Earth boring – well treating – and oil field chemistry – Well treating – Contains organic component
Reexamination Certificate
1998-05-29
2001-05-01
Alexander, Lyle A. (Department: 1743)
Earth boring, well treating, and oil field chemistry
Well treating
Contains organic component
C507S201000, C507S267000, C507S276000, C166S270000, C166S300000, C428S403000, C428S404000, C428S407000
Reexamination Certificate
active
06225262
ABSTRACT:
FIELD OF INVENTION
This invention relates to encapsulated breakers. More specifically, this invention relates to slurries of encapsulated breakers.
BACKGROUND OF THE INVENTION
Hydraulic fracturing of oil subterranean formations is a well known technique for increasing the permeability of subterranean formations. In this technique, a viscous fluid (“fracturing fluid,” commonly an aqueous fracturing fluid, most commonly guar) is introduced to the wellbore, pressure is applied to induce fracture, and proppants in the fluid (most commonly sand) maintain the fractures in an open state. The viscous fluid must then be removed, and oil is harvested from the thus opened subterranean formation. In order to facilitate the quick removal of the fracturing fluid, chemicals are used to reduce or “break” the viscosity of the fracturing fluid; these chemicals (most commonly oxidizers, and in particular persulfates) are known as “breakers.” The chemical reaction of the breaker with the fracturing fluid is undesirable prior to completion of the fracturing operation. Therefore, it is advantageous to encapsulate or coat the particles with a polymer to delay the release of the breaker, and hence to delay the breaking of the fracturing fluid.
Many materials have been used in the art to encapsulate breakers for fracturing fluids. For example, U.S. Pat. No. 4,506,734 (Nolte) describes a breaker within a crushable glass or ceramic coat that ruptures upon closure of the induced fractures. U.S. Pat. No. 4,741,401 (Walles, et.al.) teaches that a polymer can be applied to a solid breaker chemical, most preferably by air suspension coating. The polymers of Walles are most typically homopolymers and copolymers of polyolefin and ethylene oxides. This patent describes the release of the breaker by rupture of the membrane, either by the force of closure of the fractures within the subterranean formation or by the osmotic pressure of water diffusing into the shell. In U.S. Pat. No. 5,164,099 (Gupta, et.al.), a polymer is applied to a solid particle of a breaker chemical by interfacial polymerization. Typically, this polymer is a polyamide or a crosslinked cellulosic material. This patent states that breaker is released from the capsules described therein by diffusion through the membrane of the encapsulation.
U.S. Pat. No. 5,591,700 (Harris, et.al.) relates to encapsulated breakers that are coated by surfactants that are solid at ambient surface conditions and which dissolve at elevated temperatures in the subterranean formation. The surfactants are mixed in from the dry state.
Alkyl-2-cyanoacrylate monomers polymerize immediately in the presence of a weak base, and as such have been widely used for encapsulation and particle coating in the pharmaceutical industry, primarily for the purpose of drug delivery. For example, Kante et al. (Int. J. Pharm., 1980, 7, 45.) have described a method for preparing actinomycin D nanoparticles using poly(butylcyanoacrylate).
U.S. Pat. No. 4,452,861 (to RCA Corporation) describes a method for coating luminescent, inorganic phosphors using polymeric cyanoacrylates. The procedure outlined involves a five stage process which requires complete evaporation of the nonaqueous solvent during each of the coating stages and a final step which calls for washing of the coated particles.
SUMMARY OF THE INVENTION
An encapsulated breaker slurry composition is provided for reducing the viscosity of a fracturing fluid. The breaker is enclosed within a coating and is provided as a slurry, together with a liquid that is selected such that the overall slurry has a flashpoint above 93° C. The liquid is also selected such that it does not facilitate release of the breaker from the coating. The slurry also contains a suspension aid in an amount sufficient to maintain the suspension of the encapsulated breaker in the liquid. Methods of use of these encapsulated breaker slurry compositions are also provided.
DETAILED DESCRIPTION OF THE INVENTION
While certain encapsulated breakers have been utilized before in the prior art, a new format for delivery of encapsulated breakers has been discovered, which format provides surprising advantages over previous formats. It has been found that providing encapsulated breakers in a slurry format has distinct advantages, because these compositions are easier to ship, mix and pump than prior art compositions. Because the slurry composition preferably contains surfactant, the fracturing fluid will readily flow from the fractured formation after breaking, and thereby provides easier cleanup of the subterranean formation.
The slurry compositions of the present invention are easy to handle at the pumping site because there is no need to use solid metering devices. Time is thereby saved at the work site because liquids mix faster, more completely and easier than solids with liquids. Because the breaker in a slurry composition may be more readily and completely mixed with the fracturing fluid, the breaker will be more uniformly distributed in the fluid pumped downhole, and will thereby also be more effective in completely breaking the fracturing fluid in even the remotest parts of the subterranean formation.
As an additional advantage, the slurry format allows the user to greatly simplify their logistics in maintaining work supplies. Because the materials may now be provided in a simple to use, stable, premixed format, the number of components that must be ordered and kept in inventory and or supply at the drilling site by the user may be substantially reduced. Further, the likelihood of error on the part of workers at the site in mismeasuring or improperly mixing components is substantially reduced as well.
Providing the encapsulated breaker in a slurry format allows the optional surfactant to be present in an available liquid phase at time of mixing with the fracturing fluid. This format aids in providing easy and complete mixing of the breaker throughout the fracturing fluid.
Additionally, because the encapsulated breaker is provided as a slurry for mixing with the fracturing fluid, the material to be pumped downhole is more homogeneous in distribution of both the breaker and any surfactant in the composition. Use of the present invention therefore may result in fewer agglomeration problems, especially downhole, while removing the fracturing fluid from the formation, because the breaker is available throughout the fracturing fluid and the surfactant is available at the site of the breaker to assist in removal of agglomerates in the broken fracturing fluid from the formation.
The preferred breaker material for aqueous-based fracturing fluids can comprise, for example, enzymes such as hemicellulase, oxidizers such as sodium or ammonium persulfate, organic acids or salts, such as citric acid or a citrate, fumaric acid, liquids adsorbed on a solid substrate, solid perborates, solid peroxides or other oxidizers, mixtures of two or more materials and the like. Most preferably, the breaker material is potassium persulfate. For gelled hydrocarbon fracturing fluids, preferred breakers include calcium oxide, calcium hydroxide, p-nitrobenzoic acid, triethanolamine, sodium acetate, sodium bicarbonate, and the like.
The coating of the encapsulated breaker is any coating suitable for protecting the breaker from premature reaction with the fracturing fluid, while providing an appropriate release mechanism to allow the breaker to reduce the viscosity of the fracturing fluid in the desired time frame, thereby allowing removal of material from the subterranean formation. Examples of suitable coating materials include glass coatings, ceramic coatings, homopolymers and copolymers of polyolefin and ethylene oxides, polyamides, crosslinked cellulosic materials, polyureas, polyurethanes, dehydrated gums (such as guar), surfactant coatings, and the like.
The encapsulated breaker of the present invention is preferably enclosed within a hydrolytically degradable polymer coating. By “hydrolytically degrade” is meant that the polymer coating will react with water to chemically break down the poly
Irwin Michael J.
Muir David J.
3M Innovative Properties Company
Alexander Lyle A.
Bjorkman Dale A.
Cross LaToya I.
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