Patent Publication Number: US-8978787-B2

Title: Turbine driven reaming bit with blades and cutting structure extending into concave nose

Description:
FIELD OF THE INVENTION 
     The field of the invention is reaming bits and more particularly those used on high speed, low torque turbines or motors attached to the leading end of a casing or liner string. The bits having profile characteristics that reduce torque fluctuations due to unpredictable variations in weight on bit. 
     BACKGROUND OF THE INVENTION 
     When running a casing or liner into a predrilled bore hole, it is desirable that the bore hole will have been drilled with the intended shape, to its designed diameter, and without marked deviations, such as doglegs, along its path. Unfortunately, due to unstable, heterogeneous formations, irregularities such as stringers within a formation, poor drilling practices, damage and wear of drill bits and bottom hole assemblies (BHA) and various other factors, the ideal bore hole is rarely achieved. 
     Therefore, it is desirable to provide the casing or liner string being run into the existing bore hole with a cutting structure at the leading end thereof to enable enlargement, as necessary, of portions of the bore hole so that the casing or liner may be run smoothly into the bore hole to the full extent intended. Initially the entire liner or casing string was rotated while it was being lowered into the borehole, which required powerful and complex drive systems at the surface. More recent projects use a hollow turbine or motor at the leading end of the casing string which are driven by drilling fluid pumped from the surface. It provides for a more efficient and economical transfer of power from the surface to the drill bit but it also limits the amount of torque that can be delivered to the bit and most of the power is in the form of high rotational speed. This most recent approach of using high speed turbines to provide a casing or liner string with a reaming capability has yielded inconsistent results with conventional, bullet shaped reaming bits. 
     In U.S. Pat. No. 7,621,351 a reamer bit having a substantially tubular body and a nose portion with a concave center extends from the nose portion to the side wall through a tapered shoulder region. The reaming tool further comprises a cutting structure for enlarging, also termed “reaming,” of a bore hole through contact with the side wall thereof. The term “tool” is used herein in a non-limiting sense, and embodiments of the present invention may also be characterized as a reaming bit or reaming shoe. In some embodiments, the nose portion of the reaming tool has at least one port therethrough extending to the interior of the body. In some embodiments, a plurality of circumferentially spaced, spirally configured blades extend on the exterior of the body from proximate the shoulder transition region to the gage and define junk slots there between. An axially leading end of each blade commences with substantially no standoff from the body and tapers radially outwardly to a portion having a substantially constant standoff and having a radially inwardly extending, beveled, axially trailing end. A plurality of cutting elements are disposed along a rotationally leading edge of each blade. The nose of this tool can be drilled out in a related method to allow further completion of the well. 
     In the past reaming tools that were surface driven turned typically in an RPM range of about 40-80 RPM and the large diameter, stiff casing was able to transmit high levels of torque. Turbines or high speed motors driven at speeds of 300-600 RPM and higher can only supply a fraction of the torque provided by top drives or rotary tables. Due to the lower torque capacity of the turbines the reaming tools that were previously serviceable experienced a great deal of stalling, reduced rates of penetration and generally unreliable performance. Typically these reamers had a bullet shaped profile  10 , shown in  FIG. 1 , from the cylindrical gage dimension  12  to the center  14  of the concave cone section  16  that featured a long tapered segment  18  sandwiched between a curved segment  20  that had one or two radii  22  and a lower curved transition  24  having a radius  26  that forms the leading part or nose of the profile and then continues in a bottom taper  28  that defines a recessed, concave cone  16 . In  FIG. 1  the profile length (PL) is defined as the distance along the profile between and not including the gauge dimension  12  and the center  14  of the cone  16 . The nominal diameter or bit size (BS) is double the distance from the centerline  30  to the gauge dimension  12  in a plane perpendicular to the centerline  30 . The range of PL/BS ratios of existing reamer tools that were run in the typical RPM range of 40-80 RPM was in the order of 0.76 to 1.27 for a range of BS of 5.5 to 19.25 inches. In addition to the profile length the inclination a of the long tapered section with respect to the reamer axis  30  is important. It forms a conical wedge in the borehole which provides a mechanical advantage by producing high lateral forces for small changes in axial forces or weight on bit (WOB). The mechanical advantage is proportional to 1/tan α and therefore is quite significant for smaller angles. This is desirable in applications where it is difficult to deliver sufficient WOB to advance the reamer but becomes the source of high torsional oscillations in applications where WOB control is difficult or erratic due to a complex well trajectory, borehole tortuosity, formation heterogeneity and many other operational variables. 
     While the various reamers described above functioned fairly well at higher torque and slower RPM, the recent advent of a turbine driving a reamer with less torque at significantly higher speeds of 300-600 RPM and above produced an unacceptable level of torque fluctuation and stalling of the turbines. The present invention was developed to address this situation and enhance the performance of reamers in turbine applications by making modifications to the profile and other design features as will be described below. One of the approaches was the profile modification and shortening of the PL by using a plurality of arcuate surfaces between the gauge dimension  12  and bottom taper  28  and eliminating the long, low angle, tapered segment  18  of  FIG. 1 . Another variation was to retain it but reduce its length and increase the angle of the tapered segment  18  to more than 30 degrees which reduces the aggressiveness and brings the PL/BS ratio to below 0.75. A different source of undesirable vibrations and torsional oscillations at low torque and high rpm is a perfectly symmetrical spacing of blades. Even small variations in the angular spacing between blades will significantly reduce these harmonic vibrations without having to affect the mass balance of the reamer itself. Another feature to assure reliable performance of the reamer was to extend the reamer blades into the concave cone section  16  and add additional fluid ports to enhance bottomhole cleaning. Thus the reamer is capable to effectively drill a full diameter borehole in case the pilot hole gets completely obstructed, is irregularly shaped or is backfilled with cave-ins and/or a cuttings bed in inclined, extended reach wells. Other features were added to the blade structure to protect the outer casing when running the casing or liner string through an already cased upper hole section. The long, spiraled gage pads which are extensions of the blades along the cylindrical section of the reamer bit are designed with smooth but highly wear resistant surfaces to minimize the borehole wall contact stresses and stabilize the bit at high speeds. The upper or trailing end of the gage pad is provided with a single row of active cutting elements for back-reaming while the casing string is moved up and down to condition the borehole and keep the reamer from getting stuck. At the transition from the gage pads to the leading, actively cutting blades the outer surface of the blades includes a peripheral step to allow greater exposure of the primary cutting elements. A series of projections rotationally behind the primary cutting elements limit the depth of cut to further control unintended weight on bit spikes, torsional oscillations and stalling in interbedded, mixed strength formations. These and other features of the present invention will be more readily apparent to those skilled in the art from a review of the detailed description of the preferred embodiment and the associated drawings while recognizing that the full scope of the invention is to be determined from the appended claims. 
     SUMMARY OF THE INVENTION 
     A reaming bit designed to operate with low torque fluctuation when driven with a turbine at speeds in the order of 300-600 RPM and above features a profile that is arcuate from the gage dimension to the nose area or alternatively has a greater than 30 degrees, straight taper section and a profile length (PL) to bit size (BS) ratio of under 0.75. The blade spacing is asymmetrical but the reamer itself is mass balanced. The blades extend into a concave cone section towards the center and the cutting structure and nozzle arrangement cover the entire profile to ensure continued drilling if the reamer encounters an obstructed bore hole and/or has to disperse a built-up of cuttings. The blades start with long, smooth and partially spiraled gage pads on the periphery of the reamer and transition into the blade cutting structure with increased exposure, primary cutting elements on the leading edge. An array of protrusions are disposed behind the primary cutting elements to limit depth of cut to further enhance high speed stability and to protect the outer casing on run in. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a profile view of an existing reamer tool where the ratio of the profile length to the bit size is over 0.76; 
         FIG. 2  shows the reamer tool with an arcuate profile from the gage portion to the nose; 
         FIG. 3  shows the reamer tool with a straight, high angle taper in the profile and where the ratio of the profile length to the bit size is under 0.76; 
         FIG. 4  is a front view of the reamer tool; 
         FIG. 5  is a rotated front view from the  FIG. 4  orientation showing the rupture disc location; 
         FIG. 6  is a top view showing the concave cone section of the reamer tool; 
         FIG. 7  is a perspective view of the reamer tool. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 2  shows a profile  32  that begins below the gage segment  34 . The next segment is shown as a single segment  36  with a single radius  38  which is preferably tangent to gage segment  34  but it can also be a plurality of arcuate segments with differing radii, which blend into each other. Transition segment  40  is adjacent to segment or segments  36  and curves around with a radius  42  into the leading part or nose of the profile and joins the tapered segment  46  to define the concave cone  44 . Segment  46  extends to the centerline  50 . Radius  38  and  42  can also be combined into large, single radius. The profile length (PL) is defined as the sum of the lengths of segment(s)  36 ,  40  and  46 . The bit size (BS) is defined in a plane perpendicular to the centerline  50  and is twice the distance from the centerline  50  to the gauge segment  34  that is preferably cylindrical. The use of an arcuate profile from the gage segment  34  to the nose  44  and the elimination of long, low angle tapered sections allows the reamer tool  48  to be considerably shortened and be less aggressive which is directly related to a reduction of torque fluctuation at the higher speeds and lower depth of cut of a typical turbine drive system. The difference can be readily seen in a comparison of  FIGS. 1 and 2 . 
       FIG. 3  takes the prior design of  FIG. 1  but reconfigures it to address the torque fluctuation issue at the higher speeds of the turbine driver, shown schematically as T in  FIG. 5 , by still retaining the straight taper as before but by making it more blunt and shortening it to the point that the ratio of PL/BS is less than 0.75. By changing the taper angle from about 7 degrees to more than 30 degrees the mechanical advantage or aggressiveness is reduced by about 5:1. As before there is a gauge section  60  that is cylindrical. Two arcuate sections  62  and  64  are shown having respective radii of  66  and  68 . As an option a single arcuate section with a single radius can be used instead of the two that are illustrated. A blunt, straight tapered section  70  disposed at an included angle  71  of at least 60 degrees follows leading to an arcuate shoulder transition section  72  with a nose radius  74  followed by a concave, straight segment  76  leading to the centerline  78 . As before the bit size is measured in a plane perpendicular to the centerline  78  and is twice the distance from the centerline  78  to the gage section  60 . The difference between  FIGS. 1 and 3  is that the straight tapered section is greater than 30 degrees and reduced in length to shorten the bit length to the point where the ratio of PL/BS is less than 0.75. In this instance it is the recognition that shortening the PL for a given size which is preferably accomplished with blunting the taper and shortening the straight tapered segment results in a measurable decrease in torque fluctuation and stalling when rotating with a turbine or other comparable driver that attains speeds of 300-600 RPM or higher. 
     In the preferred embodiment the profile between the gage section and the nose is fully arcuate but an alternative can be a reconfiguration of the existing profile for a reamer tool shown in  FIG. 1  by blunting the taper and shortening the PL to get the ratio of PL/BS of less than 0.75. Apart from altering the profile as discussed above, the reaming tool of the present invention has additional features discussed below to facilitate the reaming of partially obstructed or tortuous boreholes, the cleaning of debris and cuttings, protection of the existing or outer casing while tripping and finally additional secondary means to further increase the tool stability at the high rotational speeds when using turbines or similar drivers. 
     Referring to  FIGS. 4-7  three blades  80 ,  82  and  84  extend into the central, concave cone section  86 . This configuration ensures that drilling is feasible and material can be removed from the central portion of the reamer when the borehole is tortuous and/or severely compromised such as with debris or cuttings in an inclined or horizontal borehole, or where there has been a hole cave in or collapse due to tectonic stresses or inherently weak and damaged formations. The cutting structure of hard metal or polycrystalline diamond (PDC) inserts  88  in the central part  86  of the reamer and the junk slots and nozzles between blades  80 ,  82  and  84  promote cutting and adequate borehole cleaning through such obstructions. 
     Referring to  FIG. 4  the gage pads  98  extend from the top end  100  to the lower end  102 . At the top end  100  there is a cutting element  90  at the leading side and near the top of all the blades which provides the ability to back-ream when removing the reamer in the event of hole collapse behind the reamer while drilling. This allows for easier up and down movement of the reaming tool and reduces the chances of getting stuck when short-tripping or conditioning the borehole through a tight section. 
     At the lower end there is a diametrical step up  104  of about 0.050 to 0.110 inches to transition to the blades  80  which have cutting elements on their leading side. The gage pads  98  are radially slightly smaller than the adjacent, actively cutting blades to assure smooth, passive contact with the borehole wall during rotation. They are partially spiraled with a bend  106  at the transition to the straight portion. The spiraling provides more circumferential contact and with the smooth surface and slight recess adds lateral stability to the reamer tool at high rotational speeds. An array of wear resistant, hard metal inserts  108  are inserted into the gage pad surface to provide wear resistance and maintain the critical gage diameter over the life of the reamer. 
     At step up  104  the gage pads transition into the actively cutting blades with primary hard metal or PDC cutting elements  88  at the leading edge. For drilling at high speed it is desirable to limit and control the depth of cut (DOC) or advance per revolution of the reamer to dampen both axial and torsional vibrations in mixed and interbedded formations. To control the depth of cut, a series of protrusions  112  and  114  are located generally behind and rotationally in line with the primary cutting elements  88 . The exposure of these protrusions is less than that of primary inserts  88  and is adjustable based on the particular application. The protrusions  112  and  114  also protect the already existing, outer casing that the reamer may need to traverse before reaching the open hole segment to be reamed,&#39; limit the side cutting aggressiveness and thus improve directional stability in inclined and horizontal wells. The protrusions can be hard metal or PDC inserts or appropriate shapes of hardfacing material welded to the outer surface of the blades. Another way to reduce the exposure of the primary cutting elements  88  is by depositing of a layer of hardfacing material across the entire outer blade surface or parts thereof. 
     Another important feature to reduce harmful torsional and lateral accelerations is the asymmetrical spacing of the blades to prevent the formation of a repetitive pattern on the borehole bottom and prevent the harmonics produced by evenly spaced blades. This is accomplished by having a standard deviation of at least 5 degrees in the angular spacing between blades. 
     The concave shape of the central part  86  of the reamer assures that it can be milled or drilled-out from the center to the shoulder without the risk of leaving any un-drilled parts downhole which could damage the next bit or bottom hole assembly. 
     One or more rupture discs  92  are provided with communication to the internal passages that lead to inner nozzles  94  and outer nozzles  96  so that in the event there is a nozzle obstruction and pressure builds up the rupture discs  92  will break and fluid circulation can continue uninterrupted. The inner nozzles are particularly important to assure adequate cleaning when the borehole is filled with excess cuttings from the reaming process itself or accumulation of cuttings in front of the reamer. 
     Those skilled in the art will appreciate that the reaming tool of the present invention designed to operate at speeds in the order of 300-600 RPM and higher has features that limit torque fluctuation using an arcuate profile between the gage section and concave cone section so as to eliminate an aggressive tapered section and shorten the profile length. An alternative design retains a straight tapered segment in the profile but the taper is greater than 30 degrees and the PL/BS ratio is smaller than 0.75 to shorten the height of the reaming tool and thus reduce torque fluctuation and stalling tendencies at high rotational speeds. The ability of the reamer to drill-out fully or partially obstructed holes is greatly enhanced by extending at least some of the blades with PDC cutting elements into the concave cone section and near to the center. Other features that aid the dynamic stability are asymmetrical spacing of the blades, depth of cut control through reduced exposure of the primary cutting elements and smooth spiraled and slightly recessed gage pads to restrict lateral motion. The row or rows of protrusions behind the primary PDC cutters promote not only dynamics stability but also reduce the side cutting aggressiveness when reaming an inclined wellbore and protect already existing outer casing when the next casing string with the turbine/reamer at its leading end is run into the borehole. 
     The above description is illustrative of the preferred embodiment and many modifications may be made by those skilled in the art without departing from the invention whose scope is to be determined from the literal and equivalent scope of the claims below: