Fractal Analysis of Tight Sandstone Petrophysical Properties in Unconventional Oil Reservoirs with NMR and Rate-Controlled Porosimetry

Characterization of the pore size distribution of tight sandstone is of great significance for the effective development of tight oil resources. This paper analyzed the fractal characteristics of pore spaces of tight sandstones from the Yanchang Formation in the Ordos Basin of China with NMR and rate-controlled porosimetry (RCP). A new NMR fractal analysis method that considers the movable fluid distribution was proposed. Compared with the conventional NMR fractal analysis metrics, the fractal dimensions of the pore spaces occupied by completely movable fluid and partially movable fluid have stronger correlations with the tight sandstone petrophysical properties. This work shows that the fractal dimensions of pore spaces with partially movable and completely movable fluids are negatively correlated with the permeability and reservoir quality index (RQI), and as the fractal dimensions approach 3, the tight sandstone petrophysical properties become less favorable. The fractal dimensions of the pore spaces detected by RCP were calculated with three different fractal models, the thermodynamic model, the 3D capillary model, and the wetting phase model. The research results show that compared with the results of the other two fractal models, the fractal dimensions obtained from the 3D capillary model can be used to more accurately evaluate tight sandstone petrophysical properties, and the calculated fractal dimensions are strongly negatively correlated to the pore radius, sorting coefficient, skewness, and permeability. The fractal dimensions obtained from the thermodynamic model have no obvious relationship with the tight sandstone properties, and the wetting phase model gives opposite fractal analysis results, so these two fractal models are not recommended for fractal analysis of tight sandstone pore size distributions via RCP. Finally, the models for predicting tight sandstone permeability with NMR and RCP were evaluated and screened.

Automated summary

This study analyzed the fractal characteristics of pore spaces in tight sandstones from the Ordos Basin in China, using NMR and RCP techniques. It showed that the fractal dimensions of pore spaces with movable fluids were negatively correlated with permeability and reservoir quality index, with fractal dimensions approaching 3 indicating less favorable petrophysical properties. Different fractal models were compared for calculating fractal dimensions, with the 3D capillary model found to be the most accurate for evaluating tight sandstone properties.