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BACKGROUND 
   This disclosure relates to a system and method for treating a subterranean formation penetrated by a wellbore, and, more particularly, to such a system and method for removing downhole tools that are inserted into the wellbore to perform various operations in connection with recovering hydrocarbon fluids from the formation. 
   Various types of downhole tools are inserted in a well in connection with producing hydrocarbon fluids from the formation surrounding the well. For example, tools for plugging, or sealing, different zones of the formation are often inserted in the wellbore to isolate particular zones in the formation. After the operation is complete, the plugging or sealing tools must be removed from the wellbore which is usually accomplished by inserting a drilling tool into the wellbore and mechanically breaking up the tools by drilling, or the like. However this removal process is expensive and time consuming. 
   The present invention is directed to a system and method for removing tools from a wellbore that is an improvement over the above techniques. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an partial elevational/partial sectional view, not necessarily to scale, of a well depicting a system for recovering oil and gas from an underground formation. 
       FIG. 2  is a sectional view of a example of a tool that is inserted in the well of  FIG. 1  then removed according to an embodiment of the present invention. 
       FIGS. 3–6  are enlarged sectional views of the well of  FIG. 1  illustrating several steps of inserting and removing the tool of  FIG. 2  according to the above embodiment. 
   

   DETAILED DESCRIPTION 
   Referring to  FIG. 1 , the reference numeral  10  refers to a wellbore penetrating a subterranean formation F for the purpose of recovering hydrocarbon fluids from the formation F. To this end, and for the purpose of carrying out a specific operation to be described, a tool  12  is lowered into the wellbore  10  to a predetermined depth, by a string  14 , in the form of wireline, coiled tubing, jointed tubing, or the like, which is connected to an upper end of the tool  12 . The tool  12  is shown generally in  FIG. 1  but will be described in detail later. The string  14  extends from a rig  16  that is located above ground and extends over the wellbore  10 . The rig  16  is conventional and, as such, includes support structure, a motor driven winch, and other associated equipment for receiving and supporting the tool  12  and lowering it to a predetermined depth in the wellbore  10  by unwinding the string  14  from a reel, or the like, provided on the rig  16 . 
   At least a portion of the wellbore  10  can be lined with a casing  20 , and the casing  20  is cemented in the wellbore by introducing cement  22  in an annulus formed between an inner surface of the wellbore  10  and an outer surface of the casing  20 , all in a convention manner. 
   For the purpose of example only, it will be assumed that the tool  12  is in the form of a plug that is used in a stimulation/fracturing operation to be described. To this end, and with reference to  FIG. 2 , the tool  12  includes an elongated tubular body member  32  having a continuous axial bore extending through its length for passing fluids in a manner to be described. A cage  34  is formed at an upper end of the body member  32  for receiving a ball valve  36  which prevents fluid flow downwardly through the body member  32 , as viewed in  FIG. 1 , but permits fluid flow upwardly through the body member  32 . 
   A plurality of angularly spaced packer elements  40  are mounted around the body member  32 , and a plurality of angularly spaced slips  42  are mounted around the body member  32  just below the packer elements  40 . A tapered shoe  44  is provided at a lower end of the body member  32  for the purpose of guiding and protecting the tool  12  as it is lowered in the wellbore  10 . 
   The above components, as well as most other components making up the tool  12  which are not shown and described above, are fabricated from at least one metal selected from the group consisting of magnesium, aluminum, zinc, iron, tin, and lead or from carbon, with the exceptions of the ball valve  36  and any elastomers utilized in the packer elements  40  or in any other sealing components that may be included in the tool  12 . Otherwise, the tool  12  is conventional and therefore will not be described in further detail. 
     FIGS. 3–6  depict the application of the tool  12  in an operation for recovering hydrocarbon fluids from the formation F. In particular, and referring to  FIG. 3 , a lower producing zone A, an intermediate producing zone B, and an upper producing zone C, are all formed in the formation F. A plurality of perforations  20   a  and  22   a  are initially made in the casing  20  and the cement  22 , respectively, adjacent the zone A. This can be done in a conventional manner, such as by lowering a perforating tool (not shown) into the wellbore  10 , performing the perforating operation, and then pulling the tool from the wellbore  10 . 
   The area of the formation F adjacent the perforations  20   a  and  22   a  can then be treated by introducing a conventional stimulation/fracturing fluid into the wellbore  10  such as by pumping, so that it passes through the perforations  20   a  and  22   a  and into the formation F. This stimulation/fracturing fluid can be introduced into the wellbore  10  in any conventional manner, such as by lowering a tool containing discharge nozzles or jets for discharging the fluid at a relatively high pressure, or by passing the stimulation/fracturing fluid from the rig  16  directly into the wellbore  10 . In either case, the stimulation/fracturing fluid passes through the perforations  20   a  and  22   a  and into the zone A for stimulating the recovery of production fluids, for example hydrocarbons such as oil and/or gas. The production fluids pass from the zone A, through the perforations  20   a  and  22   a , and up the wellbore  10  for recovery at the rig  16 . If the stimulation/fracturing fluid is discharged through a downhole tool as described above, the latter tool is then removed from the wellbore  10 . 
   The tool  12  is then lowered by the string  14  into the wellbore  10  to a position where its lower end portion formed by the shoe  44  is just above the perforations  20   a  and  22   a , as shown in  FIG. 4 . The slips  42  and the packer elements  40  are set to lock the tool  12  to the casing  20  and to seal the interface between the tool  12  and the casing  20  and thus isolate the zone A. The string  14  is disconnected from the tool  12  and returned to the rig  16 . The production fluids from the zone A then pass through the perforations  20   a  and  22   a , into the wellbore  10  and through the aforementioned bore in the body member  32  of the tool  12 , before flowing up the wellbore  10  for recovery at the rig  16 . 
   A second set of perforations  20   b  and  22   b  are then formed, in the manner discussed above, through the casing  20  and the cement  22 , respectively, adjacent the zone B just above the upper end of the tool  12 . The zone B can then be treated by the stimulation/fracturing fluid, in the manner discussed above, causing the recovered fluids from the zone B to pass from through the perforations  20   b  and  22   b  and into the wellbore  10  where they mix with the recovered fluids from the zone A before flowing up the wellbore  10  for recovery at the ground surface. 
   As shown in  FIG. 5 , another tool  12 ′ is provided, which is identical to the tool  12  and thus includes identical components as the tool  12 , which components are given the same reference numerals. The tool  12 ′ is lowered by the string  14  into the wellbore  10  to a position where its lower end portion formed by the shoe  44  is just above the perforations  20   b  and  22   b . The slips  42  and the packer elements  40  of the tool  12 ′ are set to lock the tool  12 ′ to the casing  20  and to seal the interface between the tool  12 ′ and the casing  20  and thus isolate the zone B. The string  14  is then disconnected from the tool  12 ′ and returned to the rig  16 . 
   A third set of perforations  20   c  and  22   c  are then formed in the casing  20  and the cement  22  adjacent the zone C and just above the upper end of the tool  12 ′, in the manner discussed above. The zone C can then be treated by the stimulation/fracturing fluid, also in the manner discussed above, causing the recovered fluids from the zone C to pass through the perforations  20   c  and  22   c  and into the wellbore  10  where they mix with the recovered fluids from the zones A and B before passing up the wellbore  10  for recovery at the ground surface. 
   It can be appreciated that additional producing zones, similar to the zones A, B, and C, can be provided above the zone C, in which case the above operations would also be applied to these additional zones. 
   After the above fluid recovery operations are terminated, the tools remaining in the wellbore  10 , which in the above example are tools  12  and  12 ′, must be removed from the wellbore  10 . To this end, a mineral acid, such as hydrochloric acid or sulfuric acid, is introduced into the wellbore  10  in any conventional manner. For example, as shown in  FIG. 6 , the string  14  can be formed by coiled tubing and a discharge head  50  is attached to the end of the string  14  and lowered into the wellbore  10  until the discharge head  50  is just above the tool  12 ′. The mineral acid is introduced into the upper end of the string  14  from a source at the rig  16  and passes through the string  14  before it discharges from the discharge head  50  onto the tool  12 ′. 
   As stated above, the tools  12  and  12 ′ are comprised of a metal that chemically reacts with the mineral acid and, in particular, by at least one metal selected from the group consisting of magnesium, aluminum, zinc, iron, tin, and lead or from carbon. The mineral acid is introduced in sufficient quantities so as to react with the metal in a conventional manner to corrode and eventually completely break up or dissolve the metal. This leaves only the components of the tools  12  and  12 ′ not fabricated of the metal, which, in the example above, are the ball valves  36 , as well as any elastomers utilized in the packer elements  40  or any other sealing components that may be included in the tool  12 ′. 
   After the metal components of the tool  12 ′ are dissolved in the above manner, additional mineral acid from the rig  16  is introduced into the wellbore  10  in the above manner so as to react with the metal components of the tool  12  and dissolve the latter components, as discussed above. It is understood that the string  14 , and therefore the discharge head  50 , can be lowered as necessary in the wellbore  10  to a position extending just over the tool  12 . 
   The non-metallic components from the tools  12  and  12 ′ could then be pumped or dropped to the bottom of the wellbore  10  into a rat hole, or the like (not shown). 
   The method of the above embodiment thus permits tools located in a wellbore to be easily and quickly removed with a minimum of expense. 
   VARIATIONS AND ALTERNATES 
   The cement  22  can be eliminated. 
   The type of downhole tool utilized and treated in the above manner can be varied. 
   The mineral acid introduced to the tools  12  and  12 ′ to break up or dissolve the components of the tools can be a pure mineral acid or a mineral acid based solution. 
   The type of materials forming the tools as well as the type of acid that breaks up or dissolves the materials can be varied. For example, an organic acid such as formic acid can be used to break up or dissolve the components of the tool. 
   The mineral acid can be discharged into the wellbore  10  in manners other than that described above. 
   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 of treating a subterranean formation penetrated by a wellbore, according to which a tool is fabricated of a material that breaks up or dissolves in the presence of a fluid and is inserted in the wellbore for performing a function in the wellbore. The fluid is then introduced to the tool to break up or dissolve portions of the tool and the remaining portions of the tool fall to the bottom of the well.