Numerical investigation on 2-D NMR response mechanisms and the frequency conversion of petrophysical parameters in shale oil reservoirs

Characterizing the petrophysical properties holds significant importance in shale oil reservoirs. Two-dimensional (2-D) nuclear magnetic resonance (NMR), a nondestructive and noninvasive technique, has numerous applications in petrophysical characterization. However, the complex occurrence states of the fluids and the highly non-uniform distributions of minerals and organic matter pose challenges in the NMR-based petrophysical characterization. A novel T₁-T₂ relaxation theory is introduced for the first time in this study. The transverse and longitudinal relaxivities of pore fluids are determined based on numerical investigation and experimental analysis. Additionally, an improved random walk algorithm is proposed to, on the basis of digital shale core, simulate the effects of the hydrogen index (HI) for the organic matter, echo spacing (T_E), pyrite content, clay mineral type, and clay content on T₁-T₂ spectra at different NMR frequencies. Furthermore, the frequency conversion cross-plots for various petrophysical parameters influenced by the above factors are established. This study provides new insights into NMR-based petrophysical characterization and the frequency conversion of petrophysical parameters measured by laboratory NMR instruments and NMR logging in shale oil reservoirs. It is of great significance for the efficient exploration and environmentally friendly production of shale oil.