Abstract

It has recently become possible to make pulsed nuclear magnetic resonance (NMR) measurements of subsurface geological formations in situ. The longitudinal relaxation time, T1, has long been the parameter of greatest interest to petrophysicists. However modeling shows that T1 measurements are not repeatable when NMR logging tools move past bed boundaries. Therefore measurements of the transverse relaxation time, T2, must be relied upon. Unfortunately, reservoir rocks have properties that make T2 measurements difficult to interpret. We discuss the relationship between T1 and T2, and the measurement conditions for which T2 gives meaningful petrophysical information.

Introduction

Nuclear magnetic resonance (NNM) can provide important information about sedimentary rock formations including the porosity, movable fluid porosity, permeability to fluid flow, and pore size distribution. In order to perform these measurements in situ, where they are of most value, a number of modern borehole NMR instruments have been designed and built.

Historically, laboratory NMR investigations of rocks have been confined to measurements of T1 at proton Larmor frequencies of 10 MHz and above. In contrast, modern NMR borehole logging tools make measurements of T2 at 1 to 2 MHz. To establish a connection between large extant data bases and the new borehole measurements, we have made careful comparisons of T1 relaxation spectra at 2 MHz, T1 relaxation spectra at 10 MHz, and T2 relaxation spectra at 2 MHz. The echo spacing dependence of T2 at 2 MHz has also been determined.

2. Importance of the T1 Measurement
2.1 Permeability

A connection between the NMR relaxation time T1 of water-saturated sandstones and their permeability to fluid flow was first proposed by Seevers. This hypothesis was substantiated by a large body of experiments culminating in the work of Kenyon et al. and others, who found a correlation of wide applicability. The correlation appears to depend on constancy of the intrinsic surface relaxation rate across a wide variety of sandstones, a surprising conclusion; progress is being made in understanding the microscopic nature of this surface interaction.

Attempts to establish similar correlations with carbonates have met with mixed success. The microgeometry of carbonates is much more diverse than that of sandstones. NMR measurements are sensitive to pore size, and when there are large differences between pore and throat sizes, correlations with permeability are not always possible.

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