Gamma Ray Log

GAMMA RAY LOG

Gamma Rays are high-energy electromagnetic waves which are emitted by atomic nuclei as a form of radiation.

Gamma ray log is measurement of natural radioactivity in formation verses depth.

It measures the radiation emitting from naturally occurring U, Th, and K.

It is also known as shale log. GR log reflects shale or clay content. Clean formations have low radioactivity level. Correlation between wells, Determination of bed boundaries, Evaluation of shale content within a formation, Mineral analysis, Depth control for log tie-ins, side-wall coring, or perforating. Particularly useful for defining shale beds when the sp is

featureless, GR log can be run in both open and cased hole.

Gamma rays can react with matter in three distinct manners:

· Photoelectric effect, where a gamma ray collides with an electron, is absorbed, and transfers all of its energy to that electron. In this case, the electron is ejected from the atom.

· Compton scattering, where a gamma ray collides with an electron orbiting some nucleus. In this case, the electron is ejected from its orbit and the incident gamma ray loses energy.

· Pair production, where a gamma ray interacts with an atom to produce an electron and positron. These will later recombine to form another gamma ray.

Photoelectric interaction can be monitored to find the lithology-related parameter, Pe. For the conventional density measurement, only the Compton scattering of gamma rays is of interest.

Conventional logging sources do not emit gamma rays with sufficient energies to induce pair production, therefore pair production will not be a topic of this discussion.

GAMMA RAY LOGS:

Of the 104 (108?) naturally occurring elements, 83 have more than one form, or isotope. Isotopes are inherently unstable and, over time, decay to the lower energy, stable form. The half life of an isotope may be millions of years, days, or even milliseconds.

The most common isotopes are the uranium series, the thorium series, and potassium, which has only one isotope. These elements are found in nature, and amongst other things, emit natural gamma rays that can be detected by a logging tool in a borehole.

Each of the above elements naturally emits gamma rays which are distinctive in both number and energy. One gram of potassium 40 emits an average of 3.4 photons per second at a fixed 1.46 MeV energy. But an equal weight of either thorium or uranium produces respectively 12,000 or 26,000 gamma rays per second with a spectrum of energies that average 0.5 MeV.

In the logging industry, gamma ray flux has been recorded in micrograms Radium equivalent per ton (ug Ra equiv / ton) prior to about 1960. After that time, logs were calibrated in API units based on known radiation levels of artificial formations in test pits located in Houston. The usual scale for old style logs was 0 to 10 ug Ra and 0 to 100, 0 to 120, or 0 to 150 API units for newer logs.

There is an exact conversion between ug Ra and API units but since the old logging tools were rarely calibrated, this conversion is seldom useful. The pragmatic solution is to multiply ug Ra by 10 to obtain an approximate API units scale.

The counting rate at the detector in a gamma ray logging tool is naturally influenced by the tool itself and the borehole environment. However, the primary response will be related to the number of atoms per unit mass emitting gamma rays.

In gamma ray spectral logging, the three main gamma ray contributors, potassium, thorium, and uranium, give gamma rays of different energy levels. By appropriate filtering, the total gamma ray flux can be separated into the three components. This aids log analysis as thorium is a good shale indicator when uranium masks the total GR response. Thorium-potassium ratio and other combinations of curves can be used for mineral identification and clay typing. Finally, uranium counts can be subtracted from the total counts to give a uranium corrected gamma ray curve that is easier to use and to correlate from well to well.

Figure 19.46: Spectral breakdown of total GR into its three major components

Gamma rays emitted by the formation rarely reach the detector directly. Instead, they are scattered and lose energy through three possible interactions with the formation; the photoelectric effect, Compton scattering, and pair production. Because of these interactions and the response of the sodium iodide scintillation detector, the spectra are degraded to the rather “smeared” spectra shown in Figure 19.46.

The high-energy part of the detected spectrum is divided into three energy windows, W1, W2, and W3; each covering a characteristic peak of the three radioactivity series. Knowing the response of the tool and the number of counts in each window, it is possible to determine the amounts of thorium 232, uranium 238, and potassium 40 in the formation. There are relatively few counts in the high-energy range where peak discrimination is best; therefore, measurements are subject to large statistical variations, even at low logging speeds.

Figure 19.47: Gamma Ray Spectral Log Presentation. Note difference between standard gamma ray (SGR) and uranium corrected gamma ray (CGR).

By including a contribution from the high-count rate, low-energy part of the spectrum (Windows W4 and W5), these high statistical variations in the high-energy windows can be reduced by a factor of 1.5 to 2. The statistics are further reduced by another factor of 1.5 to 2 by using a filtering technique that compares the counts at a particular depth with the previous values in such a way that spurious changes are eliminated while the effects of formation changes are retained.