Resistance Mechanism of Sodium Methallyl Sulfonate–Acrylamide Copolymer Against High-Concentration Divalent Cations in Polymer Flooding
Professional English Translation:
Why can the copolymer of sodium methallyl sulfonate (SMAS) and acrylamide (AM) resist the detrimental effects of high-concentration divalent cations (e.g., Ca²⁺, Mg²⁺) during oil displacement? The fundamental reason lies in the unique chemical stability and anti‑precipitation ability of the sulfonate group (–SO₃⁻) introduced by SMAS. Compared with the carboxylate group (–COO⁻) in conventional polymers such as partially hydrolyzed polyacrylamide (HPAM), the sulfonate group exhibits distinctly different ionic behavior. The specific mechanisms are as follows:
1. The Sulfonate Group Does Not Form Insoluble Precipitates with Divalent Cations
- Drawback of the carboxylate group: The –COO⁻ in HPAM strongly interacts with divalent cations like Ca²⁺ and Mg²⁺, forming insoluble calcium/magnesium carboxylate precipitates. This not only consumes the polymer but also blocks pore throats, leading to a sharp viscosity drop and reduced oil displacement efficiency.
- Advantage of the sulfonate group: The ion pairs formed between –SO₃⁻ and Ca²⁺/Mg²⁺ are highly soluble and do not precipitate. Even in the presence of high concentrations of divalent ions, the SMAS copolymer remains a homogeneous solution, avoiding phase separation or polymer loss.
2. Strong Hydration Layer Shields Electrostatic Bridging by Divalent Cations
- The sulfonate group possesses a high charge density and strong electronegativity, attracting numerous water molecules through hydrogen bonding and ion‑dipole interactions, thereby forming a dense and stable hydration shell around itself.
- This hydration layer acts as a physical barrier that prevents divalent cations from approaching and bridging anionic sites on adjacent polymer chains. Consequently, even at high Ca²⁺/Mg²⁺ concentrations, interchain crosslinking or aggregation is not induced, and the polymer chains remain extended and well‑dispersed.
3. Minimal Impact of Divalent Ions on the “Salt‑Thickening” Effect of Sulfonate Groups
- In HPAM systems, Ca²⁺ and Mg²⁺ not only cause precipitation but also screen electrostatic repulsion, leading to chain collapse and a drastic viscosity decrease.
- For SMAS‑AM copolymers, although divalent cations exert some screening effect, the strong hydration and low compressibility of the sulfonate group result in only minor conformational changes. Moreover, moderate ionic screening may even promote slight intermolecular associations (i.e., the antipolyelectrolyte effect), maintaining or even slightly increasing solution viscosity. Experimental results show that at Ca²⁺/Mg²⁺ concentrations as high as tens of thousands of ppm, the viscosity retention of SMAS copolymers is far superior to that of HPAM.
4. Synergistic Role of Acrylamide Units
- Acrylamide (AM) provides the polymer backbone with water solubility and baseline thickening ability, while the AM units themselves are uncharged and unaffected by divalent ions.
- SMAS delivers the core function of divalent‑ion resistance. When copolymerized, the resulting polymer exhibits both good viscosifying power and high tolerance to Ca²⁺/Mg²⁺.
Conclusion
The copolymer of SMAS and acrylamide resists the deleterious effects of high‑concentration divalent cations because the sulfonate group does not form insoluble precipitates, creates a strong protective hydration layer, and maintains a stable chain conformation in the presence of divalent ions. This property enables the displacement fluid to retain effective viscosity in hard water or high‑salinity reservoirs rich in Ca²⁺ and Mg²⁺, thereby ensuring sweep efficiency and oil recovery during polymer flooding.
