Two-dimensional nuclear magnetic resonance method for wettability determination of tight sand

The wettability of reservoir rocks is important for oil recovery and reserve calculations. However, current methods for evaluating the wettability of rocks are time-consuming and expensive. Previous work has shown that low-field nuclear magnetic resonance (NMR) is a potentially useful and non-invasive technique for rock wettability determination. However, for rocks with strong internal magnetic field gradients, the current method is less efficient. In this study, the bipolar pulsed field gradient (PFG)-Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence was applied to the study of rock wettability. This method can suppress the effect of the internal magnetic field gradient in rocks and accurately extract wettability information. The diffusion-transverse relaxation time (D-T₂) method was employed to quantitatively estimate the wettability of rocks. Results of Amott wettability tests and NMR T₁-T₂ maps were combined to provide a more complete wettability characterization of tight sand. The results demonstrate the feasibility of the new method for characterizing wettability. The proposed method and workflow is of significance to the development of oil fields.