US3545552A - Cavitational drilling utilizing an acoustic generator and an acoustic concentrator - Google Patents

Description

United States Patent llll 3,545,552

3,251,424 5/1966 Brooks inventor Frank A. Angona Dallas, Texas Appl. No. 783,226 Filed Dec. 12, 1968 Patented Dec. 8, 1970 Assignee Mobil Oil Corporation a corporation of New York CAVITA'IIONAL DRILLING UTILIZING AN ACOUSTIC GENERATOR AND AN ACOUSTIC CONCENTRATOR 3 Claims, 3 Drawing Figs.

u.s. Cl 175/65, l66/ l 77 Int. Cl E21b7ll8 Field olSearch l75 /55- References Cited UNITED STATES PATENTS 3,302,720 2/1967 Brandon l75/56X 3,405,770 10/1968 Galle et al... l75/65X 3.438.352 4/1969 Brandon 166/ 177X Primary Examiner-David H. Brown Atlorneys--William J. Scherback, Frederick E. Dumoulin,

William D. Jackson, Andrew L. Gaboriault and Sidney A. Johnson ABSTRACT: This specification discloses an improved method and apparatus for drilling a liquid-filled borehole into the earths crust with cavitational energy. Acoustic energy is generated at a first region in the borehole of an amplitude less than that required to produce cavitation in this region. This acoustic energy is then focused to a second region downhole of the first region and concentrated to an amplitude greater than the amplitude required to produce cavitation in the second region. The second region is moved about the bottom of the borehole to effect cavitational drilling of the borehole.

CAVITATIONAL DRILLING UTILIZING AN ACOUSTIC GENERATOR AND AN ACOUSTIC CONCENTRATOR BACKGROUND OF THE INVENTION This invention relates to the drilling of boreholes into the earths crust. More particularly, it relates to the drilling of such boreholes by using acoustic energy to effect cavitation in the drilling liquid and utilizing the resulting shock waves from the cavitation to fracture earth materials at the bottom of the borehole. 1

Numerous techniques are available for drilling boreholes into the earths crust. A relatively new drilling technique which is showing increasing promise utilizes cavitation of a liquid within the borehole. Cavitation is a phenomenon whereby under certain conditions cavities form and violently collapse within a liquid. A shock wave results from the cavitation and may cause considerable mechanical damage to neighboring solid surfaces. Cavitation takes place within a liquid when the pressure within the liquid is reduced by an amount commonly termed the cavitational threshold. The cavitational threshold is a function of the pressure on the liquid and is also influenced by various other factors such as the presence of gas or solid particles which serve as nuclei for cavitation. The cavitational threshold is the value of the acoustic pressure required to produce cavitation and in general the cavitational threshold of a drilling liquid at a particular location within a borehole -is somewhat less than the hydrostatic pressure of the drilling liquid at this location.

Normally in cavitational drilling, a drilling liquid is circulated downward through a drill string and thence upward to the surface of the earth through the annulus formed between the outer wall of'the drill string and the borehole wall. Cavitation is induced in the drilling liquid by any suitable technique such as by the generation of acoustic energy. The resulting shock waves function to break up the rock surfaces at the bottom of the borehole. The rock fragments thus produced are entrained in the circulating drilling liquid and withdrawn from the borehole with this liquid.

Various cavitational drilling techniques are known in the art. For example, in U.S. Pat. No. 3,315,755, to W. B. Brooks,

there is disclosed a technique for carrying out cavitational drilling through the use of acoustic energy. In this technique, a piston is vibrated to impart acoustic energy to the drilling liquid. The acoustic energy induces cavitation in the drilling liquid and shock waves resulting therefrom are utilized in drilling the borehole. Another technique in which acoustic energy is utilized to efiect cavitation in a drilling liquid is disclosed in U.S. Pat. No. 3,387,672,-to E. l... Cook. In this technique, the cavitational threshold of the drilling liquid an amount of gas which goes into solution therein.

SUMMARYOF THE INVENTION In accordance with the present invention there are provided new and improved methods and systems for practicing cavitational drilling. In practicing the method of the present invention, drilling liquid is circulated through the borehole. Acoustic energy is generated at a first region inthe borehole of an amplitude less than the amplitude required to produce cavitation in the drilling liquid in this first region. This acoustic energy is 'focused'through a liquid path to a second or focal region in the borehole and concentrated to a second amplitude greater than the amplitude required to produce cavitation in the drilling liquid in this second region. Cavitation is thus produced in this second region and the resulting shock waves are utilized to break up'earth materials at the bottom of the borehole.

In accordance with another embodiment of the invention, there is provided apparatus for drilling a borehole by cavitational energy. This apparatus is comprised of an elongated housing having a passageway therein for the circulation of drilling liquid. Included in the housing are a flexible coupling, an acoustic transducer, focusing means located below the coupling and transducer for focusing acoustic energy generated by the transducer through a path extending below the housing, and deflector means below the coupling for moving the focusing means laterally within the borehole. BRIEF DESCRIPTION OF THE DRAWINGS;

FIG. 1 is an illustration showing a drilling tool formed in accordance with one embodiment of this invention and located in a borehole;

FIG. 2 is a cross-sectional view of a biconical concentrator attached to an acoustic transducer and located in a borehole; and

FIG. 3 is an illustration of the bottom of the borehole showing a trace of the path that the focal region follows over the bottom of the borehole during operation of the drilling tool.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, there is shown a borehole tool, more specifically a cavitational drilling tool, attached to a drill string located in a borehole. A fluid passageway is provided through the drill'string and drilling tool whereby drilling liquid may be circulated downward through the drill string and upward through the annulus formed between the drill string and the wall of the borehole. An acoustic transducer or generator is actuated by the circulating drilling liquid to vibrate a piston and produce acoustic energy within the drilling liquid at a first region in the vicinity of the piston. This acoustic energy is of a first amplitude which is less than'the amplitude required to produce cavitation in the drilling liquid in this first region. The acoustic energy is concentrated by a focusing means to a second region whereby cavitation'is produced in the drilling liquid. The drilling tool further includes means for moving the focusing means laterally within the borehole whereby the second region is moved over the bottom of the borehole for improved drilling efficiency.

More particularly and with reference to FIG. 1, borehole 3 is shown extending into the earths crust 2. This borehole is initially started and drilled into the earths crust by any suitable technique, such as a rotary procedure, for at least a depth greater than the length of the cavitational drilling tool of this invention. Normally the borehole will be drilled to some depth at which it becomes more economical to continue drilling the borehole with cavitational energy rather than by rotary techniques. The rotary drilling tool is then withdrawn from the borehole and the cavitational drilling tool of this invention is attached to the lower end of the drill string and lowered into the borehole.

In FIG. 1 a cavitational drilling tool is shown positioned in the vicinity of the bottom of borehole 3 and secured to drill string 11. The cavitational drilling tool is comprised of an elongated housing 1 having a passageway therein for the circulation of drilling liquid 6. The following elements of the cavitational drilling tool are secured into elongated housing 1 by, for example, threaded connections; flexible coupling 9, acoustic transducer or generator 5, focusing means 7, and deflector means which include deflector 10 keyed into grooves 8 which are formed in a tapered portion of the outside case of elongated housing 1. Focusing means 7 is located in housing 1 below flexible coupling 9 and acoustic transducer 5. The deflector means is located in housing 1 below flexible coupling 9.

In a preferred cavitational drilling tool, flexible coupling 9 forms the upper part of housing 1 and is adapted at its upper end for connecting to a drill string. Acoustic transducer 5 is secured to the lower end of coupling 9 and focusing means 7 is secured to the lower end of transducer 5. The outside case of focusing means 7 is tapered and grooves 8 are formed in it. Deflector I0 is keyed into grooves 8 to complete the deflector means. When this preferred embodiment is utilized in drilling a borehole, piston 12 is located immediately adjacent the upper end of the focusing means and the deflector means moves the focusing means laterally within the borehole by flexing coupling 9.

means 7. Also, the deflector means maybe located at other positions inhousingl than .in the outside case of focusing means 7 so long as it is located belowcoupling means 9.

Acoustic generators "may beanytype of transducer which" iscapable of being operated to produce acoustic energy at the first region in the borehole of afirst amplitude that is lessthan the amplitude to. produce cavitation in the drilling liquid within-the borehole at thisfirst region. Stated otherwise, this'first amplitude is lessthan the cavitational threshold of the drilling liquid in the first-region.

In the preferred -.ernbodiinent of the invention illustrated, the acoustic generator '5 comprises a vibrating piston 12 which is driven by a turbine (notshown). By circulating drilling liquid through the drill string 11 and housing 1, the turbine is actuated andvibrates piston 12 thereby generating acoustic energyin' the drilling liquid. Suitable acoustic generators including a vibratory piston forusein the present invention.arethe turbine-powered acoustic pressure pulse generators and jet: edge generators disclosed in US. Pat. No. 3,315,755 to'B'rooks:

A suitable flexible coupling means for use in the invention is on e'c oniprised of reinforcedflexible'hydraulic hose lined on the inside with a-collapse-r'esisting spring and enclosed within a burst-resistant spring. For a' detail description ofxsuch a I coupling reference is made tofNew Concept in Drilling Rigs Designed to Lower wen Costs, wonmoin 114, Dec. 1967: I

.ln drilling a welliin accordance with thewpresent invention; drilling liquid 6 is. circulated downward through drill string 11 and upwardthtough annulusd, defmedby the outer wall of drill string ll and-the borehole 3.'Acoustic generator is actuated bytht: circulating drilling liquid. to produce at a first region in the borehole acoustic energy of afi'rst amplitude that is less-than thamplitude required to produce cavitation in the drilling liquidin' this first region. The acoustic energy at this first region'is then focused by focusing means 7 to a second or focal region 13 located. at approximately the bottom of borehole 3. The acoustic energy 'is' concentrated by the focusing'means atthe second region 'and'the amplitude is thus increased to a second amplitude which is greater than. the amplitude requiredto produce cavitation inthe drilling liquid in this second region: Cavitationv is thus produced within this secondr'egion'andthe shock waves resulting from this cavitation are utilizedto :break up theearth materials at the bottom of the borehole;

The focusing means 7 may be of any suitable type but preferably takes the forrn'of'a biconical concentrator 30, as shown'in FIG. 2.' reference to FIG. 2, the biconical concentrator 'is attached bythreads 32 to acoustic generator 5 and positioned in the lower portion of borehole 3. The biconical concentrator is comprised of two conical frusta 34 and 36 joined end-to-end by-their smaller apertures at 38. Acoustic energy producedby acoustic generator 5 is illustrated by ray paths 40. This acousticenergy is concentrated by the biconical concentrator to a focal point 14 within a second region 13v in the vicinity of the borehole bottom. The amplitude of the acoustic energyproduced'by acoustic generator 5 at the first region in the borehole is thusrincreased to a second amplitude in focal region. 13"and cavitation iseffectedin the drilling liquid. The importance offocusing and concentrating this energy in a'secondregion is twofold. First, the amplitude of the acousticenergy-generated by the acoustic generator is increased, th'ere'byimproving the efficiency of the generator particularly for drilling deep boreholes; and second, the region of cavitation is removed from the immediate vicinity of the acoustic generator-,"thereby avoiding excessive wear of the The deflector. functions to move the focusing meam laterally within the borehole. Spiral grooves 8 are provided in the outside surface of tapered biconical concentrator 30. Deflector means 10 is slidably keyed in grooves 8, and rubs against the wall of borehole 3. when deflector-means 10 is at the highest position of spiral grooves 8, focal region 13 extends to its most lateral position in borehole 3 where cavitation in this region functions totbreak up the earth material along-the outside edges of the borehole. When deflector 'means 10 is inits lowermost position in spiral grooves 8, focal region 13 is centered about a point 14 which is offset a short lateral distance from the longitudinal axis of the borehole. Thus, flexible coupling means 9 biases the cavitational drilling .tool toward the longitudinal axis of the borehole while deflector means 10 biases it toward the wall thereof.

In operation, drilling liquidis circulated through elongated housing 1 and then upward through annulus M0 the surface of the earth. Acoustic energy is produced at a first region by acoustic generator 5 which acoustic energy is focused to a focal region 13 where cavitation is initiated. Rotational torque is applied to drill string '1 l by conventional drilling machinery (not shown). This Rotational torque is transmitted by flexible couplingmeans 9 to the cavitational drilling tool causing it to be rotated. This rotation of the cavitational drilling tool and differential friction between deflector means 10 and the wall of borehole 3 cause deflector means 10 to move upwardv and downward along spiral grooves 8,'thereby laterally moving focusing 7 within the borehole. Concomitantly, flexible coupling 9 exerts. a centeringforcewhich tends to center zfocusing means 7 along the longitudinal axis of borehole 3'whiledeflector means .10 produces a lateral offset of focusing means 7 'from'the longitudinal axis of borehole 3 even when deflector means 10 is in its lowermost position in-v spiral grooves 8. The result of these forces-and counteracting forces is that focusing means 7'is moved laterally within the borehole concomitantly? with the rotation of the drill string. Thus focal region'13 is moved about the bottom of borehole 3 to effect drilling of the borehole.

Referring to FIG. 3,"there is illustrated a plan view of the bottom of borehole 3 .having superimposed thereon a trace of the path that focal region 13 follows as it sweeps the borehole bottom. Focal point 14, which is the center of focal region :13, is shown offset a short distance from the center of borehole 3. As the cavitational drilling tool is rotated, deflector means 10 sliding in spiralgrooves S-laterally moves focusing means 7 within the borehole 3 and causes the focal region 13 to be moved about the bottom of the borehole in a spirallike pattern indicated by broken line 15 until deflector means loreaches the higherrnost point of spiral grooves 8. Focal point 14 is then located at a laterally displaced position indicated, by reference numeral 16.' Thereafter, with continual rotation the'trace of focal region 13.spirals back toward the center of borehole 3 .until deflector means 10 reaches its lowermost position and.

focal. point 14 is again in its initial position or in a similar laterally offset position from the center of borehole 3.

I claim:

1. A method of drilling a borehole into the earths crust focusing said acoustic energy through a liquid path to a second region in said borehole to concentrate said acoustic energy to a .second amplitude greater thanthe amplitude required to produce cavitation in said drilling liquid in said second region, whereby cavitation is produced within said second region to break up earth material at the bottom of said borehole.

' material at said bottom of said borehole.

3. The method of claim 2 wherein said second region is moved over the bottom of said borehole to trace a spirallike pattern.

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