Review of CO2–CH4 clathrate hydrate replacement reaction laboratory studies – Properties and kinetics

Monitoring the developmental trend and competitive landscape of natural gas hydrate through patent analysis

Natural gas hydrate (NGH) is a significant alternative energy resource in achieving carbon neutrality. The developmental trend and competitive landscape of NGH exploitation and production are crucial for policymakers in government, managers of enterprises, and researchers. This study introduces a novel framework for conducting an in-depth analysis of NGH, integrating patentometrics, technology evolution, and correlation relationships to monitor developmental trends and competitive landscape through patent analysis. The results indicate that China, the USA, and Japan have distinct technology advantages. Current technological developments in the NGH field focus primarily on extraction technologies, equipment, and processing systems. The co-opetition analysis among countries reveals that the most extensive international cooperation network is primarily in Europe and the USA, with national partnerships in Asia concentrated in China and Japan. Institutional cooperation remains limited, primarily within universities in China, while both the USA and Japan foster collaboration between enterprises. The competitive landscapes of key NGH-related technologies among countries and institutions are also examined. This study contributes not only to monitoring the developmental trend and competitive landscape in NGH but also to providing policy recommendations for government and enterprises regarding strategic management and collaborative innovation.

Terahertz Signatures of the Methane Replacement Reaction in Hydroquinone Clathrates

We report a comprehensive experimental and computational study into low-frequency vibrational dynamics of hydroquinone clathrate during in situ gas loading, in order to monitor replacement of carbon dioxide with methane in its atomic-level pores. We used terahertz time-domain spectroscopy, because terahertz modes are highly sensitive to the identity and structure of enclathrated guest molecules. Through ab initio simulations, we determined that the replacement reaction is not completed. Instead we observed the formation of a heterogeneous material, with methane molecules occupying approximately one-third of available adsorption sites. While the structure of the methane-hydroquinone clathrate system has been previously determined, our observations suggest the reported symmetry is incorrect due to methane molecules weakly interacting with the framework, resulting in dynamic (as opposed to positional) disorder of guests, unlike the related fully ordered carbon dioxide clathrate. This work puts us on the path to quantitatively tracking gas loading in porous materials using terahertz spectroscopy.

Facilitation of Hydrate Dissociation and Structural Evolution by Major Marine Anions under Static Electric Fields

Electric fields have been proven to be capable of significantly affecting the equilibrium state of hydrates. In this study, the thermodynamic properties and structural changes of methane hydrate (MH) in various anion solutions in an electric field at 0.7 V/nm were investigated by molecular dynamics simulations. The presence of anions significantly enhances the instability of methane hydrates under electric fields, leading to a staged dissociation process. First, the anions coexist with MH to form a temporary metastable structure under the action of an electric field. Then, the migration of anions causes the dissociation of nearby hydrates and the formation of flow channels in the hydrate layer, which leads to the complete dissociation of MH after a period. The promotive effects of F-, Br-, I-, and Cl- ions were close, while SO42- was relatively weak. The anions are still in hydration shells in the MH phase, but the structure of the hydration shells differs slightly from that in solution (the coordination numbers of I- and SO42- ions increased). The migration resistances of multiple anions to cross the surface of the hydrate layer are similar. However, inside the hydrate phase, the anions with a larger radius have a higher migration resistance. It is difficult for SO42- ions to migrate inside the hydrate phase, and they tend to form a metastable structure on the hydrate surface. Combining our previous studies, SrCl2 solution has the best hydrate promotion under an electric field environment.

Molecular Dynamics Simulation of the Three-Phase Equilibrium Line of CO2 Hydrate with OPC Water Model

The three-phase coexistence line of the CO2 hydrate was determined using molecular dynamics (MD) simulations. By using the classical and modified Lorentz-Berthelot (LB) parameters, the simulations were carried out at 10 different pressures from 3 to 500 MPa. For the OPC water model, simulations with the classic and the modified LB parameters both showed negative deviations from the experimental values. For the TIP4P/Ice water model, good agreement with experimental equilibrium data can be achieved when the LB parameter is adjusted based on the solubility of CO2 in water. Our results also show that the influence of the water model on the equilibrium prediction is much larger than the CO2 model. Current simulations indicated that the H2O-H2O and H2O-CO2 cross-interactions' parameters might contribute equally to the accurate prediction of T3. According to our simulations, the prediction of T3 values showed relatively higher accuracy while using the combination of TIP4P/Ice water and EPM2 CO2 with modified LB parameter. Furthermore, varied χ values are recommended for accurate T3 estimation over a wide pressure range. The knowledge obtained in this study will be helpful for further accurate MD simulation of the process of CO2/CH4 replacement.

Dissociation mechanism of methane hydrate by CaCl2: an experimental and molecular dynamics study

The formation of gas hydrate is a serious threat to the safe and effective completion of deepwater drilling and transportation operations, although it is considered as a potential energy resource. The inorganic salts are generally used as thermodynamic inhibitors; CaCl₂ as a common additive in drilling fluids exhibits unique properties. In this study, we explored the dissociation mechanism of CH₄ hydrate in CaCl₂ solutions at the macroscopic and microscopic scale using experiment and molecular dynamics (MD) simulation. The experimental results showed that CaCl₂ accelerated the dissociation rate of CH₄ hydrate. The dissociation rate of CH₄ hydrate increased with the increase of CaCl₂ concentration at large depressurization pressure and was mainly affected by pressure when the depressurization pressure was lower. MD simulations were used to give an atomic scale interpretation of the macroscopic results obtained from the experiment. The results showed that the addition of CaCl₂ destroyed the resistance liquid film formed during CH₄ hydrate dissociation, thus accelerating the dissociation process, in good agreement with experimental results. HIGHLIGHTS: • The amount of CaCl₂ affects CH₄ hydrate dissociation at large depressurization pressure. • The dissociation of CH₄ hydrate at low depressurization pressure is dependent on pressure. • Ca²⁺ destroys effectively the resistance liquid film produced during hydrate dissociation. • MD simulation results are in agreement with those of the experiment.

Numerical Simulation on the Dissociation, Formation, and Recovery of Gas Hydrates on Microscale Approach

Investigations into the structures of gas hydrates, the mechanisms of formation, and dissociation with modern instruments on the experimental aspects, including Raman, X-ray, XRD, X-CT, MRI, and pore networks, and numerical analyses, including CFD, LBM, and MD, were carried out. The gas hydrate characteristics for dissociation and formation are multi-phase and multi-component complexes. Therefore, it was important to carry out a comprehensive investigation to improve the concept of mechanisms involved in microscale porous media, emphasizing micro-modeling experiments, 3D imaging, and pore network modeling. This article reviewed the studies, carried out to date, regarding conditions surrounding hydrate dissociation, hydrate formation, and hydrate recovery, especially at the pore-scale phase in numerical simulations. The purpose of visualizing pores in microscale sediments is to obtain a robust analysis to apply the gas hydrate exploitation technique. The observed parameters, including temperature, pressure, concentration, porosity, saturation rate, and permeability, etc., present an interrelationship, to achieve an accurate production process method and recovery of gas hydrates.

Enhanced CH4-CO2 Hydrate Swapping in the Presence of Low Dosage Methanol

CO2-rich gas injection into natural gas hydrate reservoirs is proposed as a carbon-neutral, novel technique to store CO2 while simultaneously producing CH4 gas from methane hydrate deposits without disturbing geological settings. This method is limited by the mass transport barrier created by hydrate film formation at the liquid–gas interface. The very low gas diffusivity through hydrate film formed at this interface causes low CO2 availability at the gas–hydrate interface, thus lowering the recovery and replacement efficiency during CH4-CO2 exchange. In a first-of-its-kind study, we have demonstrate the successful application of low dosage methanol to enhance gas storage and recovery and compare it with water and other surface-active kinetic promoters including SDS and L-methionine. Our study shows 40–80% CH4 recovery, 83–93% CO2 storage and 3–10% CH4-CO2 replacement efficiency in the presence of 5 wt% methanol, and further improvement in the swapping process due to a change in temperature from 1–4 °C is observed. We also discuss the influence of initial water saturation (30–66%), hydrate morphology (grain-coating and pore-filling) and hydrate surface area on the CH4-CO2 hydrate swapping. Very distinctive behavior in methane recovery caused by initial water saturation (above and below Swi = 0.35) and hydrate morphology is also discussed. Improved CO2 storage and methane recovery in the presence of methanol is attributed to its dual role as anti-agglomerate and thermodynamic driving force enhancer between CH4-CO2 hydrate phase boundaries when methanol is used at a low concentration (5 wt%). The findings of this study can be useful in exploring the usage of low dosage, bio-friendly, anti-agglomerate and hydrate inhibition compounds in improving CH4 recovery and storing CO2 in hydrate reservoirs without disturbing geological formation. To the best of the authors’ knowledge, this is the first experimental study to explore the novel application of an anti-agglomerate and hydrate inhibitor in low dosage to address the CO2 hydrate mass transfer barrier created at the gas–liquid interface to enhance CH4-CO2 hydrate exchange. Our study also highlights the importance of prior information about methane hydrate reservoirs, such as residual water saturation, degree of hydrate saturation and hydrate morphology, before applying the CH4-CO2 hydrate swapping technique.

Effect of Clay Content on the Mechanical Properties of Hydrate-Bearing Sediments during Hydrate Production via Depressurization

The effects of sediments with different clay contents on the mechanical properties of hydrate deposits were studied using a high-pressure, low-temperature triaxial apparatus with in-situ synthesis, as well as the mechanical properties of self-developed hydrate sediments. Through multi-stage loading, triaxial compression tests were conducted by adding quartz sand with different clay contents as the sediment skeleton, and the stress–strain relationship of the shearing process and the strength of sediments with different clay contents were determined. Volumetric changes were also observed during shearing. The results show that the strength of hydrate sediments decreases with the increasing clay content of sediments; in the processes of depressurization and shearing, the hydrate samples exhibited obvious shear shrinkage, regardless of the sediment particle size.

Hydrate Stability and Methane Recovery from Gas Hydrate through CH4–CO2 Replacement in Different Mass Transfer Scenarios

CH4–CO2 replacement is a carbon-negative, safer gas production technique to produce methane gas from natural gas hydrate reservoirs by injecting pure CO2 or other gas mixtures containing CO2. Laboratory-scale experiments show that this technique produces low methane volume and has a slow replacement rate due to the mass transfer barrier created due to impermeable CO2 hydrate layer formation, thus making the process commercially unattractive. This mass-transfer barrier can be reduced through pressure reduction techniques and chemical techniques; however, very few studies have focused on depressurization-assisted and chemical-assisted CH4–CO2 replacement to lower mass-transfer barriers and there are many unknowns. In this work, we qualitatively and quantitatively investigated the effect of the pressure reduction and presence of a hydrate promoter on mixed hydrate stability, CH4 recovery, and risk of water production during CH4–CO2 exchange. Exchange experiments were carried out using the 500 ppm sodium dodecyl sulfate (SDS) solution inside a high-pressure stirred reactor. Our results indicated that mixed hydrate stability and methane recovery depends on the degree of pressure reduction, type, and composition of injected gas. Final selection between CO2 and CO2 + N2 gas depends on the tradeoff between mixed hydrate stability pressure and methane recovery. Hydrate morphology studies suggest that production of water during the CH4–CO2 exchange is a stochastic phenomenon that is dependent on many parameters.

Experimental Study of Mixed Gas Hydrates from Gas Feed Containing CH4, CO2 and N2: Phase Equilibrium in the Presence of Excess Water and Gas Exchange

This article presents gas hydrate experimental measurements for mixtures containing methane (CH4), carbon dioxide (CO2) and nitrogen (N2) with the aim to better understand the impact of water (H2O) on the phase equilibrium. Some of these phase equilibrium experiments were carried out with a very high water-to-gas ratio that shifts the gas hydrate dissociation points to higher pressures. This is due to the significantly different solubilities of the different guest molecules in liquid H2O. A second experiment focused on CH4-CO2 exchange between the hydrate and the vapor phases at moderate pressures. The results show a high retention of CO2 in the gas hydrate phase with small pressure variations within the first hours. However, for our system containing 10.2 g of H2O full conversion of the CH4 hydrate grains to CO2 hydrate is estimated to require 40 days. This delay is attributed to the shrinking core effect, where initially an outer layer of CO2-rich hydrate is formed that effectively slows down the further gas exchange between the vapor phase and the inner core of the CH4-rich hydrate grain.

Computational study of the interplay between intermolecular interactions and CO2 orientations in type I hydrates

Carbon dioxide (CO₂) molecules show a rich orientation landscape when they are enclathrated in type I hydrates. Previous studies have described experimentally their preferential orientations, and some theoretical works have explained, but only partially, these experimental results. In the present paper, we use classical molecular dynamics and electronic density functional theory to advance in the theoretical description of CO₂ orientations within type I hydrates. Our results are fully compatible with those previously reported, both theoretical and experimental, the geometric shape of the cavities in hydrate being, and therefore, the steric constraints, responsible for some (but not all) preferential angles. In addition, our calculations also show that guest-guest interactions in neighbouring cages are a key factor to explain the remaining experimental angles. Besides the implication concerning equation of state hydrate modeling approximations, the conclusion is that these guest-guest interactions should not be neglected, contrary to the usual practice.

Direct phase coexistence molecular dynamics study of the phase equilibria of the ternary methane–carbon dioxide–water hydrate system

Grown mixed carbon dioxide–methane hydrate with molecular dynamics.

Carbon dioxide-induced liberation of methane from laboratory-formed methane hydrates

This paper reports a laboratory mimic study that focused on the extraction of methane (CH4) from hydrates coupled with sequestration of carbon dioxide (CO2) as hydrates, by taking advantage of preferential thermodynamic stability of hydrates of CO2 over CH4. Five hydrate formation-decomposition runs focused on CH4–CO2 exchange, two baselines and three with host sediments, were performed in a 200 mL high-pressure Jerguson cell fitted with two glass windows that allowed visualization of the time-resolved hydrate phenomenon. The baseline pure hydrates formed from artificial seawater (75 mL) under 6400–6600 kPa CH4 or 2800–3200 kPa CO2 (hydrate forming regime), when the bath temperature was maintained within 4–6 °C and the gas/liquid volumetric ratio was ∼1.7:1 in the water-excess systems. The data show that the induction time for hydrate appearance was largest at 96 h with CH4, while with CO2 the time shortened by a factor of four. However, when the secondary gas (CO2 or CH4) was injected into the system containing preformed hydrates, the entering gas formed the hydrate phase instantly (within minutes) and no lag was observed. In a system containing host Ottawa sand (104 g) and artificial seawater (38 mL), the induction period reduced to 24 h. In runs with multiple charges, the extent of hydrate formation reached 44% of the theoretical value in the water-excess system, whereas the value maximized at 23% in the gas-excess system. The CO2 hydrate formation in a system that already contained CH4 hydrates was facile and they remained stable, whereas CH4 hydrate formation in a system consisting of CO2 hydrates as hosts were initially stable, but CH4 gas in hydrates quickly exchanged with free CO2 gas to form more stable CO2 hydrates. In all five runs, even though the system was depressurized, left for over a week at room temperature, and flushed with nitrogen gas in between runs, hydrates exhibited the “memory effect”, irrespective of the gas used, a result in contradiction with that reported previously in the literature. The facile CH4–CO2 exchange observed under temperature and pressure conditions that mimic naturally occurring CH4 hydrates show promise to develop a commercial carbon sequestration system.

DFT calculation of the potential energy landscape topology and Raman spectra of type I CH4and CO2hydrates

Type I methane and carbon dioxide hydrates analyzed using DFT calculations: Raman spectra and potential energy landscape.

Quantitative measurement and mechanisms for CH4 production from hydrates with the injection of liquid CO2

Illustration of the potential mechanisms for CH₄ production based on “exchange” and “no exchange” between CO₂ and CH₄ hydrates.