Patent Publication Number: US-8973682-B2

Title: Wellbore obstruction clearing tool and method of use

Description:
FIELD OF THE INVENTION 
     Embodiments herein related to apparatus and methods for clearing obstructions in wellbores during casing of the wellbores and more particularly to apparatus connected at a bottom of a typically non-rotating tubular string for clearing obstructions encountered in the wellbore as the tubular string is run into an open hole, such as prior to cementing. 
     BACKGROUND OF THE INVENTION 
     In the oil and gas industry, following drilling of a vertical or horizontal wellbore into a formation for the production of oil or gas therefrom, the wellbore is typically cased and cemented to line the length of the wellbore to ensure safe control of production of fluids therefrom, to prevent water from entering the wellbore and to keep the formation from “sloughing” or “bridging” into the wellbore. 
     It is well known that during the running in of a tubing string, such as casing and particularly the production casing, the casing may encounter tight spots and obstructions in the open wellbore, such as that created by sloughing of the wellbore wall into the open hole or as a result of the casing pushing debris ahead of the bottom end of the casing along the open hole until it forms a bridge. Such obstructions prevent the advance of the casing and require the open hole to be cleared in order to advance the casing to the bottom of the hole. This is particularly problematic in horizontal wellbores. 
     Should the casing string become sufficiently engaged in a mud pack formed at the obstruction, differential sticking may also occur, making advancing or removal of the casing from the wellbore extremely difficult. 
     While casing strings have been rotated to assist with moving past or through an obstruction, high torque created by trying to rotate a long string of casing may result in significant damage to the threads between casing joints and may cause centralizers and the like to drag and ream into the wellbore. While rotation of casing may be a viable option in a vertical wellbore, albeit fraught with problems, it is extremely difficult, if not impossible in a horizontal wellbore. 
     One option is to employ a washing technique, pumping fluids through the casing while the casing is axially reciprocated uphole and downhole. The fluids exiting the downhole end of the casing bore act on the obstruction in the wellbore to wash out or erode the wellbore obstruction creating debris which is lifted or conveyed through the annulus to surface by fluid circulation therein. Should the washing technique be unsuccessful, it is known to trip out the casing and run in a mud motor on a drill string to drill out or ream the obstruction from the wellbore. Such repeated running in and tripping out of tubulars is time consuming, labor intensive and, as a result, very expensive. Alternatively, others have contemplated providing teeth on the bottom of the casing string or on a shoe at the bottom of the casing string to assist with cutting away the obstruction as the casing is advanced during running in. Typically, the casing is also reciprocated or stroked during the clearing operation, or, in some cases, at the same time as the casing is rotated. 
     Further, it has been contemplated to attach costly apparatus, such as mud motors, jetting or reaming tools, to the bottom of the casing string, however the apparatus is not retrievable thereafter from the wellbore and adds significantly to the cost of the casing operation. 
     Ideally, what is required is a relatively simple and inexpensive apparatus that can be incorporated into the casing string for clearing wellbore obstructions without the need for rotating the casing string. Ideally, the apparatus could be left downhole, after the casing and cementing operations are complete, without a significant increase in operational costs. 
     SUMMARY OF THE INVENTION 
     A wellbore obstruction-clearing tool is fit to a downhole end of a string of tubulars, such as a casing string or a string of coiled tubing (CT). The tool comprises a tubular mandrel having a rotatable tubular sleeve concentrically fit thereabouts. A helical drive is positioned between the mandrel and the sleeve, permitting the sleeve to reciprocate axially along the mandrel and to rotate relative thereto. The sleeve is driven to extend or retract axially and to rotate relative to the mandrel through axial reciprocation of the tubulars and the mandrel in the wellbore, commonly referred to as stroking of the tubulars within the wellbore. At least the rotation of the sleeve engaging the wellbore obstructions causes the obstructions to break up or erode, forming debris therefrom which is conveyed to surface by fluids circulated downhole through the string and uphole to surface in an annulus between the tubulars and the wellbore. The fluids can also aid in hydraulically extending the sleeve during the upstroke and fluidly eroding wellbore obstructions. 
     In a broad aspect, a wellbore obstruction-clearing tool is fit to a downhole end of a tubing string for advancing the tubing string through obstructions in a wellbore. The tubing string has an axial bore therethrough for communicating fluids to an annulus between the tubing string and the wellbore for circulation to surface. The obstruction-clearing tool comprises ad tubular mandrel a tubular sleeve and a helical drive therebetween. The tubular mandrel connects to the downhole end of the tubing string, the mandrel having a mandrel bore extending axially therethrough, and the mandrel bore being fluidly connected to the axial bore. The tubular sleeve has a sleeve bore extending axially therethrough and fit concentrically fit about the mandrel, the sleeve bore being fluidly connected with the mandrel bore, and a downhole end for engaging the wellbore obstructions. The helical drive arrangement acts between the mandrel and the sleeve for driving the sleeve axially and rotationally along the mandrel between a retracted position and an extended position in response to reciprocating axial movement of the tubing string and mandrel. The engagement of the downhole end of the sleeve creates debris from the wellbore obstructions, and wherein the fluids from the sleeve bore convey debris along the annulus to surface. 
     The obstruction-clearing tool enables methods for clearing obstructions in a wellbore and advancing a tubing string therein without rotation of the tubing string. Such method comprises running a wellbore obstruction-clearing tool on a downhole end of the tubing string, such as casing or CT, the wellbore obstruction-clearing tool having a tubular mandrel for connection to the tubing string and tubular sleeve which is axially and rotationally moveable therealong between a retracted position and an extended position; and when the wellbore obstruction-clearing tool encounters a wellbore obstruction. In operation, the method comprises stroking the tubing string uphole and downhole so as to drive the tubular sleeve to rotate and reciprocate axially between the retracted position and the extended position for engaging the wellbore obstruction and creating debris therefrom; and discharging fluid through contiguous bores in the tubing string, the mandrel and the sleeve for conveying debris to surface. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a fanciful schematic sectional view of an embodiment of obstruction-clearing tool connected to a downhole end of a casing string; 
         FIG. 2  is a cross-sectional view of the tool of  FIG. 1 , taken along section lines II-II, and illustrating guide pins on an inner surface of a sleeve engaging helical grooves on an outer surface of a mandrel; 
         FIG. 3  is a longitudinal sectional view of a tapered discharge of a tool of  FIG. 1 , the tool having centralizing ribs formed on a sleeve and having a flow restrictor; 
         FIG. 4A  is a longitudinal side view of a mandrel having helical grooves with a uniform pitch of about 45 degrees; 
         FIG. 4B  is a longitudinal side view of a mandrel having helical grooves having a pitch that varies from 60 degrees to 45 degrees, from 45 degrees to 30 degrees, from 30 degrees to 45 degrees, and from 45 degrees to 60 degrees; 
         FIG. 5  is a longitudinal perspective view of an embodiment of the obstruction-clearing tool a PDC equipped bit at a downhole end of the sleeve; 
         FIG. 6A  is a longitudinal partial sectional view of the embodiment of  FIG. 5 , illustrating the mandrel in side view and the sleeve in cross-sectional view and in an extended position; 
         FIGS. 6B and 6C  are detailed partial sectional views of the mandrel&#39;s uphole end and downhole end respectively, according to  FIG. 6A ; 
         FIG. 7  is a perspective view of a PDC equipped bit embodiment according to  FIG. 5 , the bit having a plurality of openings for the passage of fluids therethrough; 
         FIG. 8  is a perspective sectional view of the bit according to  FIG. 7 , showing an uphole face and the plurality of openings for fluid passage; 
         FIGS. 9A ,  9 B and  9 C illustrate another embodiment of an obstruction-clearing tool which is optimized for horizontal wellbores and drillable embodiments: 
         FIG. 9A  is a longitudinal side view of the tool in the extended position; 
         FIG. 9B  is a partial sectional view of  FIG. 9A  with the mandrel is side view and the sleeve in cross-sectional view; 
         FIG. 9C  is a partial sectional view of  FIG. 9B  with the sleeve retracted over the mandrel; 
         FIG. 10A  is a longitudinal partial sectional view of the embodiment of  FIG. 9A , illustrating the mandrel in side view and the sleeve in cross-sectional view and in an extended position; 
         FIGS. 10B and 10C  are detailed partial sectional views of the mandrel&#39;s uphole end and downhole end respectively, according to  FIG. 10A ; 
         FIG. 11  is a perspective view illustrating the tubular bit of  FIG. 10A ; 
         FIG. 12  is a sectional view of the tubular bit of  FIG. 1 ; 
         FIG. 13  is a longitudinal partial sectional view illustrating an embodiment of a drill-throughable bit having a less competent bit insert and a locking mechanism between the mandrel (shown in side view) and the bit at the downhole end of the sleeve (shown in section); 
         FIG. 14  is a perspective view of an embodiment of the mandrel having a first castellated profile at a downhole end for forming a locking mechanism; 
         FIG. 15  is a perspective sectional view of a downhole end of the sleeve, illustrating a tubular bit having a second castellated profile for correspondingly interlocking with the first castellated profile of  FIG. 14  to form a locking mechanism; 
         FIG. 16  is a perspective view of an alternative form of a locking mechanism comprising a screw head-type interlocking interface; 
         FIG. 17A  is a longitudinal partial sectional view of the embodiment of  FIG. 13  illustrating a drill-throughable wellbore obstruction-clearing tool having a casing shell extending over the mandrel (is seid view) and the sleeve (in sectional view), the sleeve being in the retracted position; 
         FIG. 17B  illustrates the sleeve of  FIG. 17A  its fully extended position and the casing shell surrounding the mandrel for providing a guide for a subsequent pr secondary drill string; 
         FIG. 18  is a schematic representation illustrating a six-step progression of a wellbore obstruction-clearing tool engaging an obstruction in a vertical wellbore and being activated by shearing of shear pins; 
         FIG. 19  is a schematic representation illustrating a five-step progression of a wellbore obstruction-clearing tool engaging an obstruction in a horizontal wellbore, the sleeve being axially extended through fluid hydraulics; 
         FIG. 20  is a schematic representation illustrating a six-step, left-to-right progression of a downstroke of the casing and wellbore obstruction-clearing tool acting against an obstruction in a vertical wellbore; 
         FIG. 21  is a schematic representation illustrating a six-step, right-to-left progression of an resetting, upstroke of the casing and wellbore obstruction-clearing tool; and 
         FIGS. 22A and 22B  are schematic representations of a drill-throughable tool according to  FIG. 17A , which is cemented in a wellbore and then being drilled out by a secondary drill string respectively, for extending a previously cased wellbore. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of a wellbore obstruction-clearing tool are connected to a downhole end of a string of tubulars, such as casing or coiled tubing (CT), to aid in advancing or removing the tubulars within a wellbore. Thus, the obstruction-clearing tool obviates the need to rotate the casing thereby, substantially avoiding problems associated therewith, such as torque build up along the casing. For the purposes of the description which follows, Applicant has described the tool in the context of use with casing. Those of skill in the art will appreciate however, that embodiments disclosed herein are not limited for use with casing and are suitable for use with other tubulars having a bore formed therethrough and for which rotation is to be avoided. 
     In embodiments, a tubular sleeve is caused to rotate while extending and retracting along a mandrel connected to the downhole end of the casing. Axial reciprocation and rotation of the sleeve along the mandrel is initiated by axial reciprocation of the casing in the wellbore, commonly referred to as stroking of the casing. At least the rotation of the sleeve within the wellbore clears any obstruction, creating debris, the debris being conveyed to surface by circulation of fluids downhole through the casing and uphole to surface through an annulus between the casing and the wellbore. When the obstructions are removed from the wellbore, the casing can be lowered to a target depth such as prior to cementing the casing into place in the wellbore. 
     In embodiments, fluid, such as a drilling fluid, is injected or pumped downhole through the casing. The mud is circulated up the annulus for conveying the debris to surface. Further, extending or resetting of the tubular sleeve can be through hydraulic impetus from the drilling fluid and gravity depending on the wellbore orientation. The fluids discharging from the casing can also aid in clearing obstructions by fluidly engaging the wellbore obstructions, such as in a jetting action, fluidly eroding the wellbore obstructions for creating debris therefrom. A velocity of the fluids discharged can be increased for enhancing the fluid erosion. The downhole end of the sleeve can also physically disrupt the obstructions for creating debris therefrom. 
     In more detail, and having reference to  FIGS. 1 and 2  of one embodiment, an obstruction-clearing tool  100  is connected at a downhole end  12  of a tubing string, such as casing  10  or coiled tubing (CT) for clearing obstructions  119  from a wellbore  14 . 
     The obstruction-clearing tool  100  comprises a tubular mandrel  120 , connected, such as by threading, to the downhole end  12  of the casing  10  and having a mandrel bore  121  which is fluidly connected to an axial bore  11  of the casing  10 . 
     A tubular sleeve  110  having a sleeve bore  115  is fit concentrically about the tubular mandrel  120  and is axially displaceable therealong between a fully retracted position, wherein a downhole end  112  of the sleeve  110  is adjacent a downhole end  127  of the mandrel  120 , and a fully extended position, wherein the downhole end  112  of the sleeve  110  is displaced axially away from the downhole end  127  of the mandrel  120 . 
     In embodiments, fluid F is pumped through the contiguous bores of the casing&#39;s axial bore  11 , the mandrel bore  121  and the sleeve bore  115 . The fluid F discharges from the sleeve bore  115  and into the wellbore  14 . The fluid F is circulated along an annulus  20 , between the casing  10  and the wellbore  14 , to surface through the annulus  20 . 
     A drive arrangement  118 , co-operates between the mandrel  120  and the sleeve  110 , and permits the sleeve  110  to be rotated as the sleeve  110  is axially displaced along the mandrel  120 . Thus, the sleeve  110  is axially and rotationally displaceable between the extended and retracted positions. 
     The tubular sleeve  110  engages obstructions  119  in the wellbore  14 . Applicant believes that at least the engagement of the sleeve  110 , and rotational movement thereof, aids in agitating or disrupting the obstructions  119 . The fluids F discharged through the sleeve bore  115  convey the debris from the wellbore  14  as the fluid F is circulated to surface through the annulus  20 . Fluid F, where discharged so as to contact the obstruction  119 , further acts to fluidly erode the obstructions  119 , enhancing the production of debris therefrom. 
     In greater detail, as shown in  FIGS. 1 ,  2 ,  4 A and  4 B, the drive arrangement  118  is a helical drive arrangement formed between the mandrel  120  and the sleeve  110 . One or more helical slots or grooves  122  cooperate with one or more protrusions  111 , such as buttons, pins or the like, for guiding the sleeve  110  rotationally and axially relative to the mandrel  120 . In an embodiment, the one or more helical grooves  122  are formed on either of an inner surface  115  of the sleeve  110  or on an external surface  126  of the mandrel  120 . The one or more protrusions or guide pins  111  extend radially from the other of the outer surface of the mandrel  120  or the inner surface of the sleeve  110 . 
     Referring again to  FIGS. 1 to 3 , in an embodiment, the helical drive arrangement  118  comprises three helical grooves  122 ,  122 ,  122 , equally spaced apart in the external surface  126  of the mandrel  120 , and three corresponding guide pins  111 ,  111 ,  111  spaced equally apart and extending radially inwardly from an inner surface  115  of the sleeve  110 . Each pin  111  engages a corresponding helical groove  122 . Use of the three helical grooves  122 ,  122 ,  122  and corresponding guide pins  111 ,  111 ,  111  acts to centralize the mandrel  120  within the sleeve  110 . As the sleeve  110  is extended or retracted along the mandrel  120 , the sleeve  110  rotates as the pin  111  follows the path of the helical groove  122 . The three pins  111 ,  111 ,  111  are positioned adjacent the uphole end  114  of the sleeve  110  to permit full axial extension of the sleeve  110  along the mandrel  120 . The tolerance between the sleeve  110  and the mandrel  120  is sufficiently tight such that the each guide pin  111  remains in the corresponding helical groove  122 , when the tool  100  is assembled. The direction of the helical grooves  122 ,  122 ,  122  ensures that rotational loading on the mandrel  120  is compatible with conventional threaded connection of the mandrel  120  to the casing  10  to avoid separation of the obstruction-clearing tool  100  from the casing  10  during use. 
     With reference to  FIGS. 4A and 4B , a pitch of each helical groove  122  may be uniform along the path of the helical grooves  122 , being substantially a length of the mandrel  120  ( FIG. 4A ) or may vary ( FIG. 4B ) to change the speed of rotation and the corresponding effort to initiate rotation of the sleeve  110  as the sleeve  110  moves axially along the length of the mandrel  120 . 
     In an embodiment as shown in  FIG. 4B , the pitch of the helical grooves  122  is about 60 degrees, measured from a transverse plane, at a location adjacent the uphole end  128  of the mandrel  120 , which decreases to about 45 degrees, then to about 30 degrees and thereafter increases again from 30 degrees, to about 45 degrees and then to about 60 degrees at the downhole end  127  of the mandrel  120 . Thus, the sleeve  110 , as it extends or retracts axially along the length of the mandrel  120 , begins to easily and slowly rotate at either the uphole or downhole end  128 ,  127  of the mandrel  120 . As the sleeve  110  moves axially along the mandrel  120 , the rotational speed increases as the sleeve  110  passes through the about 45 degree section and then the about 30 degree section. Thereafter, as the sleeve  110  continues to move axially and enters the subsequent about 45 degree section, rotation of the sleeve  110  begins to slow and as the sleeve  110  enters the about 60 degree section, the sleeve  110  has slowed once again to the easy, slow rotation. 
     Axial movement of the mandrel  120 , fixed to the casing  10 , causes the sleeve  110  to reciprocate along the mandrel  120 . A downhole stroke of the casing  10  causes the sleeve  110  to rotate in one direction and an uphole stroke of the casing causes the sleeve  110  to rotate in the opposite direction. The downhole stroke causes the sleeve  110  to retract along the mandrel  120  and the uphole stroke permits the sleeve  110  to extend along the mandrel  120 . The impetus to retract the sleeve  110  relative to the mandrel  120  is by resistance encountered at the sleeve, such as by the obstruction  119 , or a tight wellbore  14 . The impetus to extend the sleeve  110  relative to the mandrel  120  is by hydraulic force created by the fluid F on the downhole end of the sleeve and gravity depending on the orientation of the wellbore, being most effective in vertical wellbores. 
     In one method of manufacture the sleeve  110  is slipped over the mandrel  120  and the pins  111  are installed through the sleeve  110  to engage the helical grooves  122 . The pins  111  are retained therein, such as by deformation of the installation hole, or use of a cap screw or welding. 
     In an embodiment of the invention, the mandrel  120  is threadably connected to a last joint of casing  10 . The uphole end  128  of the mandrel  120  has a box end which is threaded to a conventional pin end at the downhole end  12  of the casing  10 . A thickness of the tubular mandrel  120  is generally greater than a thickness of the casing  10  to permit machining of the helical grooves  122  therein. 
     As shown in  FIG. 1  and in greater detail in  FIGS. 6B ,  6 C,  10 B and  10 C, at least one stop is formed between the sleeve  110  and the mandrel  120  to limit the axial movement of the sleeve  110  along the mandrel  120  and to retain the sleeve  110  thereon. 
     As shown in  FIGS. 6C and 10C , an uphole stop  113  is formed at the uphole end  114  of the sleeve  110 . A downhole stop  123  is formed between the downhole end  127  of the mandrel  120  and the uphole end  114  of the tubular sleeve  110  for retaining the sleeve  110  on the mandrel  120  when in the fully extended position. Similarly, as shown in  FIGS. 6B and 10B , an uphole stop  125  is formed between an uphole end  128  of the mandrel  120  and the sleeve&#39;s uphole stop  113  for retaining the sleeve  110  on the mandrel  120  when in the fully retracted position. 
     Annular seals are positioned to fluidly seal between the sleeve  110  and the mandrel  120 . A downhole annular seal  124  is positioned such that the downhole seal  124  becomes sandwiched axially between the mandrel&#39;s downhole stop  123  and the sleeve&#39;s uphole stop member  113  when the sleeve  110  is in the fully extended position. An annular seal  126  is positioned such that it becomes sandwiched axially between the uphole stop  125  and the sleeve&#39;s uphole stop member  113  when the sleeve  110  is in the fully retracted position. 
     In an embodiment, a shipping or shear pin  129  is employed to maintain the sleeve  110  in the axially retracted position during shipping. Depending on operator technique, the shear pins can also maintain the sleeve  110  in the axially retracted position running-in of the casing  10  and the tool  100 . The shear pin  129  extends radially inwardly from the stop member  113  on the uphole end  114  of the sleeve  110  to engage the uphole end  128  of the mandrel  120 . When removed after shipping, or if retained, when sheared in the wellbore, the sleeve  110  is freed to reciprocate as described herein in response to the axial reciprocation of the casing  10  and mandrel  120 . 
     As shown in  FIGS. 1 and 3 , the downhole end  112  of the sleeve  110  may be tapered, such as to a truncated cone shape, so as to narrow the cross-section area of the sleeve bore  115  to increase the velocity of fluids F exiting therefrom. The increase in velocity acts to increase the degree of agitation caused by the fluids F exiting therefrom. Alternatively, the sleeve bore  115  can be configure to affect the fluid F issuing therefrom for forming an extending force and for jetting fluids therefrom. 
     Having reference again to  FIG. 3 , in an embodiment, the downhole end  112  of the sleeve bore  115  is fit with a flow restrictor  140 . The flow restrictor  140  reduces the diameter of the sleeve bore  115  or forms one or more openings  142  of smaller diameter therein for increasing the extending force acting on the sleeve and for increasing velocity of the fluid F discharged therethrough. The higher velocity causes the discharged fluid F to increase the degree of agitation caused by the fluids F exiting therefrom and to engage the obstructions  119  with greater force to further aid in erosion of the obstructions  119 . 
     In vertical wellbores, stroking the casing  10  uphole permits gravity to act on the sleeve  110  for causing axial extension of the sleeve  110  along the mandrel  120 . In the case of horizontal wellbores, there is little to no gravitational impetus to cause axial extension of the sleeve  110 . In this case, the flow restrictor  140  further acts to create an uphole face or shoulder  141  upon which the fluid F pumped through the sleeve bore acts, creating a backpressure and an extending force or impetus for hydraulic extension of the sleeve  110 . 
     Optionally, as shown in  FIG. 3 , ribs  116  may be formed on an outer surface  117  of the sleeve  110  to act as centralizers for avoiding contact between the sleeve  100  and the wellbore  14 , preventing reaming of the wellbore  14 . In an embodiment, the ribs  116  are helical and are formed on the outer surface  117  of the sleeve  110  to minimize reaming should the ribs  16  come into contact with the wellbore  14 . Further, helical ribs  116  provide a passage for fluids circulated in the annulus  20  to surface and therefore do not block the annulus  20  to the passage of fluids therethrough, permitting fluid F and debris to be directed up the annulus  20  to surface. 
     Further, in the case of horizontal wellbores, the centralizing ribs  116  may engage and drag in the wellbore  14  during uphole stroking of the casing  10 , assisting with axial extension of the sleeve  110  relative to the mandrel  120 . 
     In an embodiment, as shown in  FIG. 3 , the downhole end  112  of the sleeve  110 , further comprises a plurality of protrusions  131  ( FIG. 3 ), such as teeth, extending outwardly therefrom. The plurality of protrusions  131  act to either physically engage the obstruction for disrupting the obstruction and forming debris therefrom or to agitate fluid about the obstructions for fluidly eroding the obstruction or a combination thereof. The plurality of protrusions  131  are made from tungsten carbide or are coated with tungsten carbide to increase the strength and to enhance the cutting ability of the plurality of protrusions  131 . The plurality of protrusions  131  are formed on the downhole end  112  of the sleeve  110 , are welded to the downhole end  112  of the sleeve  110  or are replaceably threaded to the downhole end  112  of the sleeve  110 , such as on a threaded shoe  130 , as shown in  FIG. 1 . 
     Similarly, as shown in  FIGS. 7 ,  12  and  13  the protrusions  131  can be various forms of teeth  161 . The plurality of protrusions  131  or teeth  161  are positioned circumferentially about the downhole end  112  of the sleeve  110 . As shown  FIG. 1 , the plurality of protrusions  131  can be generally offset from one another, such as radially set, or opposingly oriented circumferentially, or both, to aid in engaging and agitating obstructions, aiding in the erosion thereof. Further turbulence aids in keeping the debris from settling out of the fluid F so as to lift the debris with the fluid F to surface. 
     With reference to  FIGS. 5 to 12 , and in an embodiment, the protrusions  131  are provided by mechanical means, such as conventional cutters or teeth  161 , on a drill bit  150  fit to the downhole end  112  of the sleeve  110 . The drill bit  150  has one or more openings  151  therein for discharging the fluid F therefrom. 
     As shown in  FIGS. 7 and 8 , and in an embodiment, the drill bit  150  is a PDC-equipped drill bit comprising a tapered or bullet-shaped leading surface  152  and PDC cutter elements  153 . A tapered or bullet contoured leading surface  152  aids in tracking of the wellbore such as in horizontal wells. The leading surface  152  of the drill bit further comprises at least one opening  151  for permitting fluid F to pass therethrough from the sleeve bore  115  to the annulus  20 . The at least one opening  151  functions similarly to the flow restrictor  140  and acts to restrict the flow of the fluid F passing therethrough for increasing the velocity of the fluid F. Further, an uphole face  154  created by the leading surface  152  aids in increasing the backpressure acting thereon for extension of the sleeve  110  to the extended position. 
     With reference to  FIGS. 9A-12 , the drill bit  150  is a tubular drill bit  160  having an open bore  162  which is contiguous with the sleeve bore  115  for delivery of fluids F therethrough and a plurality of teeth  161  ( FIGS. 11 and 12 ) extending downwardly therefrom for forming the protrusions  131 . The tubular drill bit  160  further comprises flow restrictor  140 . The flow restrictor  140  is positioned within the bore  162  for increasing the velocity of the fluids passing therethrough and provides uphole surface  154  for hydraulically extending the tubular sleeve  110 . 
     In the case of horizontal wellbores  14 , the teeth  161  formed about the open bore  162  can engage and ream the wellbore  14 . An alternate embodiment of bit  179  is shown in  FIG. 13 . 
     In some embodiments, there may be an objective to drill through the obstruction-clearing tool  100 . In a conventional casing operation, casing is advanced into the wellbore  14  until the casing  10  is landed at the target depth. The casing  10  is cemented into place. In embodiments, for use where there is no expectation to extend the wellbore  14  after cementing the casing  10 , the obstruction-clearing tool  100  is manufactured of robust 4140 steel. 
     In embodiments, for use where the depth of the wellbore  14  is to be extended following cementing of at least a first section of casing  10 , at least portions of the obstruction-clearing tool  100  are made to be drillable. Due to the nature of the tool  100  to have relative rotatable components, accommodations are made to avoid reactive rotation of one or more portions of the tool  100  when drilling through the tool  100 . 
     Generally, the drillable portions are made of less competent materials, such as aluminum and aluminum composites, which facilitate being drilled out. In such cases, the portions that are made drillable are generally internal components which would otherwise interfere with or retard passage of a drill string therethrough. The bit  150  can also be drillable or its design accommodates passage of a drill string therethrough, such as in the tubular drill bit  160  embodiment of  FIG. 12 , which minimally obstructs the bore  115  of the sleeve  110 . 
     For example, the mandrel  120  may be formed of aluminum and the guide pins  111  may be made of bronze while the remaining components such as the sleeve  110  are made of 4140 steel. The bit  150  is also made of less competent materials permitting drilling therethrough. 
     In an embodiment, shown in  FIG. 13 , a drillable bit incorporates robust characteristics used for engaging and clearing the wellbore obstructions  119 , yet permits drilling out for passage of a subsequent drill string therethrough for extending the wellbore  14  beyond the initial target depth. The bit  150  comprises a tubular bit body  170  made of robust steel construction including polycrystalline diamond compact (PDC) cutter elements (not shown), which are not readily drilled through. The tubular bit body  170  has a bit bore  171  formed therein through which the drill string may pass, the bit  170  body being substantially avoided. A less competent bit insert  173  is fit within the bit bore  171 , the bit insert  173  having a leading bit surface  174  comprising the plurality of protrusions  131  such as teeth of cutters  175  formed thereon. The plurality of cutters  175  engage the obstructions  119  much like the protrusions  131  and drill bits  150 ,  160  of the previously described embodiments. The bit insert  173  further forms the flow restrictor  140 , as previously described both for increasing the velocity of fluid F discharged therefrom and for hydraulic extension of the sleeve  110 . 
     The bit body  170  is manufactured from robust 4140 hardened steel. The bit insert  173  and the flow restrictor  140  are manufactured from 6061 aluminum, which is suitable to withstand the rigors of the casing stroking operation yet are drillable. 
     The drillable embodiment of the obstruction-clearing tool  100  is connected to the downhole end  11  of the casing  10  and casing  10  is lowered to the target depth, the obstruction-clearing tool  100  acting as a landing tool. The casing  10  is thereafter cemented into with wellbore  14  using conventional cementing operations. Cement is pumped through the casing  10  and is discharged from the downhole end  112  of the sleeve  110  and into the annulus  20 . The cement hardened about the sleeve  110  prevents any further axial or rotational movement of the sleeve  110  about the stationary mandrel. 
     In drill-through operations, a secondary drill string and drill bit can damage or drill out the helical drive connection between the mandrel  120  and the sleeve  110 . Free rotation of the mandrel ahead of the secondary drill string nullifies the drilling operation. Several features are provided in one or more embodiments, to minimize problems when drilling through the tool  100 . 
     In one embodiment, shown in  FIGS. 13-16 , a locking mechanism  180  connects between the mandrel  120  and sleeve  110  in the fully retracted position, preventing independent rotation of the mandrel  120  should the connection between the mandrel  120  and the casing  10  and the mandrel  120  and the sleeve  110  be compromised. As shown in greater detail in  FIGS. 14 and 15 , the locking mechanism  180  is an interlocking interface, such as a castellated interface, between the downhole end  127  of the mandrel  120  and the downhole end  112  of the sleeve  110  for interlocking the components and preventing relative rotational movement therebetween. The downhole end  127  of the mandrel  120  comprises a first castellated profile  181  ( FIG. 14 ) having a plurality of circumferentially-spaced axially-extending projections  182  formed thereon and a plurality of recesses  186  therebetween. Similarly, the downhole end  112  of the sleeve  110  comprises a second castellated profile  183  ( FIG. 15 ) having a plurality of circumferentially-spaced, axially-extending projections  184  formed thereon and a plurality of recesses  188  therebetween. In an interlocked position, with the first and second castellated profiles  181 ,  183  being face-to-face, the projections  182  of the first castellated profile  181  are engaged in the recesses  188  of the second castellated profile  183 . Accordingly, the projections  184  of the second castellated profile  183  are engaged in the recesses of the first castellated profile  181 . In the interlocked position, the mandrel  120  is prevented from rotating. 
     The mandrel  120  and the sleeve  110  may not be in the interlocked position when the drilling operation begins, such as when the sleeve  110  is in the axially extended position when cemented in. In such instances, when the mandrel  120  becomes free to rotate with the drill string, the remaining portion of the mandrel  120  having the first castellated profile  181  is pushed downhole by the secondary drill string. The first castellated profile  181  is caused to engage with the second castellated profile  183  of the sleeve  110  in the interlocked position preventing further rotational movement of the mandrel  120  and permitting the drilling operation to continue. 
     In an embodiment as shown in  FIGS. 13 and 16 , the locking mechanism  180  comprises a uni-directional, screw-head-type interlocking cog-like interface having cooperating and rotationally ramped axial faces  185 ,  186  for arresting co-rotation of the mandrel  120  during drilling out. 
     In an embodiment which minimizes deviation of the extended wellbore when drilling through the tool, the mandrel and sleeve are provided with a casing shell  190  which guides the second drill through the tool  100 . 
     Having reference to  FIGS. 17A and 17B , an obstruction-clearing tool  100  having a drillable bit  170 , further comprises a casing shell  190  manufactured from materials that are resistant to drilling or milling, such as 4140 hardened steel. The casing shell  190  shields the mandrel  110  for guiding the second drill string along a drilling path substantially in alignment with the mandrel  120  and into the sleeve  110 . The casing shell  190  is fit concentrically over the mandrel  120 , and concentrically and slidably over the sleeve  110 , and extends along a length of the mandrel  120  from about the mandrel&#39;s upper end  128  to the mandrel&#39;s downhole end  127 . The casing shell  190  is secured to the mandrel&#39;s upper end  128  by an upper collar  191  and slidable over the sleeve  110 . The casing shell  190  is stationary with the mandrel  120  during axial extension of the sleeve  110 . A downhole end  192  of the casing shell  190  is slidably and rotatably stabilized about the sleeve  110  by a downhole collar  192 . As shown in  FIG. 17B , the sleeve  110  passes through the downhole collar  192  when the sleeve  110  is axially extended, the casing shell  190  remaining substantially surrounding the mandrel  120 . 
     As one of skill in the art will appreciate, the obstruction-clearing tool  100  can be sized appropriately depending upon the size of the casing  10  being utilized. That is, the obstruction-clearing tool  100  can be adapted to operatively and fluidly connect to tubulars commonly used in the industry, such as 4½ inch, 5½ inch, 7 inch, or 9⅝ inch casings and 2⅞ inch coiled tubing, or can be custom sized for any size casing  10  or CT. 
     As shown in  FIGS. 5 and 6A  to  6 C, an obstruction-clearing tool  100 , particularly suited for use in vertical wellbores with 5½ inch casing  10 , comprises a mandrel  120  having a diameter of about 4.25 inches and a length of about 68 inches (about 1.73 m) and a sleeve  110  having a length of about 92 inches (about 2.34 m). The sleeve  110  has an inside diameter of about 4.89 inches (about 12.42 cm) forming a clearance fit concentrically about the mandrel  120  and an outside diameter of about 5½ inches (13.97 cm). Three, 1 inch (about 2.43 cm) diameter guide pins (not shown) are provided at about 120 degrees apart for engaging three parallel and helical grooves  122  in the mandrel  120 . Annular seals  124 ,  126 , such as rubber cushions or large O-rings, are fit about the mandrel&#39;s uphole end  128  and downhole end  127  as cushions between the mandrel  120  and sleeve  110  when the sleeve  110  bottoms at each end of the stroke. The resulting stroke of the obstruction-clearing tool  100  is about 68.5 inches or about 5 feet (1.52 m) the sleeve  110  rotating approximately 4.9 revolutions about the mandrel  120  per stroke. 
     With reference to  FIGS. 9A to 9C ,  10 A to  10 C,  11  and  12 , an embodiment well-suited for passing through and cleaning deviated or horizontal wellbores is shown. In  FIGS. 9A to 9C , a shorter or stubby embodiment comprises a mandrel  120  having a length of about 32 inches (about 81.28 cm) a corresponding sleeve  110  having a length of about 54.38 inches (about 1.38 m). When sized for use with a 7 inch casing, the mandrel  120  has a diameter of about 5.7 inches (about 14.48 cm) and the sleeve  110  has an outside diameter of 7 inches (about 17.78 cm) and an inside diameter of about 6.37 inches (about 16.18 cm). The stroke length is about 32 inches (81.28 cm) and the sleeve  110  makes about 2 revolutions about the mandrel  120  per stroke. 
     In Operation 
     Embodiments of the wellbore obstruction-clearing tool  100  are used during casing of an open hole or wellbore  14  which has been drilled in a previous drilling operation. A survey can log obstructions, including tight spots, requiring clearing. The wellbore obstruction-clearing tool  100  is connected to a bottom of a joint of conventional casing and the casing is run into the wellbore. 
     Some operators prefer to remove the shipping or shear pin or pins  129  and run the tool  100  in extended, possibly operating passively and periodically on the trip downhole. In other cases the shear pin or pins  129  remain in place to retain the sleeve  110  in the retracted position during tripping into the wellbore  14 . 
     As shown in  FIG. 18 , with the shear pins  129  in place, and in a vertical wellbore, the casing  10  and tool  100  are lowered into the wellbore at ( 1 ) and ( 2 ) to an obstruction  119  at ( 3 ). A downhole shear force, such as a downhole set-down load of about 1000 lbs, is applied to the tool  100  at ( 4 ), sufficient to shear the shear pins  129 , permitting the sleeve  110  to be free to move relative to the mandrel  120 . 
     Once the sleeve  110  is free to move axially and rotationally relative to the mandrel  120 , the casing  10  and mandrel  120  are lifted or stroked uphole at ( 5 ) with sleeve  110  moving rotationally towards its extended position. The casing is stroked upwardly and the sleeve  110  reaches the extended position at ( 6 ). The stoke of the casing can be controlled and is not necessarily stroked to the full extension or the full retraction. 
     The stroking of the casing  10  continues uphole and downhole so as to drive the tubular sleeve to rotate and reciprocate axially between the retracted position and the extended position for engaging the wellbore obstruction, creating debris and is repeated until the obstruction is cleared and the tool  100  can be landed at target depth, or the next obstruction. 
     In a vertical wellbore, extension of the sleeve  110 , as the mandrel  120  is stroked uphole, is largely under the influence of gravity and thus lifting of the casing  10  may be sufficient to cause the sleeve  110  to extend. Fluid F is typically used as well for removal of debris and for extension of the sleeve  110 . 
     With reference to  FIG. 19 , in a horizontal wellbore where gravity provides no gravitational impetus for the sleeve  110  to extend along the mandrel  120 , the fluid F hydraulically extends the tubular sleeve to the extended position as the tubing string is stroked uphole. In this case, as the casing  10  is stroked uphole at ( 3 ), the fluid F forces the sleeve  110  to remain downhole, while rotating and may be engaged against the obstruction  119 . 
     With reference to  FIG. 20 , in a typical clearing operation as shown from left to right, whether the wellbore  14  is vertical or horizontal, the casing  10  is stroked downhole from an extended position at ( 1 ) to a retracted position at ( 6 ). The stroking of the casing and mandrel  120  causes the sleeve  110  to axially and rotationally retract along the mandrel  120 . The rotation of the sleeve  110  engages the obstruction  119  and creates debris therefrom. The fluids F circulated uphole through the annulus  20  convey the debris to surface. 
     Thereafter, as shown from right to left in  FIG. 21 , and beginning with the tool at the retracted position at step ( 7 ), the casing  10  and mandrel  120  are lifted clear of any remaining obstruction  119 . As shown in steps ( 8 ) through ( 12 ), as the sleeve  110  extends along the mandrel  120  the sleeve  110  rotates in the opposite direction to that when the sleeve is retracted along the mandrel  120 . The sleeve  110  resets for a subsequent downstroke of  FIG. 20 , but also continues to rotate and discharge fluid F for engaging the obstruction. 
     The operation of  FIGS. 20 and 21  is repeated as many times as is necessary to clear the obstruction  119 , and for each and any subsequent obstructions, sufficient that the casing  10  can be advanced thereby until the casing  10  reaches the target depth. As will be appreciated by those of skill in the art the tool  100  according to embodiments of the invention acts as a casing landing tool. Thereafter, such apparatus as may be required to cement the casing into the wellbore is run into the casing  10 . 
     With reference to  FIGS. 22A and 22B , in a drillable embodiment using a form of tool  100  set forth in  FIGS. 17A and 17B , a length of a wellbore  14  is extended, As secondary drill string  200  and drill bit  201 , has an outer diameter smaller than the inner diameter of the sleeve  110 . At least a portion of the mandrel  120 , the bit  150  and the flow restrictor  140  are drilled through for gaining access to the formation below the previously cased wellbore  14  and drilling an extension of the wellbore therein. 
     EXAMPLE 
     An embodiment of the invention was tested during casing of a vertical wellbore in which normal casing operations were first attempted and had failed. Obstructions were encountered at about 1 kilometer downhole preventing passage of the casing to the target depth. 
     Previously, a drilling fluid was circulated through the casing and adjacent the obstructions in an attempt to hydraulically clear the obstruction. The process lasted three successive days, at great expense, and was ultimately unsuccessful in clearing a first obstruction. The casing was tripped out and a mud motor was run downhole to mechanically drill through the first obstruction. The conventional mandrel, drill bit and bottom sub of the expensive mud motor were eventually lost downhole without successfully clearing the first obstruction. The bottom sub of the mud motor was eventually recovered by a fishing operation. Several weeks were lost and the first obstruction was still not cleared. 
     Thereafter, an obstruction-clearing tool  100  was operatively and fluidly connected to the casing and run downhole. The obstruction-clearing tool was actuated when the first obstructions was reached. The casing and the tool were stroked fully, uphole and downhole, three times. The obstruction was successfully cleared and the casing advanced thereby.