Correction of Echo Spacing and Advanced Permeability Modeling for Key NMR Parameters in Overpressure and Low-Permeability Reservoirs of the YGH Basin

Low-field nuclear magnetic resonance (NMR) acts as an indispensable borehole logging method for the pore size characterization and formation evaluation, including the estimation of the reservoir parameters and fluid discrimination.

The YGH basin , located in the northern South China Sea, is characterized by rapid subsidence rate, high geothermal gradient, and high formation pressure coefficient. The overpressure and low-permeability reservoirs of the YGH Basin is featured with Fine-grained lithology, elevated shale content, minute pore sizes, and complex pore structures . The NMR responses are significantly affected by measurement parameters such as echo spacing. Notable discrepancies exist between the core NMR T₂ spectrum and the NMR logging T₂ spectrum. Key parameters derived from NMR logging, such as the T₂ geometric mean and fractal dimension, fail to accurately represent the true characteristics of the rock, thereby posing substantial challenges for precise permeability evaluation.

For better estimating the reservoir permeability using the low field NMR logging data, we conducted comprehensive petrophysical measurements such as NMR, CT scanning, grain size analysis, and mercury injection capillary pressure(MICP), specifically tailored to the characteristics of overpressure and low-permeability reservoirs. Pore structure parameters were extracted from these petrophysical experiments, and the self-organizing map (SOM) unsupervised clustering method was employed to classify the pore structures of overpressure and low-permeability reservoirs. Based on the principle of phase control, multiple sets of echo spacing core NMR experiments were conducted on representative samples of different pore structure types. Systematic analysis of echo spacing 's impact on T₂ spectra lead to the development of an empirical model relating T₂ geometric mean, fractal dimension, and echo spacing, with a focus on the shortest echo spacing of 0.2 ms. This model provides essential data support for correcting T₂ geometric mean and fractal dimension derived from NMR logging. Building on this research, the traditional Timur formula was refined by fractal dimension, significantly enhancing the accuracy of permeability calculations for overpressure and low-permeability reservoirs.

The research findings indicate that there exists a negative correlation between the T₂ geometric mean value and fractal dimension, with respect to the echo interval. As the pore diameter of the rock diminishes, the porosity intensifies, accompanied by an augmentation in clay content, leading to a greater influence of echo spacing on the geometric mean of T₂ and fractal dimension. By employing an enhanced permeability model, the derived permeability values demonstrate a high degree of consistency with measured data, achieving an average relative error of approximately 15%. This level of accuracy fulfills the criteria for assessing low-permeability reservoirs.