Yuechao Zhao

MRI measurements of CO2 hydrate dissociation rate in a porous medium

After obtaining experimental data of CO(2) hydrate formation and dissociation in a porous medium using magnetic resonance imaging (MRI), the purpose of this study was to analyze the different dissociation rate of CO(2) hydrate using two heating rates. Images were obtained by using a fast spin-echo sequence, and the field of view was set to 40×40×40 mm. The vessel pressure was monitored during hydrate formation and dissociation, which was used to compare with MRI mean intensity. The result indicated that the MRI could visualize hydrate formation and dissociation, and the MRI mean intensity of water was in good agreement with the vessel pressure changes. The hydrate formation and dissociation rates were also quantified using the MRI mean intensity of water. The experimental results showed that the higher heating rate caused the rapid hydrate dissociation.

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.

Magnetic resonance imaging study on near miscible supercritical CO2 flooding in porous media

CO2 flooding is one of the most popular secondary or tertiary recoveries for oil production. It is also significant for studying the mechanisms of the two-phase and multiphase flow in porous media. In this study, an experimental study was carried out by using magnetic resonance imaging technique to examine the detailed effects of pressure and rates on CO2/decane flow in a bead-pack porous media. The displacing processes were conducted under various pressures in a region near the minimum miscibility pressure (the system tuned from immiscible to miscible as pressure is increasing in this region) and the temperature of 37.8 °C at several CO2 injection volumetric rates of 0.05, 0.10, and 0.15 ml/min (or linear rates of 3.77, 7.54, and 11.3 ft/day). The evolution of the distribution of decane and the characteristics of the two phase flow were investigated and analyzed by considering the pressure and rate. The area and velocity of the transition zone between the two phases were calculated and analyzed to quanti...

Branched ZnO nanotrees on flexible fiber-paper substrates for self-powered energy-harvesting systems

In this paper, branched ZnO nanotrees (NTs) have been synthesized on flexible fiber-paper substrates by introducing a multistep hydrothermal approach for realizing high-performance piezoelectric nanogenerators. With this method, a significant enhancement in output voltage of the NGs ranging from 14 mV to 0.1 V was achieved, with a nearly 20 times enhanced power density compared to the vertically grown ZnO NWs. This is the first demonstration of fabricating branched ZnO NTs-coated fiber paper for energy harvesting devices, which may provide guidelines for designing high-performance piezoelectric energy harvesting.

Estimation of minimum miscibility pressure (MMP) of CO2 and liquid n-alkane systems using an improved MRI technique

Minimum miscible pressure (MMP) of gas and oil system is a key parameter for the injection system design of CO2 miscible flooding. Some industrial standard approaches such as the experiment using a rising bubble apparatus (RBA), the slim tube tests (STT), the pressure-density diagram (PDD), etc. have been applied for decades to determine the MMP of gas and oil. Some theoretical or experiential calculations of the MMP were also applied to the gas-oil miscible system. In the present work, an improved technique based on our previous research for the estimation of the MMP by using magnetic resonance imaging (MRI) was proposed. This technique was then applied to the CO2 and n-alkane binary and ternary systems to observe the mixing procedure and to study the miscibility. MRI signal intensities, which represent the proton concentration of n-alkane in both the hydrocarbon rich phase and the CO2 rich phase, were plotted as a reference for determining the MMP. The accuracy of the MMP obtained by using this improved technique was enhanced comparing with the data obtained from our previous works. The results also show good agreement with other established techniques (such as the STT) in previous published works. It demonstrates increases of MMPs as the temperature rise from 20 °C to 37.8 °C. The MMPs of CO2 and n-alkane systems are also found to be proportional to the carbon number in the range of C10 to C14.

The influence of oxygen functional groups on gas-sensing properties of reduced graphene oxide (rGO) at room temperature

Three methods were used to prepare reduced graphene oxide (rGO) with various ratios of oxygen functional groups, such as –OOH, –OH and CO, to study their effects on the NO2 sensing properties at room temperature. It was found that the percentages of oxygen-containing species such as –OH in the rGO nanosheet, significantly affected the sensitivity of rGO to NO2 at room temperature. The –OH species can promote the sensitivity and also the recovery ability with respect to NO2. This may be because –OH has a greater attraction to NO2 and obtains electrons from it, whereas the separation of NO2 from –OH is also slightly easier, enabling a better recovery compared to other oxygen functional groups such as –OOH and CO.

A rapid method for the measurement and estimation of CO2 diffusivity in liquid hydrocarbon-saturated porous media using MRI

In this study, magnetic resonance imaging (MRI) was used to dynamically visualize the diffusion process of CO2 in porous media saturated with liquid hydrocarbon. Based on the assumption of semi-infinite media, effective CO2 diffusivity was obtained directly by the nonlinear fitting of one MR profile during the diffusion process. These experimental findings obtained based on MRI method showed a close agreement with the conventional pressure-volume-temperature method. The novel MRI-based technique is a time-saving approach that can reduce the duration of CO2 diffusivity measurement more than 90%, and realize rapid and accurate measurement and estimation of CO2 diffusivity.

A Temperature-Mapping Method for TetraHydrofuran Hydrate Formation in Porous Media

Temperature mapping provides important information for thermal characteristic analysis. There is no effective method for temperature mapping in porous media. A magnetic resonance imaging method is proposed to obtain temperature maps for hydrate formation in porous media in this study. The experimental results show that there is a high-temperature region around the hydrate formation zones. A threshold value of the mean intensity is necessary for temperature mapping. The effects of temperature on the mean intensity are negligible after the tetrahydrofuran solution completely forms hydrates. It is also shown that hydrate dissociation is confined by heat transfer and that there are three parameters affecting the hydrate saturation.

Dynamic stability characteristics of fluid flow in CO2 miscible displacements in porous media

The dynamic characteristics of fluid flow are important in miscible displacement processes in carbon dioxide enhanced oil recovery (CO2-EOR) projects. And the stability of the in situ mixing zone greatly influences the oil recovery factor, which deserves further research. We investigated CO2 miscible displacement processes using magnetic resonance imaging (MRI) apparatus. The CO2 miscible displacement flows were performed at a low injection rate of 0.1 ml min−1 with reservoir conditions of 8.5 to 9.5 MPa and 37.8 °C. The oil saturation evolution, the length of the in situ mixing zone, and the mixing-frontal velocity and CO2-frontal velocity were quantified. The experimental results showed that the residual oil saturation decreased with pressure and the mixing zone length was independent of pressure. The mixing-frontal velocity and the CO2-frontal velocity were nearly the same and increased with pressure. The critical velocity of the CO2/n-decane (CO2/nC10) system was 1.105 × 10−5 m s−1. Although the whole mixing zone length had no obvious change with pressure, a higher pressure compressed the mixing zone and led to an unstable mixing front above the critical velocity. The longitudinal dispersion coefficient was calculated by fitting the experimental data with an error function, which had no obvious change with pressure. Additionally, a three-dimensional lattice-Boltzmann method (LBM) was used to simulate pore-scale miscible fluid flows in upward vertical displacements. A front fingering occurred at a low kinematic viscosity ratio (νCo2 : νo = 1 : 1). At a large kinematic viscosity ratio (νCo2 : νo = 1 : 15), the high kinematic viscous oil restrained the buoyancy of supercritical CO2, but also impeded the displacement with a pore-scale backflow which might lead to a low oil recovery factor.

A vibration-driven nanogenerator fabricated on common paper substrate for harvesting energy from environment

There are numerous sources of mechanical energy in our environment, such as ultrasonic waves, body movement, and irregular air flow/vibration. Here, we present a simple, cost-effective approach for fabricating a flexible nanogenerator and apply it to harvest energy from environmental mechanical vibrations. The nanogenerator was based on ZnO nanorods grown on common paper substrate using a low-temperature hydrothermal method. Piezoelectric currents were measured by attaching the nanogenerator on the surface of a cantilever and a wind-up drum, respectively. At the same time, the vibrations of the cantilever and wind-up drum could also be characterized by the corresponding output signals. This is a practical and versatile technology with the potential for converting a variety of environment energy into electric energy, and also with the application for pre-warning of emergency, such as earthquake and burgling.