Effect of Shear Rate on the Activity of Enzymes Used in Hydraulic Fracture Cleanup of Tight Unconventional Reservoirs
- Reza Barati (University of Kansas) | Chris Ouyang (University of Kansas)
- Document ID
- Society of Petroleum Engineers
- SPE Unconventional Resources Conference-USA, 10-12 April, The Woodlands, Texas, USA
- Publication Date
- Document Type
- Conference Paper
- 2013. Society of Petroleum Engineers
- 1.8 Formation Damage, 4.3.1 Hydrates, 2.5.2 Fracturing Materials (Fluids, Proppant), 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 5.4.7 Chemical Flooding Methods (e.g., Polymer, Solvent, Nitrogen, Immiscible CO2, Surfactant, Vapex), 5.8.4 Shale Oil, 4.1.2 Separation and Treating
- Fracture Cleanup, Tight Unconventional Reservoirs, Shear Effect, Fracturing fluids, Breakers
- 1 in the last 30 days
- 395 since 2007
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Injection 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).
Introduction and Background
Injection 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. (1) Slick water, linear gel or hybrid treatments include the injection of different polymer concentrations. Moreover, cross-linked fluids are still being used often for hydraulic fracturing of tight oil and gas formations. (2)
In addition to the propagation of a main fracture, polymeric fluids open the already existing network of microfractures and extend the network connected to the main fracture. (3) The very low permeability of tight oil and gas formations results in the formation of filter cake when using low concentrations of polymer. Higher flow rates used in order to prevent poor proppant transport (4) may cause a filter cake on the faces of fractures and microfractures. Considering the very low permeability of the formations and small width of the fractures in the network, and also considering the importance of fracture cleanup and effective length during the production phase, even a small residual concentration of polymeric fluid will affect the conductivity of the fracture network in tight and ultra-tight formations. (5)
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