Abstract

Polymer gel treatment has been widely applied for improving sweep efficiency and controlling excessive water and gas production. Their performance depends on gelation time and final gel strength. In most studies, brine salinity is used to describe the effect of formation water on gel behavior. However, changing salinity also changes ionic strength and ion composition at the same time. Because of this coupling, it is difficult to identify the mechanisms controlling gelation, which has led to inconsistent trends in the literature. Increasing salinity has been reported to either slow or accelerate gelation and to weaken or strengthen gels depending on system chemistry and conditions. This study evaluates gelation behavior using ionic strength as the main controlling parameter, separated from total salinity, for a CO2-resistant polymer gel based on containing 20% 2-acrylamido-2-methylpropane sulfonic acid (AMPS) crosslinked with chromium. Gel systems were prepared using brines containing single Na+, K+, Mg2+, or Ca2+ salts. Two experimental approaches were applied: (1) varying the overall ionic environment and (2) isolating ion specific effects by independently controlling cation or anion concentration. Gelation time was monitored, and gel strength was evaluated using rheological measurements. The results show that strengthening the ionic environment generally accelerates gelation and increases gel strength by reducing electrostatic repulsion between negatively charged polymer chains. When ion-specific effects are isolated, Na+ and K+ behave similarly, Mg2+ consistently enhances gel strength and shear resistance, and Ca2+ produces weaker gel networks even under enhanced charge screening, explaining why gel systems may fail in some formation waters despite favorable salinity conditions. Overall, gelation is primarily governed by charge screening, but ion type can strengthen or weaken the gel. Using ionic strength instead of salinity provides a clearer and more reliable basis for selecting polymer gels for conformance control in reservoirs with complex brine compositions under CO2 injection.

Department(s)

Geosciences and Geological and Petroleum Engineering

Second Department

Chemical and Biochemical Engineering

Third Department

Chemistry

Fourth Department

Materials Science and Engineering

Publication Status

Available Access

Comments

PetroChina, Grant None

Keywords and Phrases

Cation and anion; Conformance control; Gelation kinetics; Ionic strength; Polymer gel

International Standard Book Number (ISBN)

978-196452313-2

International Standard Serial Number (ISSN)

0271-7026

Document Type

Article - Conference proceedings

Document Version

Citation

File Type

text

Language(s)

English

Rights

© 2026 Society of Petroleum Engineers, All rights reserved.

Publication Date

01 Jan 2026

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