ODP Leg 204 Nuclear Magnetic Resonance Data Processing
The measurements made by Schlumberger's Nuclear Magnetic Resonance Logging-While-Drilling tool (MRT) are explained here. The basic technology behind this tool is similar to modern wireline nuclear magnetic resonance technology (Allen et al., 2000; Horkowitz et al., 2002), and is based on measurement of the relaxation time of the magnetically induced precession of polarized protons.
The NMR tool measures the nuclear magnetic resonance properties of hydrogen in the formation. Initially, the hydrogen atoms are aligned in the direction of a static magnetic field (B0). The hydrogen atoms are then tipped by a short burst from an oscillating magnetic field that is designed so that they precess in resonance in a plane perpendicular to B0. The precession of the hydrogen atoms induces a signal in the tool antenna, and the decay of this signal is measured as the transverse relaxation time, T2. Because the formation contains hydrogen in different forms (in water in large pores, small pores, and bound in clay minerals, and in methane hydrate), there is a distribution of T2 times, here given from 3 ms to 3 s. The T2 distribution is the basic output of NMR measurement. It is further processed to give the total pore volume (the total porosity) and pore volumes within different ranges of T2, such as the bound and free fluid volumes.
Presence of gas hydrate is indicated by deviation of the magnetic resonance porosity log from other porosity logs (such as density porosity). See the Initial Reports volume for further information and interpretation of the NMR-LWD logs.
The Schlumberger-Anadrill NMR tool (MRT) used during Leg 204 was an experimental tool; processing was performed onshore by Sclumberger in Sugarland, Texas.
The MRT data quality is high throughout most of the interval in all nine holes drilled during Leg 204. MRT data quality may be degraded, however, when the distance between the tool sensor and the wall of the borehole is greater than 1 inch. The differential caliper log (DCAL) is recorded by the LWD density tool and provides a measure of this distance. DCAL logs from the LWD drill holes measure values < 1 inch over 90-95% of the total sections drilled. The uppermost 10-30 mbsf of each hole typically washed out to >1 inch due to drilling disturbance of the soft sub-seafloor sediments. Deeper intervals in only one hole (Hole 1244D) shows deflections of the DCAL measurement of up to 1 inch where borehole breakouts occurred below 250 m depth. The NMR measurements may be degraded in these intervals.
The NMR-LWD data are presented in two ascii files:
1) Files named like 1249A-nmr.dat contain the following 1-D NMR log data:
DEPTH (mbsf): sub-bottom depth
MRP (%) magnetic resonance porosity
BFV (%) bound fluid volume
FFV (%) free fluid volume
T2LM (msec) Log mean T2 relaxation time
2) Files named like 1249A-t2dist.dat contain the T2 distribution (spectra):
DEPTH (mbsf) sub-bottom depth
T2 (%): at each depth, values of % volume are given for 30 relaxation times (T2) ranging from 3 to 3000 msec. The scale is logarithmic, and T2 is given both in msec and as the log10 of the time in msec, from 0 to 29 (because this is a whole number, = 10 * log10(T2/3) ≠ 0.5)
Allen, D., et al., 2000, Trends in NMR Logging. Oilfield Review, Autumn 2000 issue, p2-19.
PDF file available:
Collett, T.S., 2001, Review of well-log analysis techniques used to assess gas-hydrate-bearing reservoirs: In Natural Gas Hydrates: Occurrence, Distribution, and Detection, American Geophysical Union, Geophysical Monograph 124, p. 189-210.
Horkowitz, J., Crary, S., Ganesan, K., Heidler, R., Luong, B., Morley, J., Petricola, M., Prusiecki, C., Speier, P., Poitzsch, M., Scheibal, J.R., and Hashem, M., 2002, Applications of a new magnetic resonance logging-while-drilling tool in a Gulf of Mexico deepwater development well: Proceedings of the Society of Professional Well Log Analyst Annual Logging Symposium, June 2-5, 2002, Paper EEE, 14p.