Page 345 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

314 Reservoir Engineering

Geophyslcal Well Logging Tmhnlques
Geophysical well logging has the advantage of being an in-situ measurement
and is able to give a continuous estimate of residual oil saturation versus depth
[343,344]. These features allow the calibration of the measurements in known
water-saturated formations. A more detailed discussion of well logging is given
earlier. An evaluation of logging techniques for measurement of Sm was provided
by Fertl [344].

Logging Devlces
Five measurements that have potential application are:

1. Electrical resistivity. Many devices of different depths of investigation are
available [58, 3451. These devices cannot be used in cased holes unless one
uses some nonconducting casing.
2. Puked prtzrtrrm capture. This name (PNC) covers logs commercially available
such as the Dresser Atlas Neutron Lifetime Log (NLL) and the Schlumberger
Thermal Decay Time (TDT). The PNC has the virtue of being useful in
cased holes.
3. Carbon-oxygen. This measurement has the virtues of being directly sensitive
to carbon and of working in cased holes.
4. Nuclear magnetism. This service is not routinely used but has the unique
virtue of being sensitive only to formation fluids.
5. Dielectric constant. This service is now routine but is limited to open holes.
Its main advantage over resistivity measurements is that water salinity need
not be known.

Electrical Resistivity. Resistivity measurements provide a great range of choice
as to the volume of formation to be sampled, ranging from a few cubic inches
to many cubic feet. Interpretation of the measurements for residual oil saturation
requires a determination of the relation between water saturation and resistivity.
If the true formation resistivity R, and the water resistivity R, are known, then
for clean sandstones [42],

R, = F,R$: (5-252)

where FR is the formation factor. If after measuring &, the fluid around the
hole is replaced by water of resistivity R: (through chemical flushing followed
by displacement with water of resistivity R:), then from Equation 5-252 it
follows that

(5-253)

Clearly, n, the saturation exponent, must also be known. It is often taken to be
n = 2. If cores or other logs are available, better estimates of n can be made.