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CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of application Ser. No. 09/448,216, filed Nov. 23, 1999, now abandoned, which is a continuation of application Ser. No. 08/903,226, filed Jul. 22, 1997, now U.S. Pat. No. 6,029,746, and also claims priority from provisional application Ser. No. 60/167,119, filed Nov. 23, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to tools that are used for drilling water and hydrocarbon wells and, more particularly, to a drill bit sub that generates a pulsating jet flow in drilling fluid circulated through a conventional drill bit. The drill bit sub of the invention can be used with either drill pipe or coiled tubing. Use of the subject drill bit sub to achieve greater reaches during horizontal drilling is also disclosed. 
     2. Description of Related Art 
     It has been suggested in the prior art to use acoustic energy for stimulating producing wells. A fluidic oscillator may be used to create pressure fluctuations to induce stress in the walls of the perforation tunnel, thereby increasing production and cleaning perforations as disclosed in U.S. Pat. Nos. 5,135,0531 and 5,228,508 issued to Facteau. The pressure fluctuations of the Facteau tool are generated from an oscillation chamber with two outlet ports. A similar fluidic oscillation chamber with dual outlet ports is disclosed in U.S. Pat. No. 5,165,438 also issued to Facteau. Another stimulation tool using acoustic energy is disclosed in U.S. Pat. No. 3,520,362 issued to Galle. Although the above recited tools seemed feasible, there exists a practical difficulty of delivering sufficient acoustic power to the producing formation for the desired stimulation and/or to the area to be cleaned. 
     IADC/SPE paper 27468 teaches the use of Helmholtz oscillator theory for generating a pulsating jet flow in drill bits. The pulsating jet flow is said to produce much higher instantaneous jet velocity and impact pressure than experienced with non-pulsating flow, resulting in significant improvements in hole cleaning and drilling rate. Pulsed high pressure water jets are known to have advantages over continuous jet streams for use in cutting materials, especially brittle materials. By exerting an alternating load on materials, pulsed jets can produce not only extremely high momentary pressures (i.e. water hammer effect) in the materials, but also absolute tensile stress, which gives rise to unloading destruction of brittle materials, through reflection of the stress waves. Pulsating jet flow is also known to reduce the hydraulic hold-down effect, thereby increasing the cutting and cleaning action of the jet. IADC/SPE 27468 discloses a new nozzle that generates the pulsating jet flow by a self-oscillation of fluid in the oscillation chamber. A “bump surface” at the bottom of the oscillation chamber, shown for example in FIG. 3, is said to be a key factor in the quality of the pulsating jet flow. The article concludes that the nozzle shape, manufacturing technique, and the bit structure may be further studied to improve the pulsating jet flow quality. 
     A discussion of design parameters for self-excited oscillation jet nozzles is included in a paper entitled “Nozzle Device for the Self-Excited Oscillation of a Jet” presented as Paper  19  at the 8th International Symposium on Jet Cutting Technology held in Durham, England, Sep. 9-11, 1986 and available from BHRA, the Fluid Engineering Centre, Cranfield, Bedford MK430AJ, England. 
     A drill bit sub is needed, however, that can create a self-excited, pulsating flow of drilling fluid in an oscillation chamber above a conventional drill bit of the type typically employed for drilling water, oil and gas wells using either a standard drilling string or coiled tubing. The desired drill bit sub should be useable with cone, blade or PDC bits having single or multiple ports. 
     SUMMARY OF THE INVENTION 
     The present invention comprises a self-excited drill bit sub that creates a pulsating flow of drilling fluid utilizing Helmholtz oscillation theory prior to entering the drill bit. The pulsating stream is caused by vortices which are created inside the tool by structural features not disclosed in the prior art. The vortices create pressure pulses as they leave the sub and travel into and through the ports in the drill bit. The cyclic pressure pulses facilitate cutting and removal of cuttings from the interface between the drill bit and the formation. In contrast to drill bits using jet nozzles that generate pulsed flow, the drill bit sub of the invention provides a longer flow path for the pulsed flow prior to exiting the bit, providing a more steady flow through the bit, and causing less erosion of the nozzles. Use of the drill bit sub disclosed herein also insures that all bit nozzles are pulsing in unison, and reduces the likelihood of forming a destructive standing wave when rotating while pulsating during drilling. 
     The drill bit sub of the invention desirably includes an elongated tubular first member adapted on an upper end for connection to a running string. The first member includes an upper portion with a central bore open to a top of the tool, and a lower portion having a cylindrical shaped cavity open to a bottom surface of the first member. The cylindrical cavity is internally threaded in a lower portion and has an internal diameter larger than the diameter of the central bore of the upper portion. The cylindrical cavity has an interior wall height of the unthreaded portion that is less than the diameter of the cylindrical cavity. The central bore of the first member is open to the cylindrical cavity for delivering drilling fluid supplied through the running string. 
     The drill bit sub further includes a second elongated tubular member having an upwardly directed, male threaded end and a central bore having a diameter larger than the diameter of the central bore of the first member but less than the diameter of the cylindrical cavity of the first member. Importantly, the top end surface of the second tubular member comprises a truncated annular conical projection having a blunt annular horizontal surface adjacent to the bore, a conical shoulder extending downward and radially outward from the blunt surface, and another annular horizontal surface extending radially outward between the base of the conical shoulder and the threaded edge of the male end. The male threads on the top of the second tubular member are receivable in the internally threaded lower portion of the first member. The second tubular member also preferably includes a downwardly facing, female threaded box end adapted to receive the male threaded pin end of a drill bit so that, when connected, the bit sub discharges pulsating drilling fluid through the bit exit ports and against the formation. When the drill bit sub of the invention is assembled with the male end of the second member threaded into the lower female end of the first member, an internal oscillation chamber is formed. 
     According to another preferred embodiment of the invention, a drill bit sub is provided that comprises an oscillation chamber disposed above a drill bit, the drill bit sub having at least two radially and axially spaced, annular impingement surfaces interconnected by an inclined annular surface. The oscillation chamber is preferably coaxially aligned and in fluid communication with upper and tubular bores that each have diameters less than the diameter of the chamber. According to a particularly preferred embodiment of the invention, the oscillation chamber has a diameter from about three to about five times the diameter of the upper tubular bore and a height between 1.6 and 5.6 times the diameter of the upper tubular bore. The diameter of the lower tubular bore is preferably about 1.3 times the diameter of the upper tubular bore. 
     In accordance with the present invention, the subject drill bit sub is easily fabricated and assembled, has no moving parts, and significantly increases the drilling rate as compared to a drill string using the same bit without the bit sub of the invention. In drilling, the subject drill bit sub achieves a higher penetration rate due to the higher impact pressure created from pulsing the jet stream and the reduction of the “hydraulic hold-down effect” on the cuttings that is caused by conventional straight jets. The drill bit sub of the invention not only aids in the break-up of hard, brittle material but also aids in the cleaning of the bit and removal of debris from the hole bottom. The ability to keep the bit clean and the hole bottom free of debris causes the drill bit/hole bottom contact to be greatly increased, thereby increasing the bit penetration rate. Use of the subject drill bit sub requires no other changes in drilling procedure. 
     Some of the advantages of locating the oscillator of the invention in a separate drill bit sub above the drill bit are: 
     the sub can be made in various sizes to adapt to any size bit and collar combinations; 
     there is ample space to machine the oscillator directly into the collar; 
     the porting can be large enough to eliminate the “washing” effect due to high pressures and abrasive material; 
     by using no moveable parts, the oscillator will operate at its maximum efficiency; 
     “plugging” of the ports is virtually eliminated due to the port sizing; and 
     the drill bit sub can be machined from 4140 heat treated and hardened to s 2832 Rockwell C hardness, which is the same hardness as material used to make drill collars. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The apparatus of the invention is further described and explained in relation to the following figures of the drawings in which: 
     FIG. 1 is a simplified cross-sectional elevation view of a preferred embodiment of the self-excited, pulsating drill bit sub of the invention as employed on a drill string inside a well bore; 
     FIG. 2 is a cross-sectional elevation view of the upper member of the subject drill bit sub; 
     FIG. 3 is a cross-sectional elevation view of the lower member of the subject drill bit sub; 
     FIG. 4 is an enlarged, diagrammatic view showing the flow of drilling fluid within the oscillation chamber and the vortices that are created within the oscillation chamber to cause the pulsating flow to the drill bit; and 
     FIG. 5 is an enlarged, diagrammatic view showing the flow of pulsating drilling fluid out of the drill bit sub and into and through the exit ports of a drill bit (shown in dashed outline). 
    
    
     Like reference numerals are used to designate like parts in all figures of the drawings. 
     DETAILED DESCRIPTION 
     Referring to FIGS. 1-3, the present invention is a self-excited drill bit sub  10  that creates a pulsating jet flow within a conventional drill bit  12  utilizing Helmholtz oscillation theory. Drill bit  12  can be, for example, a cone, blade or PDC bit having single or multiple outlet ports. Drill bit sub  10  is suspended in well bore  14  on a running string  16  comprising either coiled tubing or conventional tubing that extends upwards to the surface. It will be understood by those skilled in the art that the running string may include conventional 2 ⅜ inch or 2 ⅞ inch diameter upset tubing, 1 inch macaroni string tubing or coiled tubing. In some applications, crossovers, as well known in the art, may be needed to connect the drill bit sub  10  to the running string  16 . 
     Drill bit sub  10  of the invention preferably comprises upper tubular member  18  and lower tubular member  20 . Referring to FIGS. 1 and 2, upper tubular member  18  further comprises female threaded box end  22  adapted to receive the male threaded pin end of a drill collar on running string  16 . Central bore  24  extends axially through upper tubular member  18  from box end  22  to a substantially cylindrical section bounded by sidewall  26 , which desirably has a diameter greater than that of central bore  24  and, together with annular divider wall  27 , forms the side and top walls, respectively, of oscillation chamber  44 . Referring to FIG. 2, the lower end  54  of upper tubular member  18  preferably comprises another substantially cylindrical section having female Acme threads  48 . 
     Referring to FIGS. 1 and 3, lower tubular member  20  of drill bit sub  10  further comprises central bore  34  and terminates at its upper end in an annular, truncated conical projection having a flat annular ledge surface  28  surrounded on the outside by tapered conical shoulder  30 , and a flat, upwardly facing annular surface  32  that abuts against downwardly facing shoulder  46  of upper tubular member  18  between cylindrical section  26  and female Acme threads  48 . Annular surfaces  28 ,  32  of lower tubular member  20  and inclined annular shoulder  30  between them cooperate with cylindrical sidewall  26  and annular divider wall  27  to define the walls of oscillation chamber  44 . Central bore  34  extends axially through lower tubular member  20  from its top end to a female-threaded, downwardly facing box end  36  that is adapted to receive a cooperatively threaded pin end of drill bit  12  (shown in dashed outline in FIGS. 1 and 5 ) having at least one discharge port  40  connected by a passageway  42  to central bore  34  of lower tubular member  20 . 
     Referring to FIG. 3, the outside diameter of lower tubular member  20  is preferably reduced near its top end and is desirably provided with male Acme threads  50  that cooperatively engage female threads  48  of upper tubular member  18 . Referring to FIGS. 2 and 3, shoulder  52  of lower tubular member  20  and lower end  54  of upper tubular member  18  are also placed in abutting contact when upper and lower tubular members  18 ,  20 , respectively, are threaded together to form oscillation chamber  44 . 
     When drill bit sub  10  is assembled with the upper portion of lower tubular member  20  threadedly engaged in the lower portion of upper tubular member  18  as shown in FIG. 1, internal oscillation chamber  44  is formed. In order for oscillation chamber  44  to function in the desired manner to form vortices and excite a pulsating flow within and through drill bit sub  10 , there are preferred ratios for the dimensions of different portions of the sub. Referring to FIG. 4, the diameter of central bore  24  extending axially through upper tubular member  18  is identified as D 1 . The internal diameter of sidewall section  26  of upper tubular member  18  is identified as D 2  and the height of sidewall section  26  is identified as H. The diameter of central bore  34  of lower tubular section  20  is identified as D 3 . Although it should be appreciated that the drawing figures are not drawn to scale, for preferred generation of vortices in oscillation chamber  44 , D 2  ranges from about 3 to about 5 times D 1 ; height H 1  ranges between 1.6 and 5.6 times D 1 , and most preferably about 3 times D 1 ; and D 3  is most preferably about 1.3 times D 1 . Tapered conical shoulder  30  preferably extends downwardly and outwardly at an angle  2  that is about 30 degrees from vertical. The width of annular surface  28  is desirably at least about ⅛ inch. While the recited proportions and dimensions are preferred for use in the invention, it will be appreciated that beneficial results can also be achieved where the dimensions vary within reasonable limits from those recited. 
     Referring to FIGS. 4 and 5, drilling fluid directed downward through the tubing string as indicated by arrows  57  spreads outwardly upon entering oscillation chamber  44 , as indicated diagrammatically by arrows  58 . That portion of the fluid flow that contacts blunt annular surface  28 , conical surface  30  and annular surface  32  is redirected, forming vortices  60  that cause the fluid to pulse as it travels downwardly into and through central bore  34 . Smaller vortices  62  in pulsed turbulent flow  63  continue downwardly through bore  34 . Referring to FIG. 5, additional vortices  64  are formed when flow  63  expands radially upon entering the conical section bounded by sidewall  70  above female threaded box portion  36  of lower tubular member  20  and contacts annular end  68  of the pin end of a drill bit  12  (partially shown in phantom outline) when engaged with the box end of member  20 . The pulsed flow containing vortices  62  then continues downwardly through bore section  38 , passages  42  and exit ports  40  of the drill bit, as shown by flow lines  66 ,  68 . Passages  42  and exit ports  44  are preferably sized so that there is no restriction in cross-sectional area as the drilling fluid passes through bore  38 , passages  42  and ports  40  of bit  12 . 
     Table A includes dimensions D 2 , D 1 , D 3  and H (inches) and D 1  and D 3  Areas (square inches) for selected embodiments of the present invention. It will be understood by those skilled in the art that the present invention is not limited to the disclosed preferred embodiments as listed in Table A below: 
     
       
         
               
               
               
               
               
               
             
           
               
                 TABLE A 
               
               
                   
               
               
                 D 2   
                 D 1   
                 D 3   
                 H 
                 D 1  AREA 
                 D 3  AREA 
               
               
                   
               
             
             
               
                 1.0000 
                 0.3438 
                 0.4469 
                 1.0314 
                 0.0928 
                 0.1569 
               
               
                 1.0000 
                 0.3750 
                 0.4875 
                 1.1250 
                 0.1104 
                 0.1867 
               
               
                 1.0000 
                 0.4062 
                 0.5281 
                 1.2186 
                 0.1296 
                 0.2190 
               
               
                 1.0000 
                 0.4375 
                 0.5688 
                 1.3125 
                 0.1503 
                 0.2541 
               
               
                 1.0000 
                 0.4688 
                 0.6094 
                 1.4064 
                 0.1726 
                 0.2917 
               
               
                 1.0000 
                 0.5000 
                 0.6500 
                 1.5000 
                 0.1964 
                 0.3318 
               
               
                 1.0000 
                 0.5312 
                 0.6906 
                 1.5936 
                 0.2216 
                 0.3745 
               
               
                 1.0000 
                 0.5625 
                 0.7313 
                 1.6875 
                 0.2485 
                 0.4200 
               
               
                 1.0000 
                 0.5938 
                 0.7719 
                 1.7814 
                 0.2769 
                 0.4680 
               
               
                 1.0000 
                 0.6250 
                 0.8125 
                 1.8750 
                 0.3068 
                 0.5185 
               
               
                 1.0000 
                 0.6562 
                 0.8531 
                 1.9686 
                 0.3382 
                 0.5715 
               
               
                 1.0000 
                 0.6875 
                 0.8938 
                 2.0625 
                 0.3712 
                 0.6274 
               
               
                 1.0000 
                 0.7189 
                 0.9346 
                 2.1567 
                 0.4059 
                 0.6860 
               
               
                 1.0000 
                 0.7500 
                 0.9750 
                 2.2500 
                 0.4418 
                 0.7466 
               
               
                 1.0000 
                 0.8125 
                 1.0563 
                 2.4375 
                 0.5185 
                 0.8762 
               
               
                 1.0000 
                 0.8750 
                 1.1375 
                 2.6250 
                 0.6013 
                 1.0162 
               
               
                 1.0000 
                 0.9375 
                 1.2188 
                 2.8125 
                 0.6903 
                 1.1666 
               
               
                 1.0000 
                 1.0000 
                 1.3000 
                 3.0000 
                 0.7854 
                 1.3273 
               
               
                 1.0000 
                 1.1250 
                 1.4625 
                 3.3750 
                 0.9940 
                 1.6799 
               
               
                 1.0000 
                 1.2500 
                 1.6250 
                 3.7500 
                 1.2272 
                 2.0739 
               
               
                 1.0000 
                 1.3750 
                 1.7875 
                 4.1250 
                 1.4849 
                 2.5095 
               
               
                 1.0000 
                 1.5000 
                 1.9500 
                 4.5000 
                 1.7672 
                 2.9865 
               
               
                 1.0000 
                 1.6250 
                 2.1125 
                 4.8750 
                 2.0739 
                 3.5050 
               
               
                 1.0000 
                 1.7500 
                 2.2750 
                 5.2500 
                 2.4053 
                 4.0649 
               
               
                 1.0000 
                 1.8750 
                 2.4375 
                 5.6250 
                 2.7612 
                 4.6664 
               
               
                 1.0000 
                 2.0000 
                 2.6000 
                 6.0000 
                 3.1416 
                 5.3093 
               
               
                 1.0000 
                 2.2500 
                 2.9250 
                 6.7500 
                 3.9761 
                 6.7196 
               
               
                   
               
             
          
         
       
     
     In constructing drill bit sub  10 , the dimension D 1  is desirably selected first, based on the desired flow rate and pressure drop to be encountered through the tool. The dimensions D 3  and H are then calculated according to the preferred design parameters described above. 
     Although the present invention is not limited by the following description, it is believed that because the diameter D 2  of the oscillation chamber is much larger than the diameter D 1  of the inlet bore  24 , the speed of the fluid in oscillation chamber  44  is slower than that of the inlet jet. This difference in fluid speed creates fierce shear movement at the interface between the fast and slower moving fluids in chamber  44 . Because of the viscosity of the fluid there must be a momentum exchange between the two fluid components through the interface. The shear flow results in vortices. With the inlet bore being round, the vortex lines take the shape of a circle; i.e., the vortices come about and move in the form of a vortex ring. The impingement of orderly axis-symmetric disturbances, such as the vortex ring, in the shear layer on the edge of the discharge bore generates periodic pressure pulses. These pressure pulses propagate upstream to the sensitive initial shear layer separation region and induce further fluctuations in the vortices. The inherent instability of the shear layer amplifies small disturbances imposed on the initial region. This amplification is selective; i.e., only disturbances with a narrow frequency range get amplified. Where f=frequency; U 0 =velocity at the jet axis; D 1 =diameter of the inlet bore; and SD=dimensionless frequency=fD 1 /U o ; if the frequency of a disturbance is f=S D U O /D 1  the disturbance will receive maximum amplification in the jet shear layer between the initial separation region and the impingement zone. The amplified disturbance travels downstream to impinge on the edge again. Thereupon the events above are repeated in a loop consisting of emanation, feedback and amplification of disturbances. As a result, a strong oscillation is developed in the shear layer and even in the jet core. A fluctuation pressure field is set up within the oscillation chamber  44 . The velocity of the jet emerging from bore  34  varies periodically, thus a pulsed jet is produced. The oscillation is referred to as self-excited oscillation because it comes into being without any external control or excitation. Low frequency, self-excited oscillation is observed when the oscillation chamber height H, varies in the range of 1.6&lt;H/D 1 &lt;5.6. Low frequency oscillation has a relatively high pressure fluctuation rate. In a desired range of operation bit sub  10  creates pressure pulsations between 100 and 245 cycles per second. 
     The following are results of a test which took place in a field in Texas. The controls of the test were to install the drill bit sub of the invention in the drill string and drill as normal the type of bit and formation in the area. 
     
       
         
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 BIT 
                   
                   
                   
                   
                   
               
               
                   
                 SIZE 
                 IN 
                 OUT 
                 FOOTAGE 
                 HR&#39;S. 
                 FPH 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 VORTECH #1-A 
                 7 ⅞″ 
                 3610 
                 5100 
                 1490 
                 46.25 
                 32.22 
               
               
                 OFF-SET #1 
                 7 ⅞″ 
                 3606 
                 5150 
                 1544 
                 57 
                 27.09 
               
               
                 IN-FIELD #4 
                 7 ⅞″ 
                 3587 
                 5110 
                 1523 
                 52.5 
                 29.01 
               
               
                 OFF-SET #3 
                 7 ⅞″ 
                 3610 
                 5130 
                 1520 
                 68.75 
                 22.11 
               
               
                   
               
             
          
         
       
     
     All of the wells were drilled with the same drilling rig and crew. The results of the bit inspection indicated there was no adverse effect on the drill bit. The caliper logs showed no wash-outs in the drilled hole size that were abnormal for the field. There was no change in the degree of deviation beyond that normal for the field. The cuttings which were examined during the drilling process appeared no different than in the other wells. 
     Another example of the efficiencies achievable through use of the drill bit sub of the invention involved an application where the drill bit sub was used in drilling out 10,500 feet of cemented well bore casing. The drilling operation was expected to take 8-10 days using normal (no bit sub) procedures. With the bit sub of the invention, the job was completed in 61 hours. A further indication of the increased rate of penetration (“ROP”) achieved through use of the invention is that experienced members of the operation said that whereas they normally would only be able to drill three 30 ft. pipe joints an hour, use of the invention increased the ROP to 10 joints per hour. The rate was actually restricted to enable cuttings being produced to circulate out of the well bore. 
     Although preferred and alternate embodiments of the invention have been illustrated in the accompanying drawings and described in the foregoing detailed description, it will be understood the invention is not limited to the embodiments disclosed but is capable of numerous modifications without departing from the scope of the invention as claimed.

Summary:
A drill bit sub attachable to a drilling string above the drill bit, the sub having an oscillation chamber with at least two radially and axially spaced, annular impingement surfaces interconnected by an inclined annular surface. The oscillation chamber is preferably coaxially aligned and in fluid communication with upper and tubular bores that each have diameters less than the diameter of the chamber. The oscillation chamber preferably has a diameter from about three to about five times the diameter of the upper tubular bore and a height between 1.6 and 5.6 times the diameter of the upper tubular bore. The diameter of the lower tubular bore is preferably about 1.3 times the diameter of the upper tubular bore.