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TECHNICAL FIELD 
       [0001]    The present disclosure relates generally to oilfield equipment, and in particular to downhole tools, drilling and related systems and techniques for drilling, completing, servicing, and evaluating wellbores in the earth. More particularly still, the present disclosure relates to an improvement in systems and methods for jarring operations. 
       BACKGROUND 
       [0002]    During the drilling, completion, servicing, or evaluation of an oil or gas wellbore or the like, situations are encountered wherein a downhole tool or a component of drill string or other conveyance becomes lodged in the wellbore. When the static force necessary to move a component exceeds the rig&#39;s capabilities or the tensile strength of the conveyance, the component is stuck and can no longer be moved or rotated. A jar is a tool that may be prepositioned within the string or other conveyance, such as wireline, e-line, slickline, etc., to free any component which may become stuck. Jars may also be used to shear pins, push or pull tools, actuate tools, et cetera. 
         [0003]    A jar operates by releasing stored potential energy. Jarring is the process of converting potential energy into kinetic energy concentrated at a given point. In a typical jar, the potential energy available comes from over-pull (tensile) or set-down (compressive) forces applied to the drill pipe at the surface. 
         [0004]    A typical jar may include a mandrel, which slides within a sleeve, and a detent mechanism. The mandrel functions as a hammer, and the sleeve functions as an anvil. The detent mechanism restricts the movement of the mandrel before releasing it (“firing”), so that sufficient potential energy is accumulated and transferred to the mandrel to cause, upon firing, the mandrel to rapidly move and strike the sleeve. This impact creates an impulse and the kinetic energy is transmitted as shock wave that travels up and down the tool string, drill string, or other conveyance to free a stuck tool or pipe, to shear pins, or to perform some other desired function. 
         [0005]    A jar tool may be a double acting jar that can provide jarring force both upwards and downwards. The separate functions of jarring upward or downward may be accomplished in any sequence; that is, up only, down only, or alternately up and down. A jar may be classified as either of two types based on the detent mechanism: hydraulic and mechanical. 
         [0006]    A hydraulic jar moves a piston with a fluid-filled hydraulic cylinder. Fluid passes from one side of the piston to the other through an orifice, triggering valve, or similar restriction which initially limits flow to create a time delay during the loading phase and then opens the flow path to trip the detent mechanism and fire the jar. In some hydraulic jars, the pressure piston must move a predetermined distance in order to bypass the restriction or open the triggering valve. The built-in delay is designed to allow the operator sufficient time to apply the desired tensile or compressive force to the drill string before the flow restriction is cleared or the triggering valve is opened. Therefore, varying the metering rate of the fluid through the restriction varies the magnitude of impact. 
         [0007]    In contrast, a mechanical jar is actuated using a series of springs, locks, and rollers with release mechanisms. A mechanical jar fires upward at a preset tensile force and downward at a preset compressional force, which normally exceed the forces reached during drilling. Firing does not depend on the duration of the loading phase. A mechanical jar is typically either non-adjustable and made to deliver a preset amount of jarring force, or field-adjustable allowing setting at the surface before the jar is run into the wellbore. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    Embodiments are described in detail hereinafter with reference to the accompanying figures, in which: 
           [0009]      FIG. 1A  is an elevation view in partial cross section of a drilling system that employs a drill string with drill pipe, and at least one controllable, powered bidirectional jarring tool assembly according to an embodiment; 
           [0010]      FIG. 1B  is a block-level schematic diagram of a well servicing or like system according to an embodiment, showing downhole tools including powered bidirectional jarring tool assembly suspended by wireline in a well; 
           [0011]      FIG. 2A  is an elevation view of a stroker of  FIG. 1A or 1B  according to an embodiment, shown in partial cross section along a longitudinal axis thereof to reveal a pump and hydraulic ram for moving an interface rod; 
           [0012]      FIG. 2B  is an elevation view of a jar of  FIG. 1A or 1B  according to an embodiment, shown in partial cross section along a longitudinal axis thereof to reveal a central anvil, first and second hammers urged toward the anvil by first and second springs, and an interface rod carrying a catch for engaging the hammers; 
           [0013]      FIG. 3A  is an enlarged axial cross section of the catch of  FIG. 2B  according to an embodiment, showing the catch in a disengaged position; 
           [0014]      FIG. 3B  is an enlarged axial cross section of the catch of  FIG. 3A , showing the catch in an engaged position; 
           [0015]      FIGS. 4A-4F  are simplified axial cross sections of the jar of  FIG. 2B  that illustrate the operational sequence for creating a jarring force in a first axial direction; 
           [0016]      FIGS. 5A-5F  are simplified axial cross sections of the jar of  FIG. 2B  that illustrate the operational sequence for creating a jarring force in a second axial direction; and 
           [0017]      FIG. 6  is a flow chart showing a method for creating a jarring force along a drill string for use with jar tools such as those illustrated in  FIGS. 4A-4F and 5A-5F . 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    The foregoing disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. Further, spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper,” “uphole,” “downhole,” “upstream,” “downstream,” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the apparatus in use or operation in addition to the orientation depicted in the figures. 
         [0019]      FIG. 1A  is an elevation view in partial cross-section of a drilling system  20  including a bottom hole assembly  90  according to an embodiment. Drilling system  20  may include a drilling rig  22 , such as the land drilling rig shown in  FIG. 1A . However, teachings of the present disclosure may be used in association with drilling rigs  22  deployed on offshore platforms, semi-submersibles, drill ships, or any other drilling system for forming a wellbore. 
         [0020]    Drilling rig  22  may be located proximate to or spaced apart from well head  24 . Drilling rig  22  may include rotary table  38 , rotary drive motor  40 , and other equipment associated with rotation of drill string  32  within wellbore  60 . Annulus  66  is formed between the exterior of drill string  32  and the inside wall of wellbore  60 . For some applications drilling rig  22  may also include top drive motor or top drive unit  42 . Blowout preventers (not expressly shown) and other equipment associated with drilling a wellbore may also be provided at well head  24 . 
         [0021]    The lower end of drill string  32  may include bottom hole assembly  90 , which may carry at a distal end a rotary drill bit  80 . Drilling fluid  46  may be pumped from reservoir  30  by one or more mud pumps  48 , through conduit  34 , to the upper end of drill string  32  extending out of well head  24 . The drilling fluid  46  then flows through the longitudinal interior  33  of drill string  32 , through bottom hole assembly  90 , and exits from nozzles formed in rotary drill bit  80 . At bottom end  62  of wellbore  60 , drilling fluid  46  may mix with formation cuttings and other downhole fluids and debris. The drilling fluid mixture then flows upwardly through annulus  66  to return formation cuttings and other downhole debris to the surface. Conduit  36  may return the fluid to reservoir  30 , but various types of screens, filters and/or centrifuges (not expressly shown) may be provided to remove formation cuttings and other downhole debris prior to returning drilling fluid to reservoir  30 . Various types of pipes, tube and/or hoses may be used to form conduits  34  and  36 . 
         [0022]    Bottom hole assembly  90  may include a downhole mud motor  82 , which may have a bent housing. Bottom hole assembly  90  may also include various other tools  91 , such as those that provide logging or measurement data and other information from the bottom of wellbore  60 . Measurement data and other information may be communicated from end  62  of wellbore  60  using measurement while drilling techniques using electrical signals or other communication media that can be converted to electrical signals at the well surface to, among other things, monitor the performance of drilling string  32 , bottom hole assembly  90 , and associated rotary drill bit  80 . 
         [0023]    Drill string  32  includes a jarring tool assembly  99  with jar  100 . Jarring tool assembly  99  may be located in bottom hole assembly  90  or elsewhere along drill string  32 . Although not illustrated, stroker tool  120  and/or drill string  32  may include an anchoring device, such as an inflatable packer. In some configurations, multiple jarring tool assemblies  99  may be included in drill string  32 . Jar  100  includes a housing  102  and a stroke rod  110 , which is linearly stroked with respect to jar housing  102  to transfer potential energy to jar  100  and to cock jar  100  for firing, as described in greater detail below. 
         [0024]      FIG. 1A  illustrates two jarring tool assemblies  99  according to various embodiments of the present disclosure. According to a first embodiment, interface stroke rod  110  of jarring tool assembly  99   a  is connected to a stroker tool  120 , which functions downhole to stroke rod  110  as described below. According to a second embodiment, stroke rod  110  of jarring tool assembly  99   b  is connected directly to drill string  32 , which may function to stroke rod  110  relative to housing  102 . 
         [0025]    Although the embodiments of  FIG. 1A  are described as a drilling system using a drill string, the present disclosure is not limited to a single embodiment. Accordingly, the term “drilling system” broadly includes various production systems, completion systems, workover systems, and the like used with wellbores. Likewise, the term “drill string,” as used herein broadly encompasses any conveyance for downhole use, including working strings, completion strings, evaluation strings, other tubular members, wireline systems, and the like. For example,  FIG. 1B  shows a system view of a well servicing system  200  including a wireline cable deployment of the present disclosure according to an embodiment. 
         [0026]    Referring to  FIG. 1B , a wireline cable  211  suspends jarring tool assembly  99  in wellbore  213 , including jar  100  optionally connected to stroker tool  120  at stroke rod  110 . Wellbore  213  may have been drilled by a drill bit on a drill string as illustrated in  FIG. 1A , and wellbore  213  may be uncased or lined with casing. Wellbore  213  can be any depth and the length of wireline cable  211  may be any length appropriate for the depth of wellbore  213 . 
         [0027]    Various other types of tools  201  may also be suspended by wireline cable  211  in wellbore  213 . Jarring tool assembly  99  and other tools  201  may be supported by a downhole power supply  215 , multiplexer  216 , and/or communications module  217 . Communications module  217  may include an uplink communication device, a downlink communication device, a data transmitter, and/or a data receiver. 
         [0028]    Well servicing system  200  includes a sheave  225  which is used for guiding wireline cable  211  into wellbore  213 . Wireline cable  211  is spooled on a cable reel  226  or drum for storage. Wireline cable  211  connects with the downhole equipment and is spooled out or taken in to raise and lower the tools in wellbore  213 . 
         [0029]    Wireline cable  211  may include electrical, optical, or hydraulic conductors that connect with surface-located equipment, which may include a DC power source  227  to provide power to downhole power supply  215 , a surface communication module  228  having an uplink communication device, a downlink communication device, a data transmitter, a data receiver, a surface computer  229 , a display  231 , and/or one or more recording devices  232 . Sheave  225  may be connected to surface computer  229  by a suitable communication means to provide depth measuring information. Surface computer  229  may provide output to display  31  and/or recording device  32 . 
         [0030]      FIGS. 2A and 2B  together show an elevation view of a jarring tool assembly  99 , which includes jar  100  and may include a stroker tool  120 , according to an embodiment. With the exception of interface stroke rod  110 , jar  100  and stroker  120  are shown in axial cross section to illustrate the operation of jarring tool assembly  99 . 
         [0031]    Jarring tool assembly  99  as shown in  FIGS. 2A and 2B  is arranged for connection within drill string  32  ( FIG. 1A ). Accordingly, jar  100  and stroker tool  120  include connections  150 ,  152 , which may be threaded pins or the like. Jarring tool assembly  99  may include a flow path, such as that illustrated by arrows  154 , for drilling fluid or other well treatment to pass through the tool. Fluid from drill string  32  ( FIG. 1A ) may enter the housing  122  of stroker  120  at connection  152 . Stroker  120  may include a hollow piston rod  112 , which is fixed to a hollow stroke rod  110 . Fluid may then enter the distal end of piston rod, flow through piston rod  112  into stroke rod  110 , and exit the distal end of stroke rod  110 . Fluid flow may continue through jar housing  102 , exiting into drill string  32  ( FIG. 1A ) through connection  150 . 
         [0032]    Jarring tool assembly  99  may be arranged for transmitting drill string torque. According to an embodiment, stroke rod may have a portion with a circular outer wall  114  and a portion with a hexagonal outer wall  116 . The medial end  123  of housing  122  of stroker  120  may include an aperture  124  having a hexagonal shape dimensioned so that hexagonal outer wall  116  can slide therein and torque applied to housing  122  is transferred to stroke rod  110 . Similarly, the medial end  103  of housing  102  of jar  100  may include an aperture  104  having a hexagonal shape dimensioned so that hexagonal outer wall  116  can slide therein and torque applied to stroke rod  110  is transferred to housing  102 . Circular outer wall  114  is dimensioned so that it will not pass through hexagonal aperture  104 , thereby preventing stroke rod  110  from being removed from jar  100 . Although a hexagonal shape is described herein, other spline profiles that allow sliding movement and torque transmission may be used as appropriate. 
         [0033]    Jar  100  includes a shaped anvil  130 , which is located on the interior wall  105  of housing  102 . Anvil  130  has a longitudinal bore  131  formed therethrough, and in an embodiment bore  131  is coaxial with housing  102 . Anvil  130  is rigidly fixed to housing  102  so that jarring forces acting upon anvil  130  are transferred to drill string  32  ( FIG. 1A ). In an embodiment, anvil  130  may be integrally formed with housing  102 . Jar  100  also includes a first hammer  140 , which is axially movable within housing  102 , and a first spring  142  which urges first hammer  140  in a first axial direction (indicated by arrow  156 ) against anvil  130 . First hammer has a longitudinal bore  141  formed therethrough, and in an embodiment bore  141  is coaxial with housing  102 . 
         [0034]    Stroke rod  110  includes a catch  118 , which may be located at or near the distal end of stroke rod  110 . As described in greater detail below with respect to  FIGS. 3A and 3B , in a disengaged position, catch  118  is not engaged with first hammer  140  and in an engaged position, catch  118  is engaged with hammer  140 . In one or more embodiments, catch  118  may be radially extended into the engaged position and retracted into the disengaged position. Regardless of whether catch  118  is in the engaged or disengaged position, stroke rod  110  is dimensioned so as to pass freely through anvil bore  131 . When catch  118  is in a disengaged position, stroke rod  110  is dimensioned so as to pass through first hammer bore  141 . However, when catch  118  is extended in the engaged position, catch  118  will not pass through first hammer bore  141 . In this manner, as described in greater tail below with respect to  FIGS. 4A-4F , by selectively engaging catch  118 , stroke rod  110  may be used to move first hammer  140  in a second axial direction (indicated by arrow  155 ) away from away from anvil  130  to compress first spring  142 , storing potential energy and putting jar  100  in a cocked state; then by selectively disengaging catch  118 , first hammer is released and accelerated by first spring  142  against anvil  130 , creating a jarring effect in the first axial direction  156 . The extent to which first hammer  142  is moved to compress first spring  142  determines the resultant jarring force. 
         [0035]    In a preferred embodiment, as illustrated in  FIG. 2B , jar  100  is capable of bidirectional jarring. In such an embodiment, anvil  130  may be located approximately midway along the axial length of housing  102  and may have first and second sides  132 ,  133  forming obverse striking surfaces for bidirectional jarring. In other embodiments, separate anvils  130  may be provided for each hammer. First hammer  140  is located to a first side of anvil  130  and is urged against striking surface  132  by first spring  142 . A second hammer  145 , which is axially movable within housing  102 , and a second spring  147  which urges second hammer  145  in the second axial direction  155  against striking surface  133  of anvil  130 , are provided. Second hammer  145  has a longitudinal bore  146  formed therethrough, and in an embodiment bore  146  is coaxial with housing  102 . As with first hammer  140 , when catch  118  is in a disengaged position, stroke rod  110  is dimensioned so as to pass through second hammer bore  146 , and when catch  118  is extended in the engaged position, catch  118  will not pass through second hammer bore  146 . Accordingly, as described in greater tail below with respect to  FIGS. 5A-5F , stroke rod  110  may also be used to move second hammer  140  in the first axial direction  156  away from away from anvil  130  to compress second spring  147  and then release second hammer  145  for creating a jarring effect in the second axial direction  155 . 
         [0036]    According to an embodiment, as shown in  FIG. 2A , jarring tool assembly  99  includes stroker tool  120 , which is operable to move stroke rod  110  with respect to jar housing  102 . Stroker tool  120  may include a hydraulic ram actuator  160  having a piston  162  connected to piston rod  112 . Piston  162  is dynamically sealed and moves linearly in a hydraulic cylinder defined by inner wall  161  and first and second end caps  163 ,  164 . Piston rod  112  is dynamically sealed within apertures formed through first and second end caps  163 ,  164 . 
         [0037]    Piston rod  112 , piston  162 , inner wall  161 , and first end cap  163  define a first hydraulic chamber  168 , and piston rod  112 , piston  162 , inner wall  161 , and second end cap  164  define a second hydraulic chamber  169 . First and second hydraulic chambers  168 ,  169  are fluidly coupled via hydraulic pump  170  and flow channels  171 ,  172 . Pump  170  may be selectively actuated to transfer fluid from chamber  169  to  168 , thereby moving piston  162 , piston rod  112 , and stroke rod  110  in the second axial direction  155  (referred to above with respect to jar  100 ). Conversely, pump  170  may be selectively actuated to transfer fluid from chamber  168  to  169 , thereby moving piston  162 , piston rod  112 , and stroke rod  110  in the first axial direction  156  (referred to above with respect to jar  100 ). 
         [0038]      FIGS. 3A and 3B  show enlarged axial partial cross sections of catch  18  of carried by stroke rod  110  according to an embodiment, showing the catch in disengaged and engaged positions, respectively. The disclosure is not limited to a particular configuration of catch  118  so long as catch  118  functions to engage and disengage hammers  140 ,  145 . In one or more preferred embodiments, catch  118  may include one or more radially movable fingers  180  that are actuated to move radially inward and outward relative to the stroke rod  110 . In one or more embodiments, such fingers are balls  180  movably captured within tapered radial apertures  181  formed in stroke rod  110 . As shown most clearly in  FIG. 3B , apertures  181  are dimensioned so that ball(s)  180  may extend partially beyond the outer circumference of stroke rod  110  but cannot completely pass through aperture  181 . In the embodiment illustrated in  FIGS. 3A and 3B , four balls  180  are provided at 90 degree spacing, however, a greater or lesser number of balls  180  may be provided as appropriate. 
         [0039]    A cone  184 , which may broadly include a frustroconical- or pyramid-shaped structure having tapered or curved surfaces for engaging balls  180  is provided, whereby the tapered surfaces form a first portion of the cone  184  with a surface of a larger diameter and a second portion of the cone  184  with a surface of a diameter smaller than the first diameter. Cone  184  may be moved axially by an actuator  186 , which may be a solenoid, or hydraulic piston-cylinder arrangement, for example. Actuator  186  may include control/power lines  187 , which may pass through stroke rod  110  to stroker  120  or another device via drill pipe  32  ( FIG. 1A ), for example. Cone  184  may include one or more flow passages  189  formed therethrough for accommodating drill string fluid flow. 
         [0040]    While one embodiment of catch is described as having a cone  184  and balls  180 , in other embodiments, for example, fingers  180  may be pins seated in a piston which pins can be moved radially outward under pressure from within the piston. 
         [0041]    As shown in  FIG. 3A , when catch  18  is in the inward, disengaged position, a second portion of cone  184  having a smaller diameter engages balls  180  thereby allowing balls  180  to be substantially contained within stroke rod. In this position, cone  184  still captures balls  180  partially within apertures  181 . Should any ball  180  continue to extend radially outward from stroke rod  110  after catch  18  is disengaged, any external force acting thereon, such as from first or second hammer  140 ,  145  ( FIG. 2B ), will urge the ball inward. As shown in  FIG. 3B , when catch  18  extends radially outward from said stroke rod  110 , in the engaged position, a first portion of cone  184  having a larger diameter engages balls  180  forces balls  180  to extend radially outward from stroke rod  110 . 
         [0042]    While the figures illustrate catch  18  engaging a hammer  140 ,  141  at an end of the respective hammer, in other embodiments, catch  18  may engage the respective hammer anywhere along the length of the hammer. For example, a seat or cavity (not shown) may be provided at any point along the length of longitudinal bore  141  for receipt of catch  18 . 
         [0043]      FIGS. 4A-4F  are simplified axial cross sections of jar  100  according to an embodiment that illustrate the process of creating a jarring force in first axial direction  156  using first hammer  140 .  FIG. 6  is a flowchart describing a method for creating a jarring force in a wellbore. Referring to  FIGS. 4A-4F and 6 ,  FIG. 4A  shows stroke rod  110  of jar  100  in an initial condition according to step  300  where stroke rod  110  is positioned to align with anvil  130 . Catch  18  is disengaged so that stroke rod  110  can move axially independently of hammers  140 ,  145 . In step  304 , first hammer  140  is engaged by stroke rod  110 . More specifically, stroke rod  110  may be positioned adjacent first hammer  140  and catch  18  is actuated so as to extend radially outward to engage first hammer  140 .  FIG. 4B  illustrates this state. 
         [0044]    At step  308 , stroke rod  110  is used to move first hammer  140  in a second axial direction  155  opposite the first axial direction  156 , thereby causing the first hammer  140  to compress a first spring  142 , as shown in  FIG. 4C . To fire jar  100 , at step  312 , catch  18  is disengaged ( FIG. 4D ) from first hammer  140 , so that at step  316 , first hammer  140  is allowed to rapidly accelerate under the force of spring  142  to strike anvil  130 , thereby creating a jarring force in first axial diction  156  ( FIG. 4E ). At step  320 , stroke rod  110  is moved to place jar  100  in the initial condition again, with catch  18  axially aligned with anvil  130  ( FIG. 4F ), and the process may be repeated, starting with step  304 , to provide jarring in first direction  156 , as just described, or to provide jarring in second direction  155 , as described below. 
         [0045]      FIGS. 5A-5F  are simplified axial cross sections of jar  100  according to an embodiment that illustrate the process of creating a jarring force in second axial direction  155  using second hammer  145 . Referring to  FIGS. 5A-5F and 6 ,  FIG. 5A  shows stroke rod  110  of jar  100  in an initial condition according to step  300  where stroke rod  110  is positioned to align with anvil  130 . Catch  18  is disengaged so that stroke rod  110  can move axially independently of hammers  140 ,  145 . In step  304 , second hammer  145  is engaged by stroke rod  110 . More specifically, stroke rod  110  may be positioned adjacent second hammer  145  and catch  18  is actuated so as to extend radially outward to engage second hammer  145 .  FIG. 5B  illustrates this state. 
         [0046]    At step  308 , stroke rod  110  is used to move second hammer  145  in first axial direction  156 , thereby causing second hammer  145  to compress a second spring  147 , as shown in  FIG. 5C . To fire jar  100 , at step  312 , catch  18  is disengaged ( FIG. 5D ) from second hammer  145 , so that at step  316 , second hammer  145  is allowed to rapidly accelerate under the force of spring  147  to strike anvil  130 , thereby creating a jarring force in the second axial diction  155  ( FIG. 5E ). At step  320 , stroke rod  110  is moved to place jar  100  in the initial condition again, with catch  18  axially aligned with anvil  130  ( FIG. 5F ), and the process may be repeated, starting with step  304 , to provide jarring in first direction  156  or second direction  155 . 
         [0047]    In summary, a jarring tool assembly, jarring system for use in a wellbore, and a method for creating a jarring force have been described. Embodiments of the jarring tool assembly may generally have: A generally cylindrical housing; an anvil fixed within the interior the housing; a first hammer movably disposed within the housing at a first side of the anvil; a first spring disposed within the housing so as to urge the first hammer towards the anvil in a first axial direction; a stroke rod at least partially disposed and axially movable within the housing, the stroke rod being selectively movable with respect to the first hammer; a catch carried by the stroke rod and being radially movable with respect to the stroke rod between a disengaged position and an engaged position; and an actuator coupled to the catch so as to selectively move the catch between the disengaged and engaged positions; wherein in the disengaged position, the stroke rod is freely movable with respect to the first hammer in the first axial direction and in a second axial direction opposite the first axial direction; and in the engaged position, the catch is positioned for engagement with the first hammer so that the stroke rod becomes fixed to the first hammer during movement in the second axial direction. Embodiments of the jarring system for use in a wellbore may generally have: A conveyance; and a jar carried by the conveyance and disposed within the wellbore, the jar including an anvil fixed within the interior of a housing, a first hammer movably disposed within the housing and positioned on a first side of the anvil, a first spring disposed within the housing to urge the first hammer towards the anvil in a first axial direction, a stroke rod at least partially and movably disposed within the housing, the stroke rod being selectively movable with respect to the first hammer, a catch carried by the stroke rod and radially movable with respect to the stroke rod between a disengaged position and an engaged position, and an actuator coupled to the catch so as to selectively move the catch between the inward and outward positions, wherein in the disengaged position, the stroke rod is freely movable with respect to the first hammer in the first axial direction and in a second axial direction opposite the first axial direction, and in the engaged position, the catch is positioned for engagement with the first hammer so that the stroke rod becomes fixed to the first hammer during movement in the second axial direction. Embodiments of the method for creating a jarring force may generally include: Engaging a first hammer by a stroke rod; moving the first hammer by the stroke rod in a second axial direction opposite a first axial direction so as to compress a first spring; and then disengaging the stroke rod from the first hammer so as to allow the first spring to move the first hammer into striking engagement with an anvil, thereby providing the jarring force within the wellbore in the first axial direction. 
         [0048]    Any of the foregoing embodiments may include any one of the following elements or characteristics, alone or in combination with each other: A second hammer movably disposed within the housing at a second side of the anvil opposite the first side; a second spring disposed within the housing so as to urge the second hammer toward the anvil in the second axial direction; the stroke rod is selectively movable with respect to the second hammer; in the disengaged position, the stroke rod is freely movable with respect to the second hammer in the first and second axial directions; in the engaged position, the catch is positioned for engagement with the second hammer so that the stroke rod becomes fixed to the second hammer during movement in the first axial direction; a stroker tool connected to the stroke rod and operable to selectively move the stroke rod in the first and second axial directions with respect to the housing; the actuator is coupled to the stroker tool so that the stroker tool controls the actuator; the first hammer has a longitudinal bore formed therethrough; the stroke rod is axially movable through the longitudinal bore; the first hammer is cylindrical; the anvil is cylindrical; a tapered aperture radially formed in a wall of the stroke rod; a finger movably captured within the tapered aperture by a cone, the cone being axially movable within the stroke rod by the actuator; when the catch is in the disengaged position, a smaller portion of the cone engages the finger thereby allowing the ball to be substantially contained within the stroke rod; when the catch is in the engaged position, a larger portion of the cone engages the finger thereby forcing the ball to be partially located outside of the stroke rod; the finger is a ball; the housing is arranged for connection along a string; a second hammer movably disposed within the housing and positioned on a second side of the anvil opposite the first side; a second spring disposed within the housing to urge the second hammer toward the anvil in the second axial direction; the stroke rod is selectively movable with respect to the second hammer; in the disengaged position, the stroke rod is freely movable with respect to the second hammer in the first and second axial directions; in the engaged position, the catch is positioned for engagement with the second hammer so that the stroke rod becomes fixed to the second hammer during movement in the first axial direction; a stroker tool disposed along the drill string and connected to the stroke rod so as to selectively move the stroke rod in the first and second axial directions with respect to the housing; the actuator is coupled to the stroker tool so that the stroker tool controls the actuator; the first hammer has a longitudinal bore formed therethrough; the stroke rod is axially movable through the longitudinal bore; the first hammer is cylindrical; the anvil is cylindrical; a tapered aperture radially formed in a wall of the stroke rod; a ball movably captured within the tapered aperture by a cone having a first portion with a first diameter and a second portion with a second diameter, the cone being axially movable within the stroke rod by the actuator; when the catch is in the disengaged position, the second portion of the cone engages the ball thereby allowing the ball to be substantially contained within the stroke rod; when the catch is in the engaged position, the first portion of the cone engages the ball and urges the ball to extend radially outward from the stroke rod; the conveyance is a string; the conveyance is a wireline cable; engaging a second hammer by the stroke rod; moving the second hammer by the stroke rod in the first axial direction so as to compress a second spring; disengaging the stroke rod from the second hammer so as to allow the second spring to move the second hammer into striking engagement with the anvil, thereby providing the jarring force within the wellbore in the second axial direction; actuating a stroker tool to move the stroke rod; moving a catch carried by the stroke rod to a disengaged engaged position so that in the disengaged position, the stroke rod is freely movable in the first and second axial directions with respect to the first hammer; moving the catch to an engaged position so that the stroke rod becomes fixed to the first hammer during movement in the second axial direction; moving a cone axially within the stroke rod so that a first portion of the cone engages a finger to force the finger to at least partially extend outward from the stroke rod; moving the cone axially within the stroke rod so that a second portion of the cone engages the finger thereby allowing the finger to be urged radially inward into the stroke rod by the first hammer; moving the cone by an actuator located within the stroke rod; and passing a portion of the stroke rod through the first hammer. 
         [0049]    The Abstract of the disclosure is solely for providing the reader a way to determine quickly from a cursory reading the nature and gist of technical disclosure, and it represents solely one or more embodiments. 
         [0050]    While various embodiments have been illustrated in detail, the disclosure is not limited to the embodiments shown. Modifications and adaptations of the above embodiments may occur to those skilled in the art. Such modifications and adaptations are in the spirit and scope of the disclosure.

Summary:
A method for jarring allowing the operator to selectively control the jarring force while the tool is downhole and a powered bidirectional mechanical jar therefor, is disclosed. The jar includes a housing, an anvil fixed within the interior the housing, first and second hammers movably disposed within the housing at obverse sides of the anvil, first and second springs disposed to urge the hammers towards the anvil, and a rod with a radial catch that selectively engages and disengages the hammers so as move the hammers to compress the springs and thereafter release the hammers to be accelerated against the anvil. A actuator operates the catch. By controlling the movement of the rod, the spring compression and resultant jarring intensity can be controlled. A stroker tool may be provided to move the rod.