A method to determine nuclear magnetic resonance T2 cutoff value of tight sandstone reservoir based on multifractal analysis

Nuclear magnetic resonance (NMR) T-2 cutoff value is an important parameter for pore structure evaluation. It is complicated and uneconomical to obtain T-2 cutoff value by an experimental method; therefore, it is necessary to explore a prediction method of T-2 cutoff value. In this paper, 10 samples of tight gas reservoirs in the eastern Ordos Basin were selected, and then saturation and centrifugal experiments of nuclear magnetic resonance were carried out. On this basis, multifractal theory was introduced to calculate the multifractal characteristics of the NMR T-2 spectrum of each sample, and the relationship between multifractal parameters and T-2 value was analyzed. The influencing factors of the T-2 cutoff value were clarified, and the prediction model of the T-2 cutoff value was constructed accordingly. The results show that the T-2 spectra of sandstones in the study area can be divided into three types: single steeple peak, double steeple peak, and irregular double peak. The pore diameter of the three types is 1 nm 3x10(4) nm, 1 nm 10(4) nm and 1 nm 4x10(3) nm, respectively. The T-2 cutoff value ranges from 9.72 to 35.16 ms. The correlation analysis suggests that the symmetrical fractal dimension difference and symmetrical multifractal dimension ratio (Dmin-Dmax, D-min/D-max) shows a positive linear correlation with the T-2 cutoff value. The value of T-2 cutoff gradually decreases with the increase of the flow zone indicator (FZI). Therefore, three parameters, including symmetrical fractal dimension difference, symmetrical multifractal number ratio, and FZI are optimized, and the prediction model for the NMR T-2 cutoff value of sandstone samples in the study area is proposed. The introduction of porosity-related parameters compensates for the shortcomings of previous T-2 cutoff value prediction models. At the same time, the prediction model is proven to be accurate and reliable by testing the measured data of the samples near the study area. The results of this paper can be used for further study of the NMR T-2 cutoff value prediction of tight sandstone reservoirs in different areas.