Experimental Measurement of the Persistence of Permeability Reduction in Porous Media Treated With Xanthan/Cr(III) Gel Systems

Abstract

Summary A series of long-term experiments was conducted to determine the persistence of flow resistance in unconsolidated sandpacks treated with xanthan/Cr(m) gels. Gels were prepared with 2, 000 ppm xanthan and Cr(m) concentrations from 25 to 200 ppm. These systems exhibit various degrees of swelling or syneresis. The experiments were conducted by first allowing a gel to set up in a sandpack and then continuously flushing the sandpack with brine for up to 4 months at a constant pressure drop of 13.3 psi/ft. Flow rate was monitored to calculate permeability changes with time. Flow experiments indicated that the measurement of swelling and syneresis in bulk-gel tests is not a good predictor of gel response in unconsolidated sandpacks. Excellent permeability reduction was obtained in sandpacks when gels that exhibited 35% to 60% reduction in volume in bulk tests owing to syneresis were used. Gels were most effective at retaining flow resistance in the range of 35 to 75 ppm Cr(m), where the sandpacks regained < 0.1 % of their original 4, 000 md permeability during the experiments. These observations were supported by experimens in which the pH of the injected brine was varied between 3.0 and 6.5. The swelling tests on the bulk gel indicated that permeability would decrease as pH increased and would increase as pH decreased. This occurred; however, the magnitude of the permeability change was less than inferred from bulk-gel measurements. Other experiments indicated that residual oil does not affect permeability modification in sandpacks. The amount of Cr(m) removed from the sandpack during brine flushing ranged from 38% for the 25-ppm-Cr(III) gel to 70% for the 200-ppm-Cr(III) gel. In all systems, effluent Cr(III) concentration declined exponentially as flushing continued, suggesting that the Cr(III) recovered never reacted with the polymer.