b.
In preparing the log, the 0.25 foot reading, normal, arrangement should be plotted at the depth as read
from the marked cable and the 2.5 ft readings about one foot above this point. This is because the cable
markings have been measured from the current electrode. For the lateral arrangement, the values are
plotted just as is the case for the normal arrangement. If the 10 ft normal is used, its reading should be
plotted about 5 ft above the marked cable reading.
2-6.2 Significance of 0.25 ft Spacing. The reading obtained with the 0.25 ft spacing is heavily influenced by the fluid in
the well bore and hence it reads only some fraction of the formation resistively. However, the short spacing enables you
to see changes in resistively with greater detail. With this electrode spacing, formations having a thickness of about 6
inches or greater can be detected. Because of this ability to see small detail, the 0.25 ft curve should be used to "pick"
formation boundaries.
2-6.3 Significance of 2.5 ft Spacing. The 2.5 ft electrode spacing provides you with very nearly the true formation
resistively for wells having diameters up to about 16 inches and for formations thicker than about 5 ft. For larger
diameter wells or thinner formations, the measured resistively will depart somewhat from the true. For qualitative
interpretation this departure is not significant. Because the 2.5 ft curve provides you with the formation resistively, it is
used to identify the type of material penetrated.
2-6.4 Significance of the Lateral Log. The lateral log obtained with the equipment is made by a combination of either the
0.25 or 2.5 ft electrode with the 10 ft. electrode. Because the 10 ft electrode is at a distance fairly large compared with
either of the other two, the interpretation is essentially the same as for the normal log after using the appropriate
correction factors.
a.
For the 0.25 lateral log, the meter factor is 1.025.
b.
For the 2.50 lateral log, the meter factor is 13.33.
2-6.5 Interpretation of Resistively Values.
a.
In interpreting the resistively values obtained, clays and shales will be low resistive and sands, gravels,
sandstones and limestones will be high resistive. Igneous and metamorphic rocks (such as granites and
gneisses) will most generally be extremely high resistive.
b.
The exact range of numerical values will depend upon the:
1.
Type of earth material making up the formation.
2.
Degree of cementation of the formation.
3.
Water quality of the formation water.
4.
Porosity of the formation.
5.
Diameter of the well bore.
6.
Resistively of the fluid in the well bore.
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