Effect of High Shear Rate on In-Situ Gelation of a Xanthan/Cr(III) System

Abstract

ABSTRACT An experimental investigation of the effect of high shear rates on the mechanisms of in situ gelation and development of flow resistance of a xanthan/Cr(III) gel system in sandpacks was made to simulate the conditions around a wellbore during a gel treatment. Gel solutions were mixed in-line to give an injected solution concentration of 1,500 ppm xanthan and 50 ppm Cr(III). Such a gel system has a gel time in bulk in the neighborhood of 15 hours. A 15 foot length of unconsolidated sand was used as the porous medium. The effect of high shear rates was examined by varying the frontal velocity from 12 to 120 ft/d. During each gel displacement, pressure drops across sub-sections of the sandpack were recorded as well as data on the effluent. Gelation behavior and Cr(III) concentrations of effluent samples were measured. In gel displacements conducted at linear velocities between 11 to 35 ft/d. a high flow resistance developed at a specific location in the sandpacks confirming earlier work of Hejri et al.1,5. The build-up in flow resistance (in situ gelation) was accompanied by a reduced polymer concentration downstream of this location, which was inferred from reduced pressure drops and reduced effluent viscosity. The location of this region of high flow resistance was correlated linearly as a function of frontal velocity. Studies performed on effluent samples indicated that a large amount of Cr(III) and polymer were retained in the sandpacks. Moreover, the high flow resistances developed at a location where the age of the flowing solution was around 10 hours, corresponding approximately to the point where the viscosity starts to increase for this gel system.1. At frontal velocities of 83 and 118 ft/d no significant flow resistance developed in these sandpacks. The residence time was apparently too small and the shear rate too large for in situ gelation to occur. The results of the experiments indicate that in situ gelation is mainly controlled by the kinetics of the gelation reaction and gel/polymer retention.