A setting tool is provided for positioning in a subterranean wellbore. The tool carries a pre-charged, pressurized chamber, preferably filled with inert gas. A force-balanced piston assembly, with the piston chamber initially at atmospheric pressure, is in selective fluid communication with the pressurized chamber. A release mechanism, rupture disc, or valve is selectively operable to open the pressurized chamber and allow fluid flow to the piston chamber. The pressurized gas drives the piston which, in turn, drives a power rod for setting a downhole tool. Preferably a flow restrictor is incorporated in the gas flow path to meter the fluid and control the setting speed. In a preferred embodiment, the pressurized chamber is opened by rupturing a disc. A pyrotechnic device, which qualifies as a non-explosive device and is triggered by a low-powered battery, drives a piercing member into and through the rupture disc.

CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    None. 
       FIELD 
       [0002]    Methods and apparatus are presented for a force-balanced setting tool operable independent of wellbore hydrostatic pressure, and more particularly, to a force-balanced setting tool having a pre-charged, fluid chamber for force generation. 
       BACKGROUND 
       [0003]    Without limiting the scope of the present inventions, their background is described with reference to setting tools, downhole force generators and downhole power units and improvements thereto. It is typical in hydrocarbon wells to “set” or actuate downhole tools, such as packers, bridge plugs, high-expansion gauge hangers, straddles, wellhead plugs, cement retainers, through-tubing plugs, etc. Additionally, some of these tools are later “unset” for retrieval. Setting tools are run-in, and in some cases retrieved, using various conveyance methods such as a wireline, slickline, or coiled tubing. The generic name for the running tool which provides the large setting forces required is a setting tool. 
         [0004]    Several types of setting tool and downhole force generators are known in the art, including those operated mechanically, electrically, chemically, explosively, hydraulically, electro-mechanically, etc. One type of DFG uses electro-mechanical power, where the DFG converts electrical power, typically provided by a battery unit, into mechanical movement, typically rotary or longitudinal movement of a shaft or power rod. One such setting tool is the DPU (trade name) Downhole Power Unit available from Halliburton Energy Services, Inc. 
         [0005]    Additionally, industry standard setting tools, for example, the Baker E4 or Baker 20 setting tool and the Halliburton “Shorty,” operate utilizing a force generated by rapidly burning chemicals, typically in a pyrotechnic charge, to create a high-pressure gas. Such explosive tools are referred to generically as “pyrotechnic” setting tools or force generators. These tools create and contain high pressure gas by igniting a pyrotechnic charge in a closed chamber. The pyrotechnic charge is initiated by electrical current supplied from the surface down an electric cable or from batteries carried downhole with the setting tool and used in conjunction with associated pre-programmed timers, electronics package, etc. The chamber containing the high pressure gas features a floating hydraulic piston with an oil filled chamber below. The hydraulic oil is pressured by the expanding gas, providing hydraulic power which performs the setting task. Disadvantages to such pyrotechnic setting tools include the necessity of transporting a gas-pressured container to the surface after use and releasing the pressure in a controlled and safe manner Such venting is hazardous and conducted under strictly controlled conditions. Further, extensive and costly regulations require special shipping and handling of the pyrotechnic tools by trained personnel, storage on licensed premises, third party notification when shipping, inspections by official personnel, and routine inspections. 
         [0006]    Hydrostatic setting tools convert ambient hydrostatic pressure in a wellbore into hydraulic force to set the downhole tool. The setting tool is equipped with a series of pistons which each have atmospheric pressure on both sides of the piston. The piston series provides motive force. When a valve is opened (by signal or timer) well pressure acts on one side of the pistons causing a pressure imbalance. Bottom hole pressures are typically too small produce sufficient hydraulic power to set a tool, so the force-multiplier pistons generate the pressures needed. Typically, a 1 to 5 multiplier may be required. Such tools can be unwieldy due to the required length necessary for the series of pistons and performance is only marginal in certain circumstances. 
         [0007]    Hydraulic setting tools operate based on operator-increased pressure in the tool string. Typically a mandrel is connected to a work string, a stationary piston connected to the mandrel and dividing an interior chamber into two hydraulic chambers, and a hydraulic cylinder is slidingly mounted on the mandrel. An inlet port allows fluid into the bottom hydraulic chamber, which in turn urges the cylinder away from the stationary piston. As the cylinder moves downward, fluid flows out of the top hydraulic chamber via an outlet port. The movement of the cylinder is used to actuate or set other tools. Hydraulic setting tools can be damaged by hostile environments. Extreme hydrostatic pressure and imbalances between interior and exterior pressures can impair subsequent operation by deforming tool parts. 
         [0008]    Disclosure relating to downhole force generators, their operation and construction can be found in the following, which are each incorporated herein for all purposes: U.S. Pat. No. 7,051,810 to Clemens, filed Sep. 15, 2003; U.S. Pat. No. 7,367,397 to Clemens, filed Jan. 5, 2006; U.S. Pat. No. 7,467,661 to Gordon, filed Jun. 1, 2006; U.S. Pat. No. 7,000,705 to Baker, filed Sep. 3, 2003; U.S. Pat. No. 7,891,432 to Assal, filed Feb. 26, 2008; U.S. Patent Application Publication No. 2011/0168403 to Patel, filed Jan. 7, 2011; U.S. Patent Application Publication Nos. 2011/0073328 to Clemens, filed Sep. 23, 2010; 2011/0073329 to Clemens, filed Sep. 23, 2010; 2011/0073310 to Clemens, filed Sep. 23, 2010; and International Application No. PCT/US2012/51545, to Halliburton Energy Services, Inc., filed Aug. 20, 2012. 
         [0009]    It is an object of the invention then, to provide a pressure-actuated setting tool with a self-contained motive force generator. It is a further object of this invention to provide a setting tool which is not subject to the regulations and restrictions of typical pyrotechnic setting tools. It is a further object of the invention to provide a setting tool with regulated setting speeds. It is a further object of this invention to provide a setting tool which is force-balanced. It is a further object of this invention to provide a setting tool of reasonable length. Other objects and benefits will be apparent to those of skill in the art. 
       SUMMARY 
       [0010]    In aspects, the present disclosure provides methods and apparatus for setting a tool positioned in a subterranean wellbore. In one embodiment, the tool carries a pre-charged pressurized chamber, preferably with an inert gas. A force-balanced piston assembly, with the piston chamber initially at atmospheric pressure, is in selective fluid communication with the pressurized chamber. A release mechanism is selectively operable to open the pressurized chamber and allow fluid flow to the piston chamber. The pressurized gas drives the piston which, in turn, drives a power rod for setting a downhole tool. Preferably a flow restrictor is incorporated in the gas flow path to meter the fluid and control the setting speed. In a preferred embodiment, the pressurized chamber is opened by rupturing a disc or other removable barrier. A pyrotechnic device, which preferably qualifies as a non-explosive device for purposes of transport, etc., is used to drive a piercing member into and through the rupture disc. The pyrotechnic initiator is triggered by a low-powered charge, preferably from a battery carried on the setting tool. A check valve or the like can be used in some embodiments. 
     
    
     
       DRAWINGS 
         [0011]    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: 
           [0012]      FIG. 1  is a schematic view of a well system including an embodiment of the invention positioned in a subterranean wellbore; 
           [0013]      FIG. 2  is a cross-sectional schematic view of an exemplary booster-based, force-balanced setting tool assembly  100  according to an aspect of the invention and in an initial position; and 
           [0014]      FIG. 3  is a cross-sectional schematic view of an exemplary booster-based, force-balanced setting tool assembly according to  FIG. 2  in an actuated or set position. 
       
    
    
       [0015]    It should be understood by those skilled in the art that 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. Where this is not the case and a term is being used to indicate a required orientation, the Specification will state or make such clear. 
       DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0016]    It is to be understood that the various embodiments of the present invention described herein may be utilized in various orientations, such as inclined, inverted, horizontal, vertical, etc., and in various configurations, without departing from the principles of the present invention. The embodiments are described merely as examples of useful applications of the principles of the invention, which is not limited to any specific details of these embodiments. 
         [0017]    In the following description of the representative embodiments of the invention, directional terms, such as “above,” “below,” “upper,” “lower,” etc., are used for convenience in referring to the accompanying drawings. In general, “above,” “upper,” “upward” and similar terms refer to a direction toward the earth's surface along a wellbore, and “below,” “lower,” “downward” and similar terms refer to a direction away from the earth's surface along the wellbore. 
         [0018]      FIG. 1  is a schematic view of a well system including an embodiment of the invention positioned in a subterranean wellbore. A well system  10  is depicted having a wellbore  12  extending through a subterranean formation  14 , shown having casing  16 . The invention can be used in cased or uncased wells, vertical, deviated or horizontal wells, and for on-shore or off-shore drilling. A tubing string  18  is shown having a plurality of tubing sections  20 , a settable downhole tool  30 , a downhole force generator (DFG) assembly  40 , and a force multiplier assembly  50 . A mechanical linkage assembly  60  between the DFG and the downhole tool is provided for transferring the power generated by the DFG into longitudinal or rotary movement, such via a shaft, piston, sleeve, etc. The DFG assembly preferably includes a processor to operate the tool, measure environmental and tool parameters, etc. The settable downhole tools operable by DFG units are not described herein and are well known in the art. For ease of discussion, and by way of example, settable downhole tools such as settable tool  30 , shown as a packer, may be utilized in sealing and anchoring the tubing string at a downhole location. The packer has sealing elements  32  which may be set, along with slips, anchors, etc., as is known in the art. 
         [0019]      FIG. 2  is a cross-sectional schematic view of an exemplary booster-based, force-balanced setting tool assembly  100  according to an aspect of the invention.  FIG. 3  is a cross-sectional schematic view of an exemplary booster-based, force-balanced setting tool assembly according to  FIG. 2  in an actuated or set position. The Figures are discussed in conjunction. The setting assembly  100  can be used in conjunction with any settable tool or tool requiring a mechanical movement in a downhole environment. The movement most frequently used is a linear axial stroke, in either direction. The embodiment of the setting assembly shown provides an axially upward movement of a selected stroke length. As those of skill in the art will recognize, other embodiments can provide a downward setting stroke. Additionally, the setting assembly can be used to provide other types of mechanical motion, such as rotational, etc., with appropriate mechanical parts to translate motion, as will be recognized by those of skill in the art. The embodiment is discussed in terms of a setting tool for use in linear actuation of a downhole tool, however, it is understood that the invention disclosed herein can be used in other types of tool assemblies and for providing non-axial motive force. 
         [0020]    The setting tool assembly  100  has an upper connector subassembly  102 , shown configured for connection at threads  104  to a sucker rod (not shown) or similar. It is understood that the upper connector can be selected for connection to a tool string, wireline, coiled tubing, etc. The upper connector  102  has lower threads at  110  which mate with the housing  108  of the control assembly. 
         [0021]    The control assembly  106  has a housing  108 , preferably a tubular body, connected to the upper connector sub  102  at threads  110  and connected at threads  112  to connector subassembly  130 . The control assembly  106  houses an electronic control module  114  having, in a preferred embodiment, a power source, such as batteries, an electric-powered timer or timing device, and indicators  118  for start-up and timer set values. The indicators can be LED or other indicators as known in the art. The timer and battery packs are not discussed in detail and are known in the art. An electrical connector  116  is preferably provided for e-line start. It is also possible to provide electrical power via power line from the surface for signaling initiation, powering the initiator or actuator, etc. Further disclosure regarding timers, batteries, etc., can be found in the references incorporated herein. A hermetic connector  120  is positioned between the control module  114  and connector sub  130  to provide a hermetically sealed section for housing the control module. 
         [0022]    A connector subassembly  130  has a connector body  132  with a bore  134  defined therein and extending axially therethrough. The bore  134  houses communication lines, such as electrical wiring, necessary for transmitting a signal from the control module to the actuator  154 . The connector sub attaches to housing  108  at its upper end and to housing  142  at its lower end. 
         [0023]    A booster assembly  140  has a housing  142  attached at threads  144  to the connector sub  130  and at threads  146  to connector sub  180 . The booster assembly  140  defines a booster chamber  148  which is pre-charged with a pressurized fluid, preferably an inert gas to an actuation pressure. A charge port  151  and charging valve  150  are provided, with appropriate fluid passageways to the chamber, for supplying the pressurized gas to the chamber. In the embodiment shown, the charging valve and port are positioned in connector sub  180 , although they can be positioned in connector sub  130  or as part of the booster assembly  140 . 
         [0024]    Positioned in the booster assembly is an initiator  154 , actuator retainer  152 , rupture disc  160 , and pin actuator  158 . The initiator  154  is electrically connected via wire extending from the actuator retainer  152 , through a conduit or similar which is in threaded connection to the passageway  134  of connector assembly  130 , and the control electronic control module  114 . The initiator is triggered by a small electrical charge. The actuator retainer  152  houses the initiator  154 . The rupture disc retainer and actuator guide  156  is mounted to the tool assembly, for example, to the connector assembly  180 , as shown, via threaded connection or similar. Alternately, the retainer can be mounted to the housing, etc. The initiator  154  is positioned adjacent or proximate a rupture disc  160  that initially blocks fluid flow from the pressurized chamber. 
         [0025]    Small, pyrotechnic initiators  154  are available from commercial vendors known in the art, such as SDI, Inc. The pyrotechnic initiator utilizes a small amount of pyrotechnic material, triggerable by a low electrical charge, to drive a thruster pin  158  longitudinally into and rupturing the rupture disc. The pin is preferably hollow with a relief port on the stem such that if the disc fails to rupture after the pin has pushed through the disc, a fluid path is available through the pin. Note that the pyrotechnic initiator does not provide the motive force for movement of the setting rod. The tool assembly is not a pyrotechnic setting tool. The initiator only provides motive force to move the pin actuator to rupture a rupture disc. The motive force for setting the tool is provided by the release of pressurized gas in the booster chamber. Because such a low amount of force is required of the initiator, and such a small amount of chemical or pyrotechnic required to provide the force, the preferred pyrotechnic initiator is classified by DOT and BATF as a non-explosive for purposes of transportation and shipping. 
         [0026]    In addition to the preferred pyrotechnic initiator, other initiators can be used, preferably low-powered and classified as non-explosive. For example, such initiators include electrical, chemical, thermal, and other initiators. The initiators can open the pressurized chamber by opening, melting, dissolving, burning, etc., a fluid barrier. Further, the initiator can be used to power or actuate a variety of available actuators, such as a thruster pin, a check-valve, other valves, etc., to open the pressurized chamber to fluid flow. 
         [0027]    Power to trigger the initiator is provided from the battery pack or power source in the electronic control module  114  of the control assembly  106 . Since the preferred initiator is small and requires low power to initiate, it is ideal for low-powered battery activation. With a small power requirement, the timer can be small and low power and included within the timer module (e.g., a single CFX battery from Contour Energy; rated to 160 C and higher). The timer module can be small and used for the various tools for the different setting tools. The small timer module can thermally insulated, for example, for use in higher temperature operations within the larger housings of the bigger setting tools. The timer module is preferably switch-selectable and can include an electrical start port for either e-line or a pressure/temperature switch. Additional features could be added to the timer (pressure, temperature, motion, etc.), however, this would result in a larger electronics and battery assembly. 
         [0028]    The rupture disc  160  can be selected from those known in the art and alternative discs and rupture assemblies will be apparent to those of skill in the art. The disc can be made of ceramic, metal, plastic, etc. The disc can be ruptured, punctured, dissolved, melted, etc., depending on the selected initiator and actuator. The preferred assembly utilizes a rupture disc which is physically punctured or broken by the extendable pin of the initiator. The rupture disc  160  initially blocks fluid flow from pressurized chamber  148  into passageway  184  of connector assembly  180 . In a preferred embodiment, the rupture disc is mounted to the housing, connector assembly or retainer  156 . The disc assembly is positioned in a bore  157  designed for that purpose in the connector assembly  180 . Seals  161  are provided as necessary to facilitate assembly and fluid isolation. The retainer  156  provides and maintains positioning of the disc. Upon rupture, fluid communication is provided between the pressurized chamber  148  and the passageway  184  through connector assembly  180 . 
         [0029]    The initiator assembly, in a preferred embodiment, is a thruster assembly for rupturing discs. Actuator assemblies are commercially used by Halliburton Energy Services, Inc., and disclosure regarding their structure and use can be found in the following, which are each hereby incorporated by reference for all purposes: U.S. Pat. No. 8,235103, to Wright, issued Aug. 7, 2012; U.S. Patent Application Publication No. 2011/0174504, to Wright, filed Jan. 15, 2010; and U.S. Patent Application Publication No. 2011/0174484, to Wright, filed Dec. 11, 2010; U.S. Patent Publication No. 2011/0265987, to Wright, filed Apr. 28, 2010; and U.S. Patent Application Serial No. PCT/US12/53448 filed Aug. 31, 2012, to Fripp, et al. Additional actuator assemblies are known in the art and will be understood by persons of skill in the art. Additional actuator assemblies are known in the art and will be understood by persons of skill in the art. Key components are the rupture disc, an electrical power source, and an electrically-initiated method of breaching the barrier disc. In the preferred embodiment, the electrical power source is a battery, and a thruster assembly is used to puncture the disc. 
         [0030]    Connector assembly  180  is attached to a vent chamber assembly  190 , preferably by threaded connection to a vent chamber housing  192 . The vent chamber  194  defined within the vent chamber assembly contains fluid at hydrostatic pressure as it is open to fluid flow between the chamber and the exterior of the tool (the wellbore). One or more ports  196  provide fluid communication between chamber and exterior. A thick-walled tube  198  extends from the passageway  184  to a force-balance piston rod  216 , providing communication of the released pressurized gas from the pressurized chamber  148  to the piston passageway  218 . As piston rod  216  moves upward into the vent chamber, pressure is equalized in the vent chamber  194  as fluid flows out of the chamber through ports  196 . Note that the setting section is force balanced by hydrostatic pressure acting on the power rod  230  from below, so the setting action is independent of hydrostatic pressure. 
         [0031]    A flow restrictor  164  is preferably positioned across the passageway  182  of the connector assembly  180 . The speed of setting is controlled by the flow restrictor. The flow restrictor can be positioned elsewhere along the flow path from the pressurized chamber to the piston head. Flow restrictors and use thereof to control setting speed is known in the art. The flow restrictor can be a flow nozzle, orifice, plate, inflow control device, autonomonous inflow control device, tortuous path etc, as known in the art. 
         [0032]    A connector assembly  200  provides flow connection between the vent chamber assembly  190  and the force-balance piston assembly  210 . The connector assembly body  202  is threadedly attached to the vent chamber housing  192  and to a piston housing  212 . An axial passageway  204  is defined through the connector body, the piston rod  216  axially slidable therein. Seals  206  are provided for sealing engagement between passageway wall and piston. Further, rod-wipes  208 , or similar, are mounted to wipe the exterior surface of the piston as it moves through the passageway  204 . 
         [0033]    A piston assembly  210  is attached to the connector assembly  200  at housing  212 . The housing defines a piston chamber  214  which is divided into two spaces by piston head  220 . The chamber  214  is preferably at atmospheric pressure initially. Piston rod  216  defines an axial passageway  218  therein providing fluid communication from the tube  198  to a passageway  222  through the piston head  220 . The piston rod  216  is mounted to the piston head  220 . A power rod  230  is attached to the lower end of the piston head  220 . Appropriate porting  224  provides fluid communication from the passageway  218  of the piston rod to the chamber  214  below the piston head  220 . When pressurized gas is released from pressurized chamber  148 , the gas flows through the various passageways and tubes, through passageway  218  of the piston rod, through passageway  222  of the piston head  220 , and through porting  224  to the chamber  214  below the piston head. The pressurized gas forces the piston head upward. Upward movement of the piston head causes piston rod  216  to slide upwardly through the connector assembly  200  and into vent chamber  194 . Movement of the piston head also pulls power rod  230  upwardly through a bore  232  defined in the lower end of the piston housing sub  210 . Appropriate seals  234  and wipers  236  can be employed. 
         [0034]    Movement of the power rod, axially, provides the necessary motion to set (or un-set) the settable tool positioned below the setting assembly. The setting force is supplied by the pre-charged fluid in the booster chamber. Carrying the setting force with a gas pre-charge means a large motor and battery arrangement, typical in many downhole force generators, is not required. 
         [0035]    The entire assembly is compact, reducing the overall length of the tool assembly. This can be important in negotiating long, deviated or horizontal wellbores. Preferably, the length of the setting tool assembly is on the order of six feet for every eight inches of stroke. 
         [0036]    Greater setting force can be provided by utilizing a force-multiplying piston having varying surface areas on either side of the piston head, as is known in the art. Further disclosure relating to force-multiplier piston assemblies can be found, for example, in U.S. Pat. Pub. No. 2006/0076144 to Shammai; U.S. Pat. Pub. No. 2006/0022013 to Gaudron; U.S. Pat. Pub. No. 2003/0075339 to Gano; U.S. Pat. No. 8,006,952 to Wygnanski; U.S. Pat. No. 6,966,370 to Cook; U.S. Pat. No. 7,000,705 to Buyers; each of which is incorporated herein by reference for all purposes. 
         [0037]    A person skilled in the art would, upon a careful consideration of the above description of representative embodiments of the invention, readily appreciate that many modifications, additions, substitutions, deletions, and other changes may be made to the specific embodiments, and such changes are contemplated by the principles of the present invention. Accordingly, the foregoing detailed description is to be clearly understood as being given by way of illustration and example only, the spirit and scope of the present invention being limited solely by the appended claims and their equivalents.