Characterizing Mixed vs. Fractional Wettability in Porous Media Using Multifrequency Spectral Induced Polarization

Summary

Distinguishing between mixed wettability, in which different pore systems or pore sizes exhibit different wetting preferences, and fractional wettability, in which oil-wet and water-wet surfaces are randomly distributed throughout the system, is critical for accurately predicting multiphase flow, residual saturation, and therefore the amount of recoverable hydrocarbon. However, both systems share an intermediate wettability index, making it difficult to differentiate between them using traditional wettability evaluation methods. In this study, we explore the potential of the multifrequency spectral induced polarization (SIP) method to distinguish between mixed-wet and fractional-wet systems under controlled laboratory conditions. Two granular porous media were used to mimic these wettability distributions. The mixed-wettability system was created using dual-porosity silica gel, where the macropores were altered to oil-wet through asphaltene deposition, while the micropores remained water-wet. In contrast, calcite particles, along with silica gel grains, were used to represent the fractional-wettability system by mixing chemically treated calcite (oil-wet) and untreated calcite (water-wet). Nuclear magnetic resonance (NMR) measurements were then used to validate the wettability alteration in both systems, while SIP was performed to analyze the complex conductivity signature across the full frequency range (0.01–10,000 Hz) under three different conditions: water-wet at full brine saturation, water-wet at mixed saturation (50% oil), and mixed- or fractional-wet at mixed saturations for silica gel and calcite, respectively. The results show that the mixed-wettability system exhibits strong frequency-dependent behavior, producing negative dispersion in real conductivity, and an increase in imaginary conductivity due to the heterogeneous wetting state and fluid distribution. In contrast, the fractional-wettability system exhibits constant real conductivity across all frequencies and significantly lower imaginary conductivity magnitudes, reflecting continuous bulk conduction and relatively weaker polarization effects. These observations suggest that SIP responses are highly sensitive to wettability and fluid distribution. Therefore, the technique shows promise for differentiating wettability distributions in porous media, which could improve reservoir characterization.