Pore scale insights into salinity driven foam stability and conformance control in heterogeneous porous media

The potential application of foam for conformance control in heterogeneous reservoirs is critically dependent on brine salinity, yet the underlying pore-scale mechanisms involving residual oil have remained unclear. This study provides a pore-scale analysis, using a two-layer heterogeneous micromodel, to unravel how salinity (5,000 vs. 35,000 ppm NaCl) governs the complex interplay between foam and in-situ generated emulsions using sodium dodecyl sulfate (SDS) surfactant. In the absence of oil, low-salinity conditions produced a homogeneous, fine-textured SDS foam with high stability, leading to improved sweep efficiency across both high- and low-permeability layers. High salinity, by compressing the electrical double layer, yielded a coarse, heterogeneous foam with reduced stability due to enhanced coalescence and Ostwald ripening. The presence of residual oil revealed a critical paradigm shift: ultimate performance is not dictated by foam stability alone. At high salinity, the formation of coarse, unstable oil-in-water emulsion droplets, a consequence of ionic shielding, proved decisive. These large droplets acted as dynamic diverting agents, synergizing with foam to create temporary blockages in high-permeability pore throats via the Jamin effect. This mechanism effectively diverted flow into the low-permeability zone, significantly improving conformance. In contrast, the fine, stable emulsions formed at low salinity failed to block high-permeability pathways and instead caused unintended blockages within the low-permeability layer itself. Consequently, high-salinity foam injection achieved a final oil recovery of 88.02%, a significant 9.20-percentage-point improvement over the 78.81% recovery at low salinity.  These results provided the first direct pore-scale evidence that optimizing salinity can tune the balance between foam stability and in-situ emulsion generation to maximize conformance control. This insight is crucial for designing effective foam strategies for enhanced oil recovery and also gas-based storage in saline, heterogeneous formations.