Methane Hydrate Dissociation Process Research Articles

Development of an explicit pressure explicit saturation (EPES) method for modelling dissociation processes of methane hydrate

ABSTRACT Methane gas is a fossil clean fuel since the products of the combustion are only carbon hydrate and water. Methane hydrate is a potential source of methane gas. This ice-like methane source can be found in deep seafloors and permafrost regions, characterized by high pressure and low-temperature conditions. In this work, we simulate the process of methane hydrate dissociation by depressurization in a cylindrical sandstone core by means of a two-dimensional axis-symmetric model, focusing on the transport phenomena involved in the process. Our simulations indicate a first rapid dissociation phase due to depressurization itself, and a longer subsequent one due to the thermal exchange with the external environment. Our numerical results match well with experimental data found in the literature, without showing any significant pressure or temperature delay among different sections compared to other numerical studies.

Visual study on methane hydrate formation and depressurization-induced methane hydrate dissociation processes in a micro-packed bed reactor

Natural gas hydrate has become one of the most promising green alterative energies. The safe and efficient exploitation of natural gas hydrate is of importance. Depressurization method has been considered as one of the most feasible hydrate exploitation approaches. The methane hydrate dissociation by depressurization has been widely investigated by using a variety of reactors or simulators. However, the micro-packed bed reactor has been rarely used for the hydrate-related studies. The packing materials in the micro-packed bed reactor are useful for simulating the porous condition of sediments. The optical transparency of the microreactor can provide the direct visualization of phase behavior, morphology, interfacial phenomenon and fluid migration in pore spaces. Thus, a high-pressure transparent micro-packed bed reactor recently developed was used to investigate the methane hydrate formation and depressurization-induced methane hydrate dissociation behaviors in porous media in this study. The hydrate formation results identify the different patterns of methane hydrate formation caused by the various methane bubble dispersion conditions, such as size and distribution in pores. The depressurization-induced methane hydrate dissociation behavior was investigated by continuously extracting fluids after the methane hydrate formation in the micro-packed bed reactor. The hydrate reformation phenomenon was observed during the early stage of depressurization process. This was triggered by the methane escaping out of the collapsed hydrate shell. The impact of fluid extraction rate of depressurization was studied. The results indicate higher fluid extraction rate leading to more severe hydrate shell collapsing and significant gas migration, resulting in more significant methane hydrate reformation. The hydrate dissociation process and dramatic fluid migration phenomena in the pore spaces were also visualized during the later stage of depressurization.

Nuclear magnetic resonance study of the formation and dissociation process of nature gas hydrate in sandstone

In this work, the authors monitored the formation and dissociation process of methane hydrate in four different rock core samples through nuclear magnetic resonance (NMR) relaxation time (T2) and 2D imaging measurement. The result shows that the intensity of T2 spectra and magnetic resonance imaging (MRI) signals gradually decreases in the hydrate formation process, and at the same time, the T2 spectra move toward the left domain as the growth of hydrate in the pores of the sample accelerates the decay rate. The hydrate grows and dissociates preferentially in the purer sandstone samples with larger pore size and higher porosity. Significantly, for the sample with lower porosity and higher argillaceous content, the intensity of the T2 spectra also shows a trend of a great decrease in the hydrate formation process, which means that high-saturation gas hydrate can also be formed in the sample with higher argillaceous content. The changes in MRI of the sample in the process show that the formation and dissociation of methane hydrate can reshape the distribution of water in the pores.©2022 China Geology Editorial Office.

Visual Study on Methane Hydrate Formation and Depressurization-Induced Methane Hydrate Dissociation Process in a Micro-Packed Bed Reactor

Visual Study on Methane Hydrate Formation and Depressurization-Induced Methane Hydrate Dissociation Process in a Micro-Packed Bed Reactor

Effect of NaCl concentration on depressurization-induced methane hydrate dissociation near ice-freezing point: Associated with metastable phases

Methane hydrate (MH) has been viewed as an earth-abundant potential energy resource and attracted worldwide research attentions. Among such studies, the metastable performance of gas hydrates systems, i.e. gas hydrates decompose at slow rates or preserve for long periods of time out of the corresponding stable pressure-temperature conditions, was claimed to lead to the retarded hydrates dissociation and affect the recovery efficiency and sediment safety during exploitations. A mechanism proposed that MH decomposes and transform to supercooled water and gas below ice-freezing point before the formation of ice, which is a dominant factor of the related metastability. In this work, the influence of NaCl concentration on depressurization-induced MH dissociation behavior in porous sediments and the associated metastable property of the MH system near ice-freezing point were experimentally investigated. Effects of the metastability on methane gas recovery were studied. The results show that possible metastable states of supercooled water-MH-gas system can exist during the dissociation process of MH with NaCl concentrations of 0 wt%, 1 wt%, and 3 wt% under production pressures of 2.0 MPa and 1.9 MPa, leading to a prolonged gas production process. While a higher NaCl concentration of 5 wt% and a lower production pressure of 1.8 MPa showed no obvious system metastability. The formation of ice would drive the system out from the metastable region and accelerate the production of gas. The effect of NaCl concentration on formation of ice at 2.0 MPa and 1.9 MPa seems to be stochastic, but exhibit negative effect on the temperature-increase amplitudes of the sediment by ice formation at 1.8 MPa. This work present fundamental lab-scale experimental results of effects of NaCl concentrations on possible metastability of the MH system and ice formations for exploitation of MH under submarine brine environment.

Pore-scale study of multicomponent multiphase heat and mass transfer mechanism during methane hydrate dissociation process

The understanding of the multiple physicochemical and thermal processes in methane hydrate dissociation is fundamental to accurately predict the methane recovery with the hydrate depressurization-induced technique. Pore-scale investigation can enlighten the accurate REV (Representative Elementary Volume) scale simulation models to take heat transfer and mass transfer limitation into account for better recovery forecast. In this work, a pore-scale numerical model based on the lattice Boltzmann method was developed to couple the multicomponent multiphase flow, heat and mass transfer, heterogeneous reaction, and solid structure evolution during the dissociation of gas hydrates. Numerical simulation of the methane hydrate dissociation in an idealized square cavity and a realistic porous structure was studied to understand the influence of mass transport, water saturation, and heat transfer. This study distinguished the hydrate dissociation characteristics in the kinetics-limited and diffusion-limited regimes. The mass-transfer-limitation in the diffusion-limited regime indicated that the enriched methane on the hydrate-water interface decreased the hydrate dissociation rate with the increasing water saturation. The methane concentration gradient across the water layer varied with the water layer thickness and contributed to the decomposition nonuniformity. The decreasing temperature gradually reduced methane recovery efficiency and yield during the endothermic decomposition due to decreasing the equilibrium pressure and intrinsic kinetic rate. Some implications were addressed from the pore-scale understanding of the transfer and kinetics characteristics. The local thermal equilibrium at the solid–fluid interface implied that the one-temperature equation is sufficient to model the heat transfer at the REV scale. The present study proposed a corrected reaction model based on the equivalent water layer thickness and introduced the implementation procedure in the REV-solution to account for the mass-transfer-limitation on the hydrate dissociation rate.

The interface evolution during methane hydrate dissociation within quartz sands and its implications to the permeability prediction based on NMR data

Nuclear magnetic resonance (NMR) is an effective method used for measuring and predicting the properties of methane hydrate (MH)-bearing sediments. In this work we present the results of MH-related interface evolution during the MH dissociation process obtained by X-ray computed tomography (CT) imaging technique. Some implications of interface evolution for the NMR-based property predictions are analysed based on the experimental measurements. The results show that the areas of MH surface and MH-brine interface decrease slowly in the early stage of dissociation, but turns to decrease rapidly in the later stage. A conceptual model for determining the NMR transverse surface relaxivity (ρ2) in the hydrate-bearing sediment is proposed by considering these changes of MH-related interface. Predictions based on this model show that the NMR transverse surface relaxivity and effective permeability are both influenced by the influencing weight and surface area proportions of the MH-brine surface and sand-brine surface.

Study on the spatial differences of methane hydrate dissociation process by depressurization using an L-shape simulator

The spatial difference of hydrate dissociation by depressurization was investigated in an L-shape hydrate simulator to enhance the exploitation efficiency of natural gas hydrates. Hydrates were found to dissociate slower near the mining well than far from the mining well due to the higher water saturation. The mass transfer rate of methane molecules in the water phase was much slower than that in the gas phase. In both vertical and horizontal directions, pore water migrated from the location far from the mining well to that near the mining well in the hydrate reservoir under the pressure difference. This led to a lower dissociation rate for the hydrate distributed at the location near the mining well, which was different from the results in gas/petroleum reservoirs and numerical simulations. The spatial difference was more pronounced in the vertical direction than in the horizontal direction. Gravity caused more water migration in the vertical direction, which led to more uneven dissociation of the hydrates. Sediments with higher permeability and lower mining pressure could result in more pronounced spatial differences. These findings can provide deep insights into the spatial evolution of multiple fields during the exploitation of natural gas hydrates.

Examination of amino acid inhibitor effect in methane hydrate dissociation via molecular dynamics simulation

Amino acids as a new kind of hydrate kinetic inhibitor can effectively prevent the hydrate formation. In this work, molecular dynamic (MD) simulation is carried out to investigate the effect of amino acids on the methane hydrate dissociation, where different kinds of amino acids including glycine, serine and valine are compared. The interaction between amino acids and methane hydrates is further evaluated. The results reveal that the adsorption energy of serine is higher compared to those of glycine and valine. The adsorption energy has a nonlinear increase with the amount of amino acids. Furthermore, the methane hydrate dissociation process with amino acids is discussed. The radial distribution functions under different inhibitors are compared. It is found that the serine has a better inhibition effect on hydrate compared to glycine as well as valine under the combined effect of electrostatic interaction caused by side chains and hydrogen bonds.

Non-Isothermal Kinetic Model of the Methane Hydrate Dissociation Process at Temperatures Below Ice Melting Point

The paper proposes a new model of gas hydrate particles dissociation at high heat fluxes. In this model, the process of hydrate decomposition occurs under the conditions of competition between heterogeneous kinetics and gas filtration. An analysis of the experimental data gives new values ​​of the kinetic coefficients for the hydrate dissociation at low temperatures. The calculation results make it possible to reproduce experimental data on the dynamics of methane hydrate powder dissociation (including dissociation under the conditions of gas burning above the surface) and to describe the phenomenon of self-conservation in terms of changes in the pore structure of the ice crust. The submodel of dissociation of a single particle is embedded in the mathematical model of transport processes in the powder layer, which allows analyzing the heterogeneity of heating and the collective effects of dissociation.

Direct measurement of pore gas pressure and water/gas phase transitions during methane hydrate dissociation

Methane hydrate deposits worldwide are vast potential sources of natural gas. Although field tests, and many laboratory studies, of hydrate dissociation have been performed, long term gas recovery from hydrate deposits still requires a comprehensive knowledge of the pore gas pressure and related phenomena. Pore gas pressure can significantly affect the safety and efficiency of gas production from hydrate deposits. Capillary-trapped residual gas saturation, known to cause elevated pore gas pressure during methane hydrate dissociation, was measured by magnetic resonance. Elevated pore gas pressure was estimated to be 8500 psi. Different molecular species and fluid environments produced during the methane hydrate dissociation process were discriminated. The results show that the majority of gas is initially confined as capillary-trapped gas upon dissociation. The evolution of water and gas saturations was measured as a function of time. Water migration, bed failure, and crack growth, related to elevated pore gas pressure, were observed both spatially and temporally resolved. Hydrate dissociation proceeded from the sand pack exterior to the interior, in a shrinking core manner, due to heat transfer effects. It was observed that hydrate dissociation resulted in pronounced water migration toward the low-pressure surface. This study was undertaken with advanced magnetic resonance imaging (MRI) and magnetic resonance (MR) methods employing a MR/MRI-compatible metallic core holder. A hydrate-bearing sand pack, with 96% initial hydrate saturation, underwent dissociation by depressurization at 290 psi and 4 °C.

Pore‐Scale Investigation of Methane Hydrate Dissociation Using the Lattice Boltzmann Method

AbstractFundamental understanding of pore‐scale methane hydrate dissociation in porous media is important to evaluate submarine slope stability and potential utilization of methane resources. In this paper, a general pore‐scale framework based on the lattice Boltzmann (LB) method is established for reactive transport coupled with nonisothermal multiple physicochemical processes in porous media. The framework combines the gas hydrate dissociation kinetic model, the single‐phase flow LB model, the mass transport LB model, and the conjugate heat transfer LB model. The pore‐scale framework is validated by several benchmark problems and then employed to investigate the endothermic dissociation process of methane hydrate with pore‐filling and grain‐coating habits in porous media. The methane hydrate endothermic dissociation behavior coupled with nonlinear nonisothermal multiple physicochemical processes involving intrinsic dissociation dynamics, gas flow, mass transport, phase change heat transfer, and conjugate heat transfer is well captured by the framework. The phase change of methane hydrate dissociation and pore structure evolution for different pore habits of hydrate are well depicted, and some insights about the dissociation front advancement and temperature distributions are also obtained. In addition, the effects of temperature field, inlet temperature, and inlet pressure on methane hydrate dissociation are investigated. The pore‐scale methane hydrate dissociation helps to advance our understanding of permeability‐saturation variation relation for continuum models.

Experimental investigation into the dissociation of methane hydrate near ice-freezing point induced by depressurization and the concomitant metastable phases

Methane hydrate (MH) has been viewed as an important potential energy resource, drawing attention of its efficient exploitation. An intriguing phenomenon that gas hydrates dissociate in a slow rate and can preserve for long periods of time out of the stable region was considered as a performance of hydrates metastability. A mechanism that supercooled water can lead to the retarded dissociation of hydrates was proposed by researchers, which will affect the gas production efficiency and sediment safety during the exploitation of MH. In this work, experiments on MH dissociation and production of methane gas from porous MH sediment near ice-freezing point induced by depressurization were performed to obtain the evidence of the metastability of the gas hydrate system. A series of experiments at different production pressures, depressurization rates and initial water saturations were carried out to study the effects of supercooled water on gas production performance. The results show that the existence of supercooled water below the freezing point of water would retard the dissociation of MH, by which the gas production rate was slowed down. The duration of this metastable state of the system was dependent on the induction time of the ice formation during the MH dissociation process. The formation of ice would promote gas production from the sediment, while the formation was observed as a stochastic process at 2.0 MPa and 2.2 MPa. The method employed should be adjustable and amendable to investigate the metastability of the gas - liquid - gas hydrate system and the related effects on gas production during depressurization-induced MH dissociation process below ice-freezing point in pilot scale.

DSC Measurement on Formation and Dissociation Process of Methane Hydrate in Shallow Sediments from South China Sea

Marine methane hydrates exploitation has become one of the hottest research topics in recent years. In this study, we investigated thermodynamic characteristics on methane hydrates in shallow sediments of South China Sea (SCS). The formation and dissociation process of methane hydrate in SCS sediments were conducted by use of a high pressure micro-differential scanning calorimeter (μDSC). The ice melting peak and hydrate dissociation peak were well distinguished by DSC and the repeated cycles proved the accuracy of DSC measurement. According to the DSC thermograms and pressure profiles, we found the hydrate formation from ice and meltwater respectively in two heating stages. The stage of transformation from ice to hydrate had obvious pressure decrease without little heat effect. Hydrates may come unstable when close to the dissociation temperature and affected the heat flow and pressure irregularly and frequently by a small degree. Comparing the heat flows of different pressures, we also noticed the decrease of ice melting point and the increased of hydrate dissociation point along with the pressure got higher. This study aimed to provide fundamental data for spot exploitation of marine methane hydrate reservoirs.

Effects of pressure and sea water flow on natural gas hydrate production characteristics in marine sediment

Natural gas hydrates (NGHs) widely exist in continental permafrost or marine sediment, and with a carbon quantity twice that of all fossil fuels combined, they are a potential energy source for the future. The efficient exploitation of NGHs has been a popular topic of research worldwide. Currently, existing NGH exploitation methods each present characteristic defect. In this study, by combining visualization studies with sea water phase migration, which is a crucial factor influencing NGH exploitation, the method of water flow erosion was utilized to enhance the driving force of methane hydrate (MH) dissociation. The influence of seawater migration on MH dissociation was systematically and visually studied by controlling different back pressures and seawater flow rates. There was no observed influence of temperature or pressure variation during the MH dissociation process. The results showed that the chemical potential difference between the hydrate phase and aqueous phase caused MH dissociation during the seawater flow process and that the rate of MH dissociation increased with decreasing backpressure and increasing water flow rate. It can be predicted that there will be no MH dissociation or time variations of absolute MH dissociation when the water flow rate is sufficiently low or high. The water migration, water phase permeability and MH dissociation strongly interacted with one another. This study combined a visualization study with theoretical analysis and first found that the gradient decrease of pressure difference lead to the increase of permeability during different stages of the seawater flow process.

The effect of electric fields in methane hydrate growth and dissociation: A molecular dynamics simulation

Molecular dynamics simulation was employed to examine the growth and dissociation process of methane hydrate in the presence of static/oscillation electric fields at T = 260 K and P = 100 bars. The intensity of electric field was in range of 1.0–2.0 v/nm, and the frequency of applied e/m field was in range of 2.45 GHz–1.0 THz. Electric field would be a factor controlling the hydrate growth and dissociation because the migration of water molecules may be affected. Total energy of simulation system, final system configurations, hydrate-like methane numbers, radial distribution functions, H-bond numbers with simulation time, and dipole moment were analyzed. There is intensity threshold for e-field to exert an effect on methane hydrate. Static electric fields above the intensity threshold (1.5 v/nm) could push hydrate dissociation by inducing the arrangement of water molecules along the field. Cosine oscillation electric fields could promote both methane hydrate growth and dissociation, and there are different critical field magnitudes for different frequency e-fields to play positive roles in methane hydrate growth or dissociation. In order to promote methane hydrate dissociation, higher the e-field magnitude will need as the e-field frequency is higher.

The second offshore production of methane hydrate in the Nankai Trough and gas production behavior from a heterogeneous methane hydrate reservoir.

Following the first attempt at producing gas from a naturally occurring methane hydrate (MH) deposit in the Daini–Atsumi Knoll in the eastern Nankai Trough area off Honshu Island, Japan in 2013, a second attempt was made in April to June of 2017 at a nearby location using two producer wells sequentially and applying the depressurization method. The operation in the first borehole (AT1-P3) continued for 12 days with a stable drawdown of around 7.5 MPa and 41 000 m3 of methane gas being produced despite intermittent sand-production events. The operation of the other borehole (AT1-P2) followed, with a total of 24 days of flow and 222 500 m3 of methane gas being produced without sand problems. However, the degree of drawdown was limited to 5 MPa because of a higher water production rate than expected in the second hole. The pressure and temperature sensors deployed in the two producers, along with the two monitoring holes drilled nearby, gathered reservoir response data and information about the long-term MH dissociation processes in the vicinity of the production holes in the temporal and spatial domains. Although the ratio of energy return to the input was considerably larger than that for the depressurization operation, some observations (e.g., the high contrast in the production rates between the two holes and the almost constant or slightly reduced gas production rates) were not predicted by the numerical models. This failure in prediction raises questions about the veracity of the reservoir characteristics modeled in the numerical simulations. This paper presents the operation summaries and data obtained with thought-experiment based-anticipated production behaviors and preliminary analysis of the obtained data as the comparison with expected behaviors. Detailed observations of gas and water production, as well as the pressure and temperature data recorded during the gas flow tests, indicate that the heterogeneous MH distribution within the reservoir was mainly responsible for the discrepancies observed between the anticipated and actual behaviors. Furthermore, the motion of the water that does not originate from MH dissociation introduces complexity, such as the occurrence of concentrated water-producing intervals and unexpected gas production responses to decreases in pressure, into the production behavior. The influence of heterogeneity should be clearly understood for the accurate prediction of gas production behavior based on MH reservoirs.

Methane hydrate formation and dissociation behaviors in montmorillonite

Abstract The methane hydrate formation and the methane hydrate dissociation behaviors in montmorillonite are experimentally studied. Through the analyses of the microstructure characteristic, the study obtains the porous characteristic of montmorillonite. It is indicated that methane hydrate in montmorillonite forms the structure I (sI) crystal. Meanwhile, molecular dynamics simulation is carried out to study the processes of the methane hydrate formation and the methane hydrate dissociation in montmorillonite. The microstructure and microscopic properties are analyzed. The methane hydrate formation and methane hydrate dissociation mechanisms in the montmorillonite nanopore and on the montmorillonite surface are expounded. Combining the experimental and simulating analyses, the results indicate the methane hydrate formation and methane hydrate dissociation processes have little influence upon the crystal structure of porous media from either micro- or macro-analysis. It is beneficial to the fundamental researches on the exploitation and security control technologies of natural gas hydrate in deep-sea sediments.

Simulation for the Effects of Well Pressure and Initial Temperature on Methane Hydrate Dissociation

Methane hydrate dissociation is a process of heat and mass transfer, and pressure and temperature are the most important parameters. The influence of pressure and temperature on the hydrate dissociate relationship between the two parameters is the determinant for gas hydrate dissociation; meanwhile, the gradients of these parameters are the sources of flow and gas production. In this study, a 1D simulator was developed for investigating the effects of well pressures (3 MPa, 5 MPa, and 8 MPa) and initial temperatures (274 K, 279 K, and 284 K) in the process of methane hydrate dissociation by depressurization. The simulation results showed that the well pressure and initial temperature have significant effects on pressure distribution, temperature distribution, and gas production. A lower well pressure and higher initial temperature can promote methane hydrate dissociation. The combined effect of hydrate dissociation and fluid flow can cause more substantial changes in pressure distribution, temperature distribution, and gas production, especially in the initial stage of the methane hydrate dissociation process. However, the changes of the parameters tend to disappear as mining time goes on. There is a difference in the influences of exploitation well pressure and initial temperature on the stability time of gas production.

Measurement of water phase permeability in the methane hydrate dissociation process using a new method

Natural gas hydrates (NGHs) are a new, clean and effective energy source with great potential for exploitation. The permeability of hydrate-bearing sediment is one of the crucial factors for evaluating the efficiency of gas hydrate production. Many researchers have investigated the variation in permeability, but there is still no effective method for identifying the variation in permeability during hydrate dissociation. In this study, a new method was proposed by using a constant step of 0.5 K in the temperature-increasing process to control hydrate dissociation, which achieved a continuous measurement of permeability. Different initial water saturations (0%, 25%, 30%, 40%, 50%, 55%, and 60%) were used to investigate water phase permeability in the porous media during hydrate dissociation. The results showed that there was little MH in the reservoir after 277.65 K under a constant volume in the simulation process of hydrate extraction. Additionally, the distribution of permeability tended to be gradually centralized by dispersal during the increase in temperature under various experimental conditions. There is a variation in the permeability curve from an exponential distribution to a logarithmic distribution with an increase in temperature. The higher the initial water saturation, the lower the initial water phase permeability. Furthermore, in the process of hydrate dissociation, the permeability decreased exponentially with the increase in saturation. But when the hydrate saturation was less than 3%, the exponential correlation is not valid.