Abstract:
A downhole abrading tool having a body with a first end for connection to a drill string, a cutting end, a drilling fluid passageway, a restrictor disposed within drilling fluid passageway, and an indicator chamber is disclosed. Upon exposure of the indicator chamber to a well environment due to excessive wear on the cutting end, the restrictor is actuated. Actuation of the restrictor restricts the flow of drilling fluid from the drilling fluid passageway into the well environment. The restriction of flow of drilling fluid from the drilling fluid passageway causes a pressure increase in the drilling fluid flowing through the well that can detected by an operator of the downhole abrading tool. The pressure increase provides an indication to the operator of excessive wear on the cutting end of the downhole abrading tool so that the downhole abrading tool can be removed from the well and replaced.

Description:
BACKGROUND 
       [0001]    1. Field of Invention 
         [0002]    The invention is directed to downhole abrading tools utilized in oil and gas wells to abrade objects within the well and, in particular, to downhole mills that are used to abrade, among other objects, stuck tools, bridge plugs, well tubing, and well casing disposed within the well. 
         [0003]    2. Description of Art 
         [0004]    In the drilling, completion, and workover of oil and gas wells, it is common to perform work downhole in the well bore with a tool which has some sort of wearable working profile interfacing with a downhole structure. Examples would be milling a downhole metal object with a milling tool, performing a washover operation with a rotary shoe, cutting through a tubular with a cutting or milling tool, or drilling through formation with a drill bit. During the performance of these operations, it is common for the working profile of the tool, such as the cutting elements mounted on its lower or outer face, to wear away. As this wear progresses, the effectiveness of the tool decreases. 
         [0005]    It is desirable to pull the tool from the well and replace it, when the working profile has experienced a given amount of wear. The degree of wear at which it is desirable to replace the tool depends upon the type of tool and the operation being performed. Unfortunately, it is difficult or even impossible for the well operator at the surface of the well to know accurately when this given amount of wear has occurred. Often, the decision as to when to pull the tool depends substantially upon the experience of the operator. That is, the operator must estimate the amount of tool wear based on whatever is known about the time the operation has been underway, the weight on the tool, the type of downhole structure being worked, the cuttings found in the drilling fluid, or a gradual change in work string torque. None of these parameters provides a definitive indication that the wear in the working profile has progressed to a specific degree at which the operator desires to pull the tool from the well. Pulling a tool prematurely adds unnecessary trips out of the well, adding to rig time and increased costs. Pulling the tool too late gradually decreases the effectiveness of the downhole operation, also adding to rig time and increasing the cost of the operation. 
         [0006]    Accordingly, downhole abrading tools and methods of indicating to an operator of a downhole abrading tool of excessive wear on a cutting end of the downhole abrading tool have been desired in the art. As discussed herein, the present downhole abrading tools and methods of indicating to an operator of a downhole abrading tool of excessive wear on the cutting end of a downhole abrading tool effectively and efficiently identify excessive wear on the downhole abrading tool. Therefore, the operator of the downhole abrading tool is informed of when the downhole abrading tool should be removed from the well and replaced. 
       SUMMARY OF INVENTION 
       [0007]    Broadly, the invention is directed to downhole abrading tools utilized in cutting or abrading objects disposed within the well. The term “object” encompasses any physical structure that may be disposed within a well, for example, another tool that is stuck within the well, a bridge plug, the well tubing, or the well casing. 
         [0008]    The downhole abrading tools of the invention include a valve disposed within the drilling fluid passageway of the downhole abrading tool. Disposed below the valve, and within the cutting end of the downhole abrading tool is an indicator chamber. The indicator chamber is in fluid communication with the drilling fluid passageway. Initially, the valve permits drilling fluid to flow through the drilling fluid passageway, into outlets, and out of the downhole abrading tool to facilitate abrading of the object. As the downhole abrading tool abrades the object in the well, the cutting end is worn away. Upon excessive wear on the cutting end, the indicator chamber is exposed to the well environment creating a pressure differential between the pressure in the indicator chamber and the well environment. As a result of this pressure differential, the valve is actuated such that the flow of the drilling fluid through the outlets from the drilling fluid passageway is restricted. Due to the decrease in volume of drilling fluid flowing through the downhole abrading tool, the pressure of the drilling fluid, being monitored by the operator at the surface, noticeably increases to indicate to the operator that the downhole abrading tool has experienced excessive wear and should be replaced. 
         [0009]    In accordance with the invention, the foregoing advantages have been achieved through the present downhole abrading tool. The downhole abrading comprises a body for connection to a drill string and having a drilling fluid passageway therethrough with an outlet for the passage of a drilling fluid having a drilling fluid pressure; a cutting end on the body for rotation against an object in the well; a chamber having a wear-away portion that is recessed within the cutting end, the wear-away portion wearing away and exposing the chamber to a wellbore pressure when the cutting end wears a selected amount; and a movable restrictor in fluid communication with the chamber and the drilling fluid passageway, for movement from a first position to a second position, creating a pressure increase in the drilling fluid passageway when the chamber is exposed to the wellbore pressure. 
         [0010]    A further feature of the downhole abrading tool is that the restrictor may comprise a piston slidably carried within the drilling fluid passageway. Another feature of the downhole abrading tool is that the restrictor may comprise a differential area piston. An additional feature of the downhole abrading tool is that the restrictor may comprise a piston having a first pressure area in fluid communication with the chamber; and a second pressure area in fluid communication with the drilling fluid passageway, the first pressure area being greater than the second pressure area. Still another feature of the downhole abrading tool is that while in the second position, the restrictor may block at least a portion of the drilling fluid flowing through the outlet. A further feature of the downhole abrading tool is that the outlet may intersect the drilling fluid passageway at a port and, while in the first position, the restrictor is spaced below the port in the drilling fluid passageway and in the second position the restrictor at least partially blocks the port. Another feature of the downhole abrading tool is that the restrictor may comprise a movable piston that moves axially when the wear-away portion wears away and the chamber is exposed to the wellbore pressure, the movable piston having a closed first side exposed to atmospheric pressure in the chamber and a bore containing a fixed piston, the bore defining an annular second side exposed to the drilling fluid pressure in the drilling fluid passageway, so that when the first side is exposed to wellbore pressure via the chamber, the movable piston slides upward relative to the fixed piston to the second position. An additional feature of the downhole abrading tool is that the downhole tool may further comprise a rod extending downward from the fixed piston sealingly through closed first side of movable piston, the rod being stationary carried by the body. Still another feature of the downhole abrading tool is that the fixed piston may limit movement of the movable piston to the second position. A further feature of the downhole abrading tool is that the wear away portion may include at least one taggant that is releasable from the cutting end as the wear away portion is worn away. Another feature of the downhole abrading tool is that the taggant may be selected from the group consisting of a radio-frequency tag, a colored dye, a radioactive material, a florescent material, a pellet, each of the at least one pellets having an outer shell encasing a core, the outer shell being dissolvable in the drilling fluid and the core being an expandable material, and mixtures thereof. An additional feature of the downhole abrading tool is that the downhole abrading tool further may further comprise a partition disposed within the drilling fluid passageway; a sleeve disposed on an exterior surface of the body, the sleeve and the body defining an annular cavity, the restrictor being a piston disposed within the cavity, the chamber being in fluid communication with a lower end of the piston; an upper port in the body above the partition leading to leading from the drilling fluid passageway to the cavity; and a lower port in the body below the partition leading from the drilling fluid passageway to the cavity, wherein, while in the first position, the piston allows unrestricted flow of drilling fluid from the drilling fluid passageway, through the upper port and the lower port to the outlet and, in the second position, the piston at least partially blocks the lower port. 
         [0011]    In accordance with the invention, the foregoing advantages also have been achieved through a downhole abrading tool for rotatably abrading an object in a well in which the well has a surface location and a downhole location. In this embodiment, the downhole abrading tool comprises a body having a first end, a cutting end, and a drilling fluid passageway extending through the body and having an outlet for discharging a drilling fluid having a drilling fluid pressure, the first end adapted for being connected to a portion of a string for rotation, and the cutting end containing an abrading material for rotatably engaging the object in the well; a differential area piston assembly having a first pressure area in fluid communication with the drilling fluid passageway and a second pressure area that is larger than the first pressure area; and a chamber adjacent the cutting end and in fluid communication with the second pressure area of the differential area piston assembly, such that when the chamber becomes in fluid communication with a wellbore pressure due to wear of the abrading material, the wellbore pressure acts on the second pressure area to cause the differential area piston assembly to move to a position at least partially restricting the flow of drilling fluid, providing a pressure increase indication at the surface location. 
         [0012]    A further feature of the downhole abrading tool is that the differential area piston assembly may be carried within the drilling fluid passageway. Another feature of the downhole abrading tool is that the outlet may intersect the drilling fluid passageway at a port and wherein the differential area piston assembly may at least partially block the port when the chamber becomes exposed to wellbore pressure. An additional feature of the downhole abrading tool is that the differential area piston assembly may comprise a movable piston having a closed lower end that defines the second pressure area, a bore, and a fixed piston sealingly carried in the bore and secured stationary to the body, wherein the first pressure area comprises an annular upper end surrounding the bore. Still another feature of the downhole abrading tool is that the downhole abrading tool may further comprise a plug in a lower end of the drilling fluid passageway; and a rod secured to the fixed piston extending sealingly through the closed lower end of the movable piston and secured to the plug. A further feature of the downhole abrading tool is that the abrading material may include at least one taggant that is releasable from the cutting end as the abrading material is worn away. 
         [0013]    In accordance with the invention, the foregoing advantages also have been achieved through the present method of indicating wear of a downhole abrading tool. The method comprises the steps of: (a) providing a chamber with a wear-away portion that is recessed within the cutting end; (b) providing a restrictor in fluid communication with the chamber and the drilling fluid passageway; (c) during operation, causing the wear-away portion to wear away and expose the chamber to a wellbore pressure; then (d) causing the restrictor to move in response to the wellbore pressure to a position at least partially restricting the flow of drilling fluid through the drilling fluid passageway to cause an increase in the drilling fluid pressure. 
         [0014]    A further feature of the method of indicating wear of a downhole abrading tool is that step (b) may comprise applying drilling fluid pressure to a first pressure area of the restrictor and step (d) may comprise applying wellbore pressure to a second pressure area of the restrictor, the second pressure area being greater than the first pressure area Another feature of the method of indicating wear of a downhole abrading tool is that the restrictor may be slidably carried within the drilling fluid passageway, and step (d) may comprise moving the restrictor to a position at least partially blocking passage of drilling fluid from the drilling fluid passageway through the outlet. 
         [0015]    The downhole abrading tools and methods of indicating to an operator of a downhole abrading tool of excessive wear on a cutting end of the downhole abrading tool have the advantages of providing effective and efficient identification of excessive wear on the downhole abrading tool. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0016]      FIG. 1  is a cross-sectional view of a specific embodiment of a downhole abrading tool of the present invention shown disposed in a well. 
           [0017]      FIG. 2  is a cross-sectional view of the head of a specific embodiment of a downhole abrading tool of the present invention during normal milling operations. 
           [0018]      FIG. 3  is a cross-sectional view of the head of another specific embodiment of a downhole abrading tool of the present invention in which drilling fluid is not restricted from flowing from the drilling fluid passageway and through the outlets into the well environment. 
           [0019]      FIG. 4  is a cross-sectional view of the head of the embodiment of the downhole abrading tool shown in  FIG. 3  after excessive wear has occurred. 
           [0020]      FIG. 5  is a partial cross-sectional view of the head of an additional specific embodiment of a downhole abrading tool of the present invention. 
           [0021]      FIG. 6  is a cross-sectional view of another specific embodiment of a downhole abrading tool of the present invention in which drilling fluid is not restricted from flowing from the drilling fluid passageway and through the outlets into the well environment. 
           [0022]      FIG. 7  is a cross-sectional view of the embodiment of the downhole abrading tool shown in  FIG. 6  after excessive wear has occurred. 
       
    
    
       [0023]    While the invention will be described in connection with the preferred embodiments, it will be understood that it is not intended to limit the invention to that embodiment. On the contrary, it is intended to cover all alternatives, modifications, and equivalents, as may be included within the spirit and scope of the invention as defined by the appended claims. 
       DETAILED DESCRIPTION OF INVENTION 
       [0024]    Referring to  FIG. 1 , oil and gas wells  10  have surface location  11  and downhole location  12 . Object  13  is disposed within well  10 . Downhole abrading tool  20  is connected to rotating component  15  which, together with downhole abrading tool  20 , is part of drill string  16 . Downhole abrading tool  20  is placed in contact with object  13  and then rotated, using equipment known to persons skilled in the art, to abrade object  13 . 
         [0025]    As shown in  FIGS. 1-5 , downhole abrading tool  20  includes body  21 , having first end  22 , cutting end  23 , head  24 , and drilling fluid passageway  26 . First end  22  is adapted to be connected to rotating component  15  or drill string  16  to facilitate rotation of downhole abrading tool  20 . First end  22  preferably includes threads (not shown) to facilitate attachment to rotating component  15  or drill string  16 . 
         [0026]    Drilling fluid passageway  26  is disposed longitudinally within body  20  to permit drilling fluid to flow through downhole abrading tool  20 . Accordingly, drilling fluid (not shown) flows from equipment (not shown) located at surface location  11 , through drill string  16 , through drilling fluid passageway  26 , and through at least one outlet  28  into well environment  17 . Outlets  28  are inclined relative to drilling fluid passageway  26  and intersect drilling fluid passageway  26  at ports  29  disposed within the wall of drilling fluid passageway  26 . Each outlet  28  may have a nozzle (not shown). 
         [0027]    The drilling fluid facilitates cutting by downhole abrading tool  20 . Drilling fluid flows from surface location  11 , through drilling fluid passageway  26 , through outlet  28  into well environment  17  at a drilling fluid pressure that is monitored by an operator of downhole abrading tool  20 . 
         [0028]    Cutting end  23  includes abrading matrix  18  formed of an abrading material, such as hardfacing or other cutting or abrading material known in the art. The abrading material may be formed, in whole or in part, from a wear away portion that wears from cutting end  23  during abrasion of object  13 . Such abrading materials are known in the art. 
         [0029]    Disposed within abrading matrix  18  is indicator or wear-away chamber  19 , which is in fluid communication with drilling fluid passageway  26 . Indicator chamber  19  is initially sealed from the drilling fluid in the wellbore as well as the drilling fluid being pumped down drilling fluid passageway  26 . The initial pressure in indicator chamber  19  may be atmospheric. A specifically designed wear away portion may be disposed within cutting end  23  and over indicator chamber  19  which is recessed within cutting end  23 . Alternatively, the entire abrading matrix  18  may be formed from an abrading material that functions as the wear away portion. Indicator chamber  19  extends perpendicular to the axis of downhole abrading tool  20  at least part way across cutting end  23 . 
         [0030]    Plug  36  is secured into the lower end of drilling fluid passageway  26 , such as by a retainer ring, threads, or welding. A communication passage  37  extends from indicator chamber  19  vertically through plug  36 . Valve or movable piston  30  is disposed within drilling fluid passageway  26  above plug  36 . Movable piston  30  is a hollow cylindrical member with bore  30   b . Fixed piston  32  is disposed within closed bottom  30   a  and bore  30   b , defining an annular upper end  30   c . Movable piston  30  is a differential piston with a greater pressure area at closed bottom  30   a  than at upper end  30   c.    
         [0031]    As shown in  FIG. 2 , in one embodiment, fixed piston  32  may be secured to the inner wall of drilling fluid passageway  26  to prevent movement of fixed piston  32  relative to head  24 . A portion of upper end  30   c  is permitted move past fixed piston  32 ; however, the remaining portion of upper end  30   c  is restricted from upward movement past fixed piston  32 . 
         [0032]    Alternatively, as illustrated in  FIGS. 3-4 , fixed piston  32  is secured by rod  33  to plug  36  to prevent movement of fixed piston  32  relative to head  24 . Rod  33  extends sealingly through a hole in closed bottom  30   a . Fixed piston  32  also limits the upper travel of movable piston  30  to the position shown in  FIG. 4 . the location where indicator chamber  19  is in fluid communication with drilling fluid passageway is valve  30 . As shown in  FIGS. 2-5 , valve  30  is piston  31 . Piston stop  32  is also disposed within drilling fluid passageway  26 . Piston stop  32  limits the movement of piston  31  within drilling fluid passageway  26 . Piston stop is preferably disposed within drilling fluid passageway  26  such that outlet ports  29  are partially blocked. 
         [0033]    During operation, drilling fluid flows through outlets  28 . The drilling fluid pressure exerts a downward force on movable piston  30 , and more particularly, on upper end  30   c . There is no upward force component initially on piston  30  because the pressure in indicator chamber  19  and passage  37  is atmospheric. 
         [0034]    As illustrated in  FIG. 4 , when cutting end  23  experiences excessive wear  40 , indicator chamber  19  is exposed to well environment  17 . In other words, excessive wear  40  results in the wear away portion being worn off of cutting end  23 . Alternatively, excessive wear  40  results in the abrading material being worn off of cutting end  23 . When indicator chamber  19  is exposed to well environment  17  and, thus, to the wellbore pressure, the pressure in indicator chambcr  19  increases from atmospheric to wellbore pressure. The wellbore pressure acts on bottom end  30   a  of movable piston  30  while the drilling fluid pressure in drilling fluid passageway  26  exerts a downward force on piston  30 . The wellbore pressure is less than the drilling fluid pressure because of the pressure drop through outlets  28 . However, the pressure area on bottom end  30   a  is greater than the pressure area of upper end  30   c . As a result, piston  30  moves from a first position upward within drilling fluid passageway  26  to a second position partially obstructing ports  29 . In the second position, the drilling fluid flowing from drilling fluid passageway  26 , through outlet ports  29 , and through outlets  28  is restricted. 
         [0035]    As mentioned above, preferably, the drilling fluid flowing from drilling fluid passageway  26 , through ports  29 , and outlets  28  is partially restricted such that drilling fluid is still permitted to flow through outlet ports  29  and outlets  28  into the well environment. However, it is to be understood that the flow of the drilling fluid from drilling fluid passageway  26 , through outlet ports  29 , and through outlets  28  may be blocked completely. 
         [0036]    Due to the restriction of the flow of drilling fluid through drilling fluid passageway  26 , the pressure of drilling fluid, being monitored by the operator at the surface, noticeably increases to indicate to the operator that downhole abrading tool  20  has experienced excessive wear and should be replaced. 
         [0037]    In another embodiment shown in  FIG. 5 , downhole abrading tool  20  is designed and operates in the same manner discussed above with respect to the embodiment shown in  FIGS. 1-4 . In this specific embodiment, however, downhole abrading tool  20  further includes taggants  50  embedded or disposed within abrading matrix  18 . Each taggant  50  may be, for example, a colored dye, a radio-frequency tag, a radioactive material, a florescent material, or a pellet having an outer shell that is dissolvable in the drilling fluid and that encases a core formed of an expandable material such as styrofoam. As abrading matrix  18  is worn away due to excessive wear on cutting end  23  of downhole abrading tool  20 , one or more taggant  50  is released from abrading matrix  18  into well environment  17  and, thus, into the drilling fluid. As the drilling fluid circulates up well  10  in well environment  17  to surface location  11 , it carries with it each of the released taggants  50 . Upon reaching surface location  11 , taggants  50  are detected by the operator of the downhole abrading tool  20 , either visually, or using equipment designed specifically for the detection of taggant  50 . Identification of taggants  50  by the operator provides an indication that downhole abrading tool  20  has experienced excessive wear. Alternatively, the identification of taggants  50  can indicate to the operator that cutting end  23  is approaching the point at which cutting end  23  experiences excessive wear. 
         [0038]    In one specific embodiment, taggants  50  are formed integral with the abrading material that forms abrading matrix  18 . In other words, in this embodiment, taggants  50  are embedded or disposed within abrading matrix  18  during the formation of abrading matrix  18 . 
         [0039]    As shown in  FIG. 5 , different taggants  50  are disposed at different locations within abrading matrix  18 , thereby providing different indications as to the extent of wear on cutting end  23 . For example, taggants  51  are released prior to taggants  52  and taggants  52  are released prior to taggants  53 . Accordingly, the operator is provided with incremental indication as to the wear on cutting end  23 . Alternatively, taggants  51 ,  52 , and  53  can be disposed in specific areas of abrading matrix  18 , e.g., taggants  51  on the sides, taggants  52  on the bottom, and taggants  53  in the middle so that an indication can be made as to the specific area or region of cutting end  23  undergoing wear. 
         [0040]    Various combinations of the different types of taggants  50  can be used to better educate the operator as to the location of the excessive wear on cutting end  23  as well as the degree of wear occurring at various locations of cutting end  23 . For example, taggants  50  having colored dyes may be released if excessive wear occurs on the outer portions of abrading matrix  18  and taggants  50  having radio-frequency tags may be released if excessive wear occurs on the center portion of abrading matrix  18 . 
         [0041]    Referring now to  FIGS. 6-7 , in another specific embodiment, downhole abrading tool  80  includes exterior surface  81 , first end  82 , second end  83 , chamber sleeve  84 , and drilling fluid passageway  87  having partition  85  disposed therein. Chamber sleeve  84  includes chamber cavity  86  having disposed therein valve or movable piston  88 . The exterior of piston  88  seals against chamber sleeve  84  and the interior of piston  88  seals against exterior surface  81 . Downhole abrading tool  80  also includes drilling fluid passageway  87  having upper port  91  and lower port  92  disposed within the body of downhole abrading tool  80 . Upper port  91  is above partition  85  and lower port  92  id below partition  85 . Upper port  91  and lower port  92  are in fluid communication with each other by piston passageway  93  or recess in the inner diameter of piston  88  between its upper and lower seals. 
         [0042]    Downhole abrading tool  80  also includes outlet  98  (shown in dashed lines) below partition  85  providing fluid communication between drilling fluid passageway  87  and the well environment. 
         [0043]    Partition  85  requires all of the drilling fluid to flow through upper port  91 , passageway  93 , and lower port  92  as it flows to outlet  98 . 
         [0044]    Cutting end  83  includes abrading matrix  94  formed of an abrading material, such as hardfacing or other cutting or abrading material known in the art. The abrading material may be formed, in whole or in part, from a wear away portion that wears from cutting end  83  during abrasion of an object disposed within the well (such as object i 3  discussed in greater detail above). Such abrading materials are known in the art. 
         [0045]    Disposed within abrading matrix  94  is indicator or wear-away chamber  95 , which is in fluid communication with chamber cavity  86  below piston  88 . Indicator chamber  95  is initially sealed from the drilling fluid in the wellbore as well as the drilling fluid being pumped down drilling fluid passageway  87 . The initial pressure in indicator chamber  95  may be atmospheric. A specifically designed wear away portion may be disposed within cutting end  83  and over indicator chamber  95  which is recessed within cutting end  83 . Alternatively, the entire abrading matrix  94  may be formed from an abrading material that functions as the wear away portion. Indicator chamber  95  extends perpendicular to the axis of downhole abrading tool  80  at least part way across cutting end  83 . A communication passage  97  extends from indicator chamber  95  vertically to chamber cavity  86 . 
         [0046]    During operation, drilling fluid flows through drilling fluid passageway  87 , through upper port  91 , through piston passageway  93 , through lower port  92  into drilling fluid passageway  87 , through outlet  98  and into the well environment. There is no upward force component initially on piston  88  because the pressure in indicator chamber  95  and passage  97  is atmospheric. The forces acting on piston  88  due to the drilling fluid flowing through upper port  91  and lower port  92  are balanced. There is no pressure other than atmospheric pressure in chamber  86  above the upper seal of piston  88 , nor below the lower seal of piston  88 . 
         [0047]    As illustrated in  FIG. 7 , when cutting end  83  experiences excessive wear, indicator chamber  95  is exposed to well environment  17  ( FIG. 1 ). In other words, the excessive wear results in the wear away portion being worn off of cutting end  83 . Alternatively, the excessive wear results in the abrading material being worn off of cutting end  83 . When indicator chamber  95  is exposed to well environment  17  and, thus, to the wellbore pressure, the pressure in indicator chamber  95  increases from atmospheric to wellbore pressure. The wellbore pressure acts on the bottom end of piston  88  in an upward direction. The pressure area on the bottom end of piston  88  is greater than the pressure above piston  88 , which is atmospheric. As a result, piston  88  moves from a first position upward within chamber cavity  86  to a second position at least partially obstructing lower port  92 . In the second position, the drilling fluid flowing from drilling fluid passageway  87 , through upper port  91 , through piston passageway  93 , and through lower port  92  is restricted. 
         [0048]    However, preferably, the drilling fluid flowing from drilling fluid passageway  87 , through upper port  91 , through piston passageway  93 , and through lower port  92  is only partially restricted such that drilling fluid is still permitted to flow through upper port  91 , piston passageway  93 , lower port  92  and, thus, ultimately outlet  98  into the well environment. To prevent full blockage, the upper end of chamber  86  may located so that piston  88  contacts it when in its upper most position, shown in  FIG. 7 . However, it is to be understood that the flow of the drilling fluid from drilling fluid passageway  87 , through upper port  91 , piston passageway  93 , and lower port  92  may be blocked completely by allowing piston  88  to move further upward. 
         [0049]    Due to the restriction of the flow of drilling fluid through drilling fluid passageway  87 , through upper port  91 , through piston passageway  93 , and through lower port  92 , the pressure of drilling fluid, being monitored by the operator at the surface, noticeably increases to indicate to the operator that downhole abrading tool  80  has experienced excessive wear and should be replaced. 
         [0050]    It is to be understood that the invention is not limited to the exact details of construction, operation, exact materials, or embodiments shown and described, as modifications and equivalents will be apparent to one skilled in the art. For example, other arrangements for a differential area piston are feasible such as an arrangement in which the upper end could be of a smaller outer diameter than the bottom end. Accordingly, the invention is therefore to be limited only by the scope of the appended claims.