lodged within the wellbore, the method comprising the steps of:

a. opening a wash port within the work string above the lodged tool;

b. circulating fluid within the work string and the wellbore above the lodged tool to loosen debris from above the lodged tool; and

c. pulling the work string and tool to surface.

17. The method as in claim 16, further comprising the step of shearing a first shearable setting means within the work string to open the wash port.

18. The method as in claim 16, further comprising the step of shearing the lodged tool from the work string.