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BACKGROUND 
   1. Field of the Disclosure 
   Embodiments disclosed herein relate generally to apparatus and methods involved in cutting tools for oilfield applications. More particularly, embodiments disclosed herein relate to apparatus and methods for wear indicators on an expandable underreamer. 
   2. Background Art 
   In the drilling of oil and gas wells, typically concentric casing strings are installed and cemented in the borehole as drilling progresses to increasing depths. Each new casing string is supported within the previously installed casing string, thereby limiting the annular area available for the cementing operation. Further, as successively smaller diameter casing strings are suspended, the flow area for the production of oil and gas is reduced. Therefore, to increase the annular space for the cementing operation, and to increase the production flow area, it is often desirable to enlarge the borehole below the terminal end of the previously cased borehole. By enlarging the borehole, a larger annular area is provided for subsequently installing and cementing a larger casing string than would have been possible otherwise. Accordingly, by enlarging the borehole below the previously cased borehole, the bottom of the formation can be reached with comparatively larger diameter casing, thereby providing more flow area for the production of oil and gas. 
   Various methods have been devised for passing a drilling assembly through a cased borehole, or in conjunction with expandable casing to enlarging the borehole. One such method involves the use of an underreamer, which has basically two operative states —a closed or collapsed state, where the diameter of the tool is sufficiently small to allow the tool to pass through the existing cased borehole, and an open or partly expanded state, where one or more arms with cutters on the ends thereof extend from the body of the tool. In this latter position, the underreamer enlarges the borehole diameter as the tool is rotated and lowered in the borehole. 
   A “drilling type” underreamer is one that is typically used in conjunction with a conventional “pilot” drill bit positioned below (i.e. downstream of) the underreamer. Typically, the pilot bit drills the borehole to a reduced gauge, while the underreamer, positioned behind the pilot bit, simultaneously enlarges the pilot borehole to full gauge. Furthermore, it is conventional to employ a tool known as a “stabilizer” in drilling operations. In standard boreholes, traditional stabilizers are located in the drilling assembly behind the drill bit to control and maintain the trajectory of the drill bit as drilling progresses. 
   In a conventional rotary drilling assembly, a drill bit may be mounted onto a lower stabilizer, which may be disposed approximately 5 or more feet above the bit. Typically the lower stabilizer is a fixed blade stabilizer and includes a plurality of concentric blades extending radially outwardly and azimuthally spaced around the circumference of the stabilizer housing. The outer edges of the blades are adapted to contact the wall of the existing cased borehole, thereby defining the maximum stabilizer diameter that will pass through the casing. A plurality of drill collars extends between the lower and other stabilizers in the drilling assembly. An upper stabilizer is typically positioned in the drill string approximately 30-60 feet above the lower stabilizer. 
   A typical drilling apparatus  10  is shown in  FIGS. 1A and 1B . Drilling apparatus includes a drill bit  20  disposed on the distal end of a drillstring  15 , an expandable lower stabilizer/underreamer assembly  30 , a drill collar  40 , and an upper stabilizer  50 .  FIG. 1B  shows expandable underreamer  30  which includes cutting elements  32  and a stabilizer pad  34 . Expandable underreamer  30  is configured to travel along grooves  36  during expansion or retraction of the arms. 
   During underreaming operations, the cutting elements of the underreamer may wear down from the continued contact with the formation to a point at which they are no longer effective. In this condition, the cutting elements do not properly remove material from the formation, and thus affecting the gauge diameter of the bore by reducing the diameter at which it is being underreamed. This may lead to further problems when the casing is run into the bore and the gauge diameter is too small. Accordingly, there exists a need for an apparatus and/or a method to indicate to an operator when the cutting elements are no longer effectively underreaming the bore to the proper gauge diameter. 
   SUMMARY OF THE DISCLOSURE 
   In one aspect, embodiments disclosed herein relate to an expandable underreamer apparatus disposed on a distal end of a drillstring and configured to drill a formation, the expandable drilling apparatus including a substantially tubular main body adjacent a cutting head, the main body providing at least one axial recess configured to receive an arm assembly, and the arm assembly configured to translate between a retracted position and an extended position. The apparatus further includes a plurality of cutting elements disposed on the arm assembly, and a plurality of wear indicators imbedded in at least one arm assembly and positioned behind the cutting elements with respect to a direction of rotation of the arm assembly, wherein the wear indicators are configured to provide an increased torque to the drillstring upon contacting the formation. 
   In another aspect, embodiments disclosed herein relate to a method to indicate the wear of cutting elements of an expandable underreamer apparatus disposed on a drillstring, the method including running the expandable underreamer apparatus into a formation and contacting a plurality of cutting elements disposed on the underreamer with the formation. The method further includes providing a plurality of wear indicators on at least one arm assembly of the underreamer, the wear indicators configured to contact the formation when the cutting elements are worn to a specific height, monitoring a torque increase of the drillstring, wherein the torque increase is caused by contact of the wear indicators with the formation, and removing the expandable underreamer from the formation when the torque increases by a specified amount. 
   Other aspects and advantages of the invention will be apparent from the following description and the appended claims. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIGS. 1A and 1B  show a drillstring with an underreamer in accordance with prior art. 
       FIGS. 2A and 2B  show an underreamer in a retracted and expanded position in accordance with embodiments of the present disclosure. 
       FIG. 3  shows an arm assembly of an underreamer with wear indicators in accordance with embodiments of the present disclosure 
       FIG. 4  shows an arm assembly of an underreamer which includes a central stabilizer portion and with wear indicators in accordance with alternative embodiments of the present disclosure. 
   

   DETAILED DESCRIPTION 
   In one aspect, embodiments disclosed herein relate to apparatus and methods for indicating wear of the cutting elements during underreaming operations. 
   Referring to  FIGS. 2A and 2B , a section view of a lower end of a drilling assembly  100  is shown in accordance with embodiments of the present disclosure. Drilling assembly  100  is shown having a substantially tubular main body  110  having a central axis  111 , a cutting head  120 , and an expandable underreamer  130 . Cutting head  120  includes a plurality of cutting elements, or polycrystalline diamond compact (“PDC”) cutters  122 . Main body  110  of drilling assembly  100  includes a plurality of axial recesses  112  in which arm assemblies  132  of underreamer  130  are located. Arm assemblies  132  include cutting elements  134 , and in certain embodiments, also include stabilizer pads  136 . 
   Arm assemblies  132  travel from their retracted position ( FIG. 2A ) to their extended position ( FIG. 2B ) along a plurality of grooves  114  within the wall of axial recesses  112 . Corresponding grooves (not shown) of arm assemblies  132  engage grooves  114  and guide arm assemblies  112  as they traverse in and out of axial recesses  112 . One of ordinary skill in the art will understand that any number of arm assemblies  132  may be employed, from a single arm assembly  132  to as many arm assemblies  132  as the size and geometry of main body  110  may accommodate. Furthermore, while each arm assembly  132  is depicted with both stabilizer pads  136  and cutting elements  134 , it should be understood that arm assemblies  132  may include stabilizer pads  136 , cutting elements  134 , or a combination thereof in any proportion appropriate for the type of operation to be performed. 
   During drilling operations, cutting head  120  is designed and sized to cut a pilot bore, or a bore that is large enough to allow drilling assembly  100  in its retracted state ( FIG. 1 ) and remaining components of the drillstring to pass therethrough. In circumstances where the borehole is to be extended below a string of casing, the geometry and size of cutting structure  120  and main body  110  is such that entire drilling assembly  100  may pass clear of the casing string without becoming stuck. Once clear of the casing string, or when a larger diameter borehole is desired, arm assemblies  132  are extended and cutting elements  134  disposed thereupon (in conjunction with stabilizer pads  136 ) underream the pilot bore to the final gauge diameter. 
   Referring now to  FIG. 3 , a component view of an arm assembly  332  of an underreamer  330  is shown in accordance with embodiments of the present disclosure. Arm assembly  332  includes grooves  334  configured to engage grooves ( 114  in  FIGS. 2A and 2B ) when extending out of or retracting into axial recesses ( 112  in  FIGS. 2A and 2B ). Arm assembly also includes cutting elements  336  which are configured to underream the formation when arm assembly  332  is in the expanded position ( FIG. 2B ). In certain embodiments, arm assembly  332  may be coated with a high velocity oxygen fuel (“HVOF”), a thermal spray which adds increased hardness to the surface. 
   Cutting elements  336  are positioned along a length of arm assembly  332  and are configured as the “primary” cutting structures of underreamer  330 . They may be mounted in arm assembly  332  by welding, brazing, press fit, or other mounting methods known to those skilled in the art. Further, certain cutting elements  336  which are positioned at the gauge diameter section of arm assembly  332  may be configured with “pre-flats”  337  to provide a quicker cut of the formation. Pre-flats  337  may be machined onto cutting elements  336  prior to mounting cutting elements  336  on arm assembly  332 , or alternatively, may be machined after cutting elements  336  are mounted to provide “even” pre-flats  337  on cutting elements  336  as they are mounted on arm assembly  332 . 
   Arm assembly  332  further includes wear indicators  338  which are lined along the length of arm assembly  332 . Wear indicators  338  may be attached to arm assembly  332  by brazing, adhesives, or other attachment methods known to those skilled in the art. Wear indicators  338  may be diamond enhanced wear studs with PDC inserts, tungsten carbide inserts (“TCIs”), grit hot pressed inserts (“GHIs”), or other insert materials known to a person skilled in the art. Wear indicators  338  may be configured in a dome-shape or other appropriate shapes known to a person skilled in the art. 
   Wear indicators  338  are positioned “behind” cutting elements  336 ; that is, during rotation of underreamer  330 , a cutting element  336  will contact the formation ahead of a wear indicator  338 . Wear indicators  338  are also positioned at a smaller gauge diameter on each arm assembly  332  of underreamer  330  than cutting elements  336 , and thus are positioned at a lower “height” than cutting elements  336 . Height refers to the distance between the surface of arm assembly  332  and an upper surface of cutting element  336  or wear indicator  338 . The cutting element gauge diameter and the wear indicator gauge diameter may be defined as the diameter formed by the rotation of cutting elements  336  and wear indicators  338 , respectively, in the borehole. In certain embodiments, wear indicators  338  are set below cutting elements  336  by about 25% to 30% of the initial height of the cutting elements  336 . The initial height of cutting elements  336  may be defined as the height of cutting elements  336  when they are first installed on arm assembly  332  and before they have been put into use. Wear indicators  338  may be set at this particular height (25-30% below cutting elements) due to the ineffectiveness of cutting elements  336  once they are worn down 25% to 30% of their initial height, as will be known to those skilled in the art. 
   For example, in this case, cutting elements  336  must wear down to a height that is 25% to 30% of their initial height before wear indicators  338  may come into contact with the formation. Because wear indicators  338  are set lower than cutting elements  336 , initially during operation, wear indicators  338  should not be in contact with formation. As the underreaming operation continues, cutting elements  336  may wear down to a height, or to a gauge diameter equal to the wear indicator gauge diameter. When cutting elements  336  are worn down to substantially the same gauge diameter as wear indicators  338 , wear indicators  338  may then contact the formation. 
   In certain embodiments, different cutting elements  336  may be used on underreamer  330  which may exhibit varied wear characteristics. For example, various cutting elements  336  may have different profiles or may be manufactured from different materials, and therefore may have different wear rates. As such, wear indicators  338  may need to be positioned according to the wear rate of the selected cutting elements  336  in order to indicate wear of cutting elements  336 . A person skilled in the art will understand the wear characteristics of different cutting elements  336 , and be able to position wear indicators  338  at an appropriate height below cutting elements  336 . 
   In operation, cutting elements  336  of underreamer  330  contact the formation being drilled to remove material and enlarge the borehole. Due to continued contact with the formation, cutting elements  336  may gradually wear down to a point at which they are no longer effective in underreaming the borehole. Wear indicators  338  are configured to contact the formation when cutting elements  336  have worn down to a specified height. Thus, the specified height to which cutting elements  336  may wear down to corresponds to a height at which cutting elements  336  are no longer effective in properly underreaming the borehole Wear indicators  338  are configured to provide an increased torque to the drillstring after contacting the formation. The increased torque on the drillstring is caused by the increased area provided by wear indicators  338  that is now in contact with the formation. 
   In certain embodiments, wear indicators  338  may provide an increase in torque of about 20-25% of normal operating torque experienced by the drillstring. Initially, only a slight increase in torque may be experienced as cutting elements  336  engage the formation, followed by greater increases in the torque as the wear of cutting elements  336  continues. Those skilled in the art will recognize the amount of increased torque on the drillstring indicating wear of cutting elements  336 . 
   The amount of torque increase experienced by the drillstring to indicate the drillstring should be removed from the borehole may be calibrated to be large enough so as not to be affected by smaller torque increases. For example, a smaller 5% or 10% increase in the torque experienced by the drillstring will not indicate the time for removal of the underreamer from the borehole. However, a 20-25% torque increase as mentioned above, is a large enough torque increase to inform the operator that wear indicators  338  are in full contact with the formation, and the drillstring should be removed to replace worn arm assemblies of the underreamer with new arm assemblies. 
   The drillstring torque may be monitored by an operator to indicate an increase in the torque on the drillstring. The operator may then stop operation and remove the drillstring from the borehole upon receiving an indication of the torque increase. Torque monitoring of the drillstring is common in drilling and/or underreaming operations, and therefore should be well understood by those skilled in the art. Further, various torque measurement and monitoring devices may be used, as well as various indicators which show an increased torque. Torque indicators may include an analog gauge, a digital gauge, a warning light indicator, sound indicator, or other devices as will be known to those skilled in the art. 
   In certain embodiments, wear indicators  338  may be configured as “secondary” cutting elements on underreamer  330 . After primary cutting elements  336  have worn to a specified height, and wear indicators  338  have contacted the formation, an increased torque may be applied to the drillstring as previously described. Further, wear indicators  338  may provide some cutting action on the formation in an attempt to aid primary cutting elements  336  until underreamer  330  may be removed from the borehole. Still further, in certain embodiments, wear indicators  338  may be mounted on only certain arm assemblies  332  of underreamer  330 . Wear indicators  338  may be mounted on opposite arm assemblies  332  as arranged on underreamer  330  to promote balance of underreamer  330  during operation and prevent undesirable effects such as bit whirl. 
   Referring now to  FIG. 4 , a component view of an arm assembly  332  of an underreamer  330  is shown in accordance with alternate embodiments of the present disclosure. Arm assembly  332  includes cutting elements  336  which are positioned along arm assembly  332 , and wear indicators  338  which are positioned behind cutting elements  336 . As previously mentioned, certain underreamers  330  may also include a stabilizer portion  340  which is used to help control and maintain the trajectory of the drill bit as drilling and/or underreaming progresses. Stabilizer portion  340  is configured at a lower height, or gauge diameter, than cutting elements  336  and has wear indicators  338  disposed thereon. Thus, when cutting elements  336  have worn down to the height of stabilizer portion  340 , wear indicators  338  on stabilizer portion  340  may provide increased torque along with wear indicators  338  behind cutting elements  336 . Further, wear indicators  338  located on stabilizer portion  340  may be configured to help reduce wear and erosion of stabilizer pad  340  during underreaming operations. 
   As previously described, wear indicators  338  are set below cutting elements  336  by about 25% to 30% of the initial height of the cutting elements  336 . Therefore, cutting elements  336  must wear down to a height that is 25% to 30% of their initial height before wear indicators  338  may come into contact with the formation. Also, as previously described, cutting elements  336  may wear down to a specified height, at which point, wear indicators  338  may contact the formation and provide a torque increase to the drillstring, which is indicated to an operator. Wear indicators  338  may provide an increase in torque of about 20-25% of normal operating torque experienced by the drillstring. 
   Advantageously, embodiments of the present disclosure may help to provide indication that the cutting elements have worn down to a specified height and are no longer effectively underreaming the borehole. Without a wear indicator feature, it may be difficult for an operator to know when the underreamer is not properly enlarging the borehole to a fall gauge diameter. Oftentimes, the underreamer may be pulled from the borehole, after which it may be found that for some distance the borehole was not properly underreamed due to worn cutting elements. The underreamer may then be replaced and required to underream sections of the borehole that were not underreamed to a proper gauge diameter. This requires extra drilling time to go back in and properly underream the borehole. 
   Embodiments disclosed herein provide underreamers with wear indicators, such that only one drilling pass may be required and the borehole is underreamed properly the first time. At the first indication of overly worn cutting elements, the drillstring may be removed from the borehole and the arm assemblies of the underreamer replaced. This may lead to savings in drilling time, rig costs, and equipment costs. 
   While the present disclosure has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments may be devised which do not depart from the scope of the disclosure as described herein. Accordingly, the scope of the disclosure should be limited only by the attached claims.

Summary:
An expandable underreamer apparatus disposed on a distal end of a drillstring and configured to drill a formation includes a substantially tubular main body adjacent a cutting head, the main body providing at least one axial recess configured to receive an arm assembly, and the arm assembly configured to translate between a retracted position and an extended position. The apparatus further includes a plurality of cutting elements disposed on the arm assembly, and a plurality of wear indicators imbedded in at least one arm assembly and positioned behind the cutting elements with respect to a direction of rotation of the arm assembly, wherein the wear indicators are configured to provide an increased torque to the drillstring upon contacting the formation.