Effect of Shear Rate on the Activity of Enzymes Used in Hydraulic Fracture Cleanup of Tight Unconventional Reservoirs

Abstract

AbstractInjection of high or low concentration polymeric solutions in order to propagate a fracture and distribute proppants to keep the fracture open is a common practice in hydraulic fracturing of unconventional tight formations. In addition to the propagation of a main fracture, low concentration polymeric fluids open the already existing network and extend the network connected to the main fracture. Considering the very low permeability of the formation and small width of the fractures in the network, and also considering the importance of fracture cleanup and effective length during the production phase, using low concentrations of breakers is recommended to degrade the more concentrated polymeric fluid and increase the conductivity of the fracture.Enzymes have been used successfully as breakers for fracturing fluids for many years. Enzymes are polymer specific, environmentally benign, easy to handle, miscible in the fluid, equipment friendly and not consumed because they act as catalysts. These properties make enzymes much more attractive than the oxidizer class of breakers. Enzymes are known to be temperature and pH sensitive and their properties under the targeted temperature, pH and shear rate must be measured.In this study, the effect of high shear rates, applied during the injection of hydraulic fracturing fluids, on the activity of enzymes was studied. Enzyme activity was measured using a viscometric assay. Reversibility of changes in the activity of enzyme was studied by first shearing the polymer-enzyme mixture at high shear rates; then reducing the shear rate after a certain time, and finally comparing the activity with the activity of the same concentration of enzyme with lower shear rate history. Moreover, the effect of changes in the shear rate on the shear sensitivity of enzymes was investigated for a wide range of shear rate and polymer concentration.Understanding the shear sensitivity of enzymes will improve the fracturing fluid injection condition. This will assure the activity of the enzymes after reaching the fracture and the more efficient cleanup of the fracture(s).