Search Results (13)

During depressurization-based natural gas hydrate production, low-temperature and high-pressure wellbore conditions can induce secondary hydrate formation and deposition, risking flow channel blockage and production safety compromise. This study develops a predictive kinetic model for secondary hydrate formation and blockage in annular and mist flows, the flow patterns most susceptible to hydrate hazards. The model integrates multiphase flow, heat and mass transfer, hydrate phase change, and particle transport and deposition, coupling conservation equations with hydrate phase equilibrium and flow regime transition criteria to quantitatively predict the hydrate formation rate, deposition rate, and wall blockage severity. Validation against published experimental loop data shows that model-predicted pressure drop increases fall within 5.3% of measurements. Simulations based on a production well in the South China Sea’s Shenhu area provide quantitative predictions of three key operational controls: enlarging the tubing diameter compresses the hydrate stability zone; increasing the gas–liquid ratio suppresses hydrate formation by limiting free water; and the liquid production rate exerts stronger control over formation and deposition than the gas rate. The model serves as a practical quantitative tool for wellbore blockage risk assessment and operational decision support during hydrate trial production. Full article

As a finite strategic resource, helium is extracted from natural gas (NG). The concentration of helium in NG is very low, which makes helium hard to separate. The hydrate-based gas separation (HBGS) was proposed as a promising method for the separation of the NG with low helium content in this work. This work systematically investigated the HBGS of helium from simulated NG. The thermodynamic analysis reveals that the existence of 5.00 mol% tetrahydrofuran (THF) in the liquid phase decreased the gas–liquid–hydrate equilibrium pressure by 92.11%, compared to the deionized water system. The single-stage HBGS experimental results show that high THF concentration, low temperature, and high pressure benefited the gas processing capacity and helium purification, but they led to a low helium recovery rate. The best HBGS performance was limited by the “hydrate shell effect”. The decrease in gas–liquid ratio led to an increase in helium concentration without losing the gas processing capacity, but it caused a decrease in the helium recovery rate. Through three-stage HBGS optimization, the helium concentration was increased from 0.54 mol% to 13.54 mol% (a 25.07-fold enrichment), and a total helium recovery of 87.34% was achieved. The mathematical model proposed in this work accurately predicts the performance of HGBS with 2.09% average relative error compared to the experimental data. Full article

Natural gas hydrate (NGH) is a clean resource characterized by abundant potential reserves, clean combustion, and high energy density. Although significant progress has been made in the development of NGH resources all around the world, challenges still exist that hinder commercial exploitation, such as a low daily gas production rate and short steady production periods. One significant reason lies in the complex gas–liquid–solid phase transitions occurring within the formation during production, which lead to changes in flow capacity. Understanding the phase change mechanism of NGH reservoirs will help to further reveal the production increase mechanism. To address the phase transitions’ effect on production, this paper establishes a numerical simulation model for the depressurization exploitation of natural gas hydrates in order to investigate phase transition characteristics at the field scale. First, the phase equilibrium calculation method is presented and the phase equilibrium curve is modified by considering the capillary effect, soluble salt, and surface adsorption. Then, the phase transition model is successfully characterized in a simulation and the numerical simulation model is established based on the first test project parameters in the Shenhu area. The production characteristics of different sediment types (montmorillonite, South China Sea sediments, kaolin, and silt) are analyzed under the effects of water content and salinity. It is shown that lower initial water content and higher salinity result in higher gas production. The results provide a better understanding of the effects of phase transition parameters on NGH production at the field scale. Full article

This research deals with gas hydrates formation and dissociation within a marine quartz-based porous sediment and in batch conditions. Hydrates were formed with small-chain hydrocarbons included in natural gas mixtures: methane and also ethane and propane. The dissociation values were collected and provided both graphically and numerically. The results were then compared with the theoretical hydrate-liquid-vapor phase boundary equilibrium for the same species, defined according to the existing literature. The deviation of the experimental results from the ideal ones, associated with the porous sediment, was quantified and discussed. For the scope, the grain size distribution and chemical composition of the sediment were provided along with the text. The results proved that the different size of guest species and, consequently, the different hydrate structures formed, played a relevant role in determining the promoting, inhibiting or neutral behavior of the porous sediment during the process. Full article

Clathrate hydrate-based technologies are considered promising and sustainable alternatives for the effective management of the climate change risks related to emissions of carbon dioxide produced by human activities. This work presents a combined experimental and computational investigation of the effects of the operational procedures and characteristics of the experimental configuration, on the phase diagrams of CO₂-H₂O systems and CO₂ hydrates’ formation, growth and dissociation conditions. The operational modes involved (i) the incremental (step-wise) temperature cycling and (ii) the continuous temperature cycling processes, in the framework of an isochoric pressure search method. Also, two different high-pressure PVT configurations were used, of which one encompassed a stirred tank reactor and the other incorporated an autoclave of constant volume with magnetic agitation. The experimental results implied a dependence of the subcooling degree, (P, T) conditions for hydrate formation and dissociation, and thermal stability of the hydrate phase on the applied temperature cycling mode and the technical features of the utilized PVT configuration. The experimental findings were complemented by a thermodynamic simulation model and other calculation approaches, with the aim to resolve the phase diagrams including the CO₂ dissolution over the entire range of the applied (P, T) conditions. Full article

The permeability, heat conductivity, and reaction rate will be varied with the change of natural gas hydrate saturation when thermal fluid is injected into the natural gas hydrate reservoirs. In order to characterize the variation of the physical field parameters with hydrate saturation, DDF-LBM was applied to simulate the hydrate dissociation process by thermal fluid injection under pore-scale modelling. Based on the forced conjugate heat transfer case, the relaxation frequency of the thermal lattice in the pores is corrected. Based on the P-T phase equilibrium relationship of hydrates and considering the heat absorbed by the hydrate reaction, the solid–liquid state of the hydrate lattice is judged in real time, and the dynamic simulation of the heat flow solidification multi-physics field is realized. The simulation results show that the dissociation rate of the hydrates by thermal fluid injection was higher than that by heating the hydrate surface alone and was positively correlated with the hydrate saturation. On the basis of the above results, this paper provided exponential fitting equations between different hydrate saturations and average permeability, effective thermal conductivity, and inherent reaction rate. The fitting results show that saturation has a negative correlation with relative permeability and effective thermal conductivity, and a positive correlation with the inherent reaction rate. The above results can provide a reference basis for accurately describing the heat and mass transfer of natural gas hydrate under the macroscale. Full article

The purpose of this paper is to present a one-dimensional model that simulates the thermo-physical processes for methane hydrate decomposition in porous media. The mathematical model consists of equations for the conservation of energy, gas, and liquid as well as the thermodynamic equilibrium equation for temperature and pressure $\left(P-T\right)$ in the hydrate stability region. The developed model is solved numerically by using the implicit finite difference technique on the grid system, which correctly describes the appearance of phase, latency, and boundary conditions. The Newton–Raphson method was employed to solve a system of nonlinear algebraic equations after defining and preparing the Jacobean matrix. Additionally, the proposed model describes the decomposition of methane hydrate by thermal catalysis of the components that make up the medium through multiple phases in porous media. In addition, the effect of thermodynamic processes during the hydrate decomposition on the pore saturation rate with hydrates a7nd water during different time periods was studied in a one-dimensional model. Finally, in a one-dimensional model over various time intervals, $t=1,\hspace{0.17em}10,\hspace{0.17em}50\hspace{0.17em}\mathrm{s}$, the pressure and temperature distributions during the decomposition of methane hydrates are introduced and investigated. The obtained results include more accurate solutions and are consistent with previous models based on the analysis of simulations and system stability. Full article

In this work, a gas hydrate formation and dissociation study was performed on two multiphase pipeline systems containing gasoline, CO₂, water, and crude oil, CO₂, water, in the pressure range of 2.5–3.5 MPa with fixed water cut as 15% using gas hydrate rocking cell equipment. The system has 10, 15 and 20 wt.% concentrations of gasoline and crude oil, respectively. From the obtained hydrate-liquid-vapor-equilibrium (HLVE) data, the phase diagrams for the system are constructed and analyzed to represent the phase behavior in the multiphase pipelines. Similarly, induction time and rate of gas hydrate formation studies were performed for gasoline, CO₂, and water, and crude oil, CO₂, water system. From the evaluation of phase behavior based on the HLVE curve, the multiphase system with gasoline exhibits an inhibition in gas hydrates formation, as the HLVE curve shifts towards the lower temperature and higher-pressure region. The multiphase system containing the crude oil system shows a promotion of gas hydrates formation, as the HLVE curve shifted towards the higher temperature and lower pressure. Similarly, the kinetics of hydrate formation of gas hydrates in the gasoline system is slow. At the same time, crude oil has a rapid gas hydrate formation rate. Full article

Natural gas hydrates in sediments can never reach thermodynamic equilibrium. Every section of any hydrate-filled reservoir is unique and resides in a stationary balance that depends on many factors. Fluxes of hydrocarbons from below support formation of new hydrate, and inflow of water through fracture systems leads to hydrate dissociation. Mineral/fluid/hydrate interaction and geochemistry are some of the many other factors that determine local hydrate saturation in the pores. Even when using real sediments from coring it is impossible to reproduce in the laboratory a natural gas hydrate reservoir which has developed over geological time-scales. In this work we discuss the various stages of hydrate formation, with a focus on dynamic rate limiting processes which can lead to trapped pockets of gas and trapped liquid water inside hydrate. Heterogeneous hydrate nucleation on the interface between liquid water and the phase containing the hydrate former rapidly leads to mass transport limiting films of hydrate. These hydrate films can delay the onset of massive, and visible, hydrate growth by several hours. Heat transport in systems of liquid water and hydrate is orders of magnitude faster than mass transport. We demonstrate that a simple mass transport model is able to predict induction times for selective available experimental data for CO₂ hydrate formation and CH₄ hydrate formation. Another route to hydrate nucleation is towards mineral surfaces. CH₄ cannot adsorb directly but can get trapped in water structures as a secondary adsorption. H₂S has a significant dipole moment and can adsorb directly on mineral surfaces. The quadropole-moment in CO₂ also plays a significant role in adsorption on minerals. Hydrate that nucleates toward minerals cannot stick to the mineral surfaces so the role of these nucleation sites is to produce hydrate cores for further growth elsewhere in the system. Various ways to overcome these obstacles and create realistic hydrate saturation in laboratory sediment are also discussed. Full article

With a single apparatus and very short experimentation times, we have assessed phase equilibria, apparent kinetics and morphology of methane gas hydrates in the presence of thermodynamic inhibitors ethane-1,2-diol (MEG) and sodium chloride (NaCl); and kinetic hydrate inhibitor polyvinyl-pyrrolidone (PVP). Tight, local temperature control produced highly repeatable crystal morphologies in constant temperature systems and in systems subject to fixed temperature gradients. Hydrate-Liquid-Vapor (HLV) equilibrium points were obtained with minimal temperature and pressure uncertainties ($u_{T_{\mathrm{avg}}}=0.13$ K and $u_p=0.005$ MPa). By applying a temperature gradient during hydrate formation, it was possible to study multiple subcoolings with a single experiment. Hydrate growth velocities were determined both under temperature gradients and under constant temperature growth. It was found that both NaCl and MEG act as kinetic inhibitors at the studied concentrations. Finally, insights on the mechanism of action of classical inhibitors are presented. Full article

The international interest in the energy potential related to the huge amounts of methane trapped in the form of hydrates is rapidly increasing. Unlike conventional hydrocarbon sources these natural gas hydrate deposits are widely spread around the world. This includes countries which have limited or no conventional hydrocarbon sources, like for instance Japan. A variety of possible production methods have been proposed during the latest four decades. The pressure reduction method has been dominant in terms of research efforts and associated investments in large scale pilot test studies. Common to any feasible method for producing methane from hydrates is the need for transfer of heat. In the pressure reduction method necessary heat is normally expected to be supplied from the surrounding formation. It still remain, however, unverified whether the capacity, and heat transport capabilities of surrounding formation, will be sufficient to supply enough heat for a commercial production based on reduction in pressure. Adding heat is very costly. Addition of limited heat in critical areas (regions of potential freezing down) might be economically feasible. This requires knowledge about enthalpies of hydrate dissociation under various conditions of temperature and pressure. When hydrate is present in the pores then it is the most stable phase for water. Hydrate can then grow in the concentration range in between liquid controlled solubility concentrations, and the minimum concentration of hydrate in water needed to keep the hydrate stable. Every concentration in that range off concentrations results unique free energy and enthalpy of the formed hydrate. Similarly for hydrate dissociation towards water containing less hydrate former than the stability limit. Every outside liquid water concentration results in unique enthalpy changes for hydrate dissociation. There are presently no other available calculation approaches for enthalpy changes related to these hydrate phase transitions. The interest of using CO₂ for safe storage in the form of hydrate, and associated CH₄ release, is also increasing. The only feasible mechanism in this method involves the formation of new CO₂ hydrate, and associated release of heat which assist in dissociating the in situ CH₄ hydrate. Very limited experimental data is available for heats of formation (and dissociation), even for CH₄. And most experimental data are incomplete in the sense that associated water/hydrate former rate are often missing or guessed. Thermodynamic conditions are frequently not precisely defined. Although measured hydrate equilibrium pressure versus temperature curves can be used there is still a need for additional models for volume changes, and ways to find other information needed. In this work we propose a simple and fairly direct scheme of calculating enthalpies of formation and dissociation using residual thermodynamics. This is feasible since also hydrate can be described by residual thermodynamics though molecular dynamics simulations. The concept is derived and explained in detail and also compared to experimental data. For enthalpy changes related to hydrate formation from water and dissolved hydrate formers we have not found experimental data to compare with. To our knowledge there are no other alternative methods available for calculating enthalpy changes for these types of hydrate phase transitions. And there are no limits in the theory for which hydrate phase transitions that can be described as long as chemical potentials for water and hydrate formers in the relevant phases are available from theoretical modeling and/or experimental information. Full article

This article presents gas hydrate experimental measurements for mixtures containing methane (CH₄), carbon dioxide (CO₂) and nitrogen (N₂) with the aim to better understand the impact of water (H₂O) on the phase equilibrium. Some of these phase equilibrium experiments were carried out with a very high water-to-gas ratio that shifts the gas hydrate dissociation points to higher pressures. This is due to the significantly different solubilities of the different guest molecules in liquid H₂O. A second experiment focused on CH₄-CO₂ exchange between the hydrate and the vapor phases at moderate pressures. The results show a high retention of CO₂ in the gas hydrate phase with small pressure variations within the first hours. However, for our system containing 10.2 g of H₂O full conversion of the CH₄ hydrate grains to CO₂ hydrate is estimated to require 40 days. This delay is attributed to the shrinking core effect, where initially an outer layer of CO₂-rich hydrate is formed that effectively slows down the further gas exchange between the vapor phase and the inner core of the CH₄-rich hydrate grain. Full article

The knowledge of the phase behavior of carbon dioxide (CO₂)-rich mixtures is a key factor to understand the chemistry and migration of natural volcanic CO₂ seeps in the marine environment, as well as to develop engineering processes for CO₂ sequestration coupled to methane (CH₄) production from gas hydrate deposits. In both cases, it is important to gain insights into the interactions of the CO₂-rich phase—liquid or gas—with the aqueous medium (H₂O) in the pore space below the seafloor or in the ocean. Thus, the CH₄-CO₂ binary and CH₄-CO₂-H₂O ternary mixtures were investigated at relevant pressure and temperature conditions. The solubility of CH₄ in liquid CO₂ (vapor-liquid equilibrium) was determined in laboratory experiments and then modelled with the Soave–Redlich–Kwong equation of state (EoS) consisting of an optimized binary interaction parameter k_ij(CH4-CO2) = 1.32 × 10⁻³ × T − 0.251 describing the non-ideality of the mixture. The hydrate-liquid-liquid equilibrium (HLLE) was measured in addition to the composition of the CO₂-rich fluid phase in the presence of H₂O. In contrast to the behavior in the presence of vapor, gas hydrates become more stable when increasing the CH₄ content, and the relative proportion of CH₄ to CO₂ decreases in the CO₂-rich phase after gas hydrate formation. Full article