WELLOG INDUCED POLARIZATION (IP)
© 2003 - 2007 WELLOG
Induced Polarization (IP) has been used for many years by Geophysicists in search for
mineral and water deposits. In the search
for mineral deposits, an IP anomaly would on occasion lead to discovery of a
water deposit! In recent years Induced Polarization has also become a popular
solution to finding sources of petroleum contamination in water.
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 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 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 considered a nuisance factor.
Polarization will produce erroneous resistivity information. Resistivity measurements made at one
frequency will differ from resistivity measurements made at another frequency
unless they are corrected for the effect of polarization.
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.
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 = n * (n+1) * (n+2) * a
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.
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,
MORE INFORMATION:
For more information on Induced Polarization contact info@wellog.com.