Abstract:
A slip release system is disclosed which undermines support for slips by virtue of selective weakening of a structural element supporting the slips in a set position.

Description:
FIELD OF THE INVENTION 
     The field of this invention relates to slip systems and an apparatus and method for release of such slips, particularly when used with retrievable packers. 
     BACKGROUND OF THE INVENTION 
     In the past, retrievable packers have been designed with opposing slips above and below the packing element. This provides a means for transferring packing element loads due to differential pressure directly through the slips and into the casing without applying this load as a high-tensile load on the mandrel or other parts. This feature has proven to be very desirable for high-performance packers. One feature of such designs, however, is the tendency of the pack-off force applied to the packing element to be &#34;bulldogged&#34; or trapped between the two sets of opposing slip cones, which are in turn jammed under the slips. In order to release this type of a retrievable packer, the upper slip must be pulled from a position of very high engagement force between the casing and the cone which is used to guide the slip outwardly into contact with the casing. Damage can result to the tool if the slip is literally pulled off the tool, which leaves the remainder of the tool hung up in the wellbore. 
     In the past, various combinations of slips and cones have been used for urging the slips outwardly into contact with the casing. One such slip actuation mechanism is shown in U.S. Pat. No. 4,711,326. This patent illustrates vertically shiftable slips carried in slots by the side edges which engage mating profiles formed in the slots. These slots form guideways for the slips for shifting the slips upwardly and outwardly relative to the body between a set position for engaging a conduit and an unset position. Various other designs have used an upward pull to release the slips. Such designs are illustrated in U.S. Pat. Nos. 5,044,433 and 5,044,441. 
     The shortcomings of the prior designs have been in the release sequence. The prior designs have emphasized a direct pull upwardly on the uppermost slip from a mandrel to make the slips ride along the cone and retract from contact with the surrounding casing. In the prior designs, the initial contact force between the slip and the casing has been so great that shifting the mandrel to apply direct force to the slip has resulted in failure at the connection between the mandrel and the slip, leaving the slip still engaged to the casing. 
     The desired objective in the past has been to find a way to undermine the support for the slip so as to avoid the hazard of having to apply unduly high forces to make the slip release while under load. The apparatus and method of the present invention addresses this need to release the upper slip with a design that provides for selective weakening of the structural support for the engaged slips by virtue of components in the cone so that release can be accomplished with a greatly reduced force. 
     SUMMARY OF THE INVENTION 
     A slip release system is disclosed which undermines support for slips by virtue of selective weakening of a structural element supporting the slips in a set position. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is perspective of the apparatus of the present invention, shown in the set position for the slips. 
     FIG. 2 is a sectional elevational view of the slips in the set position. 
     FIG. 3 is a sectional elevational view in the released position. 
     FIG. 3A is a sectional elevational view illustrating the key and cone in the released position. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The apparatus A is shown in FIG. 1. There a series of slips 10 and 12 are illustrated. The slips 10 and are disposed preferably at 90° in an annularly shaped apparatus A which is suitable for downhole use when four slips are used. More or less slips can be used without departing from the spirit of the invention. Preferably, the slips 10 and 12 are identical in construction and in the preferred embodiment have opposed edges 18 and 20 which are guided by a cone 22. Cone 22 guides edges 18 and 20 in a manner so as to urge the slips 10 and 12 outwardly toward casing 28 as they move downwardly. When the slip, such as slip 10, advances along cone 22, it is outwardly ramped, as shown in FIG. 2, until the teeth 26 engage the casing 28, thereby suspending the mandrel 30. As previously stated, the assembly using slips can be used for a variety of downhole tools. One typical application is a packer assembly with the packing element secured above and below by slips, as illustrated in FIG. 1. At the top of each of the slips illustrated in FIG. 1 is a handle assembly 32, comprising of a pair of opposed recesses 34 and 36. Extending into recesses 34 and 36 to actuate the slip along edges 18 and 20 is sleeve 38, which is actuated for movement using mandrel 30. Upon downward movement of mandrel 30, sleeve 38 biases the edges 18 and 20 along cone 22. 
     FIG. 2 illustrates the slips in the set position. The walls of the cone 22 have disposed within cut-outs 40 a key 42 (as illustrated in FIG. 3A). Key 42 has a tapered upper end 44 whose position is fixed with respect to cone 22 by virtue of pin 46 extending transversely through bore 48 (see FIG. 3 and 3a). As will be explained below, when it is time to release the slips 10 and 12, the pin 46 is sheared with respect to bore 48 as key 42 is upwardly biased. As shown in FIGS. 2 and 3, mandrel 30 has a ring 50 secured in a groove 52. Keys 42 extend radially outwardly from cut-outs 40 such that upon upward movement of ring 50, the mandrel 30 exerts an upward force on keys 42. This is illustrated by comparing FIG. 3 to FIG. 2. As can be seen in FIGS. 3and 3A, when it is time to release the packer or tool employing the slips 10 and 12, the upper slips 10 and 12 are released by an upward pull on the mandrel 30. This upward pull brings up ring 50, which in turn via sleeve 54 pushes on lower end 56 of each of the keys 42. As keys 42 move up, pin 46, which extends through keys 42 and into cone 22, is sheared as illustrated in FIGS. 3 and 3A. Because the upper end of each of the keys 42 is tapered, the side walls 58 and 60 converge as they present themselves adjacent the top of cut-out 40 near bore 48. As a result, the circumferential support presented by cone 22 onto side walls 18 and 20 is undermined as edges 62 and 64 of cut-out 40 have the opportunity under load to flex toward each other. Once edges 62 and 64 are free to flex circumferentially toward each other, the side walls holding edges 18 and 20 can move away from each other reducing or eliminating the wedging force on slips 10 and 12, wherein an upward pull on handle 32 of the slips 10 and 12 easily dislodges teeth 26 from casing 28. Up until the time that pin 46 is sheared, the cone 22 has a rigid cylindrical structure and the slips 10 and 12 are firmly wedged along their edges 18 and 20 to the cone 22. However, with the keys 42 moved upwardly by movement of mandrel 30, the entire cone structure 22 is substantially weakened circumferentially so that the slips 10 and 12 may thereafter be easily pulled upwardly. Therefore, because of ramped guidance along edges 18 and 20, an upward pull from sleeve 38 after movement of keys 42 retracts the teeth 26 from casing 28. 
     The present design is a significant improvement over prior designs which have exhibited numerous problems in getting the upper slips to release. In prior designs, direct pulls on slips in the area of handle 32 in a situation where the slips are firmly wedged has frequently resulted in breakage of the handle 32. As a result, the entire top of the tool down to the slips and cones must be milled in order to be able to retrieve the tool from the wellbore and to allow the packing element (not shown) to release. 
     Those skilled in the art will appreciate that what has been illustrated is a specific mechanical embodiment which selectively weakens the elements used to wedge a slip in the set position. Other mechanical embodiments that weaken the cone structure so as to facilitate ultimate disengagement or defeat the wedging of the slips are also within the purview of the invention. One advantage of the design of the present invention is that the keys 42 may be reset and new pins 46 installed so that the cone 22 can be reused on another application. While a mechanical release mechanism has been illustrated, those skilled in the art will appreciate that hydraulic forces from the wellbore or from the surface alone or in combination with mechanical forces can also be used to initiate the release feature which weakens the cone structure, thereby facilitating removal of the slips in a condition where they are not mechanically or otherwise wedged against a casing 28. Other means of storing or applying a force to move keys 42 such as electrical or chemical can also be used. While a notch with a tapered wedge is illustrated as the preferred embodiment, other devices which weaken the cone wall and thus relieve, at least in part, a wedging force on the slips are within the scope of the invention. 
     The foregoing disclosure and description of the invention are illustrative and explanatory thereof, and various changes in the size, shape and materials, as well as in the details of the illustrated construction, may be made without departing from the spirit of the invention.