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Density Logs and Their Applications

Density Logs and Their Applications
Density Logs and Their Applications

Density logs are primarily used as porosity logs. Other uses include:

  • Mineral Identification in Evaporite Deposits
  • Gas Detection
  • Hydrocarbon Density Determination
  • Evaluation of Shaly Sands and Complex Lithologies
  • Oil-Shale Yield Estimation
  • Overburden Pressure Calculation
  • Assessment of Rock Mechanical Properties

Gamma Ray Interactions in Density Logging

Gamma rays emitted during density logging interact with materials via three processes:

  1. Pair Production (>3 MeV):
    • Interact with nuclei to produce an electron and a positron.
    • Low efficiency; minimal contribution to the signal.
  2. Compton Scattering (0.5 to 3 MeV):
    • Gamma rays lose energy by ejecting electrons from atoms.
    • May undergo multiple scatterings, reducing energy progressively.
  3. Photoelectric Absorption (
    • Gamma rays are absorbed, promoting or ejecting electrons.
    • Crucial for the Litho-Density tool (Figure 1).

Theory Behind Density Logging

Density logging tools emit medium-energy gamma rays into the formation via a radioactive source in a shielded skid pressed against the borehole wall. The gamma rays collide with formation electrons (Compton scattering), and the scattered rays detected indicate formation density.

The number of scatterings relates to electron density, which correlates with true bulk density, dependent on:

  • Rock Matrix Density
  • Formation Porosity
  • Pore Fluid Density

To minimize mud effects, the tool uses shields and an eccentering arm to press against the borehole wall, cutting through mudcakes (see Figure 2). Mudcake between the tool and formation must be accounted for.

Compensated Formation Density Tools

The Compensated Formation Density Tool (FDC) uses two detectors with different spacings (as in Figure 2). By plotting long-spacing versus short-spacing count rates (see Figure 3), and using the "spine and ribs" method, we can determine the corrected bulk density without measuring mudcake properties. Corrections are automated and recorded on the log (see Figure 4).