Page 93 - Well Logging and Formation Evaluation

Advanced Log Interpretation Techniques 83

Bound water mode:
• Number of echoes: 100
• Wait time: 0.2ms
• Echo spacing: 0.32ms

Total porosity mode:
• 50 measurements of 10 echoes are made
• Echo spacing: 0.32ms
• Wait time: 5ms

Note that data can also be acquired in a stationary mode. Often a few
stationary measurements, with very long wait times and numbers of
echoes, are acquired as a check that the logging speed being used is not
too fast. NMR properties of reservoir fluids vary with pressure, tempera-
ture, salinity, and viscosity. Table 5.4.1 gives some general values that
may be of use.
Through further TDAs, the software can produce the oil and gas satu-
ration. If resistivity data are input into the software, a secondary mea-
surement of hydrocarbon saturation is made. Limitations in the physics of
early-generation tool were:

1. The difficulty of generating a uniform magnetic field over the parts
of the formation from which the measurements were being made (the
T 1 and T 2 times are dependent on the strength of the static field)
2. Problems with the static magnetic field magnets at downhole
temperatures
3. Problems with logging speed when the relaxation times could be as
long as many seconds

Table 5.4.1
Typical NMR Properties of Reservoir Fluids

2
-9
Fluid T 1 (seconds) T 2 (seconds) D (10 m /s)
Brine (100kppm) at 20–25 20–25 18–22
downhole conditions
(200 bar, 125°C)
Natural gas at 2–3 — 170–180
downhole conditions
Oil, viscosity 10cp 0.1 0.1 0.1