Abstract:
Acoustic radiators for coal bed methane or shale gas production are configured to be strategically placed on drillstrings within gas production exhaust boreholes and hydraulically powered so they will radiate intense harmonic sonic waves to shake the solid media immediate to the wall areas. The gas volume output that can be realized by an exhaust well depends highly on the permeability of the media, especially at inside faces of the borehole. The shaking half-opens up fractures and pores in the solid media. Thus the permeability of the media to gas improves under such shaking and gas collection efficiencies are improved. The beneficial effects can be increased by locating two or more acoustic radiators proximate to one another in a phased relationship.

Description:
RELATED APPLICATIONS 
       [0001]    This Application claims benefit of U.S. Provisional Patent Application Ser. No. 61/675,855, filed Jul. 26, 2012, by Dmitry A. Kasyanov and Victor Zhoglikov, and titled, HYDRAULIC DRILLSTRING SOUND GENERATOR. 
     
    
     BACKGROUND 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to devices and methods for generating sound vibrations, and more particularly to intrinsically safe drillstring vibrators suitable for use in horizontal and inclined wellbores that cannot be assumed to be completely inundated with liquids. 
         [0004]    2. Description of the Prior Art 
         [0005]    Methane (CH 4 ), firedamp (CH 4 +various hydrocarbon gases), and natural gas (mostly CH 4  and ethane C 2 H 6 ), are customary constituents of every coalbed deposit, and are formed in situ by Nature when the coalbed takes shape. Such gases are adsorbed by the coal, e.g., they occupy the interior of surface areas over the entire parent coal matrix. The adsorption surface area of coal can be very large, e.g., about one billion square feet per ton of coal. Coalbeds can store significantly more gas than a typical and otherwise similarly sized natural gas deposit. 
         [0006]    Coal mining is dangerous because every coal deposit includes some amount of explosive and flammable methane and other gases. As a general rule, the amount of methane adsorbed by the coal itself is proportional to the grade of the coal. The higher the grade of coal, the higher will be the methane content. Also, the deeper the coalbed lies, the higher will be its gas content because the pressure in a coalbed is always proportional to its depth. The degree of gas sorption increases with pressure. The reduction in pressure needed to get the gas to desorb and move out to the exhaust well collectors can be predicted with a desorption isotherm for various given temperatures. 
         [0007]    A typical complication is coal beds are very often inundated with ground water. Hydrostatic pressures caused by such water will increase the pore pressure in a coalbed and its gas sorption. Desorption isotherms can be used to predict the hydrostatic pressure drop needed to recover methane from coal. 
         [0008]    In the past, good collection methods and equipment needed to harvest methane from coalbeds simply did not exist. So the methane represented a nuisance and not a potential way to make a profit. Methane poisons the air miners needed to breath, and worse, it&#39;s explosive. Even today, when modern methods and equipment can be employed to great success, serious and frequent mining explosions and disasters continue to occur that could have been avoided if the methane or firedamp had simply been removed before coal mining operations began. These accidents have been especially common recently in the coal mines of Russia, Ukraine, China and the Upper Big Branch (UBB) mine in the United States. But no coal mine in the world is immune. 
         [0009]    When removed and reinjected with water or carbon dioxide, the explosive potential can be significantly reduced. Reinjection should be made an integral part of mining best practice when mining through faults, dykes and paleochannels, and other coal seam anomalies. 
         [0010]    Methane production ahead of mining has become a widespread way to protect against methane-related accidents and to increase profits, e.g., by selling off the collected methane. The harvesting of methane can add so much to the profits, even coalbeds or strata too deep or too poor to support profitable coal production is becoming an attractive way to convert inchoate hydrocarbon reserves into real revenues. 
         [0011]    Coal mine gas production holes were once simply used to help ventilate mines and to minimize the coal-production risk due to mine gases. Now, coal mine operations recognize that profits can be made by gas production and sales. Simply releasing the gas into the atmosphere wastes resources and money, and the gas can easily pollute the environment. Recent experience in mine degasification has led to the development of gas production independent of coal mine operations. 
         [0012]    Vertical and horizontal boreholes drilled into coalbed and shale gas reservoirs are widely used methods for gas production. Coalbed deposits are best degasified for safety before starting coal mine production. Sometimes vertical or directionally drilled boreholes are drilled as gassers independent of any intent to later mine any coal involved. 
         [0013]    A second, more benign gas can be injected in a deposit to push out the methane, firedamp, and natural gas. Prior art methods use water, nitrogen, carbon dioxide, and vitiated air injections into coalbeds to force out the resource gases. A system of injection and collector holes is needed for this. 
         [0014]    The actual gas content, the pressure in the coalbed, the presence of water, and the permeability all affect how much gas can be recovered and at what cost. A fracturing pattern inside a coalbed, called “cleavage,” is one factor that determines the in-place permeability. Cleavage and stratification can ease the flow of gases and fluids inside a coal bed. 
         [0015]    For example, a coal bed with a low gas content and a high hydrostatic pressure on the desorption isotherm requires extra production of water for every unit of produced methane. Similarly, gas recovery from a coalbed with a very low permeability requires intense fracturing. In many cases, efficient gas recovery is not possible because appropriate production-enhancement technologies do not exist. 
         [0016]    Drilling-in boreholes in coal beds causes localized pressure reliefs and creates pressure gradients as the methane flows to the borehole output wells. A diffusion flux can be generated throughout the coal matrix with laminar flows through the coal bed fractures around the boreholes. Any ground water present must be pumped out to reduce the coalbed pressures enough so the gases desorb from the coal. The faster the water is removed, the faster the retained gas will be released. 
         [0017]    The volume of gas output that can be realized at any exhaust well depends on the permeability or permeability of the walls and faces of the borehole. These behave like filter matrices, with the most restrictive parts in the collector zone being not more than a few diameters from the center of the exhaust well. 
         [0018]    Embodiments of the present invention are therefore directed to enhancing the permeability of the material surrounding the exhaust wells. The more permeable that such immediate area around the exhaust borehole can be made, the higher will be the volume of gas produced, all else being equal. 
         [0019]    The formula for a pressure gradient distribution in a one-dimensional radial flow from a circular supply circuit with radius R c , and pressure P c  to a concentric borehole with effective radius r b , and face pressure P b , is as follows: 
         [0000]    
       
         
           
             
               
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         [0020]    Such describes a logarithmic pressure distribution between the supply circuit and the borehole at the center. A majority of the pressure differentials are concentrated in the walls immediately surrounding the boreholes. For example, given R c ≈100 meters, and r b ≈0.1 meter, more than one-third of the pressure difference is dropped across the last one meter to the borehole core. Over one-half is dropped across a zone of radius≈3 meters. The situation is even more pronounced for boreholes with smaller radii r b . 
         [0021]    Drilling and production can generate particles of mud filtrate and small coal that can form a filter cake that will reduce or completely shut-down an exhaust well bore. The borehole output for the same face pressure can be considerably reduced by critical-zone pore-clogging, or colmatation. For example, it is estimated a tenfold decrease in permeability in an area of radius 0.5 meter for r b ≈0.1 meter results in a threefold decrease in the output. If the same decrease in permeability takes place in an only slightly larger 0.2 meter radius zone, then the output is reduced by much less than before, e.g., 40%. 
         [0022]    A principal benefit of acoustically vibrating the inside faces of the boreholes in porous and fractured media is to increase its permeability. The more permeable that such immediate area around the exhaust borehole can be made, the higher will be the volume of gas produced, all else being equal. 
         [0023]    Vibration and acoustic effects can be used to intensify many mass-transfer processes. But all too often, no elastic-vibration devices are available that are suitable for use in the harsh environments in which they must operate. Such environments include the drilling of extended horizontal wells for preliminary methane drainage from coal beds, and the production of coal methane and shale gas. 
         [0024]    The intensifying action of elastic oscillations can be used to commercial advantage in at least two ways. The mass-exchange processes in the spaces surrounding the well are improved, e.g., as in methane output intensification of gas collection wells drilled in coal seams. The boring resistance and friction to the drill bits used during drilling can also be reduced if intense elastic oscillations are employed in drilling horizontal and/or inclined wells. 
         [0025]    The medium inside extended horizontal gas production wells is typically characterized by insignificant acoustic impedance. The insides of the wells and their walls are not usually completely flooded. Such wells are often filled with a mix of gases and liquids, where the gases predominate. The acoustic impedances in the gaps can therefore be small, and will inhibit making a good acoustic contact between the system radiators, the downhole medium, and the wellbore walls. 
         [0026]    The gas production benefits of exposing coal or shale layers to intense acoustic vibrations can be further improved by maximizing the run of the wellbore wall that actually receives the vibrations. The linear drillstring length of each of the acoustic vibrators put down a gas well should be as long as is practical. 
         [0027]    Even before a gas field is placed into production, intense acoustic vibrations from vibrators placed just behind the drill bits can be used to speed up the drilling and reduce the overall costs. Intense acoustic vibrations applied to drill-bits can reduce friction and wear. The vibrators must be able to move freely with the drillstring, and should be powered the same way as the downhole motor, e.g., liquid under pressure. 
         [0028]    Horizontal and inclined wells can be assumed to benefit from good acoustic contact inside because the acoustic vibrators will lie down and touch the wellbore wall under their own weight. 
         [0029]    The technical solutions for vibrators that have been developed are as varied as the technical fields in which they have been applied. Many conventional devices look like they could be used for partially flooded or even dry horizontal and inclined wells. However, on closer inspection it can be seen that conventional downhole elastic oscillation sources are only suited for stationary, small-scale operations in liquid-filled wells. 
         [0030]    In the construction industry, mechanical vibrators on the ends of thick hoses or on the outsides of form walls are widely used to amalgamate and strengthen concrete pours. The shaking liquefies the loose concrete pour within a circular field of action before it cures so the concrete will flow into every void and any trapped air will bubble out. 
         [0031]    Many such conventional vibrators look like they would suit the geometries found inside horizontal and inclined well. So they are often used as starting basis in vibrator designs requiring a cylindrical body with a relatively small radius. In a majority of designs, the vibrations are directly generated by rotating an eccentric mass. Various kinds of motors are employed to drive the rotations. For example, U.S. Pat. No. 2,597,505, U.S. Pat. No. 3,549,130, U.S. Pat. No. 5,564,824, and U.S. Pat. No. 6,811,297, disclose external electric drive; U.S. Pat. No. 2,891,775, U.S. Pat. No. 3,162,426, U.S. Pat. No. 3,171,634, U.S. Pat. No. 3,193,256, U.S. Pat. No. 3,365,965, U.S. Pat. No. 3,376,021, U.S. Pat. No. 4,293,231, U.S. Pat. No. 4,300,843, and U.S. Pat. No. 4,428,678, describe compressed air being used to drive a planetary or free rotor along the internal surfaces of a stator; and, U.S. Pat. No. 2,960,314, U.S. Pat. No. 3,229,961, U.S. Pat. No. 3,290,952, U.S. Pat. No. 3,318,163, U.S. Pat. No. 3,357,267, and U.S. Pat. No. 4,682,896, direct pressurized fluids to create a vortex flow inside a working volume to entrain a rotor. 
         [0032]    The advantages of using electric drive include being able to control the oscillation frequencies over a wide range. Electric drives are difficult to make safe in explosive environments, like in gas wells and coal mines. Pneumatic drive vibrators are similarly difficult to make safe, there is a high probability of forming and igniting an explosive mix of air and flammable gas. 
         [0033]    Conventional vibrators that direct tangential inputs of compressed fluid into the working volumes have problems with the longitudinal (axial) stabilization of their rotors. A lack of stabilization leads to energy losses due to rotor friction and uncontrolled collisions against inside surfaces. Destabilized rotors can generate intense random noise components. Destabilization can result when significant pressure fluctuations occur in the input flow. 
         [0034]    U.S. Pat. No. 4,682,896 by Halilovic describes an attempt to solve this problem. Unfortunately, the vibrator disclosed will not work in the horizontal or even inclined positions. The rotor is weighed to be upright in gravity. The liquid flow is fed from the bottom and lifts the rotor, starting it to spin. The flow is taken out through vertical channels. If the rotor gets tilted, the longitudinal axis will be angled with respect to gravity. The liquid flow can then only act on one end of the rotor, making planetary movement of rotor around the axis of stator impossible. When tilted, the vibrator tends to generate wideband noise, and is why the most important thing in such type of vibrators is the stabilization of the rotor movement around stator. 
         [0035]    There are some other technical solutions that were based on a fluid driven screw type motor, e.g., as originally described by Moyneau, 1939, and employed by Bodine in U.S. Pat. No. 4,824,258 and Kochnev in Russian Patent RU2162509. See, French Pat. No. 850,942 to Moyneau S. A. R. L. issued on Sep. 25, 1939, and Page 155 of  Pumps  by Kristal and Annett, McGraw-Hill Book Company, 1940. In principle, vibrators of this type for use in horizontal or inclined wells had no market. Their uses are limited by the costs of the expensive rotor and stator units they require, and the very short maintenance overhaul periods in the hundreds of hours. Well drilling screw downhole motors are economically sound, but using such motors in vibrators does not seem to be worthwhile. 
         [0036]    What is needed are vibrators suitable for use in horizontal and inclined wells. The vibrations should be driven by pressurized liquid. The vibrator design must all free travel all along horizontal or inclined wells. The vibrator design should permit serial connections of several vibrators, e.g., inline and powered from a single pressurized-liquid source. 
       SUMMARY OF THE INVENTION 
       [0037]    Briefly, acoustic radiator embodiments of the present invention for coal bed methane or shale gas production are configured to be strategically placed within gas production exhaust boreholes and hydraulically powered so they will emit intense harmonic sonic waves to shake the solid media immediate to the wall areas. The gas volume output that can be realized by an exhaust well depends highly on the permeability of the media, especially at inside faces of the borehole. The shaking half-opens up fractures and pores in the solid media. Thus the permeability of the media to gas improves under such shaking and gas collection efficiencies are optimized. The beneficial operational effects can be increased by locating two or more acoustic radiators in a linear series proximate to one another in a phased relationship. 
         [0038]    In another aspect of the present invention, acoustic radiators are placed along drill string or just behind a drillstring drill bit to reduce friction and wear. 
         [0039]    An advantage of the present invention is inert gases and liquids can more easily and effectively be re-injected into coal seams and anomalies to significantly improve miner health, safety, and productivity. 
         [0040]    These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiments which are illustrated in the various drawing figures. 
     
    
     
       IN THE DRAWINGS 
         [0041]      FIG. 1  is a cutaway perspective diagram of an underground coal deposit that is being drained of its methane, firedamp, and/or natural gas with acoustic radiator embodiments of the present invention that stimulate improved permeability of the media immediately around the exhaust well boreholes; 
           [0042]      FIG. 2  is a perspective view diagram of a planetary bushing orbiting on a shaft under the influence of spiraling flows in a general embodiment of the present invention that can produce strong audio range vibrations deep underground; 
           [0043]      FIGS. 3A and 3B  are cross sectional cutaway and exploded assembly view diagrams of a drillstring vibrator embodiment of the present invention suitable in the application diagramed in  FIG. 1 ; and 
           [0044]      FIGS. 4A and 4B  represent a shaft and a free bushing in an embodiment of the present invention that are used together to control frequency instabilities that can be caused by operating-pressure fluctuations in the hydraulic inlet feed lines. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0045]      FIG. 1  represents a coal bed deposit mining operation for methane, firedamp, and/or natural gas, and is referred to herein by the general reference numeral  100 . A pair of exploratory vertical boreholes  102  and  104  have been drilled from the ground surface to allow for electronic sensors that can imagine and characterize a coalbed  106 . A pair of directional drillstrings  108  and  110  have been used to first bore vertically to the right depth and then horizontally into the coalbed  106 . A paleochannel  112  comprising sandstone represents a typical flaw or anomaly in the coal bed  106 . 
         [0046]    The coal bed  106  has naturally occurring adsorbed methane, firedamp, and/or natural gas. It may also be inundated with groundwater. The depth of the deposit and any groundwater will pressurize the gas adsorbed by the coal. Drillstrings  108  and  110  can be used to remove the groundwater and vent the gas. Such will promote desorption and the drillstrings  108  and  110  and their boreholes are used exhaust the natural gas. 
         [0047]    Drillstrings  108  and  110  are fitted with acoustic radiators  114 - 121  and above-ground, high power hydraulic or inert gas pressurizing pumps (not shown). Inert gas or hydraulic pressure flows are sent down drillstrings  108  and  110 . The acoustic radiators  114 - 121  are each independently configured to convert the flows to strong acoustic vibrations. The acoustic radiators  114 - 121  produce oscillation modes selected to have maximum effect in gas production. Different modes can be used simultaneously, and phasing between the radiators will also have benefits. 
         [0048]    Such sound vibrations shake the coal media and increase gas permeability especially near the boreholes. Increased desorption gas flows result that can be exhausted and sold. 
         [0049]    Phased arrays can be used to focus or concentrate the sound energy. In such case, the radiators are placed within range of each other. Otherwise, they are spaced far apart to lengthen their zone of effect along the drillstring. 
         [0050]    Embodiments of the present invention are useful to degasify coal beds with borehole acoustic equipment, and inert gases and liquids can more easily and effectively be re-injected into coal seams and anomalies to enhance miner health, safety, and productivity. In particular, subjecting the near-hole area to strong sound waves improves the permeability of the media to natural gas. These further include equipment for injecting a second gas into coalbed in order to drive out the desorbing methane. 
         [0051]    The choice of what kind of acoustic radiators  114 - 121  to use and how to match the radiators with the surrounding media are practical challenges that are overcome by the present invention. Electrically operated radiators are dangerous because they can spark an explosion of the very gas being extracted. Connecting them and fitting them with an adequate power source is also problematic. Not placing the radiators in direct contact with the solid inside faces of the boreholes can result in poor acoustic impedance matching, and all the benefits can be lost because strong enough vibrations do not reach the media. 
         [0052]    Multiple acoustic radiators  114 - 117 , for example, can be mounted on pipe drillstring  108  at critical points and with critical frequency outputs compared to each other so as to produce a phasing of outputs extend or intensify the media zone in which the permeability is increased so the gases or liquids can be removed. 
         [0053]    Mechanical sound radiators not powered by electricity are attractive in this application. Herein are described mechanical sound radiator embodiments of the present invention that will be suitable for the applications described in relation to  FIG. 1 . The majority of the prior art downhole sources of elastic oscillations are not suitable for use here, since they are mainly designed for stationary small-scale operation in liquid-filled wells. 
         [0054]    Drillstring vibrator embodiments of the present invention generally include a cylindrical external housing that receives a pressurized hydraulic or inert gas flow at one end and passes the flow to the opposite end. A coaxial shaft is fitted with bushing that can eccentrically roll around in constant contact with the outer surface of coaxial shaft in a working space bordered by the external cylindrical housing. The pressurized flows are channeled and ported to cause the bushing to oscillate. This structure allows the vibrators to be organized into linear chains. 
         [0055]    Experiments with various embodiments of vibrators show the basic operating frequency solely depends on the tangential velocity of liquid in the working area, e.g., the pressure difference and geometry of its internal space. One prototype with an external diameter 80-mm, a working-space diameter 50 mm, and a vibrator pressure difference ten bar generated oscillations with a frequency of 110-Hz. 
         [0056]    The vibrator oscillation frequency is almost independent of the free-bush mass and geometry. The intensity of the vibrations generated depends on the kinetic energy developed by the planetary bushing during rotation. Whereas the dependence on the free-bushing mass is linear, the eccentricity dependence is quadratic. For example: 
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         [0000]    where m the free-bush mass, V is the linear velocity of the bush center of mass, f is the free-bush rotating frequency, and ε is the eccentricity of the free-bush center of mass with respect to the vibrator symmetry axis. The aforementioned vibrator model with the free-bush weight 450-grams and eccentricity about five millimeters and the same pressure difference developed an acceleration of forty-four m/s 2 , which is about 4.5-g. 
         [0057]      FIG. 2  represents a basic acoustic vibrator  200  in an embodiment of the present invention. A planetary bushing  202  is loosely mounted on a hollow shaft  204  and is induced to eccentrically rotate in orbit on the shaft by two spiral flows  206  and  208 . A hydraulic or inert gas inlet  210  can be used. Part of inlet  210  is directed down the hollow inside of shaft  204  to become spiral flow  208 . Exhaust ports  212  and  214  provide a means for the flows to escape a working space once they have done their jobs in a cylinder (not shown) and working space that surrounds the planetary bushing  202 . Flows from the exhaust ports  212  and  214  are combined into an outlet  216 . 
         [0058]    The basic mechanism represented in  FIG. 2  can be implemented in a number of various ways.  FIGS. 3A and 3B  represent one possible way an acoustic vibrator could be implemented and succeed in the application shown in  FIG. 1 . 
         [0059]      FIGS. 3A and 3B  represent a drillstring vibrator  300  with a hollow, a cylindrical housing  301  with drain channels  302 , a hollow shaft  303  with a central bore  304 , a first spiral a  305 , a second spiral nozzle  306 , and a free eccentrically rolling planetary bushing  307 . Bushing  307  floats loosely around on shaft  303  and orbits inside a cylindrical working space  308 . 
         [0060]    High pressure liquid, for example, is spirally introduced to working space  308  by peripheral grooves  309  and  310 , respectively disposed in the first and second spiral nozzles  305  and  306 . The flow to channel  309  comes directly from a feed line inlet  311 . Channel  310  is connected with inlet  311  by the hollow central bore  304  in shaft  303 . Exhaust flows exit working space  308  through two opposite drain channels  302  and out to drain  313 . 
         [0061]    In operation, a pressurized liquid flow F from inlet  311  is divided into two parts, F 1  and F 2 . Flow F 1  proceeds directly to working space  308  by peripheral groove  309  of spiral nozzle  305 . Flow F 2  passes through bore  304  into a chamber  312  and is turned around as flow F 3 . It then is spun into working space  308  by peripheral groove  310  on spiral nozzle  306 . Flows F 1  and F 3  enter working space  308  in balance. The longitudinal components of flows F 1  and F 3  are in the same axis and counter-propagating. 
         [0062]    Bore  304  should not be so small in diameter as to impose a significant restriction to flow F 2 . The cross-sectional area of bore  304  should be more than the total cross-sectional areas of peripheral grooves  309  and  310 . 
         [0063]    Flows F 1  and F 3  will vortex when they enter working space  308 , and their tangential components should act equally on both ends of rolling planetary bushing  307 . The vortex flows swirling around rolling planetary bushing  307  pull it into a fast orbit by viscous friction. Centrifugal forces will cause the internal bore of bushing  307  to press hard on the external surfaces of shaft  303  when orbiting. Planetary bushing  307  will accelerate in its orbit velocity to match the velocities of the swirling flows. The planetary motion generates strong audio range vibrations that are efficiently coupled outward by shaft  303 , spiral nozzles  305  and  306 , a pair of shaft supports  320  and  321 , and ultimately to housing  301 . Shaft supports  320  and  321  are each respectively ported to allow flows F 1  and F 3 , and they are pinned to spiral nozzles  305  and  306  to maintain their orientation. 
         [0064]    Rolling planetary bushing  307  is longitudinally stabilized by the counter-propagating longitudinal components of flows F 1  and F 3  at both its ends. After doing its job, the flows drain out as flow F 4  through drain  313  with flow F 5  via drain channel  302  of housing  301 . To reduce hydrodynamic resistances, the cross-sectional area of the drain channel  302  and the area of the hole connecting working space  308  and drain channel  302  significantly exceed the total cross-sectional area of all peripheral grooves which bring pressurized liquid to working space  308  of vibrator  300 . 
         [0065]    The spiral channels  309  and  310  are configured to introduce a flow with a spin into working space  308  that will ensure reliable acoustic contact of bushing  307 . The pitch of the spiral channels is selected to find a good balance between the tangential and longitudinal components of flows F 1  and F 3 . Experiments show that best results will be obtained when the pitch is in the range of 5-10°. All sharp edges should be rounded off to avoid cavitations that would otherwise occur. 
         [0066]    Experiments have shown that the optimal length of planetary bushing  307  is approximately 10% less than the inside length of working space  308 . During operation, planetary bushing  307  rolls around on shaft  303 . 
         [0067]    When hydrodynamic vibrators are driven by high-power pumps, the pressure in the feed lines can fluctuate significantly. Vibrator frequency stability can be improved by using a shaped shaft, for example, in the form of a cone of revolution with an apex a little less than 180°. Model experiments showed that complete stabilization can be realized at an angle of 177°. In this case, the internal surface of the planetary bushing  307  rolls around on the outside surface of shaft  303 . 
         [0068]    The oscillation energy, and output amplitude, can be increased by making unbalancing the planetary bushing. The oscillation energy is directly proportional to the mass planetary bushing  307 , but its dependence on the eccentricity is quadratic. 
         [0069]    Prototype vibrators were made with an external diameter of eighty millimeters (3.15″), the inside nominal diameter of working space  308  was fifty millimeters (1.97″), and the planetary bushing  307  had eccentricity about five millimeters (0.2″) and weight about 450 grams (almost a pound). The frequency of the vibrator-generated oscillations depends on the pressure difference on the vibrator. E.g., 50 Hz at two Bar rising to 110-Hz at ten Bar. The vibrators produced intense and harmonic sinusoidal oscillations. 
         [0070]    The longitudinal working position of free bushing  307  on shaft  303  can be destabilized by pressure fluctuations in the hydraulic feed lines connected to inlet  311 , especially if these fluctuations become significant. Such destabilizations can cause the operational frequency to wander, efficiencies to drop, and can be deleterious. The solution is to impart a slight longitudinal curve to the mating surfaces of free bushing  307  on shaft  303 . 
         [0071]      FIGS. 4A and 4B  represent a shaft  402  and a free bushing  404  in an embodiment of the present invention that are used together to control frequency instabilities caused by operating-pressure fluctuations in the high-pressure hydraulic feed lines. The outside surface of shaft  402  has two slightly tapered cone sections  406  and  408  on either side of a middle section  410 . The tapered cone sections  406  and  408  narrow toward respective outer shoulders  410  and  412 . The effect is to make shaft  402  slightly thicker in a middle section  410  and slightly narrower at its extremities, e.g., an angle-A will optimally be about 177°. Support ends  416  and  418  are right cylinders and the whole of shaft  402  has an end-to-end hollow  420 . Overall, shaft  402  approximately expresses a convex axi-symmetrical surface. 
         [0072]    The inside surface of free bushing  404  is slightly concave and fabricated to match and mate with the slightly convex external surface of shaft  402 . A central groove  422  is flanked by hollow cone sections  424  and  426  that taper down slightly to end sections  428  and  430 . The tapers taken together are shown here as an angle-B, about 177°. 
         [0073]    Although the present invention has been described in terms of the presently preferred embodiments, it is to be understood that the disclosure is not to be interpreted as limiting. Various alterations and modifications will no doubt become apparent to those skilled in the art after having read the above disclosure. Accordingly, it is intended that the appended claims be interpreted as covering all alterations and modifications as fall within the “true” spirit and scope of the invention.