The effects of shear and proppant on the viscosity of cross-linked fracturing fluids

Several frequently used cross-linked fracturing fluids, hereafter referred to as gels, have been evaluated as to the effects that shear and that 20/40 mesh sand proppant have on their rheological properties. A closed-loop pipe viscometer was used to evaluate the shear resistance and post-shear recovery of various gels as a function of pH, cross-linker type, polymer concentration, shear intensity (shear stress and shear rate), and shear duration. The aqueous-base gels that were tested included those made with guar, hydroxypropyl guar (HPG), and carboxymethyl hydroxyethyl cellulose (CMHEC) polymers. Selected aqueous-base gels were also evaluated as to the effect on slurry viscosity of varying concentrations of 20/40 mesh sand proppant. Test results indicate that when cross-linked with borate ions, guar and HPG gels have the greatest resistance to shear. Yet when cross-linked with various titanates, HPG gels showed significantly lower resistance to induced shear. Tests on CMHEC gels which were cross-linked with trivalent aluminum (A1+++) indicate a shear resistance between that of the HPG-borate gels and that of the HPG-titanate gels. Also tested was post-shear recovery or the percentage of viscosity increase following high shear exposure as correlated to field tubular pumping conditions. The HPG-borate gels and the CMHEC-A1+++ gels demonstrated considerablymore » higher post-shear recoveries than those attained with HPG-titanate gels. In addition, the effect of 20/40 mesh sand on the viscosity of gels is a larger increase in slurry viscosity in the fracture than that predicted from Newtonian slurry mathematical models.« less