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 shales and sandstones typically have different gamma ray levels. Shales and clays 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.
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[edit] Physics
Natural radioactivity is the spontaneous decay of the atoms of certain isotopes into other isotopes. If the resultant isotope is not stable, it will undergo further decay until finally a stable isotope is formed. The decay process usually accompanied by emission of alpha, beta, 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
[edit] Natural radioactivity in rocks
Fig1: Gamma-ray Spectra
Isotopes naturally found on earth are usually those which are stable or which 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:
- 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.
[edit] Applications
The most common sources of natural gamma rays are potassium, thorium, and uranium. These elements are found in feldspars (ie. 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 formation will 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 use in a simple sandstone-shale formation, and is subject to error when radioactive elements are present in the sand.
[edit] 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 (ie hole size), the position of the tool, its density, and composition. KCl in mud, for example, will invade permeable sections, with the net result of increase in gamma ray activity.
- Tubing, Casing, etc: Their effect depend on the thickness, density, and nature of the materials (eg. steel, aluminum). All 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 will not reflect the true value in a bed whose thickness is less than the diameter of the sphere of investigation. In a series of thin beds, the log reading will be 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 accumulated over a period of time and averaged in order to obtain a reasonable estimate of the mean. The longer the averaged period and the higher the count rate, the estimate of mean will be more precise.
Sample of corrections required for different gamma-ray tools are available from Schlumberger.
[edit] 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. 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.
Saturday, January 31, 2009
Gamma Ray Log
Gamma Ray Log adalah metoda untuk mengukur radiasi sinar gamma yang dihasilkan oleh unsur-unsur radioaktif yang terdapat dalam lapisan batuan di sepanjang lubang bor.
Unsur radioaktif yang terdapat dalam lapisan batuan tersebut diantaranya Uranium, Thorium, Potassium, Radium, dll.
Unsur radioaktif umumnya banyak terdapat dalam shale dan sedikit sekali terdapat dalam sandstone, limestone, dolomite, coal, gypsum, dll. Oleh karena itu shale akan memberikan response gamma ray yang sangat signifikan dibandingkan dengan batuan yang lainnya.
Jika kita berekerja di sebuah cekungan dengan lingkungan pengendapan fluvio-deltaic atau channel system dimana biasanya sistem perlapisannya terdiri dari sandstone atau shale (sand-shale interbeds), maka log gamma ray ini akan sangat membantu didalam evaluasi formasi (Formation Evaluation- FE).
Seperti halnya logging yang lainnya, pengukuran gamma ray log dilakukan dengan menurunkan instrument gamma ray log kedalam lubang bor dan merekam radiasi sinar gamma untuk setiap interval tertentu. Biasanya interval perekaman gamma ray (baca: resolusi vertikal) sebesar 0.5 feet.
Dikarenakan sinar gamma dapat menembus logam dan semen, maka logging gamma ray dapat dilakukan pada lubang bor yang telah dipasang casing ataupun telah dilakukan cementing. Walaupun terjadi atenuasi sinar gamma karena casing dan semen, akan tetapi energinya masih cukup kuat untuk mengukur sifat radiasi gamma pada formasi batuan disampingnya.
Seperti yang disebutkan diatas bahwa gammar ray log mengukur radiasi gamma yang dihasilkan oleh unsur-unsur radio aktif seperti Uranium, Thorium, Potassium dan Radium. Dengan demikian besaran gamma ray log yang terdapat didalam rekaman merupakan jumlah total dari radiasi yang dihasilkan oleh semua unsur radioaktif yang ada di dalam batuan. Untuk memisahkan jenis-jenis bahan radioaktif yang berpengaruh pada bacaan gamma ray dilakukan gamma ray spectroscopy. Karena pada hakikatnya besarnya energy dan intensitas setiap material radioaktif tersebut berbeda-beda.
Spectroscopy ini penting dilakukan ketika kita berhadapan dengan batuan non-shale yang memungkinkan untuk memiliki unsur radioaktif, seperti mineralisasi uranium pada sandstone, potassium feldsfar atau uranium yang mungkin terdapat pada coal dan dolomite.
Gamma ray log memiliki satuan API (American Petroleum Institute), dimana tipikal kisaran API biasanya berkisar antara 0 s/d 150. Walaupun terdapat juga suatu kasus dengan nilai gamma ray sampai 200 API untuk jenis organic rich shale.
Gambar dibawah ini menunjukkan contoh interpretasi lapisan batuan untuk mendiskriminasi sandstone dari shale dengan menggunakan log gamma ray.
Adapted from kgs.ku.edu
Dikarenakan log gamma ray memiliki kapabilitas untuk mengukur derajat kandungan shale di dalam lapisan batuan, maka didalam industri migas gamma ray log kerap kali digunakan untuk memprediksi besaran volume shale atau dikenal dengan Vshale dengan formulasi:
Gambar dibawah ini menunjukkan teknis perhitungan Vshale untuk shale A dari sebuah gamma ray log. Perhatikan bahwa penentuan nilai-nilai tersebut bersifat interpretatif.
Adapted from kgs.ku.edu
Gamma ray log memiliki kegunaan lain diantaranya untuk melakukan well to well correlation dan penentuan Sequence Boundary (SB), yakni dengan mengidentifikasi Maximum Flooding Surface (MFS) sebagai spike dengan nilai gamma ray yang tinggi. Well to well correlation ini biasanya dilakukan dengan melibatkan log-log yang lainnya seperti sonic, density, porositas, dll.
Adapted from strata.geol.sc.edu
Agus Abdullah, PhD9:17 PM
Gamma ray logging
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| Gamma ray logging |
Example gamma ray log. Blue and black lines indicate the measured gamma rays. Sand section of interest is located at bottom of log where the log moves to the left.
Gamma ray logging is a method of measuring naturally occurring gamma radiation to characterize the rock or sediment in a borehole. It is sometimes used in mineral exploration and water-well drilling, but most commonly for formation evaluation in oil and gas well drilling. Different types of rock emit different amounts and different spectra of natural gamma radiation. In particular, shales usually emit more gamma rays than other sedimentary rocks, such as sandstone, gypsum, salt, coal, dolomite, or limestone because radioactive potassium is a common component in their clay content, and because the cation exchange capacity of clay causes them to adsorb uranium and thorium. This difference in radioactivity between shales and sandstones/carbonate rocks allows the gamma tool to distinguish between shales and non-shales.
The gamma ray log, like other types of well logging, is done by lowering an instrument down the hole and recording gamma radiation at each depth. In the United States, the device most commonly records measurements at 1/2-foot intervals. Gamma radiation is usually recorded in API units, a measurement originated by the petroleum industry. Gamma logs are affected by the diameter of the borehole and the properties of the fluid filling the borehole, but because gamma logs are most often used in a qualitative way, corrections are usually not necessary.
Three elements and their decay chains are responsible for the radiation emitted by rock: potassium, thorium and uranium. Shales often contain potassium as part of their clay content, and tend to absorb uranium and thorium as well. A common gamma-ray log records the total radiation, and cannot distinguish between the radioactive elements, while a spectral gamma ray log (see below) can.
An advantage of the gamma log over some other types of well logs is that it works through the steel and cement walls of cased boreholes. Although concrete and steel absorb some of the gamma radiation, enough travels through the steel and cement to allow qualitative determinations.
Sometimes non-shales also have elevated levels of gamma radiation. Sandstone can contain uranium mineralization, potassium feldspar, clay filling, or rock fragments that cause it to have higher-than usual gamma readings. Coal and dolomite may contain absorbed uranium. Evaporite deposits may contain potassium minerals such as carnallite. When this is the case, spectral gamma ray logging can be done to identify these anomalies.
[edit] Spectral logging
Some specialized gamma radiation logging distinguishes the three component decay chains (potassium, uranium, and thorium) by the wavelengths of their characteristic gamma emissions.
The characteristic gamma ray line that is associated with each component:
- Potassium : Gamma ray energy 1.46 MeV
- Thorium series: Gamma ray energy 2.62 MeV
- Uranium-Radium series: Gamma ray energy 1.76 MeV
[edit] Use in mineral exploration
Gamma ray logs are also used in mineral exploration, especially exploration for phosphates, uranium, and potassium salts.
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