The rheological properties of hydraulic fracturing fluids, commonly used in low permeability reservoirs, has been discussed extensively in recent years because of their importance in stimulation treatment design. Rheology of these systems is primarily determined in a coaxial cylinder viscometer and reported as apparent viscosity. However, many problems have been encountered in obtaining accurate measurements of crosslinked gels due to the fluid slipping at the solid boundaries of the bob and cup. This slip creates an unknown velocity profile across the flow field; in other words, the fluid experiences an unknown shear. Since viscosity, as well as job design parameters n' and K' are the direct result of shear rate and shear stress, determination of the actual shear rate experienced by the fluid is critical. Many instances have been observed where the steady shear measurements indicate that the fluid has little or no viscosity. Yet, accounting for slip at the walls, the fluid is still a viscous crosslinked gel. Only by determining the existence and extent of wall slip of fracturing systems and how to correct for it, can the steady shear data that are utilized in treatment design models be meaningful.

This paper presents the application of a theory to determine 1). wall slip velocities in a coaxial cylinder viscometer 2). corrected shear rate not based on power law parameters and 3). true viscosity of crosslinked fracturing fluids. Data have been taken over a wide shear rate range to distinquish if wall slip is more prevalent at low or high shear rates. Data also show, that for certain mixing conditions of the polymer and crosslinking reagent, wall slip is very significant and the shear experienced by the fluid is less than expected, resulting in higher viscosity values. Methods of gel testing and preparation have been examined and those methods that minimize wall slip will be discussed. In addition, oscillatory shear measurements, which describe gel structure, are shown to further explain rheological behavior in steady shear. Viscosity values corrected for slip will be compared with viscosity values obtained on a closed loop pipe viscometer.

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