variable density log

WHAT IS VDL

A presentation of the acoustic waveform at a receiver of a sonic or ultrasonic measurement, in which the amplitude is presented in color or the shades of a gray scale. The variable-density log is commonly used as an adjacent to the cement-bond log, and offers better insights into its interpretation.

Acoustic waves are a type of longitudinal waves that propagate by means of adiabatic compression and decompression

Principle

A radioactive source, applied to the borehole wall, emits medium-energy gamma rays into the formations. These gamma rays are high velocity particles that collide with the electrons in the formation. At each collision a gamma ray loses some, but not all, of its energy to the electron, and then continues with diminished energy. This type of interaction is known as Compton scattering.

Principle(Continue)

The scattered gamma rays reaching the detector, at a fixed distance from the source are counted as an indication of formation density.

The number of Compton scattering is directly related to the number of electrons in the formation.

Tools

Long spaced detector Short spaced detector Source of radiation

Scale

The density log is generally plotted on linear scale of bulk density.

Most often its scale is in between 1.95 to 2.95g/cm3

Units

In CGS it is  g/cm3 . In SI it is kg/m3 .

Uses

Most reliable for porosity indicator of sandstone and limestone because their porosity is well known.

On the other hand the density of clay minerals such as mudstone is highly variable, depending on the depositional environment, overburden, pressure, type of clay minerals and many other factors.

Its main use to derive the total porosity of the formation.

Uses(Continue)

It is also useful in gas-bearing formations and in the recognition of evaporites.

It gives the best way to identify the lithology in borehole with collaboration of Neutron log.

Limitations

Correction only valid if mudcake or sandoff (perforating) <0.75 inch and is uniform along pad.

Abnormally low density(high permeability) is washed out (>1.7inch) hole or rough hole.