Evaluation Method of Hydrate Saturation in Porous Medium Based on kHz-MHz Electrical Spectrum:A Numerical Simulation Study
The broadband electrical spectrum contains information on conduction,polarization and its dispersion mechanisms.The dielectric constant of natural gas hydrate exhibits dispersion characteristics in the kHz-MHz frequency band.A new approach to evaluate hydrate saturation based on the electrical spectrum of this frequency band is proposed.Firstly,finite element numerical electric field models in porous media containing hydrates were established.Then,the influence of hydrate saturation,micro-distribution mode of hydrates,and electric field frequency on the broadband electrical spectrum was explored.Finally,three-phase dielectric mixing models based on a two-step method were proposed for porous media containing suspending and coating hydrates,and hydrate saturation evaluation models were established.It has been demonstrated that:(1)in the porous media containing coating hydrates,the hydrates isolate quartz sand particles from pore water,and the low-frequency(<kHz)electrical spectrum of the porous media is not affected by double layer polarization;when the frequency is higher than the dispersion frequency of hydrates,the imaginary part of the complex conductivity and the real part of the complex dielectric constant of porous media containing hydrates significantly decrease with the increase of hydrate saturation;(2)for the porous media containing suspending hydrates,the mixture of pore water and hydrates is first considered as an equivalent pore fluid phase,and the Bruggeman(BR)model is used to calculate the equivalent dielectric constant;then the equivalent pore fluid phase is mixed with the skeleton particles,and the Bruggeman Hanai(BH)model is used to calculate the equivalent dielectric constant of porous media containing hydrates;the BR-BH three-phase dielectric mixing model is used to calculate the hydrate saturation,with the maximum absolute error of 0.08 and the maximum relative error of 5.67%;(3)for the porous media containing coating hydrates,the mixture of skeleton particles and hydrates is first considered as an equivalent skeleton particle phase,and the equivalent dielectric constant is calculated using Maxwell Garnett(MG)or BR models;then the equivalent skeleton particle phase is mixed with pore water,and the BH model is used to calculate the equivalent dielectric constant of porous media containing hydrates;the hydrate saturation is calculated using the MG-BH and BR-BH three-phase dielectric mixture models,with the maximum absolute errors of 0.04 and the maximum relative errors of less than 10%.This study establishes a new method for evaluating hydrate saturation,providing a theoretical basis for developing quantitative evaluation and dynamic monitoring technologies for natural gas hydrate reservoirs.
natural gas hydrateporous mediumhydrate saturationelectrical propertybroadband electrical spectrumnumerical simulation
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