WELLOG INDUCED POLARIZATION (IP)
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 (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
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.
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.
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