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BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is directed toward a cementing tool, a casing string equipped with a cementing tool, and methods of cementing such a casing string. Particularly, the cementing tool is provided with a rupture disc assembly that upon rupture permits cement to flow from the interior of the casing string through the tool sidewall and into the annulus defined by the casing string and downhole formation into which the casing string is run. The cementing tool permits obstructions or voids within the annulus to be bypassed during cementing operations, and allows for multiple-stage cementing operations to be conducted. Further, the cementing tool, if activated during cementing operations, restores the structural integrity of the casing string that might otherwise be lost through the use of other tools or processes. 
     2. Description of the Prior Art 
     Surface casing is typically the first casing string run and fully cemented in a well. Surface casing protects fresh water-bearing sands or formations from vertical migration of well fluids that might otherwise contaminate the fresh water carried by these formations. Often too, the well blow out preventer, which is the last line of defense against an uncontrolled well, is secured to the surface casing. Further, surface casing is used to hang off the next string of casing that is run into the well. Given the many functions of surface casing, it is important for the surface casing to be well supported in order to prevent buckling and damage when loaded in this manner. 
     The purpose of cementing the surface casing is to have a competent sheath of cement to both support and seal around the casing. During cementing operations, cement is introduced into the annulus created between the casing and the formation through which the casing is run. Cement can be introduced into the annulus in a number of ways. One method is “top job” approach wherein cement is directly injected into the annulus from the surface using one or more small diameter pipes pushed down into the annulus. This method may be useful in cementing shallow casing strings, but is not always reliable in that un-cemented pockets can be left in the annulus. Another method involves the circulation of cement down through the center of the casing string and back toward the surface through the annulus. When successfully completed, this method provides a higher degree of confidence that un-cemented pockets have been avoided or minimized. However, the annulus can become obstructed, such as with a collapsed portion of a loose formation which blocks the flow of cement through the annulus. In other instances, cement may be lost from the annulus into the well formation due to the high porosity of the rock or sand that the well bore is drilled through. This loss prevents the cement from reaching the surface and is known as lost circulation or lost returns. In these instances, the casing would need to be perforated above the obstruction or region of lost circulation so that a new flow path for cement into the annulus can be established. This is undesirable as it requires compromising the casing integrity. 
     Another solution has been proposed involving the use of differential valve (DV) tools. These tools have largely been used as a part of a multistage cementing operation. These tools are typically run where the cementing is planned to be placed in multiple lifts in a single string of pipe. The bottom section of casing is cemented normally. Then the tool is opened and drilling mud is circulated. After the bottom stage of cement has been set sufficiently, the top stage is cemented through the DV tool. These tools are disadvantageous in that the cementing must be performed in stages, rather than in a single pour, thus adding additional operating time to the cementing process. Further, these tools tend to be expensive and most require some kind of actuation operation, and then be drilled out once the cementing stage is completed. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes a number of the difficulties associated with prior apparatus and methods for cementing a casing string by utilizing a cementing tool that couples adjacent casing sections and comprises an integral rupture disc assembly that can be selectively actuated so as to bypass obstructions in the annulus between the downhole formation and casing string or permit flow of cement into the annulus at a desired elevation. 
     According to one embodiment of the present invention, there is provided a cementing tool configured for attachment to a casing string. The cementing tool comprises a tubular body including a cylindrical sidewall having an interior surface and an exterior surface. The sidewall interior surface defines a central passage therethrough. At least one channel-forming member is provided that defines a channel located outboard from the central passage. The channel includes at least one open end. At least one port is formed in the sidewall that defines a path for fluid flow between the central passage and the channel. The cementing tool further comprises at least one rupture disc assembly comprising a rupture disc that, in its unruptured state, is disposed in fluid blocking relationship between the central passage and the at least one open end. 
     According to another embodiment of the present invention, there is provided a casing string that comprises at least one section of casing having a central bore and a cementing tool as described herein attached to one end of the section of casing. 
     According to yet another embodiment of the present invention, there is provided a method of cementing a casing string in a well. The method comprises positioning a casing string comprising a central bore and at least one cementing tool as described herein in a downhole formation. Next, cement is injected downhole through the casing string central bore and cement is caused to flow into an annulus located between the casing string and the formation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of a casing string comprising a plurality of cementing tools disposed in a well bore; 
         FIG. 2  is a perspective view of a cementing tool according to one embodiment of the present invention; 
         FIG. 3  is a top view of the cementing tool of  FIG. 2 ; 
         FIG. 4   a  is a cross-sectional view of the cementing tool of  FIG. 2 ; 
         FIG. 4   b  is a cross-sectional view of an alternate embodiment of a cementing tool being equipped with male and female threaded connector structure; 
         FIG. 5  is a fragmented, cross-sectional view of the port and rupture disc assembly of cementing tool of  FIG. 2 ; 
         FIG. 6   a  is a cross-sectional view of a section of the well bore wherein the annulus between the downhole formation and casing has been obstructed; 
         FIG. 6   b  is a cross-sectional view of a section of the well bore wherein a void region of lost circulation has been encountered; 
         FIG. 7   a  is a cross-sectional view of a section of the well bore containing an obstruction wherein the rupture discs carried by the cementing tool have been ruptured and the flow of cement in the annulus is resumed above the obstruction; 
         FIG. 7   b  is a cross-sectional view of a section of the well bore containing a void region wherein the rupture discs carried by the cementing tool have been ruptured and the flow of cement in the annulus is resumed above the void region; 
         FIG. 8  is a perspective view of a cementing tool according to another embodiment of the present invention; 
         FIG. 9  is a cross-sectional view of the cementing tool of  FIG. 8 ; 
         FIG. 10  is a fragmented, cross-sectional view of the port and rupture disc assembly of cementing tool of  FIG. 8 ; 
         FIG. 11  is a perspective view of a cementing tool according to yet another embodiment of the present invention; 
         FIG. 12  is a cross-sectional view of the cementing tool of the cementing tool of  FIG. 11 ; and 
         FIG. 13  is a top view of the cementing tool of  FIG. 11 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention provides apparatus and methods that are particularly suited for the running in and cementing of a casing string into a well bore. As illustrated in  FIG. 1 , a casing string  10  has been run into a well bore  12  and cemented into place by filling the annulus  14  defined by casing string  10  and the downhole formation  16  with cement. In particular, casing string  10  comprises a plurality of casing sections  18  interconnected with a plurality of cementing tools  20 , which are described in greater detail below. As shown in the illustrated embodiment, cementing tools  20  are positioned within casing string  10  across a variety of elevations within downhole formation  16 . As explained below, the precise location of cementing tools  20  can be determined as a matter of general procedure or customized depending upon the downhole formations encountered when creating the well bore. 
     Turning next to  FIGS. 2-4 , one embodiment of a cementing tool  20  in accordance with the present invention is illustrated. Generally, cementing tool  20  comprises a tubular body  22  having a cylindrical sidewall  24 . In certain embodiments, tool  20  comprises a collar or coupler that is easily inserted between adjacent casing sections. In other embodiments, tool  20  can be formed from other materials such as mechanical tubing, which may exhibit lengths much greater than that of a collar and have both male and female threaded ends. In the Figures, tool  20  is generally depicted as a collar for ease of illustration; however, this should not be taken as limiting the scope of the present invention. Sidewall  24  comprises an interior surface  26 , which defines a passageway  28 , and an exterior surface  30 , which cooperates with downhole formation  16  to define annulus  14 . When installed within casing string  10 , passageway  28  is in registry with the central bore  32  of the casing string. Thus, central bore  32  is substantially concentric with tubular body  22 . In certain embodiments, such as shown in  FIG. 6 , passageway  28  and central bore  32  have essentially the same internal diameter. 
     Sidewall  24  also comprises at least one port  34 , and in the embodiments illustrated two ports, formed therein that extend between interior surface  26  and exterior surface  30 . Thus, port  34  defines a fluid flow path between the interior and exterior of tool  20  that is substantially perpendicular to the flow path through tool  20  defined by passageway  28 . 
     In each port  34 , a respective rupture disc assembly  36  is received and secured to sidewall  24 . In the embodiment illustrated in  FIG. 5 , assembly  36  comprises a fitting  38  that is press fitted into port  34  and includes a first cylindrical portion  40  and a second cylindrical portion  42 . First cylindrical portion  40  generally has a larger diameter than second cylindrical portion  42 . Portions  40  is sized and configured to be received into an inboard portion  44  of port  34 , and portion  42  is sized and configured to be received in an outboard portion  46  of port  34 . First cylindrical portion  40  is connected to second cylindrical portion  42  by a tapered transition region  48  that is configured to abut a similarly configured tapered segment  50  of port  34  when assembly  36  is installed within port  34 . As noted above, fitting  38  is press fitted into port  34 . Thus, fitting  38  is affixed to and maintained within port  34  by frictional forces. 
       FIGS. 9 and 10  illustrate another embodiment of a rupture disc assembly  52  that comprises a two-part fitting  54  configured to be received in a port  56  formed in sidewall  24 . Fitting  54  comprises an internally threaded ferrule  58  that is secured to port  56  and an externally threaded nut  60  configured to be received within ferrule  58 . In certain embodiments, ferrule  58  is secured to port  56  by welding, although, it is within the scope of the present invention for ferrule  58  to be secured to port  56  in other ways, such as a threaded connection. In this embodiment, port  56  is of substantially uniform diameter across its entire length, as opposed to port  34  which contains differently sized inboard and outboard portions  44 ,  46 , respectively. It is also noted that rupture disc assembly  52 , when installed in port  56 , lies substantially flush with interior surface  26 , whereas in the embodiment illustrated in  FIG. 4 , rupture disc assembly  36  extends inwardly beyond interior surface  26 , although this does not necessarily need to be the case. 
     Both rupture disc assembly embodiments  36 ,  52  comprise a rupture disc  62 . In the embodiment illustrated in  FIG. 5 , rupture disc  62  is affixed to fitting  38 , and in the embodiment illustrated in  FIG. 10 , rupture disc  62  is affixed to nut  60 . Rupture disc  62  may be affixed to its respective supporting structure by welding or any other means known to those of skill in the art. Alternatively, rupture disc  62  could be commonly machined from, and thus unitarily formed with, fitting  38  or nut  60 . In both illustrated embodiments, rupture disc  62  functions, in its unruptured state, to block the flow of fluid through ports  34 ,  56 , respectively. Rupture disc  62  may also comprise structures that help define its opening characteristics, such as a line of weakness (not shown). 
     Cementing tool  20  also comprises at least one channel-forming member  64  secured to the sidewall exterior surface  30 . Member  64  cooperates with sidewall exterior surface  30  to define a channel  66  that, upon rupture of rupture disc  62 , is in fluid communication with the interior of tubular body  22 . As shown, channel  66  is longitudinal with respect to tool  20 , however, it is within the scope of the present invention for channel  66  to be oriented about different axes. As shown in  FIGS. 2-5 , channel-forming member  64  comprises an elongated segment  68  having spaced apart, longitudinal end margins  70 ,  72 , each of which are secured to sidewall exterior surface  30 . Elongated segment  68  comprises a generally V-shaped cross-sectional profile. In certain embodiments according to the present invention, channel-forming member  64  comprises a sealed end  74  and an open end  76 . As shown, channel  66  is substantially unobstructed thereby permitting, upon rupture of rupture disc  62 , free flow of a fluid or material from passageway  28  through port  34 , up channel  66  and out of open end  76 . However, it is within the scope of the present invention for channel-forming member  64  to include a check valve or other similar device, such as a screen or filter, which inhibits entry of debris or fluid into channel  66  from open end  76 . Furthermore, as illustrated in the Figures, channel-forming member  64  is disposed so that sealed end  74  is located closer to port  34  than open end  76 , but again, it is within the scope of the present invention for other configurations to be employed. 
       FIGS. 8-10  illustrate an alternate channel-forming member  78  in accordance with the present invention. Like channel-forming member  64 , channel-forming member  78  comprises an elongated segment  80  having spaced apart, longitudinal end margins  82 ,  84 , each of which are secured to sidewall exterior surface  30 . Channel-forming member  78  also comprises a sealed end  86  and an open end  88 . However, channel-forming member  78  differs from channel-forming member  64  in that it comprises an arcuate cross-sectional profile. In most other respects, channel-forming member  78  and channel-forming member  64  are configured and function similarly. 
     As noted above, cementing tool  20  is configured to be attached to at least one casing section  18 . Tool  20  includes connecting structure  90  to facilitate this attachment. In the embodiment illustrated in  FIG. 4   a , female connecting structure  90  is located at either end of tool  20  and comprises threaded connector sections  92  and  94  configured to mate with corresponding casing section connectors  96  and  98 , respectively. In the embodiment illustrated in  FIG. 4   b , tool  20 ′ comprises female/male connecting structures  90 ,  90 ′, with connector section  94 ′ being in the form of male pipe threads. Further, in particular embodiments according to the present invention, channel-forming member  64 ,  78  lies entirely outboard of an outer longitudinal margin presented the casing section  18 . In other words, channel-forming member  64 ,  78  lies within the annulus  14  defined by casing string  10  and downhole formation  16 . 
     The use of cementing tool  20  in the cementing of casing string  10  is illustrated in  FIGS. 6   a ,  6   b ,  7   a  and  7   b . In certain embodiments, casing string  10  comprises surface casing, which as noted above, performs a number of important functions. However, it is within the scope of the present invention for casing string  10  to comprise nearly any kind of pipe at any depth run into a well that will function as well casing, including drive pipe, conductor pipe, intermediate casing, drilling liner, production liner, and production casing. Surface casing, generally, can have a diameter of between 8⅝ inches up to 16 inches. 
     After casing string  10  has been run into downhole formation  16 , cement is placed in annulus  14 . In certain embodiments this is accomplished by injecting cement through casing central bore  32  toward its lowermost downhole margin  102  at which point the cement is directed into annulus  14  and flows upwardly toward the surface. In an ideal situation, cement continues to flow until the entirety of annulus  14  is filled with cement. However, it can arise that certain portions of downhole formation  16  do not possess sufficient integrity and can collapse around casing string  10  after it is run in, or alternatively a region of lost circulation may be encountered that can present a limitless void. When this occurs, an obstruction  104 , or void  105 , to the flow of cement  100  in annulus  14  is created. It is understood that the effect of either an obstruction  104  or void  105  is substantially the same in that the flow of cement upwardly through annulus  14  is impeded. Therefore, even though the following discussion is made in terms of encountering an obstruction  104 , a void  105  due to a region of lost circulation may be substituted therefor. 
     Should such an obstruction  104  (or void  105 ) be detected, the present invention advantageously permits the obstruction (or void) to be bypassed and the introduction of cement  100  into annulus  14  to continue without significant interruptions to the cementing operation, such as the need to pull or run tools downhole. If an obstruction  104  is encountered, the fluid pressure of the cement being pumped downhole may increase. In particular embodiments, the increase in cement pressure is detected by an operator, however, this does not always need to be so. At this point, a rupture disc  62  carried by rupture disc assembly  36 ,  52  may be ruptured by increasing the pressure of the cement within casing string central bore  32  proximate rupture disc  62  so that the disc opens and cement may flow through port  34 ,  56  and into the annulus thereby bypassing obstruction  104 . If cement returns to the surface are not achieved as expected, the operator may determine that a region of lost circulation has been encountered and the cement is being directed into a porous formation. The operator can then increase the pressure of the cement being flowed down through casing string central bore  32  to open rupture disc  62 . In certain embodiments, rupture disc  62  is configured to rupture at a pressure of up to 90% of the rated casing strength. This ensures that disc  62  does not rupture due to normal operating conditions experienced in the well, but rather only in response to encountering an annular obstruction or void during cementing operations. Further, if no obstruction is encountered during cementing operations, rupture disc  62  provides sufficient strength so as not to compromise the overall integrity of casing string  10 . As shown in  FIG. 7 , once ruptured, cement  100  flows from passageway  28  through port  34 , into channel  66  and into annulus  14  at a location above the obstruction  104 . Thus, avoiding the creation of an annular “void” zone where casings string  10  is unsupported. 
     Generally, cementing tool  20  should be located within casing string  10  at a higher elevation than obstruction  104 . Knowledge of the formations through which the well is being drilled can assist the operator in positioning a cementing tool  20  within casing string  10  in a location that is likely to be at a higher elevation than where an obstruction  104  or void is likely to form. In certain operations, though, it may be difficult to forecast this information. In those situations, a plurality of cementing tools  20  can be periodically installed between casing sections  18  along the length of casing string  10 . The frequency of placement of cementing tools  20  can vary depending upon the conditions expected to be encountered in the well, however, in certain embodiments cementing tools can be located within casing string  10  at a spacing of approximately at least every 100 feet, at least every 250 feet, at least every 500 feet, or at least every 1000 ft. Use of a plurality of cementing tools  20  increases the likelihood that at least one cementing tool  20  will be located at a higher elevation than the obstruction, so that the obstruction can be bypassed. 
     In embodiments which comprise a plurality of cementing tools  20  located within casing string  10 , it may be possible for an operator to detect the presence of an obstruction  104  and determine its approximate elevation within annulus  14 . Thus, by controlling the pressure within the casing central bore  32 , the operator may be able to selectively actuate the rupture disc(s)  62  carried by a particular cementing tool  20 , while leaving the other rupture disc(s) of other cementing tools intact. In other embodiments, the pressure of the cement within casing central bore  32  can be adjusted to cause the rupture of all rupture discs  62  within casing string  10 , or only those located at elevations above the obstruction  104 . In certain embodiments, in order to facilitate this selective rupturing of rupture discs  62 , rupture discs of differing burst characteristics may be employed throughout casing string  10 . 
     In other embodiments of the present invention, the bursting pressure of rupture discs  62  may be selected to automatically rupture upon encountering elevated pressures within central bore  32  that attributable to the encountering of an obstruction  104  to prevent damage to the casing. In these embodiments, actual detection and identification of the location of an obstruction is obviated and cementing operations may continue without any meaningful interruption in the flow of cement into annulus  14 . 
     In still other embodiments, a plurality of cementing tools  20  may be employed so as to carry out multistage cementing operations. In certain instances it may be desirable to selectively cement only certain elevations of the casing string  10 . For example, wells with low formation pressures may not be able to sustain the hydrostatic forces of a full column of cement. In other applications, it may be desirable to isolate certain sections of the wellbore or use different blends of cement in the wellbore. Still, in cementing deep, hot holes, cement pump times can be limited so as to prevent full-bore cementing of the casing string during a single stage. In these examples and other situations, it may be desirable to cement casing string  10  in two or more stages. 
     Typically the stage cementing operation begins as described above in that cement  100  is run cement through casing central bore  32  toward its lowermost downhole margin  102  at which point the cement is directed into annulus  14  and flows upwardly toward the surface. Even though an obstruction or void may not be encountered, once the cement has reached a desired height in annulus  14 , the flow of cement is stopped. At this point, it may no longer be possible to resume the flow of cement in annulus  14  by flowing cement down to the lowermost margin  102  and back toward the surface. Instead, the operator can actuate rupture discs  62  at a desired elevation so that the flow of cement into annulus  14  can resume, thus beginning a second stage of cementing. This process can be repeated as necessary or desired. 
     Once cementing operations have been completed, drilling within the well can be continued by merely drilling out the cement within casing string central bore  32 . There are no tools that need to be drilled out along with the cement. Alternatively, once cementing or a cementing stage is completed, any cement remaining within central bore  32  can be pumped or circulated out prior to fully curing so that the step of drilling through cement can be avoided. 
       FIGS. 11-13  illustrate another cementing tool embodiment according to the present invention. A cementing tool  106  is illustrated having a pair of channel-forming members  108  that are integrated with tool sidewall  110 . Cementing tool  106  shares certain structural and functional characteristics with the embodiments of cementing tool  20  discussed above. However, the most notable differences concern the configuration of channel forming members  108  and the placement of the rupture disc assembly  118 . Channel-forming members  108  comprise thickened regions of sidewall  110  that have channels  112  formed therein. In certain embodiments, channels  112  comprise generally circular, longitudinal bores, primarily for ease of machining, but other configurations and orientations for channels  112  also may be used. A port  114  is formed in sidewall  110  which enables fluid communication between tool central passage  116  and channel  112 . Thus, a flow path between the interior of tool  106  and the downhole annulus is established. 
     A rupture disc assembly  118  is positioned within channel  112  in normally fluid blocking relationship between port  114  and a channel outlet  120 . Rupture disc assembly  118  includes a rupture disc  122  and may be configured similarly to rupture disc assemblies  36 ,  52  discussed above. In one embodiment, rupture disc assembly  118  is threadably received and secured into a corresponding threaded portion  124  of channel  112 . When in its unruptured state, rupture disc  122  prevents fluid or cement being flowed through tool passage  116  from passing through channel out let  120  and into the downhole annulus. An optional check valve  126  may be installed toward outlet  120  to prevent fluid being circulated within the annulus or other material from entering channel  112  and interfering with the operation of rupture disc assembly  118 . 
     In one embodiment, port  114  is formed by machining a bore  128  through channel-forming member  108  and sidewall  110  until central passage  106  is reached. Likewise, channel  120  may be formed by machining a bore through channel-forming member  108  that is perpendicular to bore  128 . The orifice  130  in channel forming member  108  can later be plugged. 
     The installation and operation of cement tool  106  is similar to that described above with respect to cement tool  20 . 
     The following description sets for exemplary embodiments according to the present invention. It is to be understood, however, that these examples are provided by way of illustration and nothing therein should be taken as a limitation upon the overall scope of the invention.

Summary:
A cementing tool ( 20 ) capable of being installed in a casing string ( 10 ) and configured to selectively permit the flow of cement through one or more ports ( 34 ) in the tool is provided. The ports ( 34 ) are normally sealed by rupture disc assemblies ( 36 ) comprising a rupture disc ( 62 ) that can be opened to permit flow of cement through the casing string central bore ( 32 ) and into the annulus ( 14 ) defined by the casing string ( 10 ) and the downhole formation ( 16 ). The cementing tool ( 20 ) is particularly useful during cementing operations in which the annulus ( 14 ) has become blocked by a collapsed portion of the formation by allowing the obstruction ( 104 ) to be bypassed and the flow of cement ( 100 ) into the annulus to be continued without substantial interruption. Tool ( 20 ) may also be used in multistage cementing operations.