Study on the Formation and Disassociation Process of Hydrate in Porous Media by Permittivity Dispersion Measurement

Abstract

To develop a more efficient and environmentally friendly exploitation and production method of gas hydrate, a better understanding of the formation and disassociation process mechanism of gas hydrate in porous media is indispensable. The permittivity spectra of gas hydrate differs from water, oil, ice and gas, and thus measurement of its permittivity properties is a potential approach to detect hydrate existence and content. In this study, THF (tetrahydrofuran) was used as the hydrate former. The permittivity dispersion of hydrate in quartz sand was measured with open-ended coaxial method in the processes of formation and disassociation at a wide radio frequency band from 1MHz to 3GHz. From the dielectric measurements, the hydrate concentration in the porous media was identified with dispersion properties, and the characteristic of the permittivity response upon the hydrate formation and dissociation were also provided. Moreover, by applying dielectric mixing models, the fractions of the solution and hydrate could be estimated. These results implied that the permittivity dispersion could be an effective monitor for the formation and disassociation of the hydrate, and hydrate content could be evaluated in real time with complex permittivity spectra. INDEX TERMSComplex permittivity, dielectric measurement, dielectric dispersion, gas hydrate, open-end coaxial line, porous media.

Integrated Dielectric Model for Unconsolidated Porous Media Containing Hydrate

“…Then the temperate recorded by the RTD was kept at 4.4℃ for about 100mins in stage B, even though the temperature inside the thermotank continued dropping. Another interesting fact is that the dielectric constant at 3.5MHz kept pace with the RTD temperature, and a reverse process was found in stage D. This is because of the temperature dependence of the hydrate dielectric dispersion at low frequency, which has already been previously clarified [38]. As the hydrate formation was completed in the whole reactor, the RTD temperature remained stable around -10℃ which is the lower limit of the target temperature.…”

“…5 that the complex permittivity changed with temperature. This is due to the temperature sensitivity of the liquid solution's dielectric properties, and moreover, analogous details were discussed in our previous publication [38], which will not be repeated here.…”

“…The complex permittivities in this study were obtained with a dielectric measurement system which is the same as the equipment used in our previous work [38]. The dielectric probe acts as the key-part of the system and its detailed structure is shown in Fig.…”

“…Moreover, the dielectric dispersion of THF solution is analogous to liquid water, and it was crucially influenced by the proportion of the solution. The relationship between parameters α, τ, εs, ε∞, and the temperature were illustrated in our previous study [38]. Specifically, the parameters of the Cole-Cole model that were used in this part are presented in Table II.…”

“…Previously we reported experimental work on the permittivity dispersion spectra of the unconsolidated porous media containing gas hydrate and the permittivity variation during the hydrate agglomerating and melting process, by using the open-ended coaxial method [38]. Because of the complete miscibility of tetrahydrofuran(THF) in water, stability under atmosphere pressure, analogous electrical properties with methane hydrate, and uniform spatial hydrate distribution, THF was widely used as the hydrate former instead of methane in laboratory studies [39].…”