Perforating apparatus

April 28, 1964

R. W. BROWN ETAL

3,130,786

PERFORATING APPARATUS

Filed June 3, 1960

Sheets-Sheet 1

<img file="US3130786A_D0001.tif" />

April 28, 1964

3,130,786

R. W. BROWN ETAL

PERFORATING APPARATUS

Filed June 3, 1960

Sheets-Sheet 2

<img file="US3130786A_D0002.tif" />

April 28, 1964

R. W. BROWN ETAL

3,130,786

PERFORATING APPARATUS

Filed June 3, 1960

Sheets-Sheet 3

<img file="US3130786A_D0003.tif" />

FIG. 7

April 28, 1964

3,130,786

R. W. BROWN ETAL

PERFORATING APPARATUS

Filed June 3, 1960

Sheets-Sheet 4

<img file="US3130786A_D0004.tif" />

FIG. 10

3,130,786

Patented Apr. 28, 1964

United States Patent Office

3,130,786 PERFORATING APPARATUS

Robert Wade Brown, Dallas, and Albert H. Roebuck, Fort Worth, Tex., assignors to The Western Company of North America, Fort Worth, Tex., a corporation of ⁵Delaware

Filed June 3, 1960, Ser., No. 33,734

Claims. (Cl. 166—55)

This application is a continuation-in-part of our co- 10 pending application Serial No. 22,603 filed April 15, 1960, now abandoned.

This invention relates to a method and apparatus for producing perforations in solid materials and more particularly to a method and apparatus for creating such 15 perforations in well casings and in earth formations penetrated by a well bore hole.

In many types of wells, such as oil or gas wells, for example, a casing is placed in the well bore hole, and the casing is then perforated in the vicinity of selected produc- 20 ing zones in the surrounding earth formation to thereby condition the well for production operations. In the forming of such perforations, the resulting cavities desirably extend not only through the well casing but also through the surrounding cement and a portion of the 25 adjacent earth formation. As an illustration, it often is advantageous to fracture the earth formation adjacent the producing zones, such as by hydraulic means, for example, in order to increase the permeability of the formation, and the cavities greatly facilitate the fracturing operation. 30 One particularly advantageous method for initiating and producing fractures of this type is disclosed in U.S. patent application Serial No. 700,144, filed December 2, 1957, by Bruce Gilbert, now Patent 3,058,521, granted October 16, 1962, and employs cavities to set up stress concentra- 35 tions in the earth formation and thereby improve the effectiveness of the fracturing operation and make it possible to produce fractures in a preoriented plane.

Various methods and apparatus have been used heretofore in the perforation of a well casing and/or a selected 40 earth formation in a well bore hole. Thus, in some cases, the perforations have been produced mechanically and have also been formed through the use of explosive means, such as a perforating gun. More recently, attempts have beeen made to produce the perforations hy- 45 draulically through the use of pressurized liquids.

The prior methods and apparatus employed to form perforations of this type have exhibited certain disadvantages. For examples, it heretofore has been difficult to control the size and orientation of the perforations with 50 the degree of precision necessary to set up adequate stress concentrations in the adjacent earth formation to thereby facilitate the fracturing operation. In addition, particularly in situations in which explosive charges have been employed, irregular perforations were created or the well 55 casing was otherwise damaged. Furthermore, and this has been of special moment in cases where the perforations were formed by previous hydraulic methods, difficulties were encountered in providing satisfactory penetration of the earth formation and in controlling the extent of such 60 peneration. These latter difficulties heretofore have been of particular concern in relatively deep wells or when it was desired to simultaneously provide a series of perforations in the well casing.

One general object of this invention, therefore, is to 65 provide a novel and economical method and apparatus for producing perforations in solid materials such as well casings, earth formations and the like.

More specifically, it is an object of this invention to provide such a method and apparatus wherein the size 70 and orientation of the perforations are controlled with great accuracy.

Another object of the invention is to provide a process for efficiently and accurately forming cavities in an earth formation adjacent a well bore hole in which the degree of penetration of the cavities is increased and can be accurately controlled.

A further object of the invention is to provide a method for forming a perforation of annular configuration in a well casing, whereby the casing is cut in two.

Still another object of the invention is to provide a new and improved apparatus for producing perforations in a well casing that is economical to manufacture and thoroughly reliable in operation.

In one illustrative embodiment of the invention, a conduit having a laterally projecting nozzle or orifice is lowered into a well casing located in the bore hole of an oil well. Fluid under high pressure and carrying finely divided abrasive particles, such as sand, for example, is forced through the conduit and outwardly through the narrow orifice, to thereby form by abrasion a cavity in the wall of the casing. The flow of the abrasive fluid is then continued to extend the cavity into the adjacent earth formation.

In accordance with one feature of the invention, the abrasive fluid is continuously directed through the orifice at a controlled rate to erode the casing wall and thereby provide an extremely smooth and uniformly dimensioned perforation.

In accordance with another feature of the invention, in certain preferred embodiments, the diameter of the orifice and the ratio of this diameter to the orifice length are within well defined limits, thereby insuring accurate and extremely efficient penetration of the earth formation. In addition, in some embodiments, the conduit is provided with a plurality of orifices which simultaneously form a series of cavities in accordance with a predetermined orientation in the casing and earth formation. As a result, stress concentrations are set up which greatly facilitate the subsequent fracturing operation.

In accordance with a further feature of the invention, in certain embodiments, the ratio between the size of the abrasive particles in the fluid and the orifice diameter falls within a particular range to thereby insure effective peneration of the casing and of the earth formation without blocking or bridging the orifice.

In accordance with still another feature of the invention, in some embodiments, the conduits and its orifice are rotated to thereby form an annular perforation in the well casing and adjacent earth formation.

It is to be understood that the term perforation is used herein as including an aperture, recess, hole, cavity, opening, etc., in a well casing, earth formation or other solid material.

The present invention, as well as further objects and features thereof, will be understood more clearing and fully from the following description of. certain preferred embodiments thereof when read with reference to the accompanying drawings, in which:

FIGURE 1 is a diagrammatic view, partly in section, of a cased well bore hole in an oil bearing stratum which has been conditioned or perforated in accordance with one illustrative embodiment of the invention;

FIGURE 2 is a side elevational view of one form of tool according to the invention which is useful in the conditioning of the well;

FIGURE 3 is a vertical sectional view of the tool shown in FIGURE 2;

FIGURE 4 is a transverse sectional view taken along the lines 4—4 in FIGURE 3;

FIGURE 5 is an enlarged top view of a portion of the tool shown in FIGURE 2;

FIGURE 6 is a diagrammatic horizontal sectional view

3,130,786 &lt;3 of an earth formation including cavities formed through the use of the tool of FIGURE 2;

FIGURE 7 is a side elevational view of another form of tool according to the invention which is useful in the conditioning of a well;

FIGURE 8 is a transverse sectional view taken along the lines 8—8 in FIGURE 7;

FIGURE 9 is a diagrammatic view, partly in section, of a cased well bore hole in an oil bearing stratum which has been conditioned or perforated in accordance with another illustrative embodiment of the invention;

FIGURE 10¹ is an enlarged sectional view taken along the lines 10—10' in FIGURE 9; and

FIGURE 11 is a vertical sectional view of a portion of a well casing and tubing string, together with illustrative means for holding the string in position during the perforating operation.

Referring to FIGURE 1 of the drawings, there is shown a cross section of an underground earth formation 10 penetrated by a well bore hole 11 in a conventional manner. The bore hole 11 is provided with a well casing 12 of cylindrical configuration which is anchored to the formation 10 by means of a cement sheath ‘13. The section of the well illustrated passes through an oil bearing stratum 14 which forms a part of the earth formation 10.

Supported on the ground surface 15 of the earth formation 10 is a suitable mixing tank 16 which includes an inlet opening 17 and an outlet stem 18. The stem 18 connects the tank to the intake side of a pump 20, the outfeed side of which is connected to one end of a hose or line 21 which illustratively may be fabricated from steel, rubber, or other suitable material. The opposite end of the hose 21 is secured by a coupling 22 to the upper end of a pipe or tubing string, represented schematically in FIGURE 1 by an elongated tube 23, which extends into the well bore hole 11. The bottom portion of the tube 23 is threaded and is connected by means of a coupling 24 to the upper end of a hydraulic perforating tool 25.

As best shown in FIGURES 2 through 4, the perforating tool 25 is substantially in the form of a pipe or conduit of cylindrical construction. The upper end of the tool 25 is open and is threaded to accommodate the coupling 24, while the lower end is provided with an annular bottom plate 26 which defines an opening 27 in the bottom of the tool. The inner portion of the plate 26 adjacent the opening is beveled to thereby form a valve seat 28 which accommodates a ball valve 29. Although in the embodiment illustrated in FIGURES 2 through 4, the bottom plate forming seat 28 is integrally formed with the lower portion of tool 25, in other embodiments this tool portion illustratively may be threaded and a suitable sub including the valve seat and accompanying ball valve affixed thereto.

The outer cylindrical surface of the tool 25 is provided intermediate its ends with a first group of threaded apertures 30, 31 and 32 (FIGURE 4) and with a second group of threaded apertures 33, 34 and 35. These apertures each extend radially from the axis of tool 25 in a substantially horizontal plane, and, for purposes that will become more fully apparent hereafter, the apertures in each group are spaced thirty degrees from each other and one hundred and eighty degrees from the corresponding apertures in the opposite group.

Each of the apertures 30, 31, 32, 33, 34 and 35 accommodates a bushing 36. As best shown in FIGURE 5, the bushing 36 includes a threaded cylindrical portion 37 and a hexagonal shoulder portion 38. The threaded portion 37 of each bushing is inserted in its corresponding aperture 3®, 31, 32, 33, 34 or 35, and the shoulder portions 38 of the bushings protrude from the cylindrical side wall of the tool 25 in spaced relationship with the inner cylindrical surface of the well casing 12 (FIGURE 1).

Axially disposed in each of the bushings 36 is a cylindrical aperture 40 which is reduced adjacent its outer end to form a shoulder 41. The aperture 40 accommodates a cylindrical insert 42 which advantageously is fabricated from a relatively hard material such as tungsten carbide, although other suitable materials, such as ceramics or 5 cerametics, like may be employed with good effect. Insert 42 is reduced adjacent the outer end thereof to form a shoulder 43 which abuts the shoulder 41 and thereby prevents the insert from moving outwardly relative to its corresponding bushing 36.

The insert 42 each in bushing is provided with an axial orifice 45. In the illustrated embodiments of the invention, the cross-sections of these orifices advantageously are substantially circular, although in other arrangements the orifice cross-section may be square or may be 15 of other suitable configuration. Each orifice 45 extends in a lateral direction with respect to the tool 25, and, as shown in FIGURE 1, the outer ends of the orifices are positioned in spaced, juxtaposed relationship with the inner surface of the well casing 12 adjacent the oil bearing 20 stratum 14. In the embodiment of the invention illustrated in FIGURES 1 through 6, the axes of the orifices 45 extend in a uniform, substantially horizontal plane, and, for purposes that will become more fully apparent hereafter, the radial orientation of these axes conforms 25 to that described above with respect to the tool apertures 30, 31, 32, 33, 34 and 35.

The material to be supplied to the orifices 45 is prepared in the mixing tank 16 and comprises an abrasive which is suspended in a liquid or other fluid. Although various 30 materials may be employed as the abrasive, sand, tungsten carbide particles, or other material having a granular structure is of particular effectiveness. Because of its economy, sand is preferred. A liquid, such as crude oil, water, etc., advantageously is used as the supporting 35 fluid, although air or other gaseous fluid also may be employed with good effect. In cases where a liquid is used, the abrasive concentration has been found to be particularly effective when it is within the range of from about one-half pound per gallon of liquid to four pounds per gallon of liquid. Below this range, the time required to produce the perforations in accordance with the invention is excessive for many applications. Above this range, plugging of the orifices and interference between individual abrasive particles may occur. Best results are attained within the range of one and three and one-half pounds per gallon, and for reasons of economy concentrations in the neighborhood of one pound per gallon are preferred.

The pump 20 draws the abrasive mixture from the tank ₅₀ 16 and forces it under pressure through the hose 21, the tube 23 and the perforating tool 25. As the pressurized abrasive fluid enters the tool 25, the ball valve 29 is forced downwardly against the valve seat 28 to thereby hermetically seal the opening 27 in the bottom plate 26. 55 The pressurized mixture in tool 25 flows outwardly through the orifices 45 in six fluid streams which are disposed in a single, substantially horizontal plane. The abrasive particles in these streams strike the inner wall of the well casing 12 and erode a series of substantially ₆₀ cylindrical perforations 50 therein. As the flow of the abrasive fluid is continued, the fluid streams strike the cement sheath 13 and the oil bearing stratum 14 to thereby form six elongated cavities 51.

Upon the termination of the perforating operation, ₆5 the downward pressure exerted on the ball valve 29 by the abrasive mixture is removed, thereby permitting unobstructed upward flow of the oil or other produced fluid from the stratum 14 through the tool opening 27 and the tubing 23. In this manner, the necessity for removing γθ the tool from the well during production operations is eliminated.

Under some conditions, the abrasive mixture forced through the orifices 45 is returned to the top of the well through the annular opening between the tubing 23 and 75 the inner cylindrical surface of the well casing. In many

3,130,786 situations, however, particularly where the volume of this annular opening is relatively large, it is advantageous to remove the abrasive mixture from the well through the tool 25 and the tubing 23. This is quickly and easily accomplished by reversing the pumping action of the pump 5 20, thereby removing the ball valve 29 from its seat and drawing the mixture upwardly through the tubing. Similarly, upon the removal of the ball valve, settled sand or other fill may be circulated out of the well in a rapid and straightforward manner. 10

Under other conditions, the abrasive mixture is not returned to the top of the well after the perforating operation but is forced under pressure into the formation. In one particularly advantageous arrangement, hydraulic pressure is applied to the casing 12 and/or the tubing 23 15 to thereby squeeze the mixture into the formation.

For many applications of the invention, the pressure drop of the abrasive mixture across the orifices 45 is maintained uniform and within a specificed range, both during the formation of the perforations 50 and during 20 the formation of the cavities 51. Thus, for certain particularly satisfactory applications, this pressure differential across the orifices preferably is within the range of from about 1,500 pounds per square inch to 4,000 pounds per square inch. By maintaining the pressure differential 25 uniform and within this range, the velocity of the abrasive mixtures through each of the tool orifices is sufficient to insure that effective casing perforations and cavities in the cement sheathing and adjacent earth formation are formed without excess frictional losses resulting, from 30 high velocity flow of the abrasive particles and without causing irregularities in the perforations or damage to the casing. Although the use of pressure differentials below 1,500 pounds per square inch is acceptable in certain situations, some of which will be discussed hereafter, 35 for most applications the formation of the perforations and cavities at these lower pressure requires an excessive amount of time, and the extent of penetration of the surrounding earth formation is limited. The optimum pressure differential across the orifices depends somewhat 40 on the type of casing, the nature and depth of the earth formation, the dimensions and number of orifices in a given tool, etc. For many situations, however, it has been found that exceptional results are experienced when the pressure differential is within the range of from about 45 2,500 pounds per square inch to 3,500 pounds per square inch.

The diameter of each of the orifices in the perforating tool 25 (the distance a in FIGURE 5) advantageously is maintained within the range of from about .125 inch 50 to .25 inch, preferably from .156 to .188 inch. For orifice diameters much below .125 inch, sand or other abrasive particles of 20-40 mesh or greater tend to obstruct the orifice, and the resulting perforations, and cavities in many instances are of a diameter which is too 55 small to permit sufficient entry of the oil or other produced fluid into the well bore hole. Although adequate periorations and cavities have been formed through the use of orifices having a diameter in excess of .25 inch, and this latter situation is acceptable under certain circumstances, CO the pressure reduction occasioned by the friction caused by the use of these larger diameters tends to limit the depth a which the perforating tool is effective and restricts the number of orifices that can be used simultaneously.

Of particular importance in accordance with many applications of the present invention is the ratio between the diameter of each orifice and its length. In the embodiments illustrated in the drawings, the orifice length is determined by the length of the bushing 36 and is rep- γθ resented by the distance b. It has been found that the ratio a/b may vary from about .10 to .67 without deleterious effect, preferably from about .20 to .50, but that the use of diameter to length ratios substantially outside the broader range impairs the quality of the perforations 75 ⁶ in the well casing and the cavities in the sheathing and surrounding earth formation.

Additionally, in order to insure that the abrasive particles in the fluid do not block or bridge the orifices in the perforating tool and thereby impede -the flow of the mixture therethrough, the particle diameter preferably is. less than one-third the diameter of each orifice. For optimum results, particles having a size of from 20 to 60 mesh -are particularly effective, although even larger particles, such as 10 mesh for example, may be used for some applications without deleterious effect, provided the orifice diameter is of sufficient size. Particularly good results have been obtained using 20-40 mesh Ottawa sand.

For many applications of the present invention, it has been found desirable to add an acid, either organic or inorganic, a chelating agent, or surface active agent, to the abrasive fluid in order to increase the degree of penetration of the well casing, and surrounding earth formation. The use of such an acid, chelating agent, or surface active agent, is particularly advantageous in deep wells and in other situations where high frictional losses limit the pressure differential that can be realized across the orifices, as in cases where the tubing diameter is relatively small. Thus, in some situations, effective penetration is obtained with a pressure differential across the orifices as low as 300 pounds per square inch. The following acids in the concentrations indicated have exhibited particular effectiveness under these conditions:

Hydrochloric acid __ Greater than 0.5 percent.

Gluconic acid_____Greater than 10 parts per million.

Citric acid_________Greater than 10 parts per million.

Tartaric acid _______About 50^! parts per million.

Of course, other -acids may be employed, such as the mineral acids, including sulfuric, nitric, phosphoric, hydrobromic acids, other organic acids, including acetic, propionic, sulfamic acids, etc.

As an illustration of the effectiveness of the addition of hydrochloric acid to the abrasive fluid, one sand and oil mixture including two percent hydrochloric acid was directed through an orifice against a well casing for one minute and created a penetration which measured 500 mils. In a test under similar conditions but without the acid, the resulting penetration by the sand and oil mixture measured 37 mils. As another illustration, an emulsion of acid and oil, containing sand, likewise, was directed through an orifice against the well casing for one minute. The acid consisted of a 10 percent solution of hydrochloric acid, and the oil phase of the emulsion comprised 70 percent by volume. The resulting penetration measured 275 mils.

Of course, the foregoing -acids, and concentrations are but illustrative, and other relatively strong mineral acids, such as sulfuric acid, for example, or other acids such as sulfamic acid, or other sequestering or chelating agents, in addition to gluconic, citric and tartaric acids, such as ethylenediamine tetraacetic acid, propylenediamine tetraacetic -acid, nitrilotriacetic acid (triglycin), diaminocyclohexane tetraacetic -acid, etc., may be employed without departing from the spirit and scope of the invention. Also, other surface active agents, preferably hydrocarbon soluble, such as octadecyl carboxylic acid, dodecylbenzene sulfonic acid, polyethylene glycol oleate, polyethylene glycol ricinoleate, sorbitan, mono-laurate, polyoxyethylene lauryl alcohol, and many others, may be employed.

The acid, chelating agent or surface active agent additive may be employed in the abrasive fluid in -the form of a solution in either water, if water is employed as the suspending fluid, or -in -oil or a hydrocarbon liquid, if these liquids are employed -as the suspending fluid for the abrasive. The acid, chelating agent or surface active agent additive may also be employed in the form of a water and oil emulsion with the additive being in either the external or internal phases, preferably the external

3,130,788 phase. In the case of an emulsion, it may be desirable to employ an emulsifying agent to produce the oil-in-water or water-in-oil emulsion.

As indicated heretofore, in the embodiment of the invention illustrated in FIGURES 1 through 6 of the draw- 5 ings, the perforating tool 25 is provided with a plurality of the orifices 45 which advantageously are laterally disposed in a uniform, substantially flat plane. The orifices extend radially from the tool 25 and are arranged in two, oppositely disposed, groups. In FIGURE 4 the axes χθ of the orifices in each group are radially spaced approximately thirty degrees apart and are spaced one hundred and eighty degrees from the corresponding orifice axes in the opposite group, -although it will be understood that this radial spacing may be varied somewhat without de- jg parting from the purpose or scope of the invention. With this arrangement, the casing cavities 5® and the perforations 51 in the oil producing stratum 14 likewise extend in a single plane -and are disposed in two groups which extend in opposite directions. 20

In -order to set up -adequate stress concentrations in stratum 14 to- facilitate subsequent fracturing operations, it has been found, for practical purposes in many types of earth -formations, that the ratio between the diameter of the perforations 51 in each group and the minimum 25 thickness of the web of earth formation between these perforations may -vary from about 0.25 up to, but not including, -infinity, such as 100, 1000, etc. That is, the ratio of the distance x divided by the distance y in FIGURE 6 advantageously -is not less than 0.25 and of course 39 cannot equal infinity since the perforations would then be in direct contact and there -would be no web of earth formation therebetween. In other words, the spacing between the perforations in each group at their closest point may be -from four diameters apart to almost nothing. It 35 has been ascertained that when the arrangement of the tool orifices 45 is such that the spacing between the perforations in a particular group is greater than four diameters, adequate contol over the orientation of a fracture initiated in the vicinity of the perforations is lost. 40 The optimum -ratio x/y will depend somewhat upon the nature of the earth formation to be -fractured but preferably is in the range from about 0.75 to 1.00. The stress concentrations set up by the perforations 51 arranged in this manner greatly facilitate the fracturing of the earth ₄formation, permit fracturing in a preoriented plane, -and improve the production capabilities o-f the well.

In the embodiment of the invention illustrated in FIGURES 1 through 6, the plane of the axes of the orifices 45, and hence -the plane of the stress concentrations in the _goadjacent earth formation, is substantially horizontal. In other good embodiments, the orientation at the orifice axes may be such -that the stress concentrations are disposed in planes which extend -at an angle with respect to the horizontal, for purposes which -are more fully de- 55 scribed in Gilbert -application Serial No. 700,144, referred to above.

In accordance with many embodiments of the invention, a single orifice is employed in the perforating tool or the tool is provided with a plurality of orifices which ₆₀are positioned one above the other along the tool surface. One particularly advantageous embodiment in accordance with this latter arrangement is shown in FIGURES 7 and 8 and comprises a perforating tool 55 which is threaded at its upper end and is adapted to be affixed ₆₅to the coupling 24 (FIGURE 1) in a manner similar to the perforating tool 25. The lower end of the tool 55 is closed by an integrally formed circular plate 56. The tool 55 includes three threaded apertures 57, 58 and 59, and each of these apertures accommodates a bushing ₇θ 36, which, as indicated heretofore, includes the insert 42 and the laterally extending orifice 45. The bushing apertures 57, 58 and 59 are equally spaced along the length of the tool 55, and, as best shown in FIGURE 8, also are radially spaced one hundred and twenty degrees apart 75 around the tool. Upon the application of fluid pressure to the tool 55 in a manner similar to that described heretofore in connection with the tool 25, the abrasive mixture flowing through the orifices 45 erodes three casing perforations and corresponding cavities in the adjacent earth formation at three different levels.

The formation of cavities and perforations through the use of the tool 55 is particularly effective in situations where the permeability of the earth formation is such that little additional advantageous effect can be realized by conditioning the well for subsequent fracturing operations. In accordance with certain embodiments, a perforating tool having bushings and orifices arranged in accordance with the pattern of FIGURES 7 and 8 may be extended to include a large number of these bushings and orifices which are adapted to form a corresponding number of simultaneous perforations at different levels in the casing and surrounding earth formation. As an illustration, a perforating tool employing nine such bushings and orifices longitudinally spaced at one foot intervals and radially spaced 120° apart was successfully employed in the perforation of a well. In other tools constructed in accordance with these latter embodiments, the radial spacing between the bushings may be greater or less than 120°, depending upon the number of bushings employed and the tool dimensions.

The degree of penetration into the oil producing earth formation may be controlled to some extent by varying the amount of time during which the pump 2® is effective to force the abrasive mixture into the tubing and through the orifices 45. In one illustrative embodiment employing a perforating tool similar to the tool 55, maximum effective penetration of the earth formation occurred after approximately fifteen to thirty minutes of pumping time. In another arrangement using a tool similar to the tool 25 of FIGURES 1-5, adequate penetration occurred after three minutes of pumping time. Of course, the optimum pumping time to- effect a given degree of penetration will depend in large measure upon such factors as the nature and type of the formation, the pressure differential employed, the particular composition of the abrasive liquid, the configuration of the orifices, etc.

In order to insure the formation of accurate casing perforations and cavities in the earth formation in accordance with many embodiments of the invention, the lateral separation between the outer face of each of the bushings 36 and the inner cylindrical surface of the well casing preferably should not be less than .375 inch, and best results are obtained when this lateral separation is within the range of from about .438 inch to .750 inch. For lateral separations much below .375 inch, the deleterious effects of back splash frequently cause damage to the perforating tool and its bushings. For lateral separations greater than six-ten times the orifice diameter, losses in jet momentum tend to reduce the efficiency of the perforating operation.

Referring now to FIGURES 9 and 10 of the drawings, there is shown a perforating tool 65 which is positioned in a well bore hole 66 having the usual casing 67 and surrounding cement sheath 68. The section of the hole shown passes through an oil bearing stratum 69 forming a part of an earth formation 7®.

The tool 65 is connected to the lower end of a tubing string 71, as by a coupling 72. The opposite, upper end of string 71 is disposed above the ground surface 73 of the earth formation 70 and is rotatably secured to an elbow swivel 74. A hose or line 75 is connected between the swivel 74 and the outfeed side of a pump 76, the swivel 74 providing communication for the flow of fluids from the line 75 to the tubing string 71 and the tool 65. The intake side of pump 76 is connected to a mixing tank 77 in a manner similar to that described heretofore in connection with the pump 2® and tank 16 of FIGURE 1.

3,130,786

The perforating tool 65 is substantially in the form of an elongated, vertically disposed cylinder which is closed at its lower end, the upper end thereof being in open communication with the tubing string 71. As best shown in FIGURE 10, the tool 65 includes three threaded apertures 80, 81 and 82 which are disposed in a horizontal plane and are equally spaced around the tool at 120° intervals. Each of the apertures 89, 81 and 82 is provided with one of the orifice bushings 36, and the orifices 45 in these bushings extend outwardly in spaced relationship with the inner cylindrical surface of the casing 67.

Upon the lowering of the tool 65 into the bore hole 66 to a depth adjacent that of stratum 69, the pump 76 is actuated to force an abrasive mixture from the tank 77, through the line 75, the swivel 74, the tubing string 71, the perforating tool 65 and outwardly through the three orifices 45 therein in a manner similar to that described heretofore. The portion of the tubing string 71 immediately beneath the swivel 74 and above the ground surface 73 is then grasped by a pair of tongs 85 and is rotated to thereby rotate the tool 65 and its orifice bushings 36. As the tool revolves, the abrasive mixture flowing through the orifices 45 erodes an annular perforation 86 in the well casing 67, the cement sheath 68 and the surrounding stratum 69, thereby cutting the casing in two and providing a smooth, horizontally disposed cavity in the stratum 69. As a result, stress concentrations are set up in stratum 69 which greatly facilitate subsequent fracturing operations and enhance the production capabilities of the well. . . .,,

In certain embodiments of the invention, particularly in cases where it is desired to provide an annular perforation such as the perforation 86 in the well casing and adjacent each formation, the perforating tool advantageously is held in position along the axial center of the well bore hole during the time the abrasive mixture flows outwardly through the orifices 45. To accomplish this, there is provided a centralizer 90 (FIGURE 9) which is mounted on the tubing string 71 immediately above the coupling 72. The centralizer 90 includes two sleeves 91 and 92 which are slidably disposed in spaced-apart relationship with each other on the string 71. Rigidly affixed to the outer cylindrical surface of the lowermost sleeve 91 are the lower ends of four bowed spring members 93, and these members extend upwardly and are secured at their opposite ends to the uppermost sleeve 92. The spring members 93 are equally spaced around sleeves 91 and 92 and are prestressed in a manner such that they tend to draw the sleeves together. In their position in the well casing 67, the longitudinal midpoint of each spring member bears against the adjacent portion of the inner casing wall.

As the tubing string 71 and the tool 65 are rotated by the tongs 85, the spring members 93 remain fixed and press against the adjacent wall of the casing 67 to thereby resist sideways movement of the string and the tool away from the axial center of the well bore hole 66.

In an alternative arrangement, the sleeves 91 and 92 are fixedly secured to the rotating tubing string 71, with the result that the spring members 93 rotate with the string.

FIGURE 11 is illustrative of an alternative centralizer 95 useful in connection with the invention. The centralizer 95 is slidably disposed on a tubing string 96 which is substantially similar to the strings 23 and 71 discussed heretofore but is provided with a plurality of longitudinally extending slots 97 adjacent the lower end thereof. The string 96 is disposed in a well casing 98 and is connected to the perforating tool 65, for example, by means of a coupling 99.

Centralizer 95 rests on the coupling 99 and is substantially in the form of an elongated hollow cylinder which surrounds the slots 97 on the tubing string 96 and is of a diameter slightly less than that of the well casing 98. The ends of the centralizer 95 taper inwardly and engage the string 96 in fluid-tight relationship therewith. Bearings 100 are disposed at each end of the centralizer 95 to permit the tubing string to rotate freely therein.

A plurality of apertures 101 are disposed in the cylindrical surface of the centralizer 95 and are arranged in a series of vertically disposed rows extending therearound, four of these rows being visible in FIGURE 11. Each of the apertures 101 accommodates a laterally extending member 102 which is adapted for reciprocal sliding movement therein and protrudes outwardly toward the adjacent portion of the inner wall of the well casing.

As the abrasive mixture is pumped down the tubing string 96 and out through the orifices in the tool 65, the mixture passes through the slots 97 and into- the centralizer 95. The resulting pressure build-up in the centralizer urges each of the members 102 outwardly and into frictional engagement with the inner casing wall, thereby rigidly holding the centralizer in position. As a result, the perforating tool 95 is firmly maintained along the axial center of the well bore hole during the time the abrasive mixture is effective to penetrate the casing 98.

In some good embodiments, the centralizer 95 is maintained in fixed relationship with the tubing string 96. With this arrangement, the perforating tool is prevented from moving upwardly or downwardly with respect to the well casing as well as from moving from side to side.

In order more clearly to disclose the nature of the present invention, the following examples illustrating the invention are disclosed. It should be understood, however, that this is done solely by way of example and is intended neither to delineate the scope of the invention nor limit the ambit of the appended claims.

Example I

As an example of the effectiveness of the method and apparatus of the present invention in the conditioning of an oil well located in the Cleveland formation in Oklahoma, which prior to treatment in accordance with the invention produced a negligible quantity of oil, the well was treated as follows:

A perforating tool of the type shown in FIGURES 1 through 5 of the drawings was lowered into the well on a 2 inch diameter tubing string to a depth of 6,408 feet. The tool was provided with six orifices which were arranged as shown in FIGURE 4 and which each had a diameter of .172 inch and a length of .469 inch. The well included a 5½ inch diameter casing, and the spacing between the inner surface of this casing and the outer face of each of the orifice bushings was approximately one-half inch. A mixture of 20-40 mesh Ottawa sand and water in the ratio of 2 pounds of sand per gallon of water was pumped down the tubing and out through the six orifices at a rate such that the pressure differential across the orifices was 2,300 pounds per square inch. The water used was gelled by the addition of 75 pounds of Guar Gum per 1,000 gallons of water. After 3 minutes of pumping time, the tool was raised to a depth of 6,400 feet, and the mixture was again pumped into the tubing and outwardly through the orifices for 3 minutes at a pressure differential across the orifices of 1,600 pounds per square inch. The pumping action was then reversed to remove the ball valve from its seat and to circulate settled sand and other fill out of the well. Thereafter, using the same equipment, 500 gallons of crude oil were forced down the tubing and through the perforating tool at a surface pressure of 2,800 pounds per square inch to thereby fracture the earth formation in the plane of the perforations. Upon the completion of of the above treatment, the well produced oil at a rate of 40 barrels per day.

Example II

As another example of the effectiveness of the method and apparatus of the invention, a cased well in Carson County, Texas, which, prior to treatment in accordance

3,130,786 with the present invention, produced 3 barrels of oil per day, was treated through the use of a perforating tool of the type shown in FIGURES 7 and 8 of the drawings. The tool was two feet long and included four orifice bushings which were longitudinally spaced one-half foot apart with a radial separation of 90 degrees. The diameter of each orifice was .172 inch and the length .469 inch. The tool was lowered into the well casing on a 2 inch diameter tubing string to a depth of 3195 feet. The casing had a diameter of 7 inches, and the lateral separation between the outer face of each of the orifice bushings and the inner casing surface was about onehalf inch. A mixture of 20-40 mesh Ottawa sand and crude oil in the ratio of two pounds of sand per gallon of oil was pumped down the tubing and out through the four orifices for three minutes. The pressure differential across the orifices was 2,925 pounds per square inch. Additional three minute cuts were then made in a similar manner at depths of 3191 feet and 3171 feet with pressure differentials across the orifices of 2,750 pounds per square inch. Upon the termination of the third cut, the well began producing oil at a rate of 26 barrels per day.

Example HI

In either of the foregoing examples, when approximately 0.5% by weight of hydrochloric acid is employed in the abrasive fluid, it is possible to obtain greater penetration of the earth formation during the same period of time.

As will be apparent to those skilled in the art, other abrasives and suspending fluids may be employed in the foregoing examples. Also, acid, chelating agent or surface active agent additives, such as those disclosed hereinabove, may be used in the abrasive fluid employed in any of the foregoing examples. The additive may be dissolved in the aqueous, oily or hydrocarbon abrasive fluid or it may be present in the form of an emulsion. An example of a suitable emulsion is a water-in-oil emulsion prepared by dissolving 5 pounds of hydrochloric acid and .25 pound of an emulsifying agent in 250 pounds of water and adding to it 750 pounds of crude oil. After thoroughly blending the resulting emulsion, there was added 2% by weight of 20-40 mesh Ottawa sand.

As indicated heretofore, each of the inserts 42 in the bushings 36 is fabricated from a relatively hard tool steel or similar material. As a result, erosion of the insert orifices 45 during the perforating operation is maintained at a minimum. To reduce the adverse effects of back splash, the outer surfaces of the bushings 36 and the adjacent portions of the perforating tool advantageously are coated with an abrasive-resistant material, as illustrated by the coating 68 shown in FIGURES 2 and 5, or by the coating 61 in FIGURES 7 and 8. These coatings may be applied in any suitable manner, such as by a flame spray, hot dip process, etc. Certain illustrative coating materials that may be employed include aluminized coatings, lead coatings, carbides, plastics, laminated rubbers and the like.

Although the present invention has been shown and described as having particular utility in the formation of perforations in a well casing, it will be apparent to those skilled in the art that the invention also may be employed advantageously for other applications where the formation of perforations or cavities in solid material is either necessary or desirable. Thus, for example, in wells which are not of the casing type, the invention may be used effectively to provide elongated cavities directly in the earth formation surrounding the well bore and thereby enhance the production capacities of the well. Also, treatment of other earth formations, such as digging holes under highways for underground pipes, washing out earth in mining operations, etc., may be effectively accomplished in accordance with the invention.

The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions, of excluding any equivalents of the features shown and described, or portions thereof, it being recognized that various modifications are possible within the 5 scope of the invention claimed.