A novel method to estimate resistivity index of tight sandstone reservoirs using nuclear magnetic resonance logs

Abstract

The relationship between resistivity index and water saturation (I-Sw) is very important in formation evaluation via petrophysical data. Low permeability and complicated pore structure make it more difficult to determine I-Sw relationship through rock's electrical property measurements. Nuclear magnetic resonance (NMR) logs are alternative ways to obtain these key parameters. To establish a model to estimate resistivity index from NMR logs, 17 cores from the study area were selected for experiments. The helium porosity, air permeability, NMR T2 distribution, and resistivity of cores were measured. I-Sw relationship was evaluated accordingly. Given these variables, Archie formula, J function, Schlumberger-Doll-Research (SDR) model, and correlation between capillary pressure curve and T2 distribution (Pc-T2) were used to derive a novel method. It aims to calculate resistivity index from T2 time, porosity, and geometric mean of T2 time. The experimental data were applied and the corresponding model was established by multiple statistical regression method. Resistivity index is successfully predicted by T2 distribution. Then, the established model was tested by the experimental data not involved in the modeling. The validation shows that the model is highly precise. Finally, the model was applied to formation evaluation for a field case with satisfactory results. The proposed method and model provide a quantitative basis to analyze and predict resistivity index by well log data. They are of great significance for the accurate determination of oil and gas saturation of tight sandstone reservoirs.