Fracturing Fluid Cleanup by Controlled Release of Enzymes from Polyelectrolyte Complex Nanoparticles

Water-based polymer gels are used widely in the oil and gas industry to viscosify fluids used in the hydraulic fracturing of production wells, where they serve to increase the force applied to the rock and to improve the transport of proppants used to maintain the fracture after formation. After fracturing, the gel must be degraded to a low viscosity with enzymes or gel breakers. Existing systems add the breaker either directly to the gelant or encapsulated in beads that are crushed when the applied pressure is released and the fractures close. In the former case, the gel may be broken prematurely, and this may prevent efficient fracture propagation and proppant transport, whereas in the latter case, the breaker may not be uniformly distributed throughout the gel, with the result that the gel is incompletely broken and the hydraulic conductivity of the well is reduced. To obtain delayed release, combined with the homogeneous distribution of enzyme throughout the gel, polyethylenimine-dextran sulfate polyelectrolyte complexes were used to entrap pectinase. Such particles were originally developed to entrap pharmaceuticals, and we previously demonstrated their ability to delay the release of gel crosslinking agents for oilfield applications. The degradation of both the viscosity and viscoelastic moduli of borate-crosslinked guar gel by pectinase loaded in polyelectrolyte nanoparticles was delayed by up to 12 h, compared to about 2 h for equivalent systems where the pectinase was not entrapped. The combination of homogeneous mixing and the delayed release of enzymes packaged in polyelectrolyte complex nanoparticles showed promise for improved cleanup after hydraulic fracturing. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 121: 1292-1298, 2011