Nuclear magnetic resonance features of low-permeability reservoirs with complex wettability

The nuclear magnetic resonance T2 spectra of low-permeability reservoirs with complex wettability were studied using the samples from the Chang 8 Member, Upper Triassic Yanchang Formation, Ordos Basin, China. Abnormally high resistivity and normal resistivity core samples were selected. NMR T2 spectra under different wettability and water saturation conditions, contact angles and Amott wettability indexes were designed and tested. The test results show that under fully brine-saturated condition, the NMR T2 spectra of normal resistivity core samples reflect surface relaxation of water, while the samples with abnormally high resistivity exhibit wide unimodal T2 spectrum, consisting of both surface and volume relaxation of water, which indicates that these cores are not fully water-wet after washing oil. In the process of oil displacing water, the NMR T2 spectra of normal resistivity core samples present bimodal feature, and those of abnormal high resistivity core samples (both non-ageing and ageing) mainly show the same unimodal feature as those measured under fully brine-saturated condition. Based on these results, it can be inferred that the wettability change of abnormally high resistivity core samples to oil-wet has basically completed during oil displacing water process, and the ageing process has little effect on the wettability of abnormally high resistivity core samples. In the process of water displacing oil to residual oil, the NMR T2 spectra of abnormally high resistivity core samples generally show trimodal feature, among which, the shortest relaxation time spectrum peaks coincide with that under irreducible water saturation condition, the moderate ones reflect surface and volume relaxation of residual oil, and the longest ones reflect surface and volume relaxation of water in large pores. Key words: low-permeability reservoir, nuclear magnetic resonance, relaxation mechanism, wettability, T2 spectrum