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FIELD OF THE INVENTION  
       [0001]     The present invention relates generally to downhole tools for oil and gas wells, and more particularly to ajar for applying an axial force to dislodge equipment.  
       BACKGROUND OF THE INVENTION  
       [0002]     The sticking of drilling or production equipment in an oil or gas well bore requires that an axial blow be delivered to unstick the equipment. Downhole tools known as “jars” have been used in such situations. One type of jar is a “drilling jar.” Another type of jar is a “wire ine jar.” In the case of a wireline jar, a series of impact blows is delivered to the stuck equipment by manipulation of the wireline. Wireline jars typically have an inner mandrel and an outer housing telescopically coupled together for relative axial, sliding movement. The mandrel carries a hammer and the housing carries an anvil. By directing the hammer to impact the anvil at high velocity, a substantial jarring force may be imparted to the stuck equipment, which is often sufficient to jar the stuck equipment free. A wireline jar is shown and described in U.S. Pat. No. 6,481,495, which is hereby incorporated by reference in its entirety.  
         [0003]     There are various types of jars: mechanical, hydraulic, and mechanical-hydraulic. Each type is cocked and subsequently fired to deliver the impact blow. A trigger mechanism initiates firing of the jar by retarding relative motion of the hammer and anvil until an axial strain has been applied to the drill string pipe sufficient to actuate the trigger mechanism. Typically, an axial tensile force applied at the surface pulls on the wireline and thus the mandrel. The trigger mechanism resists the tensile force and causes potential energy to be stored. When the jar trigger mechanism fires, the stored energy is converted to kinetic energy and the hammer hits the anvil.  
         [0004]     The trigger mechanism in a mechanical jar includes a spring to resist movement of the mandrel relative to the housing. The spring has a constant response such that a certain amount of applied force applied to the mandrel is required to compress the spring a given amount. A collet is coupled the mandrel and moves with the mandrel as the spring is compressed under the applied force. The collet and a trigger sleeve keeps the mandrel engaged against the resisting force of the spring. When the applied force on the mandrel exceeds a predetermined amount (i.e., the triggering load), the spring will have been sufficiently compressed for the mandrel to have moved a sufficient distance relative to the trigger sleeve for the collet to release the mandrel, whereupon the jar “fires.” Mechanical jars having an adjustable tripping load are known, as exemplified by U.S. Pat. No. 3,685,598.  
         [0005]     The trigger mechanism in a hydraulic jar includes a piston to pressurize fluid in a chamber to resist movement of the mandrel relative to the housing. The pressurized fluid bleeds off at a predetermined rate. Eventually a pressure is reached at which a chamber seal is opened, and the compressed fluid is allowed to rush out, firing the jar by freeing the mandrel to move rapidly in an axial direction. In a hydraulic jar, the trigger mechanism is not over-pull force dependent; it will trigger at any load that is pulled following a time delay. Advantageously, a hydraulic jar as disclosed in U.S. Pat. No. 6,290,004 includes a mechanical lock preset to trigger at a load greater than the weight hanging below the jar and a hydraulic time delay that allows the jar to be actuated at loads higher than the lock setting without the need to open a chamber seal.  
       SUMMARY OF THE INVENTION  
       [0006]     Adjustment of the trigger load in a jar is afforded by provision of an adjustable trigger sleeve. The trigger sleeve is adjusted by provision of an adjustment mandrel disposed within the jar housing. To adjust the trigger sleeve, the adjustment mandrel is longitudinally translated a desired distance relative to the housing. The movement of the adjustment mandrel causes the trigger sleeve to be translated relative to the collet. Thus, the amount of movement of the mandrel under an applied force necessary to cause the collet to release the mandrel in firing of the jar can be changed on-site at the well location.  
         [0007]     More particularly, the trigger sleeve adjustment mechanism includes an adjustment mandrel and an adjustment sleeve keyed to the adjustment mandrel to prevent relative rotational movement between them but to permit relative longitudinal movement. The adjustment mechanism is disposed between two sections of the jar housing. As the adjustment sleeve is rotated relative to the housing sections, the adjustment mandrel translates longitudinally relative to the housing sections and the adjustment sleeve. The adjustment mandrel can be threaded at each end for engagement with threads on the ends of the housing sections to achieve relative translation movement. The housing sections can be rotated relative to the adjustment mandrel to bring them into a secure, abutting relationship with the ends of the adjustment sleeve.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIGS. 1A-1C  illustrate a jar having an adjustable trigger sleeve in accordance with the present invention;  
         [0009]      FIG. 2  shows the trigger sleeve adjustment mechanism of the jar of  FIGS. 1A-1C ;  
         [0010]      FIGS. 3A-3D  illustrate a hydraulic jar providing for adjustment of the compression of a biasing spring in accordance with the present invention;  
         [0011]      FIGS. 4A-4C  illustrate a jar in the cocked position and providing gas pressure equalization in accordance with the present invention when there has been no gas intrusion;  
         [0012]      FIGS. 4D-4E  illustrate the jar of  FIGS. 4A-4C  in the triggered position;  
         [0013]      FIGS. 5A and 5B  illustrate the lower portion of the jar of  FIG. 4  in the cocked position and triggered positions, respectively, when there has been gas intrusion;  
         [0014]      FIGS. 6A and 6B  illustrate an alternate configuration for the lower portion of the jar of  FIG. 4  in the cocked and triggered positions, respectively, when there has been no gas intrusion;  
         [0015]      FIGS. 7A and 7B  illustrate the alternate configuration of  FIG. 6  in the cocked position and triggered positions, respectively, when there has been gas intrusion;  
         [0016]      FIG. 8  illustrates a carrier for the biasing spring elements of the jars illustrated in  FIGS. 1-7 ;  
         [0017]      FIG. 9  illustrates a cross section of a segment of a biasing spring having spring carriers in accordance with  FIG. 8  installed thereon;  
         [0018]      FIG. 10  illustrates an alternate carrier for the biasing spring elements of the jars illustrated in  FIGS. 1-7 ; and  
         [0019]      FIG. 11  illustrates a cross section of a segment of a biasing spring having spring carriers in accordance with  FIG. 10  installed thereon. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0020]     Referring to  FIGS. 1A-1C  and  FIG. 2 , there is shown an exemplary embodiment of a jar  10  adapted to be inserted into a well borehole (not shown). The jar  10  has a mandrel  12  disposed within tubular housing  14 . The mandrel  12  is axially movable with respect to housing  14 . A bore  16  extends the length of mandrel  12 . An elongated conductor rod  20  electrically insulated from the mandrel  12  and the housing  14  by an insulating sleeve  22  extends through bore  16  to bulkhead  24 . The upper end of mandrel  12  is provided with threads  26  for connection to a wireline connector (not shown). The proximal end  28  of the conductor rod  20  projects beyond the end of mandrel  12  and above the threads  26 . The joint between the mandrel  12  and the wireline connector is sealed against fluid passage by a pair of longitudinally spaced O-rings  30  and  32 .  
         [0021]     The housing  14  has an upper or proximal tubular section  34  and a lower or distal tubular section  36 . The upper and lower tubular housing sections are secured together by an adjustment mandrel  38  having threads  37  at the proximal end and threads  39  at the distal end. An adjustment sleeve  40  is disposed between the upper (proximal) and lower (distal) tubular housing sections. The joint between the upper and lower tubular housing is sealed against fluid passage by O-ring seals  42  and  44  carried by adjustment mandrel  38 . The upper tubular housing section  34  includes an O-ring seal  46  to seal around mandrel  12  to prevent mud or other debris in the well bore from contaminating the jar. Within upper tubular housing section  34  is formed a downwardly facing annular anvil surface  48 . The mandrel  12  includes an upwardly facing, annular hammer surface  50 . As described more fully below, when the mandrel  12  is moved axially upward relative to the housing  14  at high velocity, the hammer surface  50  impacts the downwardly facing anvil surface  48  to provide a substantial upward axial jarring force.  
         [0022]     Within the upper tubular housing section and disposed around mandrel  12  is a biasing element shown as a spring  52  comprising a stack of Bellville washers. Spring  52  bears against compression ring  54 . As shown, compression  54  is disposed within the upper tubular housing section  34  and is restricted in its downward movement relative thereto by a shoulder  56 . Compression ring  54 , however, may move axially upward relative to the housing  14  to cause compression of spring  52 . As will be appreciated, spring  52  resists upward axial movement of compression ring  54  and returns compression ring  54  to the position shown in  FIG. 1  after triggering of the jar  10 . Spring  52  has a spring constant of, for example, a 1000 lb. per inch.  
         [0023]     In resisting upward axial movement of compression ring  54 , spring  52  functions to retard the upward movement of the mandrel  12  to allow a build-up of potential energy in the wireline when a tensile load is placed on the mandrel  12  from the surface. In order to retard movement of mandrel  12 , a mechanical linkage between the mandrel  12  and the housing  14  is necessary. Such a mechanical linkage includes a generally tubular collet  58  positioned within the upper tubular section  34  and around mandrel  12 . A more detailed understanding of the structure of the collet  58  can be obtained by reference to U.S. Pat. No. 6,290,004, which is hereby incorporated by reference in its entirety. As shown in  FIG. 1 , collet  58  has a plurality of inwardly facing flanges  60 . The exterior surface of mandrel  12  is provided with a plurality of external grooves  62  configured to mesh with the inwardly facing flanges  60  of the collet  58 . With the inwardly facing flanges  60  retained in physical engagement with the grooves  62 , axial force applied to the mandrel  12  will be transmitted through the collet  58  to compression ring  54 .  
         [0024]     The mechanical linkage further includes a trigger sleeve  66  positioned within housing  14  proximate the location of collet  58 . The trigger sleeve  66  is held in position relative to housing  14  by a split ring retainer  68  that is coupled to adjustment mandrel  38 . As best shown in  FIG. 2 , the lower end of trigger sleeve  66  has a downwardly facing surface  67  which seats against wave spring  69 . In turn, wave spring  69  seats against an upwardly facing surface  65  on the proximal end of adjustment mandrel  38 . The upper end of trigger sleeve  66  is provided with a plurality of grooves  70 . The grooves are sized and configured to receive the outwardly projecting flanges  72  of collet  58 .  
         [0025]     When an upward axial force is applied to mandrel  12 , collet  58  is urged to move upwardly against the resistance of spring  52  and relative to sleeve  66 . When the outwardly projecting flanges  72  of collet  58 , are in alignment with the grooves  70  of trigger sleeve  66 , the collet radially expands to seat, the flanges  72  in the grooves  70 , which releases the mechanical link between mandrel  12  and housing  14 . Mandrel  12  is allowed to rapidly accelerate upwards causing the hammer surface  50  to impact the anvil surface  48 . After firing of the jar, the applied force is released. This permits the jar to be re-cocked. In doing so, the collet reengages the mandrel. There are two forces that work together to cause the collet to reengage the mandrel. The first is that the collet has a built in retraction force. The second is the force of the spring pushing on the collet. Because of the angle of the flanges, together an inwardly directed radial force is produced on the collet.  
         [0026]     As will be appreciated, the applied load at which the jar is triggered depends upon the spring constant of spring  52  and the range of travel of collet  58  relative to trigger sleeve  66  before the flanges  72  are in registration with grooves  70 . For example, ignoring any preloading of the spring, if the spring constant is 1,000 lbs. per inch and the range of movement of collet  58  for registration with trigger sleeve  66  is one inch, the trigger load will be 1,000 pounds. If the range of movement is extended to one and one-half inches, then there will be a corresponding increase in the trigger load to 1,500 pounds. On the other hand, if the range of movement is reduced to one-half inch, then there will be a corresponding reduction in the trigger load to 500 pounds. Adjustment of the trigger load, therefore, can be accomplished by adjusting the position of trigger sleeve  66  within housing  14  to assume a different position relative to collet  58 , which effects a different range of movement necessary to place the grooves  70  and flanges  72  in registration.  
         [0027]     In order to adjust the position of trigger sleeve  66  within housing  14 , the location of adjustment mandrel  38  along the length of housing  14  can be adjusted. To do so, the threaded connections between adjustment mandrel  38  and the upper and lower housing sections  34  and  36  are loosened to permit adjustment mandrel  38  to be rotated relative to the housing sections. As will be appreciated, rotating adjustment mandrel  38  relative the housing sections will cause the adjustment mandrel to be longitudinally translated relative to them. Rotation of adjustment mandrel  38  is accomplished by adjustment sleeve  40 . As seen, particularly in  FIG. 2 , adjustment sleeve  40  surrounds adjustment mandrel  38  and is keyed, to it by key  74 , which is in elongated keyway slot  76 . The keyed connection permits rotation of adjustment sleeve  40  to cause a corresponding rotation of adjustment mandrel  38 . The keyway slot  76  for key  74  is elongated to permit the key to move longitudinally with the adjustment mandrel relative to the adjustment sleeve. As seen in  FIG. 2 , a window  78  is milled into adjustment sleeve  40  so that the extent of longitudinal movement of adjustment mandrel  38  relative to adjustment sleeve  40  and consequently housing  14  can be monitored. An index mark  77  on the mandrel is visible through the window  78 . Adjustment sleeve  40  also includes indicia in the form of marks  80 ,  82  and  84  to identify particular trigger load settings. Alignment of mark  77  with one of marks  80 ,  82 ,  84  is an indication of a high, medium, or low trigger load setting. The extent of adjustment of adjustment mandrel  38  is preferably on the order of one-half to one inch from, a nominal setting.  
         [0028]     Referring to  FIGS. 3A-3D , inclusive, there is shown an exemplary embodiment of a jar  100  adapted to be attached to a wireline and inserted into a well borehole (not shown). The jar  100  has a mandrel  112  disposed within tubular housing  114 . The mandrel  112  is axially movable with respect to housing  114 . A bore  116  extends the length of mandrel  112 . An elongated conductor rod  120  electrically insulated from the mandrel  112  and the housing  114  by an insulating sleeve  122  extends through bore  116  to bulkhead  124 . The upper end of mandrel  112  is provided with threads  126  for connection to the wireline connector (not shown). The proximal end  128  of the conductor rod  120  projects beyond the end of mandrel  112  and above the threads  126 . The joint between the mandrel  112  and the wireline connector is sealed against fluid passage by a pair of longitudinally spaced O-rings  130  and  132 .  
         [0029]     The housing  114  has an upper tubular section  134  and a lower tubular section  136 . The upper and lower tubular housing sections are secured together by an adjustment mandrel  138  (See  FIG. 3B ). An adjustment sleeve  140  is disposed between the upper and lower tubular housing sections. The joint between the upper and lower tubular housing is sealed against fluid passage by O-ring seals  142  and  144  carried by adjustment mandrel  138 . The upper tubular housing section  134  includes an O-ring seal  146  to seal around mandrel  112  to prevent mud or other debris in the well bore from contaminating the jar. Within upper tubular housing section  134  is formed a downwardly facing annular anvil surface  148 . The mandrel  112  includes an upwardly facing annular hammer surface  150 . As described more fully below, when the mandrel  112  is moved axially upward relative to the housing  114  at high velocity, the hammer surface  150  impacts the downwardly facing anvil surface  148  to provide a substantial upward axial jarring force.  
         [0030]     A spring  152  is disposed within the lower tubular housing section around mandrel  112 , which is shown to comprise a stack of Bellville springs. Spring  152  bears against the lower end of adjustment mandrel  138 . As will be described, spring  152  resists upward axial movement of mandrel  112 . In resisting upward axial movement of mandrel  112 , a build-up of potential energy in the drill string occurs when a tensile load is placed on the mandrel  112  from the surface. Spring  152  provides the jar  100  with a preload that enables the operator to apply an upward axial force on the mandrel  112 .  
         [0031]     A fluid chamber is established within the open internal spaces of housing  114  and extends generally longitudinally downward through the length of the housing  114 . The fluid chamber is sealed at its lower, end by a pressure-compensating piston  154 . The interior of the housing  114  below the pressure-compensating piston  154  is vented to the well by ports  156 . Fluid pressure is established in the fluid chamber by an actuating piston  158 . As described more fully below, actuating piston  158  restricts fluid flow within the fluid chamber, which enables a significant over-pull to be applied to the mandrel  112  followed by a gradual bleed off of fluid pressure through the piston  158  and eventual triggering of the jar  10 .  
         [0032]     The actuating piston  158  seals the fluid chamber to permit a build up of pressure therein In this way, fluid in the chamber resists the upward movement of the mandrel  112  relative to the housing  114 . Upward movement of the mandrel  112  relative to the housing  114  reduces the volume of the fluid chamber above the actuating piston  158  and causes a significant increase in the fluid pressure within that space. The fluid pressure provides an axial force to resist the relative movement of the mandrel and the housing. This resistance to relative movement creates a large potential energy.  
         [0033]     The actuating piston  158  has smooth cylindrical bore  160  and allows the mandrel  112  to slide therein. The bore  160  is sealed against the leakage of fluid around its exterior surface and past the mandrel  112  by a pair of O-rings  162  and  164  positioned proximate the outer surface and inner surface of the actuating piston  158 , respectively. The actuating piston  158  may be in accordance with that shown in U.S. Pat. No. 6,290,004. The actuating piston  158  has a flow passage  166 . The flow passage  166  permits only restricted flow of fluid from the fluid chamber above the piston  158 . The restricted flow causes the build up of pressure but also allows the actuating piston  158  to move in an upwardly direction. The tubular housing section  136  includes an upwardly facing annular shoulder  170  against which compression ring  168  bears. Shoulder  170  defines the lower limit of downward movement of the actuating piston  158 .  
         [0034]     In order to retard movement of mandrel  112 , a mechanical linkage between the mandrel  112  and the housing  114  is necessary. Such a mechanical linkage includes a generally tubular collet  172  positioned within the lower tubular section  136  and around mandrel  112 . As shown in  FIG. 3C , collet  172  has a plurality of inwardly facing flanges  174 . The exterior surface of mandrel  112  is provided with a plurality of external grooves  176  configured to mesh with the inwardly facing flanges  174  of the collet  172 . With the inwardly facing flanges  174  retained in physical engagement with the grooves  176 , axial force applied to the mandrel  112  will be transmitted through the collet  172  to compression ring  168 .  
         [0035]     The mechanical linkage further includes a trigger sleeve  180  positioned within housing  114  proximate the location of collet  172 . The trigger sleeve  180  is allowed to move slightly relative to housing  114 . The upper end of trigger sleeve  180  is provided with a plurality of grooves  182 . The grooves are sized and configured to receive the outwardly projecting flanges  184  of collet  172 . When an upward axial force is applied to mandrel  112 , collet  172  is urged to move upwardly relative to trigger sleeve  182  against the resistance of spring  152  and the fluid pressure in the fluid chamber above actuating piston  158 . When the outwardly projecting flanges  184  of collet  172  are in alignment with the grooves  182  of trigger sleeve  180 , the collet radially expands to seat the flanges  184  in the grooves  182 , which releases the mechanical link between mandrel  112  and housing  114 . Mandrel  112  is allowed to rapidly accelerate upwards causing the hammer surface  150  to impact the anvil surface  148 .  
         [0036]     In order to trigger jar  100 , an upwardly directed tensile load is applied to the mandrel  112 . As force is applied to the mandrel  112 , upward axial force is transmitted to the collet  172 . The upper annular surface of the collet is brought into engagement with compression ring  168 . If the applied load exceeds the preload of spring  152 , the actuating piston  158  moves upwardly and compresses the fluid enclosed within the fluid chamber above the piston. The upward movement of the actuating piston  158  and collet  172  is resisted by the pressure of the fluid compressed within the fluid chamber and by spring  152 , which allows potential energy in the wireline to build. Upward movement of the actuating piston  158  produces a restricted flow of fluid from the high-pressure side of the fluid chamber through the flow passage  166 . The actuating piston  158 , the collet  172 , and the mandrel  112  continue a steady but slow upward movement as fluid continues to bleed high pressure from the fluid chamber. When enough fluid has been bleed off such that the collet has moved sufficiently for the outwardly facing flanges  184  to be in alignment with the grooves  182  of trigger sleeve  180 , the collet will release the mandrel and allow it to translate upwards freely and rapidly relative to the housing  114 . The mandrel  112  accelerates upward rapidly bringing the hammer surface  150  of the mandrel  112  rapidly into contact with the anvil surface  148  of the housing  114 . If tension on the mandrel  112  is released, spring  152  urges the piston  158  downwardly and fluid is introduced into the chamber above the piston through a check valve in the actuating piston.  
         [0037]     As will be appreciated, the resistance of spring  152  establishes a “preload,” which is an amount of load that must be applied before collet  172  can begin to move relative to trigger sleeve  180 . That is, compression of spring  152  results in a reaction force that pushes down against the piston. In order to move collet  172  upwardly, an applied load to the mandrel  112  must initially overcome the reaction force due to the compression of the spring  152  or the “preload.” Thereafter, additional load must be applied to overcome the further force imposed by the spring constant plus the resisting force of the pressure of the compressed fluid in the fluid chamber. The preloading serves as a “lock” against premature triggering of the jar due to the weight of the tools suspended below the jar.  
         [0038]     In order to further understand the effect of establishing an initial compression of spring  152  and its adjustment, consider that the spring  152  has a “spring rate” that constitutes the increase in force for a given deflection. Although spring  152  could have a non-linear spring rate, preferably it has a linear spring rate. The length of spring  152  in an unloaded condition is the “free length.” The length of the spring after compression is the “stack height.” If spring  152  were to be compressed until it is flat and no further travel is possible, the spring force would be the maximum available and spring length would be its “solid height.”. The preload length of the spring is the stack height prior to applying tension to the jar. The extent of compression is the free length minus the stack height when the jar triggers. The load at any spring height is calculated by multiplying the compression and the spring rate. In an example wherein the free length is 5.25 inches, the solid height is 1.75 inches, and the spring rate is 1500 lb./inch, compression of the spring 0.25 inch, the preload would be 0.25×1500=375 lbs. In order to begin to move the mandrel, it would be necessary to pull 375 lbs. of applied load. If one inch of travel is needed to release the mandrel from the collet, so the release load is 1.25×1500=1875 lbs. The preload can be increased by additional compression of the spring. If the compression is increased by one inch to 1.25 inch, the preload becomes 1875 lbs. An additional one inch of travel to release the mandrel would require a total of 2.25 inches of spring compression and the release load would increase to 2.25×1500=3375 lbs.  
         [0039]     In order to adjust the compression or preload of spring  152 , the location of adjustment mandrel  138  along the length of housing  114  is adjusted. To do so, the threaded connections between adjustment mandrel  138  and the upper and lower housing sections  134  and  136  are loosened to permit adjustment mandrel  138  to be rotated relative to the housing sections. As will be appreciated, rotating adjustment mandrel  1388  relative the housing sections will cause the adjustment mandrel to be translated relative to them. Rotation of adjustment mandrel  138  is accomplished by adjustment sleeve  140 . As seen, adjustment sleeve  140  surrounds adjustment mandrel  138  and is keyed to it using an elongated keyway slot. The keyed connection permits rotation of adjustment sleeve  140  to cause a corresponding rotation of adjustment mandrel  138 . The keyway slot is elongated to permit the key to move longitudinally with the adjustment mandrel relative to the adjustment sleeve.  
         [0040]     A fluid-filled jar, such as that shown in U.S. Pat. No. 6,481,495, is subject to a condition known as “gas locking,” which occurs when gas in solution in the well bore enters the fluid chamber of the jar. Typically, gas locking occurs when gas permeates the elastomer seals in the jar due to a difference in the partial pressures between the gas in the well bore and the fluid inside the jar. Gas then becomes trapped in the fluid chamber of the jar. As the jar is moved uphole, the hydrostatic pressure becomes lower and the gas inside the jar will expand. The effect is that the jar will be biased (“gas biasing”), which can result in the jar triggering prematurely or not at all. The jar  200  shown in  FIG. 4  avoids gas locking by balancing the gas biasing effect. As shown in  FIGS. 4A and 4B , jar  200  is in the cocked position. In  FIGS. 4D and 4E , jar  200  is in the triggered position.  
         [0041]     Jar  200  in  FIG. 4  has a mandrel  212  and a housing  214 . A seal  216  is provided between mandrel  212  and housing  214 . A fluid chamber  202  extends through the tool. An anvil surface  208  on housing  214  is impacted by hammer  220  when the jar is triggered. As can be seen in  FIG. 4C , hammer  220  has slots  221  extending along its length to permit fluid to move within the fluid chamber  202  of the jar. Jar  200  includes biasing spring  222 , compression ring  224 , and a triggering mechanism including collet  226  and trigger sleeve  228 . The collet  226  engages flanges  227  on mandrel  212 . A floating piston  230  is disposed within cylinder bore  232  and carries inner seal  234  and outer seal  236 . Ports  238  in cylinder bore  232  below piston  230  are open to hydrostatic pressure. Above piston  230 , the cylinder bore is filled with fluid. Chamber  240  is also filled with fluid. Also provided in jar  200  are upper and lower fluid fill ports  242  and  244 , respectively. Additional ports  246  open to hydrostatic pressure are provided above fluid fill port  244 .  
         [0042]     As shown in  FIG. 4B , the mandrel  212  has a section  212 A below flanges  227 . Mandrel section  212 A is hollow and terminates in an open end  213 . The interior of hollow mandrel section  212 A is in fluid communication with chamber  215 . In the upper portion of mandrel section  212 A, fluid ports  217  are provided. The hollow interior of mandrel section  212 A is placed in fluid communication with fluid chamber  202  by ports  217 . This permits the interior and exterior of the mandrel to be pressure balanced. The exterior of mandrel  212  is surrounded by oil in fluid chamber  202 . The interior of mandrel section  212 A and the exterior portion of mandrel section  212 A exposed to chamber  215  are surrounded by oil. Additional fluid ports  219  are provided in mandrel section  212 A to oil-filled chamber  221 .  
         [0043]     In  FIG. 4 , there has been no gas invasion into the fluid chamber and the pressure-balancing piston  230  is not bottomed out within bore  232 . With floating piston  230 , which has fluid chamber pressure above and hydrostatic pressure below, the fluid chamber pressure in the jar can be balanced against the hydrostatic pressure of the well bore. As the jar moves downhole, hydrostatic pressure increases. The increase in hydrostatic pressure acts on piston  230  and causes it to be urged upwardly. This has the effect of increasing the pressure of the fluid in the chamber to balance the increase in hydrostatic pressure. As shown in  FIGS. 4D and 4E , after triggering of the jar, piston  230  remains stationary with respect to the housing and the fluid in the jar chamber remains at hydrostatic pressure.  
         [0044]     In  FIGS. 5A and 5B , the lower section of the jar shown in  FIG. 4  is shown in the cocked and triggered positions, respectively. Also, the assumption is that there has been gas intrusion to the fluid chamber of the jar and gas pressure higher than hydrostatic is present in the jar chamber. An increase in gas pressure causes floating piston  230  to move downwardly to shoulder in bore  232 . Any further increase in gas pressure increases the pressure in the jar fluid chamber. Gas pressure internal to the jar and communicated through ports  217  acts on the area A 1  of the lower mandrel section  212 A to force the mandrel  212  upwardly to “open” the jar. Internal gas pressure also acts on an area A 2 , which is substantially equal to the area A 1 . Gas pressure acting on area A 2  creates an opposing force that urges the mandrel downwardly to “close” the jar. Accordingly, the intrusion of gas into the fluid chamber of the jar can be used to create balancing forces that allow the jar to continue to function.  
         [0045]     More specifically, the end  213  of mandrel section  212 A has a cross-section area A 1  exposed to fluid pressure within the housing fluid chamber  215  that produces a force urging the mandrel in a first, upward direction. The chamber  221  forms an annulus of cross-section area A 2 . An intermediate segment  249  is within the annulus of chamber  221  and presents an annular surface of cross-section area A 2  substantially equal to the cross-section area A 1 . The annular surface of segment  249  is exposed to fluid pressure within the housing fluid chamber, which produces an opposing force that urges the mandrel in a second, opposite direction (i.e., downward) to the first direction.  
         [0046]     In  FIG. 6 , an alternate configuration for the lower section of the jar shown in  FIGS. 4 and 5  is shown. In  FIG. 6A , the jar is in the cocked position; and in  FIG. 6B  the jar is in the triggered position. As seen in  FIG. 6 , a piston  250  moves within an elongated cylindrical bore  252 . A portion of the mandrel  254  a circumferential shoulder  256  that serves as a piston stop. The mandrel portion  254  has an area A 1  and the cylindrical bore  252  has an area A 2 , which is substantially equal to A 1 . At the lower end of cylindrical bore  252  are ports  258  to hydrostatic pressure. When there is no gas invasion into the jar, piston  250  floats within cylindrical bore  252  as shown in response to hydrostatic pressure changes. An increase in hydrostatic pressure causes the piston  252  to be urged upwardly, which has the effect of increasing pressure in the jar fluid chamber to balance the hydrostatic pressure. Ports  251  in mandrel  212  provide for fluid communication between the fluid chamber  253  exterior to the mandrel and the fluid chamber  255  interior to the mandrel.  
         [0047]     In  FIG. 7 , the alternate configuration for the lower section of the jar shown in  FIG. 6  is illustrated when there has been gas intrusion to the fluid chamber of the jar and gas pressure higher than hydrostatic is present in the jar chamber. In  FIG. 7A , the jar is in the cocked position; and in  FIG. 7B  the jar is in the triggered position. As shown, piston  250  is forced against piston stop shoulder  256 . Any further increase in gas pressure increases the pressure in the fluid chamber of the jar. The internal pressure acts on area A 1  to create a force. But, the internal pressure also acts on area A 2  to create an equalizing force in opposition.  
         [0048]     As seen in the jars of  FIGS. 1, 3 ,  4  and  5 , a lower spring  300  is provided. Spring  300  is disposed between a shoulder  302  on the mandrel and a shoulder  304  on the housing. Thus, when the jar is triggered and the mandrel moves upwardly to impact the hammer against the anvil, spring  300  is compressed. When the jar is to be cocked, the spring  300  pushes downwardly on the mandrel to return it to its initial position. As can be seen in, for example,  FIG. 1C , a lower section  306  of the mandrel has an upper seal  308  and a lower seal  310 . These seals can produce drag that inhibits the return of the mandrel to the cocked position of the jar. Spring  300  facilitates movement of the mandrel to the cocked position.  
         [0049]     In  FIG. 9 , a detailed view of the spring carrier for the biasing spring, such as spring  52  in  FIG. 1A , is shown. The spring  52  is a Bellville spring comprising a plurality of “washer-like” spring elements. A Belleville washer is a compact type of spring in the shape of a washer that has been pressed into a dished shape and then hardened and tempered. In using a stack of Belleville washers as a spring, there can be a tendency to buckle and cause rubbing on either the inside diameter or the outside diameter. Rubbing causes a hysteresis effect when the spring is compressed and then released. Thus, the spring force is not constant and the triggering load of a jar cannot be repeated. The use of the spring carrier assists in reducing the hysteresis effect.  
         [0050]     The washer elements of spring  52  are loaded onto the mandrel as seen in  FIG. 1A . As seen there, the convex sides of adjacent dished shape washers are in contact. To facilitate placing the washer elements onto the mandrel, a spring carrier  53  shown in the cross section of  FIG. 8  is adapted to be placed to the inside of the spring  52  against the mandrel wail surface. As shown in the perspective view of  FIG. 9 , carrier  53  comprises a ring  51  having an external circumferential flange  55 . Adjacent washer elements  52 A and  52 B are shown in the exploded view of  FIG. 9 .  
         [0051]     In  FIGS. 10 and 11 , a detailed view of an alternate spring carrier  53 A is shown. Spring carrier  53 A comprises a ring  51 A having an internal circumferential flange  55 A. In the exploded view of  FIG. 11 , washer elements  52 A and  52 B are shown relative to carrier  53 A.  
         [0052]     Many modifications and changes may be made to the illustrated embodiments by those having ordinary skill in the art without departing from the scope and spirit of the present invention as set forth in the appended claims. For example, a coil spring may be suitable substituted for the Bellville-type spring. Also, as can be seen among the various embodiments described, a mechanical jar having the trigger load, adjustment feature such as shown in  FIG. 1  can be provided with the gas equalization features described with respect to the jar of  FIGS. 4 and 5  and the jar of  FIGS. 6 and 7 .

Summary:
A jar provided with an adjustable triggering load has a housing defining an anvil surface and a mandrel, movable relative to the housing under an applied load, defining, a hammer surface to impact the anvil surface. A trigger mechanism for releasing the mandrel to cause the hammer surface to impact the anvil surface includes a trigger sleeve axially positioned within the housing and a collet that engages the mandrel during application of a load to the mandrel. The collet releases the mandrel to cause the hammer surface to impact the anvil surface upon being moved into registration with the trigger sleeve against the resistance of the spring disposed within the housing. The triggering load is adjusted by an adjustment mechanism carried by the housing and coupled to the trigger sleeve to selectively position the trigger sleeve within the housing in establishment of a required applied load in order for the collet to be moved against the spring resistance and into registration with the trigger sleeve to thereby release the mandrel and cause the hammer surface to impact the anvil surface.