Density logging

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Density logging is a useful porosity logging tool. It is a continuous record of a formation bulk density. This is the overall density of a rock including solid matrix and the fluid enclosed in pores. Geologically, bulk density is a function of the density of the minerals forming a rock (i.e matrix) and the volume of free fluids, which it encloses (i.e porosity).

[edit] Principle

It is based on the principle of Compton scattering. A radio active source applied to the hole wall emits medium-energy gamma rays into the formation so these gamma rays may be thought of as high velocity particles ,which collide with the electrons in the formation .At each collision gamma ray loses some ,not all .of its energy to the electron, and then continues with the diminished energy .This type of interaction is know as Compton scattering. The scattered gamma rays reaching the detector ,at the fixed station from the source, are counted as an indication of formation density.

The number of Compton scattering collisions is related directly to the number of the electron in the formation. Consequently, the electron density determines the response of the density tool. Electron density is related through equation ρέ = 2ρВ ΣZ1i/M

Where ΣZ1i is the atomic numbers of all the atoms making up the molecules in the compound

M is them molecular weight of the compound.

The bulk density of a formation can be converted to porosity in the same manner as the sonic Δt.

Φ = ρma – ρb/ρma -ρf common values of ρma are

Sand & sandy lime - 2.65 Limey sands - 2.68 Limestone - 2.71 Dolomites - 2.87

When high salinity fluids are present in the pores ,a fluid bulk density ρf = 1gm/cc is appropriate where the fluid id very fresh. For flushed gas or oil reservoirs even lower ρf values should be assumed depending on the hydrocarbon density and residual saturation. In some applications hydrocarbons are indicated by the presence of abnormally high log porosities.