Formation evaluation gamma ray
Encyclopedia
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 shale
s and sandstone
s typically have different gamma ray levels. Shales and clay
s 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.
, and gamma radiation. Natural gamma ray radiation is one form of spontaneous radiation emitted by unstable nuclei. Gamma (γ) radiation may be considered either as an electromagnetic wave similar to visible light or X-rays, or as a particle of photon. Gamma rays are electromagnetic radiations emitted from an atomic nucleus during radioactive decay, with the wavelength in the range of 10-9 to 10-11cm
Isotopes naturally found on earth are usually those that are stable or have a decay time larger than, or at least a significant fraction of the age of the earth (about 5 x 109 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 1.46 MeV gamma-ray. Uranium 238 and thorium 232 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. The peak of thorium series can be found at 2.62 MeV and the Uranium series at 1.76 MeV.
Gamma-ray measurement has the following applications:
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.
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 200 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.
Shale
Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering...
s and sandstone
Sandstone
Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...
s typically have different gamma ray levels. Shales and clay
Clay
Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...
s 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.
Physics
Natural radioactivity is the spontaneous decay of the atoms of certain isotopes into other isotopes. 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, betaBeta particle
Beta particles are high-energy, high-speed electrons or positrons emitted by certain types of radioactive nuclei such as potassium-40. The beta particles emitted are a form of ionizing radiation also known as beta rays. The production of beta particles is termed beta decay...
, and gamma radiation. Natural gamma ray radiation is one form of spontaneous radiation emitted by unstable nuclei. Gamma (γ) radiation may be considered either as an electromagnetic wave similar to visible light or X-rays, or as a particle of photon. Gamma rays are electromagnetic radiations emitted from an atomic nucleus during radioactive decay, with the wavelength in the range of 10-9 to 10-11cm
Natural radioactivity in rocks
Radioisotopes with a sufficiently long halflife, and whose decay produces an appreciable amount of gamma rays are:
- Potassium 40K with half-life of 1.3 x 109 years, which emits 0 α, 1 β, and 1 γ-ray
- Thorium 232Th with half-life of 1.4 x 1010 years, which emits 7 α, 5 β, and numerous γ-ray with different energies
- Uranium 238U with half-life of 4.4 x 109 years, which emits 8 α, 6 β, and numerous γ-ray with different energies
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 1.46 MeV gamma-ray. Uranium 238 and thorium 232 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. The peak of thorium series can be found at 2.62 MeV and the Uranium series at 1.76 MeV.
Applications
The most common sources of natural gamma rays are potassium, thorium, and uranium. These elements are found in feldspars (i.e. granites, feldspathic), volcanic and igneous rocks, sands containing volcanic ash, and clays.Gamma-ray measurement has the following applications:
- Well to well correlation: gamma-ray log fluctuates with changes in formation mineralogy. As such, gamma-ray logs from different wells within the same field or region can be very useful for correlation purposes, because similar formations show similar features.
- Logging runs correlation: Gamma-ray tools is typically run in every logging tools runs in a well. Being a common measurement, logging data can be put on depth with each other by correlating the gamma-ray feature of each run.
- Quantitative evaluation of shaliness: Since natural radioactive elements tend to have greater concentration in shales than in other sedimentary lithologies, the total gamma ray measurement is frequently used to derive a shale volume (Ellis-1987, Rider-1996). This method however, is only likely to be used in a simple sandstone-shale formation, and is subject to error when radioactive elements are present in the sand.
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.
Measurement technique
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
University of Houston
The University of Houston is a state research university, and is the flagship institution of the University of Houston System. Founded in 1927, it is Texas's third-largest university with nearly 40,000 students. Its campus spans 667 acres in southeast Houston, and was known as University of...
. The artificial formation simulate about twice the radioactivity of a shale, which generates 200 API
American Petroleum Institute
The American Petroleum Institute, commonly referred to as API, is the largest U.S trade association for the oil and natural gas industry...
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.