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Nuclear Theory

Nuclear Theory

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nuke tools

Items (63)

  • What do nuclear tools measure?

    properties that indicate formation radiation, porosity, density and lithology

  • Define porosity

    the percentage of the formation that is occupied by pores or voids

  • What 2 measurements do nuclear tools make to detemine porosity?

    NEURONS DETERMINE POROSITY, GAMMA RAY DETERMINES BULK DENSITY

  • What is bulk density?

    Bulk density is the mass per unit volume of the formation. Once we know the bulk density, we can estimate the porosity.

  • Where do radioactive elements concentrate?

    These elements tend to concentrate in shale beds.

  • Define lithology, and what does it tell us?

    Lithology is the composition and character of rock. Knowing a formation’s lithology helps us indicate its porosity.

  • Can a "density tool" indicate formation lithology?

    Yes, via gamma rays.

  • What are the 3 most common natural radioactive elements?

    Th, K, Ur

  • As_Th, K, Ur decay what happens?

    They emit energy and particles. One type of energy that they emit is gamma rays.

  • What formations are least radioactive?

    Limestone, sandstone, and dolomite are generally not very radioactive.

  • What is the use of GR logs?

    correlate information from multiple logs_• identify shale beds_• determine shale type_• evaluate shale quantity_• identify radioactive sands.

  • Can you find hydrocarbons in shale beds?

    Not likely.

  • Do sand formations contain shale?

    yes, GR logs will tell us how much shale is contained in a particular formation.

  • Define "hot sands"

    Uranium dissolves easily in water. This water can invade a sandy formation. These are called radioactive, or hot, sands.

  • What do "hot sands" look like on a GR log?

    On a gamma ray log, these radioactive sands look like a shale bed

  • What are the 3 parts in a Scintillation detector?

    A scintillation detector is made up of a crystal scintillator, a photomultiplier tube and a discriminator circuit

  • How does the Scintillation detector work?_

    gamma ray passes into the detector’s crystal and collides with the crystal’s atoms. Each collision causes the atom to emit an electron. The electron is captured by the thallium in the crystal and gives off a flash of light, which is called a scintillation.

  • Define the photomultiplier (tube):

    The photomultiplier tube is made up of a series of dynodes. A voltage is applied to the tube. Each dynode is set at a higher potential than the previous one.Action: The flash of light acts on a photocathode in the photomultiplier tube. The photocathode emits electrons. The electrons are drawn across the series of dynodes where they draw additional electrons with them until the electrons reach alevel that can be measured by a conventional circuit. An anode collects the electrons and

  • Describe the Discriminator / lifier_circuit:

    The circuit contains a discriminator and an lifier.Action: Each gamma ray that enters the detector creates a pulse that is recorded by the circuit. The discriminator differentiates between pulses caused by gamma rays from the formation and pulses caused by background electrons based on the energy of the pulse<img src="theory02.PNG" />

  • What are the 2 types of Scintillation detectors?

    the plateau detector and the spectral detector

  • What does the plateau detector do?

    The plateau detector measures total gamma ray, or the number of naturally occurring gamma rays coming from the formation. The plateau detector presents its findings as counts per second, or counts/s

  • Describe the Spectral scintillation detector:

    The spectral detector counts the number of gamma rays coming from the formation, or total gamma ray. It also classifies the energy of the gamma rays. This is known as a spectral gamma ray analysis.As they decay, different elements give off gamma rays of different energies. By determining the energy of the gamma rays coming from the formation, the spectral detector can identify the radioactive elements in the formation.

  • How does the Spectral detector work?

    The spectral detector includes a small cesium (Am241) source near the crystal. The cesium constantly bombards the crystal with gamma rays. These gamma rays have an energy of 60 KeV.Gamma rays from the formation bombard the crystal at the same time.

  • How do borehole conditions affect the detector?

    if the temperature increases in the borehole, the sensitivity of the detector may change. As a result, the spectrum changes and the counts in each energy window are incorrect.

  • What is the significance of windows 1 &_ 2?

    The system constantly monitors Windows 1 and 2. It expects to see equal counts in these windows from the cesium (Cs137) source. If the counts are not equal, it indicates a change in the sensitivity of the detector.The detector receives constant feedback from Windows 1 and 2. If the counts are unequal, the detector adjusts the voltage until the counts are equal again. This is why the cesium is sometimes called a stabilization source.

  • What is the significance of windows 8 &_ 9?

    At the same time, the detector also monitors Windows 8 and 9. It expects to see equal counts in these windows from thorium in the formation. If it does not, it will adjust the voltage until they are equal.The detector constantly checks the peak in Windows 1 and 2 and the peak in Windows 8 and 9 to make sure the detector is reading correctly.

  • How does the spectral detector determine total GR?

    When the system or tool is asked for total gamma ray, it counts all the gamma rays detected in Windows 3 through 9. In other words, the counts per second (counts/s) is the total number of counts in all the windows minus the counts from the stabilization source in Windows 1 and 2.<img src="theory06.PNG" />

  • What is the GR energy given off by posassium?

    1.46 MeV

  • What is the difference between a Plateau GR log and a spectral GR log?

    Plateau = Total GR log Spectral =_can present a curve for each of the three radioactive elements: thorium, uranium and potassium. We can also present it as a ratio of one element to the other.

  • How can we identify hot sand on a spectral log?

    CGR (corrected) and SGR (spectral) curves separate

  • What happens to GR counts in an oversized wellbore?

    The mud will absorb gamma rays & the count will be too low.

  • What happens to GR counts when drilling with "Heavy mud"

    Heavy mud absorbs gamma rays. the count will be too low

  • What is the effect tool size has on GR counts?

    thick tools absorb more gamma, counts will be too low.

  • What is the effect of drilling with KCl on GR counts?

    The radiation from the potassium will cause the gamma ray reading to be too high. To correct this condition, you must enter in the concentration of potassium from the mud report.

  • How do we determine porosity?

    We must know the bulk density of the formation. We use G.R. from a radioactive source. (Cs137)_The radioactive source in the tool emits medium-energy gamma rays into the formation, where they interact with the electrons in the orbital shells of formation atoms. The type of gamma ray interaction that occurs depends on the atomic number (Z) of the element involved, the electron density of the material, and the energy of the gamma ray:• Compton scattering • photoelectric absorptions.

  • Define density:

    Density is defined as the mass per unit volume of the formation.

  • Define Bulk density

    Bulk density is the overall gross or weight-average density of a unit of the formation.

  • what is "Hmc" &_ "Pmc"

    Hmc= mudcake thickness Pmc= mudcake density

  • How do we _determine "Compensated ρb"

    The tool uses an algorithm based on the spine and rib chart to determine the correction (Δp) to arrive at the compensated ρb. Δp is added to the uncompensated ρL value to arrive at the compensated ρb. This is because ρL is less affected by the mud and mudcake.The compensated ρb is at the intersection of the rib and the spine.<img src="theory14.PNG" />

  • Can we see delta-p on a density porosity log? and what is it used for?

    Yes,_You can see Δp on a density porosity log. It is used as a quality control indicator.

  • What is PEF?

    Photoelectric absorption index

  • How do we determine bulk density?

    With Gamma rays originating from a Cs137 source in the tool.The radioactive source in the tool emits medium-energy gamma rays into the formation, where they interact with the electrons in the orbital shells of formation atoms. The type of gamma ray interaction that occurs depends on the atomic number (Z) of the element involved, the electron density of the material, and the energy of the gamma ray.

  • What is compton scattering?

    When a medium-energy gamma ray interacts with an electron, the gamma ray uses part of its energy to eject the electron from the atom’s orbital shell. The gamma ray scatters at a lower energy and interacts with other atoms.

  • What is photoelectric absorption?

    If the formation is very dense or if the atomic number is very high, the formation will contain a large number of electrons. The gamma rays will interact with many of these electrons, losing energy each time. Some gamma rays will lose so much energy, they will become low-energy gamma rays.When a low-energy gamma ray interacts with an electron, the electron is ejected from the atom’s orbital shell. The energy of the gamma ray is transferred to the electron. This energy is a function of the atomic number (Z).

  • What 2 factors have the biggest effect on the number of electrons in the formation?

    • density of the formation. Higher density = more electrons, therefor:_Density will affect the number of low- and medium-energy gamma rays that return to the tool.• lithology of the formation._<img src="theory15.PNG" />

  • How does higher atomic numbered formations (Z) relate to electron count?

    Formations with lithologies of higher atomic numbers (Z) will contain more electrons per cubic centimeter than formations with lithologies of lower atomic numbers

  • How does lithology affect GR?

    Lithology will predominately affect the number of low-energy gamma rays that return to the tool. Lithology has less effect on the number of medium-energy gamma rays that return. This is why Window 1 in the energy spectrum (low-energy window) is referred to as the lithology window<img src="theory16.PNG" />

  • What is th PEF curve based on?

    the function of the number of low-energy and medium-energy gamma rays counted

  • What is the density curve based on?

    based on the number of medium-energy gamma rays counted by the detector

  • How do we see lithology changes in the logs?

    changes in PEF and B.D.

  • What are the expected PEF values for various lithologies?

    SS-1.54-1.8, DM-2.7-3.1, LS-4.2-5.1

  • Describe the operation of a He3 detector comparator and lifier circuit

    Description: The circuit contains a comparator and an amplifier.Action: The circuit records each negative electrical pulse. The comparator discriminates between electrical pulses caused by incoming thermal neutrons and electrical pulses caused by noise and gamma rays. The lifier lifies the electrical pulses into a useful litude range.The tool counts the number of electrical pulses. It presents the results as counts per second (counts/s).

  • Describe the He3 detector voltage operating zones

    The number of electrical pulses that the detector counts is related to the voltage potential between the anode and cathode. The voltage must be set within a certain range for the detector to work correctly.The response of the He3 detector to changes in voltage can be graphed as a curve.

  • Describe the He3 detector voltage operating zone 1

    Recombination zone In this region, ions recombine before they have a chance to start the avalanche of electrons. In this case, almost no pulses are detected

  • Describe the He3 detector voltage operating zone 2

    Saturation zoneIn this area, the neutrons do interact with the He3, but there is not enough energy to cause an avalanche. As a result, the pulses created are too small to be counted.<img src="theory26.PNG" />

  • Describe the He3 detector voltage operating zone 3

    Proportional zoneAt some range of voltage in the middle of the curve, the size of each electrical pulse is proportional to the energy of the incoming thermal neutron. The comparator can look at the height of each electrical pulse to determine which electrical pulses were caused by thermal neutrons and which electrical pulses were caused by noise. As long as the voltage falls within the proportional area, the counts will be correct.<img src="theory26.PNG" />

  • Describe the He3 detector voltage operating zone 4

    Limited proportionality zoneIn this area, electrical pulses caused by thermal neutrons and fast neutrons are the same size. The comparator cannot discriminate between them. So many electrons are attracted to the anode that it becomes saturated. In this case, the counts/s will be too high<img src="theory26.PNG" />

  • Describe the He3 detector voltage operating zone 5

    Geiger zoneAt the far end of the curve, the voltage will be too high. In this region, electrical pulses caused by thermal neutrons and electrical pulses caused by noise are the same size. The comparator cannot discriminate between them. So many electrons are attracted to the anode that it becomes saturated. In this case, the counts/s will be too high.<img src="theory26.PNG" />

  • Before sending a tool downhole what is necessary for detector programming?

    Before sending a detector downhole, you will program the detector to operate at that optimum voltage.

  • What is TNRA? and what is it used for?

    The tool determines the ratio of counts at the near detectors to counts at the far detectors. The result is called the thermal neutron ratio, or TNRA. Using the ratio of the near and far detector, we can compensate for washout and rugosity

  • what is the TNRA formula

    counts@near/counts@far

  • What possible differences between calibration and hole conditions may exist?

    mud temperature• mud hydrogen index_• borehole size_• salinity effect_• lithology effect

  • What possible differences between calibration and rising hole temperatures?

    As borehole temperature increases, fewer neutrons are captured. More thermal neutrons make their way to the detector. In this case, the count will be too high. The apparent porosity will be lower than formation porosity.<img src="theory29.PNG" />