Low Detonation Velocity Explosive Composition

This invention relates to a class of low detonation velocity explosive composition exhibiting a small shock wave component based on total energy release. Such compositions have been found suitable for well stimulation inclusive of water, oil, and gas wells since they maximize fissurization while minimizing well bore damage and compression of the surrounding area.

While the technique of oil well stimulation or revival through the use of explosives such as nitroglycerin is at least 120 years old, water well stimulation using this general technique is even older, the results obtained still remain speculative in nature, success being far from assured. This is due mainly to a lack of knowledge concerning the surrounding geological structure at the active level or "pay zone" of deep wells, and also due to difficulty in assuring use of a correct amount of explosive to enlarge the well bore and uniformly open the surrounding geological formation, rather than compacting such surrounding formation and thereby decreasing its permeability to flow. In addition, it is very desirable if the amount of debris in the well bore can be minimized to avoid expensive follow up "well bailing" procedures.

Insofar as the explosives are concerned, it has been assumed, historically, that controlled amounts of high explosive material, such as nitroglycerine and TNT can best do the job. This assumption is undoubtedly due to extensive field testing and general experience with such explosives for shallow excavation such as quarry, and road cut work.

Such assumption is found to be incorrect, however, when detonation is carried out in a deep well with little overburden movement. Here, high explosives cause the nearby rock to yield (i.e. plastic flow) and the surrounding area to severely compact and then partly unload elastically, resulting in a somewhat larger well bore cavity surrounded by a residual stress field or stress cage in which deformed rock and the fines produced by the explosion are sufficiently compressed and impermeable to seal off or severely restrict the flow of gases or liquids into or out of the surrounding formation. This result clearly frustrates the purpose of the "shoot."

By way of further explanation, the detonation pressures of most high explosives are found to be far in excess of the yield stresses of the surrounding rock and, therefore, capable of causing a substantial amount of the abovedescribed irreversible plastic deformation of the surrounding rock.

In the area further away from the well bore, however, the amplitude of the stress wave caused by the explosion is mitigated by geometrical divergence effects and by other dissipating factors. Here the rock is initially displaced and then tends to return to its original position. Such return is prevented, in part, by the permanently deformed area surrounding the well bore to create the above-stated region of residual stress. In the absence of such a residual stress field and containment of the explosive gases, the resulting gasses would be expected to move into surrounding fractures and further extend them on a 360.degree. range into the surrounding untouched formation.

Formation of the above-described phenomenon can occur with the use of high explosives of widely varying charge sizes.

The stated problem has not been solved but has been minimized with varying degrees of success, depending upon (a) the surrounding geological formation, (b) the amount and placement of charge(s), and (c) the spontaneous opening up of leakage pathways into surrounding formations due to subsequent spontaneous break up of the newly formed stress field. The latter, of course, is not predictable or expected in all formations.

Placement of a charge below the "pay zone" and through use of the well bore itself as a gas or liquid flow pathway into the "pay zone" has provided some measure of control and predictability in well shooting, the most promising approach, however, appears to be achieved by charge shaping, coupled with the use of specialized propellant-type explosives which produce a maximum pressure less than the yield stress of the surrounding rock. Such compounds produce a flame front traveling more slowly than the speed of sound, and the underlying chemical reaction lags behind the flame front; as opposed to high energy explosives, which have a detonation wave which travels faster than sound and the bulk of the chemical energy is quickly released behind the detonation wave shock front. In both cases, the total chemical energy release is approximately equivalent or slightly less than that experienced with a propellant type explosive.

It is an object of the present invention to obtain an explosive composition which possesses desired propellant-type characteristics and which can successfully induce multiple fractures around a selected part of a well bore hole, while minimizing well bore hole damage and formation of a residual stress field.

It is a further object to fully utilize the benefits of a propellant-type pressure pattern while maintaining the gas generating properties of a high explosive such as

(a) low peak pressures,

(b) a shock energy comparable to a propellant deflagration,

(c) gas formation comparable to that obtained by an explosive detonation, and

(d) a substantial total energy output while still retaining cost, convenience, and packing efficiency of art recognized high explosive compositions.

It is a still further object of the present invention to minimize formation of a residual stress field and well bore hole damage during a well shoot operation

The above objects are achieved in accordance to the present invention by placing and detonating at least one explosive charge of low detonation velocity of a composition comprising

(a) an ester component of the formula

in which R and R.sup.1 are separately and individually defined as a lower alkyl group of equal or unequal chain length, inclusive of a 4-8 carbon alkyl group such as ethyl, butyl and octyl groups; A is defined as the nucleus of a substituted or unsubstituted divalent aromatic group such as a phenyl or naphthyl group, including phenylene, methyl-phenylene and napthylene moieties; R" is an alkyl group of 1-2 carbon atoms such as a methyl or ethyl group; R'" is an alkyl group of 3-8 carbon atoms; such may also contain 0-2 hydroxyl substituent groups; Ac is an acetyl group; and m is a whole number of 1-3; with

(b) a component comprising a 40-60:60-40 mixture by weight of metriol trinitrate: diethylene glycol dinitrate; and

(c) an active amount of at least one organic stabilizer component exemplified by soluble 2-nitro-diphenylamine or diethyl-diphenylurea (Ethyl Centralite), a useful ratio by weight of (a) -to- (b) for purposes of the present invention being about 9-20:91-80 and preferably 9.8-18.3:90.2-81.7 to obtain the necessary ratio between released explosive energy expressed as shock wave (S) and that expressed as gas or bubble expansion (G). Optimally the ratio of (S)-to-(G) for present purposes is kept within the range of about 20-30% (S) to 80-70%(G) to assure a maximum area of fracture with minimum amount of well damage, and minimum formation of surrounding impermeable compacted material (i.e. Residual Stress Field).

Low detonation velocity composition(s) in accordance with the present invention, when utilized in accordance with normal art-recognized well-shooting practices and equipment are found to possess a slow detonation velocity within a range of about 1200 meters/second to about 2500 meters/second and, preferably, within a range of about 1200-2100 meters/second, capable of obtaining the desired breakdown between shock wave energy(S) and gas expansion energy(G). Such compositions are found to be of particular value when used at depths in excess of 200 ft., where overburden movement is minimal or nonexistent. Such can be successfully used, for instance, at a water head pressure of 400-600 psi or higher and at a useful operating temperature range varying from about 110.degree. F. to about -22.degree. F.

Suitable components for purposes of the present invention are obtainable as follows:

(a) Ester component such as a di-lower alkyl ester of terephthalic, isophthalic, homophthalic acid and naphthalene 1,4 dicarboxylic acid can be obtained by direct reaction of the dicarboxy acid with a desired lower alkanols such as a 4-8 carbon alkanol to obtain symetrical and non-symetrical esters such as the octyl/octyl and butyl/octyl ester.

The above reaction can be conveniently carried out, for instance, by direct refluxing of phthalic anhydride with butanol, octanol or combinations thereof in desired amounts.

The esters are obtainable commercially from Reichhold Chemicals, Inc. and U.S. Steel, Chemical Division, and the tricresyl phosphate can be synthesized, for instance, by direct nitration of a corresponding Cresol intermediate using art-recognized processes.

Corresponding polyhydroxy esters of natural oils and such as triacetin are also obtainable commercially through Armek Company Industrial Chemical Division and Eastman Chemical Company.

(a) The 40-60/60-40 mixture of metriol trinitrate/diethylene glycol dinitrate is conveniently obtained, for instance, by co-nitration of the corresponding trimethylolethane and diethylene glycol with a mixture of sulfuric and nitric acids, using excess nitric acid, in the manner disclosed in U.S. Pat. No. 4,352,699 by E. H. Zeigler.

(b) The above-listed organic stabilizers suitable for use in the present invention and similar art-recognized components are commercially available, for instance, from Van de Mark Chemical Company, Inc.

Additional additive components known to the art such as sensitizers, desensitizers, gelling agents and thickening agents such as nitrocellulose or nitrocotton, woodflour, and propping agents also may be included, as desired, within compositions of the present invention to better adapt the composition to widely varying ambient and geological conditions, and to favor efficient introduction into the water, oil, or gas-bearing strata.

The following Examples further illustrate certain preferred embodiments of the instant invention.

Seven and three tenths (7.3) pounds of commercially obtained 99.6% dioctylphthalate from U.S. Steel Company, Industrial Chemicals Division and about one-half (0.5) pound of diethyl-diphenylurea obtained commercially as "Ethyl Centralite" obtained commercially from Van de Mark Chemical Company, Inc. are admixed in a 5 gallon stainless steel reactor maintained at 20.degree. C by a temperature control jacket. To this mixture is slowly added 42.2 pounds of 40/60 ratio MTN/DEGDN (metriol trinitrate/diethylene glycol dinitrate) obtained in accordance with the procedures set out in U.S. Pat. No. 4,352,699 of E. H. Zeigler, and the components allowed to remain at 20.degree. C. for about twenty (20) minutes. The resulting liquid product is found to have excellent flowability characteristics at +68.degree. F. and molasses-like characteristics at about -22.degree. F.

The resulting composition is tested for impact sensitivity using a standard Picatinny Arsenal-type of explosive impact testing apparatus with 0.1 gm of explosive and 2 Kg impact weight, and tested for velocity of reaction, using a four (4) inch diameter charge under actual detonation conditions. For the later purpose, a detonating cord downline (25 grain/ft) is used with a 1 pound booster of commercially available high brisant explosive (7000m/sec) for each 10 feet of test charge column. The test results are reported in Table I infra.

Example 1 is repeated using 7.3 pounds of dibutylphthalate and the test results evaluated as before and reported in Table I.

Example 1 is repeated using 7.3 pounds of dipentylphthalate and the test results evaluated as before and reported in Table I.

Example 1 is repeated using 7.3 pounds of dihexylphthalate and the test results evaluated as before and reported in Table I.

Example 1 is repeated using 7.3 pounds of diheptylphthalate and the test results reported in Table I.

Example I is repeated using 7.3 pounds of tricresyl phosphate in place of dioctylphthalate and the results evaluated and reported in Table I.

Example I is repeated using 7.3 pounds of triacetin in place of dioctylphthalate and the results evaluated and reported in Table I.

Example I is repeated using 0.5 pounds of Ethyl Centralite and 49.5 pounds of MTN/DEGDN but without the use of an ester "(a)" component, the results being evaluated as before and reported in Table I.

A gelled version of the Example I product is prepared using a brass Schrader Bowl maintained at 20.degree. C. by gently admixing the MTN/DEGDN component (76% by weight total composition) with dioctylphthalate (11% by weight) followed by 0.5% by weight of the Ethyl Centralite stabilizer and 4% by weight of nitrocellulose (nitrocotton). After thorough mixing, the remaining ingredients, i.e. Cab-O-Sil; (0.5%), wood flour (6%) and starch (2.5%) are mixed in, and the mixture permitted to stand for 18 hours at 20.degree. C. to gel. The resulting product is packaged in 4 inch polyethylene bags and tested for impact sensitivity (90 cm drop/2 Kg 50% detonation and reaction velocity in the manner of Example I, the results being reported in Table II below.

Example IX is repeated, employing 0.5% by weight of microballoons obtainable from Union Carbide, Inc., as UCAR phenolic microballoons in place of Cab-O-Sil. The packaged product is tested for impact sensitivity and reaction velocity, a 50% detonation rate being obtained at slightly over 100 cm travel length using a 2 Kg striker and 0.1 gm charge. Reaction velocity is reported in Table II below.

Example IX is repeated without the dioctylphthalate ester component, the tests being carried out as before to obtain an impact sensitivity of 50% detonation using a 2 Kg striker and a 0.1 gm charge at 69 cm. The reaction velocity is reported in Table II.

Example X is repeated without the dioctylphthalate ester component, the tests being carried out as before to obtain an impact sensitivity of 50% detonation using a 2 Kg striker and 0.1 gm charge at 98 cm. The reaction velocity is reported in Table II.