| Abstract: | Characterization of viscous properties of crosslinked fracturing fluids is important, but difficult. The purpose of this research was to develop laboratory methods that can be used to better quantify the viscous properties of crosslinked fracturing fluids at realistic conditions of shear rate and temperature. Several aspects concerning problems in viscosity measurement have been addressed using a standard industrial Fann Model 50C viscometer. Using state-of-the-art techniques, a test procedure that includes dynamic crosslinker injection, continuous shear during gelation, controlled shear rate and temperature histories has been developed to reproducibly measure the viscous properties of crosslinked fracturing fluids containing chemical breakers. We have found that the calculated values of viscosity for a crosslinked gel are dependent upon the size of the bob and the methods used to compute effective shear rate using specific procedures, the Fann 50 viscometer can be used to produce viscosity measurements that are comparable with the measurements from the flow loop pipe viscometer for an identical crosslinked gel. A new test method based on volumetric average shear rate in the Couette region has been developed to determine the crosslinked gel viscosity. Using this new method, we can generate reproducible viscosity data using either a EX-B5 or EX-B2 bob when the volumetric average shear rates are matched. A new method for slip correction has been developed in this research to calculate the slip velocity of a crosslinked gel at the solid boundary. The experimental study on nonhomogeneous behavior of crosslinked gels has demonstrated the occurrence of a reversible stress state transformation from low stress to high stress. The results indicate that crosslinked fracturing fluids may provide better proppant transport than expected when these fluids flow down a fracture that has rough surfaces. Degradation studies for crosslinked gels containing AP breakers illustrate that conventional oxidative breaker is effective at reducing crosslinked gel viscosity for only about 40 minutes at the final test temperature. Degradation of the crosslinked polymer at high temperature is a second order reaction dominated by thermal and mechanical degradation mechanisms. |
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