Browsing by Author "Wei, Zhoutuo"

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    Experimental study on hydrate saturation evaluation based on complex electrical conductivity of porous media
    (Elsevier, 2021-09-23) Xing, Lanchang; Niu, Jiale; Zhang, Shuli; Cao, Shengchang; Wang, Bin; Lao, Liyun; Wei, Wei; Han, Weifeng; Ge, Xinmin; Wei, Zhoutuo
    The hydrate saturation is a critical parameter in the evaluation of gas hydrate reservoirs. The complex characteristics of hydrate-bearing sediments pose challenges to the reliability of conventional geophysical techniques for hydrate saturation evaluation. In this paper, we present a study on developing a novel approach to characterize the electrical properties of hydrate-bearing porous media and to evaluate the hydrate saturation quantitatively based on parameters of the complex electrical conductivity. In the laboratory experiments we prepared samples with the tetrahydrofuran hydrate forming in sands to simulate the hydrate-bearing sediments and for measuring the complex conductivity at frequencies from 20 Hz to 100 kHz. The frequency-dispersion characteristics of complex conductivity of the hydrate-bearing samples with different saturations were analyzed, and then three types of hydrate-saturation evaluation models, denoted as the conductance-based, polarization-based and fusion models, were developed based on the in-phase conductivity, frequency-dispersion characteristic parameters of the phase angle and the combination of those two, respectively. A critical frequency (fc = 2 kHz) can be identified, where both the phase angle and imaginary component of the complex conductivity reach their minima. The Archie's formula shows its capability to model the relationship between the in-phase conductivity and hydrate saturation (i.e., conductance-based model), but the frequency higher than fc is preferred because stable Archie parameters can only be obtained in that frequency range. Linear correlations between the hydrate saturation and frequency-dispersion characteristic parameters (i.e., the logarithms of FE (frequency effect) and slope of the relation between FE and FR (frequency ratio) of the phase angle can be obtained, serving as the polarization-based models in the frequency range higher than fc. The fusion model performs the best in the perspective of low errors and high reliability for predicting the hydrate saturation, because more parameters of the complex conductivity and underlying physics of the conductance and polarization have been incorporated. In the frequency range lower than fc in contrast to that of the phase angle, the quadrature conductivity shows remarkable frequency-dispersion characteristics with the variation of the hydrate saturation, showing the great potential for developing new saturation-evaluation models in future.
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    Integrated dielectric model for unconsolidated porous media containing hydrate
    (IEEE, 2020-08-27) Wang, Bin; Zhang, Zhonghao; Xing, Lanchang; Lao, Liyun; Wei, Zhoutuo; Ge, Xinmin
    This article reports a novel dielectric model developed for estimating the complex permittivity of unconsolidated porous media containing gas hydrate. The complex permittivity spectra were experimentally obtained by using open-ended-coaxial-probes over a frequency range between 1 MHz and 3 GHz, in which the dielectric dispersion of both hydrate and liquid solution are covered. With tetrahydrofuran used as the hydrate former, reflection coefficients were recorded during the hydrate formation and dissociation processes in quartz sands and the complex permittivity spectra were inversed. Volumetric fractions estimated from the X-ray-tomography were used as the referenced values. Experimental data showed that the Maxwell-Wagner effect, surface conductance, and phase configuration can affect the bulk permittivity. The discrepancy was found to be unacceptable when the fitting was conducted with pre-existing models such as complex-refractive-index-method (CRIM) and Maxwell-Wagner-Bruggeman-Hanai (MWBH). In this study, by modifying the nested Wagner's theory, a shell-coated model was applied, and the surface of the solid sphere was assumed to possess surface conductance when it was humidified by the liquid solution. In contrast to CRIM and MWBH, the proposed model allows more accurate estimation of the volumetric fraction. By adopting this model with a range of dielectric measurements with different phase configurations, temperature, particle size, surface conductivity, and frequency, the contents of components and their influences onto the bulk permittivity can be physically estimated. The proposed model provides an essential tool for the interpretation of dielectric dispersion curves and the prediction of the volumetric fractions, which can be useful for both the field and laboratory applications.
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    Numerical study on complex conductivity characteristics of hydrate-bearing porous media
    (Elsevier, 2021-07-13) Xing, Lanchang; Qi, Shuying; Xu, Yuan; Wang, Bin; Lao, Liyun; Wei, Wei; Han, Weifeng; Wei, Zhoutuo; Ge, Xinmin; Aliyu, Aliyu M.
    The complex conductivity method is frequently used in hydro-/petro-/environmental geophysics, and considered to be a promising tool for characterizing and quantifying the properties of subsurface rocks, sediments and soils. We report a study on the complex conductivity characteristics of porous media containing gas hydrates through numerical modelling. The effects of the hydrate saturation, pore-water salinity and micro-distribution mode were studied, and hydrate-saturation evaluation correlations based on complex conductivity parameters were developed. A pore-scale numerical approach for developing the finite-element based models for hydrate-bearing porous media is proposed and a two-dimensional (2D) model is built to compute the complex conductivity responses of porous media under various conditions. We demonstrate that the simple 2D model can capture the dominant characteristics of the complex conductivity of hydrate-bearing porous media within the frequency range related to the induced polarization. The in-phase conductivity, quadrature conductivity and effective dielectric constant can be correlated with the saturation based on a power law in the log-log space, by which the hydrate-saturation evaluation models can be derived. A constant saturation exponent of the power-law correlation between the hydrate saturation and quadrature conductivity can be obtained when the pore-water conductivity exceeds 1.0 S/m. This is highly desirable in the hydrate-saturation models due to the variations of the pore-water conductivity in the processes of hydrate formation and dissociation. Within the framework of the complex conductivity analysis, the micro-distribution modes of hydrates in porous media can be categorized into two types. These are the fluid-suspending mode and grain-attaching mode. The in-phase conductivity exhibits significant variations under the same saturation and salinity but different micro-distribution modes, which can be attributed to the change in the tortuosity of the electrical conduction paths in the void space of porous media.
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    Pore-scale modelling on complex-conductivity responses of hydrate-bearing clayey sediments: implications for evaluating hydrate saturation and clay content
    (Elsevier, 2022-12-30) Xing, Lanchang; Zhang, Huanhuan; Wang, Shuo; Wang, Bin; Lao, Liyun; Wei, Wei; Han, Weifeng; Wei, Zhoutuo; Ge, Xinmin; Deng, Shaogui
    A majority of the accumulated gas hydrates exist in fine-grained unconsolidated sediments with clays, which pose challenges to reservoir evaluation with resistivity-based techniques. Characteristic electrical parameters derived from induced polarization (IP) measurements have potentials to describe complex formations of various lithology, pore-water salinities, and different depths from the borehole. However, there is still a knowledge gap in complex-conductivity properties of the hydrate-bearing clayey sediments. We present a pore-scale numerical study on modelling the low-frequency (<1 kHz) complex-conductivity spectra of clayey sediments containing hydrates based on the finite-element approach, with an emphasis on evaluating the hydrate saturation (sh) and clay content. Firstly, the influences of clay type, distribution form, content and sh on the complex conductivity of sediments containing hydrates were examined systematically. Secondly, power-law and linear correlations were established for evaluating the hydrate saturation and clay content, respectively, based on complex-conductivity parameters. (Effects of clay type) The in-phase conductivity of hydrate-bearing smectite is significantly higher than that of hydrate-bearing illite and kaolinite due to the higher cation exchange capacity (CEC) of smectite. Higher peak frequency and quadrature conductivity appear for the hydrate-bearing kaolinite case because the mobility of counterions in the Stern layer of kaolinite is about ten times of that for smectite and illite. (Effects of clay distribution form) The coating-clay case has much lower in-phase and quadrature conductivities than the dispersed- and laminated-clay cases because the coating clay isolates sand particles from the pore water and no electrical double layer (EDL) forms around the sand particles. (Effects of clay content) With an increasing content of the structural clay up to 60%, the in-phase conductivity decreases and increases in the frequency bands lower and higher, respectively, than the peak frequency corresponding to the EDL polarization. The effective dielectric constant increases consistently with the clay content due to the much higher CEC of clays than that of sands. (Effects of hydrate saturation) The in-phase conductivity decreases consistently with an increasing sh up to 0.40 due to the negligible conductivity of hydrates and blockage effect on conduction currents. Both the quadrature conductivity and effective dielectric constant in the EDL-polarization-dominant frequency band decrease with an increasing sh. In this work, it has been evidenced that complex-conductivity responses of hydrate-bearing clayey sediments can be understood theoretically and modelled numerically based on the interpretation of electrical conduction and electrochemical polarization mechanisms of EDLs. This study provides a theoretical and modelling foundation for the development of new IP-based geophysical techniques for hydrate-reservoir evaluation and monitoring in both the exploration and exploitation stages.
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    Quantitative analysis of guided wave in dielectric logging through numerical simulation
    (IEEE, 2020-10-01) Wang, Bin; Huang, Zixuan; Xing, Lanchang; Lao, Liyun; Wei, Zhoutuo; Ge, Xinmin
    A good knowledge of the electromagnetic (EM) wave propagation behaviors in dielectric logging (DL) and borehole radar (BHR) surveying is critically important for the optimization of tool design and implementation, and interpretation of the acquired logging data, as well as understanding the influences of the dielectric permittivity and conductivity of the formation on the EM waves. This letter reported a novel method for the numerical simulation and analysis of the guided wave (GW) propagating along a metallic pipe in a typical DL configuration. A numerical simulation with the 3-D finite-difference time-domain (FDTD) method was applied to the broadband DL tool to obtain the wavefield and responses of the receiver. By monitoring the wave attenuation along the metallic drill collar, the intensity of the GW and loss factor can be determined. The coupling efficiency of the GW can be obtained when the total power emitted from the transmitting antenna is known. Simulation results revealed that the coupling efficiency of the GW changes with the water saturation of the formation and frequency. The simulation also suggest, by installing a slope structure adjacent to the transmitting antenna, the energy coupled into the GW could be reduced at different levels. Finally, the relationship between the received signals' amplitude and GW's coupling efficiency showed the quantified contribution of the GW to the received signa
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    Saturation estimation with complex electrical conductivity for hydrate-bearing clayey sediments: an experimental study
    (Springer, 2023-04-04) Xing, Lanchang; Zhang, Shuli; Zhang, Huanhuan; Wu, Chenyutong; Wang, Bin; Lao, Liyun; Wei, Wei; Han, Weifeng; Wei, Zhoutuo; Ge, Xinmin; Deng, Shaogui
    Clays have considerable influence on the electrical properties of hydrate-bearing sediments. It is desirable to understand the electrical properties of hydrate-bearing clayey sediments and to build hydrate saturation (Sh) models for reservoir evaluation and monitoring. The electrical properties of tetrahydrofuran-hydrate-bearing sediments with montmorillonite are characterized by complex conductivity at frequencies from 0.01 Hz to 1 kHz. The effects of clay and Sh on the complex conductivity were analyzed. A decrease and increase in electrical conductance result from the clay-swelling-induced blockage and ion migration in the electrical double layer (EDL), respectively. The quadrature conductivity increases with the clay content up to 10% because of the increased surface site density of counterions in EDL. Both the in-phase conductivity and quadrature conductivity decrease consistently with increasing Sh from 0.50 to 0.90. Three sets of models for Sh evaluation were developed. The model based on the Simandoux equation outperforms Archie’s formula, with a root-mean-square error (ERMS) of 1.8% and 3.9%, respectively, highlighting the clay effects on the in-phase conductivity. The frequency effect correlations based on in-phase and quadrature conductivities exhibit inferior performance (ERMS = 11.6% and 13.2%, respectively) due to the challenge of choosing an appropriate pair of frequencies and intrinsic uncertainties from two measurements. The second-order Cole-Cole formula can be used to fit the complex-conductivity spectra. One pair of inverted Cole-Cole parameters, i.e., characteristic time and chargeability, is employed to predict Sh with an ERMS of 5.05% and 9.05%, respectively.