Temperature and Frequency Dependence of NMR Relaxation Properties of Oil-Based Mud Filtrate

Oil-based mud filtrate (OBMF) invasion significantly alters the petrophysical response of nuclear magnetic resonance (NMR) logging, severely compromising the accuracy of reservoir fluid identification and petrophysical evaluation. However, the NMR relaxation behavior of OBMF under extreme high-temperature (>100°C) and low-frequency (< 2MHz) conditions remains poorly understood. In this study, temperature-dependent NMR experiments were conducted from 30°C to 100°C at a fixed frequency of 21MHz, while frequency-dependent experiments were performed from 1 MHz to 21MHz at 30°C. Using combined analysis of T₂ spectra and T₂-T₁ two-dimensional spectra, the effects of temperature and magnetic field frequency on the relaxation characteristics of OBMF were systematically investigated. The results show that increasing temperature shifts the T₂ distribution toward longer relaxation times, increases T₁ values, and decreases the T₁/T₂ ratio. In contrast, decreasing frequency leads to prolonged T₂ relaxation times, shortened T₁ relaxation times, and an increased T₁/T₂ ratio. Based on these experimental findings, a dual-parameter model incorporating both temperature and frequency was established for OBMF. The proposed model provides a theoretical basis for the analysis and correction of NMR logging data under oil-based mud invasion conditions.