US3101051A

US3101051A - Apparatus for initiating fractures in earth formations

Info

Publication number: US3101051A
Application number: US675424A
Authority: US
Inventors: Gilbert Bruce
Original assignee: Western Company of North America
Current assignee: Western Company of North America
Priority date: 1957-07-31
Filing date: 1957-07-31
Publication date: 1963-08-20
Anticipated expiration: 1980-08-20

Description

Aug. 20, 1963 B. GILBERT 3,101,051

APPARATUS FOR INITIATING FRACTURES IN EARTH FORMATIONS Filed July 31, 1957 I 4 Sheets-Sheet 1 m 01 4 Q1 N Aug. 20, 1963 B. GILBERT 3,10 ,05

APPARATUS FOR INITIATING FRACTURES IN EARTH FORMATIONS Filed July 31, 1957 4 Sheets-Sheet 2 I .1 llllll 42 ml l l lllllffl FIG.3

B. GILBERT 3,101,051

APPARATUS FOR INITIATING FRACTURES IN EARTH FORMATIONS 4 Sheets-$heet 3 Aug. 20, 1963 Filed July 51, 1957 Aug. 20, 1963 r B. GILBERT 3,101,051

APPARATUS FOR INITIATING FRACTURES IN EARTH FORMATIONS Filed July 51, 1957 4 Sheets-Sheet 4 United States Patent 3,101,051 APPARATUS FOR INITlATlNG FRACTURES EN EARTH FURMATIONS Bruce Gilbert, Dallas, Tex., assignor to The Western Company of North America, a corporation of Delaware Filed July 31, 1957, Ser. No. 675,424 6 Claims. (Cl. fill-2d) This invention relates broadly to apparatus for initiating or producing fractures in earth formations and particularly to apparatus for creating fractures having a predetermined orientation in such formations. Still more particularly, the invention relates to apparatus for initiating fractures at a predetermined depth and in la redetermined orientation in an earth formation penetrated by a well bore hole.

It is frequently desired to create and extend fractures in earth formations to facilitate mining or quarrying operations or the recovery of underground fluids such as petroleum, natural gas or water. The purpose of such fractures in mining and quarrying operations is generally to separate masses of rock or mineral from large formations. In petroleum, gas or water recovery operations, the ultimate purpose is similar, ire. to produce a fracture or separation in a rock or mineral formation, but the techniques employed are somewhat different. In petroleum production, for example, it is common practice after drilling a well bore hole into an oil bearing earth formation to attempt to increase the permeability of the formation by creating therein artificial fractures which facilitate the flow of petroleum from the formation into the well. For many years such fractures were produced by detonating massive charges of nitroglycerine in the Well adjacent to the desired formation. This method has been superseded in recent years by hydraulic fracturing techniques, such as those described in US. Reissue No. 23,733. In hydraulic fracturing methods a viscous liquid, either hydrocarbon or water base, is injected into the case well under sufficient hydraulic pressure to cause the exposed formation at the bottom of the well to break down thus creating and extending fractures which increases the permeability of the oil bearing stratum. Send or other granular material is commonly incorporated in the fracturing fluid as a propping agent to prevent closore of the newly created drainage channels by the pressure of the overburden of earth. The fluids used in hydraulic fracturing are generally viscous liquids containing chemical agents which cause the viscosity of the. liquid to decrease after a period of time so that the fracturing fluids can be flushed from the newly created fractures either by fluid produced from the formation or by fluid injected into the formation for that purpose. This fracturing procedure generally causes a significant increase in the production of the treated well.

A hydraulic fracturing operation ordinarily produces only a single fracture since it is usually impossible to pump sufficient fracturing fluid to raise the treatment pressure enough to create a second fracture while the initial fracture is making fluid. For this reason techniques have been developed for isolating and treating short sections of a well bore individually in order to produce fractures at different levels in the well. straddle packers, for example, have come into common use for this purpose making it possible for the operator to control the location at which an artificial fracture is initiated in an underground earth formation. It has long been recognized, however, that it would be highly desirable not only to control the 4 hydraulic fracturing operation so as to produce a single fracture at the exact depth in the well at which oil has been determined to be present but, in addition, to orient the plane of the fracture so that it falls entirely within the producing formation. Positive control of both the depth and orientation at which an artificial fracture is created would of course make it possible to create and extend only those fractures which would be most likely to provide the greatest increase in the useful production of a well. The utility of a fracture initiation apparatus providing such positive control is apparent. For example, a more or less horizontal petroleum producing stratum is often located close to a similarly disposed Water producing stratum. When a well drilled through formations of this type is fractured by conventional methods, it frequently happens that the randomly created fracture is oriented in a more or less vertical plane cutting both the petroleum and water producing strata and consequently flooding the well with water. It is obvious that if it were possible to create a [generally horizontal fracture at the desired depth in such a well adjacent to the petroleum bearing stratum there would be no communication with the adjacent water bearing stratum and con sequently the hydraulic fracturing treatment would produce a valuable oil well rather than a worthless water well.

The apparatus of the present invention provides the desired positive control of the plane of orientation of a fracture created in an earth formation. This apparatus may be employed according to the method disclosed in my copending application for United States Letters Patent, Serial No. 700,144, filed December 2, 1957, now Patent No. 3,05 8,521 entitled, Novel Method of Initiating Fractures in Earth Formations.

It is an object of the present invention to provide novel apparatus for creating a fracture having a preetermined orientation in an exposed earth formation such as the wall of a quarry or underground tunnel.

It is a particular object of the invention to provide apparatus for treating an exposed earth formation in a well bore hole to establish the plane of any fracture created or extended by a subsequent hydraulic fracturing operation.

It is a particular object of the present invention to to provide novel apparatus for initiating a fracture in an earth formation penetrated by a well bore hole at a predetermine depth in the earth and in a predetermined orientation with respect to the bore hole.

It is a further particular object of the invention to provide an apparatus for the creation or initiation of a generally horizontal fracture in an earth formation, i.e., a fracture in a plane more or less parallel with the horizon, or in different planes, such as one about 45 from a plane parallel with the horizon.

It is another object of the invention to provide an apparatus containing a plurality of shaped explosive jet charges which, when simultaneously detonated, produce a plurality of elongated cavities critically spaced in a row lying in a predetermined plane in an adjacent earth formation.

It is an additional object of the invention to provide a fracture initiation apparatus which i ru ggedly constructed to withstand use in the field by semiskilled Workers and which is capable of being run down a well casing to a predetermined depth at high speeds to the time required-to treat a well.

It is a further object of the invention to provide an inexpensive, easily assembled, expendable fracture initiation apparatus.

These and other objects of the invention are attained by providing a fracture initiation apparatus containing a plurality of shaped explosive jet charges arranged in at least one straight row of at least two jet charges and means for detonating the charges simultaneously; the straight row or rows of shaped charges being permanently or adj-usta-bly oriented in a predetermined plane. Apparatus according to the invention for the creation of fractures in earth formations penetrated by a well bore hole is designed and constructed to be lowered into ordinary well casing at high speeds. Individual shaped explosive jet charges of the novel apparatus are designed to penetrate the charge container, Well casing, surrounding cement sheath and the adjacent earth formation to produce an elongated generally cylindrical or conical cavity in the latter; The plurality of' shaped explosive charges in the fracture initiation apparatus are spaced apart or shielded to prevent intercharge interference. when the charges are detonated simultaneously. The jet charges are arranged to pro-duce a plurality of cavities in a single plane in the adjacent earth formation in a spatial arrangement which establishes the orientation of the plane of the resulting fracture. 'Fract-ures created by the apparatus of the present invention may be extended by conventional hydraulic fracturing techniques.

I Additional objects and advantages of the invention.

will be apparent from the following detailed description. of typical embodiments thereof taken in conjunction with the accompanying drawings in which:

. FIG. 1; is a diagrammatic sectional view of an earth formation penetrated by a cased well bore hole containing a fracture initiation apparatus of the present invention suspended on an electrically conductive cable;

. FIG. 2 is an exploded perspective view partially in section of the fracture initiation apparatus of FIG. 1;

FIG. 3 is a side elevational view of a shaped charge container which forms, a part of the apparatus of FIG. 1;

FIG. 4 is a cross sectional view partially broken awayof the charge container of FIG. 3 taken along the line 4-4 showing six shaped charges therein;

5 is an elevational view in section of a second embodiment of the fracture initiation apparatus of the present invention;

FIG. 6 is a cross sectional view of a shaped charge container which forms a, part of the fracture initiation ppa atus. ofFIG. 5;

FIG. 7 is a cross sectional view of the apparatus of FIG. 5 taken along the line -'l--7; and p FIG. 8 is an elevationalview in section of the apparatus of FIG. 5 showing additional details.

FIG. 1 shows a typical well which be an oil, gas or water injection or production well in actual practice although described herein as. an oil production Well for purposes of illustration. An earth formation 11 is shown penetrated by a well bore hole 12 containing well casing 13 anchored-at the. bottom of the well by means of a sheath ofcement 14. A conventional well head 15 is attached to the upper end of the casing 13 which extends above ground level 1 6. A typical fracture initiation apparatus 17 of the present invention is shown lowered into the well to a predetermined depth adjacent to an oil hearingstratum 18 of earth formation 11. Fracture initiation apparatus 17 is connected to the lower end of a heavy metal sinker bar .19 attached to cable head 2th of conductor cable. 21. The conductor cable 21 is suitably a steel wire cable, containing insulated electrically conductive core, of the type in common use for lowering well servicing equipment into oil wells and for transmitting electrical impulses tosuch equipment. The electrical conductor incable 21.1is electrically connected to the fracture initiation apparatus. 17 through cable head 2%) and a conductor passing through sinker bar 19. When it is desired to produce a fracture in the oil bearing stratum 18, an electrical impulsefrom a source on the surface is passed down the conductor cable -21 to detonate fracture initiation apparatus 17 causing penetration of the well casing 13, cement sheath 14 and the surrounding earth formation to produce therein a fracture having a predetermined orientation within the plane of the oil bearing stratum as will be explained in detail below. The fracture thus initiated 4 can then be extended by known hydraulic fracturing techni ues.

FIG. 2 is an exploded view of fracture initiation apparatus 17 of FIG. 1 showing its component parts and their relationship to the apparatus as a whole. A detouator housing 22 is threaded externally at each end; threads 23 serving to engage the sinker bar 19 as shown in FIG. 1 and threads 24 serving to engage a one-to-three adapter 25. The detonator housing 22 is bored axially throughout its length to provide a detonator chamber 26 and a booster charge chamber 27. A detonator 28 which may suitably be an electrical blasting cap, provided with positive and negative

electrical leads

29 and 30 respectively, is shown in detonator chamber 26. A plurality of booster charges 31; which may suitably be lightly compressed RDX explosive, are shown in chamber 27; each booster charge being crimped to one of a plurality of equal lengths of explosive fuse 32 which may suitably be Primacord fuse containing PETN (pentaerythrityl tetranitrate) or RDX (cyclotrirnethylenetrinitramine). explosive. Adapter 25 contains an axially bored well 33 threaded internally to engage threads 24 of detonator housing 22. Well 33' communicates with a plurality of fuse passages 34 running through-the adapter papalllel to its axis and tel minating in ports 35-. A portion of each of the fuse passages 34 adjacent to ports 35 is threaded internally to receive and engage an externally threaded nipple 36 of a high pressure fitting. A plurality of fuse tubes 37 fitted with compression fittings and threaded at each end are; connected to adapter 25, one fuse tube being associated with each fuse passage 34, by seating the end :of the tube against the end of an externally threaded nipple 36 screwed into port 35 and tightening a nut 38 over threads onto the nipple to form a seal. :Fuse tubes 37- each contain a pair of olfsets 39 to provide passages of equal length for the equal lengths of fuse 32. A cylindrical jet charge container 40 is provided with a plurality of fuse passages.

(mown inFIG. 3) originating from ports. 41 the centers of which ports are spaced along a diameter of the upper flat surface of the charge container. A portion of each of the fuse passages adjacent to the ports 41 is threaded to engage nipples 36' which are connected -to the ends of fuse tubes 37 by means of nuts 38 over compression fittings in the same manner as the fuse tubes are connected to the adapter 25. The main charge container 40 is PTO!- vided with two circumferential grooves 42; one groove, being adjacent and parallel to, but slightly spaced from, each end of the cylindrical container. The grooves 42 in container 40 are provided with O ring gaskets 43.,

A hollow cylindrical sleeve 44 surrounds the; container 40 between the gaskets 43 A curved \guide 45 is press fitted over each end of the container 40* against an O ring gasloet 43 and the edge of the s] ve 44. The plurality (three in this case) of equal lengths of explosive fuse 32 crimped to the booster charges. 31 emerge from booster charge chamber 27, enter well 33 of adapter 25 a group and then pass individually through fuse passages 34,

fuse tubes 37 and ports 41- into the main charge container 40 wherein they serve to detonate charges of the fracture initiation apparatus. Inasmuch as the explosive fuse 32 employed in the apparatusof the present invention .detonatesat a constant rateperunit of length, the use of equal lengths of fuse initiated by a common detonator 28 insures simultaneous detonation of the jet charges in container 40. Mechanical joints ofvarious types can be substituted for those shown and dew. scribed in the apparatus of FIG. 2. It is essentialhowever, that all joints be liquid tight if the apparatus is to.

come in contact with liquids which would desensitize or otherwise interfere with the proper functioning of the explosive charges or fuses.

FIG. 3 is a side elevational viewof the. shaped charge container 40 of the apparatus illustrated in- FIG. 2. Circumferential grooves 42 are shown minus 0 ring gas lcets 43*. The ports 41 shown in FIG. 2 open into inter the main explosive) nally threaded sockets 46 which are adapted to receive the nipples 36' of FIG. 2. Fuse passages 47 communicate with sockets 46. Six shaped charge chambers 48, of which three are visible in FIG. 3, are provided in change container 40. A portion of the metallic web between adfaced to fire in one direction while those in the other row are faced to fire in the opposite direction. One of the three fuse passages 47 is disposed between the bases of each pair of opposed shaped changes 50 so that a single fuse can detonate two charges. While the shaped explosive charges are shown in a fixed position, they can be installed with means for making them adjustable, as will be apparent to those skilled in the art.

The individual jet charges 50 are composed of a metal cone 51, a shaped mass of explosive 52. packed around the exterior surface of the cone in the form of a cylinder having a convex base, a concave heavy metal base 53 containing a centrally located ignition opening 54, an explosive booster charge 55 positioned in opening 54, a metal foil booster charge wrapper 56 surrounding the booster charge on all sides except that in contact with the main charge 52, and a metal foil shaped charge wrapper 57 encasing the entire shaped charge 50. The shaped charges 50 shown in the apparatus of FIG. 4 are similar to the jet charges currently employed in the perforation of well casing although modified in several important respects due to the space limitations imposed upon the fracture initiation apparatus by the internal dimensions of conventional well casing. In apparatus of the present invention for use in cased well bore holes it is desirable to employ short-coupled shaped charges, ize., those in which the diameter of the metal cone 51 or face of the charge is maximized while the overall length of the charge is minimized. The explosive mass of conventional jet charges is more or less conical in shape whereas in the short'coupled shaped changes shown in FIG. 4 the point of the cone of explosive has been eliminated resulting in a blunt based charge which would not function properly if it were not for the addition of the dense metal base 53. Such changes must be initiated at the concentrically located booster charge in opening 54 in order to function properly. The dense metal base also serves to protect the explosive charge from premature initiation by the explosive fuse which lies across the base of the charge.

A preferred embodiment of the fracture initiation apparatus illustrated in FIGS. 1, 2, 3 and 4, which is designed to be lowered into /2" outside diameter well casing at high speeds, has a diameter of 4% at the sleeve 4-4. Preferred short coupled shaped charges for use in this apparatus have a Zinc base 53 and

aluminum foil wrappers

56 and 57 and are 1" in diameter and about 1%" long. The charges are spaced 1.155".apart between centers in the arrangement shown inFIG. 4.

Thus, with the shaped charges being 1 inch in diameter and spaced 1.155 inches apart between centers, the space between the charges is 0.155 inch and the ratio of charge diameter to distance between the charges is 6.45. The air chambers in such an apparatus are 0.155" wide and 0.375 deep and extend inward from the peripheryof the charge container to the point where the explosive mass of the adjacent shaped charge first contacts the heavy metal base of the shaped charge. It has been found that the removal of sufficient metal web to form air chambers of these dimensions between adjacent charges causes the jets from the outside shaped charges in a row of three to converge at an angle of about 12 when adjacent charges in the apparatusare arranged with their axes parallel.

When a row of three 1" diameter shaped charges spaced 1.155" between centers is fired it produces a row of cavities in the target material having a ratio of cavity diameter to web thickness between cavities varying from about 0.25 to 1.0. This is within the operable range of ratios of cavity diameter to web thickness Which is necessary to achieve controlled fracture initiation in the majority of earth formations with apparatus of the present invention. An operable range of ratios of cavity diameter to web thickness produced by the apparatus of the invention comprises at least 0.25 up to, but not including, infinity, such as 10 00 or 100. The desired ratio will depend somewhat upon the nature of the target material. A ratio of infinity would produce a continuous opening and at this value no web material would remain to separate the holes in which stress would be produced. At a value of less than 0.25 the stress concentration produced in the web is infinitesimal and the value of the perforation is'largely lost. The preferred ratio is between 0.75 and 1.0.

The jet charges employed in the apparatus of the present invention when fired into a steel target at stand-off (air space) from the target produce a perforation or conical cavity through three inches of steel; the diameter of the cavity tapering from about /2" at entry to about A3 at the exit. This tapering of the cavity is also obtained in other target materials and since it is necessary as noted above to maintain a ratio of cavity diameter to web thickness within the range from 0.25 to less than infinity it is important to offset the variation in the ratio caused by the tapering of the cavities. convergence of the outside jets from a row of three adjacent shaped charges spaced as noted :above serves this purpose since as the tapered cavities become smaller the outer cavities converge maintaining a relatively constant ratio of cavity diameter to web thickness between the cavities.

As noted above it is essential for proper operation of the fracture initiation apparatus that all shaped charges therein be detonated simultaneously. This can be accomplished by providing equal lengths of detonating or explosive fuse having a constant rate of burning per unit of length. :These equal lengths of fuse as noted above in the description of FIG. 2 are crimped to booster charges 31 in chamber 27 in contact with the detonator 28. Initiation of the detonator or blasting cap 28 sets off the booster charges 31 simultaneously initiating the equal lengths of fuse. The separate lengths of fuse burn at the same rate thus initiating the booster charges and detonating shaped charges 50 simultaneously. Without simultaneous detonation of the charges the necessary stresses do not result between the holes in the earth formation.

FIG. 5 is an elevational view in section of a second and preferred embodiment of the present invention which is designed for use in a cased well. Contact sub 60, which like sinker bar 19, serves both to connect the conductor cable to the fracture initiation apparatus and as a weight to facilitate lowering the apparatus into a well, is provided with a socket 61 adapted to receive conductor cable head 62, a socket 63 adapted to engage a fracture initiation gun assembly, and a chamber 64 providing communication through the contact sub between the two sockets. Chamber 64 contains a conductive core 65 surrounded by an insulator 66 which serves to hold the core firmly in place so that the uninsulated end thereof protrudes into socket 61 where it makes electrical contact with the internal insulated conductor (not shown) of cable head 62. A detonator assembly consisting of insu lator plug 67, two

electrical leads

68 and 69 and a blasting cap 70 is held in place in the lower part of chamber 64 by means of plug 67. Electrical leads 68 and 69 are each connected to blasting cap 70 at one end and run The 12 7 through insulator plug 67 without contacting each other. The blasting cap 70 may be any of the commercially available high temperature (stable at well temperatures) ibla'sting caps used in oil well operations, such as Du Pont E-2B. EB. caps. The other end of lead 68 is formed into a coil spring which contacts the conductive core 65 of the contact sub. The other end of lead 69 is pinched between insulator plug 67 and the wall of chamber 64 of the contact sub. The electrical circuit is completed through the body of the contact sub 60 which is in contact 'with the outer conductor of the cable head 62.

The fracture initiation gun assembly comprises a detonator housing 71 and a charge container consisting of a sleeve 72 and a bottom plate 73. Detonator housing 7.1 is adapted to. engage socket 63 of the contact sub 60. Shoulder 74 of the housing 71 contains a circumferential groove 75 adapted to receive an O ring gasket 76 which forms a seal. between shoulder 74 and seat 77 of socket 63. The detonator housing 71 contains a concentric bore 78, large enough to receive blasting cap 70* which is suspended from electr-ical leads 68 and 69 imbedded in plug 6-7 in the contact sub 60. The bore 78 opens into a booster charge chamber 79. containing three-to-one adapter 80 which consists of a cylindrical metal case partially closed at one end only and having an initiation port 81 in the center ofthat partially closed end, the three-to-one adapter 80' contains four booster charges '82, one protruding through the initiator port 81 to contact blasting cap 70 in bore 78 and the others disposed around the centrally positioned booster charge. Each of the three outer booster charges 82 has. one of a plurality of equal lengths of explosive fuse 83 crimped thereto.

Socket 84 of sleeve 72 of the main charge container is adapted to receive and engage the detonator housing 71, a. seal being provided by O ring gasket 76 in groove 85 circumscribed around socket 84 in the top of sleeve 73. The charge container is completed by means of bottom plate 73, flange 86 of which is sized to be press fitted into the open end of sleeve 72. A liquid tight seal be tween the sleeve and bottom plate-is provided by O ring gasket 76" in circumferential groove 87 in flange $6. A plurality of equally spaced slots 88 are provided in the skirt 89 of bottom plate 73. The slots 88 serve to aid in causingthe fragmentation. of skirt 89. A plurality of shaped explosive charges 90 are shown in the charge block 91 within the charge container composed of sleeve 72 and bottom plate 73. a

FIG. 6 is a cross sectional view of the apparatus of FIG. taken along the line 66 showing the circular cross section of sleeve 72 of the charge container, two charge blocks 91 disposed back to back along a diameter of the sleeve 72 and; shaped explosive charges 90 disposed in bore holes 92 of the charge blocks 91. Each charge block 91 contains three bore holes 92, arranged in a straight row, the axis of the central bore hole being perpendicular to the back of the. charge block and a diameter of the circular cross section of sleeve 72. The axes of the two outside bore holes in the charge block make an; angle of 8 with the, axis of the central bore hole. The, shaped charges 90; are inserted; in the bore holes 92 of. the, charge blocks 91 to form two opposed rows of chargeswas described-above'in theapparatus of FIG; 4.

The shaped charges 90 employed in the preferred apparatus of FIG. 5 are somewhat difierentvfromrthose employedin the apparatus of FIG. 2; These jet charges are short; coupled: as are the shaped charges 50 of FIG. 4 since both are designed for fracture initiation apparatus useful, in. well bore holes. Charges 90, however, differ from charges. 50 in that. they are provided. with a heavy dense, metal case 93, suitably of zinc, which surrounds the shaped charge on all sides except thev face of the charge. The great density of the zinc case prevents absorption of the shockwfromthe adjacent charge and per mits. the tips of the charges to bealmost in contact with each, other since the amount of explosive at the tips is small. The heavy metal case contains an ignition opening- 94 in the center of its base, a booster charge 95 in the ignition opening, a main explosive charge 96, and a metal cone 97, suitably a 60 copper cone, similar to those employed in the shaped charges 50 described above. The explosive fuse 83 is shown in contact with the booster charges 95 of opposed pairs of shaped charges 90. Air standoff spaces 98 between the shaped charges and the sleeve 72 of the charge container are necessary for proper functioning of the charges.

FIG. 7, which is a cross sectional view of the apparatus of FIG. 5 taken along the line 7-7, shows the circular cross sections of the detonator housing 71, three-to-one adapter 80, and booster charge chamber 79, and the arrangement of the four booster charges 82 within the latter.

FIG. 8 is an elevational view in section of the apparatus of FIG. 5 taken at a angle with respect to FIG. 6. The configuration of charge blocks 91 is clearly shown by a comparison of the end view of said blocks in FIG. 8 with the side view of FIG. 5. In FIG. 5 the charge blocks are shown to be co-extensive with the cross section of the void space of the charge container consisting of sleeve 72 and bottom plate 73 whereas in FIG. 8 it is seen that the blocks 91 occupy only the central portion of the charge container leaving air space for sufiicient stand off to insure proper functioning of the charges. I

As noted above fracture initiation apparatus of the 7 present invent-ion consists essentially of at least one straight row of at least two critically spaced jet charges and means for detonating the charges simultaneously. Apparatus of the invention must be adapted to produce at least two and preferably three, more or less parallel elongated cavities spaced along a straight line on the. face of the earth formation to be fractured since it is the plane defined by such cavities which determines the plane of the.

fracture. Apparatus having from two to five shaped charges per row is suitable for use in well bore holes although three charges per row is prefered since this number is sufficient to define the desired plane'of the fracture with. certainty and yet can be fitted within the narrow confines of an apparatus capable of being lowered into a conventional well casing. Of course. any number of charges per row from two on up may be employed in apparatus where there is no limitation as to size. Additional control of the plane of fracture is afforded in appar-atus for use in a well bore hole by employing two rows of charges faced in opposite directions in the same plane. When the'two opposed rows of charges are detonated, rows of cavities are produced in opposite sides of the bore hole thus creating fractures which when extended by known hydraulic fracturing methods may meet to form a single fracture radiating in all directions 360 around the bore hole.

The spacing of individual shaped charges in a row of charges in the apparatus is an important feature. of the invention. The chargesrmay be spaced as close together as possible, i.e., to the point where adjacent charges are in direct contact,'so long as the charges can be fired without intercharge interference. The greatest spacing which is permissible between adjacent charges in a row is, therefore, the most significant limitation. Although this distance varies depending upon the strength, homogeneity and character of the earth formation to be fractured it is possibleto set practical limits for the majority of formations. ilt has been found that apparatus of general utility must be capable of producing a row of cavities in which the ratio of cavity diameter, conveniently measured at the entrance, to the thickness of the web between adjacent cavities lies in the range from 0.25 to less than infinity. Apparatus of; the present invention using shaped charges of the character described will produce a row of cavities having this ratio of cavity diameter to web thickness.

It is necessary that all jet charges be detonated simultaneously since premature. detonation of one charge ina row normally causes side initiation of the adjacent charge. This results in a malformed jet which fails to produce the required cavity. If it were possible, however, to prevent side initiation of adjacent jet charges confined in the required spacing it would be unnecessary to detonate the charges simultaneously. lntercharge interference is avoided in jet charges 50 of FIG. 4 by providing air spaces 49 between adjacent charges. The metal eliminated to make air chambers 49 is removed to prevent transmission of sufficient shock to cause intercharge interference. The problem of intercharge interference is solved in jet charges 9d of 'FIG. 6 by the use of a heavy metal case around the charge.

The apparatus of the present invention is designed to be operated by semiskilled personnel who need not understand the internal workings of the mechanism in order to achieve the desired results. Apparatus for use in mining or quarrying operations is placed adjacent to the face of the quarry or other formation either by suspension on a cable or by attaching the apparatus directly to the formation by any suitable means. The apparatus is then detonated by energization of the electric conductor cable after clearing the area of personnel. The detonation of the shaped charges produces a plurality of elongated conical cavities and a communicating fracture in a predetermined orientation in the target formation and in most cases completely destroys the fracture initiation apparatus. Fracture initiation apparatus designed for use in well bore holes can be lowered into the well on a conductor cable unwound from a drum mounted on a suitably equipped truck. The fracture initiation apparatus is positioned at the desired depth by measuring the length of cable used. Additional confirmation of the position of the apparatus in a cased well can be obtained by means of an electronic collar counter. This device registers on an indicator at the surface when the fracture initiation apparatus passes the juncture between lengths of well casing. Inasmuch as the length of a section of casing and the number of sections in the well is usually known, the apparatus can be positioned accurately at any desired depth by measuring from the nearest casing collar the exact depth of which can be calculated. When the apparatus has been placed at the desired depth in the well it is detonated by energizing the conductor cable. The incipient fracture created in the formation at the bottom of the well is then extended by a hydraulic fracturing operation after retrieving the conductor cable from the well.

The apparatus of the invention may be employed in either cased or uncased wells. Best results to date have been obtained with cased wells. Erratic results are sometimes obtained with uncased wells where the earth formation contains great variations in strength in the exposed zone which is intended to be fractured by the apparatus of the invention. In such cases fracture may take place in some unintended plane which is substantially weaker than the plane intended to be fractured.

The specific embodiments of the fracture initiation apparatus of the present invention shown in the drawings and described above are merely illustrative and are not to be construed as limiting the scope of the invention or the appended claims inasmuch as numerous modifications of the apparatus within the ambit of the invention will be readily apparent to those skilled in the art.

What is claimed is:

1. A fracture initiation gun assembly adapted to be lowered into a well bore hole which assembly comprises a charge container and a plurality of shaped explosive charges; said charge container being provided with a plurality of cylindrical charge chambers arranged in two straight parallel rows in a single plane, the axes of adjacent charge chambers in each of said rows being substantially parallel, a plurality of fuse passages, one of said fuse passages being disposed between the adjacent ends of each pair of charge chambers in the charge container consisting of one chamber from each of said parallel rows, and a plurality of air spaces between adjacent charge chambers in each of said rows of chambers, said air spaces having a smaller cross section than said charge chambers, said cylindrical charge chambers having suflicient depth to contain a shaped charge at the inner end thereof and provide a remaining stand-off space between the face of said shaped charge in the charge chamber and the end of the charge chamber at the periphery of said charge container, said shaped charges disposed in one row of charge chambers being faced to fire in one direction while those in the other row of charge chambers are faced to fire in the opposite direction and said fuses being adapted to fire said charges simultaneously.

2. A fracture initiation gun assembly adapted to be lowered into a well bore hole which assembly comprises a cylindrical charge container and six shaped explosive charges; said charge container being provided with six cylindrical charge chambers arranged in two straight parallel rows of three chambers each in a single plane with the axes of adjacent chambers in each of said rows parallel, three fuse passages, one of said fuse passages being disposed between the adjacent ends of each of the three pairs of charge chambers in the charge container consisting of one chamber from each of said parallel rows, and four air spaces one of which is disposed between each pair of adjacent charge chambers in each of said rows of chambers, said air spaces having a smaller cross section than said charge chambers, said cylindrical charge chambers having sufficient depth to contain a shaped charge at the inner end thereof and provide a remaining stand-01f space between the face of said shaped charge in the charge chamber and the end of the charge chamber at the periphery of said charge container, the shaped charges disposed in one row of charge chambers being faced to fire in one direction while those in the other row of charge chambers are faced to fire in the opposite direction and said fuses being adapted to fire said charges simultaneously.

3. A fracture initiation gun assembly adapted to be lowered into a well bore hole which assembly comprises a charge container and a plurality of shaped explosive charges, said charge container being provided with two charge blocks each containing three charge chambers, said charge blocks being disposed back to back in said charge container with the charge chambers of said blocks facing in opposite directions, the axes of the terminal charge chambers in each of said charge blocks making an angle of 8 with the axis of the central change chamber in said block, said shaped explosive charges being disposed in said charge chambers with the shaped charges in one charge block faced to fire in one direction and the charges in the other charge block faced to fire in the opposite direction and a stand-off space between the shaped charges and the perimeter-of the main charge container.

4. An apparatus for initiating a fracture in a predetermined plane in an adjacent earth formation comprising a main charge container, a plurality of shaped explosive charges and means to fire said charges simultaneously; said main charge container containing two charge blocks disposed therein in back-to back relationship, each of said charge blocks defining at least two charge chambers for holding said explosive charges, said explosive charges be ing arranged in the same plane and spaced apart substantially parallelwise at intervals such that the ratio of charge diameter to distance between charges is about 6.45.

5. An apparatus for initiating a fracture in. a horizontal plane in an adjacent earth formation comprising a main charge container, a plurality of shaped explosive charges, and means to fire said charges simultaneously, said main charge container containing two charge blocks, said two charge blocks being disposed within said container in backto-back parallel relationship, each of said change blocks comprising a plurality of charge chambers for holding said shaped explosive charges, said charge chambers being arranged in a single plane and oriented to lie with their axes in a plane at right angles to the vertical axis of said main charge container, said shaped explosive spaced apart substantially parallelwise at intervals equal to about- 1.1-5 inches between char-ge centers, said backto-back relation of charge, chambers, further defining a stand-off space before the inner wall of said main charge container.

' 6. An apparatus as in claim 5 wherein the end change chambers on each side of the central charge chamber are oriented with an angular toe-in of 12 toward said central charge chamber.

Mims Apr. 27, 1926' 12 Malskat et al. Jan; 10, Kaltenberger May 9, Sweetman Feb. 26,. Sweetman Feb. 23, Church et a1. July 6, Bryant etc a1. Feb. 7, 'Ifurechek Apr. 24, Church et a1. Aug. 7, Klot-z Jan. 29, Church at all June 24, Stewart July 15, Forsyth June 23,

Borins et a1. Aug. 4,

Claims

3. A FRACTURE INITIATION GUN ASSEMBLY ADAPTED TO BE LOWERED INTO A WELL BORE HOLE WHICH ASSEMBLY COMPRISES A CHARGE CONTAINER AND A PLURALITY OF SHAPED EXPLOSIVE CHARGES, SAID CHARGE CONTAINER BEING PROVIDED WITH TWO CHARGE BLOCKS EACH CONTAINING THREE CHARGE CHAMBERS, SAID CHARGE BLOCKS BEING DISPOSED BACK TO BACK IN SAID CHARGE CONTAINER WITH THE CHARGE CHAMBERS OF SAID BLOCKS FACING IN OPPOSITE DIRECTIONS, THE AXES OF THE TERMINAL CHARGE CHAMBERS IN EACH OF SAID CHARGE BLOCKS MAKING AN ANGLE OF 8* WITH THE AXIS OF THE CENTRAL CHARGE CHAMBER IN SAID BLOCK, SAID SHAPED EXPLOSIVE CHARGES BEING DISPOSED IN SAID CHARGE CHAMBERS WITH THE SHAPED CHARGES IN ONE CHARGE BLOCK FACED TO FIRE IN ONE DIRECTION AND THE CHARGES IN THE OTHER CHARGE BLOCK FACED TO FIRE IN THE OPPOSITE DIRECTION AND A STAND-OFF SPACE BETWEEN THE SHAPED CHARGES AND THE PERIMETER OF THE MAIN CHARGE CONTAINER.