Well Log Interpretation Density Log

1
Well Log InterpretationDensity Log
Earth Environmental Science University of
Texas at Arlington
2
Density Log
The Density log measures electron density in the
rock and fluids, which is related to the bulk
(combined) density of the rock and fluids
together (rb or RHOB)
3
Density Log
Electrons scatter gamma rays and the electron
density is estimated by measuring those scattered
gamma rays. There are two energy levels
measured high energy scattering which is related
to electron density, and a low energy scattering
due to the photoelectric effect.
4
Density Log
The Logging tool has a gamma ray source (usually
radioactive Cobalt or Cesium) and a gamma ray
spectrometer which measures the energy levels of
the scattered gamma rays.
5
Density Log
The depth of penetration is small, so the logging
tool must be next to the side wall which has mud
cake on it. As a result, the tool measurement
must be corrected for bore hole effects.
6
Density Log
The borehole correction is done automatically by
the logging software, but the correction factor
is reported on the log as DRHO (Dr). If this
correction factor is more than 0.2, the density
reading is probably wrong.
7
Density Log

• The density log is used to
• Estimate porosity
• Estimate lithology from the PE log
• Detect gas bearing zones
• Identify high density zones such as evaporites.

8
Density Log
Porosity can be calculated from bulk density
because rb is the sum of the weighted components
of the bulk density fluid density, rfl, and the
matrix density, rma
9
Density Log
Rearranging
Produces the equation for porosity (F)
10
Density Log
The required fluid densities (rfl) and matrix
densities (rma) for common lithologies are shown
at right.
11
Density LogAssignment
Make a spreadsheet Similar to Fig. 4.5 in the
text (below) and to compute FD (DPHI) assuming
dolomite and limestone. Use the the log on the
next slide and take readings of RHOB at depths
shown on the table at left. Assume salt mud.
12
Density LogAssignment
On the spreadsheet, add readings from the sonic
log of the previous assignment at the same depths
shown at left. Is there much shale at any of
these depths? Why?