NUCLEAR MAGNETIC RESONANCE LOGGING-WHILE-DRILLING
The measurements made
by Schlumberger's Nuclear Magnetic Resonance Logging-While-Drilling (proVISION)
tool 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.
NUCLEAR
MAGNETIC RESONANCE measurement
The proVISION 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's 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 proVISION 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. Processing was performed onshore by Schlumberger in Sugarland, Texas.
DATA QUALITY
The presence of gas hydrate is indicated by deviation of the magnetic resonance porosity log from other porosity logs (such as density porosity). A limitation of this measurement is that the magnetic resonance porosity may be underestimated in clay-rich formations that have small pores with fast T2 relaxation times. Murray et al. (2006) note that this underestimation of porosity is most likely to take place when the NMR echo spacing is greater than 200 microseconds. The ProVISION tool is limited to a minimum echo spacing of 800 microseconds, and the NMR porosities measured by this tool may be underestimated.
PROCESSED DATA
The PROVISION-LWD data are presented in two ascii files:1) Files named like Hole#-nmr.dat contain the following 1-D 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 Hole#-nmr-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.) |
https://connect.slb.com/~/media/Files/resources/oilfield_review/ors00/aut00/p2_19.pdf
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.
Murray, D. R., R. L. Kleinberg, B. K. Sinha, M. Fukuhara, O. Osawa, T. Endo, and T. Namikawa, 2006, Saturation, Acoustic Properties, Growth Habit, and State of Stress of a Gas Hydrate Reservoir from Well Logs, Petrophysics, 47(2), p. 129-137.