Quantitative NMR assessment of pore size dependent wettability in tight sandstone and shale

Wettability controls fluid distribution, saturation, and relative permeability in tight reservoirs, but conventional measurements cannot resolve mixed wetting behavior across pore sizes. This study develops a pore-size-dependent wettability evaluation method that combines the Amott index with nuclear magnetic resonance measurements using heavy water. Signal changes during spontaneous and pressurized imbibition were used to quantify wettability indices for discrete relaxation-time intervals, corresponding to pore-throat radii of 0.0037-101 μm. The results reveal strong pore-scale wettability heterogeneity. Tight sandstones are mainly oil-wet in small pores and water-wet in larger pores, whereas shales show the opposite trend. Mineral composition explains these differences, with clay-rich inorganic pores favoring water-wet behavior and organic matter favoring oil-wet behavior. Average pore-scale wettability indices agree well with gravimetric and macroscopic nuclear magnetic resonance results, demonstrating a reliable approach for evaluating tight reservoir wettability and improving recovery assessment.