NO119829B - - Google Patents

Description

Non-self-detonating plastic explosive.

The invention relates to an improved plastic explosive. Plastic explosives have been difficult to detonate due to the high heat capacity of the water, which in most cases is used as a base in the liquid phase. A certain improvement in sensitivity has previously been achieved by incorporating voids in the form of air, foamed ammonium nitrate or other materials containing gaseous substances. However, such explosives have failed to retain sufficient sensitivity when used under elevated pressure.

When detonation under pressure is required, such as in underground blasting, it has been practice to use high-explosive products, e.g. nitroglycerol, or plastic explosives containing self-detonating organic explosives, e.g. TNT. However, it is obvious that such explosives are dangerous to handle, and in some cases incomplete detonation of the explosive in a borehole can occur.

US patent 2,671,400 relates to a shaped cartridge explosive, which, due to its special design, makes it possible to determine the direction of the main effect of the explosive action upon detonation. The cartridge has been given such a shape that it provides an area with reduced density

for the explosive effect that follows. Generally, an explosive charge is provided, e.g. dynamite, gelatin explosive, liquid nitroglycerol or TNT, in a cartridge filled with air-filled cells or globules made of plastic, aluminium, glass or the like. It appears from the patent document that the air-filled particles provide air that helps the explosive effect of the explosive charge in the casing when it is detonated under elevated pressure. The effect of the air-filled particles is to direct the explosive force upon detonation in a predetermined direction. The cartridge can be provided in different forms, depending on the desired use. According to the patent, however, only self-detonating explosives are used, which, as mentioned, are dangerous to handle.

US Patent 3,101,288 describes the use of certain specific hollow balloons of water-insoluble, thermosetting, synthetic resin in explosives of the dynamite type, here also only explosives of the self-detonating type.

A plastic explosive has now been found which is harmless to handle and which has the ability to spread practically completely when detonated under pressure, by incorporating hollow glass particles of a certain size into the explosive. The hollow glass particles are characterized in that their size varies from 10 to 250 µm, and they should have sufficient wall thickness so that the particles are not crushed before detonation. It has been found that particles whose size varies as indicated, and which have a particle density of from 0.1 to 0.9 g/cm 3 , usually have sufficient strength to withstand pressures of at least 70 kg/cm 2 Particles with this density are therefore preferred.

In order to ensure practically complete propagation upon detonation, the hollow glass particles are used in an amount sufficient to give a density of from 1.0 to 1.5 g/

cm^ in the product. In general, but depending on the particular explosive used, the amount of glass particles required falls,

in the range from 1 to 20% by weight of the total product. It has been found that practically constant densities can be maintained at

pressure of up to 1400 kg/cm 2 at the same time that no harmful loss of sensitivity occurs when hollow glass particles are incorporated into plastic explosives, and thus plastic explosives are obtained that can be detonated under elevated pressure without the use of sensitizing agents which themselves consist of a self-detonating material. When using the explosives according to the invention, these are placed in a borehole that goes underground, or are preferably pumped into cracks in a rock formation and detonated.

In practice, the improvement according to the invention can be used

in connection with any of the known plastic explosives whose components will not have a detrimental influence on the properties of the hollow glass particles. The products according to the invention contain one or more inorganic oxidizing salts, one or more non-self-detonating fuels, water and usually also a gelling or thickening agent.

Examples of salts used in connection with the invention are ammonium nitrate, alkali metal and alkaline earth metal nitrates, perchlorates and mixtures thereof. Specific examples of typical salts are ammonium nitrate, sodium nitrate, calcium nitrate, sodium perchlorate, potassium perchlorate and the like. Usually the explosive contains from 10 to 90% by weight, calculated on the total composition, of the inorganic oxidizing salt. Preferably, ammonium nitrate and sodium nitrate are used, with ammonium nitrate being used in amounts of from 30 to 90% by weight of the total composition and sodium nitrate in amounts of from 0 to 30% by weight of the total composition.

Typically, the improved explosive uses a non-explosive carbon or particulate metal fuel. Non-self-detonating fuels are preferred, although self-detonating such as e.g. organic nitrate or nitro compounds, nitramines or explosive nitrocellulose can be co-used under certain conditions to provide the necessary speed, increased sensitivity at low temperatures and similar properties.

Examples of carbonaceous non-explosive fuels include finely divided coal, carbonaceous vegetable products such as corn starch, sugar or wood pulp, organic liquids such as hydrocarbon oils, fatty oils or vegetable oils, urea, crude oil or other liquid hydrocarbons and preferably water-soluble petroleum liquids of low volatility. In general, up to 20% of the non-explosive carbonaceous fuels are used

fer in the products and preferably from 5 to 15%.

As indicated, particulate meta11 fuels can also be used, either alone or in a mixture with other fuels. Metallic fuels that can be used in the new product include aluminium, magnesium, silicon, alloys of aluminium, silicon and magnesium, iron, alloys of iron, ferro-silicon, ferro-phosphorus and mixtures and alloys of these. The particle size of the metal is usually in the range from 8 to 200 mesh (0.074-2.38 mm) and preferably from 20 to 200 mesh (0.074-0.84 mm). About. 50% by weight metallic fuels can be used in the invention. However, preferably only from 10 to 30% is used.

As indicated, water-bearing products can be used in the invention. Usually up to 25% water can be used. However, preferably only from 5 to 15% water is used and usually at least 8%.

Various thickeners can be used in the invention, and the resulting products can vary from pourable solutions, slurries or dispersions to thick, castable plastic masses. It is preferred that the explosive is kept in such a state to ensure a stable mixture where the various components are kept in suspension for significant periods of time and which are pumpable without the need for additional pump pressure. Different thickening and gelling agents can be used. Those commonly used include natural rubber, e.g. gum arabic, guar gum or karaya gum and synthetic polymers including polyacrylamides and polyvinyl alcohols. In general, from 0.1 to 10% by weight and preferably from 0.5 to 5% by weight of a thickener is used, based on the total weight of the product, depending on the particular agent chosen and on the desired consistency of the explosive.

The invention is particularly adapted to underground blasting and especially at depths where elevated pressure is encountered. In particular, it finds application when blasting oil and gas wells to provide passage from a borehole with the intention of increasing the production of oil or gas from the well.

Blasting of oil wells can be accomplished by placing pre-formed charges in the borehole. However, it is preferred to pump explosives of a plastic type into the borehole under pressure to give the explosives the opportunity to penetrate cracks in the rock and thus achieve a more complete and effective blasting of the rock layers at a greater distance from the borehole.

Plastic explosives are compressed under the elevated pressures encountered at depth in most oil wells, especially those with a high hydrostatic head. For example, the pressure at 300 m below a hydrostatic spring is equal to approx. 35 kg/cm 2. Until now, plastic-type explosives could not be fired at this pressure, even using extremely powerful boosters, and even plastic explosives containing self-detonating fuel can be ineffective at such pressures.

The method using an explosive according to the invention consists in placing the plastic explosive containing hollow glass particles in an underground formation, either in pre-formed charges or by preferably pumping plastic explosives into the borehole and into cracks in the geological formation which surrounds the borehole. A small amount of explosive is usually added back to the borehole to aid in the ignition of the explosive mass. The borehole is usually filled with a liquid, e.g. water or binder, and the explosive is detonated with a suitable detonating charge. Due to the presence of the hollow glass particles in the explosive, the explosive can be detonated with virtually complete propagation using standard detonating equipment.

The pressures encountered in underground formations such as in oil wells, can range from 7 to 1400 kg/cm 2 or more, depending on the depth of the formation. When using the new explosives according to the invention, the blasting can be carried out successfully using plastic explosives which are of a non-self-exploding nature.

A particularly suitable explosive for the mentioned improvement, and which can be used for underground blasting, contains by weight up to 60% alkali metal nitrate, ammonium nitrate from 10 to 85%, particulate aluminum from 5 to 40%, water from 5 to 25%, water-swellable gelling agent or thickener from 0.2 to 2.5%. This improvement includes the addition of hollow glass particles to the explosive. For complete propagation, the density of the explosive should be kept below 1.4 g/cm^ for 30% metal and at 21°C. Lower metal concentrations and lower temperatures normally require reduced density.

Preferably, the product contains from 5 to 40% sodium nitrate, from 15 to 80% ammonium nitrate, from 10 to 30% particulate aluminum, from 6 to 16% water, from 2 to 16% liquid organic, water-miscible liquid diluent, from 0.5 to 2.5% of water-swellable rubber and a cross-linking agent, as well as hollow glass particles with a particle density of from 0.1 to 0.9 g/cm 3 and with a size • varying from 10 to 250 , u and i sufficient amount to keep the density of the product in the range from 1.1 g/cm 3 to 1.4 g/cm 3 , which usually necessitates the use of from 2.0 to 10% by weight of hollow glass particles.

Preferably, the aluminum has a particle size of from 30 to 150 mesh (0.104-0.59 mm). Generally, the aluminum metal used has a particle size and caliber distribution as indicated in Table I.

Preferably, the metal is in the range from 0.625-125^u.

Aluminum here means aluminum and aluminum alloys that contain at least 60% aluminum by weight.

Liquid organic diluents which may be used include low molecular weight monohydric alkanols, ethylene glycol, propylene glycol, glycerol, formamide and other similar diluents which preferably have such a carbon/oxygen ratio that there is no detrimental competition between the carbon and the metal for attainable oxygen in the system. Generally, methanol, ethylene glycol, glycerol, formamide or mixtures of these are preferred in connection with the invention.

Any water resistant natural rubber or a synthetic gelling or thickening agent can be used. Examples of particularly suitable agents include guar gum, karaya gum and mixtures thereof, and a number of conventional cross-linking agents can also be used.

Many different improved explosives can be made by incorporating a sufficient amount of hollow glass particles with the unique properties described herein to give the particular product a density best suited for complete and rapid detonation of the explosive. Usually, from 1 to 10% by weight of the hollow glass particles is necessary. Preferably, an excess of hollow glass spheres is not used beyond what is necessary to provide the desired density, since an excess may provide unnecessary improvement in sensitivity and may actually produce less effective explosives.

The following examples illustrate the invention.

Example 1

A plastic explosive was produced with the following composition:

The explosive was prepared by first dispersing the gelling agent in propylene glycol. Water and formamide were mixed and added to the mixture of propylene glycol and gelling agent. Then sodium nitrate and ammonium nitrate were added and mixed. The hollow glass particles were then added to the mixture, and the entire mixture was warmed to room temperature, after which the particulate aluminum was added and mixed. Finally, the cross-linking agent was added and the explosive was allowed to gel. They hu-

le glass particles had a size that varied from 10 to 250 µm and had a particle density of approx. 0.26 g/cm 3. The final explosive mixture had a density of approx. 1.1 g/cm 3. The explosive was then tested for detonation properties under elevated

they press.

The explosive was placed in a standard steel pipe with a diameter of 5 cm which had a cover, a welded jacket and a pressure pipe connection. The cover was sealed with a standard neoprene gasket.

A 37 gram high-explosive incendiary was also placed on the bottom

of the steel pipe. The explosive was separated from the cover by a plastic disk to prevent oil from mixing with the explosive before detonation.

The steel pipe was buried in the ground with a small shaped charge close to the amplifier. The pressure in the steel tube was raised to 39.2 kg/cm 2 and the shaped charge was detonated.

Inspection of the vessel showed that it was completely cracked, the jacket had broken, which made it probable that the entire explosive charge had detonated.

Example 2

Some of the same explosive was used in another test which was carried out in much the same way as described in Example 1. For this test a pressure of 66.5 kg/cm<2 > was applied to the product before igniting it shaped charge.

The explosive charge detonated again completely.

Comparative example 1

An explosive with the following composition was produced in a similar way to that described in the production of the explosive in example 1:

A further 1% expanded polystyrene beads were added to this explosive to reduce the density of the mixture to approx. 1.3.

The mixture was detonated in the same way as described in example 1 and at a pressure of o 7 kg/cm 2. The explosive failed with regard to complete propagation.

Some of the explosive described here, but without beads, was whipped to introduce air. The mixture failed in terms of propagation when subjected to a pressure of 7 kg/cm2.

Claims (1)

Non-self-detonating, plastic explosive based on inorganic oxidizing salts and capable of practically complete propagation when detonated under pressure, characterized in that it contains hollow glass particles of sizes varying from 10 to 250/U and with a particle density varying from 0.1 to 0.9 g/cm , in an amount sufficient to give the explosive a density of from 1.0 to 1.5 g/cm 3 .