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The Gamma Ray Log

The Gamma Ray Log is one of the most useful and versatile surveys available.  It was introduced commercially in 1939 by Well Surveys Inc. (WSI), and was exclusively acquired by Lane-Wells (eventually Lane-Wells absorbed WSI).  The Gamma Ray Log can be summarized as the continuous measurement of the natural, or in some cases introduced, radioactivity in a well.  It can be recorded in open and cased holes, separately or in conjunction with virtually any other log, or with perforating guns.  It can be conducted as the common "gross count" log, or the more exotic "spectral" log producing separate curves for various isotopes.  The Gamma Ray Log can be run in any type of borehole liquid (water, oil, or mud), or the hole can be air or gas filled.

The Gamma Ray Log is a measurement of the natural radiation of various formations penetrated by a well (or in some cases artificially placed sources of radiation).  Dolomites, limestones, sandstones, anhydrites, and salts typically exhibit a low level of radiation, while shales, clays, certain cherts, and rocks of igneous origin typically have higher levels of radiation.  It is these differences that make the Gamma Ray log very useful in determining lithology.  Gamma rays occur to a lesser or greater extent in most formations, and arise primarily from the radioactive decay of certain elements within the rocks belonging to the radioactive families of Uranium, Thorium, and the radioactive isotope of Potassium, K-40 (this threesome is usually referred to as "KUT").  Gamma rays are essentially short bursts of electromagnetic energy of extremely high frequency with no mass.  These rays are absorbed gradually as they pass through matter, but the more energetic ones are quite penetrating.

The Gamma Ray Log is used in various forms for a wide variety of purposes.  It is used for correlation between open hole and cased hole logs, usually in conjunction with the magnetic casing collar locator (CCL).  It can be used alone or with a CCL for depth control purposes; sometimes sources of radiation are placed in wells as depth markers.  Gamma ray logging is the base technology behind the radioactive tracer survey (RTS or as we prefer RATS); see our RATS overview.  It is used in mineral extraction industries for qualitative and quantitative detection of valuable minerals such as uranium and coal.  It can be used for evaluation of the shale volume in zones of interest.  In older fields where wells were never logged and driller's logs are missing or suspect, the Gamma Ray Log can be a lifesaver (sometimes in conjunction with the Neutron Log) for bed definition.

Modern logging tools use two main types of gamma ray detectors.  Geiger MŁeller (G-M) tubes measure incident gamma rays by gas ionization and are typically less sensitive than the other main type, the scintillation detector.  However, G-M tubes tend to be less costly, more rugged, and can be manufactured to withstand very high temperatures.  The scintillation detector typically uses a sodium iodide crystal coupled to a photomultiplier tube (PMT) to detect tiny flashes of light associated with penetrations of the crystal by gamma rays.  Scintillation detectors are often preferred because of their superior sensitivity, but they have temperature limitations, are somewhat fragile, and are more costly than G-M detectors.  Gamma ray tools have been manufactured in many sizes down to as small as .75 inch.

Gamma ray count rates are generally displayed in the left hand track (track one) 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 gamma ray logging, but higher speeds are commonly used by companies logging deeper wells.  Unstable nuclei of radioactive elements do not disintegrate in a predictable manner; 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.

High Count Rates

Certain applications of the gamma ray tool can generate very high count rates.  Gamma ray tools are often set for divide by 8 or 16 operation when configured for RATS work.  AnaLog Services, Inc. can install a "divide by" circuit in most gamma ray tools not already equipped with same.  Divide by 1, 2. 4, 8, 16, 32, 64, and 128 are easily jumper or switch selectable.


The foregoing is an oversimplified discussion of the Gamma Ray Log; contact us if you need more detailed information.


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09-23-00
Last 10-20-10