An Experimental Study of the In-Situ Gelation of Chromium(+ 3)/Polyacrylamide Polymer in Porous Media

Abstract

Summary Gelled polymers are being used increasingly as reservoir flow control agents to improve the water-to-oil ratio (WOR) in waterflooding. To date, little has been published in the literature on the in-situ gelation of polymers in porous media. This work was undertaken to study the process of in-situ gelation of a gel system consisting of a polyacrylamide polymer, sodium dichromate, and thiourea. The experiments polyacrylamide polymer, sodium dichromate, and thiourea. The experiments consisted of continuous injection of gel solutions into 1-ft [0.3-m] unconsolidated sandpacks. Pressure distributions along the flow direction were used to interpret the nature of gelation in the porous media. We found that gelation occurred much earlier than porous media. We found that gelation occurred much earlier than anticipated from beaker tests where gel solutions were under conditions of no shear-e.g., a gel solution with a nominal 10-day gel time in a beaker gelled in 85 hours in a porous-medium run. Shear was considered one of the major factors affecting the in-situ gelation rate, and we determined that gelation times decreased with increasing in-situ shear rates. Under the same in-situ shear rate, the in-situ gelation times were approximately equal in sandpacks prepared from different grain sizes. The order in which in-situ gelation occurred along the flow direction indicated that the shear history affected gelation time. Effluents from displacements with gel solutions were analyzed to evaluate the change in the properties of a gel solution as it moved through the porous media. The chemical analysis indicated that the retention of polymer was marked after the in-situ gelation developed. Introduction Volumetric sweep efficiency is an important factor in secondary and EOR displacement processes. Volumetric sweep is primarily a function of the process mobility ratio(s) and reservoir heterogeneity. Permeability variations can occur both areally and vertically, and vertical heterogeneities caused by differences in depositional environment during rock formation are common. Where heterogeneities are severe, process economics can be significantly impacted. Several methods to control reservoir flow through insitu permeability modification have been proposed. These include (1) cement slurries, (2) injection of particulate solids, (3) silica gels, (4) monomers that polymerize in situ, (5) water-soluble polymers, and (6) gels polymerize in situ, (5) water-soluble polymers, and (6) gels or colloidal structures formed by certain metal ions and polymers. Previous research in our program has polymers. Previous research in our program has focused on the last approach: the formation of gels through the interaction of Cr(III) and polyacrylamide polymers. Studies have involved the effects of process variables on gelation times as measured in beaker tests, the effect of temperature on gelation time, reaction kinetics related to the gelation process, and rheological properties of the gels. Earlier work has shown that gelation time for the Cr(III)/polyacrylamide polymer systems in beaker-type experiments depends on several parameters, including concentration of the oxidation-reduction chemical system (e.g., sodium dichromate/thiourea) and polymer concentration, type, and temperature. Systems can be designed for which the gelation time is relatively long, from days to even months. Thus the possibility exists to develop systems that can be mixed at the surface, can be injected as slugs, and will gel in situ at a predetermined time. For systems with relatively long gelation times, in-depth treatment around injection or production wells can be accomplished and presumably permeability modification would be more effective. Previous research has also shown that the gelation time is affected by the shear rate imposed on the gel solution during the gelation process. Thus the process of injecting a solution and the flow through a porous medium could alter gelation time from that predicted on the basis of beaker-type tests. Also, the presence of the rock matrix in a porous material could affect the gelation behavior. The present work was undertaken to study the gelation process for Cr(III)/polyacrylamide polymer systems as it process for Cr(III)/polyacrylamide polymer systems as it occurs in situ in a porous medium. An objective was to obtain data on the relationship between in-situ gelation time and gelation time as measured in beaker-type experiments. Another objective was to obtain insight into the mechanisms involved in in-situ gelation. SPERE P. 583