Abstract:
A cavitation apparatus (CHRS) for use in subterranean drilling with a drilling rig, the rig having a drill bit and a drilling string mounted thereto through which a drilling fluid flows to the drill bit. The cavitation apparatus (CHRS) is adapted to be mounted in-line with the drilling string through which the drilling fluid flows. The CHRS has no moving parts and produces controlled low frequency, longitudinal fluctuations of at least a portion of a drill string to enhance the drilling rate of penetration (ROP). Optionally the entire drill string can be subjected to such controlled low frequency, longitudinal fluctuations.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to subterranean drilling. In particular, the invention relates to a cavitation apparatus and method, the cavitation apparatus is termed herein a Cavitating Hydraulic Resonator System (CHRS). The CHRS has no moving parts and produces controlled low frequency, longitudinal fluctuations of at least a portion of a drill string to enhance the drilling rate of penetration (ROP). Optionally the entire drill string can be subjected to such controlled low frequency, longitudinal fluctuations. 
         [0003]    2. Description of the Prior Art 
         [0004]    Referring to  FIG. 1  herein, in subterranean drilling, a well is produced by rotating and pushing down a drill bit  20  which is affixed to the leading end of a long metal rod called a drill string  30  into the drilled formation  40 . The drill string  30  includes, for example, a drill pipe, drill collars and drilling tools, such as reamers and shock tools, with the drill bit  20  being located at the extreme bottom end. The drill bit  20  is rotated either by rotating the entire drilling string  30  using special motors  50  located at the top of the drilling rig  60  or by special motors (not shown in  FIG. 1 ) installed inline of the drilling string  30  rotating only part of the drilling string  30 . In all cases, the drill bit  20  breaks the formation  40  either by a crushing or shearing action, depending on the type of drill bit  20  used. 
         [0005]    During the course of drilling operations, drilling fluid, often called drilling mud, is pumped downwardly through the hollow drill string  30 . The drilling fluid exits the drill string  30  at the drill bit  20  and flows upwardly along the well bore to the surface. The drilling fluid carries away cuttings, such as rock chips. 
         [0006]    Over the years, the depth of wells drilled has increased dramatically. Drilling a deep underground well is an extremely expensive operation. Great cost savings can be achieved if the drilling process can be made more rapid. A large number of factors affect the penetration rate that can be achieved in drilling a well. 
         [0007]    It is known that that the efficient destruction of hard rocks in drilling is through vibration-rotary drilling. It has been found useful in drilling such wells to introduce a longitudinal vibration to the drill bit  20 . In substantially vertical wells, the presence of such longitudinal vibration of the drill bit  20  has been shown to increase the drilling rate of penetration (ROP). Additionally, such longitudinal vibration to the drill bit  20  reduces the drilling string  30  torsional vibrations which cause the drill bit  20  to drift and move laterally in the well, distorting the normally circular shape of the well hole. 
         [0008]    A technique for introducing a longitudinal vibration to the drill bit  20  consists of setting up high frequency and high-intensity longitudinal vibrations on the rock cutting tool. The advantage of this method consists of the fact that it combines the advantages of vibration drilling and rotary drilling. In combined methods of rock destruction, the rock is acted upon not only by static forces, but also by dynamic shock pulses (short-time loads). Under the action of these forces, the rock not only is broken and chipped off under the rock cutting tool when struck, but also is cut off or chipped off due to the rotation and axial static loading. 
         [0009]    Such techniques are particularly useful in directional or inclined drilling applications, as shown in  FIG. 1 . Such wells have long horizontal or highly inclined well sections, there is a much larger contact area between the drilling string  30  and the drilled formation  40 , and, consequently, increased friction between the drilling string  30  and the drilled formation  40 . Such increased friction limits the overall measured depth of the well and causes the drilling string  30  to stick in the well. To alleviate such drill string  30  sticking and to increase the depth of the well there have been attempts and it has been found useful to maintain the drilling string  30  in a dynamic state by imparting a controlled vibration to the drilling string  30 . It has been found that the introduction of such a controlled vibration to the drilling string  30  significantly reduces the static frictional forces between the drilled formation  40  and the drilling string  30  to a dynamic frictional force, thus allowing the drilling string  30  to move easier in the horizontal or inclined sections of the well. 
         [0010]    Various concepts have been used to maintain the drilling string  30  in a dynamic state during the drilling process to achieve such benefits. The most common approach is to introduce a mechanical device in line with the drilling string  30  which is powered by a drilling fluid. Typically, these mechanical devices work by introducing a discontinuity in the flow of the drilling mud, which in turn leads to the shaking or agitation of the drilling string  30 . 
         [0011]    Downhole vibrating tools known as mud hammers have also been developed in an effort to introduce longitudinal vibrations to increase drilling penetration rates. This effect is very similar to the “water hammer” effect in plumbing systems. A typical mud hammer comprises a striker hammer which is caused to repeatedly apply sharp blows to an anvil. The sharp blows are transmitted, through the drill bit  20  to the teeth of the drill bit  20 . This has been found to increase drilling penetration rates. 
         [0012]    Mud hammers have several drawbacks. They are expensive to operate as drill bit  20  life is significantly reduced. Additionally, most mud hammers rely on a combination of relatively complex and precise moving parts which are subject to wear by abrasion from debris that forms in the drilling string  30 . Additional drawbacks include operation complexity (due to the necessity of adjustment according to drilling depth), inadequate reliability, higher wear of the pump because its hydraulic part is affected by pressure oscillations, dependence of the downhole power input and the efficiency of the machine on the parameters of hydraulic waveguide passages and low frequency of shock pulses. A further drawback is that any discontinuity in the mud flow interferes with data transfers between measuring-while-drilling (“MWD”) tools and the drilling rig  60  control systems. 
         [0013]    Various other downhole devices exist which also exploit the water hammer effect to create pulsations in the flow of drilling mud to enhance the hydraulic action of the drilling fluid. Such devices have a positive effect on rock chip removal and, consequently, drilling penetration rates. Such devices also induce vibrations in the drill string  30  and more specifically in the drill bit  20  which has a positive effect on drilling penetration rates. 
         [0014]    There have been attempts to introduce cavitation powered devices inline of the drill string  30  as a means for imparting longitudinal vibration to the drill bit  20 . Such cavitation-powered devices are generally, those relying on a combination of pistons, valves and springs to harness energy of cavitation and those relying on a Venturi effect. The need for precisely machined moving parts in the former makes them subject to abrasion and wear, resulting in an unreliable system for providing a longitudinal vibration to a drill bit  20 . Devices relying on a Venturi effect to produce a longitudinal vibration to the drill bit  20  have been shown to be reliable but they emit acoustic waves spread over a large frequency spectrum, thus making them ineffective and/or inefficient. 
         [0015]    Applicant is aware of the following references: 
       US Patents and Publications 
       [0000]    
       
         US 2012/0048619 to Seutter et al describes a drilling agitator tool that facilitates axial movement of a drill string in a well. The tool has a valve assembly and controller therefore that provide pulses of fluid pressure in the drill string. 
         U.S. Pat. No. 3,174,561 to Sterret describes a method for improving the rotary drilling penetration rate in a well drilling system which includes adding to the circulating drilling fluid hollow and frangible capsules that are freely carried by the drilling fluid through the drilling string and create cavitation when the capsules are broken by the drill bit at the bottom of the well. 
         U.S. Pat. No. 3,603,410 to Anona describes a liquid-filled borehole wherein the hydrostatic pressure at the borehole bottom is periodically reduced. Simultaneously, high frequency acoustic energy is imposed in the drilling liquid and cavitation is effected at least during the period of reduced hydrostatic pressure. 
         U.S. Pat. No. 4,185,706 to Baker, III et al describes the use of cavitation inducing nozzles. The cavitation nozzles enhance the drilling rate by creating catastrophic implosion waves which erode solid material at the bottom of the hole while reducing the localized pressure at the interface between the rock and drill bit tooth. The localized pressure reduction reduces the tendency for the cuttings to adhere to the bottom of the hole. 
         U.S. Pat. No. 5,009,272 to Walter describes a flow pulsing apparatus for down hole drilling that is connected in a drill string above a drill bit. The apparatus includes a constriction means through which the flow is accelerated increasing the flow velocity, followed by a downstream region of fluid deceleration. In order to effect the periodic interruption of the flow, a control means is movable between an open, full flow position, and a closed flow interrupting position. This control means is responsive to alternating differential fluid pressures acting on opposing sides thereof so as to move or vibrate the control means rapidly between the above-noted positions to pulsate the flow creating a water-hammer effect created upstream of the control means coupled with a pressure drop on the downstream side. 
         U.S. Pat. No. 5,125,582 to Surjaatmadja describes a cavitating jet for a fluid jetting system for cleaning and machining operations. 
         U.S. Pat. No. 6,053,261 to Walter describes a device that is placed in a drill string to provide a pulsating flow—“water hammer” effect—of the pressurized drilling fluid to the jets of the drill bit to enhance chip removal and provide a vibrating action in the drill bit itself to provide an efficient and effective drilling operation. The main flow passage includes a valve that is positively closed and opened by differential pressure of the mud flow. The device produces pronounced negative pulses, each preceded by a positive pulse. 
         U.S. Pat. No. 7,059,426 to Walter describes a drilling method and apparatus that generates intense pressure pulses at a location at the surface. Acoustic pulses are generated by interrupting the flow of the drilling mud thereby causing water hammer in the conduit. By rapidly blocking the flowing drilling mud in conduit the, flow interrupting valve generates water hammer pulses which propagate upstream in conduit. The pressure pulses propagate down through a drill string to a drill bit. Additionally, the apparatus has multiple pistons arranged in series. High pressure pulses move the pistons to generate strong mechanical vibration in the drill string. 
       
     
       Foreign Patent Literature 
       [0000]    
       
         CN Publication No. 203296733, published Nov. 20, 2013, Drilling tool capable of generating axial impact vibration (FP1, Translation FP1T) describes a drilling tool capable of generating axial shock and vibration that includes a spring loaded throttle lever slidably sleeved in the tool and a sliding impact device capable of generating axial impact on the lever to produce an axial vibration. 
         CN Publication No. 203296732, published Nov. 20, 2013, Well drilling tool capable of producing arial impact vibration (FP2, Translation FP2T) describes a drilling tool capable of generating an axial shock and vibration that includes an anvil shaft sleeved in the tool and a sliding impact device (hammer) capable of generating axial impact on the lever to produce an axial vibration. 
       
     
       Non-Patent Literature 
       [0000]    
       
         V. V. Pylypenko et al,  High - frequency downhole hydrovibrator for enhancing the effectiveness . Presented at the AADE 2005 National Technical Conference and Exhibition, held at the Wyndam Greenspoint in Houston, Tex., Apr. 5-7, 2005 (NPL1) describes a hydrovibrator for drilling wells that has no moving or rotating parts and requires no special energy supply. The device is described broadly as “ . . . part of a drill string, and its specially shaped flow passage will realize the regime of periodically detached cavitation in the drilling mud flow. It can be installed at a considerable distance from the rock cutting tool (for example, over the core barrel), and it will transform the steady-state fluid flow into a pulsating one. The pressure oscillations will act on the part of the drill string between the hydrovibrator and the rock cutting tool thus imparting longitudinal vibrations to the rock cutting tool . . . ” The devices are described as having a “ . . . converging-diverging section of the flow passage thus providing different fluid flow rates at the same inlet pressure . . . ” 
         Bit Tooth Energy: Waterjetting 14c—Intensifying Cavitation. http://bittooth.blogspotcom/20131101waterjetting-14c-intensifying-cavitation.html (NPL2) describes jets coming from an orifice to drill holes in rock (not admitted prior art). 
         Li, S. C. et al, CAVITATION  RESONANCE: THE PHENOMENON AND UNKNOWN. Conference of Global Chinese Scholars on Hydrodynamics , p 356-362, Available online at www.sciencedirect.com (undated) (NPL3) describes cavitation associated with, for example, hydropower plants, pumping station, rocket engine fuel system, and water distribution networks (not admitted prior art). 
         HYDRODYNAMIC CAVITATION TOOL, TECHNICAL INFORMATION, NPO Special Technologies Ltd. (undated) (NPL4) describes drilling boreholes with a drill bit that includes generating strong pulses hydro-dynamically by means of resonance waves that can be tuned for optimum performance in varying drilling conditions (not admitted prior art). 
         Hydrovibrator for Vibro-Rotary Well Drilling, dated Mar. 31, 2015 (not admitted prior art). (NPL5) describes a hydrovibrator that transforms the steady drilling mud flow into pulsating flow. 
         Li, S. C. et al,  Cavitation Resonance . Transactions of the ASME. 031302-4/Vol. 130, MARCH 2008 (NPL6) is a study on cavitating flow and cavitation clouds. 
       
     
         [0032]    None of these references describe the specific apparatus and method of this invention for providing controlled vibrations or longitudinal oscillations of a drilling string  30  to provide enhanced drilling rates of penetration. 
       OBJECTS AND SUMMARY OF THE INVENTION 
       [0033]    It is an object of the present invention to provide an improved flow pulsing apparatus and method for various applications wherein vibrating and/or flow pulsing effects are desired. 
         [0034]    It is another object of the present invention to provide an improved flow pulsing apparatus and method to provide a vibrating and/or flow pulsing effect in a drill string  30  to enhance the drilling rate and to pulse the flow of drilling fluid emitted from the drill bit  20  jets to further enhance the drilling rate. 
         [0035]    It is a further object of this invention to provide an apparatus and method of providing controlled vibrations or longitudinal oscillations of a drilling string  30  and/or drill bit  20  that does not depend on the use of a discontinuity in mud flow through the drilling string  30 . 
         [0036]    It is another object of this invention to provide an apparatus that provides controlled vibrations or longitudinal oscillations of a drilling string  30  and drill bit  20  that may be installed in the drilling string  30  does not does not depend on the use of a discontinuity in mud flow through the drilling string  30  and has no moving parts. 
         [0037]    It is yet a further object of this invention to provide an apparatus and method for selectively providing controlled vibrations or longitudinal oscillations to at least a portion of drill string  30 . 
         [0038]    It is yet another object of this invention to provide an apparatus and method for selectively providing controlled vibrations or longitudinal oscillations to the entire length of the drill string  30 . 
         [0039]    It is yet another object of this invention to provide an apparatus and method of selectively providing controlled vibrations or longitudinal oscillations to at least a portion of drill string  30  to enhance the drilling rate of penetration (ROP), minimize drill bit  20  drift, and/or significantly reduce the static frictional forces between the drilled formation  40  and the drilling string  30 . 
         [0040]    It is yet another object of this invention to provide an apparatus and method of selectively providing controlled vibrations or longitudinal oscillations at selected locations in the drilling string  30  that does not interfere with MWD signals travelling upstream to the rig  60  from MWD measuring devices installed in-line with the drilling string  30 . 
         [0041]    All of the foregoing objects are achieved by the cavitation apparatus (CHRS) of this invention for use in subterranean drilling with a drilling rig, the rig having a drill bit and a drilling string mounted thereto through which a drilling fluid flows to the drill bit. The cavitation apparatus (CHRS) is adapted to be mounted in-line with the drilling string through which the drilling fluid flows. 
         [0042]    The cavitation apparatus (CHRS) comprises:
       a. a hollow cylindrical body having an external diameter no greater than the external diameter of the drill string and an inlet end and an outlet end, each end adapted to be in fluid connection with the drilling string, the inlet end for receiving the drilling fluid from a downstream portion of the drill string and the outlet end through which the drilling fluid exits to an upstream portion of the drilling string;   b. the hollow cylindrical body having therein:
           i. a Venturi tube having an inlet opening and an outlet opening located between the inlet end and the outlet end of the cylindrical body, the Venturi tube having an interior diameter;   ii. at least one downstream cylindrical conical section having an inlet for receiving the drilling fluid from the inlet end of the hollow cylindrical body and an outlet for transferring the drilling fluid to the inlet opening of the Venturi tube, the diameter of the inlet to the downstream cylindrical conical section being greater than the outlet to the downstream cylindrical conical section, the outlet to the downstream cylindrical conical section in fluid communication with the Venturi tube inlet opening and of a diameter substantially equal to the Venturi tube interior diameter;   iii. an upstream cylindrical conical section having an inlet for receiving the drilling fluid from the outlet opening of the Venturi tube and an outlet for transferring the drilling fluid to the outlet end of the hollow cylindrical tube, the diameter of the outlet to the upstream cylindrical conical section being greater than the inlet to the upstream conical section, the inlet to the upstream conical section in fluid communication with the Venturi tube outlet opening and of a diameter substantially equal to the Venturi tube interior diameter;   
               
 
         [0048]    When the drilling fluid passes through the drill string, the cavitation apparatus produces low frequency longitudinal fluctuations in at least a portion of the drill string enhancing the drilling rate (ROP) of the drilling string. 
         [0049]    Preferably, there are a plurality of downstream cylindrical conical sections positioned in series and in fluid communication with each other and the inlet opening of the Venturi tube, see  FIG. 3 . Each downstream cylindrical conical section has an inlet for receiving the drilling fluid from downstream that entered the inlet end of the hollow cylindrical body and an outlet end for transferring the drilling fluid upstream toward the inlet opening of the Venturi tube. The diameter of the inlet end of a downstream cylindrical conical section is greater than the outlet end of the downstream cylindrical conical section. The outlet to the downstream cylindrical conical section is in fluid communication with the inlet of another downstream cylindrical conical section adjacent to it and positioned in closer proximity toward with the Venturi tube inlet opening. 
         [0050]    Other objects and advantages of the invention will become apparent in the following description and accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0051]    The novel features believed to be characteristic of the present invention, together with further advantages thereof, will be better understood from the following description considered in connection with the accompanying drawings in which several embodiments of the invention are illustrated by way of example. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only and are not intended as a definition of the limits of the invention. 
           [0052]      FIG. 1  is a schematic depiction of a typical subterranean drilling rig  60  comprised of a drill string  30  and drill bit  20  drilling through a formation  40 . 
           [0053]      FIG. 2  is a generalized schematic cross-sectional side view of the cavitation apparatus of this invention, termed herein a Cavitating Hydraulic Resonator System  100  (CHRS), indicating the direction of flow and nomenclature used herein. 
           [0054]      FIG. 3  is cross-sectional side view of a specific CHRS  100  apparatus specifying preferred ranges of dimensions for this apparatus and showing, in particular, the plurality of downstream cylindrical conical sections, i.e., resonators, in fluid communication with each other and in series downstream of the Venturi tube. 
           [0055]      FIG. 3A  is an axial view of the inlet to the CHRS  100  depicted in  FIG. 3 . 
           [0056]      FIG. 3B  is an axial view of the outlet to the CHRS  100  depicted in  FIG. 3 . 
           [0057]      FIG. 3C  is an axial view of the central resonator Rv depicted in  FIG. 3D  that is inserted into the Lv section (Venturi Section) of the CHRS  100  depicted in  FIG. 3 . The preferred range of the interior diameter of the central resonator Rv is from 4-7 mm, the wall thickness is preferably 5-8 mm and the preferred length is 30-65 mm. 
           [0058]      FIG. 3D  is a perspective view of the central resonator Rv depicted that is in the Lv section of the CHRS  100  depicted in  FIG. 3 . 
           [0059]      FIG. 4  is a generalized schematic cross-sectional side view of the central section of the CHRS  100  depicting the location of the central resonator Rv; 
           [0060]      FIG. 5  is graph of an acoustic wave or pressure fluctuation amplitude as a function of frequency distribution for a Venturi tube with and without the resonators placed inline of the drilling string  30 . 
           [0061]      FIG. 6  is a schematic depiction of a subterranean drilling rig  60  comprised of a drill string  30  and drill bit  20  drilling through a formation  40  and the positioning of a plurality of CHRS  100  devices as part of the drilling string  30  assembly. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0062]    It shall be noted that like reference numerals are used throughout the drawing figures and the drawings are representative of various embodiments that may be employed within the scope of the present invention. 
         [0063]    As detailed above, in many instances of subterranean drilling imparting controlled longitudinal oscillations to the drill bit  20 , a section of the drilling string  30  or the entire drilling string  30  is highly desirable. The apparatus and method of this invention can be used to impart a controlled longitudinal oscillation of a desired or selected predetermined frequency to the drill bit  20 , a section of the drilling string  30  or the entire drilling string  30  and/or maintain a section of the drilling string  30  or the entire drilling string  30  in a dynamic state with the drilled formation  40 . 
         [0064]    The cavitation apparatus of this invention is termed herein a Cavitating Hydraulic Resonant System  100 , (“CHRS”). Referring for example to  FIGS. 2 and 3 , the apparatus  100  is comprised of a plurality of what are termed herein acoustic resonators Rx  70  on the inlet (downstream) and outlet (upstream) sides of a Venturi tube  72 . Optionally and preferably, there is an additional resonator Rv within the Venturi tube  72 . The number and properties of resonators  70  are selected based on the desired operational parameters of the CHRS  100 . Each resonator  70  R x  is characterized by its critical properties, such as length L x , angle θ x , diameter d x . These properties are determined by the characteristics of the hydraulic system such as drilling mud density, drilling mud flow and drilling mud static pressure at the inlet of CHRS  100 . 
         [0065]    In  FIG. 3  we depict a preferred CHRS  100  configuration with the preferred dimensional parameters. In particular this CHRS  100  has a plurality of resonators on the inlet (downstream) of the Venturi section and on the outlet (upstream) of the Venturi section and, referring to  FIG. 3A-C  and in particular  FIG. 3D , another resonator internal to the Venturi section. The resonators on the inlet to the Venturi section are each reduced in cross-section as they approach the inlet of the Venturi, the drilling fluid flow gradually increasing in velocity as it approaches the Venturi section. The resonators on the outlet of the Venturi section are each increased in cross-section as they get further away from the outlet of the Venturi, the mud flow gradually decreasing in velocity as it leaves the Venturi section. 
         [0066]    Referring, for example to  FIGS. 2 and 4 , when the drilling fluid, passes through the CHRS  100  device and is forced through the Venturi tube  72 , a cavitating region CR is formed at or near the exit of the Venturi tube  72 . 
         [0067]    As seen in  FIG. 5 , this cavitating region CR becomes a source or exciter of acoustic waves or pressure fluctuations with the frequency distributed across a wide spectrum. Using just the Venturi tube  72  inline in the drilling string  30  the pressure fluctuations are relatively low in amplitude and, beyond noise, such fluctuations do not produce much of an effect in the hydraulic system. However, when a Venturi tube  72  is combined into a system with one or more appropriately tuned resonators  70 , as shown for example in  FIG. 3 , such a system becomes a source of one or more resonant pressure fluctuation peaks with an amplitude exceeding that of the amplitude with just the Venturi tube  71 . 
         [0068]    Another way to describe operation of the CHRS  100  is that of a collection of acoustic heterodynes, with each such heterodyne producing a single lower frequency peak from two or more higher frequencies generated by the cavitating region CR. These lower resonant peak frequencies produce high amplitude pressure fluctuations which, originating in CHRS  100 , propagate subsequently throughout the entire hydraulic system of the drill string  30 . Such high amplitude, peak frequency pressure oscillations will be present in the hydraulic system as long as the value of liquid velocity through the Venturi tube  71  is at or above critical value Va. 
         [0069]    The number, frequency and amplitude of such resonant pressure fluctuation peaks are determined by the properties of the hydraulic system (drilling mud flow, pressure, density), by the properties (specifications) and the total number of resonators  70  and their location relative to the Venturi tube  72 . The CHRS  100  can be built to produce single, dual or multiple resonant frequency peaks. The total number of frequency peaks and maximum peak amplitude is limited only by the allowable pressure differential across the CHRS  100 . Critically to subterranean drilling application, each individual resonator  70  can be tuned to somewhat different combination of drilling mud pressure and flow thus ensuring that at least one resonator  70  will be in operation as downhole conditions vary with depth or other factors. 
         [0070]    The presence of cavitating region CR or cavitating cloud at the exit of the critical area of the Venturi tube  72  becomes a source of damage to the walls of the resonators  70  and to the walls of the CHRS  100 . To reduce or avoid cavitation-induced damage to resonantor walls  70  or to the body of CHRS  100 , we employ the concept of standing waves to reduce or eliminate cavitation damage to internal surfaces. One of the characteristics of a standing wave is the absence of particle transfer in the direction of wave propagation. When a standing wave is formed in a resonator  70 , it will limit the transfer of particles, or in our case, cavitating bubbles, in the direction of the wave propagation. A standing wave at the internal resonator wall will also limit transfer of cavitating bubbles to the resonator wall. In this way CHRS  100  relies on the standing waves on the internal resonator  70  surfaces to create a boundary layer: a layer of drilling liquid devoid of cavitating bubbles. To be effective, such layer need not exceed the width of 0.1 to 1 mm and will reduce or eliminate cavitation damage to the internal resonator  70  walls. 
         [0071]    Referring to  FIGS. 1 and 6 , this invention includes the use of the CHRS  100  as a component of drill string  30  assemblies in subterranean drilling applications. As shown in  FIGS. 1 and 6 , a typical drilling string  30  is generally a long metal string extended down and/or sideways from a subterranean drilling rig  60 . One end of such drill string  30  assembly will always be fixed at the drilling rig  60 . The other end of the drill string  30  is extended into the drilled formation  40  and fitted with a special cutting device, i.e., a drill bit  20 . The length of drill string  30  is usually hundreds of feet and the diameter is typically 4 to 8 inches. Such drill strings typically have one end fixed and the other end suspended or free. The drill string  30  behaves elastically in length and, due to its limited diameter, can be assumed to behave in-elastically in width/diameter. 
         [0072]    Referring to  FIG. 6 , this invention contemplates at least one CHRS  100  located either at the free end of the drilling string  30 , juxtaposed on or near the drill bit  20 , see  FIG. 6 , Position 1, or at any distance from the free end (drill bit  20 ) along the drill string  30 , e.g., Positions 2, 3, 4 and 5. A plurality of CHRS  100  may be used along the length of the drill string  30 . When the drilling rig  60  is in operation, the drilling rig  60  pump will direct drilling fluid downward through the drill string  30  and through CHRS  100  to the output holes in drill bit  20 , i.e., jets. When the hydraulic system conditions such as liquid velocity V 1  and pressure differential across CHRS  100  ΔP 1  for given viscosity of liquid are right, a cavitating cloud or cavitating region CR is formed at the exit of the Venturi area  72  and the CHRS  100  becomes operational. The cavitating cloud CR interacts with one or more resonators  70  and becomes a source of one or more high amplitude or peak hydraulic pressure fluctuations. Because CHRS  100  is a component of the drilling string  30  and the drilling string  30  behaves elastically along its length, these peak pressure fluctuations originating at the CHRS  100  will create longitudinal mechanical oscillations of portions of the drill string  30  or the entire length of drill string  30 . 
         [0073]    It has been found that that the resonance or peak frequencies located in 300-20000 Hz range produce the best outcome in imparting or conveying longitudinal fluctuation to the drill string  30 . One selects number and values of peak frequencies depending on the length and mechanical properties on the drill string, intended operational range such as well depth, width, and location of CHRS  100  relative to the free end of the drilling string. Because each CHRS  100  can be made to work at one or more individual peak frequencies, more than one CHRS  100  can be installed on a drilling string without the danger of creating damaging mechanical resonance on the drilling string section between each CHRS  100 . CHRS  100 , thus, allows one to introduce controlled longitudinal hydraulic and mechanical drilling string oscillations for sections of a drilling string where such longitudinal oscillations are required the most. 
         [0074]    For directional and highly inclined drilling applications, CHRS  100 , placed either immediately on or in close juxtaposition to the drill bit  20  or anywhere between the drill bit and drilling rig  60 , changes the predominantly static nature of friction force to predominately dynamic nature, thus providing critical improvements to the efficiency of subterranean drilling process. The nature of these improvements on subterranean drilling process depends on the location and the number of CHRS  100  units employed. 
         [0075]    For all drilling applications, a shortest possible CHRS  100  installed immediately on or in close juxtaposition to the drill bit  20  will provide improvements in rate of penetration, reduction of torsional vibration and reductions of drill bit  20  drift. Torsional vibration occurs when drill bit  20  stops rotating for a short while, or gets stuck, due to local variations in drilled formation  40  physical properties. At the same time, the top of the drill string  30  will keep turning around its axis being driven by the drilling rig  60  motors. When this occurs, the drill string  30  undergoes elastic deformation along its length or twisting. Eventually, the formation  40  gives way to the drill bit  20  and the energy of elastic deformation stored in the drill string  30  is released, resulting in momentary higher rotational speed of the drill bit  20  and the lower part of the drill string  30 . Such variations in rotational speed during drilling will be reduced or eliminated with the application of the use of the CHRS  100  due to the presence of longitudinal oscillations preventing sticking of drill bit  20  against the formation  40 . The drill bit  20  drift is the propensity of drill bit  20  to drift sideways during drilling resulting in the well shape that is not circular or is larger than specified in the well design. Application of the CHRS  100  reduces this propensity of the drill bit  20  to drift, resulting is higher efficiency of drilling operation and much better lateral control of drill bit  20 . 
         [0076]    The following are non-limiting Examples of the use of the apparatus and method of this invention. 
       Example 1 
       [0077]    Test location: Geological (exploration) well located in Poltava Region, Ukraine. The drilled strata consisted predominantly of granite.
 
Well description: Exploration well diameter 85 mm. Drill bit  20  type: tri-cone.
 
       Test Description: 
       [0078]    The goal was to evaluate the effect of CHRS on rate of penetration for a typical vertical drilling well. This area contained a number of exploration wells drilled using standardized equipment hence a stable baseline drilling rate was already available. 
         [0079]    The standard set up for exploration in Poltava Region, Ukraine, includes a vertical 86 mm well with drilling pipe diameter 70 mm. The drill bit  20  was of tri-cone type. No other specialty equipment was integrated into the drill string. Typical drilling depth was down to 3000 meters. 
         [0080]    The test consisted of introducing a CHRS apparatus between the drill bit  20  and the drilling pipe and evaluating resultant impact on the drilling rate of penetration (ROP). Below is summary of the results: 
         [0081]    Beginning test depth: 2341 meters 
         [0082]    End test depth: 2387 meters 
         [0083]    Total drill time (with CHRS): 23 hours, 25 min 
         [0084]    ROP with CHRS installed: 31 min/meter or 1.93 meter/hour 
         [0085]    Baseline ROP (same well ROP without CHRS), two samples:
       Sample 1: 1.2 meter/hour   Sample 2: 1.14 meter/hour       
 
       Summary of Test Results: 
       [0088]    This test showed the average ROP improvement of 60% over baseline when using the CHRS apparatus. 
       Example 2 
       [0089]    Test location: The test well was located in the Rocky Mountain Oil Technology Center (RMOTC) in Casper, Wyo., USA 
         [0090]    Well description: Test well diameter 8.5 inches. A flow only test (no drilling) was performed. 
         [0091]    Test Description: 
         [0092]    The well was active, but equipment idle. The well was filled with drilling and flow testing was performed in order to ascertain the integrity and operational reliability of a scaled up CHRS from the CHRS use in Example 1. 
         [0093]    The test consisted of introducing the CHRS apparatus between the drill bit  20  and the drilling pipe and suspending it in the well at a depth of 160 feet. The drill string was connected to the mud pump and mud was circulated through the drill string at various flow rates. Stand pipe pressure readings were taken. The following data was obtained: 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                 Flow, gal/min 
                 Pressure, psi 
               
               
                   
                   
               
             
             
               
                   
                 400 
                 235 
               
               
                   
                 500 
                 450 
               
               
                   
                 600 
                 550 
               
               
                   
                   
               
             
          
         
       
     
         [0094]    In the 360 to 460 gal/min flow range a clear longitudinal vibration of the drill string was observed. Because the entire assembly was suspended as opposed to being pressed against formation, this longitudinal vibration was a low frequency “bounce” in that the entire drill string was bouncing up and down in the well with an amplitude of about 10 mm and at times higher e—(20 mm). The peak bounce was observed in the 420-440 gal/min range which correlated well with a calculated operational range for this unit. Estimated bounce frequency was about 1 Hz. Within 420-440 gal/min flow regime a strong acoustic signature with the frequency very close to that of the bounce was observed in the flexible hydraulic line. This looked like violent shake (single) of the line and sounded like a small caliber hand gun firing. Both the sound and shaking of the line disappeared as the flow moved out of 420-440 gal/min range. 
       Example 3 
       [0095]    Test goal: Field test scaled up prototype CHRS. 
         [0096]    Test plan:
       a. Established baseline ROP.   b. Added CHRS above drill bit  20 , drilled, recorded test ROP (ROP with CHRS installed).       
 
         [0099]    Test location: 
         [0100]    Test was conducted at Catoosa, Okla. Drilling was performed at Rhonda  4 C test well at the starting depth of 1885 feet. 
         [0101]    Test medium: dolomite. 
         [0102]    Test setup:
       a. Well diameter 8.5 inches.   b. Well depth at starting point: 1885 feet.   c. CHRS dimensions: length 750 mm, diameter 155 mm, calculated operational flow set point was 400 gpm.       
 
         [0106]    Test results:
       a. Baseline ROP (no CHRS): 22.8 feet per hour   b. Test ROP (with CHRS): 27.4 feet per hour       
 
         [0109]    Test lasted for 20 minutes. 
         [0110]    This was a scale up of the CHRS size (in this case from 70 mm diameter to 155 mm diameter) and flow. 
         [0111]    These tests confirm the benefits from the use of a CHRS as part of drilling string for the purposes of changing the static nature of friction during horizontal or highly inclined drilling to a dynamic nature. 
         [0112]    To summarize, the CHRS of this invention is used in a subterranean drilling string that includes a drilling rig, a drill bit, drilling pipes, including drilling collars, connecting the drill bit to the drilling rig. A mud pump supplies drilling mud to the drilling pipes, including drilling collars. The CHRS is installed as part of the drilling string between the drill bit and the drilling rig and acting as a source of peak frequency pressure fluctuations and, in doing so, imparting or conveying longitudinal pressure and mechanical fluctuations onto a part of or entire the drilling string. 
         [0113]    As depicted, the CHRS is a cylindrical body made of steel or other suitable material with an external diameter not exceeding that of a drilling pipe or string. Inside of the CHRS, approximately in the middle, is a Venturi tube characterized by minimum diameter Dv and length Lv with the values Dv and Lv selected in such a way as to form cavitating conditions at the exit of the Venturi tube. The Venturi tube dimensions are pre-selected to create the cavitating conditions when the drilling fluid flows through the device. The Venturi tube properties are selected in such a way that fluid velocity through the Venturi tube reaches critical value V σ  and static pressure at the inlet of the Venturi tube is at or above P inσ  required to create stable cavitating conditions at the exit of the Venturi tube. 
         [0114]    When the cavitating conditions are created during the hydraulic system operation, the Venturi tube becomes a source of pressure fluctuations with the pressure fluctuation frequency distributed across a wide spectrum, with the frequency spectrum dependent on selected combination of V σ , P inσ . 
         [0115]    One or more internal surfaces inside the CHRS is shaped cylindrically or conically relative to the longitudinal axis of the apparatus and having the shape being a circle in any part of the surfaces in cross-section, with the direction of such cross-section being perpendicular to the longitudinal axis of the apparatus, with the surfaces located immediately before, inside of and after Venturi tube. Such conically shaped surfaces are at the distance not exceeding 20 lengths Lv of the Venturi tube, with each the internal surface characterized by a specific length LRx, diameter DRx and incident angle θ Rx  relative to the longitudinal axis of the CHRS. The surfaces when combined with Venturi tube and during operational (drilling) conditions act as acoustic resonators or as acoustic heterodynes in concert with the Venturi tube and serve to create one or more frequency peaks of resonant nature inside CHRS. 
         [0116]    With onset of the resonance conditions inside CHRS it becomes source of one or more pressure fluctuation peaks with amplitude ΔP, where ΔP is the difference between the pressure peak and trough, and peak frequency f px′ , determined by the specific resonator properties LRx, DRX, θ Rx , the properties of the Venturi tube and properties of the overall hydraulic system. 
         [0117]    One or more internal surfaces inside CHRS, with the internal surface having their physical dimensions selected expressly for the purpose of creating standing acoustic wave or waves, with the standing wave used to reduce or prevent cavitating cloud from coming in contact with any of the internal surfaces of CHRS and keeping the cavitating cloud from coming into contact with inside walls of resonators downstream of the Venturi tube and losing energy of the cavitating cloud on the walls and damaging the walls in the process. 
         [0118]    Generally, the internal surfaces have their physical dimensions and shapes selected expressly for the purpose of creating acoustic resonant conditions or resonant frequency peaks in 100-20,000 Hz range, with the acoustic resonant conditions, or frequency peaks, spaced across 100 to 20,000 Hz spectrum in 500-10000 Hz intervals. 
         [0119]    In the event when main parameters of hydraulic system such as mud flow (determining mud velocity V σ  across Venturi tube), specific viscosity μ, pressure at the entry and exit of the Venturi tube P inσ  and P exσ , drilled formation type and density, vary due to operational conditions, the lower frequency peaks spaced as they are will ensure that there will always be a condition in the CHRS for stable presence (operation) of at least one of the frequency peaks or resonant conditions. 
         [0120]    The CHRS has its length selected in such a way as to allow the smallest possible distance between the location of cavitating cloud and the downstream end of the CHRS in such a way as to allow the cavitating cloud to retain some of its energy at the point of exit of the CHRS with the purpose of enabling the cavitating cloud to discharge energy of the cavitating cloud on the surface of drilled material and in such a way to assist drill bit in breaking down the drilled material during drilling process. 
         [0121]    Preferably two or more CHRS devices are installed as part of drilling string between the drill bit and drilling rig, with distance between each other being from 100 to 5000 meters, with each the systems tuned to same or different resonance frequencies in 100 to 20,000 Hz range, each the systems operating individually or simultaneously and each the systems acting as a source of peak frequency pressure fluctuations and, in doing so, inducing longitudinal hydraulic and mechanical fluctuations in a part of or entire the drilling string. 
         [0122]    Preferably the CHRS is installed as part of the drilling string adjacent to the drill bit and acting as a source of peak frequency pressure fluctuations and, in doing so, imparting longitudinal pressure and mechanical fluctuations onto the drill bit to assist the drill bit in breaking down drilled material during drilling process. 
         [0123]    The CHRS is used to aid horizontal or inclined subterranean drilling process, at any stage of the drilling process, by changing significantly the predominantly static type or nature of friction force of the drilling string or any of components of the drilling string against drilled formation, to significantly or predominately dynamic type or nature of friction force by inducing longitudinal pressure (hydraulic) and consequently, mechanical fluctuations in a part of or entire the drilling string. 
         [0124]    The CHRS is used to change significantly the predominantly static type or nature of friction force of drilling string or any of its components against drilled formation in horizontal or inclined drilling applications, to significantly or predominately dynamic type or nature of friction force by inducing longitudinal pressure and mechanical fluctuations in a part of or entire the drilling string. 
         [0125]    The CHRS when located adjacent to the drill bit assists the drill bit in mechanical destruction of drilled formation by imparting longitudinal pressure and mechanical fluctuations of desired frequency onto the drill bit. 
         [0126]    The CHRS when located adjacent to the drill bit assists the drill bit in mechanical destruction of drilled formation by delivering some of the cavitating cloud to the face of formation that is being drilled. 
         [0127]    The CHRS reduces or eliminates the occurrence of torsional vibration of the drilling string by inducing longitudinal pressure and mechanical fluctuations of desired frequency onto the drill bit. 
         [0128]    The CHRS imparts longitudinal pressure and mechanical fluctuations onto the drill bit; with the longitudinal pressure and mechanical fluctuations reducing or eliminating propensity of drill bit to drift laterally during drilling process and improving lateral well dimension control. 
         [0129]    Although this disclosure describes illustrative embodiments of the invention in detail, it is to be understood that the invention is not limited to the precise embodiments described. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense. Various adaptations, modifications and alterations may be practiced within the scope of the invention defined by the appended claims.