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Unformatted text preview: Petrophysics MSc Course Notes Formation Density Log Dr. Paul Glover Page 121 13. THE FORMATION DENSITY LOG 13.1 Introduction The formation density log measures the bulk density of the formation. Its main use is to derive a value for the total porosity of the formation. It s also useful in the detection of gas-bearing formations and in the recognition of evaporites. The formation density tools are induced radiation tools. They bombard the formation with radiation and measure how much radiation returns to a sensor. 13.2 Theory The tool consists of: A radioactive source. This is usually caesium-137 or cobalt-60, and emits gamma rays of medium energy (in the range 0.2 2 MeV). For example, caesium-137 emits gamma rays with a energy of 0.662 MeV. A short range detector. This detector is very similar to the detectors used in the natural gamma ray tools, and is placed 7 inches from the source. A long range detector. This detector is identical to the short range detector, and is placed 16 inches from the source. The gamma rays enter the formation and undergo compton scattering by interaction with the electrons in the atoms composing the formation, as described in Section 9.3. Compton scattering reduces the energy of the gamma rays in a step-wise manner, and scatters the gamma rays in all directions. When the energy of the gamma rays is less than 0.5 MeV they may undergo photo-electric absorption by interaction with the atomic electrons. The flux of gamma rays that reach each of the two detectors is therefore attenuated by the formation, and the amount of attenuation is dependent upon the density of electrons in the formation. A formation with a high bulk density, has a high number density of electrons. It attenuates the gamma rays significantly, and hence a low gamma ray count rate is recorded at the sensors. A formation with a low bulk density, has a low number density of electrons. It attenuates the gamma rays less than a high density formation, and hence a higher gamma ray count rate is recorded at the sensors. The density of electrons in a formation is described by a parameter called the electron number density , n e . For a pure substance, number density is directly related to bulk density, and we can derive the relationship in the following way. The number of atoms in one mole of a material is defined as equal to Avagadros number N ( N 6.02 10 23 ). The number of electrons in a mole of a material is therefore equal to NZ , where Z is the atomic number (i.e., the number of protons, and therefore electrons per atom). Petrophysics MSc Course Notes Formation Density Log Dr. Paul Glover Page 122 Since the atomic mass number A is the weight of one mole of a substance, the number of electrons per gram is equal to NZ / A ....
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