Page 212 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

Formation Evaluation 181

Sidewall Cores. After drilling, cores from the side of the borehole can be taken
by wireline core guns or drills. Guns are less expensive but do not always recover
usable cores. Sidewall drilling devices have become quite common in the last
few years. Up to 20 cores may be cut and retrieved on one trip into the hole.

Cased Hole Logs. Cased hole logs are run to evaluate reservoir performance,
casindcement job quality, and to check flow rates from producing intervals.
The reader is referred to Bateman's book [217] on cased hole logging which
provides a more detailed discussion than is possible in this summary.
Cased hole logs can be broadly divided into two classes:

1. Logs that measure formation parameters through the casing.
2. Logs that measure the parameters within and immediately adjacent to
the casing.

These logs are all combined to monitor fluids being produced, monitor
reservoir performance, and monitor production-string deterioration with time. They
differ from open-hole logs in that the majority of cased hole logs merely monitor
f hid production rather than provide extensive data on formation characteristics.

Cased Hole Formation Evaluation. Two tools are currently being used to
provide formation evaluation in cased holes:
1. Pulsed neutron logs.
2. Gamma spectroscopy tools (GST) logs.
Pulsed Neutron Logs. Pulsed neutron logs are used to monitor changes in fluid
content and water saturation with respect to time. Current tools also provide a
means of estimating porosity They are particularly valuable for [217]:
1. Evaluating old wells when old open-hole logs are poor or nonexistent.
2. Monitoring reservoir performance over an extended period of time.
3. Monitoring the progress of secondary and tertiary recovery projects.
4. Formation evaluation through stuck drill pipe (generally a last resort).

Theory. A neutron generator that consists of an ion accelerator fires deuterium
ions at tritium targets. This produces a burst of 14 keV neutrons which pass
through the borehole fluid (must be fresh water), casing, and cement. The burst
then forms a cloud of neutrons in the formation which are rapidly reduced to
a thermal state by collisions with the atoms in fluids in the rock (made up
primarily of hydrogen atoms). Once in a thermal state, they are most liable to
be captured by chlorine (or boron). The capture process will produce a gamma
ray of capture which is then detected by a scintillometer in the tool. The time
it takes for the neutron cloud to die during the capture process is a function
of the chlorine concentration in the formation fluid. This is then related to
water saturation. Rapid disappearance of the thermal neutron cloud indicates
high water saturation. Slower disappearance of the cloud indicates low water
saturation (i.e., high hydrocarbon saturations). The rate of cloud decay is
exponential and can be expressed by:
N = N e(-") (5-116)