Page 129 - The Geological Interpretation of Well Logs

- THE DENSITY AND PHOTOELECTRIC FACTOR LOGS -

Table 9.4 Unwanted environmental effects - density log.

Factor Effect on log Severity*

Caved or rough hole Decrease in formation densily to approach Frequent
a drilling mud density value

Barite in the drilling mud Automatically corrected in the tool- when mud-cake {Rectified}
is thick, gives the cake density

*When the effect makes the log reading unusable. Ratings: frequent, common, present, rare.

BULK DENSITY COMPONENTS
a

—é
| __ PORE FLUIO
Density 11 gfem4
BULK
| MATRIX (grains)
DENSITY
2:5 g/em3 Density 2-65 g/em>

TOOL MEASURED
Figure 9.7 Tool measured bulk density and a visualization of the derivation of the porosity component. The figures are for a
sandstone with 10% porosity.
BULK DENSITY g/cm?
9.5 Quantitative uses 22 24 24 2a
10
Porosity caiculation o-
The density log is used to calculate porosity and it may
also, with difficulty, be used to calculate hydrocarbon
density. To calculate porosity from Jog-derived bulk
Oma # 2-68 g/em® (quartz)
density it is necessary to know the density of all the indi-
Ptl =149/cnr {salt water}
vidual materials involved. The density tool sees global
(bulk) density, the density both of the grains forming the
rock and of the fluids enclosed in the interstitial pores
(Figure 9.7). As an example, if the tool measures a bulk 7
density of 2.5g/cm’ in a salt-water-bearing formation
(fluid density 1.1g/em? [as seen by the tool]) we can
interpret any of the following: Grain Density
2 Histogram

BULK DCuguty
Lithology Grain density Porosity toa
Sandstone 2.65g/em? 10% 80
Limestone 2.71 g/cm? 13%
Dolomite 2.87 g/cm? 21%
Of course, if we know the grain (matrix) density and the
fluid density we can solve the equation that gives porosi-
ty from the summation of fluid and matrix components
soon 4
(Figure 9.7). For example,
bulk density (p,) = porosity () X fluid density (p,)
+(1-—) X matrix density (p,)

When solved for porosity this equation becomes:
~
porosity $ = Pm Po
Pina — Pe
Figure 9.8 Close correspondence between the bulk densily
where p, | = matrix (or grain) density, p, = fluid density log and core-measured porosity in an orthoquartzite. The bulk
and p, = bulk density (as measured by the tool and hence density can be converted to porosity using a matrix density of
includes porosity and grain density). 2.68g/cm?, as indicated by the inset histogram,