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Publication numberUS3155163 A
Publication typeGrant
Publication dateNov 3, 1964
Filing dateDec 22, 1960
Priority dateFeb 20, 1956
Publication numberUS 3155163 A, US 3155163A, US-A-3155163, US3155163 A, US3155163A
InventorsBodine Jr Albert G
Original AssigneeBodine Jr Albert G
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Method and apparatus for soinc jarring with reciprocating masss oscillator
US 3155163 A
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Description  (OCR text may contain errors)

Nov. 3, 1964 A. a. BODINE, JR 3,155,163

METHOD AND APPARATUS FOR SQNIC JARRING WITH RECIPROCATING MASS OSCILLATOR Original Filed Feb. 20, 1956 2 Sheets-Sheet l FIG.1 L

y/llllllflylll l I v INVENTOR. ALBERT G. BODINE JR.

UZMMW ATTORNEYS Nov. 3, 1964 A. G. BODINE, JR

METHOD AND APPARATUS FOR SONIC JARRING WITH RECIPROCATING MASS OSCILLATOR Original Filed Feb. 20, 1956 2 Sheets-Sheet 2 Tia."

lBl

INVENTOR.

ALBERT G. BODINE JR.

ATTORNEYS United States Patent 3,155,163 METH-QD AND APPARATUS FGR SQNIC EARRING WITH PEWROCATENG MASS GSCKLLATOR Albert G. llcdine, in, Sherman Oaks, Calif. (7877 Woodiey Ave, Van Nuys, fialif.)

Original application Feb. 2%, 1956, Ser. No. 566,628, now latent No. 2,972,380, dated Feb. 21, 1961. Divided and this application Dec. 22, 1%0, Ser. No. 77,669

5 (Jlainrs. (til. 166-46) This invention relates generally to methods and apparatus for loosening and/or pulling or removing elongated objects stuck or frozen by static friction in other objects, for example, pipe such as liners, casing, drill pipe, sucker rods and pump plungers, or other objects or fish which have become stuck by static friction in oil wells. It relates, in a still broader aspect, to longitudinal movement of elongated members in a surrounding medium, and accordingly, is applicable not only to removal of stuck members, as pipe fast in a well bore, but to driving frictionally bound members downwardly in the well, and, in another application, to driving of piles into the earth.

One primary object of the invention is the provision of improved methods and apparatus for loosening and/or moving frictionally stuck or bound members by transmitting acoustic waves therethrough under conditions of standing wave resonance, utilizing novel apparatus and procedures by which very tightly bound members may be readily broken loose and removed, or driven deeper.

This application is a division of my parent application, Serial No. 566,628, filed February 20, 1956, entitled Acoustic Method for Moving Objects Held Tight within a Surrounding Medium, now Patent No. 2,972,380.

The invention provides for setting up in the elongated stuck object a resonant acoustic longitudinal standing wave of high amplitude, thereby causing a cyclic force in a longitudinal direction to be excited by the object in the cement, or other surrounding holding medium, at the stuck point, as well as a cyclic elastic contraction and expansion of the object at the stuck point, which is at a velocity node of the standing wave. Impedance mismatch and phase difference between the object and the surrounding media in which the same is frozen prevent the surrounding media from contracting and expanding either in step with or to the same degree as that which the stuck region of the object may readily undergo under proper drive. Frictional resistance losses in any media also results in failure to transmit the alternate expansion and contraction from the stuck object to the media without material loss of amplitude. The lesser amplitude of the periodic contraction and expansion of the surrounding media means that the stuck object is clear of the media for a time during each contraction half cycle. Moreover, phase difference between the expansion and contraction cycles of the stuck object and the surrounding media means that the object is undergoing contraction during at least a part of the time that the surrounding media is undergoing its expansion half-cycle, with resulting still greater momentary clearance. In effect, the surrounding media stand back while the stuck object periodically contracts. Such action rapidly loosens the stuck or frozen object, and it may then be elevated by pulling upwardly on it. It is often desirable to pull upwardly on the object while it is undergoing its cyclic motion relative to the surrounding media, with the result that it moves upwardly by small increments as successive momentary clearances occur. It is clear that the object moves when the cyclic impulse force exerted on the column together with the pull or upward bias force exerted on the object exceed the holding strength, or resistance to movement exhibited by the surrounding media under the conditions of longitudinal Fatented Nov. 3, 1964 "ice standing wave action in the stuck object. In other cases it is found preferable to permit the object, or a large longitudinal fraction thereof, to stand in compression, so as to create a downward bias force, with the result that the object moves downward as it is loosened. After complete loosening, it may be readily elevated. A feature of the invention is the control of the tension and/ or compression in the casing to accomplish most advantageous performance in any given case.

in order to achieve the above described loosening action, I have found it necessary to transmit relatively powerful acoustic waves down the frozen rods, pipe, fish, etc., to the site where the object is stuck in the cement, or other holding medium, whereby to develop cyclic forces working against and overcoming the holding strength of the cement or other holding medium on the stuck object, and to accomplish this, certain novel relationships between the source of the acoustic waves, the stuck object, and any supporting means for the equipment, have been found to be of prime importance. In this connection, I utilize the concept of mechanical impedance, which, in this case, signifies the ratio of total cyclic peak force to displacement velocity at any given point in the acoustic wave system. The stuck region of the object is a region of high impedance, being at a velocity node of the standing wave system, while the upper end of the object is a region of lower impedance, being at a velocity antinode of the standing wave system. The acoustic wave source may be a very powerful mechanical vibrator. Now, I have found that it is of utmost importance, if the necessary substantial power for tightly stuck jobs is to flow from the wave generator into the object in which the standing wave is to be developed, that the coupling between the generator and the object have an acoustic impedance which is of at least as high an order of magnitude as that of the upper end portion of the object at the coupling point. This impedance requirement may be met by use of a firm or rigid coupling means, for example serrated wedge slips, or mechanical clutch jaws, acting directly between the generator case or frame and the object. A low impedance coupling, i.e., of lower order than that of the upper end of the object, involving, for instance, a flexible element, such as a common suspension cable, or the like, lacks the ability to transmit the high cyclic force that is available from the necessary high power generator, and apply it to the pipe. Thus a high impedance coupling, i.e., one of an impedance magnitude of an order comparable to that of the upper end portion of the object at the point where the coupling is to be made thereto, is an absolute requisite, and is satisfied by any mechanical coupling device of sufficiently high impedance. I sometimes prefer for this purpose a coupling device comprising well-known serrated wedge slips rigidly connected with the generator and engaged directly with the object.

Also any supporting or suspension means for the acoustic wave generator must, I have found, have a relatively low mechanical impedance, and may be a flexible cable, desirably including a spring vibration-isolator, or other relatively compliant element, so as to prevent material transmission of vibratory power through such supporting or suspension means.

Still further, I have found it highly important to employ an acoustic wave generator which is operable at a resonant frequency of the stuck object, and one whose output impedance is comparable to the impedance of the object at the point of coupling thereto.

An important incident of the setting up of the described resonant acoustic standing wave in the pipe or other memher to be loosened is an energy storage property inherent in resonant systems of the character in question. A large quantity of acoustic energy is periodically delivered to the vibrating pipe or rod string, stored therein, and periodical- 1y released therefrom so as to deliver large cyclic forces to the stuck point. The resonant pipe or rod string thus inherently functions as a largeenergy storage reservoir, giving the system a high Q factor, i.e., large flywheel eifect, comparable to the tank circuit of an electric oscillator, and assuring large energy delivery and corresponding force application to the stuck region of the object to be loosened.

A particular form of my invention specifically covered in this divisional application embodies a mechanical wave generator or oscillator characterized by an oscillatory mass that reciprocates linearly, axially of the stuck object such as pipe or rod string. Many otherwise efiective mechanical wave generators involve a pair of rotating mass elements, whose components of lateral force are intended to be balanced. However, perfect balance is mechanically difficult of accomplishment; and a prevailing unbalanced component of lateral force sets up lateral vibration in the pipe or rod string to be loosened and removed from the well, tending to deform or break threads at the couplings in the string. Lateral vibration is also disadvantageous owing to dissipation of large quantities of energy otherwise available for the desired longitudinal vibration wave. The present invention cures these problems by the provision of a linearly reciprocating mass element in the vibration generator, with the line of reciprocation parallel to the rod or pipe string.

The invention will be further understood from the following detailed description of a number of related illustrative embodiments, reference for this purpose being had to the accompanying drawings, in which:

FIG. 1 is a view, partly in elevation and partly in vertical medial section, showing an illustrative form of the invention adapted for loosening a string of well pumping sucker rods;

FIGS. 2 and 3 are sections on lines 22 and 3-3 respectively, of FIG. 1;

FIG. 4 is an elevational view of another embodiment of the invention;

FIG. 5 is a section taken on line 5-5 of FIG. 4.

FIGS. 1-3 illustrate an embodiment of the invention designed for the purpose of loosening an elongated elastic object or column, in this case an oil well sucker rod string and or pump plunger frozen in a well by sand or other conditions. An upper end extension 129 of the sucker rod string S, above well-head 130, and above the polished rod 131, has clamped thereto as by means of clamp 132, an acoustic Wave generator or oscillator 133. It will be understood that the sucker rod string and deep well oil pump below the well head may be of any conventional type. Numeral 134 designates a hook understood to be suspended by conventional derrick tackle, by which it may be raised and lowered, and by which the aforementioned pull may be exerted during operation, and this hook engages a bail 135 carrying the casing 13s of a low impedance spring isolator 136a. A suspension rod 137 is reciprocable through an opening in the bottom of casing 136 and has at its upper end a head 139 supported on the upper end of coil spring 140 whose lower end is supported on the lower end of casing 138. The lower end of rod 137 carries a socket 142 which receives the upper extremity of rod 129, and this socket is equipped with a clamp 143 adapted to clamp rod 129 rigidly within the socket. As here shown, the socket may have a half round bore 144 adapted to snugly receive the upper extremity of the rod 129, and with an enlarged half round bore 145 adapted to accommodate arcuate clamp member 146 which engages the rod and clamps it by means of screws 147.

The vibration generator 133 incldes a housing 150 formed with a half round seat 151 (FIG. 3) adapted to engage the rod 129, and with a projecting arm 152 in which is mounted clamp screws 153 adapted to engage arcuate clamping member 154 engaging the rod 129 and clamping it tightly to its seat 151.

The generator housing 15% has a vertical fluid cylinder 155 formed with a bore 156 in which is fitted a linearly reciprocating piston 157 positioned between coil compression centering springs 158 bearing against caps 159 screwed into opposite ends of bore 156. The caps 159 are bored and threaded to receive air hose fittings 160, which are connected by air hoses 161 to opposite ends of air cylinder 162. A piston 163 in cylinder 162 is reciprocated by connecting rod 164 driven from eccentric 165 on a shaft 166, which may be powered by any suitable prime mover, not shown, for instance, an internal combustion engine.

On each stroke of piston 163, air under pressure for moving piston 157 is delivered to the space in cylinder 156 at one end of piston 157 and removed from the space at the other end thereof. On the opposite stroke of piston 163, the reverse takes place. Accordingly, piston 157 is caused to oscillate linearly within cylinder 156 between springs 15%, which alternately elongate and contract to accommodate the action, and act to retard and stop the piston at the ends of its stroke.

The described linear oscillation of piston 157, whose mass acts on the end caps 159 of the cylinder through springs 158, results in alternating vertical reactive forces being exerted against the generator housing, and therefore on the sucker rod string to which it is rigidly clamped. The upper end portion of the sucker rod string is thereby vertically oscillated, and longitudinal acoustic waves are accordingly set up in the sucker rod string, and transmitted to the site where the string or pump plunger is frozen within the well. The suspension means as described comprises a low impedance support, the spring isolator preventing transmission of vibration energy to the hook and derrick tackle above. A biasing force or tension can be exerted in the sucker rod string, for reasons described hereinabove.

The acoustic waves, transmitted down the rod string as described above from the oscillator to the point where the rod string or pump plunger is stuck in the well, are reflected from the stuck (stationary) point and reflected back up the rod string. In terms of impedance the point where the rod string or plunger is stuck is a point of high mechanical impedance, and the cyclic stresses in the string or plunger are at maximum amplitude in that region, longitudinal motion or displacement being minimized, or approaching zero. The prime mover driving the air piston 163 is then operated at a speed which, through the fluid drive system, reciprocates piston 157 of oscillator 133 at a frequency establishing in the rod string between the stuck point and said oscillator, a resonant acoustic standing wave of an odd number of quarterwave-lengths, such as represented in FIG. 4 of my aforementioned parent application Serial No. 566,628, now Patent No. 2,972,380. The operator readily recognizes that he has set the prime mover to drive the system at this condition of resonance by obvious and well-known physical manifestations of resonance, including large increases in amplitude of vibration with small variations in frequency, as in all vibratory systems operating at or approaching resonance.

It should be understood that when a resonant longitudinal standing wave is established in a column, which is stuck or immobilized against longitudinal motion at a given point, a velocity node of the standing Wave must necessarily occur at the stuck or immobilized point, and that a cyclic longitudinally-directed force will be exerted in the column, at the node, against the immobilizing medium. It should be understood that forces at a node at a tree point in a column are opposed and dynamically balanced. When, however, a given column is held fast at an arbitrary point, the resonant standing wave set up in the column will have a node at the fast point, and, in general, the cyclic forces in the column on the acoustically free side of the fast point will not be dynamically balanced by like forces in the other side thereof, but will be exerted against and opposed by the immobilizing medium. Additionally, as mentioned heretofore, the elastic column, in which a longitudinal standing wave is sustained, alternately elastically dilates and contracts at the velocity antinode in step with alternating compressive and tension phases of the wave in the column. This follows from a phenomenon generally referred to as Poissons Ratio, and consists in a radial dilation or contraction of an elastic column in response to compression or tension exerted in the column. Returning to a consideration of FIGS. 1-3, the longitudinally-directed cyclic forces thus exerted by the rod string on the medium in which the rod string is stuck, act to overcome the holding strength of the medium. The alternating dilations and contractions of the rod string act also to reduce the ability of the medium to hold onto the rod string. In connection with the latter effect, i.e., alternating dilation and contraction of the rod string, owing to impedance mismatch between the rod string and the medium in which it is frozen, frictional losses between the members, and cyclic phase displacements, the cyclic contractions of the rod string are not followed up by like contractions of the medium in which the rod string has become frozen, and the bond is quickly broken. The performance is thus that the longitudinal vibration pattern in the rod string produces therein a longitudinal cyclic impulse force which, combining with the bias force, acts to overcome the holding resistance exerted by the surrounding media; and at the same time, this longitudinal vibration pattern reduces the ability of the media to hold to or bind the rod string.

The linearly reciprocating piston mass 157 of the vibration generator produces longitudinal acoustic waves in the rod string without any tendency for production of unwanted resultant lateral vibration, such as often subject the threaded couplings to undesirable abuse. A generator system using a linearly reciprocating mass such as that of FIGS. 1-3 is therefore particularly advantageous in situations wherein the rods are to be re-used, and preservation of the threaded couplings is important.

In FIGS. 4 and 5 is shown an alternative embodiment of the invention, designed particularly for loosening a fish stuck in a well bore during drilling operations. At 170 in FIG. 4 is indicated the lower end of a conventional drill pipe stem, understood to be suspended from the usual rotary table at the ground surface, by which it may be rotated to power the loosening operation. The lower end of the drill stem is coupled at 171 to the upper end of a driving tool 172, having a long, massive inertia collar 173, a spring section 174, and a lower section 175 equipped with means for imparting vertical reciprocation to the parts suspended therebelow. At 175 is indicated a heavy drill collar, of a length of the order of 100 feet, to the lower end of which is coupled overshot grappling tool 177. The latter may be of any conventional type, being here indicated, somewhat conventionally, as containing serrated wedge slips 178 in a downwardly tapering bowl 179 above upwardly tapering mouth 180, and it will be understood how such a device is engaged with the upper end of the fish indicated at 181, and assumed to be stuck tightly in the well at some point below, as at 182. The wedge slips 178 furnish a means for coupling rigidly to the fish, in such a manner as to permit acoustic wave transmission to and downwardly along the length of the latter to the site 182 where the fish is frozen within the well.

The lower end portion 175 of the tool 172 is formed with an upwardly extending bore or socket 184, and receives a stem 185 extending upwardly from a coupling member 186 coupled as indicated, to the upper end of collar 176. Stem 185 has rigidly mounted thereon a cam collar 187 whose upper and lower edges are contoured in the form of a sine wave. This cam collar rides between rollers 188 mounted by means of bearings 189 on stub shafts 191i projecting inwardly from the walls of member 175. Accordingly, assuming collar 176 to remain rotationally stationary, by reason of its inertia, by friction in the well bore, and by being clutched to the frozen fish 181, and assuming drill stem and tool 172 to be rotated by the rotary table at the ground surface, the rollers 1% bearing on the sinuous outline of cam collar 187 cause the tool portion to reciprocate vertically. The upper portion 173 of the tool 172 is of considerable mass, and the spring section 174 deforms as the portion 175 reciprocates, so that the massive section 173 and the drill stem above are not subject to material vibration, particularly if the spring section is tuned for the operating frequency range. The reaction of the tool section 175 (constituting a linearly reciprocating mass element) exerted through rollers 188 on cam collar 1S7, imposes a vertical oscillating force on the stem on which the cam collar is mounted. This force results in alternate elastic elongation and contraction of stem 135 and collar 175, with the result that longitudinal elastic waves or vibrations are transmitted down the collar 176, through the coupling at 177, and down the fish 181 to the site 182 where the fish is stuck in the well.

The apparatus of FIGS. 4 and 5 is equipped with means for circulating fluid to aid the loosening operation, and for this purpose the fluid is pumped down the hollow drill stem, through a circulation bore 194 in the upper section 173 of the tool 172, and thence through a tube 1% pressed into the lower end of section 173 and extending down through spring 174 into the upper end of hollow stern 185. The lower end of tube 195 is slidably fitted into the bore 196 of stem 185, and packing is provided at 197. The bore 1% continues down through coupling 185 to the circulation bore 1% of collar 176, and bore 198 leads to a bore 199 in grappling tool 177, the circulation liquid being discharged from the lower end of the latter into the well bore around the fish. This circulation fluid washes the area where the fish is stuck in the well, loosens and removes packed sand, and is returned up the well bore outside the tool string, carrying sand with it, much as practiced in rotary drilling.

The spring section 174 of the tool 172 is preferably milled out from solid stock, and, as here shown, comprises two helical elements 174a and 174b, as may be readily understood from an inspection of FIGS. 4 and 5.

Operation consists in establishing acoustic wave transmission in the collar 176 and fish by rotating the drill stern, as earlier described. This acoustic wave transmission is continued down through the coupling at 177 to and downwardly through the fish to the site where the fish is stuck in the well, setting up a standing wave as described above, and the loosening action occurs in the general manner described in connection with the earlier embodiments.

As in the embodiment of FIGS. 1-3, the apparatus of FIGS. 4- and 5 has a vibration generator or oscillator characterized by a linearly reciprocating mass element, which delivers longitudinal force impulses without proneness to creation of lateral components of Vibration, one of the chief disadvantages of which is dissipation of available energy that would otherwise go into the desired longitudinal vibration of the tool and fish.

The invention has now been described through illustration of a number of illustrative embodiments. It is to be understood, of course, that these are merely illustrative of various difierent inventive forms in which the broad invention may be embodied, and are not to be considered as exhaustive of the complete range of equivalents coming within the broad scope of the appended claims.

I claim:

1. In apparatus for loosening a fish stuck at a point below its upper end in a bore hole, the combination of: a grappling tool adapted to rigidly engage the upper end of the fish, a drill collar having a vertical longitudinal axis, said collar being rigidly coupled at its lower end to said grappling tool an acoustic vibration generator located adjacent the upper end of said drill collar comprising a mass element rotatable on and linearly reciprocable along said vertical longitudinal axis, a non-rotatable member adapted for corresponding reciprocation along said axis, cam means between said mass element and said nonrotatable member for converting rotation of said mass element into axial vibration of said mass element and of said non-rotatable members, said non-rotatable reciprocable member being coupled to said drill collar for transmission of reciprocating force to the upper end thereof to set up in said collar and fish an acoustic standing wave, an inertia collar adapted to be lowered into the bore hole on a rotatable drill pipe string, suspended from a rotary table at the ground surface, and a torque transmitting spring connecting said inertia collar and said rotatable mass element of said wave generator, said spring being yieldable in a vertical direction to isolate said inertia collar from vibration transmitted upwards from said generator.

2. A method 01' the character described, comprising: acoustically coupling, with a substantial impedance match, the linearly reciprocating output means of a vibrator of the linearly reciprocating mass element type to an elastic column system embodying a portion held tight by a holding region within the earth and a portion extend ng thereabove, said coupling being made to a portion of said column system at a point spaced above said held portion thereof, arranging said vibrator and said coupling so that said linearly reciprocating output means travels longitudinally of said column system, applying a bias force from said column system against said holding region, and simultaneously operating said vibrator at a resonant frequency of said column system which establishes a longitudinal vibration pattern therein with cyclic impulse force in said column system adjacent said holding region, and at an output level which develops said cyclic impulse force of such magnitude that the combined bias force and said cyclic impulse force exceeds the resistance to not movement experienced by said column system within the earth under said condition of longitudinal vibration pattern in said column system.

3. The subject matter of claim 2, including the further step of applying a torque to said linearly reciprocating means.

4. In a system for longitudinally moving an elastic column system having a lower portion held fast in a holding region within the earth and an upper portion extending thereabove, the combination of: a vibration generator of the type embodying a linearly reciprocating mass element and a linearly reciprocating output means, means acoustically coupling said linearly movable output means to said upper portion of said column system, with said linearly reciprocating output means oriented for movement longitudinally of said column system, so as to apply to the column system a cyclic force impulse directed longitudinally of the column system, and bias means in combination with said elastic column system for applying a bias force from said column system against said holding region in a direction longitudinally of said column system, said vibration generator and said bias means in combination being adapted to develop a longitdinal moving force on said held portion of said column system which is of a magnitude exceeding the resistance to net movement experienced by said column system within the earth.

5. The subject of claim 4, wherein said vibration generator embodies a housing having a cylindrical bore extending in the direction of said column system, said mass element comprises a piston Working in said bore, and springs between the ends of said piston and said housing.

References Cited in the file of this patent UNITED STATES PATENTS 2,667,932 Bodinc Feb. 2, 1954 2,670,801 Sherborne Mar. 2, 1954 2,743,585 Berthet et al. May 1, 1956 2,972,380 Bodine Feb. 21, 1961

Patent Citations
Cited PatentFiling datePublication dateApplicantTitle
US2667932 *Feb 17, 1948Feb 2, 1954Bodine Jr Albert GSonic system for augmenting the extraction of oil from oil bearing strata
US2670801 *Aug 13, 1948Mar 2, 1954Union Oil CoRecovery of hydrocarbons
US2743585 *Oct 31, 1950May 1, 1956Francois BerthetDriving and pulling of piles, pile planks, tubing, and the like
US2972380 *Feb 20, 1956Feb 21, 1961Bodine Jr Albert GAcoustic method and apparatus for moving objects held tight within a surrounding medium
Referenced by
Citing PatentFiling datePublication dateApplicantTitle
US3262507 *Oct 14, 1963Jul 26, 1966Christiani And Nielsen LtdDriving and extraction of piles and/or encasing structures
US4314365 *Jan 21, 1980Feb 2, 1982Exxon Production Research CompanyAcoustic transmitter and method to produce essentially longitudinal, acoustic waves
US4645017 *Apr 10, 1985Feb 24, 1987Bodine Albert GVibrational isolation system for sonic pile driver
US4667742 *Jan 31, 1986May 26, 1987Bodine Albert GDown hole excitation system for loosening drill pipe stuck in a well
US4913234 *Jan 19, 1989Apr 3, 1990Bodine Albert GFluid driven screw type sonic oscillator-amplifier system for use in freeing a stuck pipe
US5234056 *Aug 10, 1990Aug 10, 1993Tri-State Oil Tools, Inc.Sonic method and apparatus for freeing a stuck drill string
US5515922 *Dec 9, 1994May 14, 1996Rattler Tools, Inc.Recovery tool
US7066250Jan 20, 2004Jun 27, 2006Dhr Solutions, Inc.Well tubing/casing vibrator apparatus
US7264055Jun 15, 2005Sep 4, 2007Baker Hughes IncorporatedApparatus and method of applying force to a stuck object in a wellbore
EP0352979A2 *Jul 20, 1989Jan 31, 1990The British Petroleum Company p.l.c.Variable amplitude drill
WO2005014970A1 *Jun 29, 2004Feb 17, 2005Baker Hughes IncShear strength reduction method and apparatus
Classifications
U.S. Classification166/301, 166/177.1, 175/56
International ClassificationE21B7/24, E21B31/00, E21B7/00
Cooperative ClassificationE21B7/24, E21B31/005
European ClassificationE21B31/00C, E21B7/24