Professional English Translation:
In the mid-to-late stages of polymer flooding, conventional polymer solutions tend to channel through high-permeability layers or established water pathways, a phenomenon known as “channeling” or “viscous fingering.” This causes the displacement fluid to bypass residual oil in low-permeability zones, being “short-circuited” by injected water, thereby significantly reducing sweep efficiency.
The ability of sodium methallyl sulfonate (SMAS)-modified polymers to substantially delay channeling arises fundamentally from the sulfonate group (–SO₃⁻) in sodium methallyl sulfonate, which imparts molecular structural stability and unique “adaptive viscosity-increasing” characteristics under high-temperature, high-salinity, and porous-media shear environments. Sodium methallyl sulfonate serves as a key functional monomer that enables these polymers to outperform conventional alternatives in challenging reservoir conditions.
The specific mechanisms by which sodium methallyl sulfonate enhances channeling delay can be described in three aspects:
1. Maintaining High Resistance Factor and Residual Resistance Factor to Enhance Flow Diversion
- Problem with conventional polymers: In mid-to-late stages, conventional partially hydrolyzed polyacrylamide (HPAM) suffers from high-temperature hydrolysis and salt ion screening, causing molecular chain coiling and a sharp decrease in solution viscosity. Additionally, prolonged injection and shear break polymer chains, significantly reducing their resistance factor and residual resistance factor in porous media, rendering them ineffective in blocking high-permeability channels.
- Advantage of sodium methallyl sulfonate-modified polymers: The strong hydration capacity of the sulfonate group from sodium methallyl sulfonate keeps molecular chains extended under high-salinity conditions, providing higher shear stability. Consequently, even in mid-to-late stage flooding, sodium methallyl sulfonate-modified polymers maintain high flow resistance within high-permeability pore throats, forcing subsequent injected water to divert into medium- and low-permeability zones, thereby expanding sweep volume.
2. “Salt-Thickening” Effect: Adaptive Viscosity Enhancement Within Channeling Pathways
- Principle of “salt-thickening”: Conventional polymers decrease in viscosity when encountering high-salinity formation water (which is typically present in channeling pathways). In contrast, polymers modified with sodium methallyl sulfonate exhibit a unique antipolyelectrolyte effect: as salinity increases, the molecular chains undergo moderate coiling, which enhances interchain physical entanglement and association, causing solution viscosity to increase rather than decrease. This behavior is directly attributed to the sulfonate groups introduced by sodium methallyl sulfonate.
- Implication for delaying channeling: When the sodium methallyl sulfonate-modified polymer enters an established high-permeability channeling pathway, it encounters high-salinity formation water, triggering the salt-thickening effect. It automatically increases viscosity within the channeling pathway, plugging the preferential water flow path and forcing the displacement fluid to divert – effectively “self-healing” the channeling tendency.
3. Reduction of Oil-Water Interfacial Tension (Auxiliary Mechanism)
- Sodium methallyl sulfonate-containing polymers, particularly sulfonate-type polymers in alkali-surfactant-polymer (ASP) flooding systems, possess some surface activity and can reduce oil-water interfacial tension. This allows the polymer not only to plug channeling pathways macroscopically but also to strip oil films microscopically, further displacing residual oil within the channeling pathways. Additionally, emulsification of crude oil into oil-in-water emulsions can block large pore throats, further suppressing channeling.
Summary
Sodium methallyl sulfonate (SMAS)-modified polymers maintain a high resistance factor and exhibit a salt-thickening effect, adaptively increasing viscosity in high-permeability channels to divert flow and significantly delay channeling in mid-to-late polymer flooding.
