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BACKGROUND 
     This application relates to a method for removing a sealing plug from a casing or a wellbore in oil and gas recovery operations. 
     After a well is put into production, a wellhead is usually placed over the well at the ground surface and a closure device, such as a sealing cap, or the like, is provided at the wellhead to prevent the flow of production fluid from the well during certain circumstances. Sometimes, under these conditions, the closure device must be removed for replacement, repair, etc., which creates a risk that some production fluid from the well may flow out from the upper end of the well. 
     To overcome this, a sealing plug, also referred to as a packer, bridge plug or barrier plug, is usually inserted in the well and activated to plug, or seal, the well and prevent any escape of the production fluid out the top of the well. However, when it is desired to recap the well, the plug must be removed. One common technique for removing the plug is to employ a rig that is used to drill-out the sealing plug, or pull the plug from the well. However, this technique requires sophisticated equipment, is labor intensive, and therefore is expensive. 
     Another technique to remove the plug from the well is to implant a timing device in the plug to actuate an explosive in the plug after a predetermined time. However, this type of technique has drawbacks since, after these types of plugs have been set in the well, the operator may want to extend the life of the plug from the predetermined time to a longer period of time or even an indeterminate time, and to do so would not be possible. 
     Therefore, what is needed is a sealing plug of the above type which can be placed in the well to seal off the flow of production fluid as discussed above and yet can be removed at an indeterminate time in a relatively simple and inexpensive manner. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic/elevational/sectional view of an oil and gas recovery operation including a sealing plug according to an embodiment of the invention. 
         FIG. 2  is an enlarged, sectional view of the plug of  FIG. 1 . 
         FIG. 3  is a view, similar to that of  FIG. 1 , but depicting a different operational mode. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , the reference numeral  10  refers to a wellbore penetrating a subterranean formation for the purpose of recovering hydrocarbon fluids from the formation. The wellbore  10  could be an open hole completion or a cased completion, and in the latter case a casing  12  would be cemented in the wellbore  10  in a conventional manner. 
     A sealing plug, or sealing tool,  14  is disposed in the wellbore  10  at a predetermined depth and is lowered to this position by a work string  16 , in the form of coiled tubing, jointed tubing, wire line, or the like, which is connected to the upper end of the plug  14 . The plug  14  is shown generally in  FIG. 1  and will be described in detail later. 
     The work string  16  extends from a rig  18  located above ground and extending over the wellbore  10 . The rig  18  is conventional and, as such, includes a support structure, a motor driven winch, or the like, and other associated equipment for lowering the plug  14 , via the string  16 , into the wellbore  10 . 
     The string  16  extends through a wellhead  22  that is positioned over the upper end of the wellbore  10  and the casing  12  at the rig  18 . The wellhead  22  is conventional and, as such, includes a closure device (not shown), such as a cap, or the like, for preventing the flow of production fluid from the formation through the casing  12 , while permitting movement of the string  16 , in a conventional manner. 
     When the well is not in production, the above-mentioned closure device associated with the wellhead  22  is set to prevent any flow of production fluid from the formation and through the casing  12  to the rig  18 . However, if the closure device has to be removed for repair, replacement, or the like, the casing  12  must be sealed to prevent the production fluid flow. To this end, the plug  14  is lowered, via the string  16 , to a desired depth in the casing  12  adjacent to, or above, the formation, such as to the depth shown in  FIG. 1 , and the plug  14  is set in the casing  12  in a manner to be described. 
     With reference to  FIG. 2 , the plug  14  includes a mandrel  30  having an upper end  30   a  that is connectable to the lower end of the string  16  in any conventional manner. The mandrel  30  has a lower end  30   b,  and a continuous bore extends between the upper end  30   a  and the lower end  30   b.    
     A tubular liner  32  is disposed in the bore of the mandrel  30 , with the lower end of the liner  32  extending flush with the lower end  30   b  of the mandrel  30 . A cap  34  extends over the lower end  30   b  of the mandrel  30  and the corresponding end of the liner  32  to retain the liner  32  in the mandrel  30 . 
     A series of axially-spaced circumferential grooves  32   a  are formed in the outer surface of the liner  32  which receive a detonation cord  35  that extends around the liner  32 . The detonation cord  35  is of a conventional design and, as such, can be a thin, flexible, waterproof fabric tube with a highly explosive core that can transmit a detonation wave. The cord  35  is wrapped around the liner  32  and extends in the grooves  32   a,  and also is more tightly wrapped in an enlarged recess  32   b  formed in the liner  32 . A conventional detonation initiator  38  abuts the upper end of the liner  32 , and, when activated in a manner to be described, detonates the cord  35 , causing the explosive in the cord to explode. 
     A compression-set, annular sealing element  44  extends around the mandrel  30  and is axially positioned between two sets of extrusion limiters  48   a  and  48   b.  A pair of wedges  50   a  and  50   b  extend between the extrusion limiters  48   a  and  48   b,  respectively, and two sets of slips  52   a  and  52   b,  respectively. The inner surfaces of the end portions of the slips  52   a  and  52   b  adjacent the wedges  50   a  and  50   b  are beveled so as to receive the corresponding tapered end portions of the wedges  50   a  and  50   b.  The sealing element  44  can be fabricated from a conventional material that performs the sealing function to be described, and the slips  52   a  and  52   b  and the mandrel  30  are preferably fabricated from a frangible material. 
     A mechanism for expanding and setting the sealing element  44  and the slips  52   a  and  52   b  includes a pair of axially-spaced ratchet shoes  54   a  and  54   b  that extend around the mandrel  30  and abut the corresponding ends of the slips  52   a  and  52   b.  Since the extrusion limiters  48   a  and  48   b,  the wedges  50   a  and  50   b,  the slips  52   a  and  52   b , and the shoes  54   a  and  54   b  are conventional, they will not be described in further detail. 
     The sealing element  44  and the slips  52   a  and  52   b  are activated, or set, in a conventional manner by using a setting tool, or the like (not shown), to move the shoe  54   a  downwardly relative to the mandrel  30 , as viewed in  FIG. 2 , and to move the shoe  54   b  upwardly relative to the mandrel  30 . This places a compressive force on the assembly formed by the slips  52   a  and  52   b,  the wedges  50   a  and  50   b  and the sealing element  44 . As a result, the slips  52   a  and  52   b  are forced radially outwardly into a locking engagement with the inner wall of the casing  12 , and the sealing element  44  expands radially outwardly into a sealing engagement with the inner wall of the casing  12 . Thus, the plug  14  seals against any flow of production fluid from the formation through the wellbore  10 . After the plug  14  is set in the above manner, the string  16  ( FIG. 1 ) is disconnected from the plug  14  in any conventional manner, and the string  16  is brought to the ground surface by the winch of the rig  18 . 
     When it is desired to recap the well, the plug  14  is removed in the following manner. Referring to  FIG. 3 , an actuator  60  is connected to the leading end of the string  16  in any conventional manner. The string  16  is then lowered into the wellbore  10  until the actuator  60  extends above, and in proximity to, the plug  14  and, more particularly, the initiator  38  ( FIG. 2 ). The actuator  60  is adapted to transmit, and the initiator  38  is adapted to receive, a wireless signal, or code, for activating the initiator  38 . In particular, the actuator  60  includes a transmitting antenna (not shown) that is adapted to transmit the signal to the initiator  38 , and the initiator  38  includes a receiving antenna that receives the transmitted signal from the actuator  60 . The signal transmitted between the actuator  60  and the initiator  38  is adapted to activate the initiator  38  and can be of any conventional type, such as electrical, acoustical, or magnetic. 
     The activation of the initiator  38  by the above signal detonates the cord  35  and explodes the explosive associated with the cord  35 . The explosion disintegrates, or breaks up at least a portion of the plug  14  and releases the engagement of the plug  14  with the casing  12  or the wellbore  10 . The resulting fragments of the plug  14  fall to the bottom of the wellbore  10  by gravity. The string  16  ( FIG. 3 ), with the actuator  60 , is then brought to the ground surface by the winch of the rig  18  ( FIG. 1 ). 
     The above-mentioned closure device associated with the wellhead  22  is then reinstalled over the wellhead  22  and set to prevent any flow of production fluid from the formation and through the wellbore  10  to the rig  18 . 
     Thus, the plug  14  can be placed in the wellbore  10  and activated to seal off the flow of production fluid as discussed above and yet can be removed in a relatively simple and inexpensive manner at any indeterminate time. 
     According to an alternate embodiment, the initiator  38  responds to the signal from the actuator  60  and produces heat and oxygen in a manner to be described, and one or more of the components of the plug  14  are formed from a consumable material that burns away and/or loses structural integrity when exposed to the heat and oxygen. 
     In particular, the initiator  38  includes what is commonly referred to as an “exploding bridge wire” that is surrounded by a material that produces heat and oxygen when ignited by the wire. In particular the bridge wire consists of a wire that is connected across a source of high-voltage electricity so that when activated, the resulting high current generates heat in the wire that is transferred to, and is sufficient to ignite, the material. An example of such a material is thermite, which comprises iron oxide, or rust (Fe 2 O 3 ), and aluminum metal powder (Al). When ignited and burned, the thermite reacts to produce aluminum oxide (Al 2 O 3 ), and liquid iron (Fe), which is a molten plasma-like substance. The chemical reaction is:
 
Fe 2 O 3 +2Al(s)→Al 2 O 3 (s)+2Fe(1)
 
     As stated above, one or more of the components of the plug  14  is formed from a consumable material that burns away and/or loses its structural integrity when exposed to the heat and oxygen resulting from the burning of the thermite. The components of the plug  14  that may be formed of the consumable material should be suitable for service in a downhole environment and provide adequate strength to enable proper operation of the plug  14 . By way of example only, the mandrel  30  and/or the slips  52   a  and  52   b  of the plug can be fabricated of a consumable material, and an example of the latter material is magnesium metal. 
     After the plug  14  is installed in the wellbore  10 , and if it is desired to remove the plug for the same reasons as indicated in the previous embodiment, the actuator  60  is attached to the end of the string  16 , and the string  16  is lowered into the wellbore  10  until the actuator  60  extends above, and in proximity to, the plug  14  and, more particularly, the initiator  38  ( FIG. 2 ). The initiator  38  is activated by the transmitted wireless signal, or code, from the actuator  60 , as described above. 
     Activation of the initiator  38  produces a high current across the above described bridge wire which generates heat sufficient to ignite, or burn, the material, such as thermite, surrounding the bridge wire, thus producing heat and oxygen. The consumable components of the plug  14 , which in the above example are the mandrel  30  and/or the slips  52   a  and  52   b,  will react with the oxygen in the aluminum oxide (Al 2 O 3 ), causing the magnesium metal to be consumed or converted into magnesium oxide (MgO), as illustrated by the chemical reaction below:
 
3Mg+Al 2 O 3 →3MgO+2Al
 
     A slag is thus produced such that the mandrel  30  and/or the slips  52   a  and  52   b  no longer have structural integrity and thus cannot carry the load. The engagement of the plug  14  with the casing  12  or the wellbore  10  is released and the resulting slag and/or fragments of the mandrel  30  and the slips  52   a  and  52   b,  along with the remaining components of the plug  14 , fall to the bottom of the wellbore  10  by gravity. 
     The string  16  , with the actuator  60  ( FIG. 3 ), is then brought to the ground surface by the winch of the rig  18  ( FIG. 1 ). The above-mentioned closure device associated with the wellhead  22  ( FIG. 1 ) is then reinstalled over the wellhead  22  and set to prevent any flow of production fluid from the formation and through the wellbore  10  to the rig  18 . 
     Thus, as in the previous embodiment, the plug  14  can be placed in the wellbore  10  and activated to seal off the flow of production fluid as discussed above and yet can be removed in a relatively simple and inexpensive manner at any indeterminate time. 
     Variations 
     It is understood that variations may be made in the foregoing without departing from the scope of the invention. Non-limiting examples of these variations are as follows: 
     (1) The number and type of the slips  52   a  and  52   b  and the sealing element  44  can be varied within the scope of the invention. 
     (2) The type of electronic signal transmitted from the actuator  60  to the initiator  38  to activate the initiator  38  can be varied and can be generated by electrical, acoustical, or magnetic devices, in a conventional manner. 
     (3) The initiator  38  could be activated by mechanical means such as a fishing head attachment that is operated by a hook, or the like, attached to the string  16 . 
     (4) The wellbore  10  could be an open hole completion, sans the casing  12 , in which case the wellbore  10  would be sealed by the plug  14 . 
     (5) The signal transmitted to the initiator  38  could be transmitted from the ground surface. 
     (6) In the second embodiment disclosed above, components, other than the slips  52   a  and  52   b  and the mandrel  30  may be fabricated from the consumable material that loses structural integrity when exposed to heat and an oxygen source. 
     (7) The consumable components of the plug  14  can be fabricated from a material other than magnesium metal. 
     (8) Conventional blasting caps can be used in place of the bridge wire discussed above. 
     (9) The plug  14  can used in other well servicing or well treatment operations when temporary plugging of the well is needed such as in fracturing operations. 
     The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents.

Summary:
A method for removing a sealing plug from a casing or a wellbore according to which a sealing plug is adapted to expand into engagement with the casing or the wellbore. A wireless signal is sent to the plug to cause the plug to lose its structural integrity and fall to the bottom of the wellbore.