Methane Hydrate Decomposition Research Articles

Thermal-hydro coupling model of methane hydrate reformation in porous media

Methane hydrates are crystalline compounds found in marine sediments and permafrost regions. Methane hydrates remain stable under both low-temperature and high-pressure conditions. When a methane hydrate reservoir is heated or depressurized, methane hydrates become unstable and decompose into water and methane gases. The heat absorption process during the decomposition of methane hydrates influences the temperature field. Methane hydrate reformation occurs during the extraction process, significantly reducing the hydraulic conductivity of the reservoir and hindering the long-term stable extraction of methane hydrates. In this paper, a numerical model is established by coupling seepage and heat processes. The temperature variation owing to the heat absorption process of methane hydrate decomposition is quantified based on the proposed model. The effect of spatial lithology on methane hydrate conversion is also analyzed, and the results for hydrate, water, gas, and permeability in the model are summarized. The numerical model also reflects the heterogeneity of marine sediments. Finally, the sensitivities of different physical parameters (permeability and porosity) and pressure gradients to the reformation rate of methane hydrate reforming are discussed. The results of this study provide scientific data supporting the influence of secondary hydrate formation on long-term extraction efficiency in the actual engineering hydrate extraction process.

Molecular Dynamic Simulation Study on Replacement of Methane Hydrates with Carbon Dioxide Under Different Temperatures, Pressures, and Concentrations of Ethylene Glycol.

In order to alleviate the world energy resources crisis, the research and development of natural gas hydrates has a very important economic value and strategic significance. The CH4-CO2 replacement method can not only achieve geological storage of carbon dioxide but also more effectively mine natural gas hydrates. Based on molecular dynamics theory and the properties of natural gas hydrates, this paper delves into the replacement of methane hydrate with carbon dioxide under different temperatures, pressures, and concentrations of ethylene glycol (EG). We established a CO2-Hydrate model and three CO2/EG-Hydrate models with different concentrations of EG, and we simulated the radial distribution function (RDF), mean square displacement (MSD), and relative density distribution of each particle in the system in different conditions. The higher the temperature, the more unstable the methane hydrates are, and the methane hydrates are more prone to decomposition. Compared with 280 and 290 K, the temperature of 270 K is more favorable for carbon dioxide molecules to enter the hydrate layer and form carbon dioxide hydrates. The changes in pressure have little impact on the decomposition of methane hydrates, the rupture of water cages of methane hydrates, and the number of carbon dioxide molecules entering the hydrate layer under temperatures of 280 K and pressures of 1, 4, and 7 MPa. But overall, a pressure of 1 MPa is more conducive for carbon dioxide molecules to enter the hydrate layer and form carbon dioxide hydrates. Adding EG to CO2 molecules can inhibit the decomposition of methane hydrates. However, the higher the concentration of EG, the faster the decomposition of methane hydrates. The degree of fracture of the water cages in methane hydrates is greater under pure CO2 conditions. Adding EG to CO2 molecules is more conducive for CO2 molecules to enter the hydrate layer and form carbon dioxide hydrates. This review is of great significance to improve the mining efficiency and CO2 storage efficiency of the replacement of natural gas hydrates with CO2.

Molecular dynamics simulations to evaluate the decomposition properties of methane hydrate under different thermodynamic conditions

Understanding the thermodynamic effect on methane hydrate decomposition is beneficial for designing controllable methane recovery processes under conditions of continental margins and oceanic sediments. In this work, we systematically examined the effects of varying temperatures and pressures on the natural gas hydrate decomposition in the presence of 20 mol% methanol through the molecular dynamics approach. Taking advantages of key parameters of angular order parameter (AOP), radial distribution function (RDF), mean square displacement (MSD), and potential energy, we mainly aimed to explore the impact of temperature and pressure on the decomposition of methane hydrate. It can be founded that high temperature is a positive factor for the methane hydrate decomposition, and as the temperature increases from 272.15 K to 277.15 K, the promotion effect becomes more obvious, resulting in a reduction of decomposition time by 3.93 ns. On the contrary, pressure has a negative effect on the methane hydrate decomposition, and as the pressure increases from 1 bar to 200 bar, the inhibitory effect becomes smaller. Additionally, the potential energy as a function of time can be used to evaluate the decomposition rates under different thermodynamic conditions. The obtained results underscore the significant dependence of methane hydrate decomposition rates on temperature and pressure, providing essential guidelines for optimizing gas hydrate decomposition processes.

ReaxFF molecular dynamics simulations of methane clathrate combustion.

Understanding the ignition and dynamic processes for the combustion of hydrate is crucial for efficient energy utilization. Through reactive force field molecular dynamics simulations, we studied the high-temperature decomposition and combustion processes of methane hydrates in a pure oxygen environment. We found that at an ignition temperature of 2800K, hydrates decomposed from the interface to the interior, but the layer-by-layer manner was no longer strictly satisfied. At the beginning of combustion, water molecules reacted first to generate OH•, followed by methane oxidation. The combustion pathway of methane is CH4→CH3•→CH3O•→CH2O→HC•O→HCOO•→CO(CO2). During the combustion process, a liquid water layer was formed between melted methane and oxygen, which hindered the reaction's progress. When there is no heat resistance, oxygen will transform into radicals such as OH• and O•, which have faster diffusion rates, allowing oxygen to conveniently cross the mass transfer barrier of the liquid water layer and participate in the combustion process. Increasing the amount of OH• may cause a surge in the reaction. On the other hand, when significant heat resistance exists, OH• is difficult to react with low-temperature hydrate components, but it can transform into O• to trigger the oxidation of methane. The H• generated has a sufficient lifetime to contact high-temperature oxygen molecules, converting oxygen into radicals that easily cross the water layer to achieve mass transfer. Therefore, finding ways to convert oxygen into various radicals is the key to solving the incomplete combustion of hydrates. Finally, the reaction pathways and microscopic reaction mechanisms of each species are proposed.

Experimental Study on Permeability and Gas Production Characteristics of Montmorillonite Hydrate Sediments Considering the Effective Stress and Gas Slippage Effect

Summary Gas permeability in hydrate reservoirs is the decisive parameter in determining the gas production efficiency and gas production of hydrate. In the South China Sea (SCS), the gas flow in tight natural gas hydrate (NGH) silty clay reservoirs is significantly affected by the gas slippage effect and the effective stress (ES) of overlying rock. To improve the effectiveness of hydrate exploitation, it is necessary to understand the influence of gas slippage in hydrate reservoirs on the permeability evolution law. For this paper, the gas permeability characteristics and methane production of hydrate montmorillonite sediments were studied at different pore pressures and ESs. Experimental data revealed that the gas permeability of montmorillonite samples before methane hydrate (MH) formation is seriously affected by the Klinkenberg effect. The gas permeability of montmorillonite sediments before hydrate formation is generally smaller than that after hydrate formation, and the gas slippage effect in the sediments after hydrate formation is weaker than that before hydrate formation. With the change in ES, the intrinsic permeability of sediment has a power law relationship with the simple ES. The ES law coefficient n was determined using the response surface method to eliminate the influence of gas slip on gas permeability. As pore pressure decreases and MH decomposes, montmorillonite swelling seriously affects gas permeability. However, the gas slippage effect has a good compensation effect on the permeability of montmorillonite sediments after MH decomposition under low pore pressure. The multistage depressurization-producing process of MH in montmorillonite sediments is mainly 3 MPa depressurization-producing stage and 2 MPa depressurization-producing stage. In this paper, the influence mechanism of gas slippage effect of hydrate reservoir is studied, which is conducive to improving the prediction accuracy of gas content in the process of hydrate exploitation and exploring the best pressure reduction method to increase the gas production of hydrate in the process of exploitation.

Molecular Dynamics Simulation of the Effects of Different Initial Water Content on Methane Hydrate Decomposition

Gas hydrate is mainly distributed in deep sea-floor sediments and permafrost regions. The water content of these sediments varies with the type of reservoir and affects the rate of hydrate decomposition. In this work, the decomposition process of methane hydrate under four different initial water contents was investigated by molecular dynamics simulation. The results were analyzed by the system conformation, radial distribution function (RDF), and mean square displacement (MSD), which revealed the microscopic mechanism of the effect of the initial water content on the decomposition rate of hydrate. The results demonstrate that the hydrate decomposition starts from the boundary to the middle, and the cage structure is destroyed layer by layer. Methane molecules continue to escape from the hydrate cages as the hydrate decomposes, and subsequent decomposition of the hydrate is inhibited when its solubility in water reaches saturation. The higher the initial water content is, the faster the decomposition rate of hydrate is. The movement distance of methane gas is affected by the initial water content. The higher the initial water content, the smaller the MSD of methane molecules.

Rapid decomposition of methane hydrates induced by terahertz bidirectional pulse electric fields

Electric field improving hydrates decomposition has broad applications in methane extraction and transportation, which benefits from polarization of water molecular and subsequent low energy cost. However, it is still challenging whether there are approaches to further accelerate decomposition of methane hydrates. Herein, molecular dynamics simulations are performed to investigate the effects of five electric fields on methane hydrates, including terahertz bidirectional pulsed electric field (BPEF), terahertz unidirectional pulsed electric field (PEF0.5), terahertz sinusoidal electric field (SEF), terahertz sinusoidal electric field with duty cycle of 50 % (SEF0.5) and static electric field (DC). We propose that BPEF, at amplitude of 0.06 V/Å and frequency of 18.5 THz, achieve highest efficiency in hydrate decomposition. The underlying mechanism is revealed to involve BPEF-induced structural phase transition of hydrate from crystalline to liquid due to resonance of electric field with vibration modes of water. Furthermore, Amplitudes of BPEF show positive correlation in promoting methane hydrate decomposition, while frequencies close to 1.8, 7.0 and 18.5 THz exhibit superior decomposition efficiency. Increasing amplitude from 0.12 to 0.16 at 1.8 THz accelerates the hydrate decomposition by 29-fold. These findings are expected to improve hydrate extraction and transportation and promote the application of terahertz electric field to hydrate domains.

Kinetic mechanisms of methane hydrate replacement and carbon dioxide hydrate reorganization

The carbon dioxide replacement is an ideal way to mine natural gas hydrates. However, current challenges of low replacement efficiency and immature technology compromise its application spectrum. This study aims to simulate the displacement of methane hydrate by carbon dioxide in a porous media system and verify the effect of water saturation on methane hydrate through experimental tests. By constructing a molecular dynamics model to investigate the replacement process of methane hydrate with carbon dioxide, the role of free water in this process was examined. Additionally, the study elucidated the factors contributing to suboptimal methane storage rate and low carbon dioxide recovery rate from a heat exchange perspective. The results show that methane replacement from methane hydrate can be divided into four stages. When the water saturation approach to hydrate saturation, and the temperature and pressure are between the phase curves of methane hydrate and carbon dioxide hydrate, the carbon dioxide storage rate and methane recovery rate can be significantly improved. The participation of free water can make the replacement more complete. The primary factor determining the replacement effect is the area of contact between carbon dioxide and methane hydrate, which is followed by the newly created carbon dioxide hydrate, which will impact the subsequent mass transfer. Although the heat released by the formation of carbon dioxide hydrate favor the replacement reaction proceed, most of the heat may get lost with the volatilization of carbon dioxide, resulting in the inability or incomplete decomposition of methane hydrate far away from free water. This research will surely have a profound impact on how hydrate replacement mining is monitored and managed as well as the ecosystem around it.

INJECTION OF SUPERHEATED WATER VAPOR INTO A POROUS RESERVOIR SATURATED IN THE INITIAL STATE WITH METHANE AND ITS HYDRATE

The work presents a mathematical model of the process of pumping superheated water vapor into a semi-infinite natural porous reservoir, which in its initial state is saturated with gas (methane) and its gas hydrate, in a flat-one-dimensional approximation. The most general case is considered, when four zones of different composition of the saturating phases and three moving boundary surfaces separating these zones appear in a natural reservoir: between the first and second zones, on which condensation of superheated water vapor occurs, between the second and third zones, where displacement takes place condensed methane water, as well as between the third and fourth zones, where the dissociation of the gas hydrate occurs. In the considered formulation of the problem, the first zone of the porous reservoir is saturated with superheated water vapor, the second zone is saturated with condensed water, the third zone is saturated with methane and still water (released during the dissociation of gas hydrate), and the fourth zone of the reservoir is saturated with methane and its gas hydrate. On the basis of a numerical solution, the hydrodynamic and temperature fields that arise in a porous reservoir are studied. It is shown that solutions with the indicated areas and boundaries exist only at relatively low values of injection pressure and reservoir permeability. It has been established that an increase in both the injection pressure and the reservoir permeability leads to a noticeable increase in only the coordinates of the boundary separating the second and third regions; in this case, the coordinate of the methane hydrate decomposition front is practically independent of the indicated parameters. An increase in the values of these parameters leads to the confluence of the boundaries of methane displacement and gas hydrate decomposition. The dependence of the limiting value of the injection pressure on the permeability at which these boundaries merge occurs is obtained.

Study of CO2 injection to enhance gas hydrate production in multilateral wells

Multilateral wells offer promising opportunities for the commercial development of natural gas hydrates. The reservoir strength and stiffness weaken as hydrates decompose during the depressurization production process, potentially leading to formation subsidence, seabed inclination, and other associated geomechanical risks. Thus, understanding the geomechanical issues surrounding the wellbore is crucial for attaining safe and efficient hydrate development. This paper develops a three-dimensional numerical model to simulate the production capacity of hydrate reservoirs through CO2 injection in multilateral wells. The study compares the pressure and temperature response of the reservoir, the characteristics of gas and water production, the variation in hydrate and methane saturation, the productivity, and the geological subsidence patterns during the extraction process. Results indicate that hydrates generated through CO2 injection inhibit methane hydrate decomposition, with delayed injections resulting in higher cumulative yields. CO2 injection can restore reservoir pressure and mitigate formation subsidence. The combined depressurization method, which involves CO2 injection in multilateral wells, can increase hydrate production while preserving formation stability, offering a potential approach for commercial hydrate exploitation. This approach is anticipated to advance the industrialization of gas hydrate extraction and propose a new possibility for CO2 utilization to curb climate change.

Experimental study on the intrinsic dissociation rate of methane hydrate

This work developed a new method that can efficiently separate gas-liquid-hydrate mixture and obtain solid gas hydrate, making it possible to measure the intrinsic dissociation rate of gas hydrate directly. The results demonstrate that the classical Kim’s model can reflect the relationship between driving force and hydrate dissociation rate when at same temperature, but can’t accurately reveal the effect of temperature on hydrate decomposition rate when under the same driving force. In Kim’s model, the driving force of hydrate dissociation was expressed as the difference between equilibrium fugacity (feq) and the fugacity of guest molecules in gas phase (fg). A new hydrate dissociation model was established in this work, and the driving force of hydrate dissociation was modified into (feq-fg)/feq, which has a higher universality than feq-fg. The intrinsic rate constant and activation energy of methane hydrate decomposition were determined to be 18905.35 mol/(m2‧s) and 22.149 kJ/mol, respectively.

Effect of main functional groups of cement slurry additives on the stability of methane hydrate: Experiment and molecular dynamics simulation

There are a lot of hydrates in deep-water and shallow layer, which accumulate at relatively shallow depths (<100–120 m). During cementation in the deep-water shallow formation, the free inorganic ions and organic molecules in the cement slurry filtrate enter the formation via percolation, which may affect the stability of the hydrate. The free organic matter in cement slurry acts mainly as retarder, fluid loss additive and dispersant. First, the representative fluid loss additive BXF-200 L, retarder PC-H21L, and dispersant PC-F41L were selected for infrared spectroscopy. Then, the influence of BXF-200 L, which contains all the previously mentioned functional groups, was studied for its impact on hydrate stability using an experimental method. Second, a molecular dynamics simulation method was used to study the adsorption properties of the functional groups on the surface of sI methane hydrate under canonical (NVT) ensemble, which it is the most widely distributed hydrate type in nature. What is more, in order to characterize the influence of functional groups on the decomposition of methane hydrate, the molecular conformation, mean square displacement (MSD), diffusion coefficient (D) and adsorption energy (Eadsorption) were calculated using the dynamics simulation trajectory of sI methane hydrate.The experimental results showed that the main functional groups in cement slurry filtrate additives were carboxyl, sulfonate, and acylamino groups. Further, BXF-200 L can promote hydrate decomposition. Subsequently, the simulation results showed that under the same conditions, the adsorption energy of sulfonate group on the surface of methane hydrate was the largest and the adsorption capacity was the greatest. When the concentration was ≤ 1 mol%, among the three functional groups, acylamino had the most obvious promoting effect on the decomposition of sI methane hydrate, and the hydrate decomposition rate increased by 16.31%. When the concentration was more than 1 mol%, the decomposition rate of hydrate was increased under the interaction of carboxyl, acylamino, and sulfonate groups, by 12.49%, 19.46% and 27.74%, respectively, compared with that under pure water-hydrate system.

Temperature response mechanism of methane hydrate decomposition coupled with icing and melting under variational thermodynamic conditions

High-efficient decomposition is the precondition of commercial production of methane hydrates. Under variational thermodynamic conditions, icing and melting may occur during hydrate decomposition, inducing complex heat and mass transfer. In this study, hydrates formed under temperature gradient of 0.012, 0.020 and 0.026 °C/mm in a 2 L reactor, decomposed by long depressurization to 1.0 MPa at constant exhaust rate from 0.11 to 1.07 ln/min. Temperature of hydrate-bearing area was in a specific relationship called thermodynamic non-equilibrium effect with pressure, controlled by hydrate decomposition. When pressure reached around 2.1–2.3 MPa, instant icing caused rapid temperature rise, and accelerated hydrate decomposition generating pressure rise of 2.36 MPa at most coordinated by limited exhaust rate. Isothermy stage occurred in double phase transition system with hydrate decomposition and ice melting, temperature slightly raised after most hydrates had decomposed. Temperature gradient only worked on non-hydrate/ice area because the temperature response of hydrate decomposition couple with icing and melting is first dominated by thermodynamics and then affected by heat and mass transfer. The control effect of ice melting in thermodynamic is stronger than hydrate decomposition. These results are significant for systematic analysis among gas production, hydrate decomposition, temperature gradient and ice behavior of actual hydrate exploitation.

Molecular simulation study of methane hydrate decomposition in the presence of hydrophilic and hydrophobic solid surfaces

Natural gas hydrates (NGHs) exist mainly in marine muddy porous media, and it is of great significance to understand the hydrate decomposition in microporous sediments for hydrate exploration. In the paper, molecular dynamics (MD) simulations have been performed for CH4 hydrate decomposition in the slit nanopores of graphite and hydroxylated-silica surfaces. As expected, the higher the temperature, the faster the decomposition rate. Compared with the hydrate decomposition in the free system, the solid surface can enhance hydrate decomposition; the hydroxylated silica surface has better enhancement effect, and this effect is more obvious at lower temperatures. At 330 K, 340 K, 350 K and 360 K, the decomposition rates of hydrate in HS-H systems are 50.8%, 36.4%, 14.7% and 11.7%, respectively, which are higher than those in G-H systems. In the slit-nanopores, hydrate decomposition starts from the surface and then grows towards the bulk region, the hydrate near the pore surface decomposes faster. However, due to the strong ability of the hydrophilic surface to adsorb water molecules, the hydrophilic-silica pore has a more vital role in enhancing hydrate decomposition near the surface in the early stage. The higher the temperature, the stronger the diffusion ability of methane molecules and water molecules, and methane molecules diffuse more easily in pores in directions parallel to the surface than perpendicular to it. Due to the difference in wettability, the contact angles of nanobubbles on the surface are different, resulting in different forms of nanobubbles. For the G-H system, both surface cap bubbles and bulk spherical bubbles were formed at higher temperatures, while only surface cap bubbles were formed at lower temperatures. For the HS-H system, bulk spherical bubbles were formed over the temperature range studied. The formation conditions of the bulk spherical bubbles are more lenient than surface cap bubbles. The hydrates in the graphene pores proceed in an isotropic and uniform decomposition. At 330 K, the liquid methane concentration of HS-H system (0.03181–0.03397) is higher than that of G-H system (0.02452–0.02559) at t = 4–6 ns, while the decomposition rate is higher than that of G-H system, which is due to the formation of bulk spherical bubbles. Bulk spherical bubbles in the hydroxylated-silica pores leads to the uneven decomposition of the hydrates, this inhomogeneity leads to local differences in hydrate dissociation rate, and the difference in hydrate dissociation rates in different regions can be up to 4.3 times. The above findings provide the theoretical basis for the safe and efficient exploitation and transportation of hydrate in marine sediments.

Analysis of Influencing Factors and Kinetic Characteristics of Spherical Methane Hydrate Decomposition

Herein, several molecular systems are simulated by molecular dynamics to study the decomposition process and fluctuation-dissipation characteristics of spherical methane hydrates under different conditions. The spherical radius and the movement of the hydrate-liquid water interface during decomposition are measured. Different fitted formulas of the variation of methane numbers are obtained from the decomposition of spherical and bulk methane hydrates. Fluctuation-dissipation characteristics for spherical methane hydrates with different radii are analyzed, which show that increasing the scale of hydrates can increase the relaxation time and slow down the fluctuation process. The variations of the hydrogen bond and hydrogen-bond lifetime are calculated. For hydrate phase water, the peak of the hydrogen-bond lifetime lies between 8 and 10 ps. After complete decomposition, the hydrogen-bond lifetime mainly distributes in 0 and 2 ps and the peak disappears. The effects of temperature, cage occupancy, liquid phase environment, and spherical hydrate scale are explored. The decomposition activation energy for the spherical hydrate with a radius of 20 Å is calculated to be 52.23 kJ/mol. It can speed up the decomposition rate as well as the diffusion of methane and water molecules with a lower cage occupancy. For the effect of the liquid phase environment, it is found that the number of liquid water rarely affects the decomposition. However, when the Na+ and Cl- concentrations change from 0 to 10%, the decomposition time reduces from ∼511 to ∼369 ps, which indicates that there is an obviously positive impact on decomposition.

Effects of different concentrations of methanol on the decomposition of methane hydrate: insights from molecular dynamics simulations

As one of the most promising alternative energy, natural gas hydrate has aroused much attention owing to its controllable exploitation with the aid of chemical injections. Although alcohols are commonly used as promoters for hydrate decomposition, few studies have been carried out to investigate the effect of different concentrations of methanol on methane hydrate decomposition at a molecular level. For the first time, we systematically combined the parameters of angular order parameter (AOP), radial distribution function (RDF), and mean square displacement (MSD) to reveal the role of methanol in the decomposition mechanism of methane hydrates through the molecular dynamics approach. The results demonstrated that methanol concentration had an optimal value for hydrate decomposition, which was 20% methanol solution for the best efficiency for the hydrate decomposition and had been examined by analyzing the parameter of AOP. This can be attributed to the promotion effect of methanol on the deconstruction of the hydrogen bond networks of water molecules and the inhibition effect of methanol on the mass transfer effect of methane diffusion, both of which determined the decomposition of methane hydrates. These effects were further quantitatively evaluated by the structural parameters of RDF and MSD. The obtained results reveal the important influence of methanol concentrations on methane hydrates decomposition on molecular scale, and provide useful guidelines for highly efficient exploitation of natural gas hydrate reservoir.

Probing the mechanism of salts destroying the cage structure of methane hydrate by molecular dynamics simulation

Much attention has been generated in the exploitation of natural gas hydrate by injecting brine due to its low cost and eco-friendliness. However, it is not clear how inorganic salts destroy the cage structure of methane hydrate. This work focuses on the mechanism of inorganic salts destroying the cage structure of methane hydrate by molecular simulation. And the influence of cation type on methane hydrate decomposition is studied in detail. The results show that inorganic salts can effectively reduce the temperature required for methane hydrate decomposition. And inorganic salts can weaken the interaction between water molecules by reducing the dipole moment of water molecules in methane hydrate, then the cage structure of methane hydrate is destroyed. Methane hydrate both can completely decompose without and with CaCl2 solution at 307 K and 20 MPa, but the decrease rate of the dipole moment autocorrelation function (DACF) of water molecules in the system with CaCl2 solution is 6.10 times than that without CaCl2 solution, and the decomposition rate of methane hydrate increased by 5.8 times because of adding CaCl2 solution. In addition, the increasing charge and radius of cations result in greater methane hydrate decomposition which is caused by the faster destruction speed of cage structure, the faster rate of methane molecules escaping from cage structure and the faster diffusion velocity of cations are demonstrated. This study provides a theoretical basis for the efficient exploitation of methane hydrates by brine injection.

Stability Condition of Methane Hydrate in Fine-Grained Sediment

Stability condition is of critical importance for methane hydrate exploitation, transportation, and reserves. This study measured the stability conditions of methane hydrate in fine-grained sediment with different dry densities (ρd = 1.40, 1.50 and 1.60 g/cm3) and various initial water saturations by the multi-step heating method. The experimental result showed that the methane hydrate formation in fine-grained sediment required lower temperature and/or higher pressure compared to that in bulk state. At the same time, it is found that the deviation degree of P–T conditions of methane hydrate in fine-grained sediment with different dry density and initial water saturation are completely different from that in pure water. In addition, according to the nuclear magnetic resonance technique (NMR), the changes in NMR signal intensity during the formation and decomposition of methane hydrate in silt were analyzed. Regardless of formation and dissociation stages, liquid water always distributes in the small sediment pores. An empirical formula is developed to address the capillary suction of water and hydrate with respect to the unhydrated water within sediment. Furthermore, a phase equilibrium model is proposed to predict the stability conditions of hydrate-bearing fine-grained sediment.

On the Theory of Methane Hydrate Decomposition in a One-Dimensional Model in Porous Sediments: Numerical Study

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 (P−T) 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, 10, 50 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.

Methane gas hydrates of the Black Sea – environmental problem or energy source?

Purpose. The purpose of this paper is to substantiate the technological solution of equilibrium conditions in the system “methane – water phase – hydrate – R-2M”; to reveal existing ecological problems of methane gas hydrate extraction from the Black Sea bottom; to determine whether gas hydrate deposits of Black Sea methane are an ecological problem or should be considered as an energy source, to explain the necessity of introduction of the effect of forced self-preservation of methane gas hydrates into development of gas hydrates from the sea bottom.&#x0D; Results. This article analyses current research on gas hydrates specifically in the Black Sea. It shows that the necessary conditions exist for the accumulation of gas hydrates in certain areas of the deep Black Sea (one of the most favourable regions among modern sea basins). This article discusses some ideas for the development of experimental studies of the metastable state of methane gas hydrates at negative temperatures: stability and mechanisms of decomposition. Despite the great variety of technological solutions and schemes of gas hydrates application proposed by the leading researchers in the world, they have been tested practically on a small number of laboratory and model installations, mainly for water desalination and concentration of water solutions, separation of two-component gas mixtures. In fact, there is no data to calculate the processes of formation and melting of ice-gas hydrate methane. The effect of self-preservation of methane gas hydrates deserves special attention.&#x0D; Scientific novelty. An attempt was made to substantiate the issue of whether gas hydrate deposits of methane in the Black Sea are an environmental problem or should they be considered as an additional source of energy and even as a “fuel of the future”. The authors for the first time introduced the concept of “forced preservation (activation) effect of methane gas hydrates”, which makes it possible to stabilize methane hydrate decomposition when the system is transferred from the area of hydrate stability to the area of thermodynamic parameters, thus significantly reducing the environmental problems of the Black Sea.&#x0D; Practical value. This article offers a technological solution for stabilization of equilibrium conditions by hydrophobic material “Ramsinks-2M” in the system “methane-water phase-hydrate-R-2M” to regulate the self-preservation effect of methane gas hydrates