Page 103 - Well Logging and Formation Evaluation

Advanced Log Interpretation Techniques 93

Exercise 5.3. Thin Beds

1. Make a Thomas-Stieber plot using f d and f n. Identify the clean sand
and shale points and establish the types of clay that are present (struc-
tural, dispersed, laminated).
2. Do you consider that your evaluation in this well is affected by thin beds?
3. Also make a plot using f d and V p. Do you learn anything additional
from this plot?

5.7 THERMAL DECAY NEUTRON INTERPRETATION

Thermal decay time tools (TDTs) are used in cased holes in order to
detect changes in the formation saturations occurring with time. Most
commonly these changes arise from:

• Depletion of the reservoir and zones becoming swept with either water
from the aquifer or injection water
• Formation of movement of the gas cap in the reservoir

The tool works by injecting neutrons, generated in a downhole mini-
tron, into the formation. These neutrons get captured by atoms in the for-
mation, most principally chlorine, which then yield gamma ray pulses that
may be detected in the tool. Through the use of multiple detectors, the
tool is able to differentiate between the signal arising from the borehole
and that of the formation.
The components of the formation may be distinguished on the basis of
their neutron capture cross-sections, measured in capture units (c.u.),
denoted by S. The contractors provide charts to predict the values of S
for different rock and fluid types. Typical values are:

= 8c.u. (sandstone), 12c.u. (limestone)
S m
S shale = 25–50c.u.; the value of S measured by the tool in a 100% shale-
bearing interval
S oil = 8c.u.
S gas = 2–10c.u.
S water (fresh) = 22.2c.u.
S water (200kppm NaCl at 250°F) = 100c.u.