ABSTRACT

Nuclear Magnetic Resonance (NMR) tools are sensitive to proton density of the formation, and thus to the formation hydrogen index. Therefore, the measurement can be used to detect gas, in a manner analogous to the neutron log. The first part of this paper derives the Amplitude Method to evaluate formation gas volume. It quantifies the reduction of the apparent formation hydrogen index and relates it to gas volume. It has been successfully applied in the field, and an example is presented here. In addition, presence of gas has more subtle effects on the physics of NMR measurement. A recent paper has described how a pulsed NMR measurement can detect gas in the formation, by taking advantage of the static field gradient to measure the effect of diffusion on the observed T2 relaxation time. The authors proposed the ?Shifted Spectrum? method to observe diffusion from data at two different echo spacings. The method involves taking a difference between the two T2 distribution spectra, whereby the water and oil signals would cancel out, and the remaining signal would have a characteristic gas signature. It can be shown, however, that the Shifted Spectrum method can be unreliable, because of secondary distortions in the shapes of the T2 distributions. The second part of this paper describes a robust method for identification of gas via detection and measurement of diffusion, as applied to the data from Schlumberger?s Combinable Magnetic Resonance (CMR) tool. The method, called ?Echo Ratio Method?, uses the ratio of smoothed measured echo trains for two passes (at widely differing echo spacing) in a fit to an analytical expression. This expression is a function of the formation apparent diffusion coefficient and the CMR field gradient distribution. The Echo Ratio method has been successfully applied to CMR data from two commercial wells. The advantage of this method over the Shifted Spectrum method can be easily seen from these results.

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