A Review on Research on Replacement of CH4 in Natural Gas Hydrates by Use of CO2

Microscopic Insights and Optimization of the CH4-CO2 Replacement in Natural Gas Hydrates

Using the CO2 replacement method to exploit natural gas hydrates and store CO2 has great significance in energy access and environmental protection. Herein, the molecular dynamic method is utilized to analyze and evaluate the CH4-CO2 replacement at different constant temperatures and pressures. For optimization, various temperature oscillations are introduced in the CH4-CO2 replacement. It illustrates that increasing the temperature can improve the amounts of CH4 escape and CO2 capture but is unfavorable to the long-term CO2 storage and hydrate stability. The effects of pressure are not as significant and definite as those of temperature. Appropriate temperature oscillations can achieve comprehensive improvements, which benefit from both the deep diffusion of CO2 in the higher temperature stage and the rapid rebuilding of CO2 hydrate within just nanoseconds caused by the memory effects in the lower temperature stage. The results also reveal that the optimal lower temperature duration and frequency should be moderate. Decreasing the lower temperature value can distinctly enhance CO2 capture and hydrate stability. This study can help understand the mechanisms of CH4-CO2 replacement under different temperature and pressure conditions, especially at temperature transitions, and proposes a potentially effective method to achieve large-scale carbon sequestration in the hydrate.

Microsecond molecular dynamics of methane-carbon dioxide swapping in pure and saline water environment

This work aims at proposing the nondestructive methane-carbon dioxide (CH₄–CO₂) replacement mechanism as an ecofriendly energy production technique from the natural gas hydrate reserves in seafloor and permanently frozen grounds. Although the experimental data is widely available in literature, this replacement mechanism has not been elucidated at molecular level. In this contribution, we perform the microsecond level molecular dynamic simulations to evaluate two different CH₄–CO₂ replacement mechanisms: (i) direct CH₄ displacement from hydrate structure, and (ii) dissociation of existing methane hydrate followed by a reformation of mixed CH₄–CO₂ hydrates. For this, we analyze CH₄–CO₂ replacement in three different modes i.e., CO₂ as a replacing agent in (i) absence of free water molecules, (ii) presence of free water molecules, and (iii) presence of salt ions and free water molecules. Despite slow kinetics in the first mode, pure CO₂ is observed to replace the methane more efficiently, while in the second mode, CO₂ forms a new mixed hydrate layer on the existing seed crystal. However, in the third mode, salt ions help in destabilizing the methane hydrate and allow CO₂ to form the hydrates. This proves that salt ions are favorable for CH₄–CO₂ replacement.

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.

Mathematical Model of Carbon Dioxide Injection into a Porous Reservoir Saturated with Methane and Its Gas Hydrate

In this paper, the process of methane replacement in gas hydrate with carbon dioxide during CO2 injection into a porous medium is studied. A model that takes into account both the heat and mass transfer in a porous medium and the diffusion kinetics of the replacement process is constructed. The influences of the diffusion coefficient, the permeability and extent of a reservoir on the time of full gas replacement in the hydrate are analyzed. It was established that at high values of the diffusion coefficient in hydrate, low values of the reservoir permeability, and with the growth of the reservoir length, the process of the CH4-CO2 replacement in CH4 hydrate will take place in the frontal regime and be limited, generally, by the filtration mass transfer. Otherwise, the replacement will limited by the diffusion of gas in the hydrate.

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.

Selective trapping of CO2 gas and cage occupancy in CO2-N2 and CO2-CO mixed gas hydrates

Hydrate-based CO2 trapping from CO2-N2 and CO2-CO gas mixtures is shown by Raman spectroscopy - the results are of interest for new separation and capture technology. A better trapping efficiency is measured for low CO2 concentrations and N2-based gas mixtures. Moreover, it is observed that CO molecules would impede hydrate formation from ice when a CO-enriched gas mixture is considered.

Formation of a nanobubble and its effect on the structural ordering of water in a CH4-N2-CO2-H2O mixture

The replacement of methane (CH4) from its hydrate by a mixture of nitrogen (N2) and carbon dioxide (CO2) involves the dissociation of methane hydrate leading to the formation of a CH4-N2-CO2-H2O mixture that can significantly influence the subsequent steps of the replacement process. In the present work, we study the evolution of dissolved gas molecules in this mixture by applying classical molecular dynamics simulations. Our study shows that a higher CO2 : N2 ratio in the mixture enhances the formation of nanobubbles composed of N2, CH4 and CO2 molecules. To understand how the CO2 : N2 ratio affects nanobubble nucleation, the distribution of molecules in the bubble formed is examined. It is observed that unlike N2 and CH4, the density of CO2 in the bubble reaches a maximum at the surface of the bubble. The accumulation of CO2 molecules at the surface makes the bubble more stable by decreasing the excess pressure inside the bubble as well as surface tension at its interface with water. It is found that a frequent exchange of gas molecules takes place between the bubble and the surrounding liquid and an increase in concentration of CO2 in the mixture leads to a decrease in the number of such exchanges. The effect of nanobubbles on the structural ordering of water molecules is examined by determining the number of water rings formed per unit volume in the mixture. The role of nanobubbles in water structuring is correlated to the dynamic nature of the bubble arising from the exchange of gas molecules between the bubble and the liquid.

Paving the way for methane hydrate formation on metal-organic frameworks (MOFs)

The presence of a highly tunable porous structure and surface chemistry makes metal-organic framework (MOF) materials excellent candidates for artificial methane hydrate formation under mild temperature and pressure conditions (2 °C and 3-5 MPa). Experimental results using MOFs with a different pore structure and chemical nature (MIL-100 (Fe) and ZIF-8) clearly show that the water-framework interactions play a crucial role in defining the extent and nature of the gas hydrates formed. Whereas the hydrophobic MOF promotes methane hydrate formation with a high yield, the hydrophilic one does not. The formation of these methane hydrates on MOFs has been identified for the first time using inelastic neutron scattering (INS) and synchrotron X-ray powder diffraction (SXRPD). The results described in this work pave the way towards the design of new MOF structures able to promote artificial methane hydrate formation upon request (confined or non-confined) and under milder conditions than in nature.

Carbon dioxide induced bubble formation in a CH4–CO2–H2O ternary system: a molecular dynamics simulation study

The role of carbon dioxide in the formation of gas bubbles in a CH₄–CO₂–H₂O ternary system is studied using molecular dynamics simulations.