The reason why sodium methallyl sulfonate (SMAS)-modified polymers (typically copolymers with acrylamide) do not cause formation plugging or precipitation risk during oil displacement is primarily attributed to the unique physicochemical properties imparted by the sulfonate group (–SO₃⁻) in the molecular structure. The specific mechanisms are as follows:
1. No Insoluble Precipitate Formation with Divalent Cations
- In conventional polymers such as partially hydrolyzed polyacrylamide (HPAM), the carboxylate group (–COO⁻) reacts with Ca²⁺ and Mg²⁺ present in formation water to form insoluble calcium/magnesium carboxylate precipitates. These precipitates can block pore throats and cause formation damage.
- In contrast, the ion pairs formed between the sulfonate group of SMAS and Ca²⁺/Mg²⁺ are highly soluble and do not precipitate. Therefore, even in high‑hardness formation water, SMAS‑modified polymers remain as clear, homogeneous solutions without any solid precipitation.
2. Excellent Salt Tolerance and Polymer Chain Stability
- The sulfonate group possesses a strong hydration ability, forming a dense hydration layer that prevents severe chain coiling or aggregation caused by electrostatic screening from salt ions (including divalent ions).
- The polymer chains remain extended and well‑dispersed, without forming large intermolecular aggregates or gel phases, thus avoiding physical plugging.
3. No Hydrolysis-Induced Precipitation at High Temperatures
- HPAM is prone to amide group hydrolysis at high temperatures, generating additional carboxylate groups that subsequently form precipitates with Ca²⁺/Mg²⁺. The ammonia released during hydrolysis may also alter the pH.
- The sulfonate group of SMAS exhibits excellent stability against thermal hydrolysis and does not generate new precipitate‑forming groups upon thermal degradation. SMAS‑AM copolymers remain clear and free of precipitation or turbidity even after prolonged aging at high temperatures (e.g., >90 °C).
4. Good Rock Compatibility and Low Adsorption Loss
- SMAS‑modified polymers show relatively low adsorption onto rock surfaces (especially negatively charged sandstone) because the strong negative charge of the sulfonate group increases electrostatic repulsion with the rock surface. Low adsorption means that the polymer does not accumulate significantly near the wellbore or on pore surfaces, reducing the risk of cumulative plugging.
- Additionally, these polymers exhibit good compatibility with formation crude oil, formation water, and commonly used flooding additives (e.g., alkalis, surfactants), without causing phase separation or deposition.
5. Designable Molecular Weight and Shear Stability
- By controlling the ratio of SMAS to acrylamide and the polymerization conditions, a suitable molecular weight can be achieved. Excessively high molecular weight may leave micro‑gel residues after mechanical shearing. However, the molecular structure of SMAS‑modified polymers is relatively more rigid, and the degradation products upon shear are still water‑soluble small molecules that do not block pore throats.
Summary
SMAS‑modified polymers do not cause formation plugging or precipitation risk during oil displacement. The core reasons are: the sulfonate group forms soluble salts with divalent cations, preventing precipitation; the polymer chains remain stable and dispersed under high‑temperature/high‑salinity conditions without aggregation; the polymer does not hydrolyze to produce precipitates; and it exhibits good compatibility with reservoir fluids and rock surfaces.These characteristics make SMAS‑modified polymers a safe and efficient choice for polymer flooding, particularly in reservoirs prone to inorganic precipitation or requiring stringent formation protection.
