WELLOG INDUCED POLARIZATION (IP)
BACKGROUND:
Induced
Polarization (IP) has been used for many years by Geophysicists in search for
mineral and water deposits. In recent years Induced Polarization has also
become a popular solution to finding sources of petroleum contamination in
water. IP is a valuable source of information in the metal mining industry.
WHAT IS IP?
In rock-fluid systems, electric energy can be stored
for short periods of time. An applied current will induce a small
polarizing voltage which decays with time after the current is switched off.
Induced polarization is more pronounced in mineralized rocks.
Induced
Polarization is a geophysical property of formations within the earth that
causes received electrical square waves to have a gradual voltage decay rather
than the expected immediate transition from one voltage to another.
Various theories are used to explain this apparent “chargeability” of the
earth. Numerous papers have been written on the fact that water contained
in the pore space of rock formations does exhibit an “IP effect”.
Polarization
is attributed to the presence of interfaces between ionic and electronic
conduction (electrode polarization) and to the presence of unequal ionic
transport properties (membrane polarization). Sulphide minerals and graphite
are sources of electrode polarization and clays and zeolites are sources of
membrane polarization.
The induced
polarization effects are important to logging and surface geophysics in two
ways. A polarization measurement can detect the presence of conductive
minerals even when concentrations are less than 1 percent. In resistivity
measurements IP is sometimes considered a nuisance factor. Resistivity
measurements made at one frequency will differ from resistivity measurements
made at another frequency unless they are corrected for the effect of
polarization. The effect becomes beneficial in frequency domain I.P. where
percent frequency effect is measured.
It is well
known that… “Ions are the carriers of electrical current in a liquid.” It is
also well known that electronic conductivity in mineral bearing rock formations
is responsible for a larger IP effect. Because it takes time for ions to move
within a liquid, a “storage delay” occurs when a voltage is applied to a
formation containing liquid within its pore space. When a voltage is applied,
and suddenly removed, a time domain voltage decay is observed. The effect is
that when the voltage has been removed, a gradually reducing transient voltage
is still present. This time related effect is known as “Time Domain” IP.
TIME DOMAIN
MEASUREMENTS:
IP EFFECT AND
PERCENT IP:
The most simple method of measuring the IP effect in time domain
IP is comparison of the receiver transient voltage at time (t) after
transmitter voltage cutoff v(t) to the steady voltage (vc).
The result is given the terms millivolts per volt or percent IP.
IP effect = V(t)/Vc
percent IP = 100 V(t)/Vc
DECAY TIME
INTEGRAL:
When
potential is integrated over a defined period of time of the transient decay, a
decay time integral is obtained.
CHARGEABILITY:
Chargeability
(M) is defined
as:
M = 1/vc x integral of t1 to t2 x v(t) x dt
The resultant
chargeability is expressed in milliseconds.
Further
information can be found at: https://gpg.geosci.xyz/content/physical_properties/induced_polarization_physical_properties_duplicate.html
FREQUENCY
DOMAIN IP:
When
alternating currents of two or more frequencies are applied to a formation, an
IP frequency effect is observed. The IP effect related to frequency is referred
to as “Frequency Domain” IP.
Apparent resistivity (ra) at two frequencies (rdc) and (rac) are used in definition of frequency
effect (Fe).
Fe = (rdc-rac)/ rac = (rdc/rac) - 1
Percent
Frequency Effect (PFE):
PFE = 100 (rdc-rac)/ rac
IP WAVEFORMS:
The
transmitted waveform is a bipolar
waveform having an “off” period between each bipolar transition. Voltage
measurements are made during the “off” period immediately after cessation of
each bipolar period. Conventional E-Log logging tools can be used to conduct
Time domain or frequency domain IP logging when properly driven by surface
electronics.
PROGRAMMABLE
IP:
WELLOG has a
programmable IP system. It uses a Laptop computer that controls the IP
transmitter waveform and provides precision control of the measuring sample
window. Certain IP sample windows have been proven to contain specific
formation information.
SURFACE IP
SURVEY:
Electrode
arrays are placed usually in a straight line over an area to be surveyed. Two
current electrodes are used to supply current which flows into the surrounding
subsurface. Two additional non-polarized electrodes are placed at a specified
spacing from the current electrodes. The spacing can remain fixed and the array
progresses along a given line and additional parallel lines surveyed or the
spacing between electrode pairs may be incremented in order to obtain
measurements having increasing depth. The information gathered is
compiled into a two dimension pseudo section.
BOREHOLE IP
SURVEY:
IP surveys conducted in a borehole are performed
with logging tools having electrodes at fixed position. As the tool is pulled
up the length of the interval to be logged, data is stored for plotting at a
later time and/or plotted in real time.
COMBINATION
ELECTRIC SURVEY:
It is common
practice to obtain Several electric parameters simultaneously. Induced
polarization can be surveyed or logged with resistivity and SP. In the case of sulphide mineral
surveys or logging, metal factor (MF) can be derived from IP data.
With
reference to the pseudosection illustration, the IP electrodes are configured
for a dipole-dipole resistivity survey.
Resistivity
is derived as follows: (using dipole-dipole array)
ra = V/I x K, K = 2p * n * (n+1) * (n+2) * a
Note: surface
measurements use pi x 2 and subsurface (logging) applications use pi x 4 in
calculation of K.
Apparent
resistivity, (pa) is recorded so that a resistivity pseudosection may also be
plotted.
Metal factor
is derived as follows:
MF = (PFE/ra) x 1000 (1000 is used to bring MF into
the range of commonly used numbers.)
Metal Factor,
(MF) is recorded so that a Metal Factor pseudosection may also be plotted.
REFERENCES:
Mining
Geophysics, D. S. Parasnis, Elsevier Scientific
Publishing Company, 1973
Principles of
Applied Geophysics, D. S. Parasnis, Chapman and Hall,
Ltd., 1972
Basic
Exploration Geophysics, Edwin S. Robinson, John Wiley and Sons, 1988
Applied
Geophysics, W.M. Telford, L.P. Geldart, R.E. Sheriff,
D.A. Keys, Cambridge University Press, 1982
