Quantifying the Occurrence of Shale Oil in Nanoconfined Kerogen Matrices through Integrated NMR Relaxation Theory and Molecular Dynamics Simulations

Low-field nuclear magnetic resonance (NMR) is a powerful technique for characterizing fluid behavior in shale oil reservoirs. However, the abundant nanopores in shale and the limitations of experimental echo time hinder its further application in characterizing oil occurrence. This study integrates the ratio of longitudinal to transverse relaxation times (<i>T</i>₁/<i>T</i>₂) with molecular dynamics simulations to determine the distributions of dissolved, adsorbed, and free <i>n</i>-decane in kerogen nanoslits and further develops quantitative models to characterize the occurrence characteristics of <i>n</i>-decane in these confined systems. The results indicate that alkane molecules in the same state exhibit identical <i>T</i>₁/<i>T</i>₂ values, with the <i>T</i>₁/<i>T</i>₂ values of dissolved, adsorbed, and free molecules being 25.27, 18.61, and 14.86, respectively. For each occurrence state, both the self-diffusion coefficient of <i>n</i>-decane and its interaction energy with the kerogen nanoslit walls exhibit distinct linear correlations with <i>T</i>₁/<i>T</i>₂. Furthermore, three mathematical models are developed to quantify the relationships between <i>T</i>₁/<i>T</i>₂ and the slit width, the adsorption layer thickness (<i>H</i>), and the free-to-adsorbed mass ratio (<i>m</i>_f/<i>m</i>_a). When <i>T</i>₁/<i>T</i>₂ ranges from 75.21 to 53.25, <i>m</i>_f/<i>m</i>_a = 0, indicating the absence of a free state. This state corresponds to slit widths of 1-3 nm, within which <i>H</i> varies linearly with <i>T</i>₁/<i>T</i>₂. As <i>T</i>₁/<i>T</i>₂ decreases further from 53.25 to 1.00, <i>m</i>_f/<i>m</i>_a increases exponentially while <i>H</i> stabilizes at ∼2.86 nm, suggesting three-state coexistence within 3-50 nm slits. Free-state dominance occurs when <i>T</i>₁/<i>T</i>₂ approaches 1, corresponding to a slit width of approximately 50 nm. Additionally, with increasing kerogen maturity, the kerogen matrix exhibits reduced solubility but enhanced adsorption capacity for <i>n</i>-decane. For kerogen types I-A, II-D, and III-A, <i>T</i>₁/<i>T</i>₂ shows exponential correlations with both slit width and <i>m</i>_f/<i>m</i>_a, indicating free-state dominance at slit widths of 45, 50, and 58 nm, respectively. This work is expected to provide novel insights into shale reservoir evaluation and enhanced oil recovery.