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BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates in general to hybrid drill bits and, in particular, to an improved system, method, and apparatus for a hybrid drill bit having a combination of rolling cones and fixed cutter elements for cutting at a center of the drill bit. 
     2. Description of the Related Art 
     In the prior art, some drilling bits use a combination of one or more roller cones and one or more fixed blades. Some of these combination-type drill bits are referred to as hybrid drill bits. Previous designs of hybrid drill bits, such as U.S. Pat. No. 4,343,371, to Baker, III, have provided for the roller cones to do most of the formation cutting, especially in the center of the hole or bit. Other types of combination drill bits are known as “core bits,” such as U.S. Pat. No. 4,006,788, to Garner. Core bits typically have truncated roller cones that do not extend to the center of the bit and are designed to remove a core sample of formation by drilling down but around a solid cylinder of the formation before being removed. 
     Another type of hybrid drill bit is described in U.S. Pat. No. 5,695,019, to Shamburger, Jr., wherein the roller cones extend almost entirely to the center. Fixed cutter inserts  50  ( FIGS. 2 and 3 ) are located in the dome area 2 or “crotch” of the bit to complete the removal of the drilled formation. Still another type of hybrid bit is sometimes referred to as a “hole opener,” an example of which is described in U.S. Pat. No. 6,527,066. A hole opener has a fixed threaded protuberance that extends axially beyond the roller cones for the attachment of a pilot bit that can be a roller cone or fixed cutter bit. In these latter two cases the center is cut with fixed cutter elements but the fixed cutter elements do not form a continuous, uninterrupted cutting profile from the center to the perimeter of the bit. 
     Although each of these drill bits is workable for certain limited applications, an improved hybrid drill bit with enhanced drilling performance would be desirable. 
     SUMMARY OF THE INVENTION 
     One embodiment of a system, method, and apparatus for a hybrid drill bit comprises both roller cones and fixed blades. Some of the fixed cutting elements on the fixed blades are located at and near the axial center of the bit body to cut formation at the axial center. The roller cone cutting elements and the fixed cutting elements combine to define a cutting profile that extends from the axial center to the radial perimeter. The fixed cutting elements form the cutting profile at the axial center and the perimeter, while the roller cone cutting elements assist the fixed cutting elements in the midsection of the cutting profile between the axial center and the perimeter. 
     The midsection comprises a nose section and a shoulder section. The nose and shoulder sections are known to be the most vulnerable parts of a fixed cutter bit and are subject to extreme loading and wear. The nose is the leading part of the overall profile and the shoulder must resist side loading and lateral vibrations. In one embodiment, some of the roller cone cutting elements and the fixed cutting elements are axially aligned at the nose of the bit. 
     The foregoing and other objects and advantages of the present invention will be apparent to those skilled in the art, in view of the following detailed description of the present invention, taken in conjunction with the appended claims and the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the features and advantages of the present invention, which will become apparent, are attained and can be understood in more detail, more particular description of the invention briefly summarized above may be had by reference to the embodiments thereof that are illustrated in the appended drawings which form a part of this specification. It is to be noted, however, that the drawings illustrate only some embodiments of the invention and therefore are not to be considered limiting of its scope as the invention may admit to other equally effective embodiments. 
         FIG. 1  is a bottom view of one embodiment of hybrid drill bit constructed in accordance with the present invention; 
         FIG. 2  is a side view of the hybrid drill bit of  FIG. 1  and is constructed in accordance with the present invention; 
         FIG. 3  is a side view of the hybrid drill bit of  FIG. 1  and is constructed in accordance with the present invention; and 
         FIG. 4  is composite rotational side view of the roller cone inserts and the fixed cutting elements on the hybrid drill bit of  FIG. 1  and is constructed in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIGS. 1-3 , one embodiment of a system, method, and apparatus for a hybrid drill bit is disclosed. The drill bit  11  comprises a bit body  13  having an axis  15  that defines an axial center of the bit body  13 . A plurality (e.g., two shown) of roller cone or cutter support arms or bit legs  17  extend from the bit body  13  in the axial direction. The bit body  13  also has a plurality (e.g., also two shown) of fixed blades  19  that extend in the axial direction. The number of each of arms  17  and fixed blades  19  is at least one but may be more than two. In one embodiment, the centers of the arms  17  and fixed blades  19  are symmetrically spaced apart from each other about the axis  15  in an alternating configuration. As illustrated, each blade  19  has a leading edge that extends from the radially outermost or gage portion of the bit body  13  to the axial center  15  of the bit body  13 . 
     Roller cones or cutters  21  are mounted to respective ones of the arms  17 . Each of the roller cones or cutters  21  is truncated in length such that the distal ends of the roller cones or rolling cutters  21  are radially spaced apart from the axial center  15  ( FIG. 1 ) by a minimal radial distance  23 . A plurality of roller cone or rolling-cutter cutting inserts or elements  25  are mounted to the roller cones or rolling cutters  21  and radially spaced apart from the axial center  15  by a minimal radial distance  27 . The minimal radial distances  23 ,  27  may vary according to the application, and may vary from cone to cone, and/or cutting element to cutting element. 
     In addition, a plurality of fixed cutting elements  31  are mounted to the fixed blades  19 . As illustrated, fixed or fixed-blade cutting elements  31  are arranged in a row on the leading edge of each blade  19 . The row extends from the radially outermost gage portion of the blade  19  and bit body  13  to or very near the axial center  15  of the bit body  13 . At least one of the fixed cutting elements  31  is located at the axial center  15  of the bit body  13  and adapted to cut a formation at the axial center. In one embodiment, the innermost edge of at least one of the fixed cutting elements  31  is within approximately 0.040 inches of the axial center. Examples of roller cone cutting elements  25  and fixed cutting elements  31  include tungsten carbide inserts, cutters made of super hard material such as polycrystalline diamond, and others known to those skilled in the art. 
     As shown in  FIG. 4 , the roller cone cutting elements  25  and the fixed cutting elements  31  combine to define a cutting profile  41  that extends from a cone region proximal the axial center  15  (named for the cone of formation material that forms underneath this region) to a gage region radially outermost perimeter  43  with respect to the axis. In one embodiment, only the fixed cutting elements  31  form the cutting profile  41  in the cone region at the axial center  15  and the gage or radially outermost perimeter  43 . However, the roller cone cutting elements  25  overlap to produce substantially congruent surfaces or kerfs in the formation being drilled with the fixed cutting elements  31  on the cutting profile  41  between the cone region near the axial center  15  and the gage region at the radially outermost perimeter  43 . The roller cone cutting elements  25  are configured to cut at the nose  45  and part of the shoulder  47  of the cutting profile  41 , where the nose  45  is the axially leading part of the profile (i.e., located between the axial center  15  and the shoulder  47 ) facing the borehole wall and located adjacent the radially outermost perimeter  43 . In this context, “shoulder” is used to describe the transition in the cutting profile between the nose region  45  and the gage region. 
     Thus, the roller cone cutting elements  25  and the fixed cutting elements  31  combine to define a congruent cutting face in the nose  45  and part of the shoulder  47 , which are known to be the most difficult to drill parts of a fixed cutter bit profile. A reference plane  51  is located at a distal axial end of the hybrid drill bit  11 , tangent to the axially distal-most portion of bit  11 , as illustrated. At least one of each of the roller cone cutting elements  25  and the fixed cutting elements  31  extend in the axial direction at the reference plane  51  at a substantially equal dimension and, in the illustrated embodiment, are radially offset from each other even though they axially align. However, alternatively, the axial alignment between the distal-most elements  25 ,  31  is not required such that elements  25 ,  31  may be axially spaced apart by a selected distance when in their distal-most position. 
     In one embodiment, the fixed cutting elements  31  are only required to be axially spaced apart from and distal (e.g., lower than) relative to the crotch  53 . In another embodiment, the roller cones  21  and roller cone cutting elements  25  may extend beyond (e.g., by approximately 0.060-inches) the distal most position of the fixed blades  19  and fixed cutting elements  31  to compensate for the difference in wear between those components. As the profile  41  transitions from the shoulder  47  to the perimeter or gage of the hybrid bit  11 , the rolling cutter inserts  25  are no longer engaged (see  FIG. 4 ), and multiple rows of vertically-staggered (i.e., axially) fixed cutting elements  31  ream out a smooth borehole wall. Rolling cone cutting elements  25  are much less efficient in reaming and would cause undesirable borehole wall damage. 
     The invention has several advantages and includes providing a hybrid drill bit that cuts at the center of the hole solely with fixed cutting elements and not with roller cones. The fixed cutting elements are highly efficient at cutting the center of the hole. Moreover, due to the low cutting velocity in the center, the super hard material or polycrystalline cutting elements are subject to little or no wear. The roller cones are configured to enhance the cutting action of the blades in the most difficult to drill nose and shoulder areas, which are subjected to high wear and vibration damage in harder, more abrasive formations. The crushing action of the tungsten carbide roller cone inserts drives deep fractures into the hard rock, which greatly reduces its strength. The pre- or partially fractured rock is easier to remove and causes less damage and wear to the fixed cutting elements. The perimeter or gage of the borehole is generated with multiple, vertically-staggered rows of fixed cutter inserts. This leaves a smooth borehole wall and reduces the sliding and wear on the less wear-resistant rolling cutter inserts. 
     While the invention has been shown or described in only some of its forms, it should be apparent to those skilled in the art that it is not so limited, but is susceptible to various changes without departing from the scope of the invention.

Summary:
A hybrid drill bit having both roller cones and fixed blades is disclosed. The cutting elements on the fixed blades form a continuous cutting profile from the perimeter of the bit body to the axial center. The roller cone cutting elements overlap with the fixed cutting elements in the nose and shoulder sections of the cutting profile between the axial center and the perimeter. The roller cone cutting elements crush and pre-fracture formation in the weak and highly stressed nose and shoulder sections.