Jiafei Zhao

In situ observations by magnetic resonance imaging for formation and dissociation of tetrahydrofuran hydrate in porous media

Tetrahydrofuran (THF) hydrate has long been used as a substitute for methane hydrate in laboratory studies. This article investigated the formation and dissociation characteristics of THF hydrate in porous media simulated by various-sized quartz glass beads. The formation and dissociation processes of THF hydrate are observed using magnetic resonance imaging (MRI) technology. The hydrate saturation during the formation is obtained based on the MRI data. The experimental result suggests that the third surface has an effect on hydrate formation. THF hydrate crystals lean to form on the glass beads and in their adjacent area as well as from the wall of the sample container firstly. Furthermore, as the pore size diminishes, or as the formation temperature decreases, the nucleation gets easier and the formation processes faster. However, the dissociation rate is mostly dependent on the dissociation temperature rather than on the pore size.

Mechanical property of artificial methane hydrate under triaxial compression

Abstract Methane production from hydrate reservoir may induce seabed slide and deformation of the hydrate-bearing strata. The research on mechanical properties of methane hydrate is considered to be important for developing an efficient methane exploitation technology. In this paper, a triaxial test system containing a pressure crystal device was developed with the conditions to stabilize the hydrate. A series of triaxial shear tests were carried out on artificial methane hydrate specimen. In addition, mechanical characteristics of methane hydrate were studied with the strain rates of 0.1 and 1.0 mm/min, respectively, under the conditions of different temperatures ( T = −5, −10, and −20 °C) and confining pressures ( P = 0, 5, 10, 15, and 20 MPa). The preliminary results show that when the confining pressure was less than 10 MPa, the increase of confining pressure leaded to the enhancement of shear strength. Furthermore, the decreasing temperature and the increasing strain rate both caused the increase in shear strength.

Magnetic resonance imaging on CO2 miscible and immiscible displacement in oil-saturated glass beads pack

In this study, the displacement processes were observed as gaseous or supercritical CO(2) was injected into n-decane-saturated glass beads packs using a 400-MHz magnetic resonance imaging (MRI) system. Two-dimensional images of oil distribution in the vertical median section were obtained using a spin-echo pulse sequence. Gas channeling and viscous fingering appeared obviously in immiscible gaseous CO(2) displacement. A piston-like displacement front was detected in miscible supercritical CO(2) displacement that provided high sweep efficiency. MRI images were processed with image intensity analysis methods to obtain the saturation profiles. Final oil residual saturations and displacement coefficients were also estimated using this imaging intensity analysis. It was proved that miscible displacement can enhance the efficiency of CO(2) displacement notably. Finally, a special coreflood analysis method was applied to estimate the effects of capillary, viscosity and buoyancy based on the obtained saturation data.

In-situ visual observation for the formation and dissociation of methane hydrates in porous media by magnetic resonance imaging

In this work, magnetic resonance imaging (MRI) was employed to observe the in-situ formation and dissociation of methane hydrates in porous media. Methane hydrate was formed in a high-pressure cell with controlled temperature, and then the hydrate was dissociated by thermal injection. The process was photographed by the MRI, and the pressure was recorded. The images confirmed that the direct visual observation was achieved; these were then employed to provide detailed information of the nucleation, growth, and decomposition of the hydrate. Moreover, the saturation of methane hydrate during the dissociation was obtained from the MRI intensity data. Our results showed that the hydrate saturation initially decreased rapidly, and then slowed down; this finding is in line with predictions based only on pressure. The study clearly showed that MRI is a useful technique to investigate the process of methane hydrate formation and dissociation in porous media.

In situ observation of hydrate growth habit in porous media using magnetic resonance imaging

To investigate the growth law and microstructure of hydrates in sediments, the growth process of the tetrahydrofuran (THF) hydrate is observed in site using magnetic resonance imaging (MRI). The hydrate formation starts preferentially from the grain surface and then grows towards the liquid phase filling up the pore. In the final stage, the hydrate cements and stiffens the sediment. In this study, the cementing microstructure of the THF hydrate was directly observed in porous media. The extension of the observed behavior to methane hydrates gives implications for understanding their role in the seismic exploration and the stability of permafrost and seafloor.

Hydrate-based heavy metal separation from aqueous solution

A novel hydrate-based method is proposed for separating heavy metal ions from aqueous solution. We report the first batch of experiments and removal characteristics in this paper, the effectiveness and feasibility of which are verified by Raman spectroscopy analysis and cross-experiment. 88.01–90.82% of removal efficiencies for Cr3+, Cu2+, Ni2+, and Zn2+ were obtained. Further study showed that higher R141b–effluent volume ratio contributed to higher enrichment factor and yield of dissociated water, while lower R141b–effluent volume ratio resulted in higher removal efficiency. This study provides insights into low-energy, intensive treatment of wastewater.

A novel apparatus for in situ measurement of thermal conductivity of hydrate-bearing sediments

An experimental apparatus was developed to synthesize natural gas hydrates and measure the thermal conductivity of hydrate-bearing sediments in situ. The apparatus works over a temperature range varying from -20 °C to 50 °C and up to a maximum pressure of 20 MPa. This apparatus is mainly composed of a thermal conductivity test system and a reaction cell, into which a lab-fabricated thermistor probe is inserted. This thermistor has excellent temperature sensitivity and can work at high pressures. The basic principles of this apparatus are discussed, and a series of experiments were performed to verify that the apparatus can be practically applied in chemical engineering. The thermistor-based measuring method was applied successfully in a high-pressure environment both with and without porous media.

Optimal Design and Performance Analysis of a Low Concentrating Photovoltaic/Thermal System Using the Direct Absorption Collection Concept

This paper presents an optimized Concentrating Photovoltaic/Thermal system based on the direct absorption collection concept. In this system, working fluid water and solar cell separately utilizes solar radiation at different wavelengths to achieve photothermal conversion and photoelectric conversion. The system avoids the complicated fabrication technique in conventional CPV/T systems. The thermal unit has no temperature limitation from the PV module. As the incident solar irradiance increases from 800 W/m 2 to 3,600 W/m 2 , the system can produce high grade heat, almost without sacrificing electrical efficiency (around 8.9% to 10.4%), and the total exergetic efficiency of the system increases from 12.8% to 18.4%.

Study on Shear Strength of Artificial Methane Hydrate

The production of methane from hydrate reservoir may induce deformation of the hydrate-bearing strata. The research on mechanical properties of methane hydrate and establishing an efficient methane exploitation technology appear very important. In this paper, a low-temperature high-pressure triaxial test system including pressure crystal device (sample preparation system) was developed. A series of triaxial shear tests were carried out on artificial methane hydrate samples. The mechanical behavior was analyzed. The preliminary results show that the shear strength of methane hydrate increases with the increase of confining pressure and strain rate. While it increases with the decrease of temperature. Moreover, the secant modulus increases with the enhancement of strain rate and the decrease of confining pressure.Copyright

Assessment of gas production from natural gas hydrate using depressurization, thermal stimulation and combined methods

The largest sources of hydrocarbons worldwide are distributed in the permafrost and submarine sediments in the form of methane hydrates, but exploitation of these hydrocarbons is still years away from being economical, safe, and commercially viable; thus, further research is needed. To analyze the characteristics of methane hydrate (MH) dissociation and evaluate the gas production during the application of different MH decomposition methods, this study firstly compared MH dissociation during depressurization, thermal stimulation, and combined method (depressurization + thermal stimulation) treatments using magnetic resonance imaging (MRI) in situ observation. In particular, the influences of back-pressure and temperature on hydrate dissociation, the hydrate saturation, the rate of hydrate dissociation and MRI images from each of the three methods were investigated. The results proved that during application of the depressurization and combined methods at different back-pressures (2.2–2.6 MPa), the MH dissociation proceeded via radial dissociation rather than axial dissociation; moreover, during the application of the thermal stimulation method at different dissociation temperatures (278.15–288.15 K), the MH dissociated uniformly. Overall, a combination of depressurization and thermal stimulation at the initial stage of hydrate decomposition was proposed and comparison of the three methods demonstrated that the combined method had obvious advantages for methane treatment. Specifically, the combined method was capable of solving the problems related to low gas production and poor energy efficiency that were encountered when using either the depressurization or thermal stimulation method alone.