Patent Publication Number: US-10774594-B2

Title: Rotating cutting structures and structures for retaining the same

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the U.S. national stage entry of International Patent Application Serial No. PCT/US2016/049170 filed on Aug. 29, 2016, which application claims the benefit of U.S. Provisional Patent Application Ser. No. 62/234,560 filed on Sep. 29, 2015, the disclosure of which is incorporated by reference. 
    
    
     BACKGROUND 
     Various types and shapes of earth boring bits are used in various applications in the earth drilling industry. Earth boring bits have bit bodies which include various features such as a core, blades, and cutter pockets that extend into the bit body or roller cones mounted on a bit body, for example. Depending on the application/formation to be drilled, the appropriate type of drill bit may be selected based on the cutting action type for the bit and its appropriateness for use in the particular formation. 
     Drag bits, often referred to as “fixed cutter drill bits,” include bits that have cutting elements attached to the bit body, which may be a steel bit body or a matrix bit body formed from a matrix material such as tungsten carbide surrounded by a binder material. Drag bits may generally be defined as bits that have no moving parts. However, there are different types and methods of forming drag bits that are known in the art. For example, drag bits having abrasive material, such as diamond, impregnated into the surface of the material which forms the bit body are commonly referred to as “impreg” bits. Drag bits having cutting elements made of an ultra hard cutting surface layer or “table” (which may be made of polycrystalline diamond material or polycrystalline boron nitride material) deposited onto or otherwise bonded to a substrate are known in the art as polycrystalline diamond compact (“PDC”) bits. 
     PDC bits drill soft formations easily, but they are frequently used to drill moderately hard or abrasive formations. They cut rock formations with a shearing action using small cutters that do not penetrate deeply into the formation. Because the penetration depth is shallow, high rates of penetration are achieved through relatively high bit rotational velocities. 
     PDC cutters have been used in industrial applications including rock drilling and metal machining for many years. In PDC bits, PDC cutters are received within cutter pockets, which are formed within blades extending from a bit body, and are generally bonded to the blades by brazing to the inner surfaces of the cutter pockets. The PDC cutters are positioned along the leading edges of the bit body blades so that as the bit body is rotated, the PDC cutters engage and drill the earth formation. In use, high forces may be exerted on the PDC cutters, particularly in the forward-to-rear direction. Additionally, the bit and the PDC cutters may be subjected to substantial abrasive forces. In some instances, impact, vibration, and erosive forces have caused drill bit failure due to loss of one or more cutters, or due to breakage of the blades. 
     In some applications, a compact of polycrystalline diamond (PCD) (or other ultrahard material) is bonded to a substrate material, which may be a sintered metal-carbide to form a cutting structure. PCD includes a polycrystalline mass of diamonds (often synthetic) that are bonded together to form an integral, tough, high-strength mass or lattice. The resulting PCD structure produces enhanced properties of wear resistance and hardness, making PCD materials extremely useful in aggressive wear and cutting applications where high levels of wear resistance and hardness are desired. 
     A PDC cutter may be formed by placing a sintered carbide substrate into the container of a press. A mixture of diamond grains or diamond grains and catalyst binder is placed atop the substrate and treated under high pressure, high temperature conditions. In doing so, metal binder (often cobalt) migrates from the substrate and passes through the diamond grains to promote intergrowth between the diamond grains. As a result, the diamond grains become bonded to each other to form the diamond layer, and the diamond layer is in turn integrally bonded to the substrate. The substrate may be made of a metal-carbide composite material, such as tungsten carbide-cobalt. The deposited diamond layer is often referred to as the “diamond table” or “abrasive layer.” 
     An example of PDC bit having a plurality of cutters with ultra hard working surfaces is shown in  FIGS. 1 and 2 . The drill bit  100  includes a bit body  110  having a threaded upper pin end  111  and a cutting end  115 . The cutting end  115  includes a plurality of ribs or blades  120  arranged about the rotational axis L (also referred to as the longitudinal or central axis) of the drill bit and extending radially outward from the bit body  110 . Cutting elements, or cutters,  150  are embedded in the blades  120  at angular orientations and radial locations relative to a working surface and with a back rake angle and side rake angle against a formation to be drilled. 
     A plurality of orifices  116  are positioned on the bit body  110  in the areas between the blades  120 , which may be referred to as “gaps” or “fluid courses.” The orifices  116  are commonly adapted to accept nozzles. The orifices  116  allow drilling fluid to be discharged through the bit in selected directions and at selected rates of flow between the blades  120  for lubricating and cooling the drill bit  100 , the blades  120  and the cutters  150 . The drilling fluid also cleans and removes the cuttings as the drill bit  100  rotates and penetrates the geological formation. Without proper flow characteristics, insufficient cooling of the cutters  150  may result in cutter failure during drilling operations. The fluid courses are positioned to provide additional flow channels for drilling fluid and to provide a passage for formation cuttings to travel past the drill bit  100  toward the surface of a wellbore. 
     Referring to  FIG. 2 , a top view of a prior art PDC bit is shown. The cutting face  118  of the bit shown includes a plurality of blades  120 , wherein each blade has a leading side  122  facing the direction of bit rotation, a trailing side  124  (opposite from the leading side), and a top side  126 . Each blade includes a plurality of cutting elements or cutters generally disposed radially from the center of cutting face  118  to generally form rows. Certain cutters, although at differing axial positions, may occupy radial positions that are in similar radial position to other cutters on other blades. 
     Cutters may be attached to a drill bit or other downhole tool by a brazing process. In the brazing process, a braze material is positioned between the cutter and the cutter pocket. The material is melted and, upon subsequent solidification, bonds (attaches) the cutter in the cutter pocket. Selection of braze materials depends on their respective melting temperatures, to avoid excessive thermal exposure (and thermal damage) to the diamond layer prior to the bit (and cutter) even being used in a drilling operation. Specifically, alloys suitable for brazing cutting elements with diamond layers thereon have been limited to a couple of alloys which offer relatively low brazing temperatures to avoid or reduce damage to the diamond layer and high enough braze strength to retain cutting elements on drill bits. 
     A factor in determining the longevity of PDC cutters is the exposure of the cutter to heat. Polycrystalline diamond may be stable at temperatures of up to 700-750° C. in air, above which observed increases in temperature may result in damage to and structural failure of polycrystalline diamond. This deterioration in polycrystalline diamond may be due to the substantial difference in the coefficient of thermal expansion of the binder material, cobalt, as compared to diamond. Upon heating of polycrystalline diamond, the cobalt and the diamond lattice will expand at different rates, which may cause cracks to form in the diamond lattice structure and result in deterioration of the polycrystalline diamond. Damage may also be due to graphite formation at diamond-diamond necks leading to loss of microstructural integrity and strength loss, at extremely high temperatures. 
     Exposure to heat (through brazing or through frictional heat generated from the contact of the cutter with the formation) can cause thermal damage to the diamond table and eventually result in the formation of cracks (due to differences in thermal expansion coefficients) which can lead to spalling of the polycrystalline diamond layer, delamination between the polycrystalline diamond and substrate, and conversion of the diamond into graphite, causing rapid abrasive wear. As a cutting element contacts the formation, a wear flat develops and frictional heat is induced. As the cutting element continues to be used, the wear flat will increase in size and further induce frictional heat. The heat may build-up and cause failure of the cutting element due to thermal mis-match between diamond and catalyst discussed above. This is particularly true for cutters that are immovably attached to the drill bit, as conventional in the art. 
     SUMMARY 
     This summary is provided to introduce a selection of concepts that are further described below in the detailed description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter. 
     In one aspect, embodiments disclosed herein relate to a downhole cutting tool that includes a tool body defining a cutter pocket and at least one rolling cutter including an inner rotatable cutting element and a sleeve. Axial movement of the inner rotatable cutting element is limited by an external retention element disposed outside of the sleeve. 
     In another aspect, embodiments disclosed herein relate to a downhole cutting tool that includes a tool body having at least one cutting element support structure formed thereon, the at least one cutting element support structure including at least one cutter pocket formed therein. At least one rolling cutter is in the at least one cutter pocket and includes an inner rotatable cutting element partially disposed in a circumferential sleeve. The inner rotatable cutting element has a back retention portion that extends axially beyond the circumferential sleeve, and the back retention portion has a groove formed therein with a retention element in the groove. 
     In another aspect, embodiments disclosed herein relate to a downhole cutting tool that includes a tool body having at least one cutting element support structure. The at least one cutting element support structure includes at least one cutter pocket in the at least one cutting element support and extending from an opening in a leading face and formation facing surface of the cutting element support to a back face. The at least one cutting element support structure also includes at least one retention opening in the formation facing surface spaced rearward from the opening of the at least one cutter pocket. The at least one retention opening extends into the cutting element support surface to interface the back of the cutter pocket. The tool further includes at least one rolling cutter in the at least one cutter pocket. The rolling cutter is at least partially retained by a retention element in the at least one retention opening. 
     In yet another aspect, embodiments disclosed herein relate to a downhole cutting tool that includes a tool body having at least one cutting element support structure. The at least one cutting element support structure includes at least one cutter pocket. At least one rolling cutter is in the at least one cutter pocket and includes an inner rotatable cutting element. The inner rotatable cutting element has an outermost diameter that extends at least 40% of an axial length of the rotatable cutting element. The rotatable cutting further includes a groove; and a retention element is in the groove, thereby retaining the rotatable cutting element in the cutter pocket. 
     Other aspects and advantages of the claimed subject matter will be apparent from the following description and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  shows a perspective view of a conventional drag bit. 
         FIG. 2  shows a top view of a conventional drag bit. 
         FIG. 3  shows a perspective view of a rolling cutter. 
         FIG. 4  shows a perspective view of a rolling cutter. 
         FIG. 5  shows a perspective view of a retention element for an embodiment of a rolling cutter. 
         FIG. 6  shows a perspective view of a downhole cutting element support structure including a rolling cutter thereon. 
         FIG. 7  shows a cross-sectional view of a downhole cutting element support structure including a rolling cutter thereon. 
         FIG. 8  shows a cross-sectional view of a downhole cutting element support structure including a rolling cutter thereon. 
         FIG. 9  shows a cross-sectional view of a sleeve and cutting element according to one embodiment of the present disclosure. 
         FIG. 10  shows a perspective view of a downhole cutting element support structure including a rolling cutter thereon. 
         FIG. 11  shows a perspective view of a downhole cutting element support structure including a rolling cutter thereon. 
         FIG. 12  shows a cross-sectional view of a downhole cutting element support structure including a rolling cutter thereon. 
         FIG. 13  shows a perspective view of a downhole cutting element support structure including a rolling cutter thereon. 
         FIG. 14  shows a cross-sectional view of a downhole cutting element support structure including a rolling cutter thereon. 
         FIG. 15  shows a cross-sectional view of a downhole cutting element support structure including a rolling cutter thereon. 
         FIG. 16  shows a perspective view of a downhole cutting element support structure including a rolling cutter thereon. 
         FIG. 17  shows a cross-sectional view of a downhole cutting element support structure including a rolling cutter thereon. 
         FIG. 18  shows a perspective view of a downhole cutting element support structure including a rolling cutter thereon. 
         FIG. 19  shows a rotated profile view of a drill bit. 
         FIG. 20  shows a tool that may use the cutting elements of the present disclosure. 
         FIG. 21  shows a cross-sectional view of a downhole cutting element support structure including a rolling cutter thereon. 
         FIG. 22  shows a cross-sectional view of a sleeve. 
         FIG. 23  shows a cross-sectional view of an inner rotatable cutting element. 
         FIG. 24  shows a cross-sectional view of a downhole cutting element support structure including a rolling cutter thereon. 
     
    
    
     DETAILED DESCRIPTION 
     In some aspects, embodiments disclosed herein relate to drill bits and other downhole cutting tools using rotatable cutting structures (rolling cutters) and the retention of such rolling cutters. 
     Generally, rotatable cutting elements (also referred to as rolling cutters) described herein allow at least one surface or portion of the cutting element to rotate as the cutting element contacts a formation. As the cutting element contacts the formation, the cutting action may allow portion of the cutting element to rotate around a cutting element axis extending through the cutting element. Rotation of a portion of the cutting structure may allow for a cutting surface to cut the formation using the entire outer edge of the cutting surface, rather than the same section of the outer edge, as observed in a conventional cutting element. 
     Numerous variations on the cutting element capable of rotating may be used without departing from the scope of the present disclosure. For example, the rotation of the rolling cutter may be controlled by the side cutting force and the frictional force between the bearing surfaces. If the side cutting force generates a torque which can overcome the torque from the frictional force, the rotatable portion will have rotating motion. The side cutting force may be affected by cutter side rake, back rake, and geometry, including the working surface patterns disclosed herein. Additionally, the side cutting force may be affected by the surface finishing of the surfaces of the cutting element components, the frictional properties of the formation, as well as drilling parameters, such as depth of cut. The frictional force at the bearing surfaces may affected, for example, by surface finishing, mud intrusion, etc. The design of the rotatable cutters and the location and orientation of rotatable cutters on the bit disclosed herein may be selected to ensure that the side cutting force overcomes the frictional force to allow for rotation of the rotatable portion. 
     Referring now to  FIG. 3  (a back perspective view) and  FIG. 4  (a front perspective view), one embodiment of a rolling cutter is shown. Each rolling cutter  300  includes an inner rotatable cutting element  310 - 1  at least partially disposed in a sleeve  320 - 1 . Inner rotatable cutting element  310 - 1  extends an axial distance from sleeve  320 - 1 , terminating in a cutting surface  312  that interfaces the formation to be drilled. The cutting extension  314  portion of inner rotatable cutting element  310 - 1  may include an ultrahard material layer and optionally a substrate. As illustrated, the cutting extension portion  314  of inner rotatable cutting element  310 - 1  and sleeve  320 - 1  have substantially the same outer diameter. However, the present disclosure is not so limited, and the inner rotatable cutting element may instead have the same diameter along substantially the entire length of the element (except for grooves for retention) and the sleeve may have a larger diameter and envelop inner rotatable cutting element (except for a cut-away for exposure of the cutting surface along a portion of the circumference adjacent the cutting surface). Such a sleeve  320 - 3  and inner rotatable cutting element  310 - 3  combination is illustrated in  FIG. 9 . 
     Inner rotatable cutting element is retained within sleeve (with limitation on the axial movement thereof) by external retention element  330 . External retention element  330  is disposed at an axially lower position (opposite cutting extension portion  314 ) of sleeve  320 - 1 . Thus, a back retention portion  316  of inner rotatable cutting element  310 - 1  extends axially lower than sleeve  320  to limit the axial movement of inner rotatable cutting element  310 - 1  relative to sleeve  320 - 1 . In limiting the axial movement, inner rotatable cutting elements  310 - 1  may be allowed to move axially back into the sleeve  320 - 1  based on general forces experienced during drilling when weight on bit is applied, but external retention element  330  may keep the inner rotatable cutting element  310 - 1  from falling out of the sleeve  320 - 1 . 
     Referring now to  FIG. 5 , a perspective view of one embodiment of an external retention element  330  is shown. As shown in  FIG. 3 , external retention element  330  is a clamp, such as a c-clamp, that may fit into a groove in an outer circumference of back retention portion  316  of inner rotatable cutting element  310 - 1 . Retention element  330  includes two linear arms  332  extending from an arcuate connecting region  334 ; however, the present disclosure is not so limited, and the arms may be curved, and/or the connecting region may be substantially linear. In one embodiment, retention element  330  may retain inner rotatable cutting element  310 - 1  by the interfacing shapes, while other embodiments may also use compression on inner rotatable cutting element  310 - 1 . The size of retention element  334  may vary, but in some embodiments may be less than the outer diameter of the sleeve  320 - 1 , which may range, for example, from 11-22 mm. Other embodiments, where a greater strength may be desired, may use a retention element that is slightly larger than the sleeve (e.g.,  334  may be larger than an outer diameter of the sleeve), such as 5 mm or less than the sleeve diameter. 
     Inner rotatable cutting element  310 - 1 , once inserted into a sleeve  320 - 1 , together referred to as a rolling cutter  300 , may be disposed on a cutting element support structure  400 , as illustrated in  FIG. 6 . Sleeve  320 - 1  may be attached to cutting element support structure  400  using techniques such as by brazing, infiltration, casting, etc., as well as using mechanical devices, such as screws, a sleeve with threads along an outer diameter, etc. In one or more embodiments, cutting element support structure  400  may be a drill bit (such as that shown in  FIGS. 1 and 2 ) or other downhole cutting tool, e.g., a downhole tool that conventionally uses PDC cutters. 
     Referring now to  FIG. 7 , a cross-sectional view of a rolling cutter installed on a cutting element support structure, the rolling cutter includes an inner rotatable cutting element  310 - 1  partially disposed within sleeve  320 - 1 . The inner rotatable cutting element  310 - 1  includes a cutting extension portion  314  and a back retention portion  316 , each extending axially from sleeve  320 - 1  in opposite directions. Inner cutting element  310 - 1  is illustrated as including an ultrahard material layer  315  (forming part of cutting extension portion  314 ) and substrate  317  (forming part of back retention portion  316 ). A groove  318  is formed in the outer circumference of a portion of substrate  317  along the back retention portion  316 . Outer retention element  330  is disposed within groove  318  to mechanically limit the axial movement of inner rotatable cutting element  310 - 1  (to the extent discussed above). Sleeve  320 - 1  may be retained within a cutter pocket  410  formed in cutting element support structure  400  as described herein. Pocket  410  extends axially longer than sleeve  320 - 1  to accommodate inner rotatable cutting element  310 - 1 . In one or more embodiments, in addition to receiving the back retention portion  316  of inner rotatable cutting element  310 - 1 , the cutting extension portion  314  may also be supported by cutter pocket  410 , i.e., inner rotatable cutting element  310 - 1  does not extend substantially beyond a leading face  402  (in the direction of rotation of the tool) of the cutting element support structure  400  (such as a blade of a drill bit). If a portion of the inner rotatable cutting element extends beyond the leading face  402 , such extension may be less than 0.200 inch, in one or more embodiments. In operation, in one or more embodiments, the sleeve  320 - 1  may be secured to the cutter pocket  410  prior to assembly with the inner rotatable cutting element  310 - 1 , and upon installation of the inner rotatable cutting element  310 - 1  within the sleeve, outer retention element  330  may be inserted to fit within the groove  318 , thereby retaining the inner rotatable cutting element  310 - 1  in such a manner that the element is free to rotate about its axis, but has limited axial and radial movement. Further, the outer retention element  330  may optionally remain accessible and exposed, thereby allowing for replacement of the inner rotatable cutting element  310 - 1 . In one or more embodiments, the sleeve may overlap the external retention element (in that the sleeve has a larger inner diameter than the outer spread of the retention element) through an asymmetrical form at the bottom end thereof (as illustrated in  FIG. 3-4 ) so long as the retention element is accessible and exposed. 
     As shown in the cross sectional view of a rolling cutter installed on a cutting element support structure of  FIG. 8  according to embodiments of the present disclosure, in addition to the outer retention element  330  that is external to the sleeve  320 - 2 , the rolling cutter may also include an internal retention element  340  disposed in grooves formed in the inner rotatable cutting element  310 - 2  and sleeve  320 - 2  at an axial length that is entirely surrounded by sleeve  320  (i.e., it is entirely internal to the rolling cutter). Such internal retention elements may include, for example, retention rings, pins, balls, etc. In one or more particular embodiments, the internal retention element  340  may be a closed loop retention ring, such as the type discussed in U.S. patent application Ser. No. 61/794,580 and U.S. Ser. No. 13/972,465, which is assigned to the present assignee and herein incorporated by reference in its entirety. 
     Referring now to  FIG. 10 , another embodiment of a rotatable cutting element  510  is shown. As illustrated, a rotatable cutting element  510  is disposed in a cutter pocket  610  formed in a cutting element support structure  600 . Cutter pocket  610  opens to the leading face  602  (in the direction of rotation of the cutting tool). In the illustrated embodiment, the back or retention end of inner rotatable cutting element  510  is enveloped by cutter pocket  610 . A retention opening  620  may be formed in the formation-facing surface  615  (top surface that faces the formation when the cutting tool is oriented in a wellbore) spaced rearward of the opening of cutter pocket  610  to the leading face  602 . In particular embodiments, such as the one illustrated, cutter pocket  610  may be open (having an arc length of less than 360 degrees) for a first distance rearward of the leading face  602 . Retention opening  620  is located a second distance rearward from the point at which the cutter pocket transitions to being closed (extending the entire 360 degrees around the inner rotatable cutting element  510 ). Retention opening  620  extends axially inward into the cutting element support structure  600  to intersect the cutter pocket  610  near or adjacent (relative to the cutting pocket opening) to a back face of the cutter pocket  610 . While not shown in this illustration, a retention element may be disposed in the retention opening to also intersect cutter pocket  610 , and to interface an inner rotatable cutting element  510 , thereby retaining inner rotatable cutting element  510  in cutter pocket  610 . Rotatable cutting element  510  may be assembled with a sleeve, similar to those embodiments discussed above, or rotatable cutting element may be retained by a retention element directly without the use of a sleeve. 
     For example, referring now to  FIGS. 11-12 , an embodiment of a rotatable cutter without a sleeve is shown. As shown in  FIGS. 11-12 , a rotatable cutting element  510  is disposed in a cutter pocket  610  formed in cutting element support structure  600 . A retention opening  620  is spaced rearward of a portion of cutter pocket  610  that extends around the rotatable cutting element by 360 degrees. Further, the cross-sectional view in  FIG. 12  shows that cutter pocket  610  includes two diameters, a front diameter proximate the opening of cutter pocket  610  and a second, smaller diameter that is rearward of the point at which the cutter pocket  610  is closed (extends 360 degrees). Retention opening  620  extends from a formation-facing surface  615  of cutting element support structure  600  into structure  600  to intersect with cutter pocket  610  adjacent its back face  612 . A retention element  625 - 1  is disposed in retention opening and interfaces rotatable cutting element  510  around at least a partial circumference of a back retention portion  516  of rotatable cutting element  510 . As illustrated, a groove  618  may be formed in back retention portion  516 , and retention element  625 - 1  may be arranged to at least partially fit within such groove  518 . In addition to groove  618  formed in back retention portion  516 , back retention portion  516  may optionally have a reduced diameter as compared to a cutting portion  514 . As illustrated, retention element  625  is a two-piece circumferential lock  627  with a fastener  629  threaded into a hole  628  formed in lock  627 . Fastener  629  screws into hole  628  to interface groove  618  formed in rotatable cutting element  510 . As illustrated, groove  618  does not need to be circumferential around the entire back retention portion  516 , but can instead be a divot or dimple formed therein of a size large enough to fit fastener  629 . Rotatable cutting element may be retained by being inserted into cutter pocket  610  and through lock  627 . When groove  618  is aligned with hole  628 , a fastener may be inserted and fastened therein. 
     Referring now to  FIGS. 13-15  as well as  FIGS. 16-18 , a rotatable cutting element  510  similar to the one illustrated in  FIGS. 11-12  is disposed in a cutter pocket  610  also similar to the one illustrated in  FIGS. 11-12 , with the exception of retention element  625 - 2  and  625 - 3 . In the embodiment illustrated in  FIGS. 13-15 , retention element  625 - 2  may be a crimp-on retaining clip. Rotatable cutting element may be retained by being inserted into cutter pocket  610  and through the retention element  625 - 2  (in an un-crimped state). When groove  618  is aligned with retention element  625 - 2 , retention element may be crimped from the retention opening  620 . Further, in the embodiment illustrated in  FIGS. 16-18 , retention element  625 - 3  is a retaining ring that may extend less than 360 degrees. After rotatable cutting element  510  is disposed in cutter pocket, the retaining ring retention element  625 - 3  may be inserted into retention opening and fit into groove  618 . In one or more particular embodiments, a retaining ring having a conic cross-section may be used, which may aid in pushing rotatable cutting element  510  into the cutter pocket (and provide a spring effect). In the various embodiments illustrated, the retention opening is shown as having a U-shaped cross-section. Such shape may depend, for example, on the type of retention element being used. Further, in one or more embodiments, after retention element retains a rotatable cutting element, the retention opening may be filled with a filler material to reduce drilling fluid from entering the bearing spaces accessible therefrom. In one or more embodiments, the filler may be optionally be removable so that the retention element (and rotatable cutting element) can be removed and replaced. 
     Various embodiments described above (in  FIGS. 9-17 ) may allow for a largest diameter of the inner rotatable cutting element to represent a relatively large amount of the total axial length of the element. For example, such largest diameter may extend at least 40%, 50%, 60%, 70%, or 80% of the axial length of the inner rotatable cutting element. Such longer axial length is also present in the embodiment shown in  FIG. 21 . As shown in  FIG. 21 , a rotatable cutting element  910 - 1  is disposed in a cutter pocket  1010  formed in cutting element support structure  1000 . Cutter pocket  1010  opens to the leading face  1002  (in the direction of rotation of the cutting tool). The rotatable cutting element  910 - 1  includes a cutting end  912  at the full or largest diameter D 1  of the rotatable cutting element  910 - 1 , and a retention end  914  at a reduced diameter D 2 . The retention end  914  at the reduced diameter D 2  is formed by a spindle  916 - 1 . As illustrated, spindle  916 - 1  is a separate component that is brazed and/or threaded into an opening  915  on the component forming the cutting end  912  (having the full diameter D 1 ). While the embodiment illustrates the use of two components, it is also within the scope of the present disclosure that the cutting end  912  and retention end  914  may be integrally formed from a single piece. However, in embodiments using a separate spindle  916 - 1  component that is attached to the remaining portion of the rotatable cutting element  910 - 1 , use of two components may allow for use of multiple material types to optimize the needs for cutting and retention. Adjacent the back face of the cutting end  912  component of rotatable cutting element  910 - 1  is a sleeve  920 - 1 , in which the distal end of spindle  916 - 1  is inserted. A groove  918  is formed in spindle  916 - 1 , which aligns with a corresponding groove  922  in the inner diameter of sleeve  920 - 1 . A retention element  930  is within the grooves  918 ,  922 , thereby retaining the spindle  916 - 1  of rotatable cutting element  910  within the sleeve. Further, because sleeve  920 - 1  is also brazed or otherwise affixed or retained within cutter pocket  1010 , the retention element  930  also retains the rotatable cutting element  910 - 1  within cutter pocket. 
     According to some embodiments, D 1  extends along at least 40% (or at least 50, or 60%, etc.) of the axial length of rotatable cutting element  910 - 1 , which allows for the longer cutting end  912  having the full D 1  be the load bearing surface during rotation, as compared to a reduced diameter spindle portion  916 - 1 . As mentioned, the spindle  916 - 1  may be formed of a different material, such as a grade of tungsten carbide or a steel that are tougher as compared to the cutting end of rotatable cutting element  910 - 1 . The spindle  916 - 1  may have a reduced diameter D 2  that ranges from 25 to 75% of the full diameter D 1  (with embodiments having a lower limit of any of 25, 40, 50% and an upper limit of any of 40, 50, 60, 75%). Depending on the size of the spindle  916 - 1 , the sleeve  920 - 1  can have the same diameter as D 1  or can have a smaller diameter than D 1 . For example, in  FIG. 24 , the sleeve  920 - 3  can have a smaller diameter than D 1 . Sleeve  920 - 2  may be modified, as shown in  FIG. 22 , to have a tapered inner diameter surface  924  at the proximal end thereof, which, in embodiment using a retention ring such as the type described in U.S. Patent Publication No. 2014/0054094, incorporated by reference in its entirety, the tapered inner diameter surface may allow easier installation/compression of the retention ring as a spindle and retention ring are guided into the sleeve  920 - 2  and groove  922  formed in the sleeve. 
     Further, while  FIG. 21  illustrates a spindle being brazed or threaded into an opening formed in the back end of the full diameter D 1  portion of rotatable cutting element  910 - 1 , as shown in  FIG. 23 , a spindle portion  916 - 2  may be brazed to a planar back end  917  of a rotatable cutting element, without the use of an opening in the back end. As illustrated in  FIG. 23 , the spindle portion  916 - 2  may include a proximal end with a full diameter D 1  that is brazed to the cutting end  912  and a distal end with a reduced diameter D 2  that may be inserted into a sleeve. In some embodiments, the spindle may be integral with the substrate of the rotatable cutting element (e.g., where the spindle is formed at the same time as rest of the cutting element or where a one piece body is formed and the spindle is machined into the retention end of the rotatable cutting element). 
     One or more embodiments described herein may have an ultrahard material disposed on a substrate. Such ultrahard materials may include a conventional polycrystalline diamond table (a table of interconnected diamond particles having interstitial spaces therebetween in which a metal component (such as a metal catalyst) may reside, a thermally stable diamond layer (i.e., having a thermal stability greater than that of conventional polycrystalline diamond, 750° C.) formed, for example, by substantially removing metal from the interstitial spaces between interconnected diamond particles or from a diamond/silicon carbide composite, or other ultrahard material such as a cubic boron nitride. Further, in particular embodiments, the rolling cutter may be formed entirely of ultrahard material(s), but the element may include a plurality of diamond grades used, for example, to form a gradient structure (with a smooth or non-smooth transition between the grades). In a particular embodiment, a first diamond grade having smaller particle sizes and/or a higher diamond density may be used to form the upper portion of the inner rotatable cutting element (that forms the cutting edge when installed on a bit or other tool), while a second diamond grade having larger particle sizes and/or a higher metal content may be used to form the lower, non-cutting portion of the cutting element. Further, it is also within the scope of the present disclosure that more than two diamond grades may be used. 
     Thermally stable diamond may be formed in various manners. A typical polycrystalline diamond layer includes individual diamond “crystals” that are interconnected. The individual diamond crystals thus form a lattice structure. A metal catalyst, such as cobalt, may be used to promote recrystallization of the diamond particles and formation of the lattice structure. Thus, cobalt particles are generally found within the interstitial spaces in the diamond lattice structure. Cobalt has a significantly different coefficient of thermal expansion as compared to diamond. Therefore, upon heating of a diamond table, the cobalt and the diamond lattice will expand at different rates, causing cracks to form in the lattice structure and resulting in deterioration of the diamond table. 
     To obviate this problem, strong acids may be used to “leach” the cobalt from a polycrystalline diamond lattice structure (either a thin volume or entire tablet) to at least reduce the damage experienced from heating diamond-cobalt composite at different rates upon heating. Examples of “leaching” processes can be found, for example, in U.S. Pat. Nos. 4,288,248 and 4,104,344. Briefly, a strong acid, such as hydrofluoric acid or combinations of several strong acids may be used to treat the diamond table, removing at least a portion of the co-catalyst from the PDC composite. Suitable acids include nitric acid, hydrofluoric acid, hydrochloric acid, sulfuric acid, phosphoric acid, or perchloric acid, or combinations of these acids. In addition, caustics, such as sodium hydroxide and potassium hydroxide, have been used to the carbide industry to digest metallic elements from carbide composites. In addition, other acidic and basic leaching agents may be used as desired. Those having ordinary skill in the art will appreciate that the molarity of the leaching agent may be adjusted depending on the time desired to leach, concerns about hazards, etc. 
     By leaching out the cobalt, thermally stable polycrystalline (TSP)diamond may be formed. In certain embodiments, only a select portion of a diamond composite is leached, in order to gain thermal stability without losing impact resistance. As used herein, the term TSP includes both of the above (i.e., partially and completely leached) compounds. Interstitial volumes remaining after leaching may be reduced by either furthering consolidation or by filling the volume with a secondary material, such as described in U.S. Pat. No. 5,127,923, which is herein incorporated by reference in its entirety. 
     In one or more other embodiments, TSP may be formed by forming the diamond layer in a press using a binder other than cobalt, one such as silicon, which has a coefficient of thermal expansion more similar to that of diamond than cobalt has. During the manufacturing process, a large portion, 80 to 100 volume percent, of the silicon reacts with the diamond lattice to form silicon carbide which also has a thermal expansion similar to diamond. Upon heating, any remaining silicon, silicon carbide, and the diamond lattice will expand at more similar rates as compared to rates of expansion for cobalt and diamond, resulting in a more thermally stable layer. PDC cutters having a TSP cutting layer have relatively low wear rates, even as cutter temperatures reach 1200° C. However, thermally stable diamond layer may be formed by other methods, including, for example, by altering processing conditions in the formation of the diamond layer. 
     The substrate on which the cutting face is optionally disposed may be formed of a variety of hard or ultrahard particles. In one embodiment, the substrate may be formed from a suitable material such as tungsten carbide, tantalum carbide, or titanium carbide. Additionally, various binding metals may be included in the substrate, such as cobalt, nickel, iron, metal alloys, or mixtures thereof. In the substrate, the metal carbide grains are supported within the metallic binder, such as cobalt. Additionally, the substrate may be formed of a sintered tungsten carbide composite structure. Various metal carbide compositions and binders may be used, in addition to tungsten carbide and cobalt. Thus, references to the use of tungsten carbide and cobalt are for illustrative purposes, and no limitation on the type of substrate or binder used is intended. In another embodiment, the substrate may also be formed from a diamond ultrahard material such as polycrystalline diamond or thermally stable diamond. While the illustrated embodiments show the cutting face and substrate as two distinct pieces, one of skill in the art should appreciate that it is within the scope of the present disclosure the cutting face and substrate are integral, identical compositions. In such an embodiment, it may be desirable to have a single diamond composite forming the cutting face and substrate or distinct layers. Specifically, in embodiments where the cutting element is a rotatable cutting element, the entire cutting element may be formed from an ultrahard material, including thermally stable diamond (formed, for example, by removing metal from the interstitial regions or by forming a diamond/silicon carbide composite). 
     The retention element may be formed from a variety of materials. In one embodiment, the retention element may be formed of a suitable material such as tungsten carbide, tantalum carbide, or titanium carbide. Additionally, various binding metals may be included in the outer support element, such as cobalt, nickel, iron, metal alloys, or mixtures thereof, such that the metal carbide grains are supported within the metallic binder. It is also within the scope of the present disclosure that the retention element and/or substrate may also include one or more lubricious materials, such as diamond to reduce the coefficient of friction therebetween. The components may be formed of such materials in their entirely or portions of the components may include such lubricious materials deposited on the component, such as by chemical plating, chemical vapor deposition (CVD) including hollow cathode plasma enhanced CVD, physical vapor deposition, vacuum deposition, arc processes, or high velocity sprays. In a particular embodiment, a diamond-like coating may be deposited through CVD or hallow cathode plasma enhanced CVD, such as the type of coatings disclosed in US 2010/0108403, which is assigned to the present assignee and herein incorporated by reference in its entirety. 
     In other embodiments, the retention elements may be formed of tool steel or other alloy steels, nickel-based alloys, or cobalt-based alloys. One or more components may be coated with a hardfacing material for increased erosion protection. Such coatings may be applied by various techniques known in the art such as, for example, detonation gun (d-gun) or spray-and-fuse techniques. 
     The cutting elements of the present disclosure may be incorporated in various types of cutting tools, including for example, fixed cutter bits or hole enlargement tools such as reamers. Bits having the cutting elements of the present disclosure may include a single rolling cutter with the remaining cutting elements being conventional fixed cutting elements, all cutting elements being rotatable, or any combination therebetween of rolling cutters and conventional, fixed cutters. Further, cutting elements of the present disclosure may be disposed on cutting tool blades (such as drag bit blades or reamer blades) having other wear elements incorporated therein. For example, cutting elements of the present disclosure may be disposed on diamond impregnated blades. Additionally, any size cutting elements may be used. For example, in various embodiments, the cutting elements may be formed in sizes including, but not limited to, 9 mm, 11 mm, 13 mm, 16 mm, and 19 mm. 
     Further, any of the design modifications as described above, including, for example, side rake, back rake, variations in geometry, surface alteration/etching, seals, bearings, material compositions, diamond or similar low-friction bearing surfaces, etc., may be included in various combinations not limited to those described above in the cutting elements of the present disclosure. In one embodiment, a cutter may have a side rake ranging from 0 to ±45 degrees. In another embodiment, a cutter may have a back rake ranging from about 5 to 35 degrees. 
     In one or more embodiments, rolling cutters may be disposed in locations of the bit or other tool experiencing the greatest wear, such as the nose or shoulder of the bit. Referring now to  FIG. 19 , a profile of bit  10  is shown as it would appear with all blades and cutting faces of all cutting elements (including both fixed cutters such as those referenced as  150  in  FIG. 1  and rolling cutters such as those referenced as  300  in  FIG. 3 ) rotated into a single rotated profile when rotated  18  about axis  60 . In rotated profile view, blade tops of all blades of bit form and define a combined or composite blade profile  39  that extends radially from bit axis  60  to outer radius  23  of bit  10 . Thus, as used herein, the phrase “composite blade profile” refers to the profile, extending from the bit axis to the outer radius of the bit, formed by the blade tops of all the blades of a bit rotated into a single rotated profile (i.e., in rotated profile view). 
     Composite blade profile  39  (most clearly shown in the right half of bit  10  in  FIG. 19 ) may generally be divided into three regions conventionally labeled cone region  24 , shoulder region  25 , and gage region  26 . Cone region  24  includes the radially innermost region of bit  10  and composite blade profile  39  extending generally from bit axis  60  to shoulder region  25 . As shown in  FIG. 19 , in most conventional fixed cutter bits, cone region  24  is generally concave. Adjacent cone region  24  is shoulder (or the upturned curve) region  25 . In most conventional fixed cutter bits, shoulder region  25  is generally convex. Moving radially outward, adjacent shoulder region  25  is the gage region  26  which extends parallel to bit axis  60  at the outer radial periphery of composite blade profile  39 . Thus, composite blade profile  39  of bit  10  includes one concave region—cone region  24 , and one convex region—shoulder region  25 . 
     The axially lowermost point of convex shoulder region  25  and composite blade profile  39  defines a blade profile nose  27 . At blade profile nose  27 , the slope of a tangent line  27   a  to convex shoulder region  25  and composite blade profile  39  is zero. Thus, as used herein, the term “blade profile nose” refers to the point along a convex region of a composite blade profile of a bit in rotated profile view at which the slope of a tangent to the composite blade profile is zero. For most conventional fixed cutter bits (e.g., bit  10 ), the composite blade profile includes only one convex shoulder region (e.g., convex shoulder region  25 ), and only one blade profile nose (e.g., nose  27 ). In one or more embodiments, rolling cutters of the present disclosure may be located in the nose and/or shoulder region of the cutting profile, and fixed cutters may be located in the cone and/or gage of the cutting profile. In other embodiments, the rolling cutters may also be disposed in the cone and/or gage of the cutting profile. For example, in one or more embodiments, rolling cutters  300  are located in at least some of the nose and shoulder regions of the blades, while fixed cutters  150  are located in the cone and gage regions of the blade. It is also within the scope of the present disclosure that the nose and shoulder may also include fixed cutters as either primary or back-up cutting elements. 
     As described throughout the present disclosure, the cutting elements may be used on any downhole cutting tool, including, for example, a fixed cutter drill bit or hole opener.  FIG. 20  shows a general configuration of a hole opener  830  that includes one or more cutting elements of the present disclosure. The hole opener  830  includes a tool body  832 , a plurality of blades  838  disposed at selected azimuthal locations about a circumference thereof, and a plurality of cutting elements  840  on the blades  838 . The hole opener  830  generally includes connections  834 ,  836  (e.g., threaded connections) so that the hole opener  830  may be coupled to adjacent drilling tools that include, for example, a drillstring and/or bottom hole assembly (BHA) (not shown). The tool body  832  generally includes a bore therethrough (along axis  837 ) so that drilling fluid may flow through the hole opener  830  as it is pumped from the surface (e.g., from surface mud pumps (not shown)) to a bottom of the wellbore (not shown). The tool body  832  may be formed from steel or from other materials. For example, the tool body  832  may also be formed from a matrix material infiltrated with a binder alloy. The blades  838  shown in  FIG. 20  are spiral blades and are generally positioned at substantially equal angular intervals about the perimeter of the tool body. This arrangement is not a limitation on the scope of the disclosure, but rather is used merely to illustrative purposes. Any downhole cutting tool may be used. While  FIG. 20  does not detail the location of the rolling cutters, their placement on the tool may be according to any of the variations described above. 
     Although just a few embodiments have been described in detail above, those skilled in the art will appreciate that many modifications are possible in the example embodiments without materially departing from the apparatus, systems, and methods disclosed herein. Accordingly, such modifications are intended to be included within the scope of this disclosure. Additionally, it should be understood that references to “one embodiment” or “an embodiment” of the present disclosure are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. For example, any element described in relation to an embodiment herein may be combinable with any element of any other embodiment described herein. 
     In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not just structural equivalents, but also equivalent structures. Thus, although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface, in the environment of fastening wooden parts, a nail and a screw may be equivalent structures. It is the express intention of the applicant not to invoke means-plus-function for any limitations of any of the claims herein, except for those in which the claim expressly uses the words ‘means for’ together with an associated function. Each addition, deletion, and modification to the embodiments that fall within the meaning and scope of the claims is to be embraced by the claims.