About Gamma ray Logging Gamma ray (GR) logs measure the natural radioactivity in formations and can be used for identifying lithologies and for correlating zones The first gamma ray logs were run by L[.]
About Gamma ray Logging Gamma ray (GR) logs measure the natural radioactivity in formations and can be used for identifying lithologies and for correlating zones The first gamma ray logs were run by Lane Wells in 1936 This is often proportional to the amount of shale in the rocks, but there are other causes of gamma radiation The spectral gamma ray log records not only the number of gamma rays emitted by the formation but also the energy of each, and processes that information into curves representative of the amounts of thorium (Th), potassium (K), and uranium (U) present in the formation It looked similar to an SP log and was easy to use in correlating zones from well to well, gamma ray logs can be used not only for correlation, but also for the determination of shale (clay) volumes It was hailed as a great advance over the SP log because its value does not depend on formation water resistivity., and because the gamma ray log responds to the radioactive nature of the formation rather than the electrical nature, it can be used in cased holes and in open holes containing nonconducting drilling fluids (i.e., oil-based muds or air) Radiation is naturally erratic A stationary detector facing a given gamma ray flux will not see a constant stream of gamma rays To obtain a reliable count rate, measuring instruments record the total number of emissions over a period of time, known as the time constant For most gamma ray tools, the time constant is or seconds to obtain a smooth log curve The differences in count rates between one time constant and another are called statistical variations Introduction Camma ray logs are used for three main purposes: correlation evaluation of the shale content of a formation mineral analysis The gamma ray log measures the natural gamma ray emissions from radioactive formations Since many gamma rays can pass through steel casing, the log can be run in both open and cased holes Pictuce shows a gamma ray log It is normally presented in track on a linear grid and is scaled in API units Gamma ray activity in creases from left to right Modern gamma ray tools are in the form of double ended subs that can be sandwiched into almost any logging tool string Gammaray tools consist of a gamma ray detector and the associated electronics for passing the gamma ray counts or count rates to the surface Pic 1: Gamma ray log Distribution of relative radioactivity level for various rock types Origin of Natural Gamma Rays Gamma rays originate in three sources in nature These are the radioactive elements of the Uranium Group, the Thorium Group, and potassium Uranium 235, uranium 238 and thorium 232 all decay, via long chains of daughter products, to stable lead isotopes.An isotope of potassium, 4%, decays to argon and emits a gamma ray as shown in Picture It should be noted that each type of decay is characterized by a gamma ray of a specific energy (wave length, frequency, or color) and that the frequency of occurrence for each decay energy is different Picture shows this relationship between gamma ray energy and frequency of occurrence This is an important concept since it is used as the basis for analysis of data from the natual gamma spectroscopy tools Pic 2: Decay model of K40 Pic 3: Emission spectra for potassium, thorium, and uranium series 3 Abundance of NaturallyOccurring Radioactive Minerals An "average” shale contains 6ppm uranium, 12 ppm thorium and 2% potassium Since the various gamma ray sources produce different numbers and energies of gamma rays, it is more informative to consider this mix of radioactive materials on a common basis by referring to potassium equivalents (the amount of potassium that would produce the same number of gamma rays per unit of time) Reduced to a common denominator, the average shale contains uranium equivalent to 4.3% potassium, thorium equivalent to 3.5% potassium, and 2% potassium This "average” shale is a rare find A shale is a mixture of clay minerals, sand, siltsand other extraneous materials; thus, there can be no "standard” gamma ray activity for shale Indeed, the main clay minerals vary enormously in their natural radioactivity Kaolinite has almost no potassium whereas illite contains between 4% and 8% potassium Montmorillonite contains less than 1% potassium Natural radioactivity may also be due to the presence of dissolved potassium or other salts in the water contained in the pores of the shale Operating Principle of Gamma RayTools Traditionally, two types of gamma ray detectors have been used in the logging industry Geiger-Mueller and scintillation detectors Today most gamma ray tools use scintillation detectors containing a sodium iodide (NaI) crystal (Figure 10-5); newer and more efficient crystal materials are constantly being discovered but the principles of operation are the same When a gamma ray strikes the crystal, a single photon of Light is emitted This tiny flash of light then strikes a photocathode (probably made from cesium antimony or silver-magnesium) Each photon hitting the photocathode releases a bunch ofelectrons.These, in turn, are accelerated in an electric field to strike another electrode producing an even bigger "shower” of electrons This process is repeated through a number of stages until a final electrode conducts a small current through a measure resistor to produce a voltage pulse that can be measured Each detected gamma ray produces a single pulse The "dead time” of these systems vary but are typically very short, and they can register many counts/second before being "swamped” by numerous near-simultaneous gamma rays Pic 4: Scintillation gamma ray detector Precision Consider briefly some details of how a standard, gross-count-rate gamma ray tool works Most modern tools (in nuclear logging, "modern" means within the past 25 years) use a solid-state scintillator crystal (most often sodium iodide, NaI) to detect gamma rays When a gamma ray strikes the crystal, there is some probability that it will be captured That probability is mostly proportional to the size and density of the crystal If it is captured, it gives off a flash of light A photomultiplier mounted on one end of the crystal converts that light to an electrical pulse, which is then fed to an electronic pulse counter To measure a count rate with a given precision in the laboratory, one counts until enough counts are registered to give the desired level of precision (see the discussion of counting statistics above) Then, one divides that number of counts by the time it took to get that many to obtain a count rate Unfortunately, in a logging tool, all measurements are depth-based To measure a count rate, the tool counts for the length of time it takes the tool to move 1/2 ft (or whatever the depth increment is), then divides by the length of time it took the tool to move that distance This means that the precision of a nuclear-logging measurement in a given lithology is proportional to one over the square root of the logging speed Remember that the number of counts received crossing a clean 1/2 ft will be much less than the number when crossing a shaly 1/2 ft 6 Environmental distortion The simple consideration of the discussion of radiation transport helps clarify which environmental effects most seriously distort the gamma ray log Imagine what happens as borehole size increases There is less of the radiating radioactive material near the detector, and the measured count rate goes down, even though the actual level of radioactivity in the formation remains the same Further imagine the rather typical case in which the shales are eroded and broken out while the sands remain in gauge This would suppress the apparent gamma ray count rate in the eroded shales much more than in the sands, suppressing the gamma ray contrast between eroded shales and sands This is typically one of the largest environmental effects on the gamma ray count rate Again from the discussion of radiation transport, heavier materials in the path that the gamma rays must follow from the formation through the detector will absorb more gamma rays than lighter materials (as will be seen in a later section, this is the basis for the bulk density log, but that is another story and a different log) Worse yet, barite is a big absorber of gamma rays The lesson to carry away is that borehole size and fluid corrections are almost always important when running the gamma ray log