US3058521A

US3058521A - Method of initiating fractures in earth formations

Info

Publication number: US3058521A
Application number: US700144A
Authority: US
Inventors: Gilbert Bruce
Original assignee: Western Company of North America
Current assignee: Western Company of North America
Priority date: 1957-12-02
Filing date: 1957-12-02
Publication date: 1962-10-16
Anticipated expiration: 1979-10-16

Description

SLAHUH www B. GILBERT Oct. 16, 1962 2 Sheets-Sheet 1 United States Patent O 3,058,521 METHUD F lNlTlATlNG FRACTURES IN EARTH FORMATlONS Bruce Gilbert, Dallas, Tex., assignor to The Western Company of North America, a corporation of Dela- Ware Filed Dec. 2, 1957, Ser. No. 700,144 7 Claims. (Cl. 166--42) nainalnrrriaa-arid-minins. in.rherecogernorimdera .srgulnluidesastres..aetroleiirnmnatural..gas.-or...water, and A14 the 1n `tu combustion of carbonaceous deposits SUC as 011, fari. rstralitemus shaleandsnatmeenam ground disposal of radioactive or other wastes, and in other fields in which it is advantageous to create articial fissures in the earths crust. For example, the method of creating fractures in earth formations according to the invention facilitates mining or quarrying operations by aiding in the separation of masses of rock or mineral from large formations. Similarly in petroleum, gas or water recovery operations, the initiation and extension of artificial fractures in fluid-bearing earth formations facilitates drainage of such formations into a well from which the desired fluids can be recovered. In petroleum production particularly, it is common practice after drilling a well bore hole into an oil Ibearing earth formation to attempt to increase the productibility of the formation by creating therein artificial fractures which facilitate the ow of petroleum from the formation into the well. For many years it was the practice to try to increase the productibility of the formation by creating therein artiicial fractures which facilitate the flow of petroleum from the formation into the well. For many years it was the practice to try to increase the productibility of oil-bearing formations by horizontal drilling or by detonating massive charges of nitroglycerin in the well adjacent to the desired formation. Both of these practices were expensive, the latter being hazardous as well, and they have, therefore, been lsuperseded in recent years by generally more effective hydraulic fracturing techniques such as those described in U.S. Reissue No. 23,733.

In hydraulic fracturing methods, a liquid, of either hydrocarbon or water base, is injected into the well under sufficient hydraulic pressure to cause the exposed formation at or near the bottom of the well to break down, thus creating and extending fractures which increase the productibility of the oil 'bearing strata. Sand or other granular material is commonly incorporated in the fracturing fluid as a propping agent to prevent closure of the newly created drainage channels by the pressure of the overburden of earth. When viscous liquids are used in hydraulic fracturing they often contain chemical additives which cause the viscosity of the liquid to decrease after a period of time so that the fracturing fluids can be readily recovered from the newly created fractures by a uid produced from the formation. Hydraulic fracturing treatments have come into widespread use since they are frequently successful in achieving a significant increase in the production of the treated well.

Hydraulic fracturing treatments frequently produce only a single fracture in an oil producing formation. When this occurs, it is usually due to the fact that availice able high pressure pumps are limited in their capacity, making it impossible, as a general rule, to pump lsufficient fracturing fluid into the well to increase the treatment pressure sufficiently to create a second fracture in the eX- posed formation while the initial fracture is taking uid. For this reason techniques have been developed for isolating and treating short sections of a well lbore individually in order to produce fractures at different levels in the well when this is desirable. For example, a formation or casing packer, either single or dual, can be run down the well casing on tubing and located and set in a Well to isolate and confine a selected producing zone which it is desired to fracture. This practice gives the operator control over the general location at which an artificial fracture is created in an underground earth formation by means of a hydraulic fracturing operation. Multiple fractures may be created using sufcient pump capacity. When this occurs, however, there is currently no method of controlling the direction and orientation of the fracture. Thus fractures may be extended out of oil bearing rock (or the desired interval to be fractured) into a barren, or undesirable zone; also fractures may be extended into water bearing or gas bearing rock resulting in subsequent production of water or gas instead of oil. Also multiple fracturing frequently results in the formation of many small fractures immediately adjacent to the well bore (or point of initiation) instead of a few fractures penetrating deeply into the reservoir. Also frequently a reservoir may be more efficiently produced by fractures located at strategic points. Current fracturing practices are random, and do not permit selection of where fractures are to be placed.

It has long been recognized that it would be highly desirable, not only to control the 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. This has not heretofore been possible with the previously available hydraulic fracturing methods. Positive control of both the depth and the 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 the Well, or which might otherwise produce the results desired, i.e., formation of a horizontal cement barrier at a certain depth. The utility of a fracture initiation method 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 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 to the adjacent water bearing stratum and consequently the hydraulic fracturing treatment would produce a Valuable oil Well, rather than a worthless Water Well. The creation of fractures oriented in planes disposed at various angles other than horizontal is, of course, frequently desirable particularly in mining and quarrying operations and also in oil recovery operations where detection methods indicated that an oil bearing stratum lies in such a plane. Also, in secondary and tertiary recovery operations it may be desirable to employ fractures located in planes other than horizontal. For example, gravity drainage production of reservoirs may be greatly facilitated by employing a fracture at the bottom of a reservoir and extending slightly upward from its intersection With the well bore. The method of the present invention provides for the creation of fractures in any predetermined orientation in an earth formation of any type.

Prior attempts have been made to control fractures in an oil well bore. One such early attempt consisted of firing simultaneously 4 shaped charge shots spaced 90 apart in a single plane. The purpose was to create a horizontal fracture for squeeze cementing operations in the entire 360 around the casing in order to eliminate vertical migration of fluid. Another attempt consisted of creating vertical fractures by perforating a formation through a well casing with shots arranged linearly 3 inches apart along one side of a gun. Another attempt consisted of creating a wedge or circular cavity in the rock surounding a Well bore in the hope that when hydraulic pressure was applied the rock would break at the point of the Wedge. These efforts were all unsuccessful.

It is an object of the present invention, therefore, to provide a novel method for creating a fracture having a predetermined orientation in an earth formation such as concrete or the wall of a quarry, underground tunnel, or well bore hole.

It is a particular object of the invention to provide a method for treating an earth `formation surrounding a Well bore hole to establish the plane of any fracture initiated, created or extended therein by a subsequent hydraulic fracturing operation.

It is another particular object of the present invention to provide a novel method for initiating a fracture in an earth formation penetrated by a well bore hole at a single point of predetermined depth in the earth and in a predetermined orientation with respect to the bore hole without the necessity of exposing a complete zone of the well by perforating.

It is a further particular object of the invention to provide a novel method for the creation 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 any plane, such as one about 45 from a plane parallel with the horizon.

It is a further object of the invention to provide a method for reducing the pressure required to fracture an earth formation by a hydraulic fracturing treatment.

It is a further object of the invention to provide a method for the penetration of unusually thick cement sheaths surrounding a cased well bore hole by creating a fracture therein, extending it into the adjacent formation.

It is a further object of the invention to provide a method for preventing a hydraulic fracturing treatment from channeling between the well casing and the surrounding cement sheath, and/or between the cement sheath and the surrounding formation.

Additional objects and advantages of the invention will be apparent from the following description of the method of the invention taken in conjunction with the accompanying drawing in which:

FIG. 1 is a diagrammatic sectional view of a cased well bore hole showing cavities according to the invention in an oil bearing stratum;

FIG. 2 is a diagrammatic view of an earth formation containing a plurality of cavities arranged and spaced according to the invention;

FIG. 3 is a diagrammatic sectional view of the cavities of FIG. 2 taken along the line 33.

FIG. 4 is a diagrammatic view of the earth formation of FIG. 2 after the application of hydraulic pressure to the cavities has established a fracture therein;

FIG. 5 is a diagrammatic View of the fractured earth formation of FIG. 4 taken along the line 5--5; and

FIG. 6 is a section along the line 6-6 of FIG. 1 after the fracture has been initiated.

These and other objects of the invention are attained by creating in the solid material, such as an earth formation, at the point to be fractured, a plurality of critically spaced generally parallel holes or cavities oriented in the plane in which it is desired to create a fracture and creating or extending a fracture in the plane defined by the cavities by applying fluid pressure to them. The cavities are so spaced that the ratio of the diameter of a cavity to the width of the web between adjacent cavities falls Within a critical range such that when fluid pressure is applied in the cavities any fracture created in the vicinity will be established in the plane defined by the plurality of parallel cavities and extended in substantially the same plane.

More specifically, the invention is based on the discovery that when at least two, and preferably three elongated, generally parallel, cavities are created in a single plane in an exposed earth formation and the ratio of the diameter of an individual cavity to the thickness of the web of earth formation between that cavity and the adjacent cavity or cavities falls within certain critical limits, it is possible to initiate a fracture oriented in the plane defined by the parallel cavities by the application of sufficient hydraulic pressure to the cavities to fracture the formation. In the process of the present invention the plane of the fracture is controlled in a preoriented, predetermined plane by that of the cavity pattern and when the fracture is extended by hydraulic fracturing, the fracture is extended in the same plane and not in some random unwanted plane which the user did not desire. Any number of parallel cavities greater than one can be employed with success although, as noted above, three cavities are preferred since this number has been found to provide better and more consistent control of the plane of orientation than two cavities. More than three cavities may be employed but it has been found that there is seldom any advantage in doing so since three provide satisfactory control of fracture initiation in most cases. Also, in the usual small diameter well bore holes it is difficult to introduce means for simultaneously' producing many more than two sets of three parallel cavities in a single plane.

The diameter of the cavities, although not particularly critical, should be great enough to afford a reasonably large internal surface in the cavity `for the application of fluid or hydraulic pressure and of course should not be so small that the uid or hydraulic pressure will be prevented from entering the cavity either by its own viscosity or by the bridging of solid particles in the fluid across the face of the cavity. There is no critical upper limit on cavity diameter as long as a web is left between the cavities. It has been found, for practical purposes in the majority of earth formations, that cavities having a ,diameter in the range from about 0.25 inch up to several inches in diameter are satisfactory. For Work in well bore holes which are generally from 4 to 8 inches in diameter, it has been found that cavities ranging from about 0.10 to 1.0 inch in diameter and preferably about 0.20 to 0.625 inch are satisfactory.

The depth of the cavities is not particularly important so long as they are relatively elongated with respect to their diameter. For example, cavities from 1 diameter to an infinite number of diameters deep, but desirably 5 to 25 cavity diameters deep, and preferably 12 to 20 diameters deep, are satisfactory in most instances. For use in Well bore holes, cavities which are about 0.30 inch in .diameter and 6.0 inches deep have been found to give excellent results.

The ratio of .cavity diameter to the thickness of the web of earth formation between adjacent cavities is a fundamental element of the method of the present invention. It has been found that this ratio may vary from about 0.25 up to, but not including, infinity, such as 1000 or 100, or in other Words the cavities may be spaced from four diameters apart to almost touching. Of course if the plurality of cavities were in direct contact there would be no web between the cavities and the ratio of cavity diameter to web thickness would be infinity. For this reason it is the maximum spacing between adjacent cavities which is of greatest importance. The optimum ratio will depend somewhat upon the nature of the earth formation to be fractured. It has been found that when the cavities are spaced more than four diameters apart adequate control over the orientation of a fracture initiated in the vicinity of the cavities is lost. The preferred ratio of cavity diameter to web thickness is in the range from about 0.75 to 1.0. Ratios in this range are particularly suitable for use in earth formations where the cavities must be produced in a well bore hole.

With a plurality of cavities placed in an earth formation in accordance with the method of the invention the web thicknesses between the cavities are subjected to a concentration of stress which causes the web between the cavities to fail when subjected to hydraulic pressures before failure occurs anywhere else in the exposed area. This oriented failure provides initiation of a fracture in the desired predetermined plane which is extended in the same plane upon the further application of fluid or hydraulic pressure. Because initiation of the fracture in accordance with the method of the invention permits fracture extension by hydraulic means at lower pressures than are necessary for conventional hydraulic fracture formation, it is possible to complete the fracture at sufciently W pressures that reduce the likelihood of producing new uncontrolled fractures in an unwanted or unoriented plane. This, of course, contributes materially to the success of the method of the invention. In the injection of the hydraulic breakdown fluid, it is preferable to limit the pressure until a filter-cake can be developed by the breakdown fluid preventing loss of fluid into and preferential fracturing of intrinsic fractures or planes of weakness exposed to the breakdown fluid. The fracture is then initiated between the cavities by increasing the pressure until a break or a pressure drop occurs. After this initial fracture is produced between the cavities, the pressure required to extend the single fracture in the desired plane is usually less than the pressure required to create the fracture.

The method of the invention may b'e employed in either cased or uncased wells. Best results to date have been obtained in cased wells.

The cavities employed in the fracture initiation method of the present Iinvention can be produced in any manner since the method of the invention is independent of the manner in which the cavities -are created. The production of the required cavities in Widely exposed earth formations such as the wall of a mine or quarry presents no problem and is most conveniently accomplished by drilling according to known methods, such as screw drills or pneumatic drills. When the working space is limited, as for example, in a deep well bore hole, the creation of cavities becomes more diicult. Such cavities can, however, be produced lin a variety of ways including drilling, electric arcs, oxyacetylene torches, shooting with bullets, blasting with explosive agents, the use of jets of corrosive or abrasive fluid, by means of explosive jet charges. Among the suitable cavity producing devices which may be employed are the rock piercing blowpipe described in U.S. Patent No. 2,794,620 of C. S. Arnold et a'l. and the heat blasting fluid-jet tool described in U.S. Patent No. 2,772,346 of T. I. Leston et al. The use of bullets and jet charges in the perforation of well casing, cement sheathing around such casing and the surrounding earth formations is common practice in treatment of oil and similar wells. Abrasive jets based on the sand blasting ,f principle would also be suitable as would jets of corrosive 6 jet charges is described in my copending application for U.S. Letters Patent, Serial No. 675,424, filed July 3.1, 1957, entitled, Apparatus for Initiating Fractures in Earth Formations.

The cross section of the required cavities may take any form so long as it is generally rounded and is free of sharp angular crevices or configurations which might cause loss of control of the orientation of the fracture due to random failure of the formation in the angular crevice. The preferred cross section is circular although yan ovoid or irregular cross section is suitable so long as it does not contain angular crevices which would cause stress concentrations in a plane other than that desired.

Although the cavities are preferably generally parallel, it is permissible for their `axes to converge somewhat and that the cavities have a tapered configuration so that the ratio is more or less constant along their entire length.

The manner in which the method of the invention accomplishes oriented fractures in an earth formation will be illustrated in part by the attached drawings. FIG. l shows a cross section portion of an underground earth formation 11 penetrated by a conventional well bore hole containing well casing 12 anchored to the formation by lmeans of cement sheath 13. The section of lthe well illustrated passes through an oil bearing stratum 14 of earth formation 11. Elongated cavities 15 in a horizontal plane areshown penetrating the well casing 12, cement sheath 13 and oil bearing stratum 14. It should be noted that although cavities 15 are shown extending from opposite sides of `the well, the two cavities illustrated lie in the same Ihorizontal plane in which it is desired to create a fracture. Although only one cavity 15 may be seen in each side of the well in FIG. l it is essential that at least one other cavity and preferably two `additional cavities be associated with and in the same plane as each of the cavities shown in the required spacing in order to achieve the purposes of the invention.

FIG. 2 illustrates the open ends of three substantially parallel cavities 15 positioned in a single plane and spaced approximately one cavity diameter apart along a straight line in an earth formation 11. The open ends of the cavities are flush with the face of an exposed earth formation such as the wall of a mine or quarry or may be considered to be in the curved face of a well bore hole. The ratio of the diameter (a) of a cavity -15 to the thickness of the web (b) between adjacent cavities is 1.0 in the drawing since (a)=(b). In actual practice this ratio can vary from 0.25 up to, `but not including, infinity, depending upon the characteristics of the formation.

FIG. 3 is a diagram-matic sectional view of the earth formation of FIG. 2 taken along the line 3 -3 showing the depth of the cavities 15 in earth formation 11. The curved face of the well `bore hole is shown at 16. The cavities 15 are shown to be about nine diameters deep which is within the preferred range of the present invention.

FIG. 4 is a diagrammatic view of the earth formation 11 of FIGS. 2 and 3 showing an incipient fracture 17 filled with fluid 18. The vestiges of the faces of the cavities 15 may be seen as enlarged portions 19 of the fracture 17. The application of hydraulic pressure to the cavities 15 of FIGS. 2 and 3 by means of hydraulic iiuid 18 has initiated the fracture in the formation in the plane established by the substantially parallel cavities 15 and established Ia fracture 17 which can be propagated in the same plane by continued pumping of fracturing fluid.

FIG. 5 is a diagramma-tic view taken along the line 5--5 of FIG. 4 showing the manner in which the fracture 17 in earth formation 11 would be extended by continued pumping of fracturing fluid 18. Of course, the fracture 17 would be extended in the direction of the arrows as well as in all other directions in approximately a semicircle around the bore hole 16 but for purposes of illustration the front of the fracture `is shown moving through the depth ofthe cavities 15 shown in broken lines.

liG. 6 is a section taken along the line 6 6 of FIG. 1 after the fracture has been initiated showing the fracture extending radially about the full 360 around the well bore hole in a plane which is substantially perpendicular to the -bore hole. The reference numerals have the same significance as in FIGS. 1 and 5.

ln FIGS. 5 and 6 the cavities 15 are shown in broken lines to indicate only the original position of the cavities rather than the cavities themselves. As shown in FIGS. 5 and 6, the fracture has extended beyond the length of the cavities.

The fracture initiated by the plurality of critically spaced cavities in the single plane in the earth formation may be extended by introducing into the formation under hydrostatic or iluid pressure any of the fracturing liuids which are known to those skilled in the art for Ifracturing purposes. Preferred fracturing fiuids are the socalled lowpenetrating fracturing liuids. This type of fluid in general is characterized by its reduced tendency to penetrate the interstices of an earth formation. These iluids in general have a greater viscosity than average crude oil or other well fluids or they tend to produce a plaster or cake on permeable formations with which they are brought into contact. These fracturing fluids generally have a viscosity of from at least 30 centipoises to about 5000 centipoises or even higher. Among the low penetrating fracturing fluids which may be employed are those described in U.S. Reissue No. 23,733. Such uids may have either a hydrocarbon or water base. The hydrocarbon base uids are generally the most suitable for this purpose. These fluids are produced by incorporating a `bodying agent into a hydrocarbon fluid, such as crude or refined oil. The bodying agent may comprise a colloid material or a metallic soap of an organic acid; a high molecular weight oleiin polymer, particularly high molecular Weight linear polymers, such as polypropylene; or an oil or water soluble plastering agent such as blown asphalt or pitch. Other bodying agents are well known to the art. Among the metallic soaps of organic acids which may be employed as the bodying agent, are the hydroxy aluminum soaps. The bodying agent may be employed in suflicient quantity to impart to the base iiuid suicient viscosity or low penetrability. The methods and materials employed to produce low penetrating fluids are Well known to those skilled in the oil well treating industry.

As an example of the effectiveness of the method of the present invention on the operation of an oil well in the San Andres `formation in Texas, which on a production test prior to treatment in accordance with the present invention produced two barrels of oil per day, the well was treated as follows:

The well, which consisted of a 4% diameter open hole having a depth of 4,922', was first filled with 5,000 gallons of crude oil containing about 1.2% by weight of sodium dodecylbenzene sulfonate containing calcium sulfate as an inert filler. The well was next subjected to cavity formation employing a jet charge apparatus in accordance with my aforementioned copending U.S. patent application Serial No. 675,424. This apparatus comprised six shaped jet charge chambers in two opposing parallel rows of three chambers each capable of firing in opposite directions, in which the jet charge chambers were 1 in diameter and about 1% long lwith each charge in parallel alignment spaced 1.155 apart between the centers. The jet charge chambers were positioned in a plane parallel with the horizon. After simultaneous detonation of the six jet charge chambers of the fracture initiating apparatus, the tiring mechanism was removed from the well, and the well permitted to stand overnight. During this time, the iluid level in the well decreased approximately 330 feet below the surface. Approximately 8 barrels of the same crude oil, containing about 1.2% by weight of sodium dodecylbenzene sulfonate which contained calcium sulfate `as an inert iiller, were added to bring the liquid level in the well to the surface. Fracturing iiuid in the amount of 1900 gallons of crude oil containing about 1.2% by weight of sodium dodecylbenzene sulfonate ('liuid loss additive) `containing calcium sulfate as an inert filler was next introduced into the well. Of this 1900 gallons, the first 750 gallons contained no sand additive. The remaining 1150 gallons contained 0.5 lb. of sand per gallon. In introducing this 1900 gallons of fracturing fluid, the first 8 barrels were pumped before a surface pressure of 600 p.s.i. was measured. The next 10 barrels were injected slowly at first with the rate of pumping increasing to approximately 9 'barrels per minute at a surface pressure of 600 p.s.i. This injection rate at low pressure indicated that the fracture was initiated and at this stage the injection rate Was increased to about 22.5 barrels per minute using the fluid containing the sand. After the 1900 gallons had been introduced into the well, 4200 gallons of crude oil, containing sand as the only additive, was introduced into the well. Of this 4200 gallons, the rst 1500 gallons contained 0.5 lb. o-f sand per gal.; the next 1700y gallons contained 1 lb. of sand per gal.; and the last 1000 gallons contained 1.5 lbs. of sand per gal. During the addition of the oil containing sand as the only additive, an injection rate of 33.3 barrels per minute was attained. Finally, 6300 gallons of ordinary crude oil was introduced into the Well at a rate of about 29.5 barrels per minute to flush the fracturing and treating uids out of the well intothe formation. injection of liquid was continuous throughout `the fracturing operation. In this particular well, no initial pressure build-up and break was obtained upon initiating the fracturing treatment, as is often the case with the method of the invention. After approximately days a production test was conducted on the well and it lwas lfound that the well produced 30 barrels of oil and 6 barrels of water per day. The results obtained are particularly spectacular since wells in this region typically produce water when employing other means of fracturing.

As will be apparent to those skilled in the art, the `orientation of the plane of -fracture may be positioned |by locating the plurality of parallel cavities in a corresponding plane. This plane may be in any position with respect to the horizon from horizontal to vertical.

As has been expressed hereinabove, one of the important advantages of the process of the present invention is that it permits fracturing of an earth formation at a point where a fracture is desired at lower pressures than were effective by prior hydraulic fracturing treatments. This 1s a direct contribution of the concentration of stresses introduced in the web of the earth formation between the plurality of substantially parallel cavities. Another concomitant advantage of the method according to this invention over the prior hydraulic fracturing treatments is that by initiating a plurality of parallel cavitles in a single plane, a subsequent injection of a hydraulic iiuid is less likely to channel Ibetween the casing 0f an oil well and its surrounding cement sheath and/or between the cement sheath and the surrounding formation.

The terms and expressions which have been employed are used as terms of description and not of limitation, and 1t is not intended, in the use of such terms and expressions, to exclude any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed.

What is claimed is:

l. A method of fracturing an earth formation in a predetermined plane which comprises producing a plurality of spaced substantially parallel cavities in the plane in which the fracture is desired, said cavities being spaced sufiiciently close together so that the ratio of cavity diameter to the thickness of the web of earth formation between adjacent cavities is at least about 0.25, and then injecting hydraulic fluid into the cavities under such pressure as will initiate the fracture in the plane of the plurality of cavities.

2. A method of fracturing an earth formation in a predetermined plane which comprises producing a pluralily of spaced substantially parallel cavities in the plane in which the fracture is desired, said cavities being spaced sufficiently close together so that the ratio of cavity diameter to the thickness of the web of earth formation between adjacent cavities is from about 0.25 up to, but not including, infinity, and then injecting hydraulic iluid into the cavities under such pressure as will initiate the fracture in the plane of the plurality of cavities.

3. A method of fracturing an earth formation in a predetermined plane which comprises producing a plurality of spaced substantially parallel cavities in the plane in which the fracture is desired, said cavities being spaced suliciently close together so that the ratio of cavity diameter to the thickness of the web of earth formation between adjacent cavities is from about 0.75 to 1.0, and then injecting hydraulic fluid into the cavities under such pressure as will initiate the fracture in the plane of the plurality of cavities.

4. A method as defined in claim 1 wherein three cavities are employed.

5. A method as dened by claim 1 wherein the cavities are of substantially circular cross-section of from about 0.25 to 1 inch in diameter and from about 1 to an infinite number of cavity diameters deep.

6. A method as dened by claim 1 wherein the cavities 10 are of substantially circular cross-section of from about 0.25 to 1 inch in diameter and from about 5 to 25 cavity diameters deep.

7. A method of fracturin,y an earth formation in a substantially horizontal plane which comprises producing a plurality of spaced substantially parallel cavities in a substantially horizontal plane, said cavities being spaced sufficiently close together so that the ratio of cavity diameter to the thickness of the web of earth formation between adjacent cavities is at least about 0.25, and then injecting hydraulic iluid into the cavities under such pressure as will initiate the fracture in the plane of the plurality of cavities.

References Cited in the le of this patent UNITED STATES PATENTS 475,647 Weller May 24, 1892 820,849 Capps May 15, 1906 1,630,470 Clifford May 31, 1927 2,385,753 Young Sept. 25, 1945 2,494,256 Muskat Ian. l0, 1950 2,547,778 Reistle Apr. 3, 1951 2,596,843 Farris May 13, 1952 2,630,182 Klotz Mar. 3, 1953 2,699,212 Dismukes Ian. ll, 1955 2,758,653 Desbrow Aug. 14, 1956 2,766,828 Rachford Oct. 16, 1956 2,952,319 Popham Sept. 13, 1960