Gamma Ray Log PDF

College of Petroleum Engineering & Geosciences Petroleum Engineering Dept. PETE 313 Well Logging Gamma Ray Log Dr. Talal Al Shafloot Credits to Dr. Khaled Ibrahim for the slides  Gamma...

College of Petroleum Engineering & Geosciences Petroleum Engineering Dept. PETE 313 Well Logging Gamma Ray Log Dr. Talal Al Shafloot Credits to Dr. Khaled Ibrahim for the slides  Gamma Ray Log 1930s The first gamma ray logs were run by Lane Wells in 1936. Similar to an SP log and was easy to use in correlating zones from well to well. Great advance over the SP log because it doesn’t depend on Rm, Rmf, and Rw.  Gamma Ray Log Natural GR Log: What it Measures?  GR logs measure the natural GR emissions from subsurface formations. PETE 313  Measurements can be made in: – open and cased holes. – water and oil base muds. 3  Gamma Ray Log Log Presentation  Presented in track 1 on a linear grid scaled in API units. Primary scale Back-up scale  Increases from left to right. primary curve goes off-scale 4 Typical GR log  Gamma Ray Log GR Log Applications  Correlation between wells  Identify shale zones  Determination of bed boundaries  Evaluation of shale content within a formation  Mineral analysis (type of clay minerals)  Depth control for side-wall coring, or perforating  Tracking movement of radioactive tracers 5  Gamma Ray Log GR Principals Radioactive decay? Radioactive decay is a transition from one nuclear energy state to another lower one. The excess energy is shed by the nucleus, by means of α, β or γ radiation.  Spontaneous disintegration of atoms accompanied by the emission of radiation  Unstable elements emit: ― ɣ has no mass and no charge ― β has a very small mass and –ve charge ― α has mass as that of 2He4 6  Gamma Ray Log GR Principals Gamma Ray? Electromagnetic spectrum starts at radio and ends at gamma ray It is the very highest energy form. 7 Electromagnetic radiation spectrum  Gamma Ray Log Radioactivity  GRs are electromagnetic radiation or photons that are emitted by nucleus in an excited state (has no mass, no charge, but has an energy)  Half life time: is the time required for radioactive isotope to lose 50% of its activity.  The radioactive materials originally occurred at igneous rocks, and rarely in sedimentary rocks from water passes from igneous rocks through sedimentary beds  The radioactive elements restricted in shales (clay Minerals) and other fine sediments.  Carbonates have a very low radioactivity level, but during the formation of a secondary porosity or dolomitization, radioactive elements may deposit by formation water and this could show higher radioactivity. 8 8  Gamma Ray Log Natural GR Principle  Estimated age of the earth, which is about 4×109 years.  Only three isotopes of the elements with half-lives of that magnitude or greater.  The most common sources are: ― Potassium-40, a lattice constituent in many mica, feldspar, and clay minerals, ― Thorium, associated mostly with clay minerals ― Uranium-series elements deposited over geological time. K and Th are associated with most shales. 9 9  Gamma Ray Log Natural GR Principle  Three main contributors ̶ K40 with half-life 1.3x109 yrs ̶ Th232 with half-life 1.4x1010 yrs ̶ U238 with half-life 4.4x109 yrs  Sources ̶ K40 feldspar, mica, illite ̶ Th232 heavy minerals, clays ̶ U238 organic material 10  Gamma Ray Log Sources of Passive Gamma Rays  Clays: some clay minerals are naturally radioactive or have radioactive ions associated with them. – Kaolinite (very little K [potassium]) – Illite (4-8% K) – Montmorillonite (smectite) (