Abstract:

In the evaluation of low-porosity and low-permeability light oil reservoirs in the eastern South China Sea using nuclear magnetic resonance(NMR) logging data, the permeability values tend to be overestimated. To improve the accuracy of NMR-derived permeability calculations, it is essential to thoroughly analyze the causes of this overestimation and establish corresponding correction methods. This paper systematically conducted NMR transverse relaxation time (T₂) spectroscopy experiments under different saturation states, including water-saturated, bound water, oil-saturated, and residual oil states. By comparing the T₂ spectra of the same core sample under water-saturated and oil-containing states, it was observed that light crude oil causes a significant rightward shift in the distribution of macropores within the T₂ spectrum, while the distribution of micropores remains largely unchanged. This indicates that light oil is the primary cause of T₂ spectrum anomalies and NMR permeability deviations. Based on this finding, T₂ cutoff values for different reservoir types were determined through core classification, and a T₂ spectrum correction method calibrated for macropore components was established. Results demonstrate that this method effectively corrects T₂ spectra from light oil-bearing reservoirs to fully water-saturated state, significantly enhancing the accuracy of NMR-derived permeability measurements.

Key words: nuclear magnetic resonance (NMR), T₂ spectrum correction, permeability, low permeability sandstone, light crude oil