Abstract
In this study, novel star-like polymers (SLPs) with four-gradient molecular weight (Mw), consisting of nano-silica as the core and a layer of hydrophilic chains as the shell, were fabricated via mild water free-radical polymerization. The comprehensive properties of SLP flooding systems prepared with brine were investigated to elucidate their underlying applications by combining static measurements, filterable tests, two-serial cores displacement experiments, and polymer flooding tests for enhanced oil recovery (EOR). The specific injectivity index was proposed, for the first time, to demonstrate the flow behavior of SLP solutions in conglomerate rocks.The results indicated that the rigid core together with the three-dimensional microstructure bestowed SLPs with the desirable viscosification and long-term stability in harsh conditions, outperforming ultrahigh-Mw partially hydrolyzed polyacrylamide. The dynamic hydrodynamic size (Hd) of SLPs was a power function over concentration and significantly depended on Mw. Based on the limitation of specific injectivity index, the resistance factor and residual resistance factor of SLP solutions as a function of permeability were mathematically correlated. SLPs had sustainable mobility control capability with no face plugging or internal filter cake when the average pore size over Hd of SLP solutions was higher than 5.2. This is contradictory to the conventional rigid sphere packing theory. Considering permeability of conglomerate rocks, the SLP flooding systems were designed using polymer concentration and Mw. The incremental oil recovery factor (RF) achieved by the designed SLP flooding systems was 7.0% more than that achieved by the SLP flooding systems that were not well designed but had an equivalent viscosity.This study indicates that the comprehensive performance of polymer solutions associated with the compatibility between Mw and rock permeability is significant to formulate polymer flooding systems with high EOR efficiency.
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