Formation Evaluation Gamma Ray - Interpretation

Interpretation

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:

• Borehole Fluid: the influence of borehole fluid depends on its volume (i.e. hole size), the position of the tool, its density, and composition. Potassium chloride (KCl) in mud, for example, flows into permeable sections, resulting in an increase in gamma ray activity.
• Tubing, Casing, etc.: Their effect depend on the thickness, density, and nature of the materials (e.g. steel, aluminum). Steel reduces the gamma-ray level, but can be corrected once the density and thickness of the casing, cement sheath and borehole fluid are known.
• Cement: Its impact is determined by the type of cement, additives, density and thickness
• Bed Thickness: Gamma-ray reading does not reflect the true value in a bed with a thickness less than the diameter of the sphere of investigation. In a series of thin beds, the log reading is a volume average of the contributions within the sphere.

In addition, all radioactive phenomena are random in nature. Count rates 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.