Wettability effects in a sandstone reservoir and outcrop cores from NMR relaxation time distributions

An extensive literature has emerged on the use of nuclear magnetic resonance (NMR) relaxation methods for the determination of various petrophysical properties such as porosity, permeability, initial water saturation and free fluid index. Although observations were first made in the 1950s, relatively few papers have appeared, which use NMR methods to determine the wettability state of a crude oil/brine/rock (COBR) system. Differences in the relaxation time distributions (for both T₁ and T₂) at the various saturation stages in the Amott test cycle provide both qualitative and quantitative information on the wettability state in sandstone samples. Two particularly convenient ways of plotting our results are proposed based on: (a) the arithmetic mean relaxation time differences given by ?T₁=<T₁ at S_or>-<T₁ at S_wi> and ?T₂=<T₂ at S_or>-<T₂ at S_wi>; and, (b) the arithmetic mean relaxation time ratios given <T₁ at S_wi>/<T₁ at S_or> and <T₂ at S_wi>/<T₂ at S_or>. In each case, these quantities are plotted against the Amott-Harvey (AH) index and good linear relationships are found. We propose measuring the relaxation times at the S_wi and S_or stages of a flood cycle and plotting the results in this way as the basis of quantitative NMR-based tests to characterise rock wettability. We find that both the T₁ and T₂ relaxation time data correlate with the wettability condition of the system although the T₁ plots give rather better wettability indicators. This implies that both relaxation times carry broadley similar information on wettability. Although the correlations are somewhat weaker for T₂, there is some potential for indicating wettability alteration using T₂ distributions obtained from downhole NMR logging tools. Comparing the corresponding T₁ and T₂ cumulative curves before and after ageing indicates that wetting alteration first takes place in the larger pores, while the smaller pores appear to remain water-wet. The number of wetting altered pores (from water-wet ?weakly water-wet?oil-wet) increases with the increase in ageing time, indicating that the samples studied here are showing mixed wettability behaviour on ageing. © 2002 Elsevier Science B.V. All rights reserved.