Abstract:
A method for use in designing rotary drill bits comprises determining locations in which cutters are to be provided, determining likely wear rates for cutters positioned at those locations, and using the likely wear rates to chose between relatively high and relatively low wear rate cutters for each location. The cutters all have substantially the same impact toughness.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from U.S. Provisional Patent Application No. 60/234,075 filed Sep. 20, 2000, and from U.S. Provisional Patent Application No. 60/281,054 filed Apr. 2, 2001. 
    
    
     BACKGROUND OF INVENTION 
     1. Field of the Invention 
     This invention relates to a method of designing a fixed cutter rotary drill bit for use in the drilling of wellbores. 
     2. Description of the Related Art 
     It is known that certain parts of a fixed cutter rotary drill bit wear at a faster rate than other parts thereof. Such variations in wear have been countered, in the past, by providing the drill bit with additional cutters in the high wear areas thereof. In the past, drill bit designs were compromised because cutters having high abrasion resistance had low impact toughness and cutters having high impact toughness had low abrasion resistance. As a result of this trade-off such placement of the cutters may improve the abrasion resistance of the drill bit, but it is likely that the overall drilling efficiency is not optimized due to relatively poor impact toughness. 
     It is an object of the invention to provide a drill bit whereby the operating efficiency of the bit can be maintained whilst providing a bit having a desired wear profile, and to provide a method for designing such a drill bit and provide high abrasion resistance while maintaining impact toughness. 
     SUMMARY OF INVENTION 
     According to the present invention there is provided a method for designing a fixed cutter drill bit comprising determining a layout of cutter locations in which cutters are to be positioned upon a bit body, determining the likely wear rate for at least the cutters provided in some of the cutter locations and using the determined likely wear rates to determine whether to mount a cutter of a first type of relatively low abrasion resistance or a cutter of a second type of a relatively high abrasion resistance in each cutter location. The impact toughness of all the cutters is substantially the same. 
     The method of the invention is advantageous in that the positions of the cutter locations can be optimized whilst achieving an acceptable overall wear rate for the bit. 
     The cutters of the second type conveniently each comprise a table of polycrystalline diamond bonded to a substrate, the table of polycrystalline diamond defining a matrix of interstices containing a catalyzing material, the interstices of the matrix located within a volume close to a working surface of the cutter being substantially free of catalysing material. The volume preferably extends to a depth of at least around 0.25 mm from the working surface. 
     According to another aspect of the invention there is provided a method of designing a fixed cutter drill bit comprising determining a layout of cutter locations in which cutters are to be positioned upon a bit body, determining a likely wear rate for at least the cutters provided at some of the cutter locations, selecting a desired wear profile for the bit, and using the determined likely wear rates to determine the abrasion resistance of the cutter to be mounted at each cutter location to achieve the desired wear profile. 
     The invention also relates to a fixed cutter drill bit designed in accordance with the methods defined hereinbefore. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The invention will further be described, by way of example, with reference to the accompanying drawings. 
     FIG. 1 is a perspective view of a drill bit designed in accordance with an embodiment of the invention. 
     FIG. 2 is a diagrammatic sectional view of a cutter of the drill bit of FIG.  1 . 
     FIG. 3 is a diagrammatic view illustrating the structure of part of the cutters of FIG.  2 . 
     FIGS. 4 to  7  are diagrammatic representations of wear profiles. 
     FIGS. 8 and 9 are diagrammatic views of two further drill bits. 
    
    
     DETAILED DESCRIPTION 
     The fixed cutter drill bit illustrated in FIG. 1 comprises a bit body  10  having a leading face  12  and a shank  14  to permit the drill bit to be secured to the remainder of a drill string. The bit body  10  is intended to be rotated, in use, about an axis of rotation  16 . 
     Upstanding from the leading face  12  are a plurality of blades  18  upon which a plurality of cutters  20  are mounted. As shown in FIG. 2, each cutter  20  comprises a table  22  of polycrystalline diamond bonded to a tungsten carbide substrate  24 . Each cutter  20  is manufactured using a known technique involving exposing diamond powder, a catalyst material and a tungsten carbide substrate to high temperature, high pressure conditions to cause the diamond powder to undergo a structural change, becoming polycrystalline diamond, and to cause the polycrystalline diamond to bond to the substrate. 
     It has been found that where the catalyzing material used in the manufacture of the cutters is cobalt, removal of the catalyst material from a part of the cutter close to a working surface thereof results in the abrasion resistance of the cutter improving. FIG. 3 illustrates part of a cutter so treated. In FIG. 3, the polycrystalline diamond  26  defines a matrix of interstices  28  containing the catalyzing material used in the formation of the polycrystalline diamond and used in the bonding of the polycrystalline diamond to the substrate. The interstices  28  located within a volume  34  close to a working surface  32  of the diamond  26  have been rendered substantially free of catalyzing material  30  by leaching the catalyzing material  30  from the volume  34 . In the arrangement of FIG. 3, the volume  34  extends over the full cross-sectional area of the polycrystalline diamond, and extends to a depth D of approximately 0.25 mm from the working surface  32 . 
     Although leaching is the currently preferred technique for rendering the interstices free of catalyzing material, other techniques may be used. For example, the catalyzing material may be forced to undergo a structural change to take an alternative form which does not have a catalyzing effect, or may be chemically reacted to form a substance which does not have a catalyzing effect. 
     In accordance with the invention, a layout or arrangement of cutter locations in which the cutters  20  are to be mounted on the bit body  10  is chosen, and the likely wear rate to which each of the cutters  20  is to be exposed is determined. The likely wear rate for each location may be determined using, for example, a computer modelling technique, or alternatively may be derived from measurements taken from a similar, used drill bit. Using the determined likely wear rate information, either a cutter  20   a  of a first type of a relatively low abrasion resistance, or a cutter  20   b  of a second type having a relatively high abrasion resistance manufactured as described hereinbefore is chosen for each cutter location in order to achieve a desired wear profile. 
     FIGS. 4 to  7  illustrate a range of possible profiles. In each drawing, the full line  36  denotes the profile that would be achieved if all cutter locations were occupied by cutters  20   a  of the first type. In FIGS. 4 and 5, the broken line  38  illustrates the modification to the wear profile achieved by using cutters  20   b  of the second type rather than cutters  20   a  of the first type in a region  40  of the bit. 
     FIG. 6 illustrates an arrangement which makes use of cutters of a third type of higher abrasion resistance than the cutters  20   b  of the second type. These cutters are similar to the cutters  20   b  of the second type but are treated to remove the catalyst material to a greater depth, for example to a depth of 0.5 mm, from the working surface. In FIG. 6, the cutters  20   b  of the second type are located in regions  40  and the cutters of the third type are location in a region  42 . 
     In the profiles of FIGS. 4 to  6 , the use of cutters  20   b  of the second type, and in FIG. 6 the use of cutters of the third type, serves to flatten the profile, increasing the uniformity with which the cutters on the bit wear, thereby permitting the working life of the bit to be increased. FIG. 7 illustrates an alternative profile for a bit where the region which conventionally would have the highest wear rate has a lower wear rate than the remainder of the bit. It is thought that this may be beneficial in providing bits of good stability, directional control, and rate of progress. 
     In the hereinbefore described manufacturing process for improving wear resistance, as the wear resistance of a cutter improves, its ability to withstand impacts is substantially retained. However, because a thin wear-resistance lip is often desirously formed at the cutting edge cutters of the second and third types, they may be more suitably used in in cutter locations which are protected against impact. This is because the lip, which is believed to increase drilling efficiency, may become chipped during impact. FIGS. 8 and 9 illustrate, diagrammatically, two rotary drill bits of the type having a plurality of primary cutters and a plurality of secondary cutters, the secondary cutters being protected, at least to some extend, against chippage of the lip by the presence of the primary cutters. 
     In FIG. 8, the drill bit  44  defines a plurality of blades  46 , each blade  46  having mounted thereon a plurality of cutters  20 . The cutters  20  are arranged in two rows  48 ,  50  on each blade  46 . The first row  48  comprises a row of primary cutters  20   a  of the first type. The second row  50  comprises secondary cutters  20   b  of the second type. The secondary cutters are each provided immediately behind an associated primary cutter, and so are protected by the primary cutters from impacts. 
     The arrangement of FIG. 9 differs from that of FIG. 8 in that the secondary cutters are mounted upon separate blades from the primary cutters. 
     It will be appreciated that, in order to provide a bit of good abrasion resistance and good impact resistance, it may in some circumstances be desirable to position some cutters of the first type within the regions  40 ,  42  of FIGS. 4 to  7  to protect the cutters of the second type from lip chippage. 
     Although specific embodiments of the invention have been described hereinbefore, it will be appreciated that various changes may be made without departing from the scope of the appended claims. The present invention has also been described in particular relation to the drawings attached hereto, it should be understood that other and further modifications apart from those shown or suggested herein, may be made within the scope and spirit of the present invention.