Achieving high vertical and spectral resolution of nuclear magnetic resonance (NMR) logging involves much more than antenna aperture, stacking, and logging speed. A systematic approach is necessary to optimize hardware, the data acquisition sequence, and processing algorithms that are particularly suited for enhancing resolution without detrimentally affecting repeatability. This paper discusses important aspects for obtaining high-vertical-resolution or high-spectral-resolution NMR logs and demonstrates the validity of these methods using the specifications and parameters consistent to the new Xaminer® magnetic resonance (XMR™) logging tool. Moreover, the concepts, techniques, and methodologies should be applicable to any NMR logging instrument.
A preferred NMR logging tool should possess the following features:
high vertical resolution at reasonable logging speeds,
high spectral resolution to resolve fluid constituents embedded in relaxation time distributions,
the capability to acquire data with a short interecho time (TE) to capture fast decay components and increase data density,
an adequate field gradient to enable diffusivity sensitivity, and (5) robust data quality that is immune to tool motion, borehole rugosity, and environment variations. The realization of these comprehensive requirements is possible with the aid of substantial sensor, environment, and spin-dynamics simulations.
To achieve high vertical and spectral resolutions of NMR logging, a systematic approach for optimization is necessary. First, the antenna aperture is optimized to be sufficiently short for static resolution but sufficiently long to avoid the need of substantial motion correction for high-speed logging. Second, broadband multi-frequency with narrow, shaped radio-frequency (RF) pulses are used to ensure high dynamic resolution with robust logging speed and to increase the frequency-band packing density to boost the overall signal-to-noise ratio (SNR) while minimizing the need of vertical stacking. Narrow pulses also enable short-TE data acquisition, which increases the time-domain data density. Third, a highly focused antenna minimizes the sensitivity to borehole salinity, enabling adequate SNR, even in high-salinity boreholes. Fourth, a combination of narrow, shaped-RF pulses and an intermediate gradient range results in wider sensitive volumes of individual frequencies. These design considerations have been realized in the new wireline NMR logging tool: XMR.
