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CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of U.S. patent application Ser. No. 10/619,087, filed Jul. 14, 2003 now U.S. Pat. No. 7,036,602 . Each of the aforementioned related patent applications is herein incorporated by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   In the completion of oil and gas wells, there are various downhole operations in which it may become necessary to isolate particular zones within the well. This is typically accomplished by temporarily plugging off the well casing at a given point or points with a bridge plug. Bridge plugs are particularly useful in accomplishing operations such as isolating perforations in one portion of a well from perforations in another portion, or for isolating the bottom of a well from a wellhead. The purpose of the plug is simply to isolate some portion of the well from another portion of the well. However, in some instances, the bridge plug may not necessarily be used for isolation, but may be used, for example, to create a cement plug in the wellbore. The bridge plug may be temporary or permanent; if temporary, it must be removable. 
   Bridge plugs may be drillable or retrievable. Drillable bridge plugs are typically constructed of a brittle metal such as cast iron that can be drilled out. One typical problem with conventional drillable bridge plugs, however, is that without some sort of locking mechanism, the bridge plug components may tend to rotate with the drill bit, which can result in extremely long drill-out times, excessive casing wear, or both. Long drill-out times are highly undesirable, as rig time is typically charged by the hour. 
   An alternative to drillable bridge plugs is the retrievable bridge plug, which may be used to temporarily isolate portions of the well before being removed, intact, from the well interior. Retrievable bridge plugs typically have anchor and sealing elements that engage and secure it to the casing wall. To retrieve the plug, a retrieving tool is lowered into the casing to engage a retrieving latch, which, through a retrieving mechanism, retracts the anchor and sealing elements, allowing the bridge plug to be pulled out of the wellbore. A common problem with retrievable bridge plugs is the accumulation of debris on the top of the plug, which may make it difficult or impossible to engage the retrieving latch to remove the plug. Such debris accumulation may also adversely affect the relative movement of various parts within the bridge plug. Furthermore, with current retrieving tools, jarring motions or friction against the well casing can cause accidental unlatching of the retrieving tool, or re-locking of the bridge plug (due to activation of the plug anchor elements). It may also be difficult to separate the retrieving tool from the plug upon removal, necessitating the use of additional machinery. Problems such as these sometimes make it necessary to drill out a bridge plug that was intended to be retrievable. 
   Thus, there is a need in the art for a bridge plug whose performance is not impaired by undesirable conditions such as differential pressure zones or wellbore debris, and that may be removed from the wellbore without undue exertion or cost. 
   SUMMARY OF THE INVENTION 
   One embodiment of the present invention provides a bridge plug for isolating portions of a downhole casing comprising a retrievable upper mandrel assembly and a lower mandrel assembly coupled to the upper mandrel assembly, wherein the lower mandrel assembly comprises a drillable material. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     So that the manner in which the above recited embodiments of the invention are attained and can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
       FIG. 1A  is a longitudinal cross-sectional view of one embodiment of a bridge plug according to the present invention; 
       FIG. 1B  is a longitudinal cross-sectional view of the upper mandrel assembly of  FIG. 1A ; 
       FIG. 1C  is a longitudinal cross-sectional view of the lower mandrel assembly of  FIG. 1A ; 
       FIG. 2A  is a longitudinal cross-sectional view of the bridge plug of  FIG. 1A  in the set position; 
       FIG. 2B  is a longitudinal cross-sectional view of the upper mandrel assembly of  FIG. 2A ; 
       FIG. 2C  is a longitudinal cross-sectional view of the lower mandrel assembly of  FIG. 2A ; 
       FIG. 3A  is a longitudinal cross-sectional view of a second embodiment of a bridge plug according to the present invention; 
       FIG. 3B  is a longitudinal cross-sectional view of the upper mandrel assembly of  FIG. 3A ; 
       FIG. 3C  is a longitudinal cross-sectional view of the lower mandrel assembly of  FIG. 3A ; 
       FIG. 4A  is a longitudinal cross-sectional view of the bridge plug of  FIG. 3A  in the set position; 
       FIG. 4B  is a longitudinal cross-sectional view of the upper mandrel assembly of  FIG. 4A ; 
       FIG. 4C  is a longitudinal cross-sectional view of the lower mandrel assembly of  FIG. 4A ; and 
       FIG. 5  is a flow diagram illustrating a method of retrieving the bridge plug of the present invention from a wellbore. 
   

   To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. 
   DETAILED DESCRIPTION 
   The present invention aims to provide an improved bridge plug that is both retrievable and drillable. Existing bridge plugs that are either retrievable or drillable individually suffer from respective shortcomings related to plug setting and removal. The present invention provides a retrievable bridge plug having several drillable components, preferably made of composite materials, and therefore it may be retrieved, drilled, or both for removal as need dictates. 
     FIG. 1A  is a cross-sectional view of one embodiment of a bridge plug according to the present invention. While  FIG. 1A  illustrates the tool in its entirety,  FIGS. 1B and 1C  each depict roughly one half of the tool (cut along line A—A in  FIG. 1A ) so that the details of the present invention may be more clearly illustrated. The bridge plug  100  illustrated in  FIG. 1A  is in a “locked”, or inactivated position, as for running into a string of casing. In one embodiment, the bridge plug  100  comprises an upper mandrel assembly  102  and a lower mandrel assembly  104 . 
   The upper mandrel assembly  102  is illustrated in further detail in  FIG. 1B  and comprises a substantially tubular outer setting sleeve  106  having a connection  108  at an upper end  107  of the assembly  102 . The connection  108  is threaded for attachment to a hydraulic or explosive operated tool (not shown). The setting sleeve  106  houses a setting tool body  110 , which has a threaded sucker rod connection  111  at its upper end, and in turn cariles a selection tool  112  having a fishing neck  114  at an upper end  113  and a radial port  116  proximate a lower end  115  of the upper mandrel assembly  102 . Within the serection tool  112  is an upper mandrel  118 . The setting tool body  110 , selection tool  112 , and upper mandrel  118  are secured to one another by an upper shear pin  120  located proximate lower end  115  of the upper mandrel assembly  102 , distal from the sucker rod connection  111 . Furthermore, a selection tool lug  122  extends radially inward from the selection tool  112  toward the upper mandrel  118 , to engage an annular, sinuous groove  124  that extends around the outer circumference of the mandrel  118 . 
   A portion of the upper mandrel  118  that is distal from the shear pin  120  connection is surrounded by a spring housing  126 . The spring housing  126  houses a coil spring  128  that is carried around the upper mandrel  118 . An upper spring stop  130  is secured, for example by a pin  132   a , to the mandrel  118 , while a lower spring stop  134  is secured to the selection tool  112 , also by a pin  132   b . The coil spring  128  is restrained axially within the upper and lower spring stops  130 ,  134 . Below the spring housing  126 , but above the upper shear pin  120 , a radial port  136  is provided in the upper mandrel  118 . 
   The lower mandrel assembly  104  is illustrated in further detail in  FIG. 1C  and is coupled to the lower end  115  of the upper mandrel assembly  102 . The lower mandrel assembly  104  comprises a lower mandrel  138  preferably comprised of a composite material and having a first end  140  that fits within the lower end  115  of the upper mandrel  118 . Composite materials are well known in the art and typically comprise high-strength plastics containing fillers such as carbon or glass fiber. The lower mandrel  138  is secured in place by the upper shear pins  120  and  141  that secure the upper mandrel  118 , selection tool  112 , and setting tool body  110 . A second end  142  of the lower mandrel  138  terminates in a nose shoe  144 . The nose shoe  144  forms the lowermost portion of the bridge plug  100 . 
   A body lock ring housing  146  surrounds the lower mandrel  138  just below the setting tool body  110  and upper mandrel  118 . The body lock ring housing  146  may be formed of metallic or composite material and carries a lock ring  148 . The lock ring  148  comprises a plurality of teeth  150  that engage the lower end  115  of the selection tool  112  and secure the selection tool  112  to the lower mandrel  138 . 
   The lower mandrel assembly  104  further comprises upper and lower slip and cone assemblies  152 ,  154  and a resilient packer element  156 . The upper slip and cone assembly  152  comprises a slip cage  158  formed of a composite material and secured by a lower shear pin  160  to a lower end  147  of the lock ring housing  146 . The upper slip cage  158  carries a plurality of upper slip segments  162 , each of which comprises a plurality of teeth  170  and surrounds a tapered end  173  of a conical upper cone  172 , also formed of a composite material. Thus, the upper cone  172  is situated to slide upwardly beneath the upper slip segments  162 . A lower slip and cone assembly  154  is formed similarly but is oriented to oppose the upper slip and cone assembly  152 ; that is, the lower slip segments  176  slide upwardly beneath the lower cone  174 . The upper and lower slip and cone assemblies  152 ,  154  are spaced longitudinally so that a resilient packer element  156  may be retained between the upper and lower cones  172 ,  174 . 
   The operation of the bridge plug embodiment illustrated in  FIG. 1A  may best be understood with reference to  FIGS. 2A–C , which illustrates the bridge plug of  FIG. 1A  in the “set” position.  FIG. 2A  illustrates the bridge plug  100  in its entirety, while  FIGS. 2B and 2C  each illustrate roughly one half (or the upper and lower mandrel assemblies  102 ,  104 , respectively) of the bridge plug  100  shown in  FIG. 2A . 
   The hydraulic or explosive operated tool (not shown) that is coupled to the sucker rod connection  108  on the upper mandrel assembly  102  is actuated to exert a downward force on the setting tool  110 , while pulling up on the main body of the bridge plug  100 , including the slips  162 ,  176  and packer element  156 . This provides an upward force against the nose shoe  144  that moves the cones  172 ,  174  into the slips  158 ,  178 . As the cones  172 ,  174  move into the slip cages  158 ,  178 , they also are forced closer together, compressing the packer element  156  longitudinally so that it expands or extends radially outward. The travel of the cones  172 ,  174  beneath the slip cages  158 ,  178  also expands the slip segments  162 ,  176  radially outward so that the teeth  170  “bite” into and engage the inner wall  182  of the casing  180 , which secures the packer element  156  in its compressed and fully expanded condition. At the same time, the body lock ring housing  146  is forced downwardly with relation to the bridge plug body  100 , the lock ring teeth  150  bite into the body lock ring housing  146  to prevent upward movement that might release the applied downward force. 
   In order to allow flow through the tool  100 , a central conduit  184  is provided through the slips  162 ,  176  and packer  156  and part of the upper mandrel  118 . The radial port  136  in the upper mandrel  118  may be opened or closed depending on the relative axial positions of the upper and lower mandrels  118 ,  138 . To open the port  136 , first, upward force is applied to the setting sleeve  106  and the setting tool body  110  to break the shear pin  120 , thereby allowing removal of the setting sleeve  106  and setting tool body  110 . The fishing neck  114  is thus exposed for grasping by a fishing tool (not shown), supported by a wire line (not shown). Pulling upward on the fishing neck  114  exerts an upward force on the upper mandrel  118 , compressing the spring  128 . The selection tool lug  122  that extends radially inward from the selection tool body  112  engages the sinuous groove  124  that extends around the outer circumference of the upper mandrel  118 . Thus, when the upper mandrel  118  is pulled upward, the engagement of the lug  122  with the sinuous groove  124  causes relative rotation of the upper mandrel  118  and the selection tool  112 . At the same time, the spring  128  surrounding the upper mandrel  118  is compressed. 
   When the upward force is released, the spring  128  is relaxed, causing relative axial movement between the upper mandrel  118  and the selection tool  112 . Lug movement through the grooves  124  causes simultaneous relative rotation of these components, which moves the ports  116 ,  136  so that they are aligned, thereby opening the port to allow fluid to flow through the tool. 
   To retrieve the bridge plug  100  from the wellbore, a wire line (not shown) is connected to the fishing neck  114  on the selection tool  112 , and upward force is applied. This exerts an upward force that pulls on the lower mandrel  138 , which in turn pulls on the body lock ring housing  146 , which is connected to the upper slip cage  158 . The upper slip cage  158  is thereby pulled upwardly to release the radial force on the slips  162 ,  176 , allowing the upper cone  172  to move upwardly and release the compressive force on the packer element  156 . Similarly, the lower cone  174  is removed from beneath the lower slip cage  178  so that the packer element  156  relaxes. With no radial forces forcing components of the bridge plug  100  into engagement with the inner wall  182  of the casing  180 , the bridge plug  100  may be retrieved from the wellbore by pulling upwardly. 
   In the event that the slips  162 ,  176  and packer element  156  cannot be released as described above, they may be drilled out. If the application of a predetermined amount of force is not sufficient to release the slips  162 ,  176 , an emergency release is provided to disconnect the lower mandrel assembly  104  from the remainder of the bridge plug tool  100 . This release comprises the lower shear pin  160 , which breaks when a sufficient amount of force is applied. The upper mandrel  118  and upper mandrel assembly  102  may be retrieved as described above. The remaining tool components—the lower mandrel  138 , slips  162 ,  176 , cones  172 ,  174  and packer element  156 —all comprise composite material, and so a milling machine may be lowered into the well to drill out the remaining material. Thus at worst, the bridge plug tool  100  is largely retrievable, cutting down on drilling time and cost. That which might not be retrieved is made of drillable material and represents a small percentage of the overall tool material to keep the complexity and cost of removal to a minimum as well. 
   An alternate embodiment of the present invention in illustrated in  FIGS. 3A–C .  FIG. 3A  is a cross-sectional view of a second embodiment of a bridge plug according to the present invention. While  FIG. 3A  illustrates the tool in its entirety,  FIGS. 3B and 3C  each depict roughly one half of the tool (cut along line C—C in  FIG. 3A ) so that the details of the present invention may be more clearly illustrated. The bridge plug  200  illustrated in  FIG. 3A  is in a “locked”, or inactivated position, as for running into a string of casing. In one embodiment, the bridge plug  200  comprises an upper mandrel assembly  202  and a lower mandrel assembly  204 . 
   The upper mandrel assembly  202  is illustrated in further detail in  FIG. 3B  and comprises a substantially tubular setting sleeve  206  having a threaded connection  208  at its upper end  207 . The setting sleeve  206  houses a setting tool body  210 , which in turn carries a selection tool  212 . The selection tool  212  has an upper end  213  terminating in a fishing neck  214  and a lower end  215  terminating in a downward facing plunger  222 . In addition, a radial port  216  is formed in the selection tool  212  proximate the lower end  215 . 
   The lower mandrel assembly  204  is coupled to the lower end  209  of the upper mandrel assembly  202 . The lower mandrel assembly  204  comprises a lower mandrel  238  comprised of a composite material and having an upper end  240  terminating in a counterbore  224  (shown in  FIG. 3B ) defined therein. The upper end  240  of the lower mandrel  238  is secured to a setting sleeve  215  and setting tool  210  by an upper shear pin  220 . A lower end  242  of the lower mandrel  238  terminates in a nose shoe  244 . The nose shoe  244  forms the lowermost portion of the bridge plug  200 . The nose shoe  244  has a central bore  245  terminating in a conical seat  247  which receives a lower plunger  223  mounted on a rod which extends downward from the plunger  222 . 
   A body lock ring housing  246  surrounds the lower mandrel  238  just below the upper mandrel assembly  202 . The body lock ring housing  246  may be formed of a metallic or composite material and carries a lock ring  248 . The lock ring  248  comprises a plurality of teeth  250  that engage the lower end  215  of the setting tool  210  and secure it to the upper end  240  of the lower mandrel  238 . 
   The lower mandrel assembly  204  further comprises upper and lower slip and cone assemblies  252 ,  254  and at least one of resilient packer element  256 . The upper slip and cone assembly  252  includes an upper cone  258  comprising an inclined slip ramp and secured by a lower shear pin  260  to a lower end  247  of the lock ring housing  246 . The tapered end  257  of the upper cone  258  engages the tapered surface  259  of upper slip segments  262 , which comprise a plurality of teeth  270 . A recess  228  in the slip  262  is slidably engaged with an elongated end  230  of an upper compression element  272 . Thus, the upper cone  258  is designed to slide downwardly under the slip elements  262 , to force the slip elements  262  downward against the upper compression element  272  and radially outward against the inner wall  282  of the casing  280 . The slip segments  262  and cone  272  are preferably formed of a composite material. A lower slip and cone assembly  254  is formed similarly but is oriented to oppose the upper slip and cone assembly  252 ; that is, the lower cone  278  abuts the upper end  245  of the nose shoe  244 , and the slip segments  276  move downwardly so that their tapered bore  277  engages the tapered upper end  279  of the compression element  272 . The upper and lower slip and cone assemblies  252 ,  254  are spaced longitudinally so that at least one resilient packer element  256  may be retained between the upper and lower compression elements  272 ,  274 . In the embodiment illustrated in  FIG. 3C , 3 such packer elements  256  are utilized; however, a greater or lesser number may be used. 
   The operation of the bridge plug  200  is not unlike the operation of the bridge plug  100  discussed herein, and may best be understood with reference to  FIGS. 4A–C , which illustrate the bridge plug of  FIG. 3A  in a “set” position.  FIG. 4A  illustrates the bridge plug  200  in its entirety, while  FIGS. 4B and 4C  each illustrate roughly one half (or the upper and lower mandrel assemblies  202 ,  204 , respectively) of the bridge plug  200  shown in  FIG. 4A . 
   A hydraulic or explosive tool (not shown) is coupled to the threaded connection  208  on the upper mandrel assembly  202  and is actuated to exert a downward force on the setting tool  210 , while pulling up on the main body of the bridge plug  200 , including the slips  262 ,  276  and packer elements  256 . This provides an upward force against the nose shoe  244  that moves the cones  258 ,  278  further under the slips  262 ,  276  and forces the slips  262 ,  276  closer axially to the compression elements  272 ,  274 . As the slips  262 ,  276  move closer to the compression elements  272 ,  274 , they force the compression elements  272 ,  274  closer to each other, which compresses the packer elements  256  longitudinally so that they expand radially outward. The travel of the cones  258 ,  278  beneath the slip segments  262 ,  276  also expands the slip segments  262 ,  276  radially outward so that the teeth  270  “bite” into and engage the inner wall  282  of the casing  280 , which secures the packer elements  256  in their compressed conditions. At the same time, the body lock ring housing  246  is forced downward with relation to the bridge plug body  200 , and the lock ring teeth  250  bite into the body lock ring housing  246  to prevent upward movement that might release the applied downward force. 
   In order to allow flow through the tool  200 , a central conduit  284  is provided through the slips  262 ,  276  and packer elements  256  and part of the upper mandrel assembly  202  (see  FIGS. 4A–C , which show the bridge plug in the “set” condition). The radial port  236  in the selection tool  212  may be opened or closed depending on the relative axial position of the upper and lower mandrel assemblies  202 ,  204 . To open the port  236 , first, upward force is applied to the setting sleeve  206  and the setting tool body  210  to break the shear pin  220 , thereby allowing for removal of the setting sleeve  206  and setting tool body  210 . The fishing neck  214  is exposed for grasping by a fishing tool (not shown), and a wire line (not shown) is connected to the fishing neck  214  so that an upward force may be applied to the selection tool  212 . The plunger  222  on the lower end of the selection tool  212  is removed from the recess  224  in the lower mandrel  236 , so that flow f is allowed from the conduit  284 , through the recess and out the port  236 . When the upward force is released, the plunger moves back into the recess, thereby closing the port opening  236  off from flow. 
   Retrieval of the bridge plug  200  is also substantially similar to the retrieval process discussed herein with reference to the bridge plug  100 . If the slips  262 ,  276  should fail to release, sufficient upward force will break the lower shear pin  260 , thereby separating the upper and lower mandrel assemblies  202 ,  204 . The upper mandrel assembly  202  may then be pulled upwardly out of the wellbore, while the lower mandrel assembly  204 , largely comprising composite materials, may be drilled out with a milling machine. 
   Thus the present invention represents a significant advancement in the fields of oil and gas drilling and bridge plug technology. A bridge plug is provided that is largely retrievable from a wellbore. However, incorporated into the design is an emergency release that allows at least a portion of the plug to be retrieved if difficulty is encountered in removing the entire tool. In such an event, those components that remain in the wellbore are formed of a composite, drillable material that can be milled to clear the bore. Therefore, removal difficulties encountered with common existing retrievable bridge plugs are addressed. Time and cost for drilling are substantially reduced by making only a portion of the plug drillable, and by drilling only in the event that removal difficulties make retrieval of the entire tool infeasible or impossible. 
   While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.

Summary:
A method and apparatus for a bridge plug for isolating portions of a downhole casing is provided comprising a retrievable upper mandrel assembly and a lower mandrel assembly coupled to the upper mandrel assembly, wherein the lower mandrel assembly comprises a drillable material.