A Method for Calculating the Relative Permeability of Tight Sandstone Oil Reservoirs Based on Nuclear Magnetic Resonance Logging

Summary Relative permeability is a key parameter in predicting production capacity. However, conventional core analysis methods, which rely on sampling at discrete depths, fail to capture the vertical heterogeneity of the reservoir along the wellbore and within the radial detection range of logging tools. To overcome this limitation, we propose a method for predicting oil-water relative permeability based on nuclear magnetic resonance (NMR) logging data. The method enables computation of relative permeability throughout the vertical extent of the reservoir within the limited radial detection range of NMR logging tools. In this study, based on the established empirical relationship between transverse relaxation time and capillary pressure, combined with fractal theory and resistivity experiments, and by improving the NMR porosity model, we consider the effects of bound water and residual oil on the permeability of water phase and oil phase to achieve a more comprehensive characterization of the vertical heterogeneity of the reservoir. We compare the proposed method with the traditional oil-water relative permeability calculation method based on core experiments. The results show that the new method can provide a relative permeability curve that matches the experimental results. Furthermore, we investigate the potential application of the proposed method in real oilfield settings. By analyzing the production performance of perforated intervals in two wells, the results show that the pseudocapillary pressure curves and oil-water relative permeability calculated from NMR logging align well with actual production outcomes. Specifically, perforated intervals with higher oil saturation and better productivity display pseudocapillary pressure curves indicative of favorable reservoir properties, as well as higher isopermeability crossover points on the relative permeability curves. In contrast, intervals characterized by higher water saturation and lower productivity exhibit poorer reservoir properties and lower crossover points. These observations suggest that the method offers considerable value in accurately forecasting production capacity, providing a new scientific basis for the evaluation and development of tight sandstone oil reservoirs.