Performance Evaluation of High-Strength Polyethyleneimine Gels and Syneresis Mechanism under High-Temperature and High-Salinity Conditions

Abstract

Summary A high-strength gel was prepared by crosslinking a high-concentration but low molecular weight polyacrylamide-tert-butyl acrylate (PAtBA) copolymer with polyethyleneimine (PEI), and the various factors affecting the gelling were systematically evaluated. The results showed that, besides the concentration of polymer and crosslinker, the molecular weight of PEI, pH value and salinity of gelant, and temperature all affected the gelation time, gel strength, and stability. High-strength PAtBA-PEI gels could be stable for more than 240 days in synthetic brine with salinity less than 43 530 mg/L at a temperature not higher than 130°C. However, when the temperature is higher than 150°C, the gel would be broken within 60 days. When the salinity in the synthetic brine was too high, the gel also exhibited syneresis at 110°C within 15 days. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) analysis showed that the syneresis of PAtBA-PEI gel under high-salinity conditions was caused by the reaction of carboxylate group generated by the hydrolysis of the amide group with divalent cations. In addition, Cryo-scanning electron microscope (Cryo-SEM) analysis showed that high-strength PAtBA-PEI gels had high crosslinking density and poor deformation ability. Nevertheless, the results of rheological tests and flooding experiments showed that the low deformation ability of the gel did not affect the sealing ability of the gel, and the high storage modulus and yield stress of the gel enabled it to exhibit high breakthrough pressure and residual resistance factor in the core. The results of this study are instructive for the field application of PEI gels.