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Abstract

A simple, pore-level model for diffusion and NMR relaxation of oil-water mixtures in water wet pores has been developed. In addition to lending physical insight into the relaxation time behavior of mixed pore fluids, the model can be used to generate a practical tool for interpreting diffusion log data, i.e., a T2 vs. D cross plot. Locating points on the cross plot simultaneously yields the near wellbore water saturation and the rock pore size. For light oils, T2 is shown to be mainly a pore size indicator whereas D is controlled mainly by Sw. The pore size 'resolution' decreases as oil viscosity and/or Sw decreases. A preliminary verification of the model has been made with limited core data and the results of applying the model to interpreting NMR log data are encouraging. The capability to separate the effects of pore size from fluid saturation on NMR response of pore fluids indicates an important potential advantage of the T2 and D measurement combination over standard T2 logging for characterizing formation pore size and related reservoir flow properties.

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