CH4 Hydrate Formation Research Articles

Rheological behavior of deep eutectic solvent promoted methane hydrate formation

Natural gas hydrates are recognized as a crucial and sustainable energy resource. The formation, dissociation, and flow properties of CH4 hydrate from deep eutectic solvents (DESs) are investigated using a high-pressure rheometer. In this work, rheological experiments are performed using two DESs namely, tetrabutylammonium bromide + ethylene Glycol (TBAB + EG, DES1) and methyltriphenylphosphonium bromide + ethylene Glycol (MTPB + EG, DES2) at 1 wt% concentration, 274 K temperature and 8 MPa pressure. The hydrate formation starts around 1.7 h for DES1 and around 4 h for DES2. Pressure and viscosity of the slurry are analyzed while methane hydrate forms and dissociates. The pressure drop experienced during hydrate formation is roughly 1.4 MPa for DES1 and 1.7 MPa for DES2. Further, viscosity profiles are analyzed for both DESs with varying shear rate. The viscosity gradually decreases as the shear rate increases. At equilibrium conditions of pure CH4, a spike is observed during dissociation. The rheological data is further analyzed with Cross model to validate the shear-thinning behavior of methane hydrate.

Gas hydrate stability for CO2-methane gas mixes in the Pegasus Basin, New Zealand: A geological control for potential gas production using methane-CO2 exchange in hydrates

Gas hydrate is an ice-like form of water containing gas, in nature mostly methane (CH4), which requires moderate pressures and low temperatures. The replacement of CH4 by CO2, which also forms a hydrate, could allow CH4 production from hydrate while sequestering CO2. A number of recent studies have focused on theoretical background, experimental simulations, and engineering approaches related to CH4-CO2 exchange in hydrates. We here investigate a key geologic constraint for possible CH4-CO2 exchange in sub-seafloor reservoirs, hydrate stability. We analyze seismic data and gas hydrate system models from the Pegasus Basin east of New Zealand, a region with evidence for abundant gas hydrates. Pressure-temperature conditions beneath the seafloor need to be within the stability fields for both CH4 hydrate and hydrate from the resulting gas mix after CO2 injection. Based on experimental and theoretical studies, we consider 64% a benchmark for maximum achievable CH4 replacement by CO2, resulting in a mix of 64% CO2 – 36% CH4, in hydrate. Down to a water depth of 1087 m, hydrate from this gas mix is stable within the entire CH4 hydrate stability field. A gap develops in deeper water with the base of gas hydrate stability (BGHS) for CH4 being deeper than for the 64% CO2 – 36% CH4 mix. In nature, most mechanisms for CH4 hydrate formation favor high saturation near the BGHS. For an evaluation of possible CH4-CO2 exchange, it is therefore important to investigate mixed-gas hydrate stability near the CH4-BGHS and to identify CH4 hydrates closer to the seafloor.

Methane hydrate formation in amino acids / sodium montmorillonite systems

To understand the occurrence of natural gas hydrates in seabed sediments, it is crucial to examine the mechanisms of methane (CH4) hydrate formation in sodium montmorillonite (Na-Mt) systems in the presence of amino acid. Accordingly, this study employed kinetics experiments and molecular dynamics simulations to investigate CH4 hydrate nucleation and growth in an Na-Mt system containing alanine (Ala), leucine (Leu), and phenylalanine (Phe), respectively. Kinetics and Raman experiments showed that, compared with Ala, Leu and Phe enhanced hydrogen bonding between water molecules surrounding Na-Mt. This enhancement was due to the long carbon chain of Leu and the phenyl ring of Phe and facilitated CH4 hydrate formation. Moreover, in the Na-Mt system, Ala reduced CH4 consumption, whereas Leu and Phe increased CH4 consumption. Molecular dynamics simulations revealed that the strength of electrostatic interactions between the negatively charged Na-Mt surface and the functional groups of amino acids affected the distribution of amino acids, thereby altering CH4 aggregation and CH4 hydrate nucleation processes. The strong interaction between Na-Mt and Ala significantly disrupted interfacial interactions between Na-Mt and water molecules. In contrast, the weaker interactions between Na-Mt and Leu and Phe, respectively, meant that these amino acids affected CH4 hydrate nucleation in the bulk-like solution by influencing the arrangement of water molecules. These findings indicate that interfacial interactions between Na-Mt and amino acids play a crucial role in CH4 hydrate formation. Overall, this study generated insights into the formation kinetics and nucleation properties of CH4 hydrates in clay mineral–amino acid complexes that may increase understanding about the occurrence of natural gas hydrates in marine sediments.

Experimental and modeling investigations on CH4 hydrate phase equilibria in multi-ion “Haima” cold seep environment

CH4 hydrate formation represents a primary pathway for CH4 transformation within the deep-sea CH4 seepage environment, holding utmost significance for global carbon budget and marine hydrate resources distribution. As a typical active CH4 seeping, however, the influence characteristics of various ions in in-situ “Haima” cold seep on CH4 hydrate stability remains unclear. In this study, founded on the multiple ion components from the seawater of the “Haima” cold seep, the effects of salt ion categories and concentrations on CH4 hydrate phase equilibrium were investigated, and a thermodynamic model suitable for high-salinity deep-sea environment was proposed. Results indicated that, in higher concentration salt solutions (>3.45 wt%), hydrate stability was no longer solely governed by salinity, and significant differences among ion types were observed. The inhibitory strength of the ions on CH4 hydrate stability followed the order of Mg2+>Na+>Ca2+>Mn2+≈K+>Sr2+>Ba2+. The predicted CH4 hydrate phase equilibrium conditions from the proposed model exhibited strong agreement with both experimental results and data reported in the literature. The average absolute deviation of pressure (AADP) from the model prediction was 2.7% and 4.3% compared to the experimental and published literature data, respectively, which outperformed those of the CSMHYD model, Chen-Guo model and Hu-Lee-Sum correlation (4.7%–6.2%). In light of the marine temperature gradient and prediction results, it is estimated that CH4 hydrate could maintain stably below 600 m depth of the seawater in the cold seep area.

Unveiling the effect of H2S concentration in CH4 hydrate formation and growth: A molecular dynamics study

We used molecular dynamics method to simulate the formation and growth of CH4 hydrates with different concentrations of H2S. At the large scale of simulation time, 10000 ns, it is unlikely to form hydrates at 10 MPa and 260 K if there are only CH4 and water; and if there was a small proportion of H2S in the gas, hydrates would easily form. When the molar concentration of H2S in the gas is 1 %, the formed hydrate is unstable and will eventually disintegrate, leaving the box in a two-phase state of gaseous and liquid. In contrast, at molar concentrations of 5 % or higher of H2S, hydrate formation is accelerated, and the hydrate will grow continuously. However, it is not structure I type as expected. These views are supported by the changes in F3 and F4 order parameters, numbers of different cage types, and the number of water molecules belonging to different states. Moreover, since the formation of hydrates is an energy reduction process, we can easily develop inhibitors targeted at hydrates in actual production and gas transportation. According to IGMH analysis, only van der Waals interactions occur between gas molecules inside the cages and the water cage.

Effects of leucine on hydrate formation: A combined experimental and molecular dynamics study

Compared to conventional natural gas storage and transportation methods, hydrates offer a safer, more compact alternative with broad future potential. To enhance hydrate formation, there has been considerable interest in environmentally friendly and cost-effective amino acid promoters, such as leucine. This study utilized a transparent vessel for CH4 hydrate formation experiments, identifying the optimal 0.5–0.6 wt% leucine concentration, which led to enhanced gas consumption, shorter induction time, and maximum gas storage density. In the leucine system, hydrates exhibited distinctive branching growth on container walls, both upward and downward. The formation of numerous microchannels amplified capillary effects, enhancing gas–liquid contact and fostering hydrate formation. Simultaneously, isobaric-isothermal molecular dynamics (MD) simulations integrated system growth configurations, structural parameters, radial distribution functions, energy barriers and mean square displacements, unveiling leucine’s microscopic impact on hydrate formation. Simulation results unequivocally show that leucine significantly accelerates CH4 hydrate growth by momentarily adsorbing at the solid–liquid interface, expediting water molecule ordering into cage-like structures. Leucine also enhances water molecule structural order around the mobile leucine molecule. Moreover, leucine lowers the energy barrier for methane transitioning from gas to liquid, facilitating gas molecule diffusion into the liquid phase and affording plenty interaction time for water and methane molecules, thereby providing favorable conditions for hydrate formation. These findings provide crucial perspectives for future research in the field of eco-friendly hydrate promoters, paving the way for innovative solutions in natural gas storage and transportation.

Adsorption enhances CH4 transport-driven hydrate formation in size-varied ZIF-8: Micro-mechanism for CH4 storage by adsorption-hydration hybrid technology

The adsorption-hydration hybrid technology holds significant potential for future CH4 storage applications. However, the transport of CH4/H2O and the mechanisms governing hydrate formation within metal-organic framework (MOF) systems remain unclear. In this study, the mechanism of CH4 adsorption, transport, and hydrate formation within ZIF-8 pores of different sizes was delineated utilizing the magnetic resonance imaging (MRI) technique. Primarily, CH4 undergoes physisorption within the inner cavity of ZIF-8, facilitating diffusion through the pores, which preferentially accumulate in larger pore spaces. Due to favorable interactions between CH4 and pre-absorbed water on the ZIF-8 surface, ZIF-8–01 and ZIF-8–02 preferentially facilitate substantial hydrate formation within larger pore spaces. The continuous hydrate growth depletes CH4 in the pore space, enabling two-way migration of adsorbed CH4 to support ongoing hydrate formation. This limited availability of adsorption sites within ZIF-8 induces the diffusion of surrounding CH4 into smaller pore spaces, facilitating the conversion of bound water. The size of the particles considerably influences the transport of physically adsorbed CH4 and impacts hydrate formation. Similar patterns are observed in the adsorption mass transfer coefficients of ZIF-8–01 and ZIF-8–02, which range from 0.0038 to 0.0044 and 0.0031 to 0.0124, respectively. The presence of more adsorption vacancies in the early stage leads to rapid diffusion adsorption of CH4 owing to strong adsorption effects. As adsorption within ZIF-8 pore cavities approaches saturation, increased water transport elevates internal gas-liquid two-phase resistance, slowing the kinetics of late-stage CH4 adsorption. At 6 MPa, CH4 with a larger particle size preferentially overcomes the gas-liquid energy barrier, expediting transport and promoting hydrate formation. Furthermore, the optimal particle sizes of ZIF-8 substantially influence physisorption and significantly improve the kinetics of hydrate formation.

Competition between CH4 hydrate formation and phase separation in a wetted metal–organic framework MIL-101 at moderate subcooling: molecular insights into CH4 storage

Systematic molecular dynamics simulations reveal the competition between CH4 hydrate formation and phase separation of CH4 in a mesoporous metal–organic framework MIL-101 at moderate subcooling.

Kinetics behaviors of CH4 hydrate formation in porous sediments: Non-unidirectional influence of sediment particle size on hydrate formation

The kinetic characteristics governing methane conversion into solid hydrates within seabed sediments influence hydrate accumulation and resource distribution, which is significant for the evaluation of global natural gas hydrate reserves and the implication of hydrate development. However, the influence of sediment particle size on methane hydrate formation kinetics remains unclear. In this study, simulating in situ conditions of the seafloor, the kinetics behaviors of CH4 hydrate formation within saline porous sediments were investigated. The experimental results indicate that the methane hydrate formation rate does not exhibit a strictly unidirectional change with changed quartz sand particle size. The unidirectionality and magnitude of hydrate formation rate change are simultaneously influenced by both quartz sand particle size and water saturation levels. The different effective gas-liquid contact area is the primary factor underlying the occurrence of these phenomena. However, for more porous clay sediments with greater specific surface area, the theory of effective gas-liquid contact area becomes inapplicable. The addition of clay to quartz sand sediment inversely inhibits the methane hydrate formation, which is caused by water absorption. This work can advance the comprehension of the intricate interplay between hydrate growth kinetics and the attributes of sedimentary hosts.

Kinetic and Thermodynamic Influence of NaCl on Methane Hydrate in an Oil-Dominated System.

This experimental study reports the kinetic and thermodynamic inhibition influence of sodium chloride (NaCl) on methane (CH4) hydrate in an oil-dominated system. To thoroughly examine the inhibition effect of NaCl on CH4 hydrate formation, kinetically by the induction time and relative inhibition performance and thermodynamically by the hydrate liquid-vapor equilibrium (HLwVE) curve, enthalpy (ΔHdiss) and suppression temperature are used to measure the NaCl inhibition performance through this experimental study. All kinetic experiments are performed at a concentration of 1 wt % under a pressure and temperature of 8 MPa and 274.15K, respectively, whereby for the thermodynamic study, the concentration was 3 wt % by using the isochoric T-cycle technique at the selected range of pressures and temperatures of 4.0-9.0 MPa and 276.5-286.0K, respectively; both studies were conducted using a high-pressure reactor cell. Results show that kinetically, NaCl offers slightly to no inhibition in both systems with/without oil; however, the presence of drilling oil contributes positively by increasing the induction time; thermodynamically, NaCl contributes significantly in shifting the equilibrium curve to higher pressures and lower temperatures in both systems. In the oil system, the contribution of the THI to the equilibrium curve increases the pressure with a range of 0.04-0.15 MPa and reduces the temperature with a range of 1-3 K, which is due to the NaCl presence in the systems that reduces the activity of water molecules by increasing the ionic strength of the solution. At a high pressure of 9 MPa, the NaCl inhibition performance was greater than that at lower pressures <5.5 MPa because, at the high pressure, NaCl increases the activity of water, which means that more water molecules are available to form hydrate cages around gas molecules.

Geometric and Hydrophilic Effects of Oxirane Compounds with a Four-Carbon Backbone on Clathrate Hydrate Formation.

The physicochemical properties of clathrate hydrates are influenced by the chemical nature and three-dimensional (3D) geometry of the added molecules. This study investigates the effects of five oxirane compounds: cis-2,3-epoxybutane (c23EB), trans-2,3-epoxybutane (t23EB), 1,2-epoxybutane (12EB), 1,2,3,4-diepoxybutane (DEB), and 3,3-dimethylepoxybutane (33DMEB) on CH4 hydrate formation. Despite having a four-carbon backbone, these compounds differ in their 3D geometries. The structures and stabilities of CH4 hydrates containing each compound were analyzed using high-resolution powder diffraction, solid-state 13C NMR, and phase equilibrium measurements. The experimental results revealed that c23EB, 12EB, and 33DMEB act as sII/sH hydrate formers and thermodynamic promoters, whereas t23EB and DEB have opposite roles. These results were analyzed in relation to the 3D geometries and relative stabilities of various rotational isomers using DFT calculations. Hydrate structure was influenced by both the length and thickness of the added compounds. Moreover, an appropriate level of (not excessive) hydrophilicity induced by an oxirane group appeared to enhance the thermodynamic stability of the hydrates. This study provides insights into how the chemical nature of additives influences the structure and stability of clathrate hydrates.

Metastable structure II methane hydrate: A bridge to obtain high methane uptake in the presence of thermodynamic promoter

The addition of thermodynamic promoters is a promising and perhaps the only effective method to reduce hydrate formation conditions. However, this is achieved at the cost of the gas storage capacity reduction since the thermodynamic promoter will occupy some hydrate cages. To date, there is no effective method that can simultaneously limit this negative effect and obtain high water-to-hydrate conversion. In this work, we determine the methane (CH4) hydrate formation mechanism in the presence of a thermodynamics promoter, Cyclopentane (CP), and find that structure II (Str.II) CH4 hydrate, as a metastable intermediate structure of the reaction, plays a quite significant role in encapsulating CH4 molecules in some 51264 cages (Str.II hydrates) and facilitating structure I CH4 hydrate formation, thereby increasing gas uptake significantly. A CH4 uptake, as high as 141.21 mmol/molH2O, is obtained from this work. Relative to the acknowledged limit of CH4 uptake of Str.II hydrates formed commonly in the presence of same mole fractions of thermodynamics promoter (8(Promoter)·16(CH4)·136H2O)), the highest increase in CH4 uptake is up to 232.75 %. These results would stimulate the development of the hydrate reaction theory and provide a new direction for promoter design.

The effects of rhamnolipid on the formation of CH4 hydrate and separation of CH4/N2 via hydrate formation

Rhamnolipid (Rha) is an efficient green surfactant that is able to promote hydrate generation, it can be applied to the hydrate method of separating. However, the effect of rhamnolipid on the separation of methane and nitrogen by the hydrate method is not clear. Therefore, in this paper, we investigated the effect of rhamnolipid on methane hydrate formation and the separation between methane and nitrogen via hydrate method. Firstly, the effects of five concentrations (0 ppm, 200 ppm, 400 ppm, 600 ppm and 800 ppm), temperatures and pressures on methane hydrate formation were studied separately. The rate of methane hydrate formation was positively correlated with experimental pressure. The gas consumption increased rapidly with the rhamnolipid concentration, and then increased slowly to a steady state. The final gas consumption above 200 ppm rhamnolipid concentrations could be increased more than five times compared with pure water conditions. The promotion of methane hydrate production is most pronounced at 274.15 K, mainly depending on the diffusion of gas molecules in solution and the ease of hydrate nucleation. Then, the interfacial tension at different concentrations was measured by the suspension drop method, and the relationship between the interfacial tension and the rate of hydrate formation was studied. Hydrate formation rate shows a multiplicative power function with the interfacial tension. What’s more, the kinetics of hydrate formation and gas separation effect of 3:1 CH4/N2 at 274.15 K and 7 MPa were studied with five concentrations. And 200 ppm was found to be the optimum concentration for separation, the separation factor is 8.25. Finally, the kinetics and thermodynamics of hydrate formation and gas separation effect of 1:1 CH4/N2 at 274.15 K and 9 MPa were also investigated, and the separation factor reached a maximum of 12.7 at 800 ppm, and the methane gas recovery rate also reached the maximum at 40.46%. The results of this study can provide some theoretical guidance for the optimal selection of green surfactants in separation by hydrate method.

Effects of gas to water ratio and nano-Cu dosage on CH4 hydrate growth: A combined molecular simulation and experimental study

Natural gas hydrate (NGH) is a highly promising alternative for gas storage and transportation. Nevertheless, the artificial generation of gas hydrates faces considerable obstacles for large-scale commercial implementation, primarily due to its slow formation rate and inadequate storage density. This study delves into the effects of gas to water ratios and metal nanoparticle dosages on the synthesis of NGH. Molecular dynamics (MD) simulations were employed to examine the formation of CH4 hydrate under varying gas to water ratios and metal nanoparticle concentrations. The investigation assessed critical parameters of the gas–water system, such as radial distribution function, carbon storage ratio, hydrate cages, potential energy, diffusion coefficient, and thermal conductivity. For the first time, Pearson's correlation coefficient analysis was utilized to discern the interrelationships among these factors. The results demonstrated that medium gas to water ratios led to increased carbon storage, a higher number of cages, and augmented energy consumption. The nano-Cu dosage impacted the stability of hydrates and the thermal conductivity of the gas–water system, particularly at elevated gas to water ratios. Drawing on the MD simulation findings, a design factor was introduced to appraise the performance of the multicomponent system, which in turn, informs the development of a high-performance NGH formation system. The effectiveness of the design factor was corroborated through laboratory experiments, revealing its robust potential for designing high-performance NGH systems. This research paves the way for the practical realization of efficient hydrate-based production in real-world applications.

Kinetics studies of methane hydrate formation and dissociation in the presence of cyclopentane

The work systematically investigates the kinetic of methane(CH4) hydrate formation in the presence of cyclopentane（CP）by measuring induction time, hydrate formation rate, and gas uptake. The results showed that there are two steps for CH4 hydrate formation in the presence of CP: Step 1, pure CP hydrate formation. Step 2, gas enters the empty cages of pure CP hydrates to form CP/CH4 mixed hydrates. Besides, the rapid hydrate formation rate and high CH4 uptake are observed from hydrate formation in this work. At the optimum conditions of 288.15 K and CP mole fractions of 5.6 mol%, the gas uptake and the time required for completion of hydrate formation are 131.80 mmol/mol H2O and 138 min, respectively. Relative to the highest reported gas uptake obtained in the presence of thermodynamic promoter (72.0 mmol/mol H2O), the value increased by 83%. More strikingly, the work shows that high gas uptake hydrates formed at relatively low temperatures can be stored and transported at relatively high temperatures, as high as 298.15 K, without significantly reducing the gas uptake, thereby providing one of the cost-effective methods for hydrate storage and transportation, that is forming hydrates at relatively low temperatures and storing and transporting hydrates at relatively high temperatures. Further, we proposed a schematic of the process chain of SNG(storing natural gas via solid hydrate) technology in vehicular and detailed discuss the current status of SNG technology development. It can be concluded that using the strategy that storage and transportation hydrates at a low-pressure condition instead of at ambient pressure (1.0 atm) may be a more realistic choice to industrialize SNG technology.

Molecular dynamics simulation on methane hydrate formation in clay nanopores of edge surfaces

Natural gas hydrates are predominantly buried in clay sediments in natural environments, where are some edge surfaces of clay particles directly contacting the hydrates. However, the exact nature of the interaction between these surfaces and the hydrates, as well as their influence on hydrate formation, remains elusive. Herein, microsecond molecular dynamics simulations have been performed to investigate CH4 hydrates formation in nanopores consisting of clay edge surfaces, to reveal the effects of clay edge surfaces and layer charges. The simulation results show that the clay edge surfaces affect CH4 hydrate formation by changing the distribution of water and CH4 molecules via surface adsorption, mainly ascribed to the different polarities of the groups on the edge surfaces of different clays. The greater the electronegativity of the clay, the stronger the inhibition of CH4 hydrate formation, thus, the electroneutral clays are more beneficial for CH4 hydrate formation than the electronegative clays. Moreover, in the early stage of the simulation, compared with the electronegative clays, the electroneutral clays are more favorable for the diffusion of CH4 molecules from the nanopores into the bulk solution and then promote CH4 hydrate formation. On the other hand, the ions in the solution gradually aggregate together and their distribution becomes denser and more ordered. The edge surfaces of electroneutral clay are more accessible to hydrate solids than electronegative clay. These molecular insights into the formation behavior of CH4 hydrates in clay nanopores consisting of edge surfaces help to understand the formation process of natural gas hydrates in marine sediments.

Molecular dynamics simulations to investigate the effects of organic amines on biogas clathrate hydrate formation

The biological degradation of organic wastes mostly generates biogas including CO2 and CH4. Since both gases may be utilized as feedstock for various industrial applications, the alternative of gas hydrates to separate or enrich such gas species is of particular interest. To promote the hydrate-based (HB) methods, introducing environmentally friendly promoters could either enhance the formation or improve the recovery rates. In this study, the effects of pure and binary organic amines (methylamine, dimethylamine, amylamine) and urea on the kinetics of CO2 + CH4 hydrate formation through molecular dynamics simulations were explored. To characterize the kinetics of biogas hydrate formation, different analyses such as the three/four-body structural order parameters, determination of hydrate-like cages, number of hydrogen bonds, molecular distributions, and energy variations were investigated. Results indicate that the promotion effects of organic molecules were evident in which the presence of methylamine, dimethylamine, and their mixture was found to be more kinetically efficient than other studied solution systems. These molecules can also induce guest gases toward being located inside the formed cages more than in pure water. Studied molecules can also affect the distribution of CO2 and CH4 molecules during the conversion of the solution phase to a clathrate-like state which could be a useful feature to intensify the split fraction of HB processes. Nonetheless, the addition of urea in either a pure or binary mixture with other amine molecules was not recognized as the observed efficiency of combined methylamine and dimethylamine. This investigation reveals the uncovered characteristics regarding the utilization of long/short-chain amines to upgrade the process of biogas separation through hydrate.

Effect of cocoamidopropyl betaine on CH4 hydrate formation and agglomeration in waxy oil-water systems

Hydrates and waxes are the two major factors that provoke flow assurance issues in deep-water multi-phase transport units in the energy industry, and the application of eco-friendly and efficient anti-agglomerants is one of the key strategies for addressing this problem. The present work investigates the anti-aggregation performance of an environment-friendly amphoteric surfactant, Cocamidopropyl betaine (CAPB), in wax-containing oil-water systems. It is found that the anti-aggregation performance of CAPB in wax-containing oil-water systems is synergistically influenced by the concentration of CAPB and the wax content. The addition of 1.5 wt% CAPB can effectively inhibit hydrate formation and improve the flow of hydrate slurry in oil-water systems without wax. The addition of 2.0 wt% CAPB slows the growth rate of hydrates and reduces inter-particle aggregation in waxy oil-water systems containing 2.5 wt% n-Pentadecane. The wax content does not affect the amount of hydrate formation but influences the induction time and growth rate of hydrate formation. Additionally, the microscopic analysis further reveals that the addition of surfactant CAPB does not affect the crystal structure of hydrate, but has a significant influence on the distribution of methane molecules in the crystal cavities, particularly in wax-containing oil-water systems.

Experimental Study of CH4-CO2 Replacement in Gas Hydrates in the presence of nitrogen and graphene nanoplatelets

Vast methane hydrates found in seabeds dispersed throughout ocean sediments and permafrost areas are a potential source of revenue as they act as an energy source. The encapsulation of carbon dioxide into the spaces occupied by methane molecules also creates a storage solution for greenhouse gases. This replacement reaction has two purposes; the first is energy release in the form of CH4 gas and the second is CO2 storage. This article presents experimental results on the CH4-CO2 replacement process using a. laboratory scale reactor with pure water and nanoparticles. Furthermore, the addition of nitrogen, as well as the use of thermal stimulation was studied experimentally to assess the enhancement of the replacement reaction. Additives such as graphene and sodium dodecyl sulfate (SDS) were utilized in the methane hydrate formation to further enhance the rate of hydrate formation. The experimental measurements show that a gas mixture of 80 mol%N2/20 mol%CO2 yields a CH4 replacement efficiency of 17.04 %, while 28.77 % replacement efficiency was the highest CH4 replacement efficiency obtained using thermal stimulation with pressurized CO2. The highest amount of CO2 sequestrated was 57.03 %. This was achieved via thermal stimulation with pressurized CO2. The lowest CO2 sequestrated was 10.57 % with 90 mol% CO2/10 mol%N2. The results confirm the significant improvements in CH4 hydrate formation, CH4 replacement efficiency, and CO2 sequestration ratio using graphene and SDS solutions.

CH4 hydrate production coupled with CO2 sequestration and hydrate restoration employing depressurization assisted by CO2-N2 injection at marine conditions

Natural gas hydrates (NGHs) buried below the seafloor are globally abundant, energy-dense, and essential to the world’s future energy mix. The combination of depressurization and CO2-N2 injection processes for NGHs recovery has been identified as an environmental method to harvest energy, CO2 sequestration, and to maintain the mechanical stability of sediment. In this regime, partial CH4 was produced by depressurization followed by CO2-N2 injection to form CH4-CO2-N2 mixed hydrate (Mix-H). In this study, a series experiments were designed to examine the key factors in the depressurization process (i.e. bottom hole pressure BHP and CH4 production ratio) on the subsequent processes of Mix-H formation, CO2 sequestration and hydrate restoration. It was observed that Mix-H formation occurred in all cases, but the final amount of Mix-H were all lower than the initial amount of MH before depressurization. Particularly, the formation of CO2/N2 hydrate in Mix-H is positively correlated with the amount of remaining CH4 in the combined phases of gas and hydrate after depressurization. The formation of CH4 hydrate in Mix-H during the reformation process occurred in five cases when MH dissociation ratio (DMH) reaches above 45.1 % during depressurization. MH was only observed to dissociate continuously after CO2/N2 injection when DMH is low at 30.8 %, which indicates possible “CH4-CO2 replacement reaction”. Overall, increasing CH4 production at the same BHP decreases the CO2 sequestration and hydrate restoration. We first verified the feasibility of the combination method for synergistic CH4 recovery and CO2 sequestration in marine conditions. The experimental results provide possible guidance on the optimal design of the coupled processes and shed light on the relationship between CH4 recovery and CO2 sequestration that need to be wholistically balanced.