A modeling and experimental study of pre-gel aggregate growth during the gelation of a polyacrylamide-chromium(III) acetate gel system

Aqueous gel systems produced by the crosslinking of partially hydrolyzed polyacrylamide with chromium(III) ions are used to improve volumetric sweep efficiency and reduce water production in oil reservoirs. Pre-gel aggregates formed during the in situ gelation are filtered and retained in the reservoir rocks, causing high flow resistance and impeding further penetration. Filtration is dependent on the size or molecular weight of pre-gel aggregates. A fundamental understanding of the pregel aggregate growth will improve the design of gel treatments. This study was undertaken to mathematically model and experimentally characterize the growth of pre-gel aggregates during the bulk gelation of a partially hydrolyzed polyacrylamide-chromium(III) acetate gel system.;Models were developed based on the theory of branching processes and a kinetic study of the chromium-polymer uptake reaction. Through the models, the bulk gel time, the molecular weight averages and distributions of pre-gel aggregates can be estimated as functions of reaction time and initial conditions, such as concentrations of polymer and chromium(III), and molecular weight and degree of hydrolysis of the polymer.;A multi-angle laser light scattering (MALLS) instrument was used to determine the weight-average molecular weight and z-average root-mean-square (rms) radius of pre-gel aggregates as functions of reaction time. Good agreement between the experimental data and the modeled results was obtained. The weight-average molecular weight and z-average rms radius of pre-gel aggregates increased slowly in the initial gelation stage, and then increased rapidly around the gel time, in accordance with the viscosity profile during the gelation.;Molecular weight and size distributions of polyacrylamide samples and pre-gel aggregates were determined by a MALLS detector, coupled with a size exclusion chromatography (SEC) or a flow field-flow fractionator (FFFF), and a refractive index concentration detector. The measurement of molecular weight and size distributions of both non-ionic and partially hydrolyzed polyacrylamide samples was demonstrated successfully.;A suitable method was not found for fractionating pre-gel aggregates. Sample retention was observed in the SEC columns when larger aggregates formed. The unsuccessful fractionation of pre-gel aggregates in FFFF was probably due to charge repulsion.