Les appareils de diagraphie Geovista se distinguent de par leurs dimensions Sonde Gamma Ray; Sonde Gamma ray Spéctroscopique; Sonde Résistivité. (°C – MPa) – Modules: – Availabilities upon request –. Telemetry / GR · Sprectrum Gamma Ray · Compensated Neutron · Pe Density /. The formation evaluation gamma ray log is a record of the variation with depth of the natural radioactivity of earth materials in a wellbore. Measurement of natural.

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The formation evaluation gamma ray log is a record of the variation with depth of the natural radioactivity of earth materials in a wellbore. Measurement of natural emission of gamma rays in oil and gas wells are useful because shales and sandstones typically have different diagraphle ray levels.

Shales and clays are responsible for most natural radioactivity, so gamma ray log often is a good indicator of such rocks.

In addition, the log is also used for correlation between wells, for depth correlation between open and cased holes, and for depth correlation between logging runs. Natural radioactivity is the spontaneous decay of the atoms of certain isotopes into other isotopes.

diagraphie pétrole Archives – SEMM Logging – well logging services – mesures diagraphiques

If the resultant isotope is not stable, it undergoes further decay until a stable isotope forms. The decay process is usually accompanied by emissions of alpha, betaand gamma radiation.

Natural gamma ray radiation is one form of spontaneous radiation emitted by unstable nuclei. Isotopes naturally found on earth are usually those that are stable or have a decay time larger than, or at gama a significant fraction of the age dagraphie the earth about 5 x 10 9 years.


Isotopes with shorter halflifes mainly exist as decay products from longer lived isotopes, and, as in C14, from irradiation of the upper atmosphere.

Radioisotopes with a sufficiently long halflife, and whose decay produces an appreciable amount of gamma rays are:. Each of these elements emits gamma-rays with distinctive energy. Figure 1 shows the energies of emitted gamma-ray from the three main isotopes. Potassium 40 decays directly to stable argon 40 with the emission of 1. Uranium and thorium decay sequentially through a long sequence of various isotopes until a final stable isotope.

The spectrum of the gamma-rays emitted by these two isotopes consists of gamma-ray of many different energies and form a complete spectra. gaamma

Formation evaluation gamma ray

The peak of thorium series can be found at 2. The most common sources of natural gamma rays are potassium, thorium, and uranium.

These elements are found in feldspars i. Gamma-ray detected by Gamma-ray detector in an oil or gas wells, is not only a function of radioactivity of the formations, but also other factors as follows:. In addition, all radioactive phenomena are random in nature. Count gammaa vary about a mean value, and counts must be averaged over time to obtain a reasonable estimate of the mean.

The longer the averaged period and the higher the count rate, the more precise the estimate. Sample of corrections required for different gamma-ray tools are available from Schlumberger.

Gamma ray log interpretation show different peaks in well. Shale are represent the Sharp Peaks and its range is API and contain the high amount of potassium. Older gamma-ray detectors use the Geiger-Mueller counter principle, but have been mostly replaced thallium-doped sodium-iodide NaI scintillation detector, which has a higher efficiency.


NaI detectors are usually composed of a NaI crystal coupled with a photomultiplier. When gamma ray from formation enters the crystal, it undergoes successive collisions with the atoms of the crystal, resulting in a short flashes of light when the gamma-ray is absorbed. The light is detected by the photomultiplier, which converts the energy into an electric pulse with amplitude proportional to the gamma-ray energy.

The number of electric pulses is recorded in counts per seconds CPS. The higher the gamma-ray count rate, the larger the clay content and vice versa.

Primary calibration of gamma-ray tool is the test pit at the University of Houston. The artificial formation simulate about twice the radioactivity of a shale, which generates API units of gamma radiation. The detector crystal is affected by hydration and its response changes with time. Consequently, a secondary and a field calibration is achieved with a portable jig carrying a small radioactive source.

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