Study on the response mechanisms of nuclear magnetic resonance (NMR) log in tight oil reservoirs

The nuclear magnetic resonance (NMR) log responses of Permian tight oil reservoirs of X-Basin in West China are extremely complicated. To be specific, the T-2 spectra of the tight oil reservoirs with poor pore structures behave widely, which are incomprehensible and hard to interpret. What's more, the NMR log responses of the tight oil reservoirs are paid little attention currently. Based on the characteristics of tight oil reservoirs, this paper mainly study the effects of carbonate contents on the T-2 spectra and the relaxation mechanisms caused by different wettability through the experiments on three groups of samples, including NMR on different state samples, mercury injection capillary, X-ray diffraction (XRD) and Amott wettability measurements. The results indicate that both the pore size distribution and carbonate contents have influences on T-2 spectra. Although the shapes of T-2 spectra are controlled by pore distribution, the T-2, spectra show more long relaxation components and less short relaxation components with increasing carbonate contents. And the T-2 relaxation mechanism of "as received" state samples and vacuum and water saturated samples are more complex due to oil wet surface. The analysis suggests that the T-2 relaxation signals of "as received" state samples are attributed to the surface relaxation of water and the surface and bulk relaxation of oil, and that the T-2 relaxation signals of vacuum and water saturated samples are dominated by the surface and bulk relaxation of water. Further, the NMR log responses are interpreted reasonably by using those results. Besides the effects of pore size distribution, the complicated NMR log responses are mainly caused by two other factors. One is that the low relaxivity of water wet surface of carbonate minerals make the T-2 distributions of water move to long relaxation positions. The other is that the relaxivity of oil wet surface is even lower, causing the oil signals dependent on the surface and bulk relaxation. Thus, in turn, result in the wide T-2 distributions.