Characterization of pore structure of tight sandstone reservoirs based on fractal analysis of NMR echo data

Abstract

The evaluation of the pore structure of tight oil and gas reservoirs has great significance for the characterization of the reservoir properties and the prediction of reservoir productivity. Fractal analysis is an effective method commonly used to evaluate reservoir pore structure. In previous studies, the fractal dimension of nuclear magnetic resonance (NMR) transverse relaxation time (T2) distribution was calculated to evaluate reservoir pore structure. However, due to the low signal-to-noise ratio (SNR) of the NMR echo data in tight sandstone reservoirs, the T2 distribution obtained by inversion of the echo data with low SNR has great uncertainty, which affects the evaluation accuracy of reservoir pore structure. In this paper, the fractal dimension of the NMR echo data (D_ECHO) is calculated to evaluate the pore structure of tight sandstone reservoirs for the first time. Fast Fourier transform is performed with the NMR echo data and then the D_ECHO is calculated. We analyze the correlation between the fractal dimension and the physical parameters and pore structure parameters, the results showed that 1) compared with the calculation of the fractal dimension of the T2 distribution, the calculation of the D_ECHO has higher stability, which can avoid the influence of the inversion uncertainty of echo data with a low SNR on the evaluation of the reservoir pore structure; 2) the fractal dimension is closely related to pore structure parameters, D_ECHO are well suited for characterizing the pore structure of tight sandstone reservoirs. As the heterogeneity of reservoirs increases, the D_ECHO increases. Numerical simulations, core experiments and NMR logging data processing results verify the effectiveness of the proposed method.