Mixed Wettability Modeling and Nuclear Magnetic Resonance Characterization in Tight Sandstone

Rock wettability significantly impacts fluid distribu-tions in reservoirs and enhanced oil recovery. As the large variety of mixed-wet tight formations becomes increasingly evident, research evaluating wettability has become increasingly important. The nuclear magnetic resonance (NMR) technique is suitable for wettability characterization, but the simplified rock models currently used in NMR studies, that is, entirely water-wet or oil-wet or classified as water-or oil-wet based on pore geometry, may result in inaccurate pore-fluid distributions and wettability estimates. In this paper, a new pore-scale mixed-wet rock model was constructed that considers the contact angle and wetting area. A tight sandstone rock saturated with oil-water is presented as an example to investigate T2 responses and NMR T2-based wettability indices for different wettability features. In the model, a mixed-wet digital rock saturated with oil and water is constructed using the sedimentological process method and the Lattice Boltzmann method. Then, the random walk method is applied to simulate T2 responses in cases with varied wettability properties, such as contact angle, wetting area ratio, and fluid saturation. Finally, the apparent contact angle cosine is used as the wettability indicator for comparison with published NMR T2-based wettability indices. The results showed that the combination of the wetting area ratio (determined by the pore size cutoff), actual contact angle of wetting surfaces, and fluid saturation significantly impacted T2 distributions of fluid-saturated mixed-wet rocks. Simulations and experiments showed that three NMR T2-based wettability indices were inappropriate for wettability assessment in weakly wet or intermediate-wet rocks. This was because those NMR indices are based on wetting areas and do not account for the effect of local contact angles of wetting surfaces. These investigations have provided a theoretical basis for interpreting NMR T2 characterizations in rocks with mixed wettability.