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CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a regular application claiming priority of U.S. Provisional Patent application Ser. No. 61/359,718, filed on Jun. 29, 2010, the entirety of which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     Embodiments of the invention are related to systems, apparatus and methods used during cementing of tubulars in a wellbore and, more particularly, to cementing tubulars which comprise a mud motor while minimizing the amount of cement passing through the mud motor. 
     BACKGROUND OF THE INVENTION 
     In oil and gas well drilling operations it is necessary to cement various tubular members to a subterranean formation at different points during the well drilling and completion operations. This practice is well known for various purposes, such as anchoring a surface casing to the earth to provide a solid leak-free top section of the well, and, in the lower portions of the well, to provide isolation between different subterranean zones. 
     Many wells are now drilled in deviated or non-vertical directions. This practice often utilizes a mud motor to rotate the drill bit without the need to rotate the entirety of the drill string. Conventional mud motors are run on a work string and are retrieved from the wellbore before the string of tubulars, typically casing, is run in the hole. 
     Applicant is aware that a third party has developed a mud motor that is relatively inexpensive and can be abandoned in the wellbore. This disposable mud motor is run on the end of the casing string. 
     During cementing operations, it is desired that the cement slurry not be pumped through the mud motor so as to prevent the mud motor from continuing to rotate. Further, mud motors have a high pressure differential across motor which may adversely affect the rate at which the cement is pumped and delivered to the annulus between the casing and the wellbore. 
     In order to facilitate cementing around, rather than through, a mud motor, the cement must be able to pass from a bore through the casing string to the exterior of the casing string and then be able to pass around the exterior of the mud motor. To accomplish this, ports are provided in a wall of the casing to allow cement to pass therethrough. As will be appreciated by one of skill in the art, a hole drilled through the wall is insufficient. There are many steps in the drilling process where having ports open between the interior and exterior of the casing would be undesirable. It is known that the timing of opening of ports in the casing must be controllable. 
     Prior art solutions have used conventional burst disks to control the opening of the ports using a predetermined pressure. Once the burst disks, positioned above the mud motor have ruptured, cement flowing down the bore of the casing exits the casing wall through the open ports created thereby for flowing the cement around, rather than through, the mud motor. 
     Applicant has found however, that conventional burst disks do not open reliably. Further, where a plurality of burst disks are used, if a first burst disk or a relatively small number of the plurality of disks burst, the pressure in the casing bore is relieved as the fluid flows to the wellbore, and thereafter, the pressure does not meet the threshold required to burst the remainder of the burst disks. One solution has been to attempt to significantly increase the pumping rate such that the resulting pressure is adequate to result in rupture of more of the burst disks. 
     Cementing operations typically require a relatively high pumping rate to ensure cement is pumped downhole through the casing bore and returned toward surface through the annulus between the casing and the wellbore. With only a single port or a small number of ports open through the ruptured burst disk or disks, the flow rate of cement is restricted to that possible through a openings or ports created by the rupture of the single burst disk or small number of disks. 
     Clearly there is a need in the industry for apparatus that reliably opens to permit pumping of cement through the work string, at a relatively high pumping rate, so as to flow around the mud motor and into the annulus between the casing and the wellbore. 
     SUMMARY OF THE INVENTION 
     Embodiments of the invention utilize two or more burst disks located at or above a mud motor in a tubular string to permit cement to flow therethrough, once ruptured, and substantially bypass the mud motor. 
     A cap is spaced above the burst disk for forming a chamber therebetween. The chamber remains at a substantially fixed and known pressure, such as about atmospheric pressure, when the tubular string is run into the wellbore. Thus, each of the two or more burst disks is unaffected by the variable hydrostatic pressure of fluids in the annulus. As all of the rupture disks will rupture at substantially the same threshold pressure, forming two or more open ports, pumping of cement is possible at a desired, relatively high pumping rate, which is greater than a pumping rate through a single, open port formed by a single ruptured burst disk, typical of the prior art. 
     In a broad aspect, a method for cementing a tubular conveyance string in a wellbore traversing a subterranean formation, comprises drilling the wellbore with a mud motor supported on the tubular conveyance string and forming an annulus therebetween. The tubular conveyance string has a bore and two or more burst disks fit to the string at or uphole of the mud motor. Each of the two or more burst disks has a cap spaced radially outward from the burst disk for forming a chamber therebetween. The chamber is maintained at a substantially fixed and known pressure, the two or more burst disks having a same threshold pressure at which the two or more burst disks rupture. The mud motor is abandoned downhole. Cement is pumped downhole in the bore of the conveyance string. The bore is pressurized to the threshold pressure for rupturing the two or more burst disks for forming two or more open ports therethrough. Thereafter, cement is continued to be pumped downhole in the bore of the conveyance string and through the two or more open ports to the annulus. 
     In another broad aspect, a system for completion of a wellbore traversing a subterranean formation comprises a mud motor having a drill bit and supported by a tubular conveyance string having a bore and forming an annulus with the wellbore. Two or more burst disks are fit to the string at or uphole of the mud motor, each of the two or more burst disks having a threshold pressure at which the burst disk ruptures. A cap is spaced radially outward from the burst disk for forming a chamber therebetween, the chamber being maintained at a substantially fixed and known pressure. When the drilling of the wellbore is stopped and cement is pumped downhole through the bore of the conveyance string, the pressure of the cement at the two or more burst disks reaches the threshold pressure for rupturing the two or more burst disks and forming two or more open ports therethrough for delivering the cement to the annulus. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a longitudinal, partial cross-sectional view of a casing-while-drilling operation wherein a mud motor is used for driving a drill bit for advancing the wellbore and casing into a formation, ruptured burst disks, according to one embodiment, being illustrated fancifully for forming rupture ports for release of cement therethrough; 
         FIG. 2  is a longitudinal section view of a wall of a casing string having a burst port assembly comprising a burst disk according to an embodiment of the invention installed in the casing string wall, an optional protective mastic shown partially covering a cap spaced from the burst disk; 
         FIGS. 3A and 3B  are longitudinal sectional views of a wall of a casing string having a burst disk machined directly into the wall of the casing, a cap being removed for clarity; more particularly,
           FIG. 3A  illustrates a single bore having a burst disk formed at a base of the bore; and     FIG. 3B  illustrates a bore and a counterbore having a burst disk formed at a base of the counterbore;       

         FIG. 4A  is a perspective view of a tubular collar having three burst port assemblies fit in each of five fins, the fins extending radially and axially along an outer surface of the collar, the fins being spaced circumferentially thereabout; 
         FIG. 4B  is an end view according to  FIG. 4A ; 
         FIG. 4C  is a longitudinal cross-sectional view along A-A of  FIG. 4B ; 
         FIG. 4D  is a detailed, longitudinal cross-sectional view of a burst port assembly according to  FIG. 4B ; and 
         FIG. 5  is a longitudinal, partial cross-sectional view of a latching sub positioned above the mud motor for operatively engaging a wiper plug run into the wellbore in advance of cement. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     As shown in  FIG. 1 , embodiments are shown in the context of casing-while-drilling operations. A tubular conveyance string  10 , typically a string of tubulars  12  forming a liner or casing string, is advanced into a wellbore  14  using a bottom hole assembly  16  having a mud motor  18  connecting the casing string  10  to a drill bit  20 , as is known in the art. Once the casing  10  has reached a bottom  22  of the wellbore  14 , the casing  10  is cemented into place. The mud motor  18  is not retrieved from the wellbore  14 , but is, instead, abandoned at the bottom  22  of the wellbore  14 . 
     In one embodiment, as shown in  FIGS. 1 and 2 , two or more burst disks  24  are incorporated into the casing  10  uphole of the mud motor  18 . The two or more burst disks  24  are designed to rupture at substantially a same threshold pressure P for forming open ports  26  in the casing  10  to permit cement C, flowing downhole through a bore  28  of the casing  10 , to exit the bore  28  uphole of the mud motor  18 . The cement C enters an annulus  30  between the casing  10  and the wellbore  14  and flows about the mud motor  18  and uphole in the annulus  30  towards surface. 
     In an embodiment, as shown in  FIGS. 1 and 4A  to  4 D, two or more burst disks  24  are positioned in a casing collar  32  located at or uphole of the mud motor  18 . The two or more burst disks  24  can be arranged in a variety of configurations within the collar  32 . 
     A plurality of burst disks  24  can be arranged in one or more circumferentially-extending rows, each disk  24  spaced circumferentially about the collar  32 . In one embodiment, a total of fifteen burst disks  24  are arranged in three rows, each row having five burst disks  24  positioned circumferentially about the collar  32  and are spaced from about 60° to about 72° apart. In another embodiment, the disks  24  of each row are staggered circumferentially form each other burst disk  24  in adjacent rows. 
     In another embodiment, the burst disks  24  are located in axially extending, raised flanges or fins  33  ( FIGS. 4A-4D ) which are spaced circumferentially about the collar  32 . The fins  33  place the burst disks  24  closer to the wellbore  14 . Flow passages  35  are formed between the raised fins  33 , aiding in the flow of fluids in the annulus  30  past the collar  32 . The casing collar  32  can have a variety of lengths which typically range from about 18 inches to about 24 inches long. 
     More particularly, as detailed in  FIG. 2 , the two or more burst disks  24  are designed to reliably rupture at about the threshold pressure P, as described in Applicant&#39;s co-pending, published PCT application, WO 2010/148494, the entirety of which is incorporated herein by reference. As all of the rupture disks  24  will rupture at substantially the same threshold pressure P, forming two or more open ports  26 , pumping of cement C is possible at a desired, relatively high pumping rate, which is greater than a pumping rate through a single, open port  26  formed by a single ruptured burst disk  24 , typical of the prior art. 
     In greater detail, as shown in  FIGS. 2 ,  3 A and  3 B, each burst disk  24  has a thickness and material properties which determine a differential pressure across the burst disk  24  at which the burst disk  24  will rupture. The burst disk  24  can be manufactured from stainless steel or any other suitable material. 
     Best seen in  FIGS. 3A and 3B , the burst disk  24  can be formed directly in a wall  34  of the casing  10  or collar  32 , such as by machining a bore  36  in the wall  34 , leaving only sufficient material at a base  38  of the machined bore  36  for forming the rupture disk  24 . The machined bore  36  can further comprise a counterbore  37  ( FIG. 3B ) 
     Alternatively, as shown in  FIGS. 2 , and  4 A to  4 D, each burst disk  24  is housed in a burst port assembly  40  which is secured in a burst port  42  formed in the casing wall  34 . 
     A cap  44  is spaced above the burst disk  24  for forming a chamber  46  therebetween. The chamber  46  remains at a substantially fixed and known pressure, such as about atmospheric pressure, when the casing string  10  is run into the wellbore  14 . Thus, each of the two or more burst disks  24  is unaffected by the variable hydrostatic pressure of fluids in the annulus  30 . 
     In an embodiment, as the pressure in the chamber  46  can be set at surface, such as at atmospheric pressure, the differential pressure downhole is both known and elevated compared to the prior art in which the hydrostatic pressure in the annulus  30  diminishes the effective differential pressure. Therefore, where the pressure in the chamber  46  is less than the pressure in the annulus  30 , the burst disks  24  are more reactive to controlled pressure in the bore  28 . Accordingly, the differential pressure at which the burst disk  24  will rupture is determined only by the pressure in the bore  28 . As the chamber  46  has a known pressure, each burst disk  24  ruptures reliably at the same threshold pressure P as a pressure in the bore  28  of the casing  10  increases to the threshold pressure P. The pressure in the bore  28  is determined by the cement C pumped downhole therein. The cap  44  is releasably supported above the bust disk  24  such that when the burst disk ruptures, the flow of cement C therethrough into the chamber releases the cap  44 , creating the open port  26  to the annulus  30 . 
     Having reference again to  FIG. 2  and in an embodiment, the burst port assembly  40  is mounted in the casing  10  and comprises the burst disk  24  which is adjacent the bore  28  of the casing  10 . More particularly, the assembly  40  is mounted in the burst port  42  formed in the casing collar  32 . The assembly  40  is retained within the burst port  42  by a retainer ring  48 . The retainer ring  48  can be threadably engaged in the burst port  42 . Wrench-receiving slots  49  are formed in the retainer ring  48  for ease of threading the assembly  40  into the burst port  42 . Further, the retainer ring  48  has a stepped bore, having a first bore  47  adjacent the burst disk  24  and a second, larger bore  45  for releasably supporting the cap  44 . The cap  44  is press-fit into the second bore  45  of the retainer ring  48  for forming the chamber  46  between the cap  44  and the burst disk  24 . Seals  50 , such as O-rings, seal between the burst disk  24  and the casing collar  32 . Further, seals  50  are provided to seal between the retainer ring  48  and the casing collar  32 . Seals  50  are also provided to seal between the retainer ring  48  and the cap  44 . Thus, the chamber  46  is sealingly maintained at the known pressure until the burst disk  24  ruptures. 
     When the pressure within the bore  28  of the casing  10  reaches the threshold pressure P, the burst disk  24  ruptures and the cap  44  is displaced from the retainer ring  48 , opening the rupture port  26  through the burst disk assembly  40 . Cement C flowing through the casing bore  28  is permitted to pass through the rupture port  26  and into the annulus  30  between the wellbore  14  and the casing thereby substantially avoiding passing through the mud motor  18 . 
     Optionally, a displaceable, protective substance  52 , such as mastic, may be used to cover the cap  44 .  FIG. 2  illustrates a partial fill of protective substance  52  to show both embodiments, one with the protective substance  52  and one without. The protective substance  52  can substantially fill an outer portion  54  of the burst port  42 , adjacent the wellbore annulus  30  and covering the cap  44 , to ensure the cap  44  is not dislodged or damaged, such as during transport or insertion into the wellbore  14 . When the burst disk  24  ruptures, the cement flowing therethrough displaces the cap  44  and the protective substance  52  for providing the open port  26  to the annulus  30 . 
     In Use 
     As shown in  FIG. 1 , in order to access zones of interest in a formation, it is well known to drill a wellbore  14  into and traversing through a formation. Further, it is known to use a mud motor  18 , operatively connected to and supported by a tubular conveyance string  10  to drive a drill bit  20  and underreamer  21  to drill the wellbore  14 . The conveyance string  10  is advanced into the wellbore  14  as the drilling advances. An annulus  30  is formed between the wellbore  14  and the conveyance string  10 . When the wellbore  14  has been drilled to the desired depth, the conveyance string  10  is cemented into place by flowing cement into the annulus  30 . 
     In one embodiment of the system, the conveyance string  10  comprises two or more burst disks  24  as described above, and in Applicant&#39;s co-pending published PCT application, WO 2010/148494, positioned uphole of the mud motor  18 . Before drilling, the cap  44  is installed, charging the chamber  46  with a known pressure, such as atmospheric pressure. Cement is pumped downhole through the bore  28  of the conveyance string  10 . The pressure in the bore  28  increases to the threshold pressure P. The pressure can result due to resistance to flow through the mud motor  18  or some other flow restriction. The two or more burst disks  24  rupture, providing open ports  26  through the conveyance string  10 . Substantially all of the burst disks  24  rupture as a result of having the threshold pressure P acting on one side and a known pressure, such as atmospheric pressure, in the chamber  46  on the other side. The cement flows out of the open ports  26 , into the annulus  30  and around the mud motor  18 . As will be appreciated by one of skill in the art, some of the cement may pass through the mud motor  18 . 
     In another embodiment, as shown in  FIG. 5 , a plug, such as a wiper plug  60 , is run into the bore  28  of the conveyance string  10  in advance of the cement. The wiper plug  60  is engaged in the conveyance string  10  below the two or more rupture disks  24  and at or uphole of the mud motor  18 . The wiper plug  60  engages a latching sub  62  connected in the conveyance string  10 , and effectively blocks the passage of cement through the mud motor  18  therebelow. Further, as a result of pumping cement downhole against the wiper plug  60 , the pressure in the bore  28  is more effectively and reliably increased to reach the threshold pressure P. 
     Alternatively, in order to minimize flow through the mud motor  18 , the mud motor  18  can be stalled, such as by increasing the weight-on-bit (WOB) until the motor  18  stalls. While a small amount of cement might pass through the stalled mud motor  18 , pumping cement against the stalled motor  18  will more quickly generate pressure in the bore  28  to reach the threshold pressure P, causing the burst disks  24  to rupture. 
     EXAMPLE 
     A wellbore having a total vertical depth (TVD) of 1200 m and a total measured depth (TMD) of 3000 m is drilled using 4.5 inch casing and a bottomhole assembly comprising a mud motor. A hydrostatic pressure of 11.7 MPa in the wellbore results in a calculated, maximum drilling pressure of about 30 MPa. 
     At or above the mud motor, a casing collar is positioned comprising fifteen burst disks according to an embodiment of the invention. Each of the burst disks has an orifice diameter of about 0.375 inches and a thickness of about 0.006 inches and is designed to have an absolute burst pressure of about 54.6 MPa for each of the burst disks. 
     In order to rupture substantially all of the burst disks, the pressure within the casing must be increased to a pressure threshold of about 43 MPa, measured at surface, in order to exceed the absolute pressure at which the disks will burst at depth in the wellbore. The burst threshold pressure, at surface, is about 13 MPa greater than the maximum drilling pressure. The difference between the rupture threshold pressure and the drilling pressure acts as a safety margin to ensure the burst disks do not rupture during normal drilling operations. 
     Once substantially all of the burst disks have ruptured, cement, flowing through the casing bore can be delivered therethrough, bypassing the mud motor and delivering the cement to the wellbore annulus.

Summary:
A method and system for cementing a tubular and mud motor in a wellbore utilizing burst disks above the mud motor. The burst disks rupture to permit the cement to flow through the burst disks and bypass the mud motor. All of the burst disks reliably rupture at a predetermined and known threshold pressure so as to permit cement to be pumped at a desired rate through all of the ruptured burst disks. Each burst disk is provided with a cover for maintaining a chamber of a known pressure between the cap and the burst disk. All of the burst disks predictably and reliably rupture at the rated pressure.