Induced Polarization Relaxation Time Research Articles

Relationships between induced polarization relaxation time and permeability of sandstone

Relationships between induced polarization relaxation time and permeability of sandstone

Relationship between Induced Polarization Relaxation Time and Hydraulic Characteristics of Water-Bearing Sand

The induced polarization method has become a popular method for evaluating formation permeability characteristics in recent years because of its sensitivity to water and water-bearing porous media. In particular, the induced polarization relaxation time can reflect the macroscopic characteristics of the porous media of rock and soil. Therefore, in order to study the relationship between relaxation time and permeability, eight quartz sand samples of different grain sizes were used to simulate water-bearing sand layers under different geological conditions, and the induced polarization experiment and the Darcy seepage experiment were carried out on the same sand sample. The experimental results show that relaxation time and permeability are closely correlated with the grain size of quartz sand samples. According to the experimental data, the power function equation is a better fit for describing the relationship between permeability and relaxation time. It is worth noting that the equations obtained are only empirical equations for quartz sand samples, and they may not be applicable to all geological conditions.

Research into the inversion of the induced polarization relaxation time spectrum based on the uniform amplitude sampling method

The induced polarization relaxation time spectrum (RTS) reflects the distribution of rock pore size, which is a key factor in estimating the oil or water storage capacity of strata. However, as the data acquisition and transmission abilities of well logging instruments are much limited due to the underground environment, it is necessary to explore suitable sampling methods which can be used to obtain an accurate RST with less sampling data. This paper presents a uniform amplitude sampling method (UASM), and compares it with the conventional uniform time sampling method (UTSM) and logarithm time sampling method (LTSM) in terms of the adaptability to different strata, RTS inversion accuracy, and stratum vertical resolution. Numerical simulation results show that the UASM can obtain high inversion accuracy of RTS with different kinds of pore size distribution formation, with high dynamic ranges of pore size, and with a small number of sampling points. The UASM, being able to adapt to the attenuation speed of polarization curve automatically, thus has the highest vertical resolution. The inversion results of rock samples also show that the UASM is superior to the UTSM and LTSM.

A time domain induced polarization relaxation time spectrum inversion method based on a damping factor and residual correction

Relaxation time spectra (RTS) derived from time domain induced polarization data (TDIP) are helpful to assess oil reservoir pore structures. However, due to the sensitivity to the signal-to-noise ratio (SNR), the inversion accuracy of the traditional singular value decomposition (SVD) inversion method reduces with a decrease of SNR. In order to enhance the inversion accuracy and improve robustness of the inversion method to the SNR, an improved inversion method, based on damping factor and spectrum component residual correction, is proposed in this study. The numerical inversion results show that the oscillation of the RTS derived from the SVD method increased with a decrease of SNR, which makes it impossible to get accurate inversion components. However, the SNR has little influence on inversion components of the improved method, and the RTS has high inversion accuracy and robustness. Moreover, RTS derived from core sample data is basically in accord with the pore-size distribution curve, and the RTS derived from the actual induced polarization logging data is smooth and continuous, which indicates that the improved method is practicable.

Is it the grain size or the characteristic pore size that controls the induced polarization relaxation time of clean sands and sandstones?

There is a wide range of evidence to suggest that permeability can be constrained through of induced polarization measurements. For clean sands and sandstones, current mechanistic models of induced polarization predict a relationship between the low‐frequency time constant inferred from induced polarization measurements and the grain diameter. A number of observations do, however, disagree with this and indicate that the observed relaxation behavior is rather governed by the so‐called dynamic pore radius Λ. To test this hypothesis, we have developed a set of new scaling relationships, which allow the relaxation time to be computed from the pore size and the permeability to be computed from both the Cole‐Cole time constant and the formation factor. Moreover, these new scaling relationships can be also used to predict the dependence of the Cole‐Cole time constant as a function of the water saturation under unsaturated conditions. Comparative tests of the proposed new relationships with regard to various published experimental results for saturated clean sands and sandstones as well as for partially saturated clean sandstones, do indeed confirm that the dynamic pore radius Λ is a much more reliable indicator of the observed relaxation behavior than grain‐size‐based models.

Relationships between induced polarization relaxation time and hydraulic properties of sandstone

We investigated electrical and physical–chemical properties of six sandstone samples with contrasting mineralogical characteristics and with hydraulic conductivity varying in a wide range. The electrical data were obtained from time domain spectral induced polarization (IP) measurements. We inverted the IP decays to relaxation time distributions, and then compared the modal relaxation times with the dominant pore throat diameters obtained from the Mercury Injection Capillary Pressure (MICP) data. We found a positive logarithmic relationship between the relaxation time and the pore throat diameter. Also, we found the normalized chargeability (an integral IP parameter) to be positively correlated with the clay content. These two results suggest that the polarization of our sandstones is controlled by the pore throat distribution, and by the clay content. The logarithmic relationship contradicts previous theories, and is not universal. Adopting an approach of Kruschwitz and her co-workers, we calculated the effective diffusivity from IP and MICP data, and we found the effective diffusivity values ranging from 2.9 × 10−13 to 1.6 × 10−10 m2 s−1. High diffusivity values, typical of surface diffusion, were obtained for clean sandstones. Low diffusivity values were obtained for clayey sandstones, and they were one to two orders of magnitude lower than those characteristic of the surface diffusion. We proposed two mechanisms to explain the ‘slow’ diffusion: (1) the effect of surface tortuosity of pore throats filled with clay minerals and (2) the effect of pore geometry. These two effects represent an obstacle in assessing the pore throat diameter and hydraulic conductivity of sandstones with large specific surface and clay content on the basis of spectral IP measurements. However, we believe that the sandstones featuring ‘slow’ diffusion can be discriminated based on the integral polarization parameters, and that the relaxation time remains a valuable parameter for assessing hydraulic properties of clean sandstones.

Experimental study of induced polarization relaxation time spectra of shaly sands

The induced polarization decay curve contains much more information than chargeability which is often used in traditional induced polarization logging tool. The singular value decomposition algorithm makes it possible to transform induced polarization decay data into relaxation time spectrum. The spectrum logging tool can be used as an in-situ method to estimate pore structure, permeability, formation water resistivity and shaliness of reservoir with deep investigation depth and high signal-to-noise ratio. In order to use this new logging method in practice, effect of some factors on the relaxation time spectrum should be investigated. The induced polarization decay curve measurements have been performed on seven samples from Daqing Oil Field using a four-electrode technique. The effects of temperature, overburden pressure, electrolyte and salinity on the relaxation spectra of the core samples were analyzed qualitatively. When we discussed the effect of one parameter on the spectra, the others were fixed. The results show that the spectra shift to low time constants with temperature increasing, but the shape is almost the same. The temperature dependence of the spectrum can be corrected. When the overburden pressure increases, the relative amplitudes of low relaxation time constants are the same, and those of high constants decrease. When pressure is diminished again, the amplitudes of high time constants do not recover to the original distribution. The salinity has small influence on the spectra. The electrolyte has the influence on the spectra because of the diffusion coefficient different. This influence of electrolyte can also be corrected.

Multi-Exponential Inversion and Laboratory Study of Induced Polarization Relaxation Time Spectra of Shaly Sands

Abstract According to the traditional Induced Polarization (IP) logging method, the IP relaxation time spectrum can give much more information about a reservoir, such as the pore structure, permeability, formation water resistivity, and shaliness. In this paper, the Singular Value Decomposition (SVD) algorithm is used to inverse the IP multi-relaxation data of the rock samples. The appropriate number of relaxation arrangement points in the relaxation data inversion is from 32 to 64. All of the relaxation spectra show a multi-peak shape. IP decay curve measurements have been performed on 30 samples from the Daqing Oil Field using the four-electrode technique. The effects of charging current, charging and discharging time, brine salinity, and water saturation on the relaxation spectra of the rock were analyzed qualitatively, so that we could develop a new IP spectra logging tool that could be used successfully in practice. When we discussed the effect of one parameter on the relaxation time spectra, the others remained fixed. The results show that the relaxation spectrum relates with the charging time, discharging time, and depends strongly on water saturation. The salinity of NaCl brine and the charging current density are shown to have a small influence on the relaxation spectra in the special ranges. Introduction Induced polarization (IP) phenomena were observed more than 80 years ago, which may be caused by metallic minerals or clay Content(1–8). The detection of IP in the time-domain is made by measuring both the maximum voltage during the charging period of a dc current flow, and the transient decay of the electrical potential as a function of time, after the current is turned off. IP logging is traditionally used to determine the formation water resistivity and shaliness parameter (Qv), which is defined as the cation exchange capacity of the shaly sands per unit pore volume(9–12). But up to now, no IP logging has successfully been used commercially to evaluate or locate petroleum reservoirs. The theory analysis shows that the IP relaxation characteristics relate to the pore structure of the rock and the IP decay curve is a superposition of signals coming from all pores with different sizes(13). So, the curve can be fitted with multi-exponential decay function and a relaxation spectrum can be obtained. The spectrum contains much more information than the traditional IP method. The IP spectra of full water-saturated rock gives a quick, easy, and nondestructive way to characterize the rock, such as the pore structure, permeability, and Qv. On the other hand, we can develop a new time-domain IP spectra logging tool. This kind of IP spectra logging can be also used to obtain the formation water resistivity and shaliness parameter, Qv, as the traditional time-domain IP logging tool. In order to transfer the lab results downhole, the borehole effects, such as rugosity and mud, should also be considered. This can be achieved by using focusing technology as well as technologies used in traditional lateral logging tools.

Estimation of Pore Size Distribution and Permeability of Shaly Sands from Induced Polarization Time Spectra

The induced polarization decay curve contains much information, which can be decomposed to a multi-exponential decay component. The singular value decomposition method makes it possible to transform induced polarization decay data into relaxation time spectra. The appropriate number of relaxation arrangement points in relaxation data inversion ranges from 32 to 64. The induced polarization relaxation time spectrum can be used to quantitatively estimate the pore size distribution of the shaly sands saturated with the NaCl brine. Combining the average IP relaxation time constant with the porosity can markedly improve the accuracy of determining the permeability over the total porosity from core measurements.

Estimation of permeability of shaly sand reservoir from induced polarization relaxation time spectra

The induced polarization (IP) decay curve contains more information than the polarizability which was used in the conventional induced polarization logging tool. The Singular Value Decomposition (SVD) method makes it possible to transform induced polarization decay data into relaxation time spectra. The appropriate number of relaxation arrangement points in relaxation data inversion ranges from 32 to 64. The induced polarization relaxation time spectrum can be used to quantitatively estimate the pore size distribution of the shaly sands saturated with the NaCl brine. Combining the average IP relaxation time constant with the porosity can markedly improve the accuracy of determining the permeability over using total porosity from core measurements.

Estimation of permeability of shaly sand reservoir from induced polarization relaxation time spectra

The induced polarization (IP) decay curve contains more information than the polarizability which was used in the conventional-induced polarization logging tool. The singular value decomposition (SVD) method makes it possible to transform induced polarization decay data into relaxation time spectra. The appropriate number of relaxation arrangement points in relaxation data inversion ranges from 32 to 64. The induced polarization relaxation time spectrum can be used to quantitatively estimate the pore size distribution of the shaly sands saturated with the NaCl brine. Combining the average IP relaxation time constant with the porosity can markedly improve the accuracy of determining the permeability over using total porosity from core measurements.