Formation Evaluation Through Casing - Cased-Hole Log Analysis and Reservoir Performance Monitoring - page 151

Pulsed Neutron Logging
Pulsed neutron logs provide a means of evaluating a formation after the well has
been cased. It is of particular value for:
1. Evaluating old wells, where the original open-hole logs are inadequate 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, as a last resort should the drill-pipe become stuck.
It is one of the most widely used logging methods in cased holes at the present
time. Other nuclear measurements that have been developed as extensions of the
basic pulsed neutron technology include logs based on the detection of gamma rays
resulting from inelastic neutron collisions with nuclei in the formation. These
include carbon/oxygen logs and the so-called elemental concentration logs. The
same basic tool with some modification can also be used for oxygen activation
logging as a flow measuring system. Such applications are covered in Chap. 5 .
Principle of Measurement
Many different service company tools are available for use in cased wells. A typical
through tubing version will have an OD of 1-11/16″. For use in cased holes without
tubing full diameter tools (3-5/8″ OD) are also commonly used. Regardless of the
tool used, the principle of measurement remains the same although their operating
systems are all slightly different. 2
A neutron generator is turned on for a very short period of time. As a result, a
burst of fast neutrons leaves the tool; and, since neutrons can easily pass through
both the steel housing of the tool and the tubing/casing, a cloud of neutrons is
formed in the formation. Fast neutrons soon became “thermalized” by collisions
with atoms in the formation. The most effective thermalizing agent is the hydrogen
present in the pore space in the form of water or hydrocarbon. Once in the thermal
state, a neutron is liable to be captured. The capture process depends on the capture
cross section of the formation. In general, chlorine dominates the capture process.
Since chlorine is present in formation water in the form of salt (NaCl), the ability of
the formation to capture thermal neutrons reflects the salt content and hence the
water saturation. The capturing of a thermal neutron by a chlorine atom gives rise to
a capture gamma ray. Pulsed neutron tools therefore monitor these capture gamma
rays. Thus, the common elements of all commercial pulsed neutron tools are (1) a
2 Historically the first pulsed neutron tool was the Dresser Atlas Nuclear Lifetime Log (NLL) later
to become the PDK-100. Schlumberger's tool was known as the TDT and Halliburton's as the
TMD Descendents of these pioneering tools now go by such trade names as TDT-P, RPM & CRE.