CN111322024A

CN111322024A - Darts for setting frac plugs ahead of time and related methods

Info

Publication number: CN111322024A
Application number: CN201911283770.0A
Authority: CN
Inventors: 詹姆斯·金; 克林特·米奇; 布鲁克·吉利斯; 帕特里克·威廉森
Original assignee: Ncsm Co ltd; Repetitive Precision Co ltd
Current assignee: Ncsm Co ltd; Repetitive Precision Co ltd
Priority date: 2018-12-13
Filing date: 2019-12-13
Publication date: 2020-06-23
Also published as: CA3065272A1; US20200190928A1

Abstract

A rescue dart is provided for deployment down a wellbore to dislodge a pre-set fracture plug having a plug mandrel from within the wellbore. The rescue dart includes: a dart adapted to be conveyed down a wellbore via a fluid flow; and a mandrel deactivator connected to the dart body and operable to reduce the structural integrity of a portion of the plug mandrel until a structural failure of the plug mandrel occurs and the fracture plug is released and ejected. A corresponding system is also provided. The system includes a rescue dart and a pump for pumping fluid down the wellbore. A method of using the rescue dart to dislodge a previously set fracture plug is also provided.

Description

Darts for setting frac plugs ahead of time and related methods

Technical Field

The technical field relates generally to frac plugs and more particularly to an apparatus and method for driving out frac plugs previously set in a wellbore.

Background

In a multi-stage fracturing operation, frac plugs may be deployed down the well in order to isolate different well sections that are subjected to a "plug and perforation" treatment and then fractured stage by stage. However, as the frac plug travels into the well, it may become pre-set at a location upstream of its target location. The pre-set frac plug must be removed before operation can resume. Known methods of dislodging a pre-set fracture plug are expensive and typically require moving a rig at the surface to drill the plug out of the wellbore.

Thus, there is a need for a technique that overcomes at least some of the disadvantages known in the art.

Disclosure of Invention

According to a first aspect, a rescue dart is provided for deployment down a wellbore to dislodge a previously set fracture plug comprising a plug mandrel from within the wellbore. The rescue dart includes a dart body adapted to be delivered down the wellbore to a pre-set fracture plug via a fluid flow. The rescue dart also includes a mandrel deactivator connected to the dart body and operable to reduce the structural integrity of a portion of the plug mandrel until a structural failure of the plug mandrel occurs and loosens and dislodges the fracture plug.

According to a possible embodiment, the rescue dart is shaped and configured to operate in an engaged position in which the dart body engages the pre-set frac plug and extends at least partially within the axial passage of the plug mandrel to position the mandrel deactivator at a predetermined position along the axial passage.

According to a possible embodiment, the dart comprises a central section, a dart head and a dart tail, the dart head being disposed at a first end of the central section and the dart tail being disposed at a second end of the central section opposite the dart head, and wherein, when in the engaged position, the dart tail abuts the pre-set fracture plug to prevent further forward movement of the rescue dart and to position the dart body relative to the pre-set fracture plug.

According to a possible embodiment, the dart tail is adapted to be seated on a ball seat of a pre-set frac plug.

According to a possible embodiment, the dart tail comprises rollers positioned and configured to engage the pre-set frac plug to allow the rescue dart to move rotationally.

According to a possible embodiment, the dart tail comprises a wiper fin for sweeping debris downhole as the rescue dart travels along the wellbore.

According to a possible embodiment, the wiper fin has an engagement surface for engaging an inner surface of the wellbore to guide the rescue dart along the wellbore.

According to a possible embodiment, the dart tail comprises at least one fluid channel for allowing fluid to flow through the dart tail and into the axial channel of the plug spindle when the rescue dart is in the engaged position.

According to a possible embodiment, the central section is a central rod dimensioned and configured to allow fluid to flow around it and through the axial channel when in the engaged position.

According to a possible embodiment, the mandrel deactivator is positioned and configured to form a restricted passage for restricting fluid flow through the axial passage to at least partially cut a portion of the plug mandrel via fluid wear.

According to a possible embodiment, the mandrel deactivator extends radially and outwardly from the dart body to proximate the dart head so as to form a restricted passage.

According to a possible embodiment, the mandrel deactivator is tulip-shaped.

According to a possible embodiment, the mandrel deactivator is disk-shaped.

According to a possible embodiment, the mandrel deactivator is tapered.

According to a possible embodiment, the mandrel deactivator is made of a hardened material.

According to a possible embodiment, the hardness of the material of the mandrel deactivator is greater than the hardness of the material of the plug mandrel.

According to a possible embodiment, the central section is a central tube having an inner conduit in fluid communication with the at least one fluid channel, and the mandrel deactivator comprises one or more fluid outlets extending through the thickness of the dart to redirect the fluid flow towards the plug mandrel to at least partially cut a portion of the plug mandrel via the fluid jet.

According to a possible embodiment, the fluid outlet is angled such that fluid flowing therethrough causes the rescue dart to rotate within the axial channel.

According to a possible embodiment, the dart and the spindle deactivator are an integral unit.

According to a possible embodiment, the mandrel deactivator comprises at least one cutting blade operable between a retracted configuration in which the cutting blade can be introduced into the axial passage and an extended configuration for engaging and at least partially cutting a portion of the plug mandrel.

According to a possible embodiment, the spindle deactivator further comprises an actuator adapted to operate the cutting blade in the extended configuration.

According to a possible embodiment, the actuator is adapted to operate the cutting blade when the rescue dart is in the engaged position.

According to a possible embodiment, the actuator is adapted to operate the cutting blade via hydrostatic and/or hydraulic pressure.

According to a possible embodiment, the actuator is an electrically driven actuator or a battery powered actuator.

According to a possible embodiment, the mandrel deactivator comprises two or more cutting blades positioned radially around the dart body.

According to a possible embodiment, the cutting blade is adapted to radially cut the plug spindle.

According to a possible embodiment, the cutting blade is adapted to axially cut the plug spindle.

According to a possible embodiment, the dart body comprises an inner channel extending along the central section and tapering inwardly towards the dart head, and the actuator comprises a plunger engageable within the inner channel for pushing the cutting blade outwardly into the plunger spindle.

According to a possible embodiment, the dart tail defines an inner chamber, and the actuator comprises a release mechanism adapted to retain the plunger at least partially within the inner chamber when the rescue dart is not in the engaged position.

According to a possible embodiment, the release mechanism comprises a shear screw, and the actuator further comprises a plunger seat extending outwardly from the plunger and retained within the internal chamber by said shear screw, the shear screw being configured to allow the plunger seat to move forward once a pressure threshold is reached so as to engage the plunger within the internal channel.

According to a possible embodiment, the inner chamber has an upstream portion and a downstream portion defined on either side of the plunger seat, and a chamber inlet communicating with the upstream portion so as to allow a fluid to flow therein to exert pressure on the plunger seat.

According to a possible embodiment, the chamber inlet is substantially sealed to prevent fluid flooding the upstream portion before the rescue dart is in the engaged position.

According to a possible embodiment, the chamber inlet is sealed via a burst disk.

According to a possible embodiment, the chamber inlet is sealed via a second plunger extending from the plunger base opposite the plunger.

According to a possible embodiment, the mandrel deactivator is adapted to reduce the structural integrity of the portion of the plug mandrel via heat.

According to a possible embodiment, the central section and/or dart head has an internal volume and the spindle deactivator includes a heating material located within the volume and an actuator adapted to operate the heating material as a heating plug spindle.

According to a possible embodiment, the actuator comprises an igniter adapted to heat the heating material and a striker operable to activate the igniter.

According to a possible embodiment, the dart tail defines an inner chamber, and the actuator comprises a release mechanism adapted to hold the striker within the inner chamber when the rescue dart is not in the engaged position.

According to a possible embodiment, the release mechanism comprises a shear screw and the actuator further comprises a plunger seat on which the striker is positioned, the plunger seat being retained within the internal chamber by the shear screw, the shear screw being configured to release the plunger seat once a pressure threshold is reached so as to allow the plunger seat to move forward and engage the striker with the igniter.

According to a possible embodiment, the pre-set frac plug comprises an elastomeric element and the predetermined location of the mandrel deactivator is downstream of the elastomeric element.

According to a possible embodiment, the pre-set frac plug comprises a sliding member and/or a compression member, and the predetermined position of the mandrel deactivator is substantially aligned with the sliding member and/or the compression member.

According to a possible embodiment, the rescue dart further comprises a deployment mechanism connectable to the logging cable for running the rescue dart down through a wellhead of the wellbore.

According to a second aspect, a system for dislodging a pre-set fracture plug comprising a plug mandrel from within a well assembly is provided. The system comprises: a rescue dart configured to engage and dislodge a pre-set fracture plug; and a pump for providing a flow of fluid down the wellbore to run the rescue dart to the pre-set frac plug for engagement therewith.

According to a possible embodiment, the pump is located at the surface.

According to a possible embodiment, the system further comprises a logging cable detachably connectable to the rescue dart for running the rescue dart through the wellhead of the well assembly.

According to a possible embodiment, the rescue dart is run down the wellbore using only fluid flow and gravity.

According to a possible embodiment, the rescue dart is as described above.

According to a third aspect, a method of dislodging a pre-set fracture plug comprising a plug mandrel from within a wellbore is provided. The method comprises the following steps: deploying a rescue dart in the shaft; pumping a fluid within the wellbore to deliver a rescue dart toward the pre-set fracture plug via a fluid flow; engaging the rescue dart with the early set frac plug; and operating the rescue dart to reduce the structural integrity of a portion of the plug mandrel until a structural failure of the plug mandrel occurs and loosens and dislodges the fracture plug.

According to a possible embodiment, the method further comprises the following steps: pressure changes in the wellbore are monitored at the surface.

According to a possible embodiment, the method further comprises the following steps: the dislodged fracture plug is pumped further down the wellbore along with the rescue dart to allow the fracturing operation to resume.

According to a possible embodiment, the rescue dart is as described above.

Drawings

Fig. 1 is a perspective view of an example frac plug.

FIG. 2 is a partial cross-sectional view of the frac plug of FIG. 1 showing various components supported by the mandrel.

Fig. 3A is a side view of a rescue dart according to an embodiment, showing various portions of the rescue dart.

Fig. 3B is a rear view of the rescue dart showing a fluid passage provided in the tail of the rescue dart.

Fig. 4 is a side view of an embodiment of a rescue dart showing a wiper fin attached to the dart tail.

Fig. 5 is a cross-sectional side view of the rescue dart of fig. 3A engaged with a frac plug, showing a mandrel deactivator redirecting fluid flow toward the plug mandrel.

Fig. 6 is a cross-sectional side view of an embodiment of a rescue dart engaged with a frac plug, showing an exit aimed at a plug mandrel.

Fig. 7A is a cross-sectional side view of an embodiment of a rescue dart engaged with a frac plug, showing a mandrel deactivator provided with a cutting blade in a retracted configuration.

Fig. 7B is a cross-sectional side view of an embodiment of a rescue dart engaged with a frac plug, showing a mandrel deactivator provided with a cutting blade in an extended configuration.

Fig. 8A is a cross-sectional side view of another embodiment of a rescue dart engaged with a frac plug, showing an interior chamber defined in the dart tail for housing an actuator.

FIG. 8B is a cross-sectional side view of the embodiment shown in FIG. 8A, showing the actuator operating the cutting blade to engage the plug spindle.

Fig. 9A is a cross-sectional side view of another embodiment of a rescue dart engaged with a frac plug showing a heating material engaged within an axial passage of a plug mandrel.

FIG. 9B is a cross-sectional side view of the embodiment shown in FIG. 9A, showing an actuator operating the heating material as a heater plug mandrel.

Fig. 10 is a schematic of a system including a rescue dart and a pump, showing the rescue dart traveling within a wellbore toward a frac plug.

Fig. 11 is a cross-sectional side view of another embodiment of a rescue dart engaged with a frac plug, showing a dart head provided with a fuel chamber for receiving fuel to be released within a passage of a plug mandrel.

Fig. 12 is a schematic diagram of a system including a rescue dart mounted to a wireline for injection into a wellbore.

Detailed Description

As will be described below with respect to various embodiments, a rescue dart is provided for dislodging a plug that has been previously set within a wellbore. It should be understood that, as used herein, the expression "eviction" and any variations thereof with respect to setting a fracture plug ahead of time may refer to an action that removes the plug from a site and/or location. It should also be understood that the expression "set in advance" refers to a state of the plug that has been set to an engaged or operational configuration prior to being at a desired location along the wellbore. Typically, premature setting occurs when the elastomeric element of the plug prematurely swells and engages the casing of the wellbore.

Further, the plugs referred to in the following disclosure are plugs typically used in connection with multi-stage fracturing operations, often referred to as "frac plugs". However, it is understood that the plug may be any other type of plug suitable for use in various well operations, such as, for example, a bridge plug. Traditionally, frac plugs used in multi-stage frac operations include a plug mandrel around which several components are connected in order to operate and position the plug. It will be appreciated that the rescue dart can be used to dislodge a pre-set fracture plug from various well components including vertical wells, horizontal wells, slant wells, and/or wells having various structural features such as casing and tubing.

Broadly described, in one embodiment, a rescue dart is configured to be conveyed down a wellbore in order to dislodge a pre-set fracture plug from within the wellbore. The rescue dart includes a dart body shaped and configured to be delivered downwardly via a flow of fluid. It should be understood that gravity may assist the flow of fluid in carrying the rescue dart down the wellbore. In addition, the rescue dart includes a mandrel deactivator that is connected to the dart body and is operable to loosen and dislodge the pre-set fracture plug in a manner to be described below.

Referring to fig. 1 and 2, an example frac plug 1 is illustrated. As described above, the frac plug 1 includes a plug mandrel 3 that supports a plurality of plug members. In this example, the frac plug 1 includes an elastomeric element 5 mounted between a pair of compression members 7a-b, which in turn are disposed between a pair of sliding members 9 a-b. It is understood that the frac plug 1 may be operated between an operating (or set) configuration and a loose configuration. It will be appreciated that in the set configuration, the compression members 7a-b apply pressure on either side of the elastomeric element 5 which causes it to extend radially and outwardly from the plug mandrel 3 so as to engage the inner surface of the wellbore to locate the frac plug 1 along the wellbore. The plug mandrel 3 illustratively has a central bore defining an axial passage 4 extending along the length of the plug mandrel 3 so as to allow fluid flow therethrough. The plug mandrel 3 may also include a ball seat 6 defined at its uphole end for receiving a ball thereon for sealing the axial passage.

Referring to fig. 3A to 5, a rescue dart 10 is shown according to a possible embodiment. As previously described, the rescue dart 10 is configured to be conveyed down the wellbore and carried therealong until it reaches and engages the pre-set frac plug 1. More specifically, the rescue dart 10 has a substantially elongate dart body 12 shaped and configured to be conveyed down the wellbore via a fluid flow. Thus, preferably the rescue dart is delivered as a free unit that is not connected to a mechanical deployment structure such as a wireline or coiled tubing. In some embodiments, dart body 12 can be made of a material that is substantially easily ground/milled out of the wellbore (such as phenolic, composite fiberglass, cast iron, or sintered metal) or a substantially decomposed material (such as aluminum and/or magnesium).

The rescue dart 10 includes a mandrel deactivator 14 connected to the dart body 12 and operable to reduce the structural integrity of the plug mandrel 3 until a structural failure of the plug mandrel 3 occurs, which effectively loosens and dislodges the frac plug 1 from within the wellbore. As will be explained below with respect to various embodiments, the mandrel deactivator 14 may operate via hydrostatic pressure, hydraulic pressure, electrical power, or a combination thereof, depending on the nature and mode of operation of the mandrel deactivator. However, it will be appreciated that the mandrel deactivator 14 may alternatively be operated via any other suitable method.

The rescue dart 10 is configured to operate in an engaged position where the dart body engages with the pre-set frac plug 1. For example, when in the engaged position, dart 12 extends at least partially within axial passage 4 of plug spindle 3 to position spindle deactivator 14 at a predetermined location along axial passage 4 (fig. 5). It can thus be seen that the mandrel deactivator 14 may be operated to reduce the structural integrity of the plug mandrel 3 at a particular or desired location along the axial passage 4. In some embodiments, the predetermined location substantially corresponds to a portion of the plug mandrel 3 that is downstream/downhole of the elastomeric element 5 of the pre-set frac plug 1. More specifically, the predetermined position may be aligned with the sliding member 9b and/or the compression member 7b of the pre-set frac plug. It follows that once a structural failure of the plug mandrel 3 occurs close to the predetermined location, the pre-set frac plug should be loosened and dislodged from the wellbore. It will be appreciated that the predetermined location may alternatively be at any suitable location along the axial passage 4 and may be determined based on the particular structure and arrangement of the plugs to be ejected.

In some embodiments, dart body 12 includes a central section 16, a dart head 18, and a dart tail 20. As seen in fig. 3A, dart 18 is disposed proximate a first end of central section 16, and dart 20 is disposed proximate a second end of the central section opposite dart 18. However, it is understood that other configurations of dart body 12 are possible. As best seen in fig. 5, the dart head 18 is shaped and configured to be inserted into the axial passage 4 of the plug spindle 3 while the rescue dart 10 is in the engaged position. The dart tail 20 is shaped and configured to engage/seat against the pre-set frac plug 1 to prevent further forward movement of the rescue dart 10. It follows that the dart 12 can be arranged in a desired position relative to the pre-set frac plug 1. It will be appreciated that the desired location may vary depending on the design of the frac plug 1 and/or the method used to operate the mandrel deactivator 14.

Still referring to fig. 3A-5, the dart tail 20 may be adapted to engage the uphole end of the frac plug 1 in order to prevent the rescue dart 10 from moving forward once engaged with the frac plug. For example, the dart tail 20 may be shaped to cooperate with the ball seat 6 of the plug mandrel 3. More specifically, the dart tail 20 can extend outward (radially and/or at an angle) from the central section 16 such that it cannot fully enter the axial passage 4 of the plug spindle 3. However, it is understood that other configurations are possible, such as having dart 18 abut against a portion of the downhole end of frac plug 1, for example. In some embodiments, the dart tail 20 may be provided with a fluid passage 26 for allowing fluid to flow through the dart tail 20 and through the axial passage 4 of the plug spindle 3 when the rescue dart 10 is in the engaged position. In the illustrated embodiment, dart 20 includes two fluid channels 26, which may have a semi-circular shape, but it is understood that any other suitable number and shape of fluid channels 26 are possible. Depending on the configuration of the tail 20, the fluid passage can be defined as a closed passage (i.e., defined only by the structure of the dart 20) (as in fig. 3B) or an open passage (i.e., defined by the combination of the structure of the plug shaft 3 and the dart 20).

Referring more particularly to fig. 4, dart tail 20 can include one or more wiper fins 22 connected thereto and extending therefrom. The wiper fin 22 is shaped and configured to sweep and/or drag debris downhole as the rescue dart 10 travels further down the wellbore. Thus, it will be appreciated that once the previously set frac plug has been dislodged, the wiper fin 22 may help to further drag debris generated by the collapsed frac plug down the wellbore, thereby allowing a new frac plug to be installed in a desired location down the wellbore. In some embodiments, dart tail 20 includes a single wiper fin 22 that extends outward therefrom on all sides (e.g., about 360 degrees) so as to substantially cover the cross-sectional area of the wellbore to drag debris down the wellbore. Optionally, the dart 20 can include a plurality of wiper fins 22, such as those of fig. 11, that extend in different directions from the dart 20 and cooperate with one another to substantially cover the cross-sectional area of the wellbore.

In some embodiments, the wiper fins 22 may each include an engagement surface 24 for engaging an inner surface of the wellbore to help guide the rescue dart 10 as it is carried down the wellbore. It can thus be seen that dart body 12 can remain substantially aligned with the central axis of the wellbore in order to facilitate at least partial entry of dart body 12 into plug mandrel 3. In some embodiments, wiper fin 22 is made of a flexible material, such as, for example, nitrile rubber, urethane, or foam, although it is understood that other materials are possible.

In some embodiments, the rescue dart 10 may include a deployment mechanism 25 connectable to a wireline cable (not shown) configured to run the rescue dart 10 down through the wellhead of the well assembly. More specifically, the deployment mechanism 25 may be releasably connected to a wireline cable to run the rescue dart 10 down the wellhead and/or along a portion of the wellbore before releasing it to allow a flow of fluid to carry the rescue dart 10 toward the frac plug. However, as noted above, the preferred method of delivering the rescue dart is to use a fluid stream to carry it as a free body.

In various exemplary embodiments of the rescue dart 10, the mandrel deactivator 14 may be configured as a fluid-based system for accelerating the fluid to impact the plug mandrel 3 at high velocity to initiate erosion. As will be described below, the fluid-based mandrel deactivator is shaped and configured to restrict fluid flow through the axial passage 4 of the plug mandrel 3, which effectively increases the velocity of the fluid and redirects the accelerated flow to the plug mandrel 3. In some embodiments, the mandrel deactivator 14 is made of a hardened material (e.g., steel, brass, aluminum, ceramic, cast iron, tungsten carbide, cobalt alloys, degradable metal composites, etc.) in order to prevent or reduce wear caused by fluids pumped down the wellbore. It will be appreciated that the hardness of the material of the mandrel deactivator 14 is preferably greater than the hardness of the material of the plug mandrel 3 to prevent structural failure of the rescue dart 10 before structural failure of the plug mandrel 3.

Referring back to fig. 3A-5, in some embodiments, the central section 16 may be a center rod 28 adapted to extend within the axial passage 4 of the plug mandrel 3 when the rescue dart 10 is in the engaged position. As seen in fig. 5, the central rod 28 is shaped and configured to allow fluid pumped downhole to flow therearound and through the axial passage 4, i.e. the central rod 28 has a smaller cross-sectional area than the axial passage 4. The mandrel deactivator 14 may be positioned and configured to form a narrow passage 30 for restricting fluid flow through the axial passage 4 such that the velocity of the fluid flow is increased. The mandrel deactivator 14 may also be shaped and configured to redirect the accelerated fluid flow or jet toward the plug mandrel 3 so as to at least partially cut an area or portion of the plug mandrel 3 via fluid wear or fluid jet. It should be understood that the expressions "fluid abrasion" and "fluid jet" refer to the use of a jet or stream of fluid (such as water) to cut through various materials. Fluid ejection is typically accomplished by using pressurized high velocity jets of water or a combination of water and abrasive, but it is understood that other fluids or combinations thereof are possible.

In this embodiment, the mandrel deactivator 14 is positioned proximate the dart 18 in order to redirect and accelerate the fluid flowing around the central rod 28, but other configurations are possible. In some embodiments, the mandrel deactivator 14 may be tulip shaped to simultaneously redirect fluid flow and restrict the axial passage 4. However, it is understood that the mandrel deactivator 14 may have any suitable shape or combination of shapes, such as, for example, a conical shape and/or a disc shape. It is also understood that the mandrel deactivator 14 may extend from the central rod 28 at two or more locations to reduce the structural integrity of the plug mandrel 3 at multiple locations.

Referring now to fig. 6, another embodiment of a rescue dart 10 is shown. In this embodiment, the central section 16 includes a central tube 32 defining an inner conduit 34 extending therethrough. It is understood that inner conduit 34 of center tube 32 is in fluid communication with fluid passage 26 of dart tail 20 to permit fluid flow therein and toward dart head 18. In this embodiment, the mandrel deactivator 14 includes at least one fluid outlet 36 that extends through the thickness of the dart 18 so that fluid can be expelled towards the plug mandrel 3. The fluid outlet 36 is shaped and configured to restrict the flow of fluid therethrough so as to form a high velocity stream or jet for cutting the plug mandrel 3.

In this embodiment, the mandrel deactivator 14 includes a plurality of fluid outlets 36, the fluid outlets 36 being adapted to form respective jets that impinge on the plug mandrel 3 at several locations simultaneously. In addition, the fluid outlet 36 may be angled in such a way that a jet exiting the inner duct 32 via the outlet 36 causes the rescue dart 10 to rotate within the axial channel 4. It will thus be seen that the mandrel deactivator 14 may be adapted to reduce the structural integrity of the plug mandrel 3 by radially cutting the plug mandrel 3 via a rotating jet. In some embodiments, the dart tail 20 may be provided with rollers 37 adapted to seat against a surface (e.g., a ball seat) of the pre-set frac plug 1 to facilitate rotational movement of the rescue dart 10 within the axial passage 4.

Referring to fig. 7A through 8B, yet another embodiment of a rescue dart 10 is shown. In this embodiment, the mandrel deactivator 14 includes at least one cutting blade 38 adapted for insertion within the axial passage 4 and operable to extend outwardly to contact the plug mandrel 3 to at least partially mechanically cut it. In other words, the cutting blade 38 is operable between a retracted position (fig. 7A and 8A), in which the cutting blade 38 can be introduced into the axial passage 4 without contacting the plug spindle 3, and an extended position (fig. 7B and 8B) for engaging the plug spindle 3.

In some embodiments, mandrel deactivator 14 includes a plurality of cutting blades 38 positioned radially around dart body 12 such that operating cutting blades 38 cut plug mandrel 3 at a plurality of locations around the periphery of plug mandrel 3. It can be seen that the cutting blades 38 can be configured to cut the plug mandrel 3 at least radially, but it is understood that the cutting blades 38 can be adapted to cut the plug mandrel 3 in any suitable manner, such as axially, for example. In some embodiments, cutting blade 38 is fixedly attached to center section 16 and/or dart 18, but it is understood that other configurations are possible. Further, the mandrel deactivator 14 may be provided with a sleeve (not shown) adapted to cover and retain the cutting blade 38 in the retracted configuration to facilitate insertion of the mandrel deactivator 14 (i.e., the cutting blade 38 in this embodiment) into the axial passage 4.

In this embodiment, the mandrel deactivator 14 further includes an actuator 40 or actuation assembly adapted to operate the cutting blades 38 between the retracted and extended positions. The actuator 40 may be configured to operate the cutting blade 38 only when the rescue dart 10 is in the engaged position. It follows that an untimely protrusion of the cutting blade 38 can be prevented, which in turn will prevent the rescue dart 10 from effectively engaging the pre-set frac plug 1. As will be described in greater detail below, the actuator 40 may be adapted to operate the cutting blade 38 via hydrostatic pressure, hydraulic pressure, or a combination thereof. Alternatively, the actuator 40 may be an electrically powered (i.e., energy) actuator, or a battery powered actuator including, for example, a battery and a controller.

Still referring to fig. 7A-8B, the central section 16 may be provided with an inner passage 42, the inner passage 42 defining the substantially hollow central section 16. Further, inner channel 42 illustratively tapers inwardly toward dart 18 so that the tapered portion is substantially aligned with the position of cutting blade 38. The actuator 40 may include a plunger 44 shaped and configured to engage the inner channel 42 to push the cutting blades 38 outward and into the plug spindle 3. More specifically, plunger 44 can comprise a rod-like body shaped and dimensioned to extend within inner channel 42 and push against the tapered wall as it extends further toward dart 18. It is understood that central section 16 and/or dart 18 can be provided with slits extending axially therealong to allow each cutting blade 38 to be outwardly displaced independently of one another.

It should be understood that other cutting blade 38 configurations and/or methods of engaging the cutting blade 38 with the plug mandrel 3 are possible. For example, the cutting blades 38 may be independently housed within the central section 16 and displaceable in the radial direction (i.e., towards the plug mandrel 3). Thus, the cutting blade 38 may be shaped and configured to be pushed out of the central section 16 as the plunger 44 extends within the inner channel 44.

In this embodiment, the plunger 44 is positioned uphole of the center section 16 proximate the dart tail 20. In some embodiments, the actuator 40 may be adapted to hold the plunger 44 in a standby position before the rescue dart 10 engages the pre-set frac plug 1. More specifically, actuator 40 may include a release mechanism 46 configured to at least partially retain plunger 44 in an uphole position of inner channel 42 when rescue dart 10 is not in the engaged position. Release mechanism 46 is also configured to release plunger 44 once rescue dart 10 engages frac plug 1, as will be explained below. In the embodiment illustrated in fig. 7A-8B, release mechanism 46 includes a shear screw 48 that extends through the thickness of dart 20 and within plunger 44 to hold plunger 44 in a standby position proximate dart 20. The shear screw 48 may be configured to release to the plunger 44 once a pressure threshold is reached, which effectively allows the plunger 44 to move forward and engage the cutting blade 38, as described above.

In some embodiments, the dart tail 20 can be shaped and sized to define an internal chamber 50 for housing the actuator 40 while the rescue dart 10 flows down the wellbore to reach the frac plug. In this embodiment, the actuator 40 also includes a plunger seat 52 that extends radially and outwardly from the plunger 44 toward the wall of the internal chamber 50. As seen in fig. 7A-8B, shear screw 48 extends through dart tail 20 and extends within interior chamber 50 to retain plunger base 52 therein, which in turn keeps plunger 44 from entering inner channel 42.

As seen in fig. 7A and 7B, the dart tail 20 can have a single fluid channel 26 for allowing fluid flow within the interior chamber 50. The fluid passage 26 may be substantially sealed by the shear screw 48 holding the plunger seat 52 in that position. It can be seen that when the rescue dart 10 engages a pre-set frac plug, fluid flowing down the wellbore will accumulate upstream of the rescue dart 10 and push against the surface area of the plunger mount 52. In some embodiments, the plunger seat 52 may be provided with one or more O-rings 54 to improve the sealing of the plunger seat 52 to the fluid passage 26, which effectively increases the hydraulic pressure exerted on the plunger seat 52 by the flowing fluid. As the pressure increases, the shear screw 48 will eventually break, which effectively releases the plunger seat 52 and allows it to move forward to engage the plunger 44 within the inner channel 42.

Referring more particularly to fig. 8A-9A, in some embodiments, the internal chamber 50 of the dart tail 20 can include an upstream portion 50a and a downstream portion 50b defined on either side of the plunger base 52. In this embodiment, the pressure differential between the upstream portion 50a and the downstream portion 50b pushes the plunger seat 52 forward, thus engaging the plunger 44 within the internal passage 42. Additionally, fluid passage 26 of dart 20 can define a fluid inlet 56 that communicates with upstream portion 50a to allow fluid to flow therein. Once the fluid has filled the upstream portion 50a, the hydraulic and hydrostatic pressures will increase and push against the plunger seat 52 to assist in engaging the plunger 44 within the inner passage 42. Thus, it should be understood that in the present embodiment, the actuator 40 is adapted to operate the cutting blade 38 via a combination of hydrostatic and hydraulic pressures.

In some embodiments, the fluid inlet 56 may be substantially sealed to prevent fluid from flooding the upstream portion 50a before the rescue dart 10 has engaged the pre-set fracture plug. As seen in fig. 8A and 8B, fluid inlet 56 may be sealed via a second plunger 58 extending from plunger base 52 opposite plunger 44. The second plunger 58 may be shaped and configured to extend through the fluid inlet 56, which effectively seals the inlet and prevents fluid flow within the upstream portion 50 a. In some embodiments, the second plunger 58 may be provided with an O-ring 54 to increase the sealing of the fluid inlet 56. Thus, it will be appreciated that fluid pumped down the wellbore will initially push the exposed surface area of the second plunger 58 until it at least partially clears the fluid inlet 56. The fluid will then begin to flood the upstream portion 50a to push the plunger seat 52, as described above. It is understood that the fluid inlet 56 may be sealed using any suitable method and/or device, such as, for example, a burst disk 60 (fig. 8B) configured to rupture once a pressure threshold is reached.

Referring now to fig. 9A and 9B, yet another embodiment of a rescue dart 10 is shown. In this embodiment, the mandrel deactivator 14 is adapted to reduce the structural integrity of the plug mandrel 3 by applying and/or transferring heat. The plug mandrel 3 may be at least partially melted under the applied heat until structural failure occurs in order to dislodge the fracture plug. In an exemplary embodiment, center section 16 and/or dart 18 can have a common interior volume 62 for containing heating material 63. Also, the actuator 40 of the mandrel deactivator 14 may be adapted to cooperate with the heating material 63 to generate heat that heats the plug mandrel 3. More specifically, the actuator 40 may include an igniter 64 or firing mechanism adapted to heat the heating material 63, and a striker 66 operable to activate the igniter 64.

As seen in fig. 9A and 9B, actuator 40 may include a release mechanism 46, substantially identical to the release mechanism described previously, for retaining striker 66 within interior chamber 50 of dart tail 20 when rescue dart 10 is not in the engaged configuration. In other words, the striker 66 may be coupled to the plunger base 52 held in place by the shear screw 48. It can thus be seen that once the rescue dart 10 engages the pre-set frac plug, the fluid flow exerts pressure on the actuator 40, which breaks the shear screw 48 and allows the striker 66 to move toward the igniter 64 to heat the heating material 63.

Referring now to fig. 11, another embodiment of a rescue dart 10 is illustrated. In this embodiment, the common interior volume 62 of the central section 16 and/or dart 18 can be a fuel chamber 62, with fuel releasably packaged in the fuel chamber 62. Fuel chamber 62 may include any of the above-described mechanisms and/or components for containing fuel until such time as it needs to be released into the mandrel of plug 3, which effectively ignites and melts the components of plug 1 to release it. However, in the illustrated embodiment, the fuel is contained within a fuel chamber 62 using a burst disk 61 positioned at the distal end of the dart and a piston 53 (similar to the plunger base 52 of fig. 8 a-9 b described above) at the proximal end of the dart. The piston 53 may be held in the initial position via the shear pin/screw 48 until pressure is applied (e.g., via fluid flow) to break the shear pin 48 and allow the piston 53 to move forward, which effectively bursts the burst disk, thereby releasing fuel into the passage of the plug spindle 3. The fuel may be further sealed within the fuel chamber 62 using a sealing element (such as, for example, an O-ring) surrounding the piston 53 or any other suitable sealing element.

The fuel preferably burns upon contact with water so that when the burst disk 61 collapses under pressure and releases the fuel, the reaction occurs rapidly so that the energy released from ignition is maintained in the target area within the plug 1. In some embodiments, the fuel may be a solid rod of fuel material or powder packaged within the fuel chamber 62, although other configurations are possible. In this embodiment, the fuel is a mixture of sodium and lithium, which is adapted to react to melt the components of the plug 1 when contacted with water in the wellbore. The fuel may be used to melt the entire plug 1 or a target component, such as, for example, the sliding member 9. It is understood that melting certain components rather than the entire plug may require less fuel to be packaged within the fuel chamber 62.

Further, the rescue dart 10 may be provided with a metering device 68 adapted to at least partially control the rate at which fuel is injected into the passage 4 of the plug spindle 3 (i.e., the rate at which fuel is released from the fuel chamber 62). The metering device 68 can engage the dart 1 at an uphole position of the piston 53 within the central section 16 of the dart 53, which together with the dart defines an interior chamber 50 similar to that described with respect to the previous embodiments. Further, the metering device 68 may have an orifice or passage 69 that extends through the metering device to allow fluid to reach the interior chamber 50. Fluid flows through the passage 69, which causes pressure to build up in the internal chamber 50 until sufficient force is exerted on the shear pin 48 to release the piston 53.

The orifice and/or passage 69 of metering device 68 can be shaped and dimensioned to control the flow rate of fluid therethrough, thereby controlling the rate at which interior chamber 50 is filled and pressurized, which in turn provides some control over the rate at which piston 53 moves forward along dart 18. It will be appreciated that based on the rate at which the piston 53 moves within the dart 10, the injection rate of fuel within the plug passageway 4 may also be determined and/or at least approximately predicted. In some embodiments, the passage 69 may have a small diameter and/or have a tortuous configuration through the metering device 68 so as to reduce the flow rate of the fluid therethrough. Alternatively, the metering device 68 may be omitted and the fluid flow may be allowed to push directly against the piston 53 for injecting fuel into the plug 1. It should also be understood that other configurations, devices, components, and/or methods of controlling the various components passing through the rescue dart 10, such as the metering device 68, the interior chamber 50, and/or the piston 53, are possible and may be used.

It should be understood that other embodiments of the rescue dart may be used to dislodge a pre-set fracture plug from within the wellbore. For example, in some embodiments, a rescue dart may be shaped and configured to strike a frac plug in order to break at least a portion thereof to cause a structural failure. In this embodiment, chemicals pumped down with other fluids may be used to weaken the plug mandrel in order to facilitate breaking the frac plug using a ram rescue dart. A ram rescue dart may have a spherical dart head shaped and dimensioned to extend within a plug mandrel and contact the ball seat with such force as to break the ball seat and/or other components of the plug. Other embodiments may simply comprise a spherical dart or fracturing ball which falls into the wellbore and travels down the wellbore via a fluid stream to contact/impact the plug to break at least one component thereof to dislodge it. Various combinations of the foregoing embodiments are also possible for effectively dislodging a pre-set fracture plug, and each embodiment should not be construed as excluding features and/or characteristics of the other embodiments.

With broad reference to fig. 1-11, it is noted that in the above-described embodiments of the rescue dart 10, the dart body 12 or a component thereof and the spindle deactivator 14 or a component thereof may be formed as an integral unit (e.g., molded). For example, the embodiment of the rescue dart 10 shown in fig. 5 illustrates the dart body 12 and the spindle deactivator 14 being formed as an integral unit. Alternatively, the components of dart body 12 and/or mandrel deactivator 14 may be formed separately and connected to each other via any suitable method and/or fastener. For example, rescue dart 10 shown in fig. 7A illustrates a mandrel deactivator 14 including an actuator 40 that is separately formed and positioned within dart body 12.

It will be appreciated that various embodiments of the above-described rescue dart may be part of a system for dislodging a pre-set fracture plug from within a wellbore of a well assembly. As seen in fig. 10, the system may include a pump 70 configured to provide a flow of fluid down a wellbore 72 to carry the rescue dart 10 towards the frac plug 1. The fluid flow is also adapted to operate the rescue dart 10 once engaged with the frac plug. As described above, the hydraulic and/or hydrostatic pressures from the flowing fluid may operate the spindle deactivators of various embodiments of the rescue dart. In this embodiment, a pump is located at the surface for pumping fluid down the wellbore.

In some embodiments, the system includes a wireline or wireline assembly releasably connectable to the rescue dart for extending the rescue dart through a wellhead of the well assembly. It is understood that the wireline may be used to run the rescue dart down the first portion of the wellbore and allow the fluid flow to carry the rescue dart to the frac plug before releasing it. However, it should also be understood that the rescue dart may be run down the wellbore using only fluid flow or only a wireline. Fig. 12 shows a diagram of a possible embodiment of a rescue dart 10 mounted onto a wireline 80. It should be understood that using the wireline cable 80 to run the rescue dart in the wellbore 72 may allow retrieval of the rescue dart 10, i.e., the wireline cable 80 may be pulled out of the wellbore 72 along with the rescue dart 10. In some embodiments, the dart 10 may be retrieved after dislodging the plug 1 from within the wellbore, thereby allowing, for example, at least some components of the dart 10 to be reused.

It is well known that when a frac plug is deployed downhole, the plug may stop moving prematurely for unknown reasons. As a result, the frac plug may become "pre-set". In some embodiments, it may not be clear whether the frac plug has stopped moving due to debris down the wellbore, or whether the frac plug has failed and was set to an operational configuration prematurely. To securely position the fracture plug down the wellbore, a setting tool may be operated, for example, on a wireline to effectively set the plug in its desired position. The wireline may then be pulled out of the wellbore, leaving the now fully set pre-set fracture plug in place. The cable is then pulled up through the wellhead of the well assembly and into the lubricator.

At this point, the method of dislodging the pre-set fracture plug from within the wellbore will now be described using the rescue dart and associated system described above. First, a rescue dart is deployed within a wellbore. Fluid is then pumped down the wellbore to deliver a rescue dart via fluid flow toward the pre-set fracture plug. Depending on the location within the wellbore where the frac plug is pre-set, fluid delivery may be performed to deliver the rescue dart short or long distances down the wellbore, either only within the vertical wellbore section or along both the vertical and horizontal sections of the wellbore. Once the rescue dart reaches the pre-set frac plug, it engages the frac plug in a manner such that operating the rescue dart effectively reduces the structural integrity of the plug mandrel. It is understood that the structural integrity of the plug mandrel is reduced until structural failure of the frac plug occurs and it becomes loose and thereby dislodges. Additional fluid may then be pumped down the wellbore to clear debris and push the broken frac plug and rescue dart further down the wellbore to allow the fracturing operation to resume.

It is understood that placing the rescue dart below the wellhead and deploying it down the wellbore can be done in various ways. For example, after the wireline is removed from the well, the lubricator may be removed to disconnect the wireline therefrom to instead attach a rescue dart to the lubricator. The lubricator may then be replaced at the top of the wellhead so that a rescue dart is inserted through the wellhead valve and down the wellbore. Once below the wellhead, a pump may be engaged to carry the rescue dart via fluid flow down the wellbore and toward the pre-set fracture plug. In some embodiments, the rescue dart may be detachably connected to a weight bar, which in turn is attached to the lubricator and lowered below the wellhead. Thus, once fluid is pumped down the wellbore, the rescue dart is separated from the weight bar under the pressure of the fluid stream to be carried towards the frac plug.

The method may further include a monitoring step in which a parameter of the wellbore is monitored at the surface during operation of the rescue dart. For example, the temperature and/or pressure of the wellbore may be monitored. Doing so may help determine various steps in the eviction method. For example, an initial change in pressure reading may indicate that the rescue dart has successfully contacted a previously set frac plug. Also, subsequent pressure changes may indicate that the frac plug has been dislodged, which indicates that the fracturing operation may be resumed once the debris is cleared.

In the above description, like reference numerals refer to like elements. Further, for the sake of simplicity and clarity, i.e., so as not to unduly burden the figures with several reference numerals, not all figures contain reference numerals for all components and features, and reference numerals for certain components and features may be found in only one figure, and the components and features of the present disclosure illustrated in other figures may be readily inferred therefrom. The embodiments, geometries, mentioned materials and/or dimensions shown in the figures are optional and are given for illustrative purposes only.

Additionally, while the alternative configurations as illustrated in the figures include various components, and while the alternative configurations of the rescue dart as shown in the figures may be comprised of certain geometric configurations as illustrated and exemplified herein, not all of these components and geometric configurations are essential and thus should not be taken in their limiting sense, i.e., should not be taken as limiting the scope of the present disclosure. It is understood that other suitable components and cooperation therebetween and other suitable geometric configurations may be used to implement and use the rescue dart and corresponding parts without departing from the scope of the present disclosure, as briefly described and as may be readily inferred therefrom.

Claims

1. A rescue dart for deployment down a wellbore to dislodge a pre-set fracture plug comprising a plug mandrel from within the wellbore, the rescue dart comprising:

a dart adapted to be delivered via a fluid flow down the wellbore to the pre-set frac plug; and

a mandrel deactivator connected to the dart and operable to reduce the structural integrity of a portion of the plug mandrel until a structural failure of the plug mandrel occurs and the fracture plug is loosened and expelled.

2. The rescue dart of claim 1 wherein the rescue dart is shaped and configured to operate in an engaged position in which the dart body engages the pre-set fracture plug and extends at least partially within the axial passage of the plug mandrel to position the mandrel deactivator at a predetermined location along the axial passage.

3. The rescue dart of claim 2 wherein the dart comprises a central section, a dart head, and a dart tail, the dart head being disposed at a first end of the central section and the dart tail being disposed at a second end of the central section opposite the dart head, and wherein when in the engaged position the dart tail abuts the pre-set fracture plug to prevent the rescue dart from moving further forward and to position the dart body relative to the pre-set fracture plug.

4. The rescue dart of claim 3 wherein the dart tail is adapted to sit on a ball seat of the pre-set frac plug.

5. The rescue dart of claim 3 or 4 wherein the dart tail comprises rollers positioned and configured to engage the pre-set frac plug to allow rotational movement of the rescue dart.

6. The rescue dart of any one of claims 3 to 5 wherein the dart tail includes a wiper fin for sweeping debris downhole as the rescue dart travels along the wellbore.

7. The rescue dart of claim 6 wherein the wiper fin comprises an engagement surface for engaging an inner surface of the wellbore to guide the rescue dart along the wellbore.

8. The rescue dart of any one of claims 3 to 7 wherein the dart tail comprises at least one fluid channel for allowing fluid to flow through the dart tail and into the axial channel of the plug spindle when the rescue dart is in the engaged position.

9. The rescue dart of claim 8, wherein the central section is a center rod, and wherein, in the engaged position, the center rod is sized and configured to allow fluid to flow therearound and through the axial channel.

10. The rescue dart of claim 9 wherein the mandrel deactivator is positioned and configured to form a restriction channel for restricting fluid flow through the axial channel to at least partially cut the portion of the plug mandrel via fluid wear.

11. The rescue dart of claim 10 wherein the spindle deactivator extends radially and outwardly from the dart body to proximate the dart head so as to form the restricted passage.

12. The rescue dart of claim 10 or 11 wherein the mandrel deactivator is tulip shaped.

13. The rescue dart of claim 10 or 11 wherein the spindle deactivator is disc-shaped.

14. The rescue dart of claim 10 or 11 wherein the mandrel deactivator is tapered.

15. The rescue dart of any one of claims 10 to 14 wherein the mandrel deactivator is made of a hardened material.

16. The rescue dart of any one of claims 10 to 15 wherein the hardness of the material of the mandrel deactivator is greater than the hardness of the material of the plug mandrel.

17. The rescue dart of claim 8 wherein the central section is a central tube comprising an inner conduit in fluid communication with the at least one fluid channel, and wherein the mandrel deactivator comprises one or more fluid outlets extending through the thickness of the dart head to redirect a fluid flow toward the plug mandrel to at least partially cut the portion of the plug mandrel via a fluid jet.

18. The rescue dart of claim 17 wherein the fluid outlet is angled such that fluid flowing therethrough causes the rescue dart to rotate within the axial channel.

19. The rescue dart of any one of claims 1 to 18 wherein the dart body and spindle deactivator are an integral unit.

20. The rescue dart of any one of claims 3 to 7 wherein the spindle deactivator comprises at least one cutting blade operable between a retracted configuration in which the cutting blade may be introduced into the axial passage and an extended configuration for engaging and at least partially cutting the portion of the plug spindle.

21. The rescue dart of claim 20 wherein the spindle deactivator further comprises an actuator adapted to operate the cutting blade in the extended configuration.

22. The rescue dart of claim 21 wherein the actuator is adapted to operate the cutting blade when the rescue dart is in the engaged position.

23. The rescue dart of claim 21 or 22, wherein the actuator is adapted to operate the cutting blade via hydrostatic and/or hydraulic pressure.

24. The rescue dart of claim 21 or 22, wherein the actuator is an electrically driven actuator or a battery powered actuator.

25. The rescue dart of any one of claims 21 to 24 wherein the spindle deactivator comprises two or more cutting blades positioned radially around the dart body.

26. The rescue dart of claim 25 wherein the cutting blade is adapted to radially cut the plug mandrel.

27. The rescue dart of claim 25 or 26 wherein the cutting blade is adapted to axially cut the plug spindle.

28. The rescue dart of any one of claims 25 to 27 wherein the dart body comprises an inner channel extending along the central section and tapering inwardly towards the dart head, and wherein the actuator comprises a plunger engageable within the inner channel for said urging of the cutting blade outwardly into the plug spindle.

29. The rescue dart of claim 28 wherein the dart tail defines an interior chamber, and wherein the actuator comprises a release mechanism adapted to at least partially retain the plunger within the interior chamber when the rescue dart is not in the engaged position.

30. The rescue dart of claim 29 wherein the release mechanism comprises a shear screw and the actuator further comprises a plunger base extending outwardly from the plunger and retained within the inner chamber by the shear screw, the shear screw configured to allow forward movement of the plunger base once a pressure threshold is reached so as to engage the plunger within the inner channel.

31. The rescue dart of claim 30 wherein the inner chamber comprises an upstream portion and a downstream portion defined on either side of the plunger base, wherein the inner chamber further comprises a chamber inlet in communication with the upstream portion for allowing fluid to flow therein to exert pressure on the plunger base.

32. The rescue dart of claim 31 wherein the chamber inlet is substantially sealed to prevent fluid from flooding the upstream portion before the rescue dart is in the engaged position.

33. The rescue dart of claim 32 wherein the chamber inlet is sealed via a rupture disk.

34. The rescue dart of claim 32 wherein the chamber inlet is sealed via a second plunger extending from the plunger base opposite the plunger.

35. The rescue dart of any one of claims 3 to 7 wherein the mandrel deactivator is adapted to reduce the structural integrity of the portion of the plug mandrel via heat.

36. The rescue dart of claim 35 wherein the central section and/or dart head has an interior volume, and wherein the spindle deactivator comprises a heating material located within the volume and an actuator adapted to operate the heating material to heat the plug spindle.

37. The rescue dart of claim 36 wherein the actuator comprises an igniter adapted to heat the heating material and a striker operable to activate the igniter.

38. The rescue dart of claim 37 wherein the dart tail defines an interior chamber, and wherein the actuator comprises a release mechanism adapted to retain the striker within the interior chamber when the rescue dart is not in the engaged position.

39. The rescue dart of claim 38 wherein the release mechanism comprises a shear screw and the actuator further comprises a plunger base on which the striker is positioned, the plunger base being retained within the interior chamber by the shear screw, the shear screw being configured to release the plunger base once a pressure threshold is reached so as to allow the plunger base to move forward and engage the striker with the igniter.

40. The rescue dart of claim 39 wherein the inner chamber comprises an upstream portion and a downstream portion defined on either side of the plunger base, wherein the inner chamber further comprises a chamber inlet in communication with the upstream portion for allowing fluid to flow therein to exert pressure on the plunger base.

41. The rescue dart of claim 40 wherein the chamber inlet is substantially sealed to prevent fluid from flooding the upstream portion before the rescue dart is in the engaged position.

42. The rescue dart of claim 41 wherein the chamber inlet is sealed via a rupture disk.

43. The rescue dart of claim 41 wherein the chamber inlet is sealed via a second plunger extending from the plunger base opposite the striker.

44. The rescue dart of any one of claims 2 to 43 wherein the pre-set frac plug comprises an elastomeric element, and wherein the predetermined location of the mandrel deactivator is downstream of the elastomeric element.

45. The rescue dart of any one of claims 2 to 44, wherein the pre-set frac plug comprises a sliding member and/or a compression member, and wherein the predetermined position of the mandrel deactivator is substantially aligned with the sliding member and/or compression member.

46. The rescue dart of any one of claims 1 to 45 further comprising a deployment mechanism connectable to a logging cable for running the rescue dart down through a wellhead of the wellbore.

47. A system for dislodging a pre-set fracture plug comprising a plug mandrel from within a well assembly, the system comprising:

a rescue dart configured to engage and dislodge the set ahead frac plug; and

a pump for providing a flow of fluid down the wellbore to carry the rescue dart to the pre-set frac plug for engagement therewith.

48. The system of claim 47, wherein the pump is located at the surface.

49. The system of any one of claims 47-48, further comprising a logging cable removably connectable to the rescue dart for running the rescue dart through a wellhead of the well assembly.

50. The system of any one of claims 47-48, where the rescue dart is run down the wellbore using only fluid flow and gravity.

51. The system of any one of claims 47-50, where the rescue dart is according to any one of claims 1-46.

52. A method for dislodging a pre-set fracture plug comprising a plug mandrel from within a wellbore, the system comprising the steps of:

deploying a rescue dart within the wellbore;

pumping a fluid within the wellbore to deliver the rescue dart toward the pre-set fracture plug via a fluid flow;

engaging the rescue dart with the pre-set fracture plug; and

the rescue dart is operated to reduce the structural integrity of a portion of the plug mandrel until a structural failure of the plug mandrel occurs and loosens and dislodges the fracture plug.

53. The method of claim 52, further comprising the steps of: monitoring pressure changes in the wellbore at the surface.

54. The method of claim 52 or 53, further comprising the steps of: pumping the dislodged frac plug further downhole along with the rescue dart to allow recovery of a frac operation.

55. The method of any one of claims 52-54, wherein the rescue dart is according to any one of claims 1-46.

56. The rescue dart of claim 35 wherein the center section and/or dart head has an interior volume, and wherein the mandrel deactivator comprises fuel located within the volume, the mandrel deactivator being adapted to release the fuel within the plug mandrel, which causes the fuel to react with the fluid within the wellbore to melt/soften at least one component of the plug.

57. The rescue dart of claim 56 wherein the mandrel deactivator comprises a piston engaged within the volume and movable toward the distal end via fluid flow to inject fuel into the plug mandrel.

58. The rescue dart of claim 57 wherein the mandrel deactivator includes a metering device adapted to adjust the flow rate of fluid to the piston so as to control the injection rate of fuel within the plug mandrel.

59. The rescue dart of claim 58 wherein the metering device is positioned within the dart body at a location where the piston is up the bore and forms an interior chamber with the piston, the metering device comprising a channel defining a flow path that allows fluid to reach the interior chamber, the channel being shaped and dimensioned to limit the flow rate of fluid therethrough.

60. The rescue dart of claim 59 wherein the passage has a tortuous configuration through the metering device to restrict the flow rate of fluid.

61. The rescue dart of any one of claims 57 to 60 wherein the dart head comprises a bursting disk at its distal end for containing the fuel within the volume.

62. The rescue dart of any one of claims 56 to 61 wherein the fuel is a solid rod, liquid and/or powder disposed within the volume.

63. The rescue dart of any one of claims 56 to 62 wherein the fuel is adapted to react with water.

64. The rescue dart of any one of claims 56 to 63 wherein the fuel is a mixture of sodium and/or lithium.