Shale Pore Structure Based on Nuclear Magnetic Resonance Dual T2 Cutoff Values: Insights from Monofractal and Multifractal Characteristics

Summary Fractal theory has been widely applied in the field of microscopic pore structure of rocks. However, the quantitative relationship between monofractal and multifractal characteristics has not been deeply studied. In this study, we investigate seven shale samples from the Longmaxi Formation in the Sichuan Basin to determine the dual cutoff values (T2c1 and T2c2) by nuclear magnetic resonance (NMR) experiments for the classification of the fluids in the pores into fully bound fluid (T2 < T2c1), partially free fluid (T2c1 ≤ T2 < T2c2), and fully free fluid (T2 ≥ T2c2). On the basis of the analysis of shale pore structure, we discuss quantitatively the similarities and differences between monofractal and multifractal characteristics. The results show that T2c1 ranges from 0.70 ms to 1.49 ms and T2c2 ranges from 4.93 ms to 9.47 ms. Full-size pore size distribution (PSD) curves are established from which shale pores can be divided into fully bound fluid pores (PB), partially free fluid pores (PP), and fully free fluid pores (PF). In the monofractal characterization, the larger the average pore size is, the larger the fractal dimension is and the stronger the heterogeneity of the pores is. In the multifractal characterization, the heterogeneity of pore structure is stronger in the low-probability region, corresponding to PF with larger pore sizes. The pore structure is more evenly distributed in the high-probability region, corresponding to PB and PP with smaller pore sizes. The results of monofractal analysis are consistent with those of multifractal analysis. The total dimensions Dt1 and Dt2 under monofractal analysis are both less than 2. The total dimension Dt3 under multifractal analysis is basically between 2 and 3. In general, multifractal analysis can provide deeper insights to help researchers identify and understand the heterogeneity and complexity within the system. Monofractal analysis, however, is more suitable for evaluating the overall characteristics and macro trends of the system. This study provides new insights into the application of fractal theory in the field of microscopic pore structure of porous media.