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Delayed-Release Coatings for Oxidative Breakers


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AuthorsWalter Philip Watson (Kemira) | Carl W. Aften (Kemira) | David J. Previs (Kemira)DOIhttp://dx.doi.org/10.2118/127895-MSDocument IDSPE-127895-MSPublisherSociety of Petroleum EngineersSourceSPE International Symposium and Exhibiton on Formation Damage Control, 10-12 February, Lafayette, Louisiana, USAPublication Date2010Document TypeConference PaperLanguageEnglishISBN978-1-55563-276-2Copyright2010. Society of Petroleum EngineersDisciplines1.8 Formation Damage, 3 Production and Well Operations, 2.5.2 Fracturing Materials (Fluids, Proppant), 4.3.4 Scale, 1.6.9 Coring, FishingDownloads1 in the last 30 days202 since 2007Show less detail

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Abstract

Inorganic and organic coatings for solid oxidative breakers have been developed to degrade slickwater and gelled stimulation fluids, reducing the polymers' potential to cause formation damage as they concentrate in fractures.  The coatings result in a delay of 90 minutes or more before the oxidizer begins to degrade the frac fluid, and this benefit was obtained without compromising the properties of the stimulation fluid or the oxidizer.

Frac fluids consist of ultrahigh molecular weight synthetic polymers or functionalized or unadulterated polysaccharides, and can be crosslinked by borates, zirconates, titanates, or other species.  The high viscosities of these fluids allow proppants to be transported, or, in the case of slickwater fracs, provide drag reduction, diminishing the amount of energy needed to pump the fluid.  After the shut-in period, however, the polymers' presence can be problematic, creating a skin over the newly formed fracture surface and proppant bed, reducing the communication from the wellbore to the fracture/formation.  Breakers such as oxidizers and enzymes have been employed to degrade polymers, each with their own strengths and weaknesses.  Oxidizers quickly and effectively attack a wide range of synthetic polymers and polysaccharides, while enzymes work in a more controlled fashion and have to be tailored for specific fluid systems.

In order to avoid premature degradation of stimulation fluids, breakers can be added towards the end of the shut-in period, or encapsulated and added earlier.  Static and dynamic benchtop tests were conducted on polyacrylamide copolymers and linear polysaccharide gels over a range of temperatures, and the new coatings were observed to increase the time to a full break by a minimum of 90 minutes, depending on the thickness and composition of the encapsulation layer.  Ultimate performance enhancement was gained within reasonable cost constraints.

Introduction

Oxidative breakers are a class of chemicals which thermally decompose, generating free radicals that degrade the molecular weight of viscosifying polymers in solution.  The generation of radicals and reduction in viscosity are strongly influenced by temperature, and a fluid's viscosity quickly can be reduced to near that of water.  In order to reduce the rate of degradation, less oxidizer can be added, but this also prevents a complete break from occurring.

To counter the drawbacks of oxidizers, enzyme breakers were introduced.  These additives offer a slow, controlled degradation rate, while still achieving a complete break.  Still, enzymes are restricted to specific thermal, pH, and salinity conditions, although the range of environments in which they are active has been widening over time.  Additionally, enzymes, unlike oxidizers, have not been successful at breaking synthetic polymers (Economides 2000).

The first coated oxidative breakers appeared in the 1980s, allowing chemistries to be released into solution at a slow rate (Burnham 1980; Nolte 1985; Gupta 1992). Since then, the inorganic and organic coatings that have been developed can be classified according to their solubility.  Insoluble inorganic or organic shells can be crushed as a fracture closes, or ruptured from the inside due to osmotic forces or the generation of internal pressure as the oxidizer decomposes.  Semi-soluble organic coatings can plasticize or swell, and the increased molecular mobility allows slow diffusion of water and breaker across the polymeric membrane.  Finally, inorganic or organic coatings with low solubilities can slowly erode away until the breaker core is exposed, although the presence of multivalent ions in solution may also cause the inorganic coating to precipitate as scale.

petroleum-gao

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黑色金子86

谢谢petroleum-gao，你的附件已购买了。就是这篇

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