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BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The invention relates generally to the field of apparatus and methods for measuring drilling parameters and/or formation parameters during wellbore drilling. More particularly, the invention relates to structures and techniques for making such measurements in drilling operations known as “casing drilling.” 
         [0003]    2. Background Art 
         [0004]    Wellbores are drilled into the Earth&#39;s subsurface to recover hydrocarbons and other desirable materials trapped in geological formations in the subsurface. A wellbore is typically drilled by advancing a drill bit through the subsurface formations. The drill bit is attached to the lower end of a “drill string” suspended from a drilling rig. The drill string is a long string of sections of drill pipe that are connected together end-to-end to form a long shaft for driving the drill bit further into the subsurface. A bottom hole assembly (“BHA”) containing various instrumentation and/or devices to control the mechanical properties of the drill string is typically provided above the drill bit. Drilling fluid, or “mud”, is typically pumped down through the drill string to the drill bit. The drilling fluid lubricates and cools the drill bit, and it carries drill cuttings back to the surface in the annulus between the drill string and the wellbore wall. 
         [0005]    In conventional drilling, a well is drilled to a selected depth, and then the wellbore is typically lined with a larger-diameter pipe, usually called “casing.” Casing typically consists of casing sections connected end-to-end, similar to the way drill pipe is connected. To accomplish this, the drill string and the drill bit are removed from the borehole in a process called “tripping.” Once the drill string and bit are removed, the casing is lowered into the well and cemented in place. The casing protects the wellbore wall from collapse and isolates the subterranean formations from each other. After the casing is in place, drilling may continue by lowering a drill bit through the casing and continuing to drill once the drill bit reaches the bottom of the well. 
         [0006]    Conventional drilling typically includes a series of drilling, tripping (removing the drill string from the wellbore), cementing in place a pipe or casing to protect the drilled wellbore, and then resuming drilling using a smaller diameter drill bit to extend the wellbore. The foregoing process is very time consuming and costly. Additionally, other problems may be encountered when tripping the drill string. For example, the drill string may get caught up in the borehole while it is being removed. These problems require additional time and expense to correct. 
         [0007]    The term “casing drilling” refers to the use of a casing string in place of a drill string. Like drill string, a string of casing sections are connected end-to-end to form a casing string. the BHA and the drill bit are connected to the lower end of a casing string, and the well is drilled using the casing string to transmit drilling fluid, as well as axial and rotational forces, to the drill bit. Upon completion of drilling, the casing string may then be cemented in place to from the casing for the wellbore. Casing drilling enables the well to be simultaneously drilled and cased, thus eliminating one of the tripping steps necessary in conventional drilling. 
         [0008]    One technique for casing drilling is described in U.S. Pat. No. 7,004,263 issued to Moriarty, et al., and assigned to the assignee of the present invention. Another casing drilling technique is described in U.S. Pat. No. 6,705,413 issued to Tessari which includes a retrievable drill bit mounted at an end of the casing string and either a mud motor with a bent housing and/or bent sub or a rotary steerable tool used to direct the bit to drill directionally. 
         [0009]    The foregoing techniques require expensive, difficult to operate drill bits that can be retrieved through the casing after the intended wellbore depth is reached in order to provide any form of measurement or directional control, such as using measurement while drilling (“MWD”) and logging while drilling (“LWD”) devices. Such bits must drill a larger diameter hole than the casing, and then be reduced in diameter so that the BHA can be retrieved to the surface through the casing after the intended wellbore depth is reached. 
         [0010]    What is needed is a system to enable casing drilling that can eliminate the need for drilling components that are retrievable through the casing. 
       SUMMARY OF THE INVENTION 
       [0011]    A casing drilling system according to one aspect of the invention includes a casing having a drill bit at one end. The drill bit is capable of drilling subsurface Earth formations and is formed from a material susceptible to removal from the casing by at least one of drilling with another well drilling bit and chemical exposure. The chemical exposure is substantially harmless to the casing. The system includes at least one centralizer affixed to an interior of the casing. The at least one centralizer includes a receptacle therein for engaging a measurement while drilling tool. The at least one centralizer is formed from a material susceptible to removal from the casing by at least one of drilling with another well drilling bit and chemical exposure, wherein the chemical exposure is substantially harmless to the casing. The system includes a measurement while drilling tool configured to move along the interior of the casing and to engage with the at least one centralizer. The measurement while drilling tool includes at least one device to measure a drilling parameter or a formation parameter. The measurement while drilling tool includes a latch at an upper end thereof for engagement with a retrieval tool moved through the interior of the casing. 
         [0012]    A method for casing drilling according to another aspect of the invention includes rotating a casing having a drill bit at one end in a wellbore while urging the casing longitudinally along the wellbore. The drill bit is capable of drilling subsurface Earth formations and is formed from a material susceptible to removal from the casing by at least one of drilling with another well drilling it and chemical exposure. The chemical exposure is substantially harmless to the casing. The casing includes therein at least one centralizer affixed to an interior of the casing. The at least one centralizer includes a receptacle therein for engaging a measurement while drilling tool. The centralizer is formed from a material susceptible to removal from the casing by at least one of drilling with another well drilling bit and chemical exposure. The chemical exposure is substantially harmless to the casing. The casing also include a measurement while drilling tool configured to move along the interior of the casing and to engage with the at least one centralizer. The measurement while drilling tool includes at least one device to measure a drilling parameter or a formation parameter. The measurement while drilling tool includes a latch at an upper end thereof for engagement with a retrieval tool moved through the interior of the casing. At a selected depth, the rotating and longitudinally urging the casing is stopped. A retrieval tool is then inserted inside the casing. The retrieval tool is engaged to the upper end of the measurement while drilling tool and the measurement while drilling tool is removed from the casing. The centralizer and the drill bit are then removed from the casing by a least one of drilling out and chemically exposing. 
         [0013]    Other aspects and advantages of the invention will be apparent from the following description and the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  shows a cross section of one example LWD/MWD casing drilling system according to the invention. 
           [0015]      FIG. 2A  shows a cross section of one example centralizer used in the system shown in  FIG. 1 . 
           [0016]      FIG. 2B  is a top view of the example centralizer shown in  FIG. 2B . 
           [0017]      FIG. 3A  shows a cross section of one example of a centralizer used to longitudinally fix position of and rotationally orient a MWD and/or LWD probe used in the system of  FIG. 1 . 
           [0018]      FIG. 3B  is an end view of the example centralizer shown in  FIG. 3A . 
           [0019]      FIG. 4  shows a cross section of one example of a telemetry modulator used in the system shown in  FIG. 1 . 
           [0020]      FIG. 5A  shows one example of a threaded insert mounted inside a casing joint used to threadedly retain a centralizer. 
           [0021]      FIG. 5B  shows the insert of  FIG. 5A  without a centralizer engaged therein. 
           [0022]      FIG. 6  shows one example of a centralizer used in the system of  FIG. 1  having a pressure sensor arranged to measure pressure in the annular space in a wellbore outside the casing. 
           [0023]      FIGS. 7 and 8  show a method for using the system shown in  FIG. 1  during drilling a wellbore. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    One example of a casing drilling system including a measurement while drilling (“MWD”) and/or logging while drilling (“LWD”) tool  14  is shown in cross section in  FIG. 1 . The tool  14  may be disposed inside one or more “joints” (individual segments) of casing  12 . The casing  12  may be any type known in the art, and typically has threaded ends (not shown in  FIG. 1 ) to enable threaded coupling of one joint to the next to make up a casing string. The casing  12  may be made from materials ordinarily used for wellbore casing, including carbon steel. In some examples, the casing  12  may be made from non-magnetic alloy such as monel, or an alloy sold under the trademark INCONEL, which is a registered trademark of Huntington Alloys Corporation, Huntington, W. Va. Using non-magnetic material for the casing  12  may enable using magnetic directional sensing devices in the MWD and/or LWD tool  14  (described in more detail below). 
         [0025]    The casing  12  includes a drill bit  22  disposed at the lower end. The drill bit  22  includes cutting elements of types known in the art to drill through subsurface formations as the drill bit  22  is rotated by the casing  12  and is urged axially into the formations by transfer of some of the weight of the casing string. Such procedure is described, for example in U.S. Pat. No. 7,004,263 issued to Moriarty, et al., and assigned to the assignee of the present invention. In the present example, the drill bit  22  may be made from a material that is readily drilled with an ordinary well drilling bit to enable its removal from the bottom of the casing  12  when a selected wellbore depth is reached. Alternatively, the drill bit  22  may be made from a material that can be removed chemically or by any other method or technique that will not harm the casing  12  or the drilled Earth formation. 
         [0026]    A MWD and/or LWD tool  14  (referred to as “MWD tool” hereinafter for convenience but intended to cover both types of instruments or tools) is disposed inside the casing  12 . The MWD tool  14  may include one or more sensors (not shown separately) of any type known in the art for determining geometric trajectory of the casing  12  at the location of the MWD tool  14 , and/or one or more sensors not shown separately) for measuring petrophysical parameters of the wellbore or the formations surrounding the wellbore ( FIGS. 7 and 8 ). The MWD tool  14  may have a relatively narrow external diameter selected to enable retaining the MWD tool  14  generally in the center of the casing  12  while enabling relatively unimpeded flow of drilling fluid (“mud”) through the interior of the casing  12 . The MWD tool  14  may be retained in its longitudinal position inside the casing  12  by including inside the casing  12  one or more centralizers. In the present example, a lower centralizer  18  is disposed generally proximate to the drill bit  22  at the lower end of the casing  12 . The lower centralizer  18  may include features (described below with reference to  FIGS. 3A and 3B ) to fix the rotary orientation of the MWD tool  14  inside the casing  12  as well as to limit the axial motion of the MWD tool  14  inside the casing  12 . One or more middle centralizers  16  may be included in some examples to reduce the possibility of damage to the MWD tool  14  by flexing and associated vibration during drilling operations. The middle centralizer(s)  16  will be described in more detail with reference to  FIGS. 2A and 2B . An upper centralizer  24  may be disposed near the upper end of the MWD tool  14  and may include features (to be described in more detail with reference to  FIG. 4 ) for diverting mud flow through a flow modulator  26 . The flow modulator  26  may be used in some examples to transmit some of the measurements made by the MWD tool  14  to the Earth&#39;s surface by modulating the flow of drilling fluid through the interior of the casing  12 . This is known in the art as “mud-pulse telemetry.” The MWD tool  14  may include a fishing neck  28  or similar feature to enable ready retrieval of the MWD tool  14  from the interior of the casing  12  when the selected wellbore depth is reached. The fishing neck  28  includes features (not shown separately) of types well known in the art for engagement of a suitable retrieval tool (not shown) disposed on the end of a wireline, slickline, coiled tubing or pipe string. 
         [0027]    One example of the one or more middle centralizers  16  is shown in cross section in  FIG. 2A . The centralizer  14  may be made from aluminum, glass fiber reinforced plastic, or other material that can be readily removed by drilling or other treatment (similar to the drill bit  22 ) after the wellbore has reached the selected depth and the MWD tool  14  is removed from inside the casing  12 . The middle centralizer  16  may be generally cylindrically shaped, having an external diameter selected to be retained inside the casing  12  by threads  17 , for example, formed on the inner wall of the casing  12  or by interference fit within the casing  12 . A central passage inside the centralizer  16  has a diameter selected to enable the MWD tool  14  to move longitudinally therethrough but to substantially prevent lateral movement of the MWD tool  14  in the centralizer  16 . The centralizer  16  includes one or more mud passages  19  in the annular space between the MWD too  14  and the casing  12  to enable free flow of drilling mud during drilling operations. 
         [0028]      FIG. 2B  shows an end view of the centralizer  16  of  FIG. 2A . The mud passages  19  may be formed by having the central passage for the MWD tool  14  to be within a center ring  19 A formed from the material used to make the centralizer  16 . The center ring  19 A may be disposed inside an outer ring  19 C by ribs  19 B extending from the center ring  19 A to the outer ring  19 C. The outer ring  19 C may be threadedly or frictionally retained in position inside the casing  12 . The mud passages  19  result from the unfilled space between the ribs  19 B. The centralizer  16  may be machined, molded or cast from a single component into the form shown in  FIGS. 2A and 2B , thus minimizing the manufacturing cost. 
         [0029]    One example of the lower centralizer  18  is shown in cross section in  FIG. 3A  and in end view in  FIG. 3B . The lower centralizer  18  can be substantially similar in material and in structure to the one or more middle centralizers explained above with reference to  FIGS. 2A and 2B . The lower centralizer  18  preferably includes a key  21  or similar feature disposed in the central passage such that when engaged with a mating feature (not shown in  FIGS. 3A and 3B ) on the exterior of the MWD tool ( 14  in  FIG. 1 ), the longitudinal position of the MWD tool will be fixed, and the rotational orientation of the MWD tool with respect to the casing ( 12  in  FIG. 1 ) will be fixed and known. Thus it will be possible to make measurements related to the geometric trajectory of the casing and the wellbore, using techniques well known in the art. The lower centralizer  18  in  FIG. 3A  is shown disposed proximate a box connection  3  (female threaded coupling) that can be used to threadedly engage mating threads on the drill bit ( 22  in  FIG. 1 ). 
         [0030]    One example of an upper centralizer  24  is shown in  FIG. 4 . The upper centralizer  24  may be made from similar materials and have similar features to retain it in the casing  12  such as threads  17  or by interference fit as were described above with reference to the middle centralizer(s) ( 16  in  FIGS. 2A and 2B ). A central opening  25  in the upper centralizer  24  should be of a diameter and surface finish to enable sealing engagement with a modulator housing  41  disposed and surface finish to enable sealing engagement with a modulator housing  41  disposed at the upper end of the MWD tool housing  14 A. A modulator operating shaft  23  extends from the upper end of the MWD tool housing  14 A and enters the modulator housing  41 , whereupon it moves a modulator  37 . The modulator  37  cooperates with a flow passage  39  in the modulator housing  41  such that motion of the modulator  37  changes the effective cross section to flow of the passage  39 . Such change can be effected by longitudinal motion or rotation of the modulator  37  to cause pressure changes in the flowing drilling fluid. The pressure changes can be any type known in the art, including pressure increase, pressure decrease or “mud siren” type modulation. The modulation is used to transmit data from the MWD tool ( 14  in  FIG. 1 ) to the Earth&#39;s surface, where the modulation may be detected. The modulator housing  41  may be sealed to the central opening  25  using an o-ring  29  or similar annular sealing device. The fishing neck  28  is shown in the upper portion of the modulator housing  41 . 
         [0031]    Referring to  FIG. 5A , in some examples, the centralizer (middle centralizer  16  shown in  FIG. 5A ) can be retained inside the casing  12  by providing a threaded insert  30  at the selected longitudinal position of the centralizer  16  inside the casing  12 . The threaded insert may be frictionally retained, adhesively bonded or welded inside the casing  12 , or may be retained therein by any other retention device known in the art. The threaded insert  30  preferably includes threads  17 A on its interior surface shaped to engage mating threads  17 B on the exterior of the centralizer  16 . The configuration shown in  FIG. 5A  may be used for any or all of the lower centralizer ( 18  of  FIG. 1 ) the one or more middle centralizers ( 16  in  FIG. 1 ) or the upper centralizer ( 24  in  FIG. 1 ).  FIG. 5B  shows the insert  30  without the centralizer in place therein. 
         [0032]    In some examples, the one or more middle centralizers ( 16  in  FIG. 1 ) and the upper centralizer ( 24  in  FIG. 1 ) may be affixed to the exterior of the MWD tool ( 14  in  FIG. 1 ) such that the one or more middle and upper centralizers are inserted into the casing ( 12  in  FIG. 1 ) with the MWD tool when the MWD tool is inserted therein. Correspondingly, such centralizers are removed from the casing with the MWD tool when the MWD tool is removed. One example of such removal will be explained in more detail below with reference to  FIGS. 7 and 8 . In such examples, the one or more middle and upper centralizers may not be made from a drillable or otherwise readily removable material. 
         [0033]    In some applications of an MWD system, and referring to  FIG. 6 , it may be desirable for the MWD tool  14  to be able to measure fluid pressure in the annular space between the casing  12  and the wellbore wall (not shown in  FIG. 6 ). In one example, the centralizer (middle centralizer  16  shown in  FIG. 6 ) may include features to provide signal communication from a pressure sensor  32  disposed in the outer wall of the casing  12  to the MWD tool  14  when inserted into the centralizer  16 . In the present example, the pressure sensor  32  may be disposed in the wall of the casing  12  so that it is sensitive to fluid pressure outside the casing  12 . Electrical connection (for electrical pressure sensors) may be made through the wall of the casing  12  to the interior thereof using a pressure sealed electrical feed through  34 . One such feedthrough is sold under the trademark KEMTITE, which is a trademark of Kemlon Products and Development, Pearland, Tex. Corresponding structures may be used for optical signal connection if the pressure sensor  32  is an optical pressure sensor. Electrical wiring inside the drill string may be conducted through a sealed tube  34 A disposed proximate the bottom of the centralizer  16 . A transformer coil  36  may be disposed on the bottom of the centralizer  16  so that it is in proximity to a corresponding transformer coil  38  in the MWD tool  14  when the MWD tool  14  is disposed in its ordinary position inside the casing  12  during use. Signals from the pressure sensor  32  may be electromagnetically communicated between the two transformer coils  36 ,  38  so that measurements of pressure may be transferred to the MWD tool  14  for storage therein and/or communication to the surface using, for example, the mud flow modulation telemetry device explained above with reference to  FIG. 4 . 
         [0034]    Referring to  FIG. 7 , one example of a method for using the system described above will be explained. After the wellbore  1  has been drilled to a selected depth  12  by rotating the casing  12  and advancing the drill bit  22  through the subsurface, drilling operations may be temporarily stopped. The upper end of the casing  12  proximate the Earth&#39;s surface may then be opened to enable insertion therein a wireline, slickline or coiled tubing, shown in  FIG. 7  generally by  52 . The slickline, wireline or coiled tubing  52  may have at its lower end a retrieval tool  50  such as an overshot, configured to engage the fishing neck  28  on the MWD tool  14 . After engagement, the slickline, wireline or coiled tubing  52  may be retrieved, thus removing the MWD tool  14  from inside the casing  12 , leaving in place inside the casing  12  only the centralizers  16 ,  18 ,  24 , and the drill bit  22  disposed at the bottom of the casing  12 . 
         [0035]    In  FIG. 8 , after removal of the MWD tool as explained with reference to  FIG. 7 , the annular space between the wall of the wellbore  1  and the outer wall of the casing  12  may be filled with cement  5  using techniques known in the art. After the cement  5  has hardened, the casing  12  may be opened for deeper drilling using a drill bit  56  having diameter selected to freely pass through the interior of the casing  12 . The drill bit  56  may be moved longitudinally and rotated by a pipe string  54  which may be a suitably sized casing string or a string of drill pipe. One type of drill bit that may be advantageously used in some examples is a so-called “bi-center” bit that can drill a hole below the casing  12  having a diameter larger than the interior diameter of the casing  12 . One such bi-center drill bit is disclosed in U.S. Pat. No. 6,269,893 issued to Beaton et al. The drill bit  56  may drill out the centralizers  16 ,  18 ,  24  and the cemented in place drill bit  22  to enable extending the depth of the wellbore  1 . 
         [0036]    Methods and systems according to the invention may enable use of MWD and/or LWD devices with casing drilling that are inexpensive to implement and avoid the need for expensive retrievably drill bits. 
         [0037]    While the invention ahs been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.

Summary:
A casing drilling system includes a casing having a drill bit at one end. The drill bit is capable of drilling subsurface formations and formed from a material removable by drilling or chemical exposure. The chemical is substantially harmless to the casing. The system includes a centralizer affixed to an interior of the casing. The centralizer includes a receptacle therein for engaging a measurement while drilling tool. The centralizer is formed from a material removable by drilling or chemical exposure, wherein the chemical is substantially harmless to the casing. The system includes a measurement while drilling tool configured to move along the interior of the casing and to engage with the centralizer. The tool includes a device to measure a drilling parameter or a formation parameter. The tool including a latch at an upper end thereof for engagement with a retrieval tool moved through the interior of the casing.