US3603410A

US3603410A - Method and apparatus for cavitational drilling utilizing periodically reduced hydrostatic pressure

Info

Publication number: US3603410A
Application number: US781424A
Authority: US
Inventors: Frank A Angona
Original assignee: Mobil Oil Corp
Current assignee: ExxonMobil Oil Corp
Priority date: 1968-12-05
Filing date: 1968-12-05
Publication date: 1971-09-07
Anticipated expiration: 1988-09-07

Abstract

This specification discloses an improved method and apparatus for drilling a liquid-filled borehole into the earth with cavitational energy. 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. The shock waves resulting from cavitation in the drilling liquid are utilized for drilling the borehole.

Description

United States Patent 2,893,692 7/1959 Marx 175/56 3,251,424 5/1966 Brooks 175/65 3,302,720 2/1967 Brandon 175/56 X 3,315,755 4/1967 Brooks 175/56 3,405,770 10/1968 Galle et al.... 175/65 X 3,441,094 4/1969 Galle et al.... 175/56 3,460,637 8/1969 Schulin 175/56 Primary Examiner- David H. Brown Attorneys-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 earth with cavitational energy. 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. The shock waves resulting from cavitation in the drilling liquid are utilized for drilling the borehole.

PATENIEDsEP Hm 3,603'410 sum 1 or 2 PRESSURE u at P'ATENTEDSEP nan SHEET 2 [IF 2 FIG. 4

METHOD AND APPARATUS FOR CAVITATIONAL DRILLING UTILIZING PERIODICALLY REDUCED I-IYDROSTATIC PRESSURE BACKGROUND OF THE INVENTION This invention relates to the drilling of boreholes into the earths crust and more particularly to the drilling of such boreholes by using acoustic energy to effect cavitation and utilizing the resulting shock waves from the cavitation induced in a drilling liquid.

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 a borehole. Cavitation is a phenomenon whereby under certain conditions cavities form and violently collapse within a liquid. A shock wave capable of causing considerable mechanical damage to neighboring solid surfaces results from this cavitation. Cavitation takes place within a liquid when the pressure within the liquid is reduced by a value commonly termed 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 in the liquid which serve as nuclei for cavitation. In general, the value of the cavitational threshold of the drilling liquid at a particular location within the borehole is normally 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 resulting rock fragments 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 moving piston is vibrated to impart acoustic energy to the drilling liquid. The acoustic energy induces cavitation in the drilling liquid and the resulting shock waves are utilized in drilling the borehole. In another technique disclosed in US. Pat. No. 3,387,672 to E. l... Cook, acoustic energy is utilized to effect cavitation in the drilling liquid. In this technique, the acoustic pressure required to produce cavitation of the drilling liquid is reduced by introducing into the drilling liquid an amount of gas which goes into solution therein.

SUMMARY OF 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, the hydrostatic pressure of liquid in a borehole in the vicinity of the borehole bottom is periodically reduced. Simultaneously therewith, an acoustic energy field is produced in the liquid in the vicinity of the bottom of the borehole to effect cavitation in the liquid.

In one embodiment of the invention, this periodic reduction in hydrostatic pressure is achieved by alternately supporting and releasing at least a portion of the drilling liquid above the vicinity of the bottom of the borehole.

In another and preferred embodiment of the invention, the periodic reduction of hydrostatic pressure is effected by producing a second acoustic energy field within the drilling liquid in the vicinity of the borehole bottom. The frequency of this second field preferably is lower than the frequency of the first-mentioned acoustic field.

By another embodiment of the invention, there is provided apparatus for drilling a borehole by cavitational energy. A first acoustic generator responsive to fluid flow is connected to the lower end of a drill string and a second acoustic generator responsive to fluid flow is connected in the drill string above the first acoustic generator. The first acoustic generator is adapted for operating to produce acoustic energy within drilling liquid in the borehole and the second acoustic generator adapted for operating to vibrate the first acoustic generator.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic illustration of a borehole and one system which may be utilized in carrying out the present invention;

FIG. 2 is a graph illustrating the pressures imposed in the drilling liquid of a borehole in carrying out this invention;

FIG. 3 is an enlarged cross-sectional view illustrating in detail the downhole apparatus of FIG. l; and

FIG. 4 is a cross section of apparatus for carrying out a preferred embodiment of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1 there is illustrated a system for carrying out cavitational drilling. In utilizing the system of FIG. ll, drilling liquid is circulated downward through a drill string and upward through the annulus. The hydrostatic pressure at the bottom of the borehole is periodically reduced by alternately supporting and releasing at least a portion of the drilling liquid above the vicinity of the bottom of the borehole. Simultaneously, an acoustic generator is actuated to produce an acoustic field in the drilling liquid at a downhole location to effect cavitation in the drilling liquid.

More particularly, and with reference to FIG. 1, borehole l is shown extending into the earths crust 2. A conduit, or drill string, 3 extends from the surface of the earth into borehole 1. Drill string 3 is connected at the surface to a source of drilling liquid by any suitable means such as a stuffing box (not shown). The drilling liquid II is circulated downward through drill string 3 to the vicinity of the bottom of borehole I and upward through annulus 13 formed between the outer wall of drill string 3 and the wall of borehole l. Drilling liquid 11 is conducted from annulus 13 of borehole l by conventional means (not shown) to a mud pit. A 1ift should 5 which functions as an external support means is connected to drill string 3 at a downhole location. Examples of external support means which may be used are annular rings or packers adapted for supporting at least a portion of the drilling liquid in annulus 113. An acoustic generator 9 is supported by drill string 3 in the lower end thereof. A lost motion connection means 7 is operatively interposed drill string 3 intermediate lift shoulder 5 and acoustic generator 9. A surface lifting means, pump jack I5, is connected by cable 17 to drill string 3.

In operation, pump jack 15 periodically raises and lowers drill string 3 to alternately support and release at least a portion of drilling liquid 11 by lift shoulder 5, thereby periodically reducing the hydrostatic pressure of drilling liquid II at a downhole location in the vicinity of the bottom of borehole ll. Simultaneously therewith, acoustic generator 9 is actuated by flow of drilling liquid ll therethrough to generate an acoustic energy field within drilling liquid 11 in the vicinity of the bottom of borehole 1. Lost motion connection means 7 allows lift shoulder 5 to be raised and lowered without moving acoustic generator 9 from the vicinity of the borehole bottom. Cavitation is induced in the drilling liquid by the combined effects of the acoustic energy field and the reduced hydrostatic pressure at least within the time periods of reduced hydrostatic pressure.

Referring to FIG. 2, there is seen a typical pressure versus time plot illustrating the pressure variations induced in a drilling liquid by the methods of this invention. The static hydrostatic pressure is represented by horizontal line P The pressure at which cavitation takes place in the drilling liquid at a particular location in the boreholes is represented by horizontal line P The fluctuating hydrostatic pressure produced by embodiments of this invention is illustrated by curve P The fluctuating pressure produced by the relatively high frequency acoustic energy field is illustrated by curve P,. Curve P, is shown for illustrative purposes as being superimposed upon curve P and thus represents the composite effect of the low and high frequency acoustic fields in the drilling liquid. It is seen that the frequency of the acoustic energy field illustrated by curve P, is high as compared to the frequency of the periodic reduction of hydrostatic pressure shown by curve P Hereafter, P, may be referred to as a high frequency field and P may be referred to as a low frequency field. It is to be understood that the terms high frequency and low frequency are used in a comparative sense only as relative to one another.

Preferably, the ratio of the frequency of P, to the frequency of P should be greater than 5. By such ratios, problems of phase relationships between P, and P are avoided and a reduction of hydrostatic pressure by the composite of the acoustic fields represented by P, and P is insured.

It will be recalled that, in the case of drilling liquids in a borehole, cavitation is effected when the pressure in the drilling liquid is reduced by a value somewhat less than the value of the hydrostatic pressure at a particular location in a borehole. The intensity of the shock waves resulting from cavitation increases with increasing pressure on the liquid. Thus, in cavitation drilling, a relatively high hydrostatic pressure is desirable from the standpoint of the amplitude of the produced energy. However, as the pressure increases, so does the cavitational threshold and, thus, and increasing amount of energy in needed in order to induce cavitation. Accordingly, when utilizing a given acoustic generator to induce cavitation, there is a limit to the depth at which cavitation can be induced. The present invention is directed to extending the depth at which cavitation can be produced by a given acoustic generator. The depth is extended by periodically reducing the hydrostatic pressure in the drilling liquid in the vicinity of the borehole bottom while simultaneously producing a high frequency acoustic field to effect cavitation in the drilling liquid.

With further reference to FIG. 2, it is seen that under static conditions the value of the amplitude of acoustic energy required to produce cavitation in a liquid-filled borehole is P,,-P:, =P which is the cavitational threshold under these conditions. At the time of the maximum negative fluctuation of P the hydrostatic pressure is seen to be reduced to a value of P To produce cavitation at this time it is only necessary to further reduce the pressure in the drilling liquid by a value of P P P This is contrasted to the larger value P regiired to produce cavitation under static hydrostatic pressure. Thus, under conditions illustrated by FIG. 2, cavitation takes place during the time period of t,t whereas without periodically reducing the hydrostatic pressure cavitation would not be induced.

Referring now to FIG. 3, there are seen further details of the downhole portion of FIG. 1. Acoustic generator 9, illustrated as an acoustic siren, it attached by threaded sleeve 19 to lost motion connector 7 which serves to connect drill string 3 to acoustic generator 9 by means which allow lift shoulder to be reciprocated relative to acoustic generator 9. Lost motion connector 7 is comprised of a cylindrical housing 14 which is closed at its upper end about drill string 3 in a manner which allows drill string 3 to slide therethrough. An annular shoulder 10 is attached to the lower outer portion of drill string 3. Annular shoulder 10 strikes the upper shoulder 12 of cylindrical housing 14 and prevents drill string 3 from being pulled from the housing. In this manner, longitudinal motion is allowed by lost motion connector 7 between lift shoulder 5 and acoustic generator 9, thus allowing lift shoulder 5 to be longitudinally oscillated without moving acoustic generator 9 from its position with respect to the borehole bottom.

The acoustic generator 9 is comprised of an outer case 33, a mud turbine 23, a stator 27, and a rotor 29 connected operatively one with the other. Mud turbine 23 is attached to outer case 33 by a spider 24. Stator 27 is secured within the lower portion of outer case 33 and rotor 29 is held by shaft 31 in operable relationship with stator 27. In operation, drilling liquid 11 is pumped through drill string 3, hollow shaft 21, mud turbine 23, ports 25, passages 26 of stator 27, and passages 28 of rotor 29, and back up through annulus 13 to the surface. The acoustic siren is thus actuated by the fiow of drilling liquid 11 to produce a high frequency acoustic field. Outer case 33 terminates in a borehole standoff 34 which extends below rotor 29, thus protecting rotor 29 from the boreholes bottom.

In FIG. 4 there is illustrated a system for carrying out cavitational drilling utilizing two acoustic generators positioned in the drill string. Drilling liquid is circulated downward through the drill string and upward through the annulus between the drill string and the borehole wall. The uppermost acoustic generator is actuated by the circulating drilling liquid to generate low frequency acoustic energy which is transmitted down the drill string and vibrates the lowermost acoustic generator, thereby vibrating the radiating face of the piston of the lowermost generator and producing a low-frequency acoustic field in the drilling liquid. Simultaneously, the lowermost acoustic generator is actuated by the circulating drilling liquid to vibrate at a high frequency the piston of the lowermost generator and produce a high frequency acoustic field in the drilling liquid. Cavitation is thus effected in the drilling liquid by the combined effects of the low and high frequency acoustic fields in the drilling liquid.

More particularly with reference to FIG. 4, a portion of borehole l is seen extending into the earths crust 2. The lower portion of drill string carrying vibration generator 35 and mud turbine 37 which drives piston 52 is seen in borehole 1'.

The vibration generator 35 is actuated by flowing drilling liquid 11 to generate low frequency acoustic energy in the drill string. The low frequency acoustic energy is transmitted downward through the drill string and adapter 36 and vibrates mud turbine 37 the drill to the lower end thereof. Simultaneously, the mud turbine is actuated by flowing drilling liquid 1 1 to vibrate piston 52 which is operatively connected therewith to produce a high-frequency frequency acoustic field in the drilling liquid in the vicinity of the borehole bottom. Since mud turbine 37 itself is also being vibrated by the low-frequency acoustic energy produced by vibration generator 35, piston 52 is vibrated by a composite of the high-frequency acoustic energy produced by mud turbine 37 and the low-frequency acoustic energy produced by vibration generator 35 to produce the composite highand low-frequency acoustic fields in the drilling liquid illustrated by FIG. 2. The face 53 of piston 52 thus serves as a radiating face for both the mud turbine 37 and vibration generator 35.

Vibration generator 35 is suspended by its upper end to the lower end of drill string 80 and is connected through adapter 36 to mud turbine 37 and is lowered to the vicinity of the bottom of liquid-filled borehole 1. Vibration generator 35 is vibrated at a low frequency, thereby vibrating mud turbine 37 and piston 52 at said low frequency. In this manner, radiating face 53 of piston 52 imposes a low frequency acoustic field in drilling liquid 11 at the borehole bottom. Simultaneously therewith, mud turbine 37 is actuated to vibrate piston 52 at a high frequency. Thus, radiating face 53 of piston 52 serves as the radiating face or transducer for both the vibration generator 35 and mud turbine 37. The low frequency acoustic field produced by vibration generator 35 serves to periodically lower the hydrostatic pressure in the drilling liquid in the vicinity of the borehole bottom. The high frequency acoustic field produced by mud turbine 37 is superimposed on the low frequency acoustic field and effects cavitation in the drilling liquid at least during a portion of the negative cycle of the low frequency acoustic field.

A suitable vibration generator 35 for use in this combination is described in U.S. Pat. No. 3,554,005, to A. G. Bodine, Jr. Such a vibration generator is adapted to be driven by circulating drilling liquid and is comprised of a plurality of oppositely rotating eccentrically weighted rotors mounted for rotation on axes transverse of the drill string to generate alternating forces along a direction longitudinally of said drill string. As seen in FIG. 4, vibration generator 35 is comprised of a cylindrical case 55 having a fluid passageway 57 passing therethrough connected in fluid communication within drill string 80. Two heavy eccentrically weighted rotors 59, 61 are mounted within

fluid cavities

63, 65 in cylindrical case 55 in fluid communication with fluid passageway 57. Rotors 59, 61 are mounted on separate axes passing transversely through cylindrical case 55 and offset equal distances in opposite directions from the longitudinal axis of fluid passageway 57. Flexible blades 67 are connected with rotors 59, 61 so that when drilling liquid 11 passes through vibration generator 35 rotors 59, 61 are rotated in opposite directions. The flow of drilling liquid is adjusted so that the two rotors fall into a mode or rotation such that the centers of gravity of the two rotors move up and down in step with one another but oscillate laterally in opposition to one another. Thus, lateral vibrations are canceled while vertical vibrations are added to produce longitudinal vibrations within the drill string.

A mud turbine suitable for use in this combination is described in U.S. Pat. NO. 3,315,755, to W. B. Brooks. Such a mud turbine is adapted to be responsive to fluid flow through a drill string and comprises a piston and a fluid turbine motor operatively connected to the piston to reciprocate the piston at a frequency and magnitude such that when the piston is spaced from the bottom of the borehole an acoustic energy field is generated in the drilling liquid to effect drilling of the borehole. As seen in FIG. 4, mud turbine 37 is connected by threaded connector 36 to vibration generator 35. Mud turbine 37 is comprised of outer case 70 which houses a multielement fluid turbine rotor 72. The fluid turbine rotor 72 is connected by a series of shafts and gears to piston 52 having radiating face 53. Fluid communication is provided through mud turbine 37 and piston 52 whereby liquid may be circulated therethrough to longitudinally vibrated piston 52 and produce a high frequency acoustic field in the drilling liquid in the borehole.

Iclaim:

l. A method of cavitationally drilling a borehole having a liquid therein into the earths crust comprising:

periodically reducing the hydrostatic pressure of said liquid in the vicinity of the borehole bottom; and

simultaneously introducing an acoustic field in said liquid in the vicinity of said borehole bottom with an acoustic generator positioned in the vicinity of said borehole to effect cavitation in said liquid.

2. The method of claim 1 wherein the frequency of said periodic reduction of hydrostatic pressure is less than the frequency of said acoustic field.

3. A method of cavitationally drilling a borehole into the earth's crust comprising:

positioning an acoustic generator in the vicinity of the borehole bottom; circulating a drilling liquid downward through a drill string in said borehole and upward through the annulus defined by said drill string and the wall ofsaid borehole;

periodically reducing the hydrostatic pressure of said drilling liquid in the vicinity of said borehole bottom; and

simultaneously operating said acoustic generator to produce an acoustic field in said drilling liquid in the vicinity of said borehole bottom to effect cavitation in said drilling liquid.

4. The method of claim 3 wherein said periodic reduction of hydrostatic pressure is effected by producing a second acoustic field within said drilling liquid in the vicinity of the borehole bottom.

5. The method of claim 4 wherein the frequency of said first-named acoustic field is greater than the frequency of said second acoustic field.

6. The method of claim 5 wherein the ratio of the frequency of said first-named acoustic field to the frequency of said second acoustic field is greater 5. 5.

7. A method of cavitationally drilling a borehole into the earths crust comprising:

circulating a drilling liquid downward through a drill string in said borehole and upward through the annulus defined by said drill string and the wall of said borehole;

alternately supporting and releasing at least a portion of drilling liquid above the vicinity of the bottom of said borehole; and

simultaneously producing an acoustic field in said drilling liquid in the vicinity of said borehole bottom of effect cavitation in said drilling liquid. 8. A method of cavitationally drilling a borehole into the earths crust comprising:

periodically reducing the hydrostatic pressure of said drilling liquid in the vicinity of the borehole bottom by generating acoustic energy in said drill string at a point above the vicinity of said borehole bottom and transmitting said energy down said drill string to the vicinity of said borehole bottom thereby producing a first acoustic field within said drilling liquid in the vicinity of said borehole bottom; and

simultaneously producing a second acoustic field in said drilling liquid in the vicinity of said borehole bottom to effect cavitation in said drilling liquid.

9. The method of claim 8 wherein said second acoustic field is generated by a radiating face and said acoustic energy generated in said drill string is transmitted to the radiating face.

mg UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 609 410 Dated September 7. 1971 Inventor(s) Frank A Angona It is certified that error appears in the aboveidentified patent and that said Letters Patent are hereby corrected as shown below:

' Column 2, line 44, change "should" to --shoulderline 49, after "drill string 3", "in" should be --at--; line 51, after "interposed" insert --in--; line 75, "boreholes" should be --borehole--.

Column 3, line 31, "and increasing" should read --an increasing-- line 32, "in needed" should read -is needed--; line 45, "P -P3 =P should be -P -P =P line 59, "it attached"should be -is attached+ Column 4, line 12, "boreholes" should be -borehole-;

line 37, "The low frequency" should be -This low frequency--; line 39, after "turbine 37" delete "the drill" and insert --connected--; line 42, cancel "frequency" (second occurrence).

Column 5, line 49, after "borehole" insert --bottom--.

Column 6, line 22, "greater 5.5." should be --greater than 5.

line 28, after "portion of" insert -said-; line 32, "of effect" should be --to effect--.

Signed and sealed this 7th day of March 1972.

(SEAL) Abtess:

EDWARD M.FLETCHIR,JR.. ROBERT GOT'I'SCHALK Attosting Oificer' Commissioner oi Patents

Claims

2. The method of claim 1 wherein the frequency of said periodic reduction of hydrostatic pressure is less than the frequency of said acoustic field.
3. A method of cavitationally drilling a borehole into the earth''s crust comprising: positioning an acoustic generator in the vicinity of the borehole bottom; circulating a drilling liquid downward through a drill string in said borehole and upward through the annulus defined by said drill string and the wall of said borehole; periodically reducing the hydrostatic pressure of said drilling liquid in the vicinity of said borehole bottom; and simultaneously operating said acoustic generator to produce an acoustic field in said drilling liquid in the vicinity of said borehole bottom to effect cavitation in said drilling liquid.
4. The method of claim 3 wherein said periodic reduction of hydrostatic pressure is effected by producing a second acoustic field within said drilling liquid in the vicinity of the borehole bottom.
5. The method of claim 4 wherein the frequency of said first-named acoustic field is greater than the frequency of said second acoustic field.
6. The method of claim 5 wherein the ratio of the frequency of said first-named acoustic field to the frequency of said second acoustic field is greater 5. 5.
7. A method of cavitationally drilling a borehole into the earth''s crust comprising: circulating a drilling liquid downward through a drill string in said borehole and upward through the annulus defined by said drill string and the wall of said borehole; alternately supporting and releasing at least a portion of drilling liquid above the vicinity of the bottom of said borehole; and simultaneously producing an acoustic field in said drilling liquid in the vicinity of said borehole bottom of effect cavitation in said drilling liquid.
8. A method of cavitationally drilling a borehole into the earth''s crust comprising: circulating a drilling liquid downward through a drill string in said borehole and upward through the annulus defined by said drill string and the wall of said borehole; periodically reducing the hydrostatic pressure of said drilling liquid in the vicinity of the borehole bottom by generating acoustic energy in said drill string at a point above the vicinity of said borehole bottom and transmitting said energy down said drill string to the vicinity of said borehole bottom thereby producing a first acoustic field within said drilling liquid in the vicinity of said borehole bottom; and simultaneously producing a second acoustic field in said drilling liquid in the vicinity of said borehole bottom to effect cavitation in said drilling liquid.
9. The method of claim 8 wherein said second acoustic field is generated by a radiating face and said acoustic energy generated in said drilL string is transmitted to the radiating face.