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Measuring the viscosity of fracturing fluids is important to designing and supervising a fracture treatment. Viscosity is a critical parameter used to compute fracture dimensions and estimate proppant transport. Accurate knowledge of the viscous properties of crosslinked gels is also indispensable for laboratory evaluation of potential fracturing fluid systems. However, attempts to reliably measure the viscous properties of crosslinked gets have proven difficult because of the complex nature of the fluids. Studies by Prud'homme el al. have demonstrated that viscous properties of crosslinked HPG polymers are controlled by the gel network structure that is formed during crosslinking. The gel network structure is determined by chemical reaction kinetics and process conditions in which the reaction proceeds. The crosslinking reaction causes the gel to be a viscoelastic fluid. As such, temperature and shear histories, crosslinker type, solution pH, degree of hydration of the HPG solution as well as test duration can have a profound effect on the viscous properties of crosslinked HPG gels. Detailed rheological characterization of a crosslinked fracturing fluid remains a difficult and elusive challenge.

Generally, both steady shear and dynamic oscillatory measurements are required to describe the viscoelastic behavior of a crosslinked gel. In a dynamic test, a sinusoidat movement is imposed on the system that results in measurements of storage modulus, G', and the loss modulus, G". These parameters provide information about fluid elasticity and energy dissipation and can be used to determine the gel structure and evolution of structure.

In steady shear, shear stress as a function of shear rate is measured, then the viscosity is calculated. Viscosity is important to the design of a fracturing treatment. To generate meaningful viscosity data, the experiment should be conducted at conditions that represent the actual fracturing treatment. Proposed test techniques require dynamic crosslinker injection, continuous shear during gelation, plus controlled temperature and shear rate histories during the measurements.

The rheology research laboratory at Texas A and M University is equipped with a standard industry Farm 50 viscometer, in addition to the GRI/TAMU rheology flow loop. Both are capable of measuring the steady-shear viscous behavior of complex gel under simulated fracture conditions. The unique aspect of the laboratory is that different types of viscometers have been linked together in a continuous flow loop. As such, it is possible to run simultaneous tests using identical gel formulations, mixing procedures and shear rates with different geometry viscometers. The following text will briefly discuss the equipment; detailed information can be found in the literature.

This paper will address several factors that affect viscosity measurements and will present several laboratory observations concerning the viscous and degradation properties of HPG solutions crosslinked with either delayed titanium or zirconium materials. Crosslinked gels with and without ammonium persulfate (AP) breakers were tested using a Farm Model 50 viscometer. Results obtained in this study provide useful information for engineers involved in fracturing fluid design and treatment execution. Our results are also useful to anyone measuring viscous properties of crosslinked HPG fracturing fluids using a Farm 50 viscometer.

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