天然气水合物浆体分解对其在垂直管中流动特性影响的研究

针对天然气水合物浆体开采提升过程中水合物分解的问题,采用Euler多相流模型以及Finite-Rate/Eddy-Dissipation模型对天然气水合物浆体垂直管输的固-液两相流动以及气-液-固三相流流动特性进行研究。结果表明,受天然气水合物分解产生的气体影响,天然气水合物颗粒的速度分布、体积浓度分布均随高度的变化呈现出波动-均匀-波动的规律;水合物分解对浆体管道运输具有减阻作用,并提出天然气水合物浆体分解工况下,其流动速度不应低于3m·s~(-1);通过对管道的阻力特性分析,拟合出水合物分解下的水力提升阻力损失与流速的关系式,为天然气水合物浆体管道的经济提升参数提供指导。     

Structure and electric properties of sodium ion hydrate shell in nanopore with hydrophilic walls

The method of molecular–level computer simulation at the temperature of 298 K was used to study the fundamental regularities of formation of electric properties of the hydrate shell of the Na⁺ cation in a planar model nanopore with hydrophilic structureless walls in contact with water vapors. Electric polarizability changes nonmonotonously: as consistent with the changes in the molecular structure of the system. Hydration within the pore occurs in several stages, from formation of chain structures, microdrop compaction and ejection of the ion from its own hydrate shell to encapsulation and absorption of the ion by the solvent preceding formation of nanoelectrolyte. Despite the significant differences in the energy of retaining hydrate shells for Na⁺ and Cl^─ ions, polarizabilities of the two systems are close and behave similarly under variation of conditions. Strong spatial anisotropy of the polarizability tensor of the ion–hydrate complex is due to the effect of the nanopore walls on multiparticle spatial correlations in the system.     

R12 hydrate formation kinetics based on laser light scattering technique

Gas hydrates are non-stoichiometric crystalline compounds of water with gas at a certain temperature and pressure. Compared to the thermodynamics of hydrate formation, our knowledge on the kinetics aspect is rather immature. It is well known that the kinetics of hydrate formation/dissociation plays an important role in many industrial cases, such as the exploitation of methane hydrate underground, the storage and transportation of natural gas in solid hydrate state, the inhibition of hydrate i…     

Molecular simulation of gas hydrate nanoclusters in water shell: Structure and phase transitions

Gas hydrate nanoclusters surrounded by water shells are studied by the molecular dynamic method. Hydrates of methane (sI structures) and krypton (sII structures), as well an ice nanocluster in a supercooled water shell, are considered. The main attention was focused on studying the local structure and phase transitions. Variations in local partial densities with an increase in temperature are monitored. Melting points of nanosized samples of gas hydrates are determined using caloric curves. Additional information on the behavior of the considered systems is obtained from the temperature dependences of diffusion coefficients and the Lindemann criterion. Two-phase transitions are revealed for gas hydrate nanoclusters. The first phase transition at 210 K can be assigned to the melting of the ice shell. The second transition at 230–235 K is identified as the phase transition in the hydrate core. The melting of ice cluster is observed at 215 K, which corresponds to the melting point of bulk crystal upon the use of the SPC/E water model.     

Guest-Host Interaction Study in Clathrate Hydrates Using Lattice Dynamics Simulation

Lattice dynamics simulation of several gas hydrates （helium, argon, and methane） with different occupancy rates has been performed using TIP3P potential model. Results show that the coupling between the guest and host is not simple as depicted by the conventional viewpoints. For clathrate hydrate enclosing small guest, the small cages are dominantly responsible for the thermodynamic stability of clathrate hydrates. And the spectrum of methane hydrate is studied compared with argon hydrate, then as a result, shrink effect from positive hydrogen shell is proposed.     

A model for estimating flow assurance of hydrate slurry in pipelines

The problem of hydrate blockage of pipelines in offshore production is becoming ever-increasing severe because oil fields in ever-increasing unusual environments have been brought in production.HCFC-141b and THF were selected as the substitutes to study the flow assurance of the hydrates in pipelines.There are critical hydrate volume concentrations for these two slurries.Hydrate slurries behave like Bingham fluids and have high agglomerating tendency when the hydrate volume concentrations are larger than the critical ones.Based on theological behaviors of these two hydrates,a non-dimensional parameter is proposed through studying the driving forces of agglomeration among hydrate particles,which shows the agglomerating probability of hydrate particles in pipeline and can be used to judge the safety of the pipeline.Moreover,a safe model to judge the safely flow hydrate slurries was presented and verified with the experimental data,which demonstrates that the model is effective to judge whether the pipeline can be run safely or not.     

Thermodynamic characteristics of the hydrate shell of a Na+ ion in a plane nanopore with hydrophobic walls

The chemical potential, free energy, and work of hydration of a single-charged sodium cation are calculated using the Monte Carlo method for a bicanonic statistic ensemble at the molecular level at 298 K in plane model nanopores 0.5 and 0.7 nm wide. It is shown that the nanopores have a stabilizing effect on the hydrate shell of an ion. It is concluded that the crisis of stability that occurs outside a pore is transformed into an abrupt acceleration of growth with the conservation of a stable equilibrium with vapor under the conditions of plane nanopores. It is established that the mechanism of the threshold acceleration of growth inside a pore is associated with an ion being displaced from its own hydrate shell.     

Gas-hydrate formation, agglomeration and inhibition in oil-based drilling fluids for deep-water drilling

One of the main challenges in deep-water drilling is gas-hydrate plugs, which make the drilling unsafe. Some oil-based drilling fluids (OBDF) that would be used for deep-water drilling in the South China Sea were tested to investigate the characteristics of gas-hydrate formation, agglomeration and inhibition by an experimental system under the temperature of 4 ℃ and pressure of 20 MPa, which would be similar to the case of 2000 m water depth. The results validate the hydrate shell formation model and show that the water cut can greatly influence hydrate formation and agglomeration behaviors in the OBDF. The oleophobic effect enhanced by hydrate shell formation which weakens or destroys the interfacial films effect and the hydrophilic effect are the dominant agglomeration mechanism of hydrate particles. The formation of gas hydrates in OBDF is easier and quicker than in water-based drilling fluids in deep-water conditions of low temperature and high pressure because the former is a W/O dispersive emulsion which means much more gas-water interfaces and nucleation sites than the later. Higher ethylene glycol concentrations can inhibit the formation of gas hydrates and to some extent also act as an anti-agglomerant to inhibit hydrates agglomeration in the OBDF.     

Gas-hydrate formation,agglomeration and inhibition in oil-based drilling fluids for deep-water drilling

One of the main challenges in deep-water drilling is gas-hydrate plugs,which make the drilling unsafe.Some oil-based drilling fluids(OBDF) that would be used for deep-water drilling in the South China Sea were tested to investigate the characteristics of gas-hydrate formation,agglomeration and inhibition by an experimental system under the temperature of 4 ?C and pressure of 20 MPa,which would be similar to the case of 2000 m water depth.The results validate the hydrate shell formation model and show that the water cut can greatly influence hydrate formation and agglomeration behaviors in the OBDF.The oleophobic effect enhanced by hydrate shell formation which weakens or destroys the interfacial films effect and the hydrophilic effect are the dominant agglomeration mechanism of hydrate particles.The formation of gas hydrates in OBDF is easier and quicker than in water-based drilling fluids in deep-water conditions of low temperature and high pressure because the former is a W/O dispersive emulsion which means much more gas-water interfaces and nucleation sites than the later.Higher ethylene glycol concentrations can inhibit the formation of gas hydrates and to some extent also act as an anti-agglomerant to inhibit hydrates agglomeration in the OBDF.     

Molecular simulation of nanoclusters of gas hydrates in a water shell. The mechanical state of the system

The molecular dynamics method is employed to study hydrates of methane (sI), and krypton hydrate (sII), as well as an ice nanocluster in a supercooled water shell. The main attention is focused on the local structure and the mechanical state of two-phase nanosized systems, which is described using the local pressure tensor. Analysis of the temperature dependence of the local pressure allows one to compare two possible mechanisms responsible for the anomalous stability of gas hydrates at ambient pressure. According to the first mechanism, the water shell plays the role of a barrier that prevents the gas from escaping from the hydrate core. The second mechanism implies that the water shell generates additional pressure, which transfers the hydrate to a thermodynamically stable state. Results of molecular dynamics simulation indicate that both mechanisms are simultaneously involved in the stabilization of the hydrate nanocluster.     

Measurement and modeling of the effective thermal conductivity for porous methane hydrate samples

The effective thermal conductivities of gas-saturated porous methane hydrates were measured by a single-sided transient plane source (TPS) technique and simulated by a generalized fractal model of porous media that based on self-similarity.The density of porous hydrate,measured by the volume of the sample in the experimental system,was used to evaluate the porosity of methane hydrate samples.The fractal model was based on Sierpinski carpet,a thermal-electrical analogy technique and one-dimensional heat flow assumption.Both the experimental and computational results show the effective thermal conductivity of methane hydrate decreases with the porosity increase.The porosity of 0.3 can reduce the thermal conductivity of the methane hydrate by 25%.By analysis of the experimental data and the simulative result,the optimized thermal conductivity of the zero-porosity methane hydrate is about 0.7 W m-1K-1.     

Study of the permeability characteristics of porous media with methane hydrate by pore network model

The permeability in the methane hydrate reservoir is one of the key parameters in estimating the gas production performance and the flow behavior of gas and water during dissociation. In this paper, a three-dimensional cubic pore-network model based on invasion percolation is developed to study the effect of hydrate particle formation and growth habit on the permeability. The variation of permeability in porous media with different hydrate saturation is studied by solving the network problem. The simulation results are well consistent with the experimental data. The proposed model predicts that the permeability will reduce exponentially with the increase of hydrate saturation, which is crucial in developing a deeper understanding of the mechanism of hydrate formation and dissociation in porous media.     

Hydration and water-exchange mechanism of the UO2 2+ ion revisited: the validity of the "n + 1" model

Hydration and water-exchange mechanism of the UO2(2+) ion was studied by the B3LYP calculations. Relative Gibbs energies in aqueous phase of the 4-, 5-, and 6-fold uranyl(VI) hydrates were compared. A model with a complete first hydration shell and one water in the second shell was used (which is called "n + 1" model) to compare the energy of the UO2(2+) ion with different hydration numbers. The n + 1 model tends to overestimate the overall stability of the complex, and this type of model should be carefully used for the determination of the coordination number or the coordination mode such as unidentate or bidentate. A stable 5-fold uranyl(VI) hydrate goes through a very rapid water-exchange process via an associative (A-) mechanism keeping the 5-fold uranyl(VI) the dominant species.     

Flow-induced shear stress and deformation of a core–shell-structured microcapsule in a microchannel

A numerical model was developed and validated to investigate the fluid–structure interactions between fully developed pipe flow and core–shell-structured microcapsule in a microchannel. Different flow rates and microcapsule shell thicknesses were considered. A sixth-order rotational symmetric distribution of von Mises stress over the microcapsule shell can be observed on the microcapsule with a thinner shell configuration, especially at higher flow rate conditions. It is also observed that when being carried along in a fully developed pipe flow, the microcapsule with a thinner shell tends to accumulate stress at a higher rate compared to that with a thicker shell. In general, for the same microcapsule configuration, higher flow velocity would induce a higher stress level over the microcapsule shell. The deformation gradient was used to capture the microcapsule's deformation in the present study. The effect of Young's modulus on the microcapsule shell on the microcapsule deformation was investigated as well. Our findings will shed light on the understanding of the stability of core–shell-structured microcapsule when subjected to flow-induced shear stress in a microfluidic system, enabling a more exquisite control over the breakup dynamics of drug-loaded microcapsule for biomedical applications.     

Formation of methane hydrate from polydisperse ice powders

Neutron diffraction runs and gas-consumption experiments based on pressure-volume-temperature measurements are conducted to study the kinetics of methane hydrate formation from hydrogenated and deuterated ice powder samples in the temperature range of 245-270 K up to high degrees of transformation. An improved theory of the hydrate growth in a polydisperse ensemble of randomly packed ice spheres is developed to provide a quantitative interpretation of the data in terms of kinetic model parameters. This paper continues the research line of our earlier study which was limited to the monodisperse case and shorter reaction times (Staykova et al., 2003). As before, we distinguish the process of initial hydrate film spreading over the ice particle surface (stage I) and the subsequent hydrate shell growth (stage II) which includes two steps, i.e., an interfacial clathration reaction and the gas and water transport (diffusion) through the hydrate layer surrounding the shrinking ice cores. Although kinetics of hydrate formation at stage II is clearly dominated by the diffusion mechanism which becomes the limiting step at temperatures above 263 K, both steps are shown to be essential at lower temperatures. The permeation coefficient D is estimated as (1.46 +/- 0.44) x 10(-12) m2/h at 263 K with an activation energy Q(D) approximately 52.1 kJ/mol. This value is close to the energy of breaking hydrogen bonds in ice Ih and suggests that this process is the rate-limiting step in hydrate formation from ice in the slower diffusion-controlled part of the reaction.     

Kinetic of hydrate formation of propane and its mixture with methane in a circulating flow reactor

Kinetics of hydrate formation for propane and its mixture with 73% methane have been studied experimentally and theoretically at pressure up to 2 MPa and temperature up to 277.65 K in a 10 m circulating flow reactor. A mathematical model has been developed for the process of hydrate formation based on crystallization, mass transfer and thermodynamics concepts. The amounts of gas consumptions due to hydrate formation are measured experimentally and predicted by the model. The agreement between the experimental measured gas consumptions and predicted values by the mathematical model are very good and the average deviation errors in the prediction of gas consumption are less than 10%.     

Investigation of the Hydrate Plugging and Non-Plugging Properties of Oils

Three laboratories (Norwegian Institute of Science and Technology [NTNU], Institut Français du Pétrole [IFP], and the Colorado School of Mines [CSM]) determined hydrate plug formation characteristics in three oils, each in three conditions: (1) in their natural state, (2) with asphaltenes removed, and (3) with naturally occurring acids removed from the oil. The objective was to determine the major variables that affect hydrate plugging tendencies in oil-dominated systems, to enable the flow assurance engineer to qualitatively assess the tendency of an oil to plug with hydrates. In the past, it was indicated that chemical effects, for example, water-in-oil/hydrate-in-oil (emulsion/dispersion) stability, prevented hydrate plugs. For example, deasphalted oils provided low emulsion/dispersion stability and thus hydrate particles aggregated. In contrast pH 14-extracted oils were reported to remove stabilizing naphthenic acids, causing asphaltene precipitation on water/hydrate droplets, stabilizing the emulsion/dispersion to prevent aggregation and pluggage. This work suggests that in addition to chemistry, shear can enable plug-free operation in the hydrate region. High shear can prevent hydrate particle aggregation, while low shear encourages particle aggregation and plugging. As a result, flow assurance engineers may be able to forecast hydrate plug liability of an oil by a combination of chemistry and flow variables, such as: a) measurements of live oil emulsion stability, b) predictions of flow line shear, and c) knowledge of water cut. Plug formation qualitative trends are provided for the above three variables. Implications for flow assurance are given.     

Mathematical modeling of gas–liquid membrane contactors using random distribution of fibers

This work presents a mathematical model for gas absorption in microporous hollow fiber membrane contactors by using a random distribution of fibers. The chemical absorption of carbon dioxide into aqueous amine solutions and sulfur dioxide into water were simulated by this model. The nonlinear mathematical expressions of the component material balance for the liquid, membrane, and gas were solved simultaneously by using a numerical method. The results from the model were compared with four sets of different experimental data in the literature. In addition, the contactors were modeled based on the assumption of regular arrangement of fibers in the shell side by using Happel's free surface as well as plug flow models. The plug flow model was employed to compare the various available equations in the literature for the shell side mass transfer coefficient. The results indicate that the channeling of gas in the shell side decreases the efficiency of contactor significantly. It was found that the random distribution of fibers is a suitable method to simulate the commercial modules. The results also indicate that, the regular Happel's free surface model and the plug flow model are more suitable for handmade modules. The influence of shell side channeling on the contactor performance were investigated in different fiber packing densities, and in various gas and liquid flow rates.     

Search for memory effects in methane hydrate: structure of water before hydrate formation and after hydrate decomposition

Neutron diffraction with HD isotope substitution has been used to study the formation and decomposition of the methane clathrate hydrate. Using this atomistic technique coupled with simultaneous gas consumption measurements, we have successfully tracked the formation of the sI methane hydrate from a water/gas mixture and then the subsequent decomposition of the hydrate from initiation to completion. These studies demonstrate that the application of neutron diffraction with simultaneous gas consumption measurements provides a powerful method for studying the clathrate hydrate crystal growth and decomposition. We have also used neutron diffraction to examine the water structure before the hydrate growth and after the hydrate decomposition. From the neutron-scattering curves and the empirical potential structure refinement analysis of the data, we find that there is no significant difference between the structure of water before the hydrate formation and the structure of water after the hydrate decomposition. Nor is there any significant change to the methane hydration shell. These results are discussed in the context of widely held views on the existence of memory effects after the hydrate decomposition.     

Experimental study on flow characteristics of tetrahydrofuran hydrate slurry in pipelines

Tetrahydrofuran (THF) was selected as the substitute to study the flow behaviors and the mechanism of the hydrates blockage in pipelines. The slurrylike hydrates and slushlike hydrates are observed with the formation of hydrates in pipeline. There is a critical hydrate volume concentration of 50.6% for THF slurries and pipeline will be free of hydrate blockage while the hydrate volume concentration is lower than the critical volume concentration; otherwise, pipeline will be easy to be blocked. Fully turbulent flow occurs and friction factors tend to be constant when the velocity reaches 1.5 m/s. And then, constant values of friction factors that depend on the volume concentrations in the slurry were regressed to estimate the pressure drops of THF hydrate slurry at large mean velocity. Finally, a safe region, defined according to the critical hydrate volume concentration, was proposed for THF hydrate slurry, which may provide some insight for further studying the natural gas hydrate slurries and judge whether the pipeline can be run safely or not.