NMR wettability indices: Effect of OBM on wettability and NMR responses

Wettability and NMR surface relaxation are related to each other. Wettability controls the fluid distribution in porous media. NMR surface relaxation dictates that the fluid in contact with the mineral surface has a relaxation time shorter than its bulk value. In this study, first, the nature of wettability effect on NMR responses was demonstrated by a parameter ρ2, eff, the effective surface relaxivity. Quantitative changes of ρ2, eff of water or oil for unconsolidated silica flour and calcite were shown to be consistent with the expected wettability alteration based on the contact angle measurements. Based on the concept of effective surface relaxivity, a novel NMR model was then proposed to quantify rock wettability by two NMR wettability indices from either water or oil responses. This model was tested with water/oil partially saturated Berea cores at different wettability conditions. Correlations show that both NMR indices agree well with the Amott–Harvey wettability index, suggesting that quantitative information of reservoir rock wettability can be gained from NMR measurements. Finally, the effect of oil base mud (OBM) surfactants on wettability alteration and NMR responses was systematically investigated with Berea cores. Results show that the originally strongly water-wet Berea cores are altered to be intermediate-wet or oil-wet by OBM surfactants. As a result, the irreducible water saturation from NMR interpretation assuming water-wetness when wettability alteration occurs generally underestimates the measured value. The magnitude of underestimation correlates well with the Amott–Harvey wettability index.