Neutron Log

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Exploration Technique: Neutron Log

Exploration Technique Information
Exploration Group: Downhole Techniques
Exploration Sub Group: Well Log Techniques
Parent Exploration Technique: Well Log Techniques
Information Provided by Technique
Lithology: if used in conjunction with other logs, this technique can provide information on the rock type and the porosity
Stratigraphic/Structural: Corelation of rock units
Hydrological: Estimate of formation porosity
Neutron Log:
The neutron log responds primarily to the amount of hydrogen in the formation which is contained in oil, natural gas, and water. The amount of hydrogen can be used to identify zones of higher porosity.
Other definitions:Wikipedia Reegle

Neutron porosity logging tools consist of a neutron source, located near the bottom of the logging tool and one or more detector located 30 cm (12") to 40 cm (16") above the source. If there are two (double) detectors, the second one is located about 60 cm (24 ") above the source, it is a compensated neutron tool.

When the hydrogen concentration of the zone surrounding the well bore is large because there is a large amount of fluid in that zone, most of the neutrons are slowed down and captured close to the well bore. This results in a low count rate and is interpreted as an indication of high porosity meaning that a large portion of the volume is filled with fluid. If the zone surrounding the well bore has a small concentration of hydrogen because of low amounts of fluid, the neutrons must travel farther from the source before being captured. This in turn results in a high measured count rate and is interpreted as an indication of low porosity. Hard, dense dolomites and limestones with low porosities usually have high count rates, while porous zones such as sandstones usually have lower count rates.

Calibration curves developed for the oil and gas industry from porosoties calculated from core analysis have set a standard based on limestone porosity. It has been observed that limestone porosity calibration used in the logs when compaired to igneous rocks varies by about 3%.

Use in Geothermal Exploration
Identification of formation porosity.

Data Access and Acquisition
Neutron logging tools commonly use chemical sealed sources, generally Americium-241/Beryllium (AmBe). These sources emit fast neutrons that are eventually slowed down by collisions with hydrogen atoms found in fluids such as oil, gas or water until they are captured at the receiver. When the sourced neutrons are received they produce a secondary emission which is detected and counted. The higher the count means that fewer of the sourced neutrons were slowed down by hydrogen atoms between the source and the receiver.
Best Practices
It is best to use neutron logging in addition to other logs to best determine the rock type and porosity. The use with gamma ray logging can reduce the possibility of potentially mistaking a shale as a high porosity zone since shales have a different gamma ray signal than sandstones.
Potential Pitfalls
Neutron logging used without other logs can provide misleading information on formation porosity. Shales tend to have low porosities, but because a large amount of water is bound up within the shale a large number of the hydrogen atoms will be slowed down and it will result in a low count rate at the detector.

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