A B S T R A C TIt is known that the time-domain induced-polarization decay curve for a shaly sand reservoir depends on the pore structure of the reservoir, and this curve can be used to estimate permeability, which is a determining factor in making production decisions in the petroleum industry. Compared with NMR logging tools, induced polarization has several advantages, such as a deep depth of investigation and a high signal-to-noise ratio. The purpose of this paper is to establish an appropriate model using induced polarization to estimate the permeability. The curve can be modelled as a weighted superposition of exponential relaxations. The plot of weight versus the relaxation time constant is defined as the relaxation time spectrum.Induced-polarization decay-curve measurements were performed on 123 samples from the Daqing oilfield using a four-electrode technique. A singular-value decomposition method was used to transform the induced-polarization decay data into a spectrum. Different models to estimate the permeability were discussed. The results of the research indicate that the induced-polarization measurements greatly improve the statistical significance of permeability correlations. Compared with the traditional forms, Aφ C and AF C , the forms, AT B φ C and AT B F C , have lower error factors, where T, and F are the geometric mean time constant of the induced-polarization relaxation time spectrum, the porosity and the resistivity formation factor, respectively, and A, B and C are constants. The mean time constant is the decisive parameter in the permeability estimation and it is not completely independent of the resistivity formation factor. The additional use of the porosity and the resistivity formation factor leads to an appreciable improvement. It is concluded that this new model will make it possible to estimate the permeability of a shaly sand reservoir downhole.

An appropriate form of induced polarization (IP) acts as a bridge between the structure of a water-saturated core plug and its transport properties. The induced-polarization decay curves of natural rocks can be modeled as a weighted superposition of exponential relaxations. A singular-value decomposition method makes it possible to transform the induced-polarization decay data of the shaley sands into relaxation-time spectrum, defined as plot of weight versus the relaxation-time constant. We measured the induced-polarization decay curves of core samples from a formation of Daqing oil field using a four-electrode method. The decay curves were transformed to relaxation-time spectra that were used to estimate the capillary-pressure curves, the pore-size distribution, and the permeability of the shaley sands. The results show that salinity ranges from [Formula: see text] have little effect on the IP relaxation-time spectra. A pseudocapillary pressure curve can be derived from the IP relaxation-time spectrum by matching the pseudocapillary curve with that from HgAir. The best-matching coefficients of the studied cores change slightly for the samples. Defined as the value of pressure at which the injected mercury saturation is 5%, entry pressures of the cores can be estimated well from IP-derived capillary-pressure curves. Pore-size distributions generated from induced polarization and mercury capillary-pressure curves are comparable. Permeability can be predicted from IP measurements in the form of [Formula: see text], where [Formula: see text] is the estimated permeability from IP relaxation spectrum in millidarcies (md), [Formula: see text] is the porosity in percentage, and [Formula: see text] is average time constant of IP relaxation-time spectra in millis (ms). The constants and exponents from various rock formations are slightly different.

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S U M M A R YPermeability is a key parameter associated with the subsurface production and injection. This paper introduces a new method which uses the complex resistivity of rock to estimate permeability in petroleum reservoirs.Complex resistivity measurements were carried out on 53 shaly sand samples from Daqing Oil Filed with a wide variation in permeability and porosity in the frequency range from 100 Hz to 20 MHz using a four-electrode technique. This paper investigates the relationship between the complex resistivity measurements and the permeabilities of the samples. The imaginary part of the complex impedance shows a power-law relation to the frequency ranges from 100 Hz to 1.5 kHz. The results show that permeability can be estimated well with expressions of the form of A · φ B · β C , where β is the slope of the bilogarthmic plot of the imaginary part versus frequency, φ is the porosity, A, B and C are constants. The salinity of NaCl has little influence of the slope in the range of 1-10 g l −1 .The isovalue maps of the error factor δ, which show substantial regions of near-minimum values of δ, are used to analyse the behaviour of δ with the change of the exponents B and C. The permeability estimation is more sensitive to the changes of the slopes than the porosity. The slope and the porosity are not completely independent. The slope is the decisive parameter for the estimation of the permeability. The additional use of the porosity improves the formula fit to the data significantly.

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