Abstract
Three-dimensional nuclear magnetic resonance (NMR) provides more reliable fluid typing and petrophysical evaluation in tight sandstone than traditional 1D NMR. However, neglecting the effects of mud invasion saturation and rock wettability under invasion conditions can lead to misinterpretation. To address this, a numerical simulation approach was introduced to examine their effects on 3D NMR responses and the accuracy of fluid identification across three reservoir scenarios: water invaded by oil-based mud (OBM), water invaded by oil, and oil invaded by OBM, each evaluated at varying saturation levels. The numerical simulation algorithm was developed based on tight sandstone pore geometry, constituent fluid distributions, and NMR signal source parameters. Numerical solutions to simulate the longitudinal relaxation time (T1) and transverse relaxation time (T2) distribution were obtained by integrating differential and integral relationships between relaxation time and pore-scale properties. The formulation incorporated statistically distributed bulk relaxation and diffusion parameters modeled as log-normal Gaussian functions. Analysis of the 3D NMR response demonstrated that, in conditions of water-wet and oil-wet, a decrease in wetting fluid saturation increased surface relaxation, resulting in shorter T2 and T1. Meanwhile, the nonwetting fluid relaxation time remained unchanged, with only a change in amplitude observed. By contrast, fluid saturation significantly influenced the fluid phase’s relaxation time and signal amplitude under mixed-wettability conditions. Moreover, the fluid type recognition could become complex. The T1 to T2 geometrical mean (T1lm/T2lm) ratio was analyzed as a potential parameter for classifying different reservoirs and, consequently, to help identify fluid types. Although the analysis revealed a strong correlation between T1lm/T2lm and oil/OBM saturation under varying wettability conditions, the similarity of T1lm/T2lm across the simulated reservoir scenarios limits its effectiveness for precise classification.
Paper Information：
Mohmmed Ishag, Xinmin Ge*, Yiren Fan, Zhenghua Sha, Liang Wang, Cheng Zhai; Three-dimensional simulation insights into NMR responses of fluid constituents in tight sandstone reservoirs: Effects of oil-based mud invasion and wettability. Geophysics 2026, 91 (4): J1–J20. https://doi.org/10.1190/GEO-2025-0268
