This paper presents a method to correct results of laboratory measurements where we investigated the effects of wall slip, wall roughness and shear-induced slip on the viscous properties of crosslinked fracturing fluids. Our research indicates that crosslinked gels exhibit wall slip. The measurement of viscosity is dependent upon the surface roughness of the bob used in the tests. Unless an appropriate correction is made, the measured viscosity will be less than the true viscosity because of wall slip. Our study also demonstrates the occurrence of a rheological transition of polymer gel induced by shear forces. The critical shear rate, which we define as the beginning of stress transition from one state to another, may vary between 10-90 sec-1, depending upon the fluid nature, test procedure and temperature, Viscosities calculated using data from the different stress states at the same nominal shear rate can be different by a factor of 3 to 5. The gel with the higher critical shear rate exhibits more stable viscous properties when subjected to the thermal and shear degradation.
Apparent viscosity of a fracturing fluid influences created fracture width, fluid leakoff rate, proppant transport, and the final conductivity of the proppant pack. Viscous properties of crosslinked fracturing fluids have been the subject of numerous research projects for many years. While abundant information is available in the literature, we still do not fully understand these fluids. Characterizing the rheological properties of a crosslinked gel poses a difficult and elusive challenge.
Research concerning hydraulic fracturing fluid at Texas A&M University dates back to 1983. Worlow summarized the early research projects, and developed a procedure for testing crosslinked gel. The testing procedure was based on the simulation of typical shear rates and temperatures encountered during a fracturing treatment, A comprehensive GRI/TAMU rheology laboratory was established in 1990 to measure the viscous properties of fracturing fluids at insitu conditions. This rheology unit consisted of four separate, but connected systems that were designed to perform specific functions, The four systems were (1) a batch mixing system, (2) a high shear (wellbore) loop, (3) a low shear loop (pipe viscometer), and the modified Fann 50 viscometer with fluid transfer system, The rheology loop had two rotational (Fann 50 and TT100 brookfield) viscometers a pipe viscometer and a slot viscometer and was operated using a computer for data acquisition, flow rate monitoring, temperature control and shear stress (pressure differential) measurements, The detailed information concerning the rheology unit and test procedures can be found in the papers by Craig and Fan.
With the rheology unit, we conducted numerous research projects, including degradation of fracturing fluids by chemical breakers, simultaneous testing of crosslinked gel with different types of viscometers, and the development of a new testing method for crosslinked gels.
This paper presents results of one laboratory research project where we investigated the effects of wall slip, wall roughness and shear-induced transition on the viscous properties of HPG solutions crosslinked with either titanium or zirconium.
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