Density Log

History of Density Tool 1950s single detector tool developed initially to measure bulk density, ρb, insitu

as an aid to geophysicists in gravity meter interpretation Early 1960s accepted tool as a source for porosity Late 1960s dual detector system, Compensated Formation Density (FDC) to correct

for borehole effects Early 1980s development of Lithodensity Tool (LDT) to measure photoelectric effect,

Pe, with bulk density

Schematic of Density Tool

Vertical Resolution ~ 1 ft

Depth of Investigation 3 - 6”

Logging speed: 15-30 ft/min

Theory

• A radioactive source – caesium-137 or cobalt-60

• Emits gamma rays in medium energy range (0.2-2 MeV)

• A short range detector – Placed 7 inches from the source

• A Long range detector – Placed 16 inches from the source

Compton Scattering in --

• A formation with high bulk density

• A formation with low bulk density

Electron Number Density (ne)

• For a pure substance, number density is directly related to bulk density

• Atoms per mole – Avogadro Number (N) • Electrons in a mole = NZ

Z= Atomic number

• Number of electrons per gram = NZ/A A = Atomic mass number

• Number of electrons per vol (ne) = (NZ/A) ρb

• Effective electron density ρe = 2ne/N • ρe = (2Z/A) ρb

Calibration

• Calibrated in freshwater filled limestone

Uses of Density Log

• Determination of porosity – Effect of gas - Overestimation of porosity – Effect of shale

• Acoustic Impedance • Identification of lithology • Recognition of Accessory Mineralogies • Fracture recognition • Organic content of source rock

Litho-Density Tool

• Source: Caesium-137 – 0.662 MeV • Efficient detectors

– hard gamma rays: 0.25 to 0.662 MeV – soft gamma rays: 0.04 to 0.0 MeV