Abstract
AbstractNuclear magnetic resonance (NMR)T2cutoff value is an important parameter for pore structure evaluation. It is complicated and uneconomical to obtainT2cutoff value by an experimental method; therefore, it is necessary to explore a prediction method ofT2cutoff 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 NMRT2spectrum of each sample, and the relationship between multifractal parameters andT2value was analyzed. The influencing factors of theT2cutoff value were clarified, and the prediction model of theT2cutoff value was constructed accordingly. The results show that theT2spectra 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 ~ 3×104 nm, 1 nm ~ 104 nm and 1 nm ~ 4×103 nm, respectively. TheT2cutoff 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, Dmin/Dmax) shows a positive linear correlation with theT2cutoff value. The value ofT2cutoff gradually decreases with the increase of the flow zone indicator (FZI). Therefore, three parameters, including symmetrical fractal dimension difference, symmetrical multifractal number ratio, andFZIare optimized, and the prediction model for the NMRT2cutoff value of sandstone samples in the study area is proposed. The introduction of porosity‐related parameters compensates for the shortcomings of previousT2cutoff 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 NMRT2cutoff value prediction of tight sandstone reservoirs in different areas.
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