Page 148 - Petroleum Geology

The Lateral and the Microlog both have analogous devices in which the
current flow patterns are focussed between two current electrodes, the
second of which is maintained automatically at the same potential as the
normal current electrode. The potential.difference between the two measur-
ing electrodes depends then largely on the resistivity of the material in the
narrowest part of the beam.
Resistivity is the inverse of conductivity, and there is one electrical log-
ging device that measures conductivity - the Induction Log. Eddy currents
coaxial with the borehole are generated in the rocks around the borehole
by a high-frequency alternating current passing through coaxial coils in the
sonde. These Eddy currents induce a signal in receiver coils that are coaxial
with the transmitter coils, and the signal is proportional to the conductivity
of the formation. The unit of conductivity used is the millimho (= 1000/ohm),
the reciprocal of which is usually also recorded, in ohms.

Spontaneous potential (SP) log
The SP log is run concurrently with one or more resistivity devices and is
an important part of the basic log used by the petroleum geologist, or the
geologist working on any area in which borehole logs are available. It records
the potential difference between a fixed electrode at the surface and a
movable electrode in the borehole. The unit of measurement is the millivolt
(mv). The base line for the measurement is not a line on the scale of the
log, but the line recorded opposite mudstones or shales. It is known as the
shale base line or shale line. Deflexions from this are measured to the left of
this line (negative) or to the right (positive). The deflexions result from
natural electric currents in the borehole that are caused by electromotive
forces of electrochemical and electrokinetic origins. The position of the base
line is arbitrary with respect to the scale, and it is normally placed in a
position that allows deflexions (observed by the operator when running in)
to fall conveniently within the scale area.
An electrochemical potential (E, or Ech) results from the introduction
of a conductive borehole fluid (the drilling mud) across porous rocks with
fluids of a salinity different from that of the mud and its filtrate. This poten-
tial, which contributes most (if not all) of the deflexion on the log, consists
of two components in a sequence of alternating lithologies with mudstones
or shales.
Consider a porous, permeable bed between two thick, porous, but rela-
tively impermeable mudstones in a borehole (Fig. 6-10). Let us assume that
the electrolytes - the formation water, the mud and its filtrate - are NaCl
solutions, and that the mud and its filtrate are less saline than the formation
water. The composition of mudstones and shales is extremely complicated
and variable, but the clay minerals can be considered as grains in which there
are layers of Al, Si, and 0 atoms with some layers of water that are bound to