The Neutron Log was introduced commercially in 1941, two years after the Gamma Ray Log, by Well Surveys Inc. (WSI), and was exclusively acquired by Lane-Wells (eventually Lane-Wells absorbed WSI). The Neutron Log responds primarily to the amount of hydrogen in the formation, hydrogen contained in oil, natural gas, and water, and is used to identify zones of porosity. It can be recorded in open and cased holes, separately or in conjunction with virtually any other log. The Neutron Log can be run in any type of borehole liquid (water, oil, or mud), or the hole can be air or gas filled (significant log shifts are seen when logging through a liquid / gas contact in the borehole). The oldest form of the neutron tool is the single detector tool; a dual detector or compensated neutron tool has become commonplace, especially in open hole work; and more recently pulsed neutron tools have found a number of applications by the major wireline companies (the pulsed neutron tool is merely mentioned in passing and is beyond the scope of this humble effort).|
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The Neutron Log can be summarized as the continuous measurement of the induced radiation from the formations penetrated by the borehole, said induced radiation produced by the bombardment of that formation with a neutron source contained in the logging tool. In general, neutrons are not produced in nature; unlike gamma rays, neutrons are particles with a mass which is very nearly that of a hydrogen atom. Modern neutron logging tools use chemical sealed sources, generally Americium-241/Beryllium (AmBe), which sources emit fast neutrons that are eventually slowed by collisions with hydrogen atoms until they are captured (think of a billiard ball metaphor where the similar size of the particles is a factor). The capture results in the emission of a secondary gamma ray; some tools, especially older ones, detect the capture gamma ray (neutron-gamma log). Other tools detect intermediate (epithermal) neutrons or slow (thermal) neutrons (both referred to as neutron-neutron logs). Modern neutron tools most commonly count thermal neutrons with an He-3 type detector.
When the hydrogen concentration of the zone surrounding the well bore is large, 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. If the zone surrounding the well bore has a small concentration of hydrogen, the neutrons must travel farther from the source before being captured. This results in a high count rate and is interpreted as an indication of low porosity. Hard, dense dolomites and limestones usually have high count rates, while porous zones usually have lower count rates. Because shale has significant water bound up, it usually has a low count rate despite its lack of real porosity (this anomaly is actually exploited when imposing porosity scaling). Lithology is often difficult to determine from a neutron curve if it is not presented in conjunction with other logs such as gamma ray, density, etc. For example, absent other curves, a shale might be interpreted as a porous zone.
The Neutron Log is primarily used to evaluate formation porosity, but the fact that it is really just a hydrogen detector should always be kept in mind. It is used for correlation between open hole and cased hole logs, usually in conjunction with the magnetic casing collar locator (CCL) and the gamma ray log; in some areas where gamma ray logs lack character, the Neutron Log is indispensable for correlation purposes. It is used to detect gas in certain situations, exploiting the lower hydrogen density, or hydrogen index, of gas as compared to water and oil (the latter two being about the same, the reason the Neutron Log cannot usually distinguish between water and oil filled porosity). In the field of production logging, it is sometimes used to detect gas entries, leaks, and other gas movement, since the tool response is significantly affected by the presence of gas around it. The chlorine log is a special application that can distinguish between saline water (high chloride content) and oil saturation (maybe, sometimes).
Neutron count rates are generally displayed in tracks two and three of the standard American Petroleum Institute (API) log presentation. 20-30 feet per minute (fpm) has been suggested as a reasonable line speed range for neutron logging, but higher speeds are commonly used by companies logging deeper wells. Like the gamma ray log, the Neutron Log is affected by statistical variations; therefore, a "time constant" circuit is used to average the counts over a period of time, giving a more usable and repeatable log. The interaction between the logging line speed and the time constant produces a "lag" effect. Usually the logging speed and time constant values are selected to produce one foot of lag (the recorder will then be one foot behind, or deeper, than the measure point of the tool in the hole); see Logging Rules of Thumb for easy calculations.
The foregoing is an oversimplified discussion of the Neutron Log;
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