You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
BACKGROUND 
     Packers and plugs may be run into a wellbore (cased or uncased) to hydraulically isolate the sections above and below the packer and to provide a mechanical anchor to prevent the packer from sliding inside the wellbore. Packers may be set, e.g., mechanically, hydraulically, or on wireline. A mechanical-set packer may be set by applying either tension or compression on the packer. Upon setting, in many instances, an anchor or slip is biased outward towards the casing to anchor the packer. A packer forms a seal for purposes of, e.g., controlling production, injection or treatment. The packer is preferably lowered downhole into the well in an unset state. However, once in the appropriate position downhole, the packer is preferably set from the surface of the well. As an example, for a mechanically-set packer, a tubular string that extends from the surface to the packer may be moved pursuant to a predefined pattern to set the packer. In its set state, the packer anchors itself to the casing wall of the well and forms a seal in the annular region between the packer and the interior surface of the casing wall. This seal subdivides the annular region to form an upper annular region above the packer that is sealed off from a lower annular region below the packer. The packer typically includes at least one seal assembly to form the annulus seal and at least one set of slips to anchor the packer to the casing string. When run into the well, the seal assembly and the slips are radially retracted to allow passage of the packer through the central passageway of the casing string. After a particular job is complete, the slips and seals may be again retracted, allowing the packer to be removed or moved to another location in the well. 
     A straight pull release (SPR) mechanism is a preferred mechanism for retracting the seals and/or the slips to retrieve a packer, because it does not require the use of additional equipment or service tools to retrieve the packer. SPR mechanisms are often realized by a group of shear screws which are directly loaded by tensile forces and pressure-induced hydrostatic forces. Therefore, the tensile rating and pressure rating of the SPR packers are frequently limited by the shear strength of these screws. Therefore it may be desirable to protect these shear screws to prevent premature release. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a drawing of an exemplary embodiment of an SPR packer as described herein. 
         FIG. 2  is a drawing of an exemplary embodiment of an SPR packer as described herein. 
         FIG. 3  is a drawing of an exemplary embodiment of an SPR packer as described herein. 
         FIG. 4  is a drawing of another exemplary embodiment of an SPR packer as described herein. 
         FIG. 5  is a drawing of another exemplary embodiment of an SPR packer as described herein. 
     
    
    
     SUMMARY 
     Disclosed herein are methods and apparatus for releasing a releasable packer. The apparatus may comprise shear screws, means for isolating the shear screw from tensile force and wherein the means for isolating is selectively unlockable to expose the shear screw to the force. 
     The methods may comprise unlocking a mechanism protecting at least one shear screw, applying a shearing force to the at least one shear screw, and shearing the at least one shear screw. 
     DETAILED DESCRIPTION 
     As used herein, the terms up and down and above and below are used for ease of relative reference. However, it is intended that the packers described herein may be used in any spatial position. 
     There is shown in  FIG. 1 , an SPR packer comprising an anchor  10  (also called a slip), a lower cone  20 , shear screws  30 , collet fingers  40 , O-ring container  50 , spring  60 , O-ring  70 , stop ring  80 , holding collar  90 , bottom mandrel  100 , gage ring  110 , mandrel  120 , and port  130   
     In operation, in general, disclosed herein is a mechanism to protect SPR shear screws  30  and release this protection mechanism only when it is desirable to release the packer. Generally, an SPR packer as disclosed herein preferably allows the packer to achieve a higher pressure and tensile rating without comprising its ability to be set or be retrieved when required. Preferably, the SPR shear screws are protected when the annulus pressure is larger (to some threshold value) than the tubing pressure. It is not necessary that the shear screws  30  are protected when there is no pressure differential between the annulus and the tubing or when the tubing pressure is larger than the annulus pressure because the danger of premature shearing of the screws is minimal. 
     In operation, when there is no pressure differential between the tubing and the annulus, spring  60  is at its free or unloaded length, the holding collar  90  is disengaged with collet fingers  40 . 
     After the packer is set (and slips  10  are engaged), the tensile force and hydrostatic forces induced by pressure below (from pressure downward for a horizontal completion) will be loaded on the shear screws if the screw protection mechanism is not in place. When the pressure below reaches some threshold value (in applications like gravel packing, frac-packing, etc.), the combined tensile force may be large enough to shear the shear screws undesirably. 
     In the embodiment shown in  FIGS. 1-3 , when the pressure below reaches some value sufficient to overcome the spring force (which is smaller than the threshold value to shear the screws), the holding collar  90  will be shifted towards the collet fingers  40  to contact and lock the collet fingers  40  into a groove in the bottom mandrel  100 . After the collet fingers  40  are locked into the groove, the tensile force will go from the mandrel  120 , bottom mandrel  100 , gage ring  110 , lower cone  20  to the anchors  10 . The load path is taken around from the shear screws  30 , so they are protected from the shear load when desirable. 
     In situations where the tensile force is less than the nominal shear strength of the shear screws, the protective mechanism need not necessarily be in place. For example, if the force is less than 95% of the shear force of the screws or less than 90% of the shear force of the screws or less than 85% of the shear force of the screws or less than 80% of the shear force of the screws or less than 75% of the shear force of the screws or less than 70% of the shear force of the screws or less than 65% of the shear force of the screws or less than 60% of the shear force of the screws or less than 55% of the shear force of the screws or less than 50% of the shear force of the screws. 
     When it is desirable to release the packer  200 , the high pressure below the packer must first be bled off. As is shown in  FIG. 2 , the holding collar  90  shifts down by the force of spring  60 . The tubing-annulus pressure differential may assist in shifting the holding collar  90  down (e.g., through port  130 ), but is not necessary. The shifted holding collar  90  causes collet fingers  40  to unlock. Once the collet fingers are unlocket, the forces are no longer routed around the shear screws, thus allowing shear force to shear the shear screws  30  and disengage the slips  10  allowing the packer to be retrieved. After the shear screws are sheared, the holding collar  90  may be shifted up as is shown in  FIG. 3 . 
     With respect to  FIG. 4 , there is shown a further embodiment in accordance with the invention. In the embodiment of  FIG. 4 , the gage ring  110  is also equipped with a group of collet fingers. It works similarly to the embodiment described with respect to  FIGS. 1-3 , but holding collar  90  does not have a sleeve to hold collet fingers  40  into the groove. The locking mechanism of the embodiment of  FIG. 4  relies on the hydrostatic force from the annulus-tubing pressure differential. 
     With respect to  FIG. 5 , there is shown a further embodiment including a gage ring  110  that includes two or more locking segments and a holding collar  90  to hold the locking segments at the bottom mandrel groove. The holding collar  90  keeps the locking segments compressed when spring  60  is at its free length. In addition, the spring  60  is preferably an extension spring whose stiffness is preferably strong enough to withstand the force induced by the setting pressure and not to displace the holding collar so that the locking segments are released. It preferably also uses the tubing-annulus pressure differential to help retrieve the packer. 
     It is envisioned that in the SPR packer described herein that the mandrel  120  and bottom mandrel  100  may be two separate parts or combined into a single part.

Summary:
Disclosed herein are methods and apparatus for releasing a releasable packer. The apparatus may include shear screws, a mechanism for isolating the shear screw from a shearing force. The mechanism for isolating the shear screw from a shearing force is selectively unlockable to expose the shear screw to the shearing force. The methods may include unlocking a mechanism protecting at least one shear screw, applying a shearing force to the at least one shear screw, and shearing the at least one shear screw.