Page 144 - Petroleum Geology

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It is worth noting before passing on that Ohm’s law is mathematically and
physically analogous to Darcy’s law for the flow of liquids through porous
media, and that if the current electrode B were replaced by a point-source
of water of constant density and viscosity, and electrode A by a point sink,
the water would flow in a similar pattern to the electrical flow*.
In each well of a field or district, a standard electrical log is run that will
be used for comparative and correlative purposes between wells, as well as
for specific detail in each well. The choice of devices in the standard log will
depend on local conditions and preferences, but will usually consist of a
Spontaneous Potential (SP; which will be discussed shortly), two Normal
resistivity and one Lateral or Inverse resistivity devices (Fig. 6-7). These
are all run in one sonde and the logs obtained simultaneously. The logging
programme of a well will include many other types of logs, and the whole
logging operation may well take 10-20 hours of rig time.
It is common practice to run two Normal devices with spacings (AM) of
16 and 64 inches (0.41 and 1.63 m). The first is called the Short Normal,
the second, the Long Normal. The Lateral or Inverse spacing is usually 15 ft
(4.6 m) or more; the Schlumberger Inverse spacing is 16 ft 8 in. (5.69 m). The
reason for the different spacings is that depth of penetration can only be
achieved at the expense of stratigraphic or bed detail. We need this detail,
and we need to know the effects of the borehole on the formations close to
the borehole, but we also need to determine the true resistivity of the for-
mation beyond the influence of the borehole.
The Short Normal therefore gives the best bed detail, but its depth of
penetration is small. Thus, if the resistivity curves across a particular water-
bearing sand show that the Short Normal measures a resistivity larger than
that measured by the Long Normal, it is inferred that the natural formation
water salinity is greater than that of the mud filtrate (which is known).
Likewise, if the sand is oil or gas bearing, the Long Normal may indicate
much greater resistivity than the Short Normal, reflecting displacement of oil
or gas by mud filtrate close to the borehole. A sandstone of very little porosi-
ty and permeability shows large resistivities on both Normals because there
is little effect on the Short Normal from invasion by mud filtrate. The true
resistivity of an oil or gas sand may differ little from that of a tight sand (of
small porosity), but the physical effect of drilling through such sands creates
differences that are detectable with the two Normals.
Bed thickness, borehole diameter, sonde diameter, and mud resistivity all

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* Hubbert (1969, p. 11) records that his paper on the theory of ground-water motion
(Hubbert, 1940) came to be written because, while conducting an electrical resistivity
survey, he wondered what the flow pattern would be if water wells (one injecting, the
other producing at the same rate) replaced the electrodes driven into the ground. Look at
Fig. 6-5 as a map rather than a section. See also De Wiest (1965, p. 248, fig. 6-5).