In the middle and late stages of low-permeability reservoirs, the pore structure is complex, with narrow throats and strong heterogeneity, making effective water injection and profile control extremely challenging. However, Sodium methallyl sulfonate (SMAS) can still be effectively injected without causing reservoir plugging, which is mainly attributed to its unique physical and chemical properties and its synergistic effect with reservoir conditions. Particularly in reservoirs with high temperature (>90℃) and high salinity (TDS>100000 ppm) during the middle and late development stages, polymers containing Sodium methallyl sulfonate are more recommended than conventional polymers, and the specific reasons are analyzed as follows:

1. Excellent water solubility and anti-salting-out ability based on the hydration of sulfonic acid groups

Sodium methallyl sulfonate has excellent water solubility; it can be quickly dissolved in injected water to form a uniform and stable aqueous solution, even under the high-salinity and high-temperature conditions of low-permeability reservoirs in the middle and late stages. Its solubility avoids the formation of insoluble precipitates that would block the reservoir’s narrow throats. Unlike some polymers prone to aggregation and precipitation, the molecular structure of Sodium methallyl sulfonate contains a highly hydrophilic sulfonic acid group, which enhances its dispersion stability in water and prevents molecular agglomeration. The sulfonic acid group (-SO₃⁻) in SMAS has a strong hydration capacity: it can combine with a large number of water molecules to form a stable hydration shell around the molecular chain. This hydration shell can effectively isolate the polymer molecular chains from high-concentration metal ions (such as Na⁺, Ca²⁺, Mg²⁺) in high-salinity formation water, avoiding the compression of the electric double layer on the molecular chain surface and the subsequent salting-out phenomenon that often occurs in conventional polymers.

2. Good injectability and thermal stability enhanced by methyl steric hindrance effect

Sodium methallyl sulfonate has a moderate molecular weight and a linear molecular structure, which endow it with good fluidity and injectability. In low-permeability reservoirs, throat diameters are usually small (often less than 100 md), and substances with excessively high molecular weight or irregular structures are easily retained in the throats, leading to plugging. However, the molecular weight of Sodium methallyl sulfonate is controlled within a reasonable range, and its linear structure allows it to pass smoothly through narrow throats under normal injection pressure without being trapped. Meanwhile, its moderate viscosity will not significantly increase injection pressure or cause pore blockage due to excessive thickness. More importantly, the methyl group (-CH₃) in the SMAS molecular structure exerts a significant steric hindrance effect. This effect can inhibit the rotation and entanglement of the polymer molecular chain, enhance the rigidity of the molecular chain, and reduce the thermal degradation of the molecular chain under high-temperature conditions (>90℃). In contrast, conventional polymers often have flexible molecular chains that are prone to thermal scission and structural collapse at high temperatures, resulting in a sharp decrease in their viscosity and loss of profile control and flooding performance.

3. Good compatibility with reservoir rocks and fluids under harsh conditions

Sodium methallyl sulfonate has good compatibility with reservoir rocks and fluids. In the middle and late stages of low-permeability reservoirs, formation water often has high salinity and mineralization, and reservoir rocks are prone to clay swelling and particle migration, which can easily cause plugging. Sodium methallyl sulfonate can weakly adsorb on the surface of reservoir rocks; this not only inhibits clay swelling and particle migration but also does not form a dense adsorption layer that would block pores. Its sulfonic acid group can also reduce the interfacial tension between oil and water, improve reservoir wettability, and further facilitate its smooth injection without causing additional plugging. Under high-temperature and high-salinity conditions, the combined effect of the sulfonic acid group’s hydration and the methyl group’s steric hindrance enables the copolymer containing SMAS to maintain a stable molecular structure, avoiding the aggregation and precipitation caused by the dehydration of conventional polymers, thus ensuring long-term effective injectability and profile control performance.

4. Stable weak gel formation ability in harsh reservoir environments

When Sodium methallyl sulfonate is used for profile control and flooding, it can form a weak gel with appropriate crosslinking agents under reservoir conditions. This weak gel has good deformability and can be injected into the deep reservoir along with injected water. It will not prematurely form a rigid gel to block the near-wellbore area but will gradually exert a profile control effect in the deep reservoir, forcing injected water to divert to low-permeability oil-bearing zones, thereby achieving effective profile control and flooding without causing reservoir plugging. In high-temperature and high-salinity environments, the stable molecular structure of SMAS-containing copolymers also ensures the stability of the weak gel, preventing gel breakage caused by harsh reservoir conditions, which is difficult for conventional polymers to achieve.

Summary

Sodium methallyl sulfonate is injectable without plugging due to its good solubility and compatibility, and its unique sulfonic acid group and methyl group ensure its stability in high-temperature and high-salinity reservoirs, making it more superior than conventional polymers in the middle and late stages of such reservoirs.