Hydrate Dissociation Research Articles

Mechanical and acoustic characterization of carbon dioxide gas replacement of methane gas hydrate

CO2 replacement presents a promising approach for the extraction of natural gas hydrate, offering advantages in cleanliness and safety. In this study, CO2 replacement CH4 hydrate experiments were carried out by using the triaxial, hydrate specimens with varying replacement ratios were prepared through different methods, and triaxial tests were performed to evaluate the mechanical properties of sediments after replacement. The key findings are as follows: the shear damage behaviors of CH4, CO2, and their mixed hydrate specimens are similar. Acoustic data indicates that initial CO2 injection during replacement causes dissociation of CH4 hydrate, potentially reducing the failure strength of the reservoir. However, post-replacement hydrate specimens exhibited higher failure strength than their original counterparts. This study provides valuable insights into the mechanical stability of CH4 hydrate reservoirs during CO2 replacement process.

Molecular insights into methane hydrate dissociation: Role of methane nanobubble formation.

Understanding the underlying physics of natural gas hydrate dissociation is necessary for efficient CH4 extraction and in the exploration of potential additives in the chemical injection method. Silica being "sand" is already present inside the reservoir, making the silica nanoparticle a potential green additive. Here, molecular dynamics (MD) simulations have been performed to investigate the dissociation of the CH4 hydrate in the presence and absence of ∼1, ∼2, and ∼3nm diameter hydrophilic silica nanoparticles at 100bar and 310K. We find that the formation of a CH4 nanobubble has a strong influence on the dissociation rate. After the initial hydrate dissociation, the rate of dissociation slows down till the formation of a CH4 nanobubble. We find the critical concentration and size limit to form the CH4 nanobubble to be ∼0.04 molefraction of CH4 and ∼40 to 50 CH4 molecules, respectively. The solubility of CH4 and the chemical potential of H2O and CH4 are determined via Gibbs ensemble Monte Carlo simulations. The liquid phase chemical potential of both H2O and CH4 in the presence and absence of the nanoparticle is nearly the same, indicating that the effect of this additive will not be significant. While the formation of the hydration shell around the nanoparticle via hydrogen bonding confirms the strength of interactions between the water molecules and the nanoparticle in our MD simulations, the contact of the nanoparticle with the interface is infrequent, leading to no explicit effect of the nanoparticle on the dynamics of methane hydrate dissociation.

Significance of well placement and degree of hydrate reserve recovery for synergistic CH4 recovery and CO2 storage in marine heterogeneous hydrate-bearing sediments

The synergistic CH4 recovery and CO2 storage in marine natural gas hydrate reservoirs presents an emerging strategic solution for energy and environment challenges, yet the efficiency requires innovative engineering designs to enhance. In this study, a water-saturated CH4 hydrate-bearing sediment (HBS, V = 22 L, T = 284 K, and P = 11.2 MPa) was synthesized to investigate the pilot-scale combined processes of CH4 recovery by depressurization followed by CO2 storage through CO2/N2 injection. The synthesized HBS exhibited vertical heterogeneity with higher hydrate saturation observed in the upper sediment and a decreasing saturation trend in the lower sediment. The effect of well placement designs on CH4 recovery and hydrate restoration by mixed CH4/CO2/N2 hydrate (Mix-H) formation was investigated, considering two scenarios: upper well for recovery while lower well for injection (URLI), and lower well for recovery while upper well for injection (LRUI). Additionally, the impact of the degree of hydrate reserve recovery (100% and 80% hydrate dissociated) on subsequent Mix-H formation was also studied. Results indicate that the URLI setting achieves a higher CH4 recovery ratio and rate during depressurization compared to the LRUI setting which encounters low-T shielding. The T variation between upper and lower sediment highlights the influence of heterogeneity on fluid behaviors. Halting the CH4 recovery after 80% hydrate dissociation results in a halved recovery time and water production. During later CO2/N2 injection and Mix-H formation process, the LRUI setting achieves a 13–54% higher Mix-H saturation and a 33–56% higher CO2 hydrate encapsulation than the URLI setting. Moreover, 80% dissociation proves more favorable for Mix-H formation achieving nearly twice higher final amount than 100% hydrate dissociation. However, it results in a lower CO2 encapsulation. This study provides insights into the trade-off between CH4 recovery and CO2 storage efficiency, emphasizing the need for optimized well placement and controlled hydrate extraction.

Gas flow in frozen hydrate-bearing sediments exposed to compression and high-pressure gradients: Experimental modeling

Changes in temperature and pressure patterns in gas- and hydrate-saturated permafrost caused by natural geodynamic processes or human impacts can lead to the active flow of gas through unfrozen zones, and its explosive emission is often accompanied by crater formation. Gas flow and accumulation in the shallow permafrost can be explained by the conditions of gas pressure equal to or exceeding the overburden pressure and high-pressure gradients. For the first time, filtration tests were conducted on ice- and hydrate-saturated rocks under uniaxial compression at various negative temperatures using a developed methodology. The modeling of gas flow in a mixture of ice-saturated sand and 25 % montmorillonite at gas pressure gradients within 2 MPa, shows that gas flow can start at warm negative temperatures near the thaw point. Pore hydrate formation in frozen sand heated to positive temperatures and frozen back led to a linear decrease in gas permeability by up to eight times. However, the behavior of gas permeability during hydrate dissociation is nonlinear as it increased within a few hours after the onset of dissociation, but then decreased exponentially in the following 24 h.

CH4 hydrate dissociation and CH4 leakage characteristics: Insights from laboratory investigation based on stratified environment reconstruction of natural gas hydrate reservoir

Submarine natural gas hydrates (NGHs) are one of the largest methane reservoirs on Earth, offering huge potential as an alternative energy source and serving as a crucial global carbon sink. However, the substantial risk of methane leakage during NGH development seriously threatens marine and global ecological and environmental safety, presenting a major challenge for commercial NGH exploitation. Focusing on the unknown leakage mechanisms, this study employed a new simulation device to construct a stratified environment and to investigate the characteristics of hydrate dissociation and methane leakage during depressurization production, under conditions of varied fracturing in overlying sediment. A movable module was used to dynamically isolate and connect the hydrate reservoir with the overlying layer. The results showed that the intrusion of overlying water decelerated the reservoir depressurization rate, leading to a mass of hydrate reformation. The invasion locations and flow paths of the overlying water were influenced by the fracture scale of the overlying sediment. Benefiting from the additional sensible heat and reservoir space occupation of the overlying water in the later production stages, the methane recovery ratio in the fracture-containing systems were not significantly affected. However, the leakage methane increased as fracture channels enlarged, reaching up to 9.90 % of the total methane content in the hydrate reservoir. The leakage mechanism primarily involved local overpressure, buoyancy effects, and diffusion. The research, for the first time, experimentally revealed the response relationship between methane leakage and hydrate dissociation, providing foundational data and theoretical support for the safe and efficient NGH development.

Optimization of energy efficiency in gas production from hydrates assisted by geothermal energy enriched in the deep gas

Scaled-up production from natural gas hydrates is still facing the significant issues of limited gas yield and unsustainable hydrate dissociation. In this work, the deep gas with huge reserves and abundant geothermal energy was used to enhance the hydrate production efficiency. Results showed that the geotherm-assisted gas production was significantly increased by 178.22 %. Surprisingly, the hydrate dissociation rate was slightly weakened by 3.66 % due to the hindered fugacity of the gas from hydrate decomposition as a result of the fast deep gas upwelling. Besides, the energy flow analysis showed that up to 40.67 % of the deep gas injection energy was dissipated to the surroundings, with only 8.28 % utilized for the hydrate decomposition. Consequently, the multi-well deployment was used to regulate the deep gas injection rate. It was found that the multiple wells weakened the sealing effect of hydrate on migration channels, preventing the violent gas upwelling while increasing the hydrate decomposition amount under geothermal stimulation. Then, prolonging crossflow distance of deep gas in the hydrate layer enabled the sediment to absorb more injected energy, resulting in the minimum wasted energy ratio. Not limited to this, the more stable gas production throughout the whole process effectively guaranteed the safety of exploitation. Finally, the hydrate decomposition rate in the depressurization and constant pressure stages was found to be positively correlated with the utilization ratio of the injected energy and the injected rate, respectively. The results could help guide the formulation of the multi-gas joint production scenarios in the field tests to balance production efficiency and cost.

Determination of hydrate phase equilibrium (H-L-V) data for the binary CO2–CH4 gas mixture in the presence of 1,3 dioxolane

Present work details the determination of three phase hydrate equilibrium data (H-L-V) using temperature search method for the binary CO2/CH4 gas mixture (50:50 molar ratio) in presence of 1,3 dioxolane (DIOX). DIOX concentration used for phase equilibrium determination were 1,3 and 5.56 mol% respectively. In the presence of DIOX, it was observed that phase equilibrium curve was shifted right with respect to the curve using same gas mixture with no additive (pure water). This indicates that the presence of DIOX moderates the hydrate formation equilibrium conditions. Also, as DIOX concentration increases from 1 mol% to 5.56 mol%, a decrement in phase equilibrium pressure at same temperatures was observed, confirming the potential of DIOX as an effective thermodynamic promoter. Enthalpy of hydrate dissociation was calculated for CO2/CH4 gas mixture in presence of DIOX using Clausius- Clapeyron plot and it was found to be 90.81±6.55 KJ/mol which confirms the sII structure of CO2-CH4-DIOX mixed hydrate. Besides providing the phase equilibrium data of formed hydrates with CO2/CH4 gas mixture using different concentration of DIOX, the present study will aid in determining suitable experimental conditions for examining kinetics and performing separation studies of CO2/CH4 gas mixture through hydrate formation.

Hydrate Formation and Dissociation of Liquid CO2 with Seawater Containing Organic Matters in Sand

Hydrate Formation and Dissociation of Liquid CO₂ with Seawater Containing Organic Matters in Sand

Thermodynamic modelling of gas hydrate dissociation conditions in porous medium in the presence of NaCl/methanol aqueous solution

AbstractDue to the growing significance of the existence of gas hydrates in natural media like the ocean floor/permafrost regions and the extraction of natural gas from hydrate reservoirs using thermodynamic hydrate inhibitors, investigating the dissociation of gas hydrates in porous media in the presence of inhibitors is crucial. This work examines a broad range of laboratory data on the dissociation conditions of gas hydrates in the porous mediums when salt/alcohol aqueous solutions are present. The temperature of gas hydrate dissociation in the presence of pure water is calculated using the van der Waals–Platteeuw solid solution theory. The water activity in the porous medium is then calculated by taking into account a number of variables, including the radius of the porous medium, molar volume, shape factor, wetting angle, and surface tension. The Pitzer and Margules activity coefficient models are used to determine the water activity in the presence of salt and alcohol, respectively. Lastly, the gas hydrate dissociation temperature in a porous medium in the presence of salt and/or alcohol aqueous solution is determined by combining Piereon's model with an enthalpy‐based correlation that was proposed by Azimi et al. The selected package can consistently correlate the gas hydrate dissociation conditions in a porous medium in the presence of alcohol or salt aqueous solution. The average absolute deviation (AAD) of 0.67 K for the whole data bank (90 experimental data points) shows the precision of the model.

The formation of tubular seep carbonate deciphered from mineralogical and geochemical characteristics: an example from the South China Sea

As a special type of seep carbonate, the many details concerning the formation mode and mechanism of tubular seep carbonates are rarely reported. Here, new geochemical and mineralogical data regarding tubular seep carbonate (SQW-65) are reported. Sample SQW-65 had anomalously negative δ13C values and positive δ18O values, which suggested the dissociation of gas hydrate. Additionally, almost all the sub-samples showed no Ce anomaly (Ce/Ce*average = 0.93), with obvious U enrichment (21.3< UEF <240.3), which indicates that the studied tubular seep carbonate was formed in an anoxic environment. Subsequently, the formation process of the studied tubular seep carbonate is further discussed according to the variability of mineralogical and geochemical characteristics from the rim to the core of the tubular formation. In the early stage of the studied tubular seep carbonate (periphery), owing to the influence of terrigenous components, the quartz and Ti content and Y/Ho ratio were high. However, with the formation of the periphery, the influence of terrigenous components was gradually weakened. In addition, from the rim to the core, the carbon and oxygen isotope values showed a “covariation” coupling relationship, an enrichment of U, and a reduction in total rare earth element content. This is because as the outer wall thickens and the internal fluid channel narrows, the intensity of the sulphate-driven anaerobic oxidation of methane and the associated precipitation rate of carbonate also increase.

Effect of polyoxyethylene laurate series surfactants on HCFC-141b hydrate formation

To address the problems of slow crystallization and nucleation in two-phase miscibility in the application of refrigerant hydrates, adding surfactants can be taken as an effective way to promote hydrate formation. Three kinds of polyoxyethylene laurate (LAE) surfactant with different hydrophilic chain lengths (LAE-4, LAE-9 and LAE-24) were selected as accelerators to study their effects on hydrate formation. LAE series surfactants significantly reduce hydrate nucleation induction time. The hydrate induction time of the system with 4.0 wt% LAE-9 is shortest (98 min). The hydrate formation with 4.0 wt% LAE-9 has small randomness and is more stable. Micelles formed by LAE surfactants provide more nucleation sites and accelerate hydrate growth. The hydrate cold storage density is related to the hydrophilic chain length of surfactant. The system with 4.0 wt% LAE-9, which has a suitable hydrophilic chain length, achieves the largest hydrate cold storage density (246.10 kJ·kg−1). Hydrate has the fastest growth rate of 4.80 kJ·kg−1·min−1 in the system with 2.0 wt% LAE-24. There is a “memory” effect during hydrate formation and dissociation cycle, eliminating the need for induction time during hydrate re-formation. Hydrate can be re-formed quickly.

Postglacial ocean methane release from the northern South China Sea

Methane release into seawater and the atmosphere as a result of cold seepage activity is a sensitive indicator of global environmental change during the last glacial period. However, the link between postglacial ocean methane escape and possible increases in atmospheric methane concentration remains unclear, which hinders the evaluation of methane impact on climatic variability. In this study, lipid biomarkers were analyzed on samples from the Shenhu area in the South China Sea to reconstruct the marine gas hydrate variability since the last glacial time. The reconstructed methane release during the cool Last Glacial Maximum and Younger Dryas agreed well with previous findings. Interestingly, our results identified a pronounced methane outgassing at the end of Heinrich 1, which was likely triggered by the abrupt increase in intermediate water temperature. The occurrence of this gas hydrate dissociation before the drastic rise in atmospheric methane during Bølling-Allerød implied that the change in the gas hydrate reservoir could likely indirectly contribute to the initiation of the warming event, though more global evidence needs to be provided in the future.

Dissociation Behavior of Methane Hydrate Inside Different Saturation Sediments in a Sustained Depressurization Process

Summary Methane hydrate is one of the important energy storage sources, naturally distributed in marine porous sediments. However, the dissociation behavior of hydrate inside different saturation sediments during sustained depressurization remains unclear. In this study, methane hydrate-bearing sediments were synthesized using initial water saturations varying between 9.5% and 56.9% and subsequently dissociated at a gas exhaust rate of 0.77 Ls/min. The results indicate that the dissociation of hydrates is closely related to the initial hydrate distribution until the sediments get iced when the sediment pressure declines below 2.5 MPa. Due to the exothermic reaction of ice formation, the dissociation of hydrates after icing accelerates significantly, and its limiting factor becomes the gas exhaust rate. In addition, both production and monitoring pipes were used in this study to evaluate the possible plugging within the sediments, and the plugging zone within the sediments can be located by examining the thermodynamic correlation between pressure and temperature responses. It was found that all experimental cases with high saturations (47.4% and more initial water) easily induce plugging between sediments and production/monitoring pipes, with the maximum pressure gap reaching up to 2.5 MPa. These findings may aid in ensuring the safety and efficiency of the hydrate exploitation process in the future.

CO2 solubility in aqueous solution of salts: Experimental study and thermodynamic modelling

AbstractThere are many economic obstacles and complex engineering problems associated with CO2 capture and storage in saline aquifers that need to be addressed. Overcoming such challenges requires precise knowledge on the fluid phase equilibria of CO2‐brine systems. Having accurate CO2 solubility data over a wide range of temperature and pressure can greatly assist in resolving these obstacles by improving the performance and accuracy of the thermodynamic modeling and subsequent CCS engineering success.CO2 solubility in pure water and NaCl solutions has been widely studied in the literature, however, there is a lack of data on CO2 solubility at lower temperatures (below 298 K). Furthermore, limited phase equilibria data are available for CO2 solubility in CaCl2, MgCl2, and KCl solutions at elevated temperatures (i.e., T > 323.15 K).In this work, the phase equilibria of CO2 and brine systems are investigated experimentally and theoretically. In this study, solubilities of CO2 in pure water and various concentrations of NaCl (10, 15, 20, and 22 wt%), KCl (10, 15, and 22 wt%), CaCl2 (7.5, 10, 15.7, and 23.4 wt%), and MgCl2 (6.7, 11, 18, and 29 wt%) aqueous solutions are reported. All CO2 solubilities were measures at 323.15, 373.15, and 423.15 K and over various pressure ranges, while solubilities in 10 and 20 wt% NaCl aqueous solutions were also measured over the temperature range of 263 to 298 K and pressures up to the hydrate dissociation pressure of each system. Equation of state modelling using the PC‐SAFT and the Cubic Plus Association equations of state, is performed in the theoretical part of the study to validate the measured solubility data. © 2024 The Author(s). Greenhouse Gases: Science and Technology published by Society of Chemical Industry and John Wiley & Sons Ltd.

Pore-scale behaviors of CO2 hydrate formation and dissociation in the presence of swelling clay: Implication for geologic carbon sequestration

Hydrate-based CO2 sequestration (HBCS) under seafloor with huge storage capacity and long-term mechanical stability is attractive for large-scale carbon reduction. However, as representative geochemical constituents of marine sediments, swelling clay minerals still play ambiguous roles in hydrate phase transition and sedimentary structure stability. In this study, pore-scale behaviors of hydrate deposition and reservoir structure evolution in the presence of montmorillonite (MMT), a representative swelling clay, was investigated using low-field nuclear magnetic resonance (LNMR) technique. Total inter-particle interaction energy of 195.4 KbT ensured the dispersion of clay in 0.5 wt% MMT system, which facilitated homogeneous hydrate formation by providing nucleation sites and surface electric fields, improving hydrate conversion from 78.2 % to 89.6 %. Weak water activity caused by strong water absorption of swelling clay resulted in a 14.4 % reduction in CO2 storage capacity of high-concentration (10.0 wt%) MMT reservoir. Hydrate phase transition accompanied by the migration of MMT particles and liquid water synergistically contributed to sedimentary structure evolution. Migration of MMT-rich fluid with high viscosity could cause irreversible geologic damages by exacerbating sedimentary skeleton deformation. This work reveals the interaction between swelling clay and CO2 hydrate at microscopic scale, providing new perspectives for effective implementation of CO2 geologic sequestration in marine sediments.

Exploring Porous Flow Behavior of the Decomposed Gas from CH4 Hydrate in Clayey Sediments by Molecular Dynamics Simulation.

A fundamental understanding of the fluid flow mechanism during CH4 hydrate dissociation in nanoscale clayey sediments from the molecular perspective can provide invaluable information for macroscale natural gas hydrate (NGH) exploration. In this work, the fluid flow behaviors of the decomposed gas from CH4 hydrate within clayey nanopores under different temperature conditions are revealed by molecular dynamics (MD) simulation. The simulation results indicate that the key influencing factors of gas-water flow in nanoscale clayey sediments include the diffusion and the random migration of gas molecules. The influencing mechanisms of fluid flow in nanopores are closely related with the temperature conditions. Under a low temperature condition, the gas diffusion process is impeded by the secondary hydrate formation, leading to the decline in gas transport velocity within nanopores. However, it is still noteworthy that the gas-water fluid flow channels are not completely blocked by the occurrence of secondary hydrate. Under a high temperature condition, the significant phenomenon of water migration during gas flow is observed, which can be ascribed to the gas-liquid entrainment effect in nanopores of the clayey sediment. These results may provide valuable implications and fundamental evidence for improving gas production efficiency in future field tests of NGH exploitation in marine sediments.

Application of time domain reflectometry to triaxial shear tests on hydrate-bearing sediments

Various triaxial shearing apparatus are used to quantify mechanical properties of hydrate-bearing sediments which are basically required to the assessment of risks posed by dissociation of natural gas hydrates. However, the key factor, hydrate saturation, has not been well measured during triaxial shearing tests. This study aims to show how a time domain reflectometry (TDR) method nondestructively measures hydrate saturation for triaxial shear tests on hydrate-bearing sands. Hydrate saturation is determined based on the difference of volumetric water contents measured by using a flexible TDR probe with two rods after calibration. The results suggest that values of hydrate saturation acquired by the TDR method are accurate, and the flexible TDR probe can monitor the kinetic process of hydrate formation and avoid any reinforcement effects on triaxial shearing properties of test specimens.

Experimental study on the effect of PVP, NaCl and EG on the methane hydrates formation and dissociation kinetics

The problem of hydrate plug, low efficiency of hydrate dissociation and short production time in hydrate exploitation processes have significantly hindered the commercial viability of gas hydrate extraction. This study investigated the inhibitory effects of ethylene glycol (EG), EG + polyvinyl pyrrolidone (PVP), and EG + PVP + sodium chloride (NaCl) on methane hydrate formation through experiment. The hydrate inhibitory performance is evaluated by using differential of pressure curve, the amount of hydrate, and pressure drop values, and the effects of different temperatures, pressures, inhibitors, and injection time on hydrate dissociation are further studied. The experiment results indicate that the rank of inhibitors combination in terms of effectiveness is 5%EG + 0.5 wt%PVP + 3 wt%Nacl > 10%EG + 1 wt%PVP > 30% EG. At low-temperature conditions, 30% EG exhibits good inhibition of hydrate synthesis but poor dissociation efficiency. As temperature increases, the hydrates dissociation rate with 30% EG also increases. For the combination inhibitor system of EG, PVP, and NaCl, PVP will reduce the dissociation efficiency of hydrates, while EG and Nacl will improve the hydrate dissociation performance. For low production pressure, it is found that 10% EG + 10% NaCl have a good promotion effect on hydrate dissociation, whereas under high production pressure, 20% EG + 10% NaCl is more effective. Furthermore, injecting the inhibitors earlier enhances the dissociation of hydrates more effectively.

Fate of Nanobubbles Generated from CO2–Hydrate Dissociation: Coexistence with Nanodroplets—A Combined Investigation from Experiment and Molecular Dynamics Simulations

The evolution of CO2 nanobubbles generated by gas–hydrate dissociation is comprehensively studied in this research, employing a synergistic approach that combines laboratory experiments and molecular dynamics simulations. The results show that a higher concentration of nanobubbles can be observed in the early stages of hydrate dissociation, while smaller, thus‐generated, nanobubbles are less stable and prefer to amalgamate into larger bubbles through coalescence or Ostwald ripening. From the high Laplace pressure inside some nanobubbles as well as their higher local densities, they may transform into nanodroplets by densification fluctuations. Thus, the dynamic coexistence of nanobubbles and ‐droplets is confirmed from both experimental and simulation measurements. The number and size of the nanobubbles in the system affects the interaction between water molecules and their movements so that the water molecules diffuse faster upon this condition. The water–water interactions become more pronounced in the presence of nanobubbles and the hydrogen bond network is better preserved in the bulk. This study provides new insights into the microscale mechanisms of gas–hydrate dissociation and highlights the complex interactions between nanobubbles/ ‐droplets, and the aqueous environment after CO2–hydrate dissociation.

Numerical Simulation of Vertical Well Depressurization-Assisted In Situ Heating Mining in a Class 1-Type Hydrate Reservoir

In situ electric heating is an important method used to increase production capacity during the extraction of natural gas hydrates. This work numerically evaluated the sensitivity of different heating parameters on gas production behavior with a vertical well depressurization in the Shenhu Sea area hydrate reservoir, the production pressure difference of 4 MPa, and continuous depressurization for 1080 days. The results showed that the in situ electric heating method can effectively enhance production capability by promoting hydrate dissociation and eliminating secondary hydrates. Compared with scenarios without heating, implementing whole wellbore heating (100 W/m) increases cumulative gas production (Vg) by 118.56%. When intermittent heating is applied to the local wellbore (15 m) located in the three-phase layer (with an interval of 30 days) and stops heating in advance at 480 days, there is no significant difference in Vg compared to the whole wellbore heating case, and the cumulative heat input is only 4.76%. We recommend considering intermittent heating of the local wellbore and stopping heating in advance during vertical well depressurization as this approach significantly reduces heating energy consumption while simultaneously improving production capacity.