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CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit under 35 U.S.C. §119 of the filing date of International Application No. PCT/US2013/037923, filed Apr. 24, 2013. 
     TECHNICAL FIELD OF THE INVENTION 
     This invention relates, in general, to equipment utilized in conjunction with operations performed in a subterranean well and, in particular, to a positive displacement dump bailer and a method of operating the positive displacement dump bailer. 
     BACKGROUND OF THE INVENTION 
     Without limiting the scope of the present invention, its background will be described with reference to isolating pressure between two regions in a well with a cement plug, as an example. Cement plugs are commonly set in a subterranean well at a desired location inside a casing string to isolate pressure between two regions in the well. In certain installations, this is accomplished by first, installing a bridge plug at the desired location in the casing string and then, lowered a dump bailer carrying a cement slurry into the casing on a conveyance such as a slickline, a wireline, a coiled tubing or the like. Once the dump bailer is positioned in the desired location proximate the bridge plug, the dump bailer is actuated to release the cement slurry. The cement slurry is deposited on a platform formed by the bridge plug and is supported by the bridge plug during curing. 
     In one type of dump bailer, gravity is used to shift a weight through the dump bailer to dispense the cement slurry from the dump bailer. It has been found, however, that such gravity operated dump bailers often fail to fully dispense the desired volume of the cement slurry from the dump bailer, which can result in cement slurry placement in undesired locations during retrieval of the dump bailer as well as additional trips into the well to add more cement. In another type of dump bailer, explosive components are used to generate pressure to dispense the cement slurry from the dump bailer. It has been found, however, that the use of explosive operated dump bailers can be undesirable due to safety concerns and their use may not be allowed in some jurisdiction due to local regulations. In a further type of dump bailer, a surface electrical power source is used to dispense the cement slurry from the dump bailer. It has been found, however, that the use of surface electrical power operated dump bailers can be undesirable due to the high deployment costs associated with the use of electric wireline packages. 
     Accordingly, a need has arisen for an improved dump bailer operable to release a cement slurry into a casing to isolate pressure between two regions in the well. A need has also arisen for such an improved dump bailer that does not solely rely on gravity to dispense the cement slurry. In addition, a need has arisen for such an improved dump bailer that does not require explosives to dispense the cement slurry. Further, a need has arisen for such an improved dump bailer that does not require a surface electrical power source to dispense the cement slurry. 
     SUMMARY OF THE INVENTION 
     The present invention disclosed herein is directed to an improved dump bailer assembly that is operable to release a cement slurry into a casing to isolate pressure between two regions in the well. The improved dump bailer assembly of the present invention does not solely rely on gravity to dispense the cement slurry. In addition, the improved dump bailer assembly of the present invention does not require explosives to dispense the cement slurry. Further, the improved dump bailer assembly of the present invention does not require a surface electrical power source to dispense the cement slurry. 
     In one aspect, the present invention is directed to a dump bailer assembly for use in a wellbore. The dump bailer assembly includes a downhole power unit having a housing and a moveable shaft. A dump bailer body is operably associated with the housing. The dump bailer body has first and second ends. A piston is disposed within the dump bailer body. The piston is releasably coupled to the moveable shaft. An actuation assembly is disposed within the dump bailer body between the first end and the piston. A barrier is operably associated with the dump bailer body and positioned proximate the second end. A wellbore agent is disposed within the dump bailer body between the barrier and the piston. In operation, the downhole power unit retracts the moveable shaft shifting the piston toward the first end to energize the actuation assembly, the downhole power unit releases the moveable shaft from the piston such that the energized actuation assembly shifts the piston toward the second end and interaction between the piston and the wellbore agent opens the barrier, thereby dispensing the wellbore agent from the dump bailer body. 
     In some embodiments, at least one shearable member may initially couple the piston to the moveable shaft. In certain embodiments, at least one wiper seal may be operably associated with the piston. In one embodiment, the actuation assembly may include a mechanical biasing element selected from the group consisting of coil springs, compression springs and Belleville washers. In another embodiment, the actuation assembly may include an opposing magnet assembly. In some embodiments, the barrier may be a frangible disk member. In certain embodiments, the wellbore agent may be a cement slurry. In one embodiment, the downhole power unit may have a self-contained power source for providing electrical power. In this embodiment, the downhole power unit may include an electric motor and a jackscrew assembly having a rotational member connected to a rotor of the electric motor. The rotational member may be operably associated with the moveable shaft to impart longitudinal motion thereto. 
     In another aspect, the present invention is directed to a dump bailer assembly for use in a wellbore. The dump bailer assembly includes a downhole power unit having a housing, a moveable shaft, a self-contained power source for providing electrical power, an electric motor having a rotor and a jackscrew assembly having a rotational member connected to the rotor. The rotational member is operably associated with the moveable shaft to impart longitudinal motion thereto. A dump bailer body is operably associated with the housing. The dump bailer body has first and second ends. A piston is disposed within the dump bailer body. The piston is releasably coupled to the moveable shaft by at least one shearable member. An actuation assembly is disposed within the dump bailer body between the first end and the piston. A barrier is operably associated with the dump bailer body and positioned proximate the second end. A wellbore agent is disposed within the dump bailer body between the barrier and the piston. In operation, the downhole power unit retracts the moveable shaft shifting the piston toward the first end to energize the actuation assembly, the downhole power unit breaks the at least one shearable member releasing the moveable shaft from the piston such that the energized actuation assembly shifts the piston toward the second end and interaction between the piston and the wellbore agent opens the barrier, thereby dispensing the wellbore agent from the dump bailer body. 
     In a further aspect, the present invention is directed to a method for operating a dump bailer assembly in a wellbore. The method includes disposing the dump bailer assembly at a target location in the wellbore, the dump bailer assembly including a downhole power unit having a housing and a moveable shaft and a dump bailer body operably associated with the housing; operating the downhole power unit to retract the moveable shaft; shifting a piston disposed within the dump bailer body toward a first end of the dump bailer body with the moveable shaft; energizing an actuation assembly disposed within the dump bailer body responsive to shifting the piston; releasing the moveable shaft from the piston responsive to continued operation of the downhole power unit; shifting the piston toward a second end of the dump bailer body with the energized actuation assembly; opening a barrier, operably associated with the dump bailer body and positioned proximate the second end, responsive to interaction between the piston and a wellbore agent disposed within the dump bailer body between the barrier and the piston; and dispensing the wellbore agent from the dump bailer body. 
     The method may also include operating an electrical motor powered by a self-contained power source of the downhole power unit; energizing a mechanical biasing element selected from the group consisting of coil springs, compression springs and Belleville washers; energizing an opposing magnet assembly; breaking at least one shearable member initially coupling the piston to the moveable shaft; and/or dispensing a cement slurry. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the features and advantages of the present invention, reference is now made to the detailed description of the invention along with the accompanying figures in which corresponding numerals in the different figures refer to corresponding parts and in which: 
         FIG. 1  is a schematic illustration of an offshore oil and gas platform during the deployment of a dump bailer assembly according to an embodiment of the present invention; 
         FIGS. 2A-2B  are schematic illustrations of a dump bailer assembly according to an embodiment of the present invention before and after operation thereof, respectively; 
         FIGS. 3A-3B  are quarter sectional views of successive axial sections of a downhole power unit for use in a dump bailer assembly according to an embodiment of the present invention; 
         FIGS. 4A-4D  are cross sectional views of a lower portion of a dump bailer assembly according to an embodiment of the present invention in its various operating positions; 
         FIGS. 5A-5D  are cross sectional views of a lower portion of a dump bailer assembly according to an embodiment of the present invention in its various operating positions; 
         FIGS. 6A-6B  are schematic illustrations of components for an opposing magnet assembly for use in a dump bailer assembly according to an embodiment of the present invention; 
         FIGS. 7A-7B  are schematic illustrations of components for an opposing magnet assembly for use in a dump bailer assembly according to an embodiment of the present invention; 
         FIGS. 8A-8B  are schematic illustrations of components for an opposing magnet assembly for use in a dump bailer assembly according to an embodiment of the present invention; and 
         FIGS. 9A-9B  are schematic illustrations of components for an opposing magnet assembly for use in a dump bailer assembly according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     While the making and using of various embodiments of the present invention are discussed in detail below, it should be appreciated that the present invention provides many applicable inventive concepts, which can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the invention, and do not delimit the scope of the present invention. 
     Referring initially to  FIG. 1 , a dump bailer assembly of the present invention is being deployed from an offshore oil and gas platform that is schematically illustrated and generally designated  10 . A semi-submersible platform  12  is centered over submerged oil and gas formations  14  located below sea floor  16 . A subsea conductor  18  extends from deck  20  of platform  12  to sea floor  16 . A wellbore  22  extends from sea floor  16  and traverse formations  14 . Wellbore  22  includes a casing  24  that is supported therein by cement  26 . Hydraulic communication between the interior of casing  24  and formation  14  has been established by perforations  28 . 
     A tubing string  30  extends from wellhead  32  into casing  24  to a location uphole of formation  14  to provide a conduit for production fluids to travel to the surface. A packer  34  provides a fluid seal between tubing string  30  and casing  24  and directs the flow of production fluids from formation  14  to the interior of tubing string  30 . A through tubing bridge plug  36  has been previously installed in casing  24  below tubing string  30  as a first step in plugging and abandoning wellbore  22 . Extending from the surface within tubing string  30  is a slickline  38  used to convey a tool system including a dump bailer assembly  40 . Even though dump bailer assembly  40  is depicted as being deployed on a slickline, it is to be understood by those skilled in the art that dump bailer assembly  40  could be deployed on other types of conveyances, including, but not limited to, a wireline, coiled tubing, jointed tubing, a downhole robot or the like, without departing from the principles of the present invention. 
     In the illustrated embodiment, dump bailer assembly  40  includes a downhole power unit  42 . As will be described in more detail below, a particular implementation of downhole power unit  42  includes an elongated housing, a motor disposed in the housing and a sleeve connected to a rotor of the motor. The sleeve is a rotational member that rotates with the rotor. A moveable member such as the above-mentioned moveable shaft is received within the threaded interior of the sleeve. Operation of the motor rotates the sleeve, which causes the moveable shaft to move longitudinally. Accordingly, when downhole power unit  42  is operably coupled within dump bailer assembly  40  and the moveable member is activated, longitudinal movement is imparted to a piston within dump bailer assembly  40  which energizes an actuation assembly of dump bailer assembly  40  enabling dispensing of a cement slurry from dump bailer assembly  40  into casing  24  on a platform created by through tubing bridge plug  36 . Even though dump bailer assembly  40  is described as dispensing a cement slurry into casing  24 , it is to be understood by those skilled in the art that dump bailer assembly  40  could be alternatively be used to dispense other wellbore agents including, but not limited to, acids, sands or the like. 
     In one implementation, a microcontroller made of suitable electrical components to provide miniaturization and durability within the high pressure, high temperature environments which can be encountered in an oil or gas well is used to control the operation of downhole power unit  42 . The microcontroller is preferably housed within the structure of downhole power unit  42 , it can, however, be connected outside of downhole power unit  42  but within the associated tool string moved into wellbore  22 . In whatever physical location the microcontroller is disposed, it is operationally connected to downhole power unit  42  to control movement of the moveable member when desired. The microcontroller may include a microprocessor that initiates operation responsive to a timing device or other circuitry and contains a program stored in a memory. The program instructions cause the microprocessor to control operations of the downhole power unit  42 . 
     The microcontroller operates under power from a power supply, which is preferably located within downhole power unit  42 . The power source provides the electrical power to both the motor of downhole power unit  42  and the microcontroller. When downhole power unit  42  is at the target location, the microcontroller commences operation of downhole power unit  42  as programmed. For example, with regard to controlling the motor that operates the sleeve receiving the moveable member, the microcontroller sends a command to energize the motor to rotate the sleeve in the desired direction to retract the moveable member at the desired speed. One or more sensors monitor the operation of downhole power unit  42  and provide responsive signals to the microcontroller. When the microcontroller determines that a desired result has been obtained, it stops operation of downhole power unit  42 , such as by de-energizing the motor. Alternatively, the operation of downhole power unit  42  may be controlled from the surface wherein command signals may be provided to downhole power unit  42  via a wired or wireless communication protocol. Similarly, power may be provided to downhole power unit  42  from the surface via an electrical conductor. 
     Even though  FIG. 1  depicts a vertical well, it should be understood by those skilled in the art that the present invention is equally well-suited for use in wells having other configurations including deviated wells, inclined wells, horizontal wells, multilateral wells and the like. As such, the use of directional terms such as above, below, upper, lower, upward, downward and the like are used in relation to the illustrative embodiments as they are depicted in the figures, the upward direction being toward the top of the corresponding figure and the downward direction being toward the bottom of the corresponding figure. Likewise, even though  FIG. 1  depicts an offshore operation, it should be understood by those skilled in the art that the present invention is equally well suited for use in onshore operations. Also, even though  FIG. 1  depicts a cased wellbore, it should be understood by those skilled in the art that the present invention is equally well suited for use in open hole operations. 
     Referring next to  FIGS. 2A-2B , therein is schematically depicted a dump bailer assembly of the present invention that is generally designated  60 . In the illustrated embodiment, dump bailer assembly  60  includes a downhole power unit  62 , an actuation assembly  64  and a cement chamber  66 . Downhole power unit  62  has a moveable member described herein as a moveable shaft that is operably associated with and extends through actuation assembly  64  and that couples to a piston disposed within cement chamber  66 . Dump bailer assembly  60  is illustrated as having been lowered into a well  68  on a conveyance  70  such as a slickline, a wireline, coiled tubing, jointed pipe or other tubing string. 
     In the illustrated embodiment, dump bailer assembly  60  has reached its target location in well  68  at a location proximate a preinstalled bridge plug  72 . Operation of dump bailer assembly  60  may now commence. Based upon a predetermined time, a command signal from the surface or other input signal, downhole power unit  62  initiates the process by retracting the moveable shaft. This operation shifts the piston toward the top of dump bailer assembly  60 . As the piston is shifted, actuation assembly  64  of dump bailer assembly  60  is energized as will be explained in greater detail below. The continued operation of downhole power unit  62  causes the moveable shaft to release from the piston. Thereafter, the energized actuation assembly  64  acts on the piston to shift the piston toward the bottom of dump bailer assembly  60 . The impact of the piston on an upper surface of the cement slurry contained within cement chamber  66  causes a barrier on the lower end of dump bailer assembly  60  to open. The downward movement of the piston in now able to urge the cement slurry out of dump bailer assembly  60  and dispense the cement slurry into well  68  and on bridge plug  72  to form a cement plug  74 , which is allowed to cure on bridge plug  72 . Following operation, dump bailer assembly  60  can be retrieved to the surface. 
     Referring now to  FIGS. 3A-3B , therein are depicted successive axial sections of an exemplary downhole power unit that is generally designated  100  and that is capable of operations as part of a dump bailer assembly of the present invention. Downhole power unit  100  includes a working assembly  102  and a power assembly  104 . Power assembly  104  includes a housing assembly  106 , which comprises suitably shaped and connected generally tubular housing members. An upper portion of housing assembly  106  includes an appropriate mechanism to facilitate coupling of housing  106  to a conveyance  108  such as a slickline, wireline, electric line, coiled tubing, jointed tubing or the like. Housing assembly  106  also includes a clutch housing  110 , which forms a portion of a clutch assembly  112 . 
     In the illustrated embodiment, power assembly  104  includes a self-contained power source, eliminating the need for power to be supplied from an exterior source, such as a source at the surface. A preferred power source comprises a battery assembly  114  which may include a plurality of batteries such as alkaline batteries, lithium batteries or the like. Alternatively, however, power may be provided to downhole power unit  100  from the surface via an electrical conductor. Connected with power assembly  104  is the force generating and transmitting assembly. The force generating and transmitting assembly of this implementation includes a direct current (DC) electric motor  116  coupled through a gearbox  118  to a jackscrew assembly  120 . A plurality of activation mechanisms  122 ,  124 ,  126  can be electrically coupled between battery assembly  114  and electric motor  116 . Electric motor  116  may be of any suitable type. One example is a motor operating at 7500 revolutions per minute (rpm) in unloaded condition and operating at approximately 5000 rpm in a loaded condition, having a horsepower rating of approximately 1/30th of a horsepower. In this implementation, motor  116  is coupled through the gearbox  118 , which provides approximately 5000:1 gear reduction. Gearbox  118  is coupled through a conventional drive assembly  128  to jackscrew assembly  120 . 
     Jackscrew assembly  120  includes a moveable shaft  130  which moves longitudinally, rotates or both, in response to rotation of a sleeve assembly  132 . Shaft  130  includes a threaded portion  134 , and a generally smooth, polished lower extension  136 . Shaft  130  further includes a pair of generally diametrically opposed keys  138  that cooperate with a clutch block  140 , which is coupled to shaft  130 . Clutch housing  110  includes a pair of diametrically opposed keyways  142  which extend along at least a portion of the possible length of travel. Keys  138  extend radially outwardly from shaft  130  through clutch block  140  to engage each of keyways  142  in clutch housing  110 , thereby selectively preventing rotation of shaft  130  relative to housing  110 . 
     Rotation of sleeve assembly  132  in one direction causes shaft  130  and clutch block  140  to move longitudinally upwardly relative to housing assembly  110  if shaft  130  is not at its uppermost limit. Rotation of the sleeve assembly  132  in the opposite direction moves shaft  130  downwardly relative to housing  110  if shaft  130  is not at its lowermost position. Above a certain level within clutch housing  110 , as indicated generally at  144 , clutch housing  110  includes a relatively enlarged internal diameter bore  146  such that moving clutch block  140  above level  144  removes the outwardly extending key  138  from being restricted from rotational movement. Accordingly, continuing rotation of sleeve assembly  132  causes longitudinal movement of shaft  130  until clutch block  140  rises above level  144 , at which point rotation of sleeve assembly  132  will result in free rotation of shaft  130 . By virtue of this, clutch assembly  112  serves as a safety device to prevent burn-out of the electric motor and also serves as a stroke limiter. In a similar manner, clutch assembly  112  may allow shaft  130  to rotate freely during certain points in the longitudinal travel of shaft  130 . 
     In the illustrated embodiment, downhole power unit  100  incorporates three discrete activation assemblies, separate from or part of the microcontroller discussed above. The activation assemblies enable jackscrew  120  to operate upon the occurrence of one or more predetermined conditions. One depicted activation assembly is timing circuitry  122  of a type known in the art. Timing circuitry  122  is adapted to provide a signal to the microcontroller after passage of a predetermined amount of time. Further, downhole power unit  100  can include an activation assembly including a pressure-sensitive switch  124  of a type generally known in the art which will provide a control signal, for example, once the switch  124  reaches a depth at which it encounters a predetermined amount of hydrostatic pressure within the tubing string or experiences a particular pressure variation or series of pressure variations. Still further, downhole power unit  100  can include a motion sensor  126 , such as an accelerometer or a geophone that is sensitive to vertical motion of downhole power unit  100 . Accelerometer  126  can be combined with timing circuitry  122  such that when motion is detected by accelerometer  126 , timing circuitry  122  is reset. If so configured, the activation assembly operates to provide a control signal after accelerometer  126  detects that downhole power unit  100  has remained substantially motionless within the well for a predetermined amount of time. 
     Working assembly  102  includes an outer sleeve member  150  which may be threadably coupled or pinned to housing assembly  106 . At its lower end  152 , outer sleeve member  150  may be threadably coupled to other tools such as a housing member of a dump bailer body. Shaft  130  extends through sleeve member  150  and is operable for coupling to other tools such as a piston disposed within a dump bailer body as will be described below. 
     In operation, downhole power unit  100  is adapted to cooperate directly with a housing member of a dump bailer body. Specifically, prior to run in, outer sleeve member  150  of downhole power unit  100  is operably associated with an outer housing of the dump bailer body. Likewise, shaft  130  of downhole power unit  100  is operably associated with a piston disposed within dump bailer body. As used herein, the term operably associated with shall encompass direct coupling such as via a threaded connection, a pinned connection, a frictional connection, a closely received relationship and may also include the use of set screws or other securing means which may or may not be shearable. In addition, the term operably associated with shall encompass indirect coupling such as via a connection sub, an adaptor or other coupling means. As such, an upward longitudinal movement of shaft  130  of downhole power unit  100  exerts an upward longitudinal force upon the component to which it is operably associated that initiates the operation of the dump bailer assembly. 
     As will be appreciated from the above discussion, actuation of motor  116  by activation assemblies  122 ,  124 ,  126  or another device and control of motor  116  by the microcontroller results in the required longitudinal movement of shaft  130 . As described below, shaft  130  is required to retract a distance that is sufficient to energize an actuation assembly and then further to cause release of shaft  130  from the piston. Preferably, downhole power unit  100  is preprogrammed to perform the proper operations prior to deployment into the well. Alternatively, downhole power unit  100  may receive command signals from the surface via wired or wireless telemetry. Once the dump bailing operation has been performed, downhole power unit  100  and the other dump bailer components, may be retrieved to the surface. Even though a particular downhole power unit has been depicted and described, it should be clearly understood by those skilled in the art that other types of downhole power devices could alternatively be used with a dump bailer assembly without departing from the principles of the present invention. 
     Referring now to  FIGS. 4A-4D , a lower portion of a dump bailer assembly according to an embodiment of the present invention is depicted in its various operating positions and is generally designated  200 . In the illustrated embodiment, dump bailer assembly  200  includes a dump bailer body  202  in the form of a generally tubular outer housing. At its upper end, dump bailer body  202  includes a threaded connector  204  that is operable to be threadably secured to another tool such as outer sleeve member  150  of downhole power unit  100 . The upper portion of dump bailer body  202  houses an actuation assembly  206 . In the illustrated embodiment, actuation assembly  206  includes a biasing member  208  depicted as a series-parallel Belleville washer assembly. Even though a particular biasing member has been depicted and described, it should be clearly understood by those skilled in the art that other types of biasing member including, but not limited to, mechanical springs, series Belleville washers, parallel Belleville washers, compression springs, coil springs, fluid springs and the like, could alternatively be used without departing from the principles of the present invention. Actuation assembly  206  also includes a spring support member  210  that provides a surface that defines the upper limit of movement for biasing member  208 . 
     The lower portion of dump bailer body  202  houses a cement chamber  212 . In the illustrated embodiment, the lower end of cement chamber  212  is defined by a barrier  214  depicted as a disk member that may be in the form of a metal rupture disk, a frangible ceramic disk or other suitably removable disk member. The upper end of cement chamber  212  is defined by a piston  216 . Piston  216  includes one or more wiper seals  218  that preferably have a sealing engagement with the inner surface of dump bailer body  202 . A piston travel limiter  220  is disposed within dump bailer body  202  that defines the lower limit of movement for piston  216 . Piston  216  is coupled to shaft  222  via one or more shearable members depicted as shear screws  224 . Shaft  222  may be the lower end of shaft  130  of downhole power unit  100  or may be a connector that extends the length of shaft  130  of downhole power unit  100 . Even though the coupling between piston  216  and shaft  222  has been depicted and described as shear screws  224 , it should be clearly understood by those skilled in the art that other types of releasable couplings including, but not limited to, shear threads, c-rings, dogs and the like, could alternatively be used without departing from the principles of the present invention. In the running configuration of dump bailer body  202 , as best seen in  FIG. 4A , a cement slurry  226  is disposed within cement chamber  212  between barrier  214  and the lower surface of piston  216 . 
     The operation of dump bailer assembly  200  will now be described. Once dump bailer assembly  200  including downhole power unit  100  is disposing a target location in the wellbore, operation of downhole power unit  100  may commence as described above. This actuation causes shaft  222  to move upwardly relative to dump bailer body  202 . As shaft  222  is initially coupled to piston  216 , this assembly is shifted upwardly together. The upward movement compresses biasing member  208  of actuation assembly  206 , thereby energizing actuation assembly  206 , as best seen in  FIG. 4B . Continued operation of downhole power unit  100  generates the required shear force to break shear screws  224  that couple piston  216  and shaft  222 . Once piston  216  is released from shaft  222 , the energized actuation assembly  206  acts on piston  216  causing piston  216  to move downwardly relative to dump bailer body  202 . As best seen in  FIG. 4C , at or near the time piston  216  contacts an upper surface of cement slurry  226 , an opening is created in barrier  214  by breaking, shattering or otherwise removing barrier  214  responsive to the impact or pressure change created due to interaction of piston  216  with cement slurry  226 . Thereafter, continued downward movement of piston  216  relative to dump bailer body  202  responsive to the force generated by the energized actuation assembly  206  dispenses cement slurry  226  from dump bailer body  202 , as best seen in  FIG. 4D . 
     Referring now to  FIGS. 5A-5D , a lower portion of a dump bailer assembly according to an embodiment of the present invention is depicted in its various operating positions and is generally designated  300 . In the illustrated embodiment, dump bailer assembly  300  includes a dump bailer body  302  in the form of a generally tubular outer housing. At its upper end, dump bailer body  302  includes a threaded connector  304  that is operable to be threadably secured to another tool such as outer sleeve member  150  of downhole power unit  100 . The upper portion of dump bailer body  302  houses an actuation assembly  306 . In the illustrated embodiment, actuation assembly  306  includes an opposing magnet assembly depicted as lower magnet member  308  and upper magnet member  310 . 
     The lower portion of dump bailer body  302  houses a cement chamber  312 . In the illustrated embodiment, the lower end of cement chamber  312  is defined by a barrier  314  depicted as a disk member. The upper end of cement chamber  312  is defined by a piston  316 . Piston  316  includes one or more wiper seals  318  that preferably have a sealing engagement with the inner surface of dump bailer body  302 . A piston travel limiter  320  is disposed within dump bailer body  302  that defines the lower limit of movement for piston  316 . Piston  316  is coupled to shaft  322  via one or more shearable members depicted as shear screws  324 . Shaft  322  may be the lower end of shaft  130  of downhole power unit  100  or may be a connector that extends the length of shaft  130  of downhole power unit  100 . In the running configuration of dump bailer body  302 , as best seen in  FIG. 5A , a cement slurry  326  is disposed within cement chamber  312  between barrier  314  and the lower surface of piston  316 . 
     The operation of dump bailer assembly  300  will now be described. Once dump bailer assembly  300  including downhole power unit  100  is disposing a target location in the wellbore, operation of downhole power unit  100  may commence as described above. This actuation causes shaft  322  to move upwardly relative to dump bailer body  302 . As shaft  322  is initially coupled to piston  316 , this assembly is shifted upwardly together. The upward movement shifts lower magnet member  308  toward upper magnet member  310  of actuation assembly  306 , thereby energizing actuation assembly  306 , as best seen in  FIG. 5B . Continued operation of downhole power unit  100  generates the required shear force to break shear screws  324  that couple piston  316  and shaft  322 . Once piston  316  is released from shaft  322 , the energized actuation assembly  306  acts on piston  316  causing piston  316  to move downwardly relative to dump bailer body  302 . As best seen in  FIG. 5C , at or near the time piston  316  contacts an upper surface of cement slurry  326 , an opening is created in barrier  314  by breaking, shattering or otherwise removing barrier  314  responsive to the impact or pressure change created due to interaction of piston  316  with cement slurry  326 . Thereafter, continued downward movement of piston  316  relative to dump bailer body  302  responsive to the force generated by the energized actuation assembly  306  dispenses cement slurry  326  from dump bailer body  302 , as best seen in  FIG. 5D . 
     Referring now to  FIGS. 6A-6B , therein is schematic depicted components for an opposing magnet assembly for use in a dump bailer assembly according to an embodiment of the present invention. In the illustrated embodiment, the upper surface of a lower magnet member  400  is depicted in  FIG. 6A  and the lower surface of an upper magnet member  402  is depicted in  FIG. 6B . The two illustrated surfaces form facing surfaces of the opposing magnet assembly containing lower magnet member  400  and upper magnet member  402 . As such, when lower magnet member  400  and upper magnet member  402  are in their operating configuration, the facing surfaces have opposite polarity. The magnetic repulsion between lower magnet member  400  and upper magnet member  402  creates an increasing axial force that acts on the piston of the dump bailer assembly as lower magnet member  400  is shifted toward upper magnet member  402  during the energizing process. This magnetic repulsion force then serves as the energy source to shift the piston toward the cement slurry and the bottom of the dump bailer assembly after the piston is released from the shaft of the downhole power unit. 
     Referring now to  FIGS. 7A-7B , therein is schematic depicted components for an opposing magnet assembly for use in a dump bailer assembly according to an embodiment of the present invention. In the illustrated embodiment, the upper surface of a lower magnet member  410  is depicted in  FIG. 7A  and the lower surface of an upper magnet member  412  is depicted in  FIG. 7B . Lower magnet member  410  includes a four member array of magnets having alternating poles. Likewise, upper magnet member  412  includes a four member array of magnets having alternating poles. The two illustrated surfaces form facing surfaces of the opposing magnet assembly containing lower magnet member  410  and upper magnet member  412 . As such, when lower magnet member  410  and upper magnet member  412  are in their operating configuration, the facing surfaces of the magnets in each of the four member arrays have opposite polarities. The magnetic repulsion between lower magnet member  410  and upper magnet member  412  creates an increasing axial force that acts on the piston of the dump bailer assembly as lower magnet member  410  is shifted toward upper magnet member  412  during the energizing process. This magnetic repulsion force then serves as the energy source to shift the piston toward the cement slurry and the bottom of the dump bailer assembly after the piston is released from the shaft of the downhole power unit. 
     Referring now to  FIGS. 8A-8B , therein is schematic depicted components for an opposing magnet assembly for use in a dump bailer assembly according to an embodiment of the present invention. In the illustrated embodiment, the upper surface of a lower magnet member  420  is depicted in  FIG. 8A  and the lower surface of an upper magnet member  422  is depicted in  FIG. 8B . Lower magnet member  420  includes an eight member array of magnets having alternating poles. Likewise, upper magnet member  422  includes an eight member array of magnets having alternating poles. The two illustrated surfaces form facing surfaces of the opposing magnet assembly containing lower magnet member  420  and upper magnet member  422 . As such, when lower magnet member  420  and upper magnet member  422  are in their operating configuration, the facing surfaces of the magnets in each of the eight member arrays have opposite polarities. The magnetic repulsion between lower magnet member  420  and upper magnet member  422  creates an increasing axial force that acts on the piston of the dump bailer assembly as lower magnet member  420  is shifted toward upper magnet member  422  during the energizing process. This magnetic repulsion force then serves as the energy source to shift the piston toward the cement slurry and the bottom of the dump bailer assembly after the piston is released from the shaft of the downhole power unit. 
     Referring now to  FIGS. 9A-9B , therein is schematic depicted components for an opposing magnet assembly for use in a dump bailer assembly according to an embodiment of the present invention. In the illustrated embodiment, the upper surface of a lower magnet member  430  is depicted in  FIG. 9A  and the lower surface of an upper magnet member  432  is depicted in  FIG. 9B . Lower magnet member  430  includes a six member array of radially spaced magnetic rings having alternating poles. Likewise, upper magnet member  432  includes a six member array of radially spaced magnetic rings having alternating poles. The two illustrated surfaces form facing surfaces of the opposing magnet assembly containing lower magnet member  430  and upper magnet member  432 . As such, when lower magnet member  430  and upper magnet member  432  are in their operating configuration, the facing surfaces of the magnetic rings in each of the six member arrays have opposite polarities. The magnetic repulsion between lower magnet member  430  and upper magnet member  432  creates an increasing axial force that acts on the piston of the dump bailer assembly as lower magnet member  430  is shifted toward upper magnet member  432  during the energizing process. This magnetic repulsion force then serves as the energy source to shift the piston toward the cement slurry and the bottom of the dump bailer assembly after the piston is released from the shaft of the downhole power unit. 
     While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments as well as other embodiments of the invention will be apparent to persons skilled in the art upon reference to the description. It is, therefore, intended that the appended claims encompass any such modifications or embodiments.

Summary:
A dump bailer assembly includes a downhole power unit and a dump bailer body. A piston is disposed within the dump bailer body and is releasably coupled to a moveable shaft of the downhole power unit. An actuation assembly is disposed proximate a first end of the dump bailer body and a barrier is positioned proximate a second end of the dump bailer body. A wellbore agent is disposed within the dump bailer body between the barrier and the piston. In operation, the downhole power unit retracts the moveable shaft shifting the piston toward the first end and energizing the actuation assembly. Further operation of the downhole power unit releases the moveable shaft from the piston such that the energized actuation assembly shifts the piston toward the second end such that interaction between the piston and the wellbore agent opens the barrier and dispenses the wellbore agent.