Hydrate Dissociation Experiments Research Articles

Novel core–shell and recyclable gas hydrate promoter for efficient solidified natural gas storage

The storage and transportation of gases, including hydrogen, methane, and carbon dioxide in the form of gas hydrates, have recently garnered considerable attention. The practical implementation of gas hydrates crucially depends on the development of efficient and reusable promoters. In this study, novel core–shell magnetic nanoparticles (CSNs) developed as recyclable promoter to enhance gas uptake. Hydrate formation experiments indicated that the storage capacity can reach up to 132.7 ± 1.4 v/v and 160.6 ± 2.5 v/v in CSNs and CSNs + SDS solutions, respectively. Compared to pure water and SDS solution, the initial methane consumption rate (0.5972 mol/h) of CSNs increased by 12.2 times and 1.08 times at 3000 ppm, respectively. CSNs exhibited favorable characteristics in terms of cycling ability and preventing foam formation during repeated methane hydrate formation and dissociation experiments. Furthermore, the combination of CSNs and SDS resulted in a 72 % reduction in hydrate nucleation induction time and a 58.3 % decrease in foam formation ability compared to the sole use of SDS. In comparison to pure water, the addition of CSNs increased the onset temperature of hydrate formation by 4 °C and reduced the driving force required for methane hydrate formation. Additionally, molecular dynamics simulation studies revealed that CSNs effectively improved the amount of the ordered water structure, hydrate cage formation, and the growth state of methane hydrates. These properties make CSNs promising for potential use in the storage and transportation of gas, considering the economic and environmental advantages associated with the use of promoters.

Amino acid-assisted effect on hydrate-based CO2 storage in porous media with brine.

CO2 storage as hydrates in porous media is a promising method for storing carbon dioxide (CO2). However, the sluggish formation kinetics of hydrates urge the need to focus on the use of additives (promoters) to accelerate hydrate kinetics. This study investigates the effect of amino acid solutions in brine on CO2 hydrate formation and dissociation kinetics in quartz sand particles QS-2 (0.6-0.8 mm) with 38% porosity. The amino acids l-methionine (l-meth), l-isoleucine (l-iso), and l-threonine (l-threo) were studied at 0.2 wt% using an autoclave hydrate reactor at 4 MPa and 274.15 K in the presence and absence of salt (3.3 wt% NaCl) in 100% water saturation. The hydrate dissociation kinetics was studied at a temperature of 277.15 K. These conditions represent the normal seabed temperature range in Malaysia and hence were used for testing CO2 hydrate formation and dissociation kinetics in quartz sand in this study. Further, CO2 hydrate formation and dissociation experiments were conducted with sodium dodecyl sulphate (SDS) and brine systems as standards for comparison. The findings reveal the best kinetics for l-meth exhibiting the highest CO2 hydrate storage capacity. l-meth recorded a gas-to-hydrate conversion ratio of about 93% at 0.2 wt% in quartz sand with brine. Moreover, l-meth exhibited the lowest hydrate dissociation rate compared to l-iso and l-threo systems, thereby enhancing CO2 hydrate stability in quartz sand. Comparatively, l-meth enhanced the storage capacity by 36% and reduced the induction time by more than 50% compared to conventional promoter SDS in quartz sand with brine, suggesting it to be favorable for CO2 storage applications. CO2 hydrate nucleation time was predicted in quartz sand with and without the best-studied amino acid l-meth system with high prediction accuracy and an absolute average deviation of 2.4 hours. The findings of this study substantiate the influence of amino acids in promoting the storage capacity of CO2 in sediments as hydrates.

Kinetic studies of the secondary hydrate formation in porous media based on experiments in a cubic hydrate simulator and a new kinetic model

The challenge of secondary hydrate formation (SHF) is frequently encountered in hydrate formation and dissociation experiments. However, there is currently a lack of modeling theory of SHF kinetics in the field and laboratory tests. In this work, the hydrate formation experiments concerning the SHF are carried out in a cubic hydrate simulator (CHS), where the pressure profiles and temperature spatial distributions are measured. A new kinetic model of hydrate formation integrating hydrate pore-scale morphology and SHF characteristics is developed. The newly developed model with unified kinetic parameters is employed in the Tough + hydrate (T + H) simulator to duplicate the experiment processes numerically, which achieves excellent agreement with the experimental data. The results show that the evolutions of the spatial distributions of the temperature and hydrate saturation behave in heterogeneous manners in the hydrate formation processes. This also reveals that the SHF can significantly exacerbate the hydrate heterogeneity in the CHS. Two additional experiments and comparisons with currently available models have validated the feasibility and accuracy of the new model. This work provides a reliable and adaptable model for describing the entire lifecycle of the hydrate formation kinetics in the porous media.

Dual-gas co-production behavior for hydrate-bearing coarse sediment with underlying gas via depressurization under constrained conditions

Hydrate reservoirs with underlying gas show significant potential for commercial exploitation. An experimental apparatus capable of applying confining pressure and axial pressure was built to simulate the accumulation and dissociation of naturally occurring marine hydrate and a novel method for synthesizing hydrate reservoirs with underlying gas was proposed. With the formed methane hydrate-bearing coarse sediment (HBCS) with underlying gas, hydrate dissociation experiments via depressurization under constrained conditions were carried out at different HBCS proportions. The characteristics of heat transfer and dual-gas co-production during the exploitation of hydrate reservoir at various HBCS proportions were presented. Due to the influence of hydrate decomposition, the diverging temperature variations of the underlying layer and upper HBCS layer were observed at HBCS proportions of 0.21, 0.48, and 0.77. The negative effect of the outside pressure on gas production was not observed in our experimental conditions owing to the adopted coarse quartz sand deposits. The final produced gas volume increases with the HBCS proportion increasing. The produced gas percentage is negative or positive related to the HBCS proportions respectively with the extended production time. In addition, the different production behavior of the underlying gas and hydrate decomposed gas may induce the unexpected exploitation accidents such as blowouts resulting from unstable gas production rate. These findings guide the design of exploitation scheme for reservoirs with different HBCS distribution. And an exploitation method by setting the production well in the underlying gas layer/underlying sediment layer with free gas is then proposed, which can inhibit hydrate reformation and ice generation and improve the production well stability during the hydrate exploitation process.

Analysis of Methane Hydrate Dissociation Experiment in a Pilot-Scale Hydrate Simulator by a Full Implicit Simulator of Hydrate

Behaviors of gas and water production from a gas hydrate reservoir are based on fully coupled Thermal–Hydraulic–Chemical (THC) processes in porous media. Studies using experiments and mathematical models could be improved in hydrate-bearing sediments in the laboratory. The experiment of depressurization-induced methane hydrate dissociation via a single vertical well was carried out in quartz sands in a pilot-scale hydrate simulator (PHS). The multi-stage hydrate dissociation process, including the free liquid release, well shutdown, depressurization, and constant pressure, was achieved by complex manual operation of the production well in the experiment. A full implicit simulator of hydrate (FISH), a four-component, three-phase mathematical model, was developed and employed to reproduce the hydrate dissociation experiment. The kinetics of the hydrate reaction, heat/mass transfer in porous media, and mass conservation in the pressure vessel were validated by the experimental results. The key finding of this study is the acquisition of a group of unified parameters in a suitable mathematical model that could clearly elucidate the hydrate dissociation process in all stages. The mean absolute percentage error (MAPE) values of the gas and aqueous production are, respectively, as low as 2.338 and 9.630%. The strong confidence of the accuracy of the numerical simulator has been built up after the analysis of the experiment and the numerical simulation in this study, which could be used to evaluate gas recovery from marine hydrate reservoirs.

Insights on CO2 Hydrate Formation and Dissociation Kinetics of Amino Acids in a Brine Solution

CO2 sequestration as hydrates has captured enormous research interest in recent years due to its high storage capacity. However, the slow hydrate formation kinetics poses a strong challenge in its applicability and urges the need for additives (kinetic promoters) to accelerate the hydrate formation kinetics. This study investigates the performance of amino acid solutions in brine (3.3 wt % NaCl) on CO2 hydrate formation and dissociation kinetics. The kinetics of hydrate formation and dissociation was evaluated at varying concentrations (0.2, 0.5, 0.8, and 1 wt %) of l-methionine, l-isoleucine, and l-threonine solutions in brine using a high-pressure hydrate reactor at 4 MPa, and formation temperature of 274.15 K, and a dissociation temperature of 277.15 K. CO2 hydrate formation and dissociation experiments were also conducted using sodium dodecyl sulfate (SDS), pure water, and brine systems as standards for comparison. The findings show that all the studied amino acid systems show hydrate inhibition as compared to the pure water system with 35–41% reduction in CO2 storage capacity. Further, l-methionine exhibited an optimum performance with a slight promotional effect at 0.2 wt % with the lowest induction time (35.6 min) and the highest CO2 uptake (42.08 mol/mmol) among the studied amino acid systems. However, comparing l-methionine with SDS shows less induction time for SDS (28.48 min), indicating kinetic promotion of SDS over l-methionine in the presence of brine. The dissociation kinetics findings reveal the lowest dissociation rate for l-methionine and a prolonged time of CO2 release compared to l-threonine and l-isoleucine, respectively. Nevertheless, all the considered amino acid systems exhibit more inhibition or stability for hydrate dissociation as compared to the pure water system. However, compared to SDS and brine systems, the amino acids l-methionine and l-isoleucine show a slight promotion effect, unlike l-threonine which exhibits strong kinetic inhibition. The results thus suggest that amino acid solutions in brine stabilize hydrate dissociation; however, the CO2 hydrate formation inhibition in brine is quite discouraging for CO2 storage purposes. Instead, applying amino acids in flow assurance could be an additional advantage due to the presence of brine in practical reservoir fluids during production.

The double-edged characteristics of the soaking time during hydrate dissociation by periodic depressurization combined with hot water injection

The combination of depressurization and thermal stimulation is considered as a promising production method of gas hydrates. In this study, the experiments of methane hydrate dissociation by periodic depressurization combined with hot water injection (D&H) were conducted. Each cycle of D&H was consisted of the hot water injection stage, the soaking stage, and the depressurization stage (below the equilibrium pressure). The influences of the soaking time (4 min, 8 min, 12 min) on the hydrate dissociation behavior, the ratio of gas to water, and the energy efficiency were analyzed. The experimental results showed that, in the early period of depressurization combined with hot water injection stage (DHS), most of the consumed heat (70–90 %) during hydrate dissociation was provided by the heat transferred from surroundings. The proportion of the heat of the injected hot water to the consumed heat in the early period of DHS was only 10–30 %, and it quickly increased to 50–60 % in the later period of DHS. The calculation showed that the reduction of the soaking time improved the heat transfer rate from surroundings to the hydrate reservoirs at the soaking stage, thereby increasing the hydrate dissociation rate. However, the reduction of the soaking time decreased the thermal efficiency and the energy efficiency during hydrate production. For the soaking time of 4 min, 8 min, and 12 min, the range of the thermal efficiency were respectively 0.34–0.82, 0.50–1.06, and 0.54–1.20, and the range of the energy efficiency were respectively 6.24–14.89, 8.91–18.82, and 9.62–21.13. Meanwhile, the reduction of the soaking time also decreased the ratio of gas production to water production, which was adverse for hydrate production. This study makes up for the deficiency that the current understanding of soaking time in periodic depressurization combined with heat injection is insufficient. The research shows that the soaking time has a double-edged characteristic, which are helpful for choosing a suitable soaking time during gas hydrate production by periodic depressurization combined with hot water injection in actual field.

A novel core–shell structure of CuNW@PSS accelerates the process of methane hydrate formation

Considering the high thermal conductivity of copper material and the promoting effect of sulfonate ions on methane hydrate formation, in this study, copper nanowires coated with sulfonate polymer (CuNW@PSS) were used in methane hydrate experiments. The morphology of synthesized CuNW@PSS was confirmed by SEM and TEM, and the composition was analyzed by elemental analysis, it has clearly core–shell structure. The surface wettability of the material changed from hydrophilic to hydrophobic after sulfonate modification. In methane hydrate formation experiments, compared with PSS, CuNW@PSS can shorten the induction time by 78.5 %, from 61.87 ± 21.71 min to 13.28 ± 5.47 min, the growth rate increased by 130 %, from 0.6 ± 0.13 mmol/min to 1.38 ± 0.34 mmol/min. In addition, CuNW@PSS has good cycling and defoaming performance in the multi-cycle methane hydrate formation and dissociation experiments, which would have potential application for natural gas storage and transportation.

Modeling Two-Phase Flow Caused by Hydrate Dissociation in a Reservoir on the Concept of Global Pressure

Summary The classic Darcy’s two-phase flow equation has a variety of mathematically equivalent formats, such as pressure/saturation (PS), two-phase pressure (PP), and global pressure/saturation (GP). Based on the concept of global pressure, we derive a new formulation for two-phase flow caused by hydrate dissociation in porous media, coupled with the mass and energy conservation equations, thereby offering a novel theoretical frame for gas hydrate extraction simulation. The new model provides in-depth insights into complex flow fields: The dissociation of hydrates under thermal stimulation may lead to two flow fields of gas and water with different directions in the reservoir. The simulation results are in good agreement with the recorded data set of Masuda’s and Chong’s experiments, which verifies the correctness and applicability of the new model. Based on numerical simulations of the same hydrate dissociation experiments, the efficiency of the GP method was compared with the PP method published in our previous work. The results show that the GP method has more than two times the timestep size of the PP method for unsteady flow and 1.5 times for steady flow. Furthermore, the relative residual of the GP method is even two orders of magnitude lower than that of the PP method for two-phase flows with complex variations of pressure gradient and fluid flux. Therefore, the GP method is significantly more efficient than the PP method for simulating gas hydrate development. The proposed GP method improves the computational efficiency of hydrate extraction simulation at the laboratory scale and understanding the mechanisms of two-phase flow in reservoirs caused by hydrate dissociation. It may have potential advantages for field-scale simulation of hydrate development, which requires further studies to demonstrate.

Experimental study of gas-liquid-sand production behaviors during gas hydrates dissociation with sand control screen

Sand production is a great challenge for gas hydrates production, and the use of sand control screen can relieve that. However, the use of the sand control screen could also change the gas/water production behavior, thereby affecting the gas production efficiency. In this study, the experiments of methane hydrate dissociation by depressurization with different mesh sizes of sand control screen (100 mesh, 200 mesh, 300 mesh, and 400 mesh) were conducted in a novel hydrate simulator. The gas-liquid-sand production behaviors during hydrate dissociation with different mesh sizes of sand control screen were analyzed. The experimental results showed that the absence of the sand control screen resulted in an obvious subsidence of the hydrate-bearing sediments due to large amount of sand production. The use of the sand control screen could prevent the sand migration from the hydrate reservoir to the production well, thereby reducing the risk of pipeline blockage and the subsidence of the sediments. The increase of the mesh size of the sand control screen reduced the total amount and the medium particle size of the sand production. However, the existence of the sand control screen also improved the resistance of fluid flowing from the hydrate reservoir to the production well, thereby decreasing the gas production rate. Compared to the average gas production rate without sand control screen, the average gas production rate at depressurization stage (DS) with the sand control screen was reduced by 76–96%. The larger the mesh size of the sand control screen, the greater the flowing resistance. Therefore, it's necessary to combine the sand control effect and gas production rate to select an appropriate mesh size of sand control screen. In this study, the particle size of the quartz sand used to simulate the hydrate-bearing sediments was 200–400 mesh, and the most appropriate mesh size of sand control screen was around 300 mesh. The results of this study are of great significance for developing the sand controlling strategy and the efficient production of gas hydrates.

Experimental study on the influence of brine concentration on the dissociation characteristics of methane hydrate

Brine stimulation is a common method for assisting depressurization to produce hydrate, which significantly improves the efficiency of hydrate production. In this work, to study the influence of brine concentration on the natural gas hydrate production process, a novel cylindrical hydrate production simulation experimental device was established, and investigations of hydrate dissociation experiments induced by depressurization in conjunction with brine injection were carried out. The effects of different brine concentrations on the gas production rate and heat transfer during production were studied. The distribution, flow, and concentration evolution of brine during hydrate production were analyzed. Brine injection can significantly enhance cumulative gas production during the depressurization stage, increase the sensible heat for hydrate dissociation, enhance heat transfer between the hydrate reservoir and the environment during the recovery process, and significantly improve the gas production rate of hydrate. The increase in the gas production rate caused by brine injection mainly occurs at the end of the depressurization stage and the beginning of the constant pressure stage. The injected brine is mainly located in the middle and upper areas of the reactor. In the bottom area of the reactor, the hydrate has higher saturation and longer dissociation time, which is slightly affected by the injected brine. A higher hydrate saturation is not beneficial to brine injection and does not promote hydrate dissociation. Additionally, in the experiments with brine injection, the resistance decreases more obviously during the depressurization stage due to more hydrate dissociation. The resistance also shows that in the bottom area of the reactor, there is a higher hydrate saturation and a lower brine effect on hydrate.

Modeling and characterizing the thermal and kinetic behavior of methane hydrate dissociation in sandy porous media

Methane hydrates (MHs) are deemed to be a potential energy source to meet the overwhelmingly growing demand for natural gas. The production of MHs involves many controlling processes such as heat transfer, fluid dynamics and hydrate reaction kinetics. Local thermal equilibrium (LTE) between solid and fluid phases was typically assumed when modeling the hydrate production, however, in many cases the local thermal non-equilibrium (LTNE) caused by the complex heat transfer processes at the pore scale cannot be neglected. In this work, a LTNE model was adopted to reflect the multiple heat transfer forms associated with hydrate dissociation and gas-liquid flow, particularly the solid-fluid interfacial convective heat transfer and the Joule-Thomson effect. The LTNE phenomenon and Joule-Thomson effect were hinted by a hydrate dissociation experiment carried out for model verification. The results show that the average estimation errors of temperature, cumulative gas and water production profiles between experiment and LTNE model were 7.70%, 6.63% and 4.02%, respectively, which are considerably better than those of the traditional LTE model (16.44%, 11.74% and 10.89%, respectively). A tenfold increase of interfacial convective heat transfer coefficient could reduce the local solid-fluid temperature difference by about 78.5%. A special focus was given to the Joule-Thomson effect which seemed to have less impact at the experimental condition in this work but become more significant when the gas saturation exceeds 40%. This work could add further insights into the thermal performance during hydrate dissociation in porous media, help to reveal the thermo-kinetic relationship at the pore scale and provide a more accurate estimation of the hydrate dissociation dynamics. It could be potentially employed to optimize the methane hydrate production strategies, minimize the risk of hydrate re-formation and maximize the overall energy recovery efficiency.

Hydrate formation and dissociation characteristics in clayey silt sediment

The geological occurrence of natural gas hydrates in clayey silt sediments has been extensively confirmed. Compared with sandy sediments, clayey silt is characterized by a large specific surface area, high capillary pressure, and high bound water content, which leads to distinct characteristics of hydrate formation and dissociation. The formation and dissociation of methane hydrates in clayey silt sediments with illite or montmorillonite were experimentally investigated. It was determined that an increase in the water content of silt and illite-silt sediments from ~9.47% to ~28.41% caused a decrease in the water-to-hydrate conversion ratio owing to the formation of a hydrate film between water and CH4 gas. However, in montmorillonite-silt sediments, the water-to-hydrate conversion rate increased between 9.47% and 18.94% of water saturation and decreased between 18.94% and 28.41% of water saturation under the combined effects of weakly bound water content and capillary pressure. In the hydrate dissociation experiment, the temperature of sediments with ~9.47% water content decreased more, which was caused by the smaller specific heat capacity of sediments with lower water content. Meanwhile, the temperature drops below the freezing point during the hydrate dissociation process, which is caused by the strong inhibition of hydrate phase equilibrium by montmorillonite. This study demonstrates the effects of clay type and water content on hydrate formation and dissociation, which is helpful for understanding the production behavior of clayey silt hydrate reservoirs.

Dissociation experiment and dissociation rate model of CO2 hydrate

Applying CO2 hydrate (referred to as hydrate) to CO2 storage and sequestration can alleviate global greenhouse effect, which is currently a hot spot of hydrate research. In order to explore the dissociation mechanism and rate, this paper carries out hydrate dissociation experiments under different working conditions of depressurization stage, temperature and initial hydrate volume fraction by using the reactor. Then, based on the experimental results, the influences of pressure, temperature, hydrate volume fraction, mass exchange and energy exchange on the dissociation rate are analyzed. Finally, a hydrate dissociation rate model considering the influences of intrinsic dissociation dynamics, mass transfer and other comprehensive mechanisms is established. And the following research results are obtained. First, two dissociation phenomena appear in the process of hydrate dissociation, including adsorption of gas bubble and shedding of flaky hydrate from the edge. Second, hydrate dissociation is a process of intrinsic dissociation dynamic driving, mass and heat transfer. According to the dissociation rate variation, it can be divided into two stages, i.e., stable dissociation stage and dissociation weakening stage. In the stable dissociation stage, the dissociation rate is maintained constant. And in the dissociation weakening stage, the dissociation rate decreases gradually with the increase of dissociation resistance (mass exchange). Third, both pressure and temperature drive the dissociation by influencing the difference between the actual hydrate surface fugacity and the phase equilibrium fugacity. The increase of depressurization range can lead to the increase of dissociation rate while the duration of stable dissociation is extending. The increase of temperature can also lead to the increase of the dissociation rate, but its influence is less than that of depressurization. In conclusion, this newly established dissociation rate model can well predict the value and variation trend of hydrate dissociation rate. And under the working condition of 2.0 MPa, the maximum relative error of the experimental results is 5.1%.

Pore-scale investigation of hydrate morphology evolution and seepage characteristics in hydrate bearing microfluidic chip

The efficiency and safety of natural gas hydrate exploitation is significantly affected by the occurrence and distribution of hydrate and the seepage behaviors of gas and water in hydrate bearing sediments. We present the results from xenon hydrate formation and dissociation experiments with different salinities using quartz etching microfluidic chips. Direct visualization of hydrate growth morphology is obtained with the help of industrial grade charge coupled device (CCD) camera coupled with a microscope module. The results show that the hydrate growth process in microfluidic chip can be divided into four stages, which are the hydrate inducing stage, the primary hydrate nucleation stage, the rapid hydrate nucleation stage and the hydrate nucleation maturation stage, respectively. Different hydrate morphologies (dendritic, intestine-like) and aggregation patterns (grain-coating, grain-cementing) are observed during the different stage of hydrate formation. The average value of all 219 contact angles of water phase on gas-hydrate interface is 44.03°, which indicates the hydrate surface is hydrophilic. And the hydrophilicity of the hydrate surface is impaired by the increase of brine salinity, owing to the loss of attraction between the molecules caused by the salt-removing effect during the hydrate formation process. The effective water permeability in hydrate bearing microfluidic chip decreases with the decrease of the pore size and is further affected by the heterogeneity. The permeability reduction caused by the hydrate formation is more pronounced in the microfluidic chips with larger initial pores. The hydrate formation kinetics is significantly inhibited by the coupling effect of the tortuosity, the pore size and the salinity and resulted in drastic reduction in hydrate saturation.

Methane Hydrate Formation and Dissociation in Sand Media: Effect of Water Saturation, Gas Flowrate and Particle Size

Assessing the influence of key parameters governing the formation of hydrates and determining the capacity of the latter to store gaseous molecules is needed to improve our understanding of the role of natural gas hydrates in the oceanic methane cycle. Such knowledge will also support the development of new industrial processes and technologies such as those related to thermal energy storage. In this study, high-pressure laboratory methane hydrate formation and dissociation experiments were carried out in a sandy matrix at a temperature around 276.65 K. Methane was continuously injected at constant flowrate to allow hydrate formation over the course of the injection step. The influence of water saturation, methane injection flowrate and particle size on hydrate formation kinetics and methane storage capacity were investigated. Six water saturations (10.8%, 21.6%, 33%, 43.9%, 55% and 66.3%), three gas flowrates (29, 58 and 78 mLn·min−1) and three classes of particle size (80–140, 315–450 and 80–450 µm) were tested, and the resulting data were tabulated. Overall, the measured induction time obtained at 53–57% water saturation has an average value of 58 ± 14 min minutes with clear discrepancies that express the stochastic nature of hydrate nucleation, and/or results from the heterogeneity in the porosity and permeability fields of the sandy core due to heterogeneous particles. Besides, the results emphasize a clear link between the gas injection flowrate and the induction time whatever the particle size and water saturation. An increase in the gas flowrate from 29 to 78 mLn·min−1 is accompanied by a decrease in the induction time up to ~100 min (i.e., ~77% decrease). However, such clear behaviour is less conspicuous when varying either the particle size or the water saturation. Likewise, the volume of hydrate-bound methane increases with increasing water saturation. This study showed that water is not totally converted into hydrates and most of the calculated conversion ratios are around 74–84%, with the lowest value of 49.5% conversion at 54% of water saturation and the highest values of 97.8% for the lowest water saturation (10.8%). Comparison with similar experiments in the literature is also carried out herein.

The determining factor of hydrate dissociation rate in the sediments with different water saturations

In nature, gas hydrate is mainly distributed in permafrost region and deep ocean sediments. For these two types of hydrate reservoirs, water contents in the sediments were different. The water content has a large influence on the characteristics of heat transfer and fluid flow in the sediments, thereby affecting the hydrate dissociation behavior. In this paper, the experiments of hydrate dissociation by depressurization in sediments with different water saturations were conducted. The influences of water saturation on the pressure, the temperature, the gas production, the water production, and the hydrate dissociation rate were analyzed. The experimental results showed that, in the depressurization stage (DS), the rate of pressure dropping and the amount of hydrate reformation increased with the raise of the water saturation in the sediments. In the constant pressure stage (CPS), although the heat conduction rate of the sediments increased with the increase of the water saturation, the mass transport rate of the gas released from hydrate dissociation was limited due to the increase of water saturation. As a result, the hydrate dissociation rate decreased with the increase of the water saturation in the sediments. According to these experimental results, during the hydrate dissociation in the sediments with different water saturations, the hydrate dissociation rate is determined by the mass transport rate of the gas released from the hydrate dissociation. For the hydrate reservoirs with the high-water saturation, the hydrate dissociation rate is largely limited by the diffusion rate of gas released from hydrate dissociation. Hence, the key factor to improve the hydrate dissociation rate in the sediments with high water saturation is to increase the diffusion rate of gas in the sediments.

Experimental study and modeling of the kinetics of carbon-dioxide hydrate formation and dissociation: A mass transfer limited kinetic approach

Kinetics of hydrate formation and dissociation of carbon dioxide studied through experiments carried out at isothermal-isobaric condition in a constant stirred tank reactor (CSTR). Experiments performed at different temperatures and pressure levels. Constant rates of CO2 mole consumption and mole production observed during hydrate formation and dissociation experiments. Accordingly, mass transfer limited hydrate kinetic models were used and mass transfer coefficients for CO2 hydrate formation and dissociation evaluated. It was found that mass transfer coefficients for CO2 hydrate dissociation are one order of magnitude larger than mass transfer coefficients for hydrate formation. The fact indicates that CO2 hydrate dissociation takes faster than formation. The evaluated mass transfer coefficient for CO2 hydrate formation and dissociation correlated as functions of temperature and pressure. The results can be applied for numerical modeling of underground storage of CO2 and simulation of hydrate based CO2 capture processes.

Experimental study of methane hydrate dissociation in porous media with different thermal conductivities

Methane hydrate dissociation is an endothermic reaction. Therefore, one of important factors for methane hydrate dissociation is the heat transfer rate. In order to study the heat transfer characteristics of porous media on methane hydrate dissociation, experiments of methane hydrate dissociation using depressurization are conducted in porous media with different thermal conductivities, including quartz sand (0.926 W/(m·K)), white corundum (28.82 W/(m·K)) and silicon carbide (41.9 W/(m·K)). Experimental results show that, during the depressurization stage (DS), the temperature difference among different positions in quartz sand is larger than that in white corundum and silicon carbide. Since the heat transfer rate of quartz sand is smallest among three kinds of sand. A low temperature zone at the center of the reactor is observed in quartz sand compared to white corundum and silicon carbide. During the constant pressure stage (CPS), as the thermal conductivity of porous media increases, the temperature rising rate increases, and the duration of the CPS decreases. The dissociation rate of methane hydrate is controlled by the heat transfer rate of sediments during the CPS. The minimum gas production rate is obtained from the experimental in quartz sand, and the maximum gas production rate is obtained from the experiment in silicon carbide. This result indicates that the dissociation rate of hydrate increased with the increase of the thermal conductivity of porous media. Meanwhile, the overall rate constants (koverall) for different runs are calculated to quantify the dissociate rate of methane hydrate during the CPS. As the thermal conductivities of porous media increase, the overall rate constant of methane hydrate dissociation increases. The results of this study are important for understanding the effects of thermal conductivity of porous media on hydrate dissociation in actual field. Furthermore, it can also be used for the validation of numerical simulation in future.

Effect of temperature on safety and stability of gas hydrate during coal mine gas storage and transportation

Knowledge of the dissociation kinetics of gas hydrate is essential for evaluate the risk of coal mine gas (CMG) during CMG hydrate storage and transportation. In this study, the hydrate dissociation experiment is conducted by using visual hydrate formation and dissociation experimental device for two kinds of CMG samples under four different temperatures. The thermal stimulation method (above the equilibrium temperature) is applied for hydrate dissociation. Based on the experimental data, it is found that the pressure curve during gas hydrate dissociation is mainly composed of three stages, including rapid dissociation, slow dissociation, and stable stages. Temperature has a significant effect on the average dissociation rate for the gas system, where a higher temperature is related to greater average dissociation rate. The maximum average dissociation rate is determined to be 4.34 ∙ 10−3 mol/min (System II-4). The mole number of methane increases obviously with the increase of temperature, while nitrogen and oxygen change slightly. It demonstrates that methane hydrate has higher risk in storage and transportation.