US3713915A

US3713915A - Thickened nitromethane explosive containing encapsulated sensitizer

Info

Publication number: US3713915A
Application number: US00095005A
Authority: US
Inventors: C Fast
Original assignee: BP America Production Co
Current assignee: BP America Production Co
Priority date: 1970-11-23
Filing date: 1970-11-23
Publication date: 1973-01-30
Anticipated expiration: 1990-01-30

Abstract

A liquid explosive primarily based on nitromethane contains small amounts of a sensitizer (an organic amine), a thickening agent, and up to 25 percent powdered aluminum. It may additionally incorporate ammonium nitrate, either per se or in an aqueous solution emulsified into the oily ingredients. Enough of the thickening agent is used to give the resultant liquid explosive low filtrate rate. Thus there is retarded tendency of the explosive to leak away into permeable earth formations. As a result it may be used in creating fractures in fluid-bearing formations and increasing drainage areas in wells. In fact, such a liquid may be used first without detonation to fracture a subsurface formation in which to place the major part of the explosive. It also has the very advantageous property of causing detonation to progress along the explosive even when it is in relatively thin cracks in the rock. The sensitizer is preferably encapsulated in quite small capsules of a solid slowly soluble in nitromethane and essentially insoluble in the sensitizer itself. The encapsulated sensitizer is added to the other components just before pumping into the well to minimize likelihood of premature explosion.

Description

United States Patent 1 Fast 3,713,915 Jan. 30, 1973 THICKENED NITROMETHANE [75] Inventor: Clarence R. Fast, Tulsa, Okla.

[73] Assignee: Amoco Production Company, Tulsa,

Okla.

{22] Filed: Nov. 23,1970

[21] Appl. No.: 95,005

Related US. Application Data [63] Continuation-impart of Ser. No. 3,511, Jan. 16,

1970, abandoned.

{52] US. Cl. ..l49/2, 149/91, 149/89, 149/38 [51] Int. Cl. ..C06b 19/00 [58] Field of Search ..149/89, 91, 38, 92, 2

[56] References Cited UNITED STATES PATENTS 2.891,852 6/1959 Schaad ..l49/89 3,309,251 3/1967 Audrieth et al. ..149/89 3,375,147 3/1968 Sparks ct al. ..149/9OX 3,419,444 12/1968 Minnick ..l49/9O X Primary Examiner-Stephen J. Lechert, Jr. Alt0rneyPaul F. Hawley [57] ABSTRACT A liquid explosive primarily based on nitromethane contains small amounts of a sensitizer (an organic amine), a thickening agent, and up to 25 percent powdered aluminum. It may additionally incorporate ammonium nitrate, either per se or in an aqueous solution emulsified into the oily ingredients. Enough of the thickening agent is used to give the resultant liquid explosive low filtrate rate. Thus there is retarded tendency of the explosive to leak away into permeable earth formations. As a result it may be used in creating fractures in fluid-bearing formations and increasing drainage areas in wells. In fact, such a liquid may be used first without detonation to fracture a subsurface formation in which to place the major part of the explosive. It also has the very advantageous property of causing detonation to progress along the explosive even when it is in relatively thin cracks in the rock. The sensitizer is preferably encapsulated in quite small capsules of a solid slowly soluble in nitromethane and essentially insoluble in the sensitizer itself. The encapsulated sensitizer is added to the other components just before pumping into the well to minimize likelihood of premature explosion.

2- Claims, N0 Drawings THICKENED NITROMETIIANE EXPLOSIVE CONTAINING ENCAPSULATED SENSITIZER CROSS-REFERENCES TO RELATED APPLICATIONS This application is a continuation-in-part of my application Ser. No. 3,51 l,filed Jan. l6, 1970, now abandoned.

BACKGROUND OF THE INVENTION Hydraulic fracturing of earth formations has been carried out for many years. It increases the drainage area into the well by extending the relatively low pressure zone of the well back through the fracture adjacent the rock matrix which is to be drained. Ordinarily this formation fracturing is carried out by confining in a zone in the well a liquid having reduced tendency to filter into the formation compared with crude oil or water. The hydraulic pressure on the liquid is built up until fracturing occurs. Continued pumping at high pressure causes the fracture to extend as long as the leak away rate is insufficient to cause the pressure at the tip of the fracture to decrease below the breaking stress of the particular rock involved.

Experimentally it was found that the breaking strength of layered earth strata was not constant and was in fact a function of the leak away rate of the fracturing liquid. Thus a material having an extremely retarded filtrate rate against formation rock, such as an extremely viscous liquid, ordinarily causes the rock to fail axially, to produce cracks the plane of which at least approximately passes through the axis of the bore; a so-called vertical fracture.

Ordinarily the material lying on each side of the fracture so created is essentially unchanged mechanically from its condition before fracture, and accordingly, drainage into the fracture of any valuable liquid content of the porous rock (such as oil) depends upon the pressure difference between that in the rock matrix and that in the fracture, and on the permeability, i.e., the flow characteristics of the formation. Thus while the normal fracturing process described above works well in most cases in oil, gas, or water wells, the expected improvement is least in the case of extremely low permeability formations, such as, for example, the Anona Chalk Formation of Texas and Louisiana. For example, in the well-known Pine Island Field, wells drilled around 100 feet away from a well which had been producing for years and which has been fractured in this Anona Chalk Formation frequently had essentially initial reservoir pressure in the pay zone. This indicates that the previously known fracturing techniques have been inadequate in draining such a rock matrix at any substantial distance from the fracture.

Under these circumstances I believe it is desirable to detonate an explosive in the drainage fractures of a well, which both increases the total effective penetration of the fracture, but of considerably more importance, increases the number of fractures by tending to cause the material adjacent the original fracture to be rubbled by the explosion, which exposes previously untapped areas to considerably greater pressure differentials.

I prefer to do this by injecting a unique liquid explosive into fractues and fissures in the pay zone, either natural or artificial, so that the liquid before detonation extends a considerable distance from the bore. The explosive in the bore is then detonated. Of course, such liquid explosive necessarily must have the property of explosive propagation in an elongated and essentially thin, sheet-like form. Also it desirably should have low sensitivity prior to detonation so that it has extremely little tendency to detonate while being transported to the rig or while being pumped into the well. Otherwise excellent liquid explosives have caused difficulty on this score.

I am aware that various liquid explosives have been suggested generally for the use I propose. For example, the Bureau of Mines has experimented with fracturing rock outcrops in which desensitized liquid nitroglycerin was displaced into the fracture system and detonated. Subsequent well data obtained with a downhole camera on this very shallow well (of the order of 625 feet) indicated localized casing damage in the perforated interval, and extensive fracturing. However, liquid nitroglycerin, even when desensitized, is not a material handled with ease by the customary service companies. Also, it has a viscosity at field temperatures not far above water and hence can leak away into the formations.

1 am aware that Chesnut in US. Pat. No. 2,892,405 has disclosed and claimed hydraulic fracturing essentially equivalent to that which I wish to carry out, using sensitized nitromethane which is pumped into cracks and fissures adjacent a bore and detonated from the bore. This is a very much safer type explosive. How- I ever, nitromethane is a very low viscosity material, typically only a few centipoises, as measured for example with the Fann VG Meter Model 35. If maintained in the formation for any ordinary period of time it will leak off into the rock matrix and the desired results cannot be obtained. Furthermore, nitromethane is a sufficiently low viscosity liquid so that it cannot be used for creating additional rock fractures before the detonation, while still in liquid form without pumping at extremely high rates.

The Laurence US. Pat. No. 3,239,305 is also in the field of liquid explosives. The inventor never teaches or contemplates use of a thickening agent. He does mention incorporating into an explosive using sensitized nitromethane, conventional solid diluents or fillers, such as sawdust or diatomaceous earth or liquid diluents such as alcohol or benzene (Column 1, lines 62-65.) These materials are in no sense thickening agents and do not reduce the fluid loss of the liquid explosive, as specifically pointed out in applicant's broadest claim. One can convert the sensitized nitromethane (a low viscosity liquid) to a solid of the nature of dynamite by the use of the conventional solid diluents or fillers, but one does not obtain a thickened liquid explosive with substantially reduced fluid loss to permeable formations, as taught and claimed by Applicant.

Various experimenters at Dow Chemical Company have described and claimed using a slurry of a liquid explosive for hydraulic fracturing purposes in earth formations. Such material is found in US. Pat. Nos. 2,867,172 Hradel, 2,992,912, Hradel and Staadt, and 3,104,706 Eilers and Park. In the first of these, the solid in the slurry is the explosive itself, such as ammonium nitrate, TNT, RDX, and the like. Properly this should not be called a liquid explosive, since the liquid itself serves only as a carrying agent. It has no explosive property. The same comment can be made on the second patent in which the solid is ammonium nitrate, though in this case the liquid (an ammoniacal solution) is essentially saturated with ammonium nitrate. In the third, at liquid explosive is used, a mixture of two liquids, one of which is an oxidizing and the other which is a reducing agent. The solid in this case is a finely divided solid fuel, such as asphalt, carbon or tar.

Other U.S. Pats. from this same source include No. 3,075,463 Eilers et al., No. 3,336,981 Barron et al., and No. 3,336,982 Woodward et al. The first of these teaches use of a liquid explosive including two liquids which are respectively a reducing agent and an oxidizing agent, together with an emulsifying agent for causing these materials to stay together and not separate in the well. U.S. Pat. No. 3,336,981 comments that a number of the earlier patents involved hypergolic mixtures which presented hazardous problems in the surface mixing involved. It teaches that a strong oxidizing agent, for example concentrated nitric acid or fuming nitric or sulphuric acid can be gelled with a specific gelling agent, such gel being injected into the well and the fractures followed by a spacer liquid and then by a liquid fuel which forms a hypergolic mixture when in contact with the acid. The materials mix in the formation, resulting in some cases in spontaneous detonation. The materials also may be detonated by an igniter means. In the last of these patents, U.S. Pat. No. 3,336,982 Woodward et al., a fuel and an oxidizing agent are pumped into well tubing and hence into a formation with suitable spacer plugs between them, part of the fuel being relatively reactive and the other part being relatively active so that detonating conditions are found in the formation and a series of spontaneous explosions ensue.

Practical difficulties were experienced with these systems either in that initial detonation was difficult or hazardous, or more frequently that when the explosive liquid was spread out into a formation fracture, either natural or induced, the detonation did not propagate through the fracture and left considerable undetonated material. Put another way, the critical detonation diameter of the explosives (which measures the propagating ability of explosive in a sheet form) is at least a substantial fraction of an inch. Since ordinarily such fractures are of the order of V; inch or less, improper detonation or no detonation at all was found in these formation fissures. Accordingly, a material having a critical detonation diameter of the order of 1/32 inch is preferable.

It is desirable to develop a liquid explosive having a very low critical detonation diameter, little tendency to leak away into the formation, and combining both low tendency to detonate accidentally with high reliability and high generation of energy per unit volume of explosive.

In the description of my invention there is reference to microencapsulation. A procedure for such encapsulation was given in Advances in Microencapsulation Techniques" by Flinn and Nack, Battelle Tech. Rev. V. 16, n.2, Feb. 1967, p.2. The article Out of packaging Dry Liquid no author given, Modern Packaging, V. 39, n. l 0, June 1966, p. l 22, teaches encapsulation of drops of liquid such as floor polish and cleaner in plastic, the plastic capsules dissolve to release the liquid. A historical review of encapsulation is given in Microencapsulated Catalysts and Resins by Hanny, Haber and Peters, Society of Petroleum Engineering, Reinforced Plastics, Sept. 1966, p. 27, giving reference to the breaking of liquid or solids-containing capsules by heat, pressure, or both, so that the capsule contents can react chemically with the surrounding medium.

U.S. Pat. Nos. 3,264,038 and 3,302,977 describe methods for hard-coating particles, the coating being removed by dissolving in the surrounding medium.

SUMMARY The liquid explosive compositions I have invented are based on the use of a major amount of nitromethane (mononitromethane, CH N0 sometimes sensitized with a few percent (based on weight of the nitromethane) of a basic amine which can form salts with weak organic or inorganic acids. To this is added a thickening agent which greatly increases the dehydration time of the liquid explosive (this is a measure of filtrate rate) or produces high apparent viscosity. To this is preferably added an appreciable amount of powdered aluminum, which acts as a fluid loss control agent, at times as a sensitizer, and which certainly enhances the energy generated per unit volume of explosive. I also may add approximately half again as much by weight of ammonium nitrate as the powdered aluminumj This may be either added directly or may be added as a concentrated aqueous solution which is emulsified into the other materials, in which case the fluid loss into the formation is still further reduced.

i prefer to encapsulate the sensitizer in quite small capsules of a solid only slowly soluble in nitromethane and preferably at least only very slowly soluble in the sensitizer itself. The capsules are mixed in the other components just before pumping into the well so that dissolution of the capsules and mixing of the sensitizer with the nitromethane occurs after the liquid is at least far down the well, and perhaps is already out in the fractures extending from the bore. The procedure of making such small capsules (major dimension of the order of 0.01 inch) is called microencapsulation" and is already well known, as previously discussed.

DESCRIPTION OF THE PREFERRED EMBODIMENT The base material employed in this liquid explosive is mononitromethane, CH N0 To this is added a thickening agent, the primary function of which is to increase considerably either the dehydration time of the resultant liquid explosive or the apparent viscosity of the liquid explosive. In this connection, the dehydration time" is defined as the time required for air under psi pressure to blow through a filter paper closing the bottom of a cylinder originally containing a 600 ml sample of the liquid under test, using the test apparatus and procedure described in APl Code No. 29. This dehydration time for my liquid explosive should be at least three minutes. More preferably the consistency of the liquid explosive should be such that in 30 minutes liquid is still being forced through the supported filter paper. In this case one refers to the volume of collected liquid in 30 minutes as the AP! fluid loss. I prefer an AP] fluid loss of not more than 100 ml.

The apparent viscosity of the liquid explosive made in accordance with my invention is at least 30 centipoises and may be as high as several thousand. Thickened liquid explosives are not ordinarily simple Newtonian liquids, i.e., the rate of shear is not usually directly proportional to the shear stress. This is why one measures apparent rather than actual viscosity, i.e., the viscosity measured is a function of the apparatus used. A commonly used viscosimeter in petroleum production laboratories is a Fann viscosimeter, and, accordingly, the apparent viscosity as specified here is determined by this standard piece of laboratory apparatus. The greater the viscosity of this liquid explosive (as long as it is readily pumpable under field conditions) the better. As mentioned above, apparent viscosities of at least 30 centipoises are desired and preferably from at least l00 to several hundred centipoises is preferred, measured at ordinary room temperature. Viscosities as high as several thousand are still in the usable range, particularly when the liquid explosive is to be pumped at a low linear velocity, which I ordinarily prefer as involving the least risk of premature explosion. When such a high viscosity liquid is placed in fractures adjacent the bore of a well the leakaway rate is quite low compared to that from ordinary nitromethane or sensitized nitromethane, which has an apparent viscosity in the range of 0.6 centipoise.

Several materials have been found satisfactory for thickening agents for nitromethane. One can use, for example, nitrocellulose (dynamite grade) such as that identified as DuPonts D-2 Pyro which will usually be employed in a concentration of about two to about five percent by weight, based on the weight of the nitromethane. (All subsequent reference to weight percent is in terms of the weight of the nitromethane.) A second thickener which can be employed is a mixture of two materials. One is known commercially as Gantrez An-l 39, which is defined more specifically as the water soluble copolymer of ethylvinyl ether and maleic anhydride, having a specific viscosity from 1.0 to about 1.4 in a 1 percent solution in methyl ethyl ketone at C. This is manufactured by General Aniline & Film Corporation. The other material is known commercially as Polyox, defined as long chain polymers of ethylene oxide, having a molecular weight in the range of one million to ten million. This mixture should have at least 40 percent of either constituent and preferably should be in an approximately 50:50 mix. Use of approximately one-half to two weight percent of such mixture (based upon weight in relation to the nitromethane) produces a thickened liquid explosive having an API fluid loss in the range specified above.

A third kind of thickening agent for nitromethane which can be employed is either Bentone 38, or a very close equivalent Bentone 34, both of which are manufactured by National Lead Company. Bentone 38 is an organic derivative of a special magnesium montmorillonite used for thickening or gelling organic liquids. It produces thixotropic gels with high efficiency. It is supplied as a fine, creamy-white powder with a specific gravity of 1.8 and a bulking value of 15.0 lbs/gal. Bentone 34 is a finely divided light cream-colored powder which is chemically dimethyldioctadecyl ammonium bentonite, with a specific gravity of 1.8, bulking value of 15.0 lbs/gal, fineness, less than 5 percent on a 200 mesh screen, and water content of less than 3.0 percent. It also has the property ofswelling in liquid organic systems. Use of about one to five weight percent of either of these materials yields a thickened liquid explosive having the above APl fluid loss characteristic.

If the well in which the liquid explosive is to be employed has a temperature in the pay zone of the order of around 200 F. or higher, the thickened nitromethane liquid explosive may be used without sensitization. In such circumstance the thickened nitromethane is pumped slowly into the well and into the fractures extending into the pay zone and detonate. Under the elevated temperature the detonation is easily accomplished and the detonation will progress throughout the earth fractures loaded with the liquid explosive. Only if the temperature in the pay zone is distinctly less than around 200 is there a necessity to resort to sensitization of the nitromethane.

The nitromethane may be sensitized prior to thickening. As mentioned above, this can be accomplished with several organic amines, and specifically, those sufficiently basic to form salts with a weak acid, either organic or inorganic. I prefer to use ethylene diamine. Others which are found to be very effective are diethylene triamine and triethylene tetramine. Of less effectiveness are Z-amino-methyl-l-propanol furfurylamine, triethylamine, and pyridine. The quantity of the sensitizer to be employed may be from zero to five percent, the smaller quantities being used when the liquid explosive is to be employed in a formation having a temperature of the order of 200 F. or greater. Ordinarily one employs a concentration of these amines which when mixed with the nitromethane will produce an increase in gap in the ordinary USBM card gap sensitivity test, well known in explosive testing, of the order of at least 300, but not more than about 450 mils. For comparison, the fuel-oil-ammonium nitrate mixture now being widely used as a safe explosive has a gap value greater than 1,000 mils, and nitromethane by itself has a gap value of essentially 200 mils.

In order to prevent the liquid explosive from being sensitive at the time it is injected into a well, I can add the sensitizer, such as an amine or hydrozine, to the other liquid components in an encapsulated form. In this case the sensitizer is encapsulated in a solid material that is slowly soluble in the major liquid component, for instance nitromethane. Until the capsule dissolves, its contents are separated from the other liquid components. In the absence of mixing of the sensitizer with such components, there is an extremely low possibility of premature detonation, and the liquid explosive containing the encapsulated sensitizer can be placed quite safely in the well. The major dimension of such capsules should be quite small, of the order of thousandths of an inch preferably, and ordinarily not more than about thirty thousandths of an inch maximum. The wall thickness is of the order of one quarter to one sixth the maximum capsule dimension. Since capsules of such dimensions may be easily forced by flow of the liquid explosive into the fractures connected to the bore of the well, the capsules are distributed by this flow to the zones of interest, where by subsequent solution of capsules in the liquid, the liquid explosive is sensitized and ready for detonation as the shock wave from explosion in the bore propagates through the explosive placed in the fractures.

Even if the capsules dissolve in the bore before the fractures are reached, if the dissolution does not occur before the capsules are at least some distance below the well head, say a few hundred feet or so, safe pumping of the liquid explosive has been achieved.

Numerous solids for encapsulation purposes can be employed, the basic requirements being low solubility in nitromethane and very low solubility in the sensitizer. I prefer to employ such solids as cellulose acetate, cellulose triacetate and the vinyl resins, such as polyvinyl chloride or polyvinyl acetate. The procedure for encapsulation has been discussed in the references given above. No unusual technique is required.

These capsules are made up in advance of use, and are added in the mixing of the liquids at the well or lubricated in at the wellhead. This gives minimum capsule exposure to the nitromethane component above the well tubing and tends to insure that there is minimum possibility of the sensitizer contacting the liquid components until they are at least hundreds of feet below ground.

A thickened explosive compounded as described above works quite satisfactorily in the explosive fracturing of wells. It combines a very low tendency for accidental detonation with high reliability, and a very great decrease in tendency to leak off into the surrounding permeable rock formations underground. rdinarily in use it is placed in an already existing fracture or fractures in the well which may and customarily are already filled to a degree with some type of propping material which may be ordinary sand or special proppant as already taught in the fracturing art.

I prefer to add to the thickened liquid a substantial quantity of finely divided aluminum. Powered aluminum is satisfactory. Up to 25 weight percent can be used, preferably from 10 to weight percent. The aluminum acts in the explosive three ways. it tends to decrease leakage from the fractures to the adjacent rock formations since it builds a filter cake on the fracture walls. It acts to a degree as a sensitizer. Third, it enhances the energy generation upon detonation since the oxidation reaction is highly exothermic.

The reaction occurring at the time of the explosion can be made still more exothermic, and hence greater explosive force can be exerted on the formation walls, if there is added to the above mix of nitromethane, a sensitizer, a thickening agent, and powdered aluminum, and appreciable quantity of ammonium nitrate. This ammonium nitrate decomposes at time of detonation to release oxygen which reacts with the aluminum, and, of course, with the nitromethane itself. Preferably the weight of ammonium nitrate added should be between about I to 2 times the weight of the powdered aluminum added to the nitromethane, typically about 1.5 times.

Since the mixture contains a thickening agent, the viscosity of the mixture is considerably above that of the base ingredients, and as a result, both the aluminum powder and dry powdered ammonium nitrate may be added directly to the basic materials, thus forming effectively a slurry. However, I have also found that a more desirable arrangement for adding the ammonium nitrate is to make a concentrated aqueous solution of this material, using as little excess water as possible, and emulsify this aqueous solution into the nitromethane mixture by the use of a very small amount of a suitable surface active agent. This arrangement offers the additional benefit that, as an emulsion, the resultant mixture is more viscous for the same amount of primary thickening agent than a mixture in which no water was employed.

A large number of surface active agents are known which emulsify an aqueous solution in an oily material, such as sensitized nitromethane. Thus, for example, the isopropyl amine salt of dodecyl benzene sulfonate (sold for example by Atlas Chemical Company under the trade name of 0-3300) may be used as the emulsifying agent in a concentration of the order of to 3 percent of the weight of the aqueous solution of ammonium nitrate. Another suitable arrangement consists in using a nonionic complex mixture which is a surface active agent and has a hydrophile-lipophile balance (HLB) roughly of the order of 5. For example sorbitan monosterate (HLB of 5.9 to 4.7), or sorbitan monooleate (HLB 4.3) which are sold by Atlas Chemical Company under the designation of Span 62 or 60, and Span 80, respectively, are such materials. They may be used in a concentration in the order of '3 to approximately 25 percent by weight of the aqueous liquid. Many other such surface active agents could be employed, the only criteria being low net cost and relatively powerful emulsifying ability.

The following examples cover compositions made in accordance with my invention. In these explosives the weight percent of all ingredients, other than the nitromethane, is stated in terms of percentages by weight of the nitromethane.

Composition No. l

0 to 5 percent Ethylene diamine (sensitizer 0 to 25 percent Powdered aluminum, (sensitizer and fluid loss control agent) Composition No. 2

Primary Ingredient About 1 to about 5 percent 0 to 5 percent 0 to 25 percent Nitromethane, Bentone 38 (thickener) Ethylene diamine (sensitizer Powdered aluminum, (fluid loss additive and sensitizer) Composition No. 3

Primary Ingredient About 0.5 to 2 percent Nitromethane Gantrez AN'polyox 5050 mix (thickening agent) Ethylene diamine (sensitizer Powdered aluminum,

(sensitizer and fluid loss additive) 0 to 5 percent b 0 to 25 percent Composition No.

Nitromethane Primary ingredient Nitrocellulose, dynamite grade About 2 to about 5 (thickener) percent Ethylene diamine (sensitizer) Powdered aluminum (sensitizer and fluid loss additive) Ammonium nitrate, powdered (oxidizer) 0 to 5 percent It) to 25 percent 15 t 37 percent Composition No. 5

Nitromethane Primary ingredient Nitrocellulose, dynamite grade About 2 to about 5 (thickener) percent Ethylene diamine (sensitizer Powdered aluminum,

0 to 5 percent 10 to 25 percent 30 to 75 percent Anyone versed in manufacture of liquid explosives realizes that the detailed compositions listed establishes a range for a large variety of allied compositions which can be manufactured. The major emphasis in this whole development is to replace the known relatively low viscosity liquid explosives having rather high leakaway rates into permeable formations, with liquid explosives having sufficient bodying capacity or viscosity so that they have low leak-away rates, and tend to remain in place in the fractures formed in the formation prior to detonation. Accordingly, the same volume of liquid explosive made as specified above will occupy a larger ultimate volume as one cohesive body in the formation prior to the explosion than the unthickened liquid explosives already known. Accordingly, it will produce a greater shattering effect, which increases the drainage area. Also it has a greater average thickness when present in the fracture, thus requiring less stringent requirements on the critical detonation diameter of the explosive.

Incidentally, the critical detonation diameter of the compositions given above are not greater than approximately 1/32 inch and are in no case greater than l/l6 inch, using the conventional rail test. In this test the liquid explosive is placed in a substantially rectangularly shaped trough in a steel bar, the depth of the trough being great at one end, progressively decreasing to a minimum depth at the other. In the open rail test, no cover is placed over the filled trough; in the closed rail test a flat steel strip wider than the transverse trough dimension is securely mounted over the trough, furnishing an almost exact simulation of an earth channel. In either case, the explosive is detonated from the thick end under specified conditions of temperature (ordinarily ambient) and the trough is subsequently examined to determine the minimum thickness of the liquid at which detonation still occurred. Such minimum thickness is in the case of the compositions specified above, of the order of l/l6 inch to the order of [/32 inch.

On the other hand, it is found that a liquid explosive of any of my compositions has unusually low sensitivity, i.e., that it does not tend to explode under accidental jars, impacts, moderate heating, etc. The explosives made in accordance with my invention are classifiable as ICC Class B explosives.

In use, the liquid explosive as above described is pumped into the well and thence into formation fractures until only a small amount (typically occupying the order of 25 to 50 feet) remains in the wellbore. A detonator is placed in the explosive in the bore, and the explosive is ultimately detonated. Preferably the well is stemmed above the explosive with a plug set above the explosive, surmounted by at least a hundred feet of set neat cement. After the explosion the plug is drilled out, if necessary, the well cleaned out, and placed on production.

I claim:

1. A liquid explosive composition comprising nitromethane to which has been added from about 0.5

to about 5 weight percent based on weight of said nitromethane o a thickening agent capable of increasing the dehydration time (as measured by the test of API Code No. 20) to at least 3 minutes, and to which has been added a small amount (not more than about 5 weight percent) of an organic amine sensitizer encapsulated in capsules with solid walls, said walls being only slightly soluble in nitromethane and essentially insoluble in said sensitizer.

2. A liquid composition in accordance with claim 1 in which said walls of said capsules are chosen from the group consisting of cellulose acetate, cellulose triacetate, and the vinyl resins, and in which the maximum capsule dimension does not substantially exceed thirty thousandths of an inch.

Claims

1. A liquid explosive composition comprising nitromethane to which has been added from about 0.5 to about 5 weight percent based on weight of said nitromethane of a thickening agent capable of increasing the dehydration time (as measured by the test of API Code No. 20) to at least 3 minutes, and to which has been added a small amount (not more than about 5 weight percent) of an organic amine sensitizer encapsulated in capsules with solid walls, said walls being only slightly soluble in nitromethane and essentially insoluble in said sensitizer.