Effects of pH and Shear on the Gelation of a Xanthan-Cr(III) System

Abstract

Summary A study was undertaken to determine the effects of pH and shear rate on the gelation of a xanthan-Cr(III) solution of the type that has been used successfully for permeability modification of oil fields. The rate of gelation and gel strength were determined using small-amplitude oscillatory methods under zero shear and superimposed steady-shear rates approximating those anticipated during placement in the porous matrix. At zero-net shear, the rate of gelation was first order in the inverse of the hydrogen-ion concentration up to pH 6.5. At pH 6.5 and above, precipitation of chromium slowed the gelation. The effect of shear was quite pronounced. No permanent gel structure was observed while the gel was under shear, regardless of pH or shear rate. Gelation was observed after shear was removed. However, the rate of gelation was 1.5 to 2 orders of magnitude slower than observed for zero-net shear runs. The rate of gelation decreased with increasing duration of shear. This effect was more pronounced at higher pH, indicating the ratio of applied-shear duration to solution-gel time may be the factor controlling gelation. Gels were not observed when the gel system was subjected to continuous shear for times longer than the zero-shear gelation time.