Pulsed Neutron Decay Logging Tool

The pulsed neutron decay logging tool has a neutron generator which is used in a controlled manner to produce bursts of high-energy neutrons along with two detectors. These detectors detect the secondary radiation resulting from interactions between the neutrons emitted from the generator and the nuclei of the elements in the borehole region and formation. The electronic circuit then records the detection times relative to a time reference related to the neutron production bursts. The downhole and uphole circuit controls the operation of the tool, and with the surface system, processes the spectrum of die-away or decay detection times. From these data the system estimates earth formation properties which is then used in the evaluation of the reservoirs. The logging tool can be logged through open or cased hole.

The detector is designed to detect gamma rays resulting from capture of thermal neutrons which is produced as a result of slowing down and  the high-energy source neutrons getting thermalized or the thermal neutrons.

Features

• This tool is considered as a radioactive tool and is commonly used in cased hole. It can be used in open hole too. As the tool is of small diameter, it can be used through tubing. The data can be of very importance for taking decisions in development wells.
• Pulsed neutron source can be controlled by turning the generator on or off using high voltage and hence it is safe to transport and store the tool. When the tool reaches a target depth, the generator can be switched on and the radiated neutrons will be captured by water and formation.
• The tool has high accuracy and wide measurement range.
• The tool is especially applicable in high saline environment and will can be used to identify water and formation.

Applications

Extremely Fresh water and Oil has almost the same Sigma, so for fresh water environments, Sigma （Σ） logging can not be used to identify between oil and water .

For high salinity oilfield, there are four main applications:

• During conventional logging, the sigma Σ curve, can directly identify oil, water and gas. With similar method and data, it is able to calculate out Saturation of water (Sw).
• For water injection-logging, it can accurately calculate current oil saturation (So.)
• With time decay logging, the tool can identify the contact of the water, oil and gas and estimate the Sw.
• With water injection logging, residual oil saturation can be calculated. This is not affected by the salinity.

Pulsed neutron logging can be used to identify gas zones too. The Ratio of Near to Far indicates porosity and it is similar to the SS/LS ratio of a normal compensated neutron tool. By analyzing the differences between RNF, ΣFar and ΣNear, it can indicate effectively the gas layer. This can be used in any salinity environment.