Echo Spacing

Key Takeaways

• The physical basis for echo spacing's effect on T2 detection is the CPMG pulse sequence mechanism: after the initial 90-degree pulse tips the hydrogen proton magnetization into the transverse plane, the protons dephase because of magnetic field inhomogeneities (including those intentionally created by the NMR tool's permanent magnet to select a specific depth of investigation shell) and diffusion of molecules through magnetic field gradients; the 180-degree refocusing pulses applied at intervals of TE/2 rephase the protons and generate a spin echo at each TE interval, but the amplitude of each successive echo decays exponentially due to irreversible relaxation processes (T2 relaxation) that cannot be refocused; the first echo is measured at time TE after the 90-degree pulse, and if any T2 component of the formation fluids has a relaxation time shorter than approximately TE/3 to TE/2, those protons will have relaxed before the first echo and will not contribute to the measured signal, causing them to be invisible to the NMR measurement; the minimum detectable T2 is therefore approximately 0.6 to 1.5 times TE, depending on the definition of "detectable" and the specific signal-to-noise ratio of the tool.
• Clay-bound water detection requires short echo spacings because the T2 of clay-bound (interlayer) water in smectite and illite clays is typically 0.3 to 3 milliseconds, reflecting the very short correlation time of water molecules that are highly constrained by their proximity to the clay mineral surface: at a standard TE of 1.2 milliseconds (used in older NMR tools), clay-bound water with T2 below approximately 1 millisecond would be completely undetected, causing the NMR total porosity to match only the free-fluid and capillary-bound porosity and significantly underestimate the true total porosity in shaly formations; at a TE of 0.2 milliseconds (available on modern tools like the Halliburton MRIL-Prime and SLB CMR-Plus in short-TE mode), clay-bound water with T2 as short as 0.1 to 0.2 milliseconds can be detected, allowing a more complete NMR total porosity measurement that can be compared to neutron-density total porosity for clay volume estimation; the trade-off is that shorter TE requires more rapid switching of the NMR tool's RF transmitter and receiver, placing higher demands on the tool hardware and limiting the maximum logging speed if adequate echo count per T2 bin is to be maintained.
• Diffusion measurement using variable echo spacing provides a method for distinguishing oil, gas, and water in the NMR T2 distribution, exploiting the fact that the rate of signal decay due to molecular diffusion through the magnetic field gradient of the NMR tool depends on the square of the echo spacing: by acquiring two or more NMR measurements at different echo spacings (differential echo spacing or diffusion editing), the diffusion-induced decay can be separated from the surface relaxation-controlled T2 decay; gas, with its high diffusion coefficient (10 to 100 times higher than liquids at reservoir conditions), shows a much stronger dependence of apparent T2 on echo spacing than water or oil, providing a diagnostic signature for gas in the NMR measurement that can distinguish gas from water even when their T1 relaxation times overlap; this technique (the MRIL "enhanced diffusion" method or the CMR "DIFAN" diffusion analysis) has been applied to identify gas in tight formations where resistivity logs are ambiguous due to low formation water salinity or clay conductivity effects.
• Logging speed constraints from echo spacing arise because the number of echoes acquired per T2 spectrum measurement (the echo train length, typically 300 to 1,000 echoes) determines the maximum T2 that can be resolved (approximately 0.5 to 1 times the total echo train duration), and the total acquisition time per depth level must be short enough to allow adequate vertical sampling at the logging speed: at a TE of 0.2 milliseconds and 600 echoes, the echo train duration is 120 milliseconds; for the NMR measurement to sample adequate depth intervals for the formation's vertical heterogeneity, the logging speed must be limited to ensure sufficient echo trains per resolution element; typical NMR logging speeds range from 200 to 1,800 feet per hour depending on TE, echo count, and the T2 resolution required; the trade-off between short TE (required for clay-bound water detection), long echo train (required for long T2 components from large pores and hydrocarbons), and logging speed is managed by the petrophysicist in designing the NMR acquisition program for each specific formation evaluation objective.
• Optimizing echo spacing for different reservoir types requires matching the TE to the expected T2 distribution of the formation fluids and pore system: in tight gas sands and shaly formations, short TE (0.2 to 0.4 milliseconds) is required to detect capillary-bound and clay-bound water that defines the minimum T2 cutoff for permeability estimation and free-fluid index calculation; in medium-porosity sandstones with moderate water salinity, a standard TE of 0.6 milliseconds captures most of the formation water T2 distribution without the logging speed penalty of the shortest TE settings; in high-porosity carbonates with large vugs and fractures (T2 values up to 1,000 milliseconds or more), a long TE (0.9 to 1.2 milliseconds) allows longer echo trains at faster logging speeds without sacrificing resolution of the long-T2 components associated with vuggy porosity; in condensate or oil reservoirs, intermediate TE combined with diffusion analysis may be optimized for hydrocarbon identification rather than for porosity accuracy, representing a different balance of the acquisition parameters for the specific petrophysical objective.