Regimes of methane recovery from gas hydrate on injection of “warm” carbon dioxide into a porous medium

Specificities of an injection of “warm” liquid carbon dioxide into a porous medium saturated with methane and its gas hydrate are studied using a mathematical model presented in this work. The release of methane from gas hydrate during injection can proceed in two different regimes. In the first regime, the injection is accompanied by the replacement of methane with carbon dioxide in methane hydrate without the release of free water. In the second one, the injection is accompanied by the decomposition of methane hydrate into methane and water and by the formation of carbon dioxide gas hydrate. For each regime, selfsimilar solutions are constructed and critical conditions separating these regimes are found.     

On the theory of formation of gas hydrate in partially water-saturated porous medium when injecting methane

We present a planar one-dimensional theoretical model and numerical solutions for the process of the formation of methane gas hydrate by injecting gas into a porous reservoir partially saturated with water. The case where the intensity of formation of the gas hydrate is limited by the diffusion of gas through a hydrate layer formed between water and gas in the pore channel core is considered. Within this process, the kinetics of hydrate formation is determined by empirical parameter D, having the dimension of a diffusion coefficient (m²/s). The effect of the value of this parameter on the characteristics of the hydrate formation process is studied depending on the parameters that determine the initial state of the porous reservoir and its porosity and permeability characteristics. The equilibrium mechanism of hydrate formation is considered, which is a limit adopted by the diffusion pattern that corresponds to the case of D → ∞.     

In situ study of mass transfer in aqueous solutions under high pressures via Raman spectroscopy: a new method for the determination of diffusion coefficients of methane in water near hydrate formation conditions

A new method was developed for in situ study of the diffusive transfer of methane in aqueous solution under high pressures near hydrate formation conditions within an optical capillary cell. Time-dependent Raman spectra of the solution at several different spots along the one-dimensional diffusion path were collected and thus the varying composition profile of the solution was monitored. Diffusion coefficients were estimated by the least squares method based on the variations in methane concentration data in space and time in the cell. The measured diffusion coefficients of methane in water at the liquid (L)-vapor (V) stable region and L-V metastable region are close to previously reported values determined at lower pressure and similar temperature. This in situ monitoring method was demonstrated to be suitable for the study of mass transfer in aqueous solution under high pressure and at various temperature conditions and will be applied to the study of nucleation and dissolution kinetics of methane hydrate in a hydrate-water system where the interaction of methane and water would be more complicated than that presented here for the L-V metastable condition.     

DEM simulation of the undrained shear behavior of sand containing dissociated gas hydrate

A numerical simulation technique is proposed using discrete element method (DEM) to study the undrained shear behavior of sand containing dissociated gas hydrate. Dissociation of gas hydrate in sand samples under undrained conditions is simulated first, followed by undrained bi-axial compression of these samples, which consist of three phases, i.e., sand particles, water, and methane gas. The simulations demonstrate that hydrate dissociation under undrained conditions generates significant excess pore pressure and volumetric dilation, the magnitudes of which are found to be comparable with those predicted by theoretical analysis. During the undrained bi-axial compression, samples with different initial degrees of hydrate saturation exhibit markedly different shear behavior. Complete strain softening behavior, i.e., static liquefaction, is observed for the samples with relatively high initial degrees of hydrate saturation, while other samples reach quasi-steady state followed by strain hardening at large strains. The observed static liquefaction is believed to be a combined result of the looser sand structure induced by hydrate dissociation, and the essentially stiff modulus of fluid under high pore pressure, despite the existence of gas in sand pores. The influence of initial sample porosity and particle shape is also investigated.     

纳米流体中CO_2水合物生成特性实验研究

在自行设计的小型气体水合物反应装置上进行了纳米流体中CO2水合物生成特性的实验研究,探讨了纳米粒子的种类、粒径和质量分数对CO2水合物生成特性的影响。研究发现,与纯水相比,纳米粒子Cu O和Si O2增加了CO2耗气量,但延长了气体水合物生成的诱导时间。金属纳米粒子Cu和金属氧化物纳米粒子Al2O3对CO2水合物生成的诱导时间和耗气量有明显改善。对不同粒径的Al2O3纳米粒子对气体水合物生成特性的研究发现,30 nm的Al2O3纳米流体对水合物生成特性影响最大。与纯水相比,0.1%-30 nm-Al2O3纳米流体中水合物生成的诱导时间缩短了76.9%,耗气量增加了23.2%。CO2水合物耗气量随着Cu粒子质量分数的增加先增加后减少。最后进行了纳米粒子对CO2水合物生成特性的影响的理论分析。     

Thermodynamics and kinetics of methane hydrate formation and dissociation in presence of calcium carbonate

Huge amount of gas hydrate deposits are identified in deep marine sediments, which may be considered as a future source of energy. Since carbonate is one of the major components of marine sediments, in the present study attention has been given to characterize methane hydrate formation and dissociation in presence of calcium carbonate. Experiments were performed with 0%, 2%, 4%, 6% and 10% by weight of calcium carbonate in distilled water. Extensive investigations have been done on pressure-temperature equilibrium behavior of hydrate formation and dissociation at varying concentrations of calcium carbonate. Hydrate formation rate was found to vary with concentration of calcium carbonate as the solubility of calcium carbonate in water is controlled by the presence of simultaneous chemical equilibria involving a high number of species like Ca²⁺, CO₃^2?, HCO₃^?, CO₂, etc. Induction time for hydrate formation has also been measured at different concentrations of carbonate. Nucleation point for the hydrate formation was observed to be slightly higher at higher concentration of calcium carbonate due to increased heat absorption. Dissociation enthalpy of hydrates was calculated by using Clausius-Clapeyron at different measured conditions. Moles consumption of methane gas during hydrate formation at different concentrations of carbonate was measured using real gas equation and found to be minimum at 10?wt%.     

Phase Equilibrium for Structure-H Hydrates Formed with Methane and Methyl-Substituted Cyclic Ether

Clathrate hydrate formation in a (methane + either 3-methyltetrahydropyran or 2-methyltetrahydrofuran + water) system is demonstrated. The first data of the quadruple (water + structure-H hydrate + either 3-methyltetrahydropyran or 2-methyltetrahydrofuran + methane) equilibrium pressure–temperature conditions are measured over temperatures from 273 to 286 K. In the 3-methyltetrahydropyran system, the equilibrium pressures are lower by 1.6–2 MPa than those of the structure-I methane hydrate formed in the methane + water system at given temperatures. In the 2-methyltetrahydrofuran system, equilibrium pressures at temperatures below 278 K are lower than those for the structure-I methane hydrate, and at temperatures above 278 K, they are higher. These phase equilibria suggest the formation of hydrates other than structure-I methane hydrates in the two systems. The crystallographic structures of the hydrates are determined to be structure H by means of X-ray diffraction as expected from considerations of sizes and shapes of the 3-methyltetrahydropyran and 2-methyltetrahydrofuran molecules     

天然气水合物合成实验

为提高天然气水合物的生产效率及储气密度,在专门设计的水合物合成实验装置上,进行了纯甲烷水合物的合成实验。实验结果表明:对于纯净甲烷水合物,压力越高,合成速率越大;但当压力大于5 MPa时,压力的提高对生成速率的影响不大。水合物合成前抽真空时间越长,生成的水合物吸收的气体量越大,表明抽真空可以排出水中溶解的气体,提高水合物的储气密度。     

表面活性剂促进气体水合物生成的研究

天然气水合物被公认为21世纪的重要后续能源,其生成与压力、温度、气水接触面积以及添加剂等因素有关。文章概述了表面活性剂的基本性质,并从改变反应液的表面张力、形成胶束、改变水合物形貌等方面总结了表面活性剂对促进水合物生成的影响,提出了其促进水合物生成的机理。对于未来的研究,我们不仅要观察水合物的形态,也要从水合物膜的内部结构分析表面活性剂对水合物的生成影响,为水合物生成机理形成统一的认识和气体水合物进一步发展提供参考。     

The Effect of Gas Properties on Bubble Formation,Growth, and Detachment

Bubble formation and growth play an important role in various processes and industries, where the dispersion of gas bubbles in a liquid medium occurs frequently. In this paper, the formation, growth, and detachment of gas bubbles produced from a submerged needle in water are numerically and experimentally investigated. The effect of injected gas properties on bubble characteristics, including bubble diameter, contact angle, and the frequency of bubble formation, is evaluated. In particular, the changes in bubble characteristics during the injection process are investigated for three different gases to evaluate the effect of density and surface tension on the bubble detachment criteria. The present numerical results show an acceptable agreement with experiments under different operating conditions. The results show that the increase in surface tension, and the decrease in gas density result in larger bubble sizes before detachment occurs. Moreover, the bubble generation frequency is found to strongly depend on the contact angle and the surface tension.     

The Characteristics of a Gas Pipeline in the Presence of Hydrate Deposits

The paper deals with the flow of a wet hydrocarbon gas in a pipeline, which is accompanied by the process of gas-hydrate deposition (sclerosis) on the internal surface of the pipeline. The pipeline sclerosis may occur in two ways, namely, under conditions of thermal balance and under conditions of water deficit. In so doing, in the initial region behind the pipeline cross section in which the condition of gas-hydrate formation sets in, the process of deposition exhibits the former pattern. The hydrate formation causes the flow depletion of water and, as a result, starting from some cross section, the pipeline sclerosis takes the latter pattern.     

高压DSC在气体水合物形成及分解机理研究中的运用

随着海底矿物开采越来越普遍,技术人员面对越来越复杂的钻探技术挑战。特别是极端条件下的深海开采,开采矿产之前,要先处理好表层的可燃冰。通过PVT仪器直接目测来观察气体水合物的形成是一种比较常用的方法。通过记录测量过程中的温压变化来确定气体水合物的形成热力学条件。但是笨重的PVT仪使得实验受限制,并且实验过程中不能有固体微粒出现。本文通过微量热法来确定水合物的成分,研究形成及分解机理,测量比热容。为了满足高压微量热试验需求,法国塞塔拉姆公司特别提出了完全革新的测试手段（法国石油学会专利技术）,运用高压MICRODSC来建立气体水合物的形成热力学模型及生成动力学理论,表征水合物相变与时间、温度、压力的对应关系。MicroDSCVII是研究气体水合的专业高压微量热仪,一经推出就广受好评,样品量为0．7ml的全新样品池,并可与专业的高压控制面板相接,最大压力可以达到1000bars,温度范围为-45℃andl20℃。目前,高压MicroDSCVII已经被运用于各种条件下的气体水合物研究,诸如：气体水合物的形成机理,特别是甲烷气体水合物的研究;气体水合物在海底沉积物中的成藏机理;石油开采中水合物形成的抑制;天然气运输及储存过程中气体水合物的形成;冷藏过程中的气体水合的形成及分解等。     

Dynamic behaviour of the CO<Subscript>2</Subscript> bubble in a bubble column bioreactor for microalgal cultivation

Carbon dioxide (CO₂) gas is a major carbon source for microalgal cultivation. It is usually sparged into photobioreactors in the form of bubbles. The behaviour of the bubbles significantly affects mass transfer, distribution and microalgae growth. In this study, the dynamic behaviour of the CO₂ bubbles was compared between a microalgal suspension and pure water. These investigations were carried out via visual methods. The movement and distribution of microalgae at the gas–liquid interface were observed. The effects of gas flow velocity, CO₂ concentration and capillary orifice size were analysed. The results indicated that much of the microalgal cells adsorbed onto the surface of the CO₂ bubbles in the microalgal suspension, when compared with that in pure water. This resulted in an easier detachment of the bubbles in the microalgal suspension. The growth status of the bubbles were divided into two states according to changes in the Eötvös number and the behaviour of the CO₂ bubbles as influenced by gas flow velocity: steady and unsteady state. The critical gas velocity between the two states was achieved. The CO₂ bubble rising trajectory can be divided into three main phases: the vertical acceleration phase, the transition phase, and the oscillatory rising phase. During the oscillatory rising phase, the amplitude of the bubble rising trajectory was approximately two times greater than the bubble diameter. In addition, the wavelength of the bubble rising trajectory was approximately 16–18 times the bubble diameter in the microalgal suspension. A smaller capillary orifice size and larger CO₂ concentration led to a decrease in the bubble detachment diameter, an increase in velocity and an enlargement in the zone of bubble influence in the horizontal direction. These are advantageous for CO₂ transportation. These findings are beneficial for optimizing the design and operation of microalgal photobioreactors.     

Bioinspired bubble design for particle generation

In this study, we devise a method to generate homogeneous particles from a bubble suspension, with the capability to control loading and the structure of bubbles. Ideally, a process such as this would occur at the interface between daughter bubble formation (instant) and gaseous diffusion (gradual). Interestingly, the budding mechanism in micro-organisms is one that demonstrates features of the desired phenomena (although at a much slower rate), as viruses can eject and evolve structures from their membranes. With these natural concepts, a bubble''s surface can also be made to serve as a platform for particle generation, which transfers significant elements from the initial bubble coating to the newly generated structures. Here, we illustrate this by preparing coated bubbles (approx. 150 µm in diameter) using a hydrophobic polymer, which may be comparable to naturally occurring bubble coatings (e.g. organic matter forming part of bubble coatings in the sea), and dye (which can demonstrate entrapment of smaller quantities of a desired moiety) and then observe particle generation (approx. 500 nm). The process, which may be driven by a polymerosome-forming mechanism, also illustrates how additional uniform sub-micrometre-scale structures may form from a bubble''s surface, which may have also previously been attributed to gas diffusion. In addition, such methods of particle formation from a bubble structure, the incorporation of chemical or biological media via an in situ process and subsequent release technologies have several areas of interest across the broad scientific community.     

The effects of surfactant on the multiscale structure of bubbly flows

It is well known that a bubble in contaminated water rises much slower than one in purified water, and the rising velocity in a contaminated system can be less than half that in a purified system. This phenomenon is explained by the so-called Marangoni effect caused by surfactant adsorption on the bubble surface. In other words, while a bubble is rising, there exists a surface concentration distribution of surfactant along the bubble surface because the adsorbed surfactant is swept off from the front part and accumulates in the rear part by advection. Owing to this surfactant accumulation in the rear part, a variation of surface tension appears along the surface and this causes a tangential shear stress on the bubble surface. This shear stress results in the decrease in the rising velocity of the bubble in contaminated liquid. More interestingly, this Marangoni effect influences not only the bubble's rising velocity but also its lateral migration in the presence of mean shear. Together, these influences cause a drastic change of the whole bubbly flow structures. In this paper, we discuss some experimental results related to this drastic change in bubbly flow structure. We show that bubble clustering phenomena are observed in an upward bubbly channel flow under certain conditions of surfactant concentrations. This cluster disappears with an increase in the concentration. We explain this phenomenon by reference to the lift force acting on a bubble in aqueous surfactant solutions. It is shown that the shear-induced lift force acting on a contaminated bubble of 1mm size can be much smaller than that on a clean bubble.     

Some dynamical characteristics of a non-spherical bubble in proximity to a free surface

The motions of a gas bubble in proximity to a free surface with and without buoyancy force, as well as in shallow water are simulated based on a numerical time integration coupled with three-dimensional boundary integral spatial solution. The fluid is assumed to be inviscid, incompressible, and the flow irrotational. The unsteady Bernoulli equation is applied on the free surface and bubble surface as one of the boundary conditions of the Laplace equation for the potential. Improvements have been made in the mesh generation of the free surface and rigid boundary, the modeling of the toroidal bubble after the jet impact and the investigation into the combined effects on the motion of a bubble in the presence of the rigid bottom and free surface. The growth and collapse of a gas bubble together with the formation of the toroidal bubble after the jet impact are simulated. The shapes and positions of the bubble, the trajectories and velocities of the poles of the bubble as well as the pressure distributions in the fluid under different standoff distances and buoyancy parameters are obtained to better illuminate the mechanism underlying the motions of gas bubble and free surface. When a bubble is initiated sufficiently close to a free surface, the free surface spike and the second accelerating phenomenon of the free surface during the collapse phase can be observed. The buoyancy force has significant effects on the jet formation and development within the bubble and it may reverse the direction of the liquid jet when exceeding the effect of the Bjerknes force induced by the free surface. The large contortions in the shallow water and the formation of the high-pressure region between the bubble and the free surface are captured when the bubble is close enough to the rigid bottom and the free surface.     

A Perspective on Rheological Studies of Gas Hydrate Slurry Properties

Gas hydrates are solid inclusion compounds that are composed of a three-dimensional hydrogen-bonded network of water cages that can trap small gas molecules, such as methane and carbon dioxide. Understanding the rheological properties of gas hydrate crystals in solution can be critical in a number of energy applications, including the transportation of natural gas in subsea and onshore operations, as well as technological applications for gas separation, desalination, or sequestration. A number of experimental and modeling studies have been done on hydrate slurry rheology; however, the link between theory and experiment is not well-defined. This article provides a review on the current state of the art of hydrate slurry viscosity measurements from high- and low-pressure rheometer studies and high-pressure flowloops over a range of different sub-cooling (ΔT_sub = T_equil − T_exp) and fluid conditions, including for water and oil continuous systems. The theoretical models that have been developed to describe the gas hydrate slurry relative viscosity are also reviewed. Perspectives’ linkage between the experiments and theory is also discussed.     

The Dynamics and Heat and Mass Transfer of a Vapor Bubble Containing a Hot Particle

A model of dynamics of “two-phase” bubble is suggested, which takes into account the disequilibrium of phase transition. Numerical simulation is performed of processes of heat and mass transfer. The thermal conditions of growth of a vapor bubble containing a hot particle are studied. The effect of disequilibrium of phase transition in the thermal and dynamic modes of vapor layer growth is investigated.     

Influence of wettability on bubble formation in liquid

This paper presents experimental results of the surface phenomena effect on bubble formation from a single orifice in water and at nozzle in liquid aluminium with gas blowing at small flow rates. The usage of coated orifice in water and nozzles of different materials in the melt realized wide range of contact angles. The meaningful stages, termed (1) nucleation period, (2) under critical growth, (3) critical growth and (4) necking, were identified during bubble formation in a regime referring to simultaneous forced flow and surface tension control. It was revealed that bubble formation is substantially dominated by hysteresis of contact angle. Evolution of interface equilibrium and force balance conditions distinctive for bubble formation is clarified. X-ray fluoroscope was used to carry out in-situ observation of bubble formation in the melt. It was shown that bubble volume increased with wettability worsening both for aqueous and metallic systems. A further insight into the mechanism of the bubble formation was obtained by comparison of the bubble behaviour at the tip of the injection devices in liquid aluminium and at the orifice in water.     

雾流强化CO_2水合物形成特性实验研究

基于雾流强化技术,在喷雾反应器中进行了CO_2水合物生成特性的实验研究,探讨了反应釜内温度、压力和夹套内冷却液温度以及缓冲气罐的使用等对CO_2水合物生成特性的影响。研究发现,与在静态条件下生成CO_2水合物相比,雾流强化过程中的压降幅度更大,CO_2水合物生成速率更快。喷雾法将水以喷雾的形式送入气体,使水颗粒与气体的接触程度明显提高。实验结果表明,在相同工况下,增置缓冲气罐时,CO_2水合物生成结束时间是20 min左右,而未使用缓冲气罐的是40 min左右。缓冲气罐的稳流稳压作用使CO_2气体均匀、稳定地进入雾化系统,增强雾化效果,使水合物生成速率提高一倍。研究结果为CO_2水合物的生成特性分析和工程应用提供了重要参考依据。