Patent Publication Number: US-9416617-B2

Title: Downhole tool having slip inserts composed of different materials

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Appl. No. 61/763,718, filed 12 Feb. 2013, which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE DISCLOSURE 
     Slips are used for various downhole tools, such as bridge plugs and packers. The slips can have inserts or buttons to grip the inner wall of a casing or tubular. Inserts for slips are typically made from cast or forged metal, which is then machined and heat-treated to the proper engineering specifications according to conventional practices. 
     Inserts for slips on metallic and non-metallic tools (e.g., packers, plugs, etc.) must be able to engage with the casing to stop the tools from moving during its operation. On non-metallic tools, such as composite plugs, the inserts can cause the non-metallic slips to fail when increased loads are applied. Of course, when the slip fails, it disengages from the casing. On non-metallic tools, the inserts also need to be easily milled up to assist in the removal of the tools from the wellbore. 
     When conventional inserts are used in non-metallic slips, they are arranged and oriented as shown in  FIG. 1A , for example. The slip  20  is disposed adjacent a mandrel  10  of a downhole tool, such as a bridge plug, a packer, or the like. As shown in  FIG. 1B , the slip  20  moves away from the mandrel  10  and engages against a surrounding tubular or casing wall when the slip  20  and a cone  12  are moved toward one another. Either the slip  20  is pushed against the ramped surface of the cone  12 , the cone  12  is pushed under the slip  20 , or both. 
       FIG. 2A  illustrates a side cross-section of a slip  20  having holes  23  according to the prior art for inserts (not shown), and  FIG. 2B  illustrates a side cross-section of the slip  20  with inserts  30  disposed in the holes  23 .  FIG. 2C  illustrates a front view of the slip  20  with the holes  23  for the inserts (not shown). The slip  20  can have a semi-cylindrical shape. The holes  23  in the surface of the slip  20  can be an array of blind pockets. The inserts  30  are anchor studs that load into the holes  23  and can be held with a press fit or adhesive. 
     Examples of downhole tools with slips and inserts such as those above are disclosed in U.S. Pat. Nos. 5,984,007; 6,976,534; and 8,047,279. Other examples include Halliburton Obsidian® and Fas Drill® Fusion composite plugs and Boss Hog frac plugs. (OBSIDIAN and FAS DRILL are registered trademarks of Halliburton Energy Services, Inc.) 
     One particular type of downhole tool having slips is a composite fracture plug used in perforation and fracture operations. During the operations, the composite plugs need to be drilled up in as short of a period of time as possible and with no drill up issues. Conventional composite plugs use metallic wicker style slips, which are composed of cast iron. These metallic slips increase the metallic content of the plug and can cause issues during drill up in horizontal wells, especially when coil tubing is used during the milling operation. 
     Due to the drawbacks of cast iron slips, composite slips having inserts, such as described above, are preferably used to reduce the issues associated with metallic slips. Unfortunately, a large amount of metallic debris can still collect at the heel of the well and cause drill up problems when composite slips having inserts are used on tools. When composite slips are used, for example, the inserts are typically composed of carbide, which is a dense and heavy material. In other developments, it is known to use a composite slip with two different insert materials (i.e., ceramic and metallic) in the same insert, such as described in U.S. Pat. No. 6,976,534. 
     In any event, when the downhole tool having slips with carbide inserts are milled out of the casing, the inserts tend to collect in the casing and are hard to float back to the surface. In fact, in horizontal wells, the carbide inserts may tend to collect at the heel of the horizontal section and cause potential problems for operations. Given that a well may have upwards of forty or fifty bridge plugs used during operations that are later milled out, a considerable number of carbide inserts may be left in the casing and difficult to remove from downhole. 
     The subject matter of the present disclosure is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above. 
     SUMMARY OF THE DISCLOSURE 
     A downhole apparatus or tool, such as a composite bridge plug used during a fracture or perforation operations, installs in a downhole tubular, such as casing. The tool can have a mandrel with a sealing element disposed thereon. The sealing element can be compressible to engage the downhole tubular when the tool is activated by a wireline unit or the like. 
     A first slip is disposed on the tool and is movable relative to the tool to engage the downhole tubular. For example, the first slip can be disposed toward an uphole end of the tool&#39;s mandrel. Similarly, a second slip is disposed on the tool and is movable relative to the tool to engage the downhole tubular. For example, the second slip can be disposed toward a downhole end of the tool&#39;s mandrel. 
     The slips can each have one or more slip bodies, segments, or elements disposed about the mandrel. For example, the segments can be arranged around the tool and can be individual or integrated segments. Other arrangements for the slips can be used. The first and second slips can both be composed of a non-metallic material, such as a plastic, a molded phenolic, a composite, a laminated non-metallic composite, an epoxy resin polymer with a glass fiber reinforcement, an ultra-high-molecular-weight polyethylene (UHMW), a polytetrafluroethylene (PTFE), etc. 
     In one embodiment, the first (uphole) slip has only one or more first inserts composed of ceramic material in exclusion of inserts composed of other materials being used on the first slip, and the second (downhole) slip has only one or more second inserts composed of a metallic material in exclusion of inserts composed of other materials being used on the second slip. When the tool is used as a fracture plug, for example, the uphole slip with only ceramic inserts engages the downhole tubular and primarily supports the sealing element compressed. In this case, use of only the first inserts composed of the ceramic material can reduce the overall metallic content of the plug, but can still support the sealing element compressed. 
     On the other hand, the downhole slip with only the metallic inserts engages the downhole tubular with the metallic inserts and primarily supports fluid pressure downhole of the tool. In this case, use of only the second inserts composed of the metallic material can still reduce the overall metallic content of the plug. Yet, the metallic inserts on the downhole slip can better support the increased fluid pressure downhole of the tool during operations. 
     Other arrangements of inserts, slips, materials, and the like are disclosed herein. The ceramic material for the inserts of the slips can be alumina, zirconia, and cermet. As noted above, use of the ceramic material inserts on the uphole slip can reduce the overall metallic content of the tool and can facilitate milling of the tool from the downhole tubular after use. 
     The metallic material for the second inserts on the slips can use a cast iron, a carbide, a cermet (i.e., composites composed of ceramic and metallic materials), a powdered metal, or a combination thereof. In one particular embodiment, the metallic material is a sintered-hardened powdered metal steel. In one particular arrangement, the sintered-hardened powdered metal steel can consist essentially of a balance of iron, an admixture of carbon, and alloy components of molybdenum, chromium, and manganese. 
     In another embodiment, a downhole apparatus or tool for engaging in a downhole tubular has a first slip disposed on the tool and is movable relative to the tool to engage the downhole tubular. The first slip is composed of a first material. At least one first insert is exposed on the first slip and is composed of a powdered metal material. 
     In one particular arrangement, the first slip is disposed toward an uphole end of a mandrel of the tool, and the first slip comprises only one or more of the at least one first inserts composed of the powdered metal in exclusion of inserts of composed of other materials. The tool also has a second slip disposed toward a downhole end of the mandrel. The second slip has only one or more second inserts composed of a metallic material in exclusion of inserts composed of other materials, the metallic material being other than powdered metal material. 
     In another embodiment, a downhole apparatus or tool for engaging in a downhole tubular has a slip disposed on the apparatus. The slip is movable relative to the apparatus to engage the downhole tubular. At least one insert is exposed on the slip and defines at least a partial hole axially therethrough. 
     In yet another embodiment, a downhole apparatus or tool for engaging in a downhole tubular has a slip disposed on the downhole tool. The slip is movable relative to the apparatus to engage the downhole tubular, and the slip having an outside surface and first and second ends. The outside surface defines a first hole toward the first end and defines a second hole toward the second end. The first hole has a different depth in the outside surface than the second hole. 
     A first insert is disposed in the first hole, and a second insert is disposed in the second hole. The first insert has a first length and extending a first extent from the outside surface on the slip. The second insert has a second length and extending a second extent from the outside surface on the slip. 
     The various arrangements noted herein can be interchanged and combined with one another in accordance with the teachings of the present disclosure. Additionally, the slip can be an individual body or segment, a unitary ring, one of a plurality of independent segments of a slip assembly, or one of a plurality of integrated segments of a slip assembly. The material of the slip can be metallic or non-metallic. In one implementation, the slip&#39;s material comprises a plastic, a molded phenolic, a laminated non-metallic composite, an epoxy resin polymer with a glass fiber reinforcement, an ultra-high-molecular-weight polyethylene (UHMW), a polytetrafluroethylene (PTFE), or a combination thereof. 
     Although suitable for a downhole tool, such as a fracture plug discussed above, the teaching of the present disclosure can apply to any of a number of downhole tools for engaging in a downhole tubular. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  illustrates inserts used in a non-metallic slip according to the prior art. 
         FIG. 1B  illustrates the slip of  FIG. 1A  during use. 
         FIG. 2A  illustrates a side cross-section of a slip having holes for inserts according to the prior art. 
         FIG. 2B  illustrates a side cross-section of the slip with inserts disposed in the holes. 
         FIG. 2C  illustrates a front view of the slip with the holes for the inserts. 
         FIG. 3  illustrates a downhole tool in partial cross-section having slip assemblies according to the present disclosure. 
         FIG. 4A  illustrates a cross-sectional view of a slip having a first type of slip insert. 
         FIG. 4B  illustrates a cross-sectional view of a slip having a second type of slip insert. 
         FIGS. 5A-5C  illustrate top, cross-sectional, and perspective views of one configuration of slip insert. 
         FIGS. 6A-6C  illustrate top, cross-sectional, and perspective views of another configuration of slip insert. 
         FIGS. 7A-7C  illustrate top, cross-section, and perspective views of another configuration of slip insert. 
         FIGS. 8A-8B  illustrate bottom and cross-section views of yet another configuration of slip insert. 
         FIG. 9  illustrates a slip assembly having segments and having a configuration of inserts with holes and without holes. 
         FIG. 10A  illustrates a cross-section of a slip segment having different depth holes for holding inserts. 
         FIG. 10B  illustrates a cross-section of the slip segment having inserts of different heights installed in the holes. 
         FIG. 10C  is a plan view of the slip segment showing an arrangement of different depth holes. 
         FIG. 11A  illustrates a cross-section of a slip segment having holes of different widths for holding inserts therein. 
         FIG. 11B  illustrates a cross-section of the slip segment having inserts of different widths installed in the holes. 
         FIG. 11C  illustrates a plan view of the slip segment showing an arrangement of different width holes. 
         FIG. 12A  illustrates a cross-section of a slip segment having holes of different depths and widths for holding inserts therein. 
         FIG. 12B  illustrates a cross-section of the slip segment having different inserts installed in the holes of different depths and widths. 
         FIG. 12C  illustrates a cross-section of a slip segment having holes of different depths for holding inserts of the same height installed therein. 
         FIG. 13  illustrates another downhole tool in side view having slip assemblies according to the present disclosure. 
         FIG. 14A  illustrates a side view of the uphole slip assembly. 
         FIG. 14B  illustrates a side view of the downhole slip assembly. 
     
    
    
     DETAILED DESCRIPTION OF THE DISCLOSURE 
       FIG. 3  illustrates a downhole tool  100  in partial cross-section having slip assemblies  110 U,  110 D according to the present disclosure. The downhole tool  100  can be a bridge plug as shown, but it could also be a packer, a liner hanger, an anchoring device, or other downhole tool that uses a slip assembly to engage a downhole tubular, such as casing. 
     The tool  100  has a mandrel  102  having the slip assemblies  110 U and  110 D and backup rings  140  arranged on both sides of a packing element  150 . Outside the inclined cones  112 , the slip assemblies  110 U and  110 D have slips  120 . Together, the slips  120  along with the cones  112  can be referred to as slip assemblies, or in other instances, just the slips  120  may be referred to as slip assemblies. In either case, either reference may be used interchangeably throughout the present disclosure. Thus, reference herein to a slip is not meant to refer only to one slip body, segment, or element, although it can. Instead, reference to slip can refer to more than just these connotations. As shown herein, slip assemblies  110 U,  110 D can have the same types of slips  120 , but other arrangements could be used. 
     As a bridge plug, the tool  100  is preferably composed mostly of non-metallic components according to procedures and details as disclosed, for example, in U.S. Pat. No. 7,124,831, which is incorporated herein by reference in its entirety. This makes the tool  100  easy to mill out after use. 
     When deployed downhole, the tool  100  is activated by a wireline setting tool (not shown), which uses conventional techniques of pulling against the mandrel  102  while simultaneously pushing upper components against the slip assemblies  110 U,  110 D. As a result, the slips  120  of the slip assemblies  110 U,  110 D ride up the cones  112 , the cones  112  move along the mandrel  102  toward one another, and the packing element  150  compresses and extends outward to engage a surrounding casing wall. The backup elements  140  control the extrusion of the packing element  150 . In the process, the slips  120  on the assemblies  110 U,  110 D are pushed outward to engage the wall of the casing (not shown), which both maintains the tool  100  in place in the casing and keeps the packing element  150  contained. 
     The force used to set the tool  100  may be as high as 30,000 lbf and could be as high as 85,000 lbf. These values are only meant to be examples and could vary for the size of the tool  100 . In any event, the set tool  100  isolates upper and lower portions of the casing so that fracture and other operations can be completed uphole of the tool  100 , while pressure is kept from downhole locations. When used during fracture operations, for example, the tool  100  may isolate pressures of 10,000 psi or so. 
     As will be appreciated, any slipping or loosening of the tool  100  can compromise operations. Therefore, the slips  120  need to sufficiently grip the inside of the casing. 
     At the same time, however, the tool  100  and most of its components are preferably composed of millable materials because the tool  100  is milled out of the casing once operations are done, as noted previously. As many as fifty such tools  100  can be used in one well and must be milled out at the end of operations. Therefore, having reliable tools  100  composed of entirely of millable material is of particular interest to operators. To that end, the slip assemblies  110 U,  110 D of the present disclosure are particularly suited for tools  100 , such as bridge plugs, packers, and other downhole tools, and the challenges they offer. 
     As shown in  FIG. 4A , one type of slip  120  for the assemblies  110  has a slip body or segment  122  with one or more individual inserts or buttons  130  disposed therein. The segment  122  can be one of several used on a slip assembly. The segment  122  can have any number of inserts  130  arranged in one or more rows and/or one or more columns in the top surface. For instance, two rows of inserts  130  may be used, each having the same number of columns. Alternatively, two rows can be used, but one row may have two columns while the other has one column. These and other configurations can be used as will be appreciated. 
     In one arrangement, the inserts  130  can be the same size and can be disposed in equivalent sized holes in the slip segment  122 . In another arrangement, the depth of holes can vary from segment to segment or from slip assembly to slip assembly. Therefore, one or more inserts  130  can be longer than the others. Additionally, the height of the inserts  130  can be the same on the given slip segment  122  once installed, but the depth of the holes can vary. This can reduce the stress around the insert  130  in the base material. Other arrangements may have the inserts  130  at different heights and different depths relative to the slip segment  122 . A number of these configurations are described below. 
     As shown in  FIG. 4B , another type of slip  120  for the assemblies  110  can have a wickered insert  130  disposed in the slip body  122 . Still other configurations of slip inserts  130  can be used as disclosed elsewhere herein. 
     In general, the inserts  130  can be constructed from a long, wide bar or rod that is then machined to the proper length and width and given suitable faces. This technique is well suited for carbide or other hard types of materials and may also be used for other disclosed materials. Alternatively, the insert  130  can be cast or otherwise formed directly with the faces and size needed, if the material and tolerances allow for this. 
     In both cases, the slip body  122  can comprise one of several independent segments of a slip assembly, such as on assemblies  110 U,  110 D shown in  FIG. 3 . As shown in  FIG. 3 , each body or segment  122  can have the same arrangement and number of inserts  130 , although different arrangements can be used. Additionally, each segment  122  can be composed of the same or different materials from the other segments  122 , and each insert  130  on a given segment  122  may be composed of the same or different materials from the other inserts  130 . In other arrangements, the slip body  122  can be a unitary ring or can be a partially integrated ring, as disclosed herein. 
     In general, the slip body  122  is composed of a first material, and the one or more inserts  130  are composed of one or more second materials exposed in the body&#39;s outer surface. The first material of the slip body  122  can generally be metal, composite, or the like. Preferably, the slip body  122  is composed of a millable material, such as a plastic, a non-metallic material, a molded phenolic, a laminated non-metallic composite, an epoxy resin polymer with a glass fiber reinforcement, an ultra-high-molecular-weight polyethylene (UHMW), a polytetrafluroethylene (PTFE), etc. 
     The second material used for the inserts  130  can in general include metallic or non-metallic materials. For example, the inserts  130  can be composed of carbide, a metallic material, a cast iron, a composite, a ceramic, a cermet (i.e., composites composed of ceramic and metallic materials), a powdered metal, or the like. Additionally, the inserts  130  preferably have a sufficient hardness, which may be a hardness equivalent to at least about 50-60 Rc. 
     In one particular embodiment, one or more of the inserts  130  on one or more of the segments  122  for one or both of the assemblies  110 U,  110 D are made from powdered metallurgy. The physical characteristics of such a powdered metal insert  130  can be tailored for the particular implementation. The powdered metal insert  130  can be tailored to be strong and hard enough to engage with the casing to prevent the tool  100  from moving. Additionally, the powdered metal insert  130  can be made frangible enough for easy removal by milling. As noted previously, conventional inserts may be strong enough to engage with the casing, but are difficult to remove and can damage the equipment used to remove the tool  100 . The powdered metal insert  130  made with powder metallurgy can allow the tool  100  to perform correctly, but can improve the speed and ease of the removal of the tool  100  from the wellbore. 
     The powdered metal insert  130  preferably has a hardness greater than or equal to about 48 HRC and may have a hardness in the range of 48 HRC to 60 HRC. Hardness is one of the driving factors for selecting the particular powdered metal to use for the powdered metal insert  130  because casings, such as P-110 grade casing, can be significantly hard. Therefore, the powered metal used is preferably of a high grade. 
     The powdered metal used can include a sinter-hardened powder metal steel material, although other types of powder metals, such as steel, iron, or high carbon steel materials can be used. Manufacture of the powdered metal insert  130  preferably involves forming the insert  130  as a completed part without the need for significant post machining required because any post machining may require using electric discharge machining (EDM) or grinding operations. 
     The sintered-hardened powdered metal steel materials have a balance of iron and use nickel, molybdenum, chromium, and manganese as major alloying components with elemental copper and nickel used in some cases. Graphite powder (carbon) is admixed to provide a necessary level of carbon for the material. One particular sintered-hardened powder metal steel for use with the powdered metal insert  130  has the material designation according to the Metal Powder Industries Federation (MPIF) Standard 35 of FL-5305, which is composed as indicated in the chart below. 
     
       
         
           
               
               
               
               
               
               
               
               
               
             
               
                   
               
               
                 Material 
                   
                   
                   
                   
                   
                   
                   
                 Element 
               
               
                 Designation 
                 Fe 
                 C 
                 Ni 
                 Mo 
                 Cu 
                 Mn 
                 Cr 
                 (%) 
               
               
                   
               
             
            
               
                 FL-5305 
                 Bal. 
                 0.4 
                 — 
                 0.40 
                 — 
                 0.05 
                 2.7 
                 Minimum 
               
               
                   
                 Bal. 
                 0.6 
                 — 
                 0.60 
                 — 
                 0.30 
                 3.3 
                 Maximum 
               
               
                   
               
            
           
         
       
     
     Some particular hardness properties of one type of powdered metal material FL-5305-135HT includes macro-indentation hardness (apparent) of 35 HRC and a micro-indentation hardness (converted) (F) of 55 Rc. The sintered-hardened powdered metal steel may be manufactured by pressing, pre-sintering, repressing, and sintering and can be hardened during the cooling cycle following sintering. 
     The shape of the one or more powdered metal insert  130  can be the same or different from one another and any other inserts  130  composed of other materials. In general, the powdered metal insert  130  can be cylindrical as shown in  FIG. 4A  or can have other shapes, such as the wickered shape shown in  FIG. 4B . Alternatively, the powdered metal insert  130  can have different geometries, such as those disclosed in U.S. application Ser. No. 14/039,032, filed 27 Sep. 2013, which is incorporated herein by reference in its entirety. 
     For instance,  FIGS. 5A through 6C  show examples of suitable geometries for the powdered metal insert  130 .  FIGS. 5A-5C  show top, cross-sectional, and perspective views of a cylindrical shape for a powered metal insert  130  of the present disclosure. The generally cylindrical insert  130  can have a diameter of about 0.3150-in., as shown on the top  132  of  FIG. 5A . The overall height H1 can be about 0.375-in. These and other dimensions discussed herein are merely meant to provide example values. 
       FIGS. 6A-6C  show top, cross-sectional, and perspective views of another configuration for a powdered metal insert  130  for the present disclosure. This insert  130  is also generally cylindrical with a diameter of 0.375-in., as shown in  FIG. 6A . The insert  130  has an overall height H2 of about 0.423-in. The top end  132  of the insert  130 , however, is cusped. Leading and tailing sides of the top end can be angled at 45-degrees. Other possible configurations for the insert  130  are disclosed in incorporated U.S. application Ser. No. 14/039,032. 
       FIGS. 7A-7C  illustrate yet another insert  130 ′ for the present disclosure. This insert  130 ′ may also be generally cylindrical, but includes a hole  135  therethrough. In  FIGS. 8A-8B , the insert  130 ″ has a partial hole  137  therethrough. For the partial hole  137 , the closed end can be used for the gripping surface of the insert  130 ″ or can be disposed in the hole of the segment in which the insert  130 ″ positions. These configurations of inserts  130 ′ and  130 ″ with the hole  135  or partial hole  137  still provide the necessary gripping for the insert  130 ′ and  130 ″ and can be composed of ceramic, metallic, and powder metal materials. For those inserts  130 ′ and  130 ″ composed of metallic material, the hole  135  or partial hole  137  of these configurations reduce the metallic content of the slip using the disclosed inserts  130 ′ and  130 ″. 
     In general, these inserts  130 ′ and  130 ″ of  FIGS. 7A through 8B  can be made from metallic materials or non-metallic materials (e.g., ceramic, powdered metal, composite, etc.). The inserts  130 ′ and  130 ″ can be used on an upper slip assembly  110 U only, the lower slip assembly  110 D only, or both upper and lower slip assemblies  110 U,  110 D. Moreover, the insert  130 ′ and  130 ″ with the hole  135  or partial hole  137  can be using in combination with solid inserts  130  as disclosed herein and with other inserts  130 ′ and  130 ″ with holes  135  or partial holes  137  in the same given segment of a slip assembly. 
     For instance,  FIG. 9  shows a slip assembly (i.e., upper assembly  110 U) having segments  122  with inserts  130 ′ with full holes (although they could be partial) toward the ramp ends of the segments  122  and with solid inserts  130  away from the ramped ends. Not all segments  122  need to have the same arrangement of inserts  130  and  130 ′. Thus, as shown in  FIG. 9 , a given segment  122  has a front row with full hole inserts  130 ′ in two columns and has a back row with solid inserts  130  in two columns. These and other various combinations and arrangements can be used as will be appreciated. 
     As hinted to above, the height of the inserts  130  can be different as can be the depth of the holes in the slips  120 . For example,  FIGS. 10A-10B  illustrate side views of a slip body or segment  122  of a slip  120  having holes  125   a - b  of different depths, and  FIG. 10C  illustrates a plan view of the segment  122  having the holes  125   a - b . As depicted in  FIGS. 10A and 100 , the holes  125   a  toward the ramped end of the segment  122  are defined to a greater extent in the top surface of the segment  122  so that these front holes  125   a  are deeper than the back holes  125   b . A reverse arrangement could be used. 
     As shown in  FIG. 100 , the less deep holes  125   a  are disposed in a row for three inserts, while the deeper holes  125   b  are disposed in another row for three inserts in similar columns. As will be appreciated, any configuration of rows and columns can be used here and in other embodiments disclosed herein. 
     As shown in  FIG. 10B , even though the front holes  125   a  for the front insert  130   a  towards the ramp  124  may be formed slightly deeper in the outer surface of the slip  120  compared to the other holes  125   b  for the back insert  130   b , the height of the two inserts  130   a - b  may be different so that the two inserts  130   a - b  extend the same distance D above the slip&#39;s surface when installed within an appropriate tolerance for the implementation. This will produce the same outside diameters for the front and trailing inserts  130   a - b  when the slip  120  installs on a tool. 
     As one example, the hole  125   a  for the front insert  130   a  towards the ramp  124  may be 0.31-in. deep, while the hole  125   b  for the trailing insert  130   b  may be 0.25-in. deep in the insert&#39;s surface. Yet, the heights of the two inserts  130   a - b  may be different (e.g., by about 0.06-in.) so that their extent D above the slip&#39;s surface can be about the same. This reduces the required height for the trailing insert  130   b  and can reduce the necessary metallic content of the slip  120 . 
     Still further, the diameter of holes for inserts  130  in a slip  120  can vary from segment to segment or slip assembly to slip assembly. For example,  FIGS. 11A-11B  illustrate side views of a slip body or segment  122  of a slip  120  having holes  125   c - d  of different widths or diameters, and  FIG. 11C  illustrates a plan view of the segment  122  having the holes  125   c - d . As depicted in  FIGS. 11A and 11C , the holes  125   c  toward the ramped end of the segment  122  are narrower than the holes  125   d  toward the opposite end. A reverse arrangement could be used. 
     As shown in  FIG. 11B , even though the front holes  125   c - d  have different diameters, the height of the two inserts  130   c - d  may be the same or different depending of the circumstances so that the two inserts  130   a - b  extend the same distance D above the slip&#39;s surface when installed within an appropriate tolerance for the implementation. This will produce the same outside diameters for the front and trailing inserts  130   a - b  when the slip  120  installs on a tool. 
     Given the various arrangements of holes, inserts, and the like disclosed above, additional configurations can be used on the slip bodies of a tool—some of which are discussed below.  FIG. 12A  illustrates a slip body or segment  122  of a slip  120  in cross-section. The segment  122  has holes  125   e - f  of both different depths and widths. The front hole  125   e  is less deep and narrower than the back hole  125   f , although a reverse arrangement can be used. 
       FIG. 12B  illustrates the slip segment  122  in cross-section with different inserts  130   e - f  installed in the holes  125   e - f  of different depths and widths. The insert  130   e  in the front hole  125   e  is shorter than the insert  130   f  in the back hole  125   f  so that the inserts  130   e - f  have the same distance D above the top of the segment  122 . A reverse configuration can be used. As also shown, the front insert  130   e  has a full hole therethrough, while the back insert  130   f  has a partial hole therein. However, any other configuration of inserts  130  disclosed herein can be used in the same manner. 
     Finally, previous embodiments have inserts  130  of different heights installed in holes  125  of different depth so that the overall extent that the inserts  130  extend from the segment  122  are the same. As an alternative, the inserts  130  can extend different distances from the segment  122 . For instance,  FIG. 12C  illustrates a slip body or segment  122  in cross-section with holes  125   g - h  of different depths, but the inserts  130   g - h  installed in the holes  125   g - h  have the same heights. The front hole  125   g , for example, can be deeper than the back hole  125   h . Yet, the two inserts  130   g - h  can be the same height so that the back insert  130   h  extends a distance further from the segment&#39;s top surface than the front insert  130   g . The reverse arrangement can also be used. Moreover, a comparable configuration can be achieved if the holes  125  are the same depth, but the inserts  130  are different heights, or if any other different arrangement is used. 
     Testing performed on powdered metal inserts  130  (based specifically on the cylindrical shape and dimensions discussed above with reference to  FIGS. 5A-5C ) has shown favorable results. For one test, a cast iron slip base was fitted with 24 powdered metal insert. The slip was then loaded up to 86,000 lbf. This is the equivalent axial force acting on a downhole slip of a 4.5″ composite fracture plug at 8,000 psi set in 11.6# max casing ID. During the testing, none of the powdered metal inserts  130  chipped, and they made good indentations in the casing. 
     In one embodiment hinted to above, the inserts  130  of different materials, such as the powdered metal insert  130 , can be arranged on both the uphole and downhole assemblies  110 U,  110 D of the tool  100 . One, more, or all of the segments  122  of an assembly  110 U,  110 D can have inserts  130  composed of the same or different materials. For example, a slip assembly having one, more, or all of the inserts  130  composed of powdered metal, metallic material, and/or a non-ceramic material can be used as the uphole slip assembly  110 U, the downhole slip assembly  110 D, or both assemblies  110 U,  110 D of a downhole tool  100 , such as a bridge plug used during fracturing. Likewise, a slip assembly having one, more, or all of the inserts  130  composed of ceramic material can be used as the uphole slip assembly  110 U, the downhole slip assembly  110 D, or both on the downhole tool  100 . 
     In a particular embodiment shown in  FIG. 13 , a downhole tool  100 , such as a bridge plug shown, uses different insert materials on the uphole and downhole assemblies  110 U,  110 D. The uphole slip assembly  110 U has inserts  130 U composed of ceramic material or other millable material to reduce the overall metallic content of the tool  100 . The downhole slip assembly  110 D preferably has inserts  130 D composed of a metallic material, and more particularly, a powered metal material as disclosed herein. 
     As shown in  FIG. 13 , the uphole and downhole slip assemblies  110 U,  110 D each has a slip  120  with slip bodies, elements, or segments  122  composed of a composite material. Rather than having the independent segments  122  as discussed previously that fit around the mandrel, the segments  122  on these assemblies  110 U,  110 D can form slip rings having one of several integrated segments  122  of the slip  120  connected at their proximal ends. 
     The uphole assembly  110 U uses ceramic inserts  130 U disposed in the composite material of the slip  120 . The ceramic material for the ceramic inserts  130 U can include alumina, zirconia, cermet, or any other suitable ceramic. 
     The downhole slip assembly  110 D uses metallic inserts  130 D. The metallic material can include cast iron, carbide, powdered metal, or combination thereof. However, the metallic material used can also be a metallic-ceramic composite material, such a cermet (i.e., composites composed of ceramic and metallic materials). 
     During use, the tool  100  of  FIG. 13  holds pressure from above the tool  100 . This means that the downhole slip assembly  110 D holds back all of the force generated by the pressure acting on the tool&#39;s cross-sectional area. Accordingly, the downhole slip assembly  110 D preferably uses the more robust metallic inserts  130 D. Additionally, in one particular embodiment, the metallic inserts  130 D are powdered metal inserts as disclosed herein and can be composed of a sintered-hardened powder metal as disclosed herein. 
     During use, the uphole slip assembly  110 U needs primarily to hold the initial setting force on the tool  100 . Testing shows that slip inserts composed of ceramic materials may tend to chip during use so that the anchoring ability of the slip assembly is reduced. Yet, even with the chipping, the use of ceramic for the slip inserts  130 U in the uphole slip assembly  110 U can still retain enough strength to keep the tool  100  set and to perform properly. Accordingly, use of the ceramic inserts  130 U in the uphole slip assembly  110 U can still reduce the metallic content of the tool  100 , yet achieve the hold required. The ceramic material can breakup during milling procedures, and the milled ceramic material can circulate out of the wellbore easier due to its lighter specific gravity than a metallic material. 
     In another configuration of the downhole tool  100  in  FIG. 13 , the uphole slip assembly  110 U can have inserts  130 U composed of powdered metal material, while the downhole slip assembly  110 D can have inserts  130 D composed of metallic material other than powered metal. This configuration has many of the same benefits as described above in that the millable nature of the tool  100  is increased while the downhole assembly  110 D with metallic (non-powdered metal) inserts  130  can produce the required hold. 
     As shown in the detail of  FIG. 14A , the inserts  130 U of the uphole slip assembly  110 U can all have the same geometry, although this is not strictly necessary. As shown in the detail of  FIG. 14B , the same can apply to the inserts  130 D of the downhole slip assembly  110 D. The downhole inserts  130 D can also be different than those inserts  130 U used for the uphole slip assembly  110 U. Again, however, this is not strictly necessary, as other configurations can be used. 
     Various inserts  130  disclosed herein have been described as being composed of powdered metal or ceramic materials. Other conventional materials, such as steel, iron, or high carbon steel, may be used for one, more, or all of the insets  130  in a given implementation. The slips  120  and inserts  130  can likewise have other configurations and orientations, such as those disclosed in incorporated U.S. application Ser. No. 14/039,032. 
     The foregoing description of preferred and other embodiments is not intended to limit or restrict the scope or applicability of the inventive concepts conceived of by the Applicants. It will be appreciated with the benefit of the present disclosure that features described above in accordance with any embodiment or aspect of the disclosed subject matter can be utilized, either alone or in combination, with any other described feature, in any other embodiment or aspect of the disclosed subject matter. 
     In exchange for disclosing the inventive concepts contained herein, the Applicants desire all patent rights afforded by the appended claims. Therefore, it is intended that the appended claims include all modifications and alterations to the full extent that they come within the scope of the following claims or the equivalents thereof.