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===Pulsed neutron tool===

===Pulsed neutron tool===

Pulsed neutron devices are electronic devices that generate pulses of high energy neutrons. These high energy neutrons bombard the formation, losing energy as they collide with atoms of the rock. Eventually, the neutrons lose so much energy that they are captured (generally by chlorine that exists as part of the salt dissolved in the formation waters). When a neutron is captured, a gamma ray is emitted. The detectors in the pulsed neutron tool are designed to measure these “capture” gamma rays, thus, a “capture cross section” of the formation through casing can be determined. This formation property allows one to estimate the water saturation when porosity and formation water salinity are known. A typical pulsed neutron log is shown in [[:basic-cased-hole-tools_fig1.png|Figure 1]].

[[file:basic-cased-hole-tools_fig1.png|thumb|{{figure number|1}}Typical presentation of a pulsed neutron log. Copyright: Schlumberger, 1986.]]

[[file:basic-cased-hole-tools_fig1.png|thumb|{{figure number|1}}Typical presentation of a pulsed neutron log. Copyright: Schlumberger, 1986.]]

Pulsed neutron devices are electronic devices that generate pulses of high energy neutrons. These high energy neutrons bombard the formation, losing energy as they collide with atoms of the rock. Eventually, the neutrons lose so much energy that they are captured (generally by chlorine that exists as part of the salt dissolved in the formation waters). When a neutron is captured, a gamma ray is emitted. The detectors in the pulsed neutron tool are designed to measure these “capture” gamma rays, thus, a “capture cross section” of the formation through casing can be determined. This formation property allows one to estimate the water saturation when porosity and formation water salinity are known. A typical pulsed neutron log is shown in [[:file:basic-cased-hole-tools_fig1.png|Figure 1]].

The gamma rays detected by the pulsed neutron devices can also be processed in a manner similar to the compensated neutron and provide a very similar estimation of porosity. This estimation of porosity can be reasonably good when the formation water is relatively salty and the formation does not contain significant amounts of gas. This combined measurement of capture cross section and porosity from this single device allows the pulsed neutron to be used frequently as a single pass cased hole formation evaluation device. As with the compensated neutron, the presence of gas in the formation will cause the pulsed neutron to underestimate porosity significantly.

The gamma rays detected by the pulsed neutron devices can also be processed in a manner similar to the compensated neutron and provide a very similar estimation of porosity. This estimation of porosity can be reasonably good when the formation water is relatively salty and the formation does not contain significant amounts of gas. This combined measurement of capture cross section and porosity from this single device allows the pulsed neutron to be used frequently as a single pass cased hole formation evaluation device. As with the compensated neutron, the presence of gas in the formation will cause the pulsed neutron to underestimate porosity significantly.