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Christopher M. Prince1, Christopher J. Murray2
(1) Petro Image, LLC, Columbia, SC
(2) Exxon Exploration Co, Houston, TX

Abstract: The relationship between NMR T2 characteristics and macro-scale fabric in sandstones

<T2 spectral characteristics are routinely used to infer petrophysical characteristics in reservoir sandstones. While there is a continuing argument as to the role of diffusion, relaxation times are primarily a function of pore morphology. There are both intuitive and theoretical links between pore size and petrophysical information ranging from permeability to water saturation. However, none of these models account for the macro-scale structure of the pore network.

In clastic sediments, pore size is a function of grain size, diagenetic maturity, and packing. Clastic sediments consist of close-packed clusters of grains separated by packing flaws. The porosity in packing flaws is larger and more well-connected than porosity within close-packed domains and has been shown to have a controlling influence upon both single-phase permeability and distribution of multi-phase fluids within the pore network.

A comparison of porosity observed in thin section with core-derived NMR T2 spectra show a strong link between the characteristics of NMR spectra and the packing characteristics of the sediment. Analysis of Free-Fluid Index (FFI) shows a very strong relationship with the proportion of flawed porosity (PPF), and that FFI tends to overestimate the proportion of effective porosity. Similarly, there is a strong relationship between BFV and microporosity - porosity that cannot be observed in thin section (PUR). The results of this investigation also indicate that the average T2 time is a function of pore size within the flaw network. The precise slope of this relationship is a function of surface relaxivity and can be used to calibrate the spectrum. Once the calibration is complete, flaw size can be calculated directly from the T2 spectrum, and used to precisely estimate permeability.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana