Superfine PETN Thin Layer Slurry Explosive

As is known, certain explosives such as penetaerythritol tetranitrate (PETN) for example, are hazardous when subjected to modest physical abuse from impact or friction forces. Such compounds additionally do not have desired inertness toward crude oil and brine and dilute forms of many other well environmental material. Such materials additionally are hazardous in being highly inflammable.

The slurry of the present invention is a new type of cap-sensitive slurry explosive capable of propagating a higher order detonation in thin layers. The invention provides a relatively safe explosive in a slurry form which may be used in oil well and gas well formation fracturing, rock fracturing for in situ ore leaching, non-nuclear fracturing of oil shale formations, and explosive formation of fire lanes in terrain inaccessible to motorized equipment.

The slurry of the present invention is formulated to resist accidental initiation by adiabatic compression of gas bubbles which may be introduced during loading and handling, and due to the good chemical stability of the explosive formula, the slurry gives a high degree of inertness toward crude oil and brine and dilute forms of most other well environmental materials. The material of the present invention in fires is difficult to ignite, and once ignited, burns quietly with no explosion in the absence of confinement. The ingredients comprising the slurry are non-toxic prior to detonation.

According to the instant invention there is disclosed a relationship between the composition of a slurry comprising superfine particles of PETN in the slurry as compared and contrasted to a completely analogous slurry comprised of fine particle PETN, as is conventionally available. According to the particular improvements taught herein, a study was conducted to clearly differentiate the performance characteristics between superfine PETN and are regular PETN to obtain the effect of using PETN explosive which has an average particle size of approximately 6.5 microns. As is taught herein, both sets of formulations are made with identical ingredients and mixed in identical procedures except for the type of PETN used. As used herein, regular PETN can be described as that available in military grade, Class IV, having a particle size by test on screens as follows:

In distinction, the superfine PETN used in the instant slurry compositions for contrast with the regular PETN has an average particle size of about 6.5 microns in diameter. Such superfine PETN is available from such manufacturers as DuPont, and will hereinafter be referred to as superfine PETN.

It has been found that by employing superfine PETN in the slurry formulations, the propagating thickness is from three to six times smaller than that obtainable with a slurry using regular PETN as a constituent part. This decrease in thin layer propagating thickness is particularly advantageous in applications of oil and gas well stimulation, solution mining, coal and oil shale fracturing and any other application seeking successful explosive stimulation with very thin layers of explosive slurries.

FIG. 1 is schematic representation of the propagation layer thickness versus PETN content for formulations using both the regular and superfine PETN explosive.

In order to correlate the reduction in slurry thickness that could be achieved with the superfine PETN substitution for regular fine particle PETN, the following mixtures were prepared:

The formulations were detonated on text fixtures which were the same for both the superfine and the regular PETN slurries. Since the only purpose of this test was to determine the propagation layer thickness versus the PETN content, as between the regular and superfine PETN, the results shown in FIG. 1 are illustrated for comparison purposes only and not as absolute values for propagation layer thicknesses.

As illustrated in FIG. 1, the uniform detonation, for example, of 25 percent superfine PETN slurry was 0.125 inches while an exactly analogous slurry with larger size particle PETN exhibited a uniform detonation layer thickness of 0.500inches. As can be seen readily from inspection of FIG. 1, which is a logarithmic plot of propagation layer thickness versus PETN content for the two categories of PETN, the fine particle regular PETN slurries represented a uniform detonation layer thickness as shown by lines B, D and F in the text fixture, while the superfine particle PETN slurries exhibited a minimum uniform detonation layer thickness as represented by lines A, C and E.

The formulations B, D, and F and also the formulations A, C, and E were tested on a fixture known as a Benelex 41 in a uniform layer with the fixture having a masonite confining layer. The slurries G and H were tested with an aluminum confinement. The tapered aluminum wedges for compositions G and H provided for an explosive 2 1/2 inches wide by 20 inches long. The wedge was constructed of 6061-T6 aluminum with a milled slot tapered uniformly over the 20-inch length, the aluminum being approximately 3/4 inch thick on the bottom and the sides with a 1/4 inch aluminum top cover. The slurry formulations were detonated with a number 6 aluminum blasting cap, readily available from DuPont, together with a 7.2 gm sheet explosive primer. The sheet explosive primer was used only for convenience in sealing the end of the text fixture in the shots which employed superfine PETN. The sheet explosive primer also insured sufficient primer to detonate the compositions which employed regular PETN, and provided identical initiation for all compositions.

With the wedge configuration the slurry comprising 20 percent of a superfine PETN exhibited a propagation limit of 0.011 inches while the same composition with regular fine PETN exhibited a propagation limit of .068 inches. Therefore, as can be seen from an inspection of the data points of FIG. 1, the employment of a superfine PETN, having an average particle size of about 6.5 microns, resulted in a propagating thickness from three to six times smaller than that obtainable with the regular PETN.

In preparing the formulations, the approximate weight percentages also included a minute amount of cross-linking agent. A satisfactory cross-linking agent was found to be TYZOR LA, a trademark product of the E. I. DuPont de Nemours and Company, Inc., which is generically known as titanium-antimonium lactate. A ten percent solution of this cross-linking agent was added in the concentration of about five drops per 100 gram of slurry just prior to loading into the test assembly. The amount of cross-linking agent may be varied to accommodate greater or lesser amounts of gelling agent, and also to effect a greater or lesser degree of cross-linking, if desired. Likewise, the concentration of cross-linking solution may be varied from about 5% to about 50%. Additionally, the mixtures G and H were partly settled and stirred just prior to the cross-linking operation. In the above examples, the concentration of the ammonium nitrate solution is not critical. A 60 percent solution is convenient to use because the ammonium nitrate stays completely in solution above 54.degree. F, which means that handling is simplified over, for example, an 80 percent solution which has a solidification temperature of about 136.degree. F; thus requiring expensive heated storage. After blending with an anti-freeze agent, such as diethylene glycol, the solidification temperature is decreased. In the above mixes A - J, the sample weight percentages for each individual constituent part are as shown prior to adding the cross-linking agent.

These formulations are then cap-sensitive explosives capable of propagating a high order detonation in thin layers, i.e., one thirty-second of an inch when confined between masonite. More significantly, there is shown herein a relationship between the use of superfine particle PETN, where the 6.5 micron diameter particles allow for reduction of three to six times in the thin layer propagation layer thickness as a function of PETN content. These formulations are able to detonate completely in thin layers at temperatures of approximately 200.degree. F while simultaneously exposed to a nydrostatic pressure of 10,000 psi or above and do not cause handling problems since there is no liquid explosive ingredient. It has been found that the slurry explosives are not detonated by British 303 ammunition from a distance of 75 to 100 feet even when backed by steel or aluminum. Such slurry explosives will slowly decompose in a bon fire when unconfined, but by themselves will not support combustion. Therefore, the purpose of this invention is to teach a relationship between the use of superfine [extremely small] particle PETN for the reduction of detonating layer thicknesses for particular applications. This control of detonation layer thickness as a function of the composition constituents allows the explosive to be tailored to explosive fracturing of oil and gas reservoirs in order to increase formation permeability and other such controlled applications. In such applications it is critical that the explosive propagate in thin layers to be effective to stimulating oil and gas wells, solution mining and coal and oil shale fracturing operations.

Ordinarily PETN (penethaerythritol tetranitrate) is considered a hazardous explosive which is known to explode when subjected to modest physical abuse from impact or friction forces. For example, in a test apparatus a small (approximately 1/20 of a gram) sample of sensitized small particle PETN when placed on a hard tool steel anvil and impacted by a free fall hammer (or hard tool steel and weighing 2.143 kg), a detonation of the PETN will occur when the drop of the hammer is only 2 cm. In other words, the impact sensitivity of the pure dry PETN is 4.3 kg-cm. However, when this same PETN is compounded into a slurry explosive form where the PETN comprises upwards to 45 percent of the total weight of the mixture, the resulting explosive mixtures as shown herein are unaffected by repeated hammer drops, even from a drop height of 63.5 centimeters. Thus, in slurry form PETN has been found to be not exploded by an impact of 136.3 kg-cm. This is more than thirty times the energy at which the PETN alone explodes. As taught herein, by particularly using a slurry wherein the PETN particles have an average diameter of about 6.5 microns, the advantages of a slurry can be further maximized for very thin layer applications. It has been found that the superfine PETN can be shown to have a significant effect on the propagation thickness of the slurry. As particularly illustrated in FIG. 1, for the same composition the use of superfine PETN particles results in a three to six times reduction in the propagating layer thickness. As has also been shown, this reduction in thickness of layer capable of supporting detonation occurs over a wide range of formulations, anywhere from 15 to 50 percent by weight of PETN in slurries.

The sensitized superfine PETN as employed herein may be made according to the method taught in my copending application Ser. No. 434,753, filed Jan. 31, 1974. While we do not in anyway wish to be limited by theory, it is presently believes that it is a unique structure and not particle size alone which imparts significantly improved detonation properties to the Superfine PETN and thus to the slurry explosives prepared therefrom, and described in the instant invention.

When viewed under an optical microscope the particles of Superfine PETN can be seen to contain a high density of optical barriers. Furthermore, when examined under very high magnification by electron microscope techniques the Superfine PETN particles are seen to contain a large number of apparent void spaces, estimated at exceeding 1000 voids and even exceeding 2000 voids for the average diameter -particle of about 6.5 microns. These voids have an apparent diameter of about 0.18 to 0.2 micron.

It is theorized that these microvoids or crystal imperfections are in large part responsible for the improved detonation properties evidenced for Superfine PETN, in that they serve as reflection sites for an incident shock wave, and that the resultant collisions of the reflected shock waves pump heat energy into and thus shorten the detonation reaction zone, and at the same time reduce the cross-sectional area of explosive required to sustain a stable detonation.

The improved slurry of the invention herein may also be used to produce a low detonation flame temperature composition by deleting the oxidigen components - i.e. the solid crystalline nitrates of Groups I and II of the Periodic Chart of the Elements. Additional antifreeze agents such as dictrylene glycol may be needed to compensate for the additional water.

Additional compositions were prepared in which the oxidizer component was omitted and therefore comprised PETN, gelling agent, diethylene glycol, and water, along with Tyzor.RTM. LA cross-linking agent. The amount of PETN was varied from 25 per-cent to 40 per-cent and the diethylene glycol from 9.5 to 25 per-cent. The amounts of gelling agent and cross-linking agent were similar to their compositions described above.

These compositions were loaded into plastic tubing having inside diameters of from three-eighth inch to five-eighth inch with a wall thickness of one thirty-second inch to one-sixteenth inch and the loaded assemblies were found to detonate when initiated by an electric blasting cap. When laid out among dry brush and pine duff type fuels, these assemblies did not cause fires when detonated, whereas the oxidizer containing compositions cited above did start fires in every instance.

It has now been demonstrated that the Superfine PETN remains effective whether it has been dried or whether it has not been dried prior to incorporation into the slurry explosive formulation, and thus it must be assumed that the void spaces are effective in their function whether they define a gas phase or a liquid phase.

Within these slurries it is contemplated that the ammonium nitrate may be replaced by any of potassium, barium and sodium nitrate, and diethylene glycol may be substituted by formamide, dimethyl formamide or other glycols and alcohols provided their vapor pressure with temperature is substantially no greater than the aforementioned compounds of water, and further provided that these glycols or alcohols are at least partly miscible with water in combination with other ingredients in the formulation. Further the guar gum or Jaguar HP-8 may be replaced by polyacrylamide or other gelling agent such as familiar to those skilled in the art.

Manifestly, minor changes can be effected in the above-described compositions without departing from the spirit and scope of the invention as defines and are limited solely by the appended claims.