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.