Natural Methane Hydrate Research Articles

Reactive transport modeling of organic carbon degradation in marine methane hydrate systems

Natural methane hydrate has often been observed in sand layers that contain no particulate organic carbon (POC), but are surrounded by organic-rich, fine-grained marine muds. In this paper, we develop a reactive transport model (RTM) of a microbially-mediated set of POC degradation reactions, including hydrolysis of POC driven by extracellular enzymes, fermentation of the resulting high-molecular weight dissolved organic carbon (HMW-DOC), and methanogenesis that consumes low-molecular weight dissolved organic carbon (LMW-DOC). These processes are mediated by two groups of microbes, fermenters and methanogens that are heterogeneously distributed in different lithologies, with the largest numbers of microbes in the large pores of coarse-grained layers. We find that the RTM can reproduce methane hydrate occurrences observed in two different geological environments, at Walker Ridge Site 313-H (Gulf of Mexico) and IODP Site U1325 (Cascadia Margin). We also find that microbes can degrade POC even if they are physically separated, as extracellular enzymes and DOC can diffuse away from where they are produced by microbes. Microbial activity is highest at relatively early times after burial at shallow depths and near lithological boundaries, where concentration gradients transport solutes to intervals that contain the most microbes.

Multi-phase retrieval of methane hydrate in natural sediments by cryogenic x-ray computed tomography.

In this study, we observed natural methane (CH4) hydrate sediments, which are a type of unconventional natural gas resources, using x-ray computed tomography (CT). Because CH4 hydrates are formed by hydrogen bonding of water molecules with CH4, material decomposition becomes challenging when CH4 hydrates coexist with liquid or solid water in natural sediments. Tri-contrast (absorption, refraction, and scattering) imaging was performed via diffraction enhanced x-ray CT optics using monochromatic synchrotron x rays. The quantitative characterization of the contrast changes successfully enabled the decomposition of CH4 hydrates coexisting with frozen seawater (ice) in natural sediments obtained from the Okhotsk Sea. This study reveals complementary structural information about the microtexture and spatial relation among CH4 hydrates, ice, and pores by utilizing the distinct physical properties of x rays when passing through the materials. These results highlight the exceptional capabilities of high-resolution multicontrast x-ray tomography in materials science and geoscience applications.

Heterogeneous catalysis of methane hydroxylation with nearly total selectivity under mild conditions.

The efficient utilization of methane, a vital component of natural gas, shale gas and methane hydrate, holds significant importance for global energy security and environmental sustainability. However, converting methane into value-added oxygenates presents a formidable challenge due to its inert nature. Direct selective oxidation of methane (DSOM) under mild conditions is essential for reducing energy consumption and carbon emissions compared with traditional indirect routes. Achieving total selectivity in methane hydroxylation remains elusive due to the competitive CO2 formation. This feature article highlights recent advancements in methane hydroxylation using thermo-, photo-, and electro-catalytic systems. Through strategically designing the structure of catalysts to control the reactive oxygen species and optimizing reaction parameters, significant progress has been made in enhancing oxygenate selectivity and minimizing overoxidation. A comprehensive understanding of the mechanisms underlying methane hydroxylation with total selectivity offers insights for improving catalyst design and reaction parameter optimization, promoting sustainable methane utilization.

Microstructural Study on Dissolution of Natural Methane Hydrate by Multicontrast and Multiscale X-ray Computed Tomography

Microstructural Study on Dissolution of Natural Methane Hydrate by Multicontrast and Multiscale X-ray Computed Tomography

Synergistic Effect of Hyperactive Antifreeze Protein on Inhibition of Gas-Hydrate Growth by Hydrophobic and Hydrophilic Groups.

Antifreeze proteins (AFPs) are biodegradable inhibitors that effectively prevent the formation of natural gas hydrates that block pipelines. In this study, molecular dynamics simulations were employed to establish a kinetic model of the hyperactive insect antifreeze protein (Tenebrio molitor, TmAFP) and its mutants to inhibit the growth of sI natural methane hydrate. Simulations revealed that the hydrophobic and hydrophilic groups of threonine (Thr) residues at hydrate-binding sites played a synergistic role in binding hydrates. The hydrophobic groups anchored TmAFP to the hydrate surface through residues Thr39-Thr65 by migrating pendant hydrophobic methyl groups to the hydrate semicages. The hydrophilic groups stabilized TmAFP by hydrogen bonding with water molecules and integrating them into a quasi-hydrate structure, which more effectively inhibited hydrate growth. The results suggest that the hydrate growth inhibition is attributed to both the shape complementarity and the flexibility of binding residues. The synergy between hydrophobic and hydrophilic groups provides guidance for the design of more effective hydrate inhibitors.

Historical perspectives on gas hydrates and citation impact analysis

AbstractGas hydrates, or clathrate hydrates, is a multidisciplinary field of research involving potential applications to the oil and gas industry, energy security, and innovative technological applications with literature referenced back to 1810. The field of gas hydrates or clathrate hydrates has progressed over the past several decades from academic curiosity‐driven research to industrially relevant research related to flow assurance and methane hydrates in nature as an energy resource. In the recent few decades, several innovative and sustainable applications have emerged with gas hydrate or clathrate hydrate as a technology enabler. In this work, I present a bibliometric analysis of the field of gas hydrates or clathrate hydrates for the period from 1901 to 2020 from the Web of Science core collection database of Clarivate Analytics. In total, 12 152 journal publications (review and original research articles) were analyzed from Web of Science core collection database spanning 121 years (1901–2020). Top countries, top cited review articles, and original research articles along with top source titles (journals) are identified and highlighted. In addition, the field classifications and citation rate trends have been analyzed and presented. Network visualization maps are presented for countries, sources, and organizations by analysing citations in VOSviewer. Co‐occurrence analysis is performed to identify the top keywords and their links through network visualization based on VOSviewer.

Application of Methane Hydrate Critical State Soil Model on Multistage Triaxial Tests of Methane Hydrate-Bearing Sediment

Natural methane hydrate, as a potential alternative energy source to fossil energy in the 21st century, is found in abundance in deep-water sediments and permafrost regions. During the production of methane gas from these sediments, the dissociation process may induce various changes to the geotechnical properties. Thus, it is important to study the geomechanical behavior of hydrate sediments and simulate the sediment deformation patterns. In this study, the effects of different sediment types on the mechanical properties of hydrate-bearing soils were investigated using multistage triaxial tests, and the methane hydrate critical state (MHCS) soil model was calibrated using an optimization-based technique. The results revealed that: (1) the strain-hardening phenomenon in methane hydrate-bearing sediments varies with the particle sizes of the host sediments; (2) the strengths of methane hydrate-bearing sediments are higher with the host sediments being pure sand, compared with those with clay-added sediments, and the strength is inversely proportional to the clay contents in these sediments; and (3) the multistage triaxial test data were fitted with the MHCS model, which illustrates the combined effects of the abovementioned factors on the model parameters and geomechanical behavior of methane hydrate-bearing sediments.

Effects of depressurization on gas production and water performance from excess-gas and excess-water methane hydrate accumulations

• Various water saturations simulate Classes I and II hydrate deposits in nature. • Ambient temperature controls periphery dissociation in excess-gas hydrate deposits. • Sensible heat dominates spatially uniform dissociation in excess-water deposits. • Secondary hydrate formation are prevented by adjusted gas production pressure. • Optimized depressurization is crucial to improve water production and CH 4 recovery. Depressurization is considered as the most promising technique for hydrate exploitation, as it achieves the highest energy profit ratio and is the most technologically feasible. However, the exploitation of excess-water hydrate accumulations generates high water production, leading to increased cost, poor energy efficiency, and problems with sand during operation. Thus, water management is crucial to gas recovery by the depressurization of different classes of hydrate accumulations, yet relevant studies remain limited. In this study, synthetic hydrate samples were prepared to simulate two types of natural methane hydrate sediments: Class 1 accumulations (excess-gas hydrate) and Class 2 accumulations (excess-water hydrate). Hydrate dissociation was conducted using a variety of depressurization approaches, and MRI imaging was employed to characterize water performance and methane recovery. Methane hydrate preferentially dissociated along the peripheries of the excess-gas samples due to more efficient heat dissipation. Methane hydrate dissociated more uniformly in the excess-water samples because the high specific heat capacity of water enabled the supply of extra heat. Furthermore, pressure histories, mean intensity change in MRI images, and water variations were monitored to analyze the characteristics of hydrate dissociation, changes in porosity, intrinsic permeability and reservoir heat, water and gas production rates, and possible secondary hydrate formation. The results of this study suggest that an optimized depressurization approach, such as stepwise depressurization, could improve methane recovery from Class 1 and Class 2 methane hydrate accumulations.

Key Areas of Gas Hydrates Study: Review

Gas hydrates are widespread all over the world. They feature high energy density and are a clean energy source of great potential. The paper considers experimental and theoretical studies on gas hydrates in the following key areas: formation and dissociation, extraction and transportation technologies of natural methane hydrates, and ignition, and combustion. We identified a lack of research in more areas and defined prospects of further development of gas hydrates as a promising strategic resource. One of the immediate problems is that there are no research findings for the effect of sediments and their matrices on hydrate saturation, as well as on gas hydrate formation and dissociation rates. No mathematical models describe the dissociation of gas hydrates under various conditions. There is a lack of research into the renewal and improvement of existing technologies for the easier and cheaper production of gas hydrates and the extraction of natural gas from them. There are no models of gas hydrate ignition taking into account dissociation processes and the self-preservation effect.

Microscopic molecular insights into clathrate methane hydrates dissociation in a flowing system

Natural methane hydrates are estimated to be the largest source of unexploited hydrocarbon fuel. Understanding the dissociation of methane hydrates in the flow system is vital for natural gas development, especially for the gas hydrate reservoir protection and the prevention of secondary hydrate generation in the wellbore. In this work, the model of natural gas hydrate in clay mineral pores was established, and the effects of drilling fluid on hydrate stability under different intrusion velocities were investigated by molecular dynamics simulation. The simulation results show that the hydrate nucleus on hydrophilic surfaces will be more susceptible to temperature. As the temperature increases, the hydrate nuclei at the edges and corners collapse preferentially due to the large contact area. Water molecules on hydrophilic surfaces are difficult to participate in the formation of hydrates due to hydrogen bonds. Furthermore, the higher the velocity of the fluid, the greater the degree of hydrate dissociation. And the hydrate nucleus decomposes faster in the vertical flow direction than that in the parallel flow direction. The destruction of hydrate formation and decomposition equilibrium is the main reason for hydrate decomposition in the flow process. These results contribute to a better understanding of hydrate decomposition in a flowing system and implications for developing new materials for high-performance hydrate drilling fluids.

The promoting effect and mechanisms of oxygen-containing groups on the enhanced formation of methane hydrate for gas storage

The sluggish formation kinetics is a formidable challenge for the practical application of gas hydrate-based technologies for natural gas storage and transportation. Methane hydrates in both nature and labs exclusively form with the assistant of foreign additives via a heterogeneous process. Although the process is common and widely used, it remains unclear what makes a material a good methane hydrate promoter. Additionally, the molecular mechanisms underlying the promoted hydrate formation remain largely unknown. Herein, carbon monoliths (labeled as CBCM) with finely controlled surface functional groups were produced using cellulose as precursors. The oxygen-containing groups have been identified as the active sites enhancing the nucleating ability of methane hydrates. Carbonyl oxygen is pinned down as the most effective functional group in reducing the induction time and enhancing the formation kinetics of methane hydrate. The turned hydrogen bonds between water molecules, which are close to the surface of CBCM, contribute to the enhanced formation kinetics as confirmed by the Raman spectroscopy. The CBCM with optimized carbonyl oxygen significantly improve the methane hydrate formation kinetics and storage capacity with outstanding cycle stability, which are superior than most of the previously reported promoters, particularly in shorter induction time. The finding in this work paves the way for effectively designing promoters and unraveling the underlying mechanism for enhancing gas hydrate formation.

Analytical review of technologies of the industrial development of aquatic methanohydrates

Methane hydrates are one of the most powerful reserves of unconventional sources of hydrocarbons. This is clearly evidenced by the forecast estimates of world volumes of methane in the form of gas hydrates, which many times exceed the total resources of traditional natural gas. In the foreseeable future, natural methane hydrates should significantly increase the current energy balance of natural hydrocarbon fuel resources. Progress in their study can be ensured by the dialectical unity of theoretical and experimental research, focused mainly on thermodynamics, kinetics and their physical properties, as well as on the development and testing of technologies for methane production from gas hydrate deposits. Existing methods of developing gas hydrates involve their preliminary dissociation into gas and water. At the same time, the deposit depressurization method is considered the most promising. However, there is still no commercially attractive technology for the development of gas hydrates. The article presents an overview of gas hydrate research in the world, provides an analysis of prospective methods of their development, summarizes the advantages and disadvantages of current research and industrial attempts to extract methane from aqua deposits of gas hydrates, and evaluates the prospects of various technologies. Currently known examples of research and industrial development of gas hydrate deposits have demonstrated a number of problems. However, encouraging results were obtained. The analysis of processes in the oil and gas production industry shows that profitable industrial production of natural gas from gas hydrate deposits will be possible after an effective breakthrough technology appears on the market.

Improving the efficiency of storage of natural and artificial methane hydrates

Methane gas hydrate is the largest hydrocarbon reservoir of natural low-carbon energy systems. Methane hydrates are a promising environmentally friendly energy source. Currently, there is a great many of experimental data on the dissociation of natural and artificial methane hydrate at negative temperatures. However, the physical mechanisms of dissociation and their description remain unclear even today, which hinders the development of these technologies. Dissociation of methane hydrates in a wide temperature range, including the self-preservation region, is studied experimentally. In this paper, experimental and theoretical studies into the natural and artificial methane hydrate dissociation are conducted. The natural gas hydrate exhibits a lower dissociation rate compared to the artificial methane hydrate. Since it is quite difficult to influence the strength of the ice crust, it is advisable to produce methane hydrate at large sphere diameters in order to reduce the dissociation rate (in the annealing temperature window). The results obtained may be useful for improving the technology of storage and transportation of natural and artificial methane hydrate.

Influence of Large Molecular Guest Species on the Formation of CH <sub>4</sub> and CO <sub>2</sub> Hydrates

Methane hydrates in nature are considered as a future energy resource for natural gas due to their abundance in nature. The presence of all three structures of clathrate hydrates in the nature have been reported. Structural transition in clathrate hydrates in particular with regards to sH hydrates and the influence of large guest molecules is still not well understood. In the current work, we synthesize mixed hydrate of CO2 & CH4 mixture in presence of sH formers, neo-hexane (NH) and tert-butylmethylether (TBME). High-pressure in-situ differential scanning calorimetry was employed to investigate the phase transformation between soild-liquid-gas. We confirmed that CO2 lean mixture (24% CO2 & 76% CH4) forms structure H (sH) hydrate, while CO2 rich mixture (76% CO2 & 24% CH4) forms structure I (sI) hydrate. This study presents new insights to understand the dynamic preference of methane and carbon dioxide in three different types of cages in sH hydrate.

Does Confinement Enable Methane Hydrate Growth at Low Pressures? Insights from Molecular Dynamics Simulations.

Natural methane hydrates are estimated to be the largest source of unexploited hydrocarbon fuel. The ideal conditions for methane hydrate formation are low temperatures and high pressures. On the other hand, recent experimental studies suggest that porous materials, thanks to their confinement effects, can enable methane hydrate formation at milder conditions, although there has not been a consensus on this. A number of studies have investigated methane hydrate growth in confinement by employing molecular simulations; however, these were carried out at either very high pressures or very low temperatures. Therefore, the effects of confinement on methane hydrate growth at milder conditions have not yet been elaborated by molecular simulations. In order to address this, we carried out a systematic study by performing molecular dynamics (MD) simulations of methane water systems. Using a direct phase coexistence approach, microsecond-scale MD simulations in the isobaric–isothermal (NPT) ensemble were performed in order to study the behavior of methane hydrates in the bulk and in confined nanospaces of hydroxylated silica pores at external pressures ranging from 1 to 100 bar and a simulation temperature corresponding to a 2 °C experimental temperature. We validated the combination of the TIP4P/ice water and TraPPE-UA methane models in order to correctly predict the behavior of methane hydrates in accordance to their phase equilibria. We also demonstrated that the dispersion corrections applied to short-range interactions lead to artificially induced hydrate growth. We observed that in the confinement of a hydroxylated silica pore, a convex-shaped methane nanobuble forms, and methane hydrate growth primarily takes place in the center of the pore rather than the surfaces where a thin water layer exists. Most importantly, our study showed that in the nanopores methane hydrate growth can indeed take place at pressures which would be too low for the growth of methane hydrates in the bulk.

Experimental study of the dissociation of natural methane hydrate in the channel in the presence of air flow

Experimental studies have been carried out on the combustion of natural gas hydrate at different rates of oxidant U (air) in the laminar flow regime and at a fixed channel height. It is established that the dissociation rate J is related to the velocity U in a non-linear manner. With an increase in rate from 0 to 0.6 m/s, the dissociation rate increases and reaches a maximum. With a further increase in rate, J decreases. The dissociation rate does not depend on the initial concentration of methane in the sample. Experimental dependences of the effect of U on the dissociation rate of a natural hydrate in a channel have been obtained for the first time. The conducted studies are important for further development of natural hydrate combustion technologies.

Non-stationary combustion of natural and artificial methane hydrate at heterogeneous dissociation

Dissociation of natural and artificial methane hydrate at combustion was studied experimentally. Thermal imaging and Particle Tracking Velocimetry (PTV) methods were used to analyze the temperature field and gas velocity. The laminar air flow rate varied from 0 to 1.8 m/s. Previously, when modeling the combustion of gas hydrate, a simplified model for a standard laminar velocity profile had been considered. Expectedly, in the combustion region (in the vicinity of the wall) the gas flow velocity was much lower than that at the outer surface of the boundary layer, and calculations showed the presence of the stoichiometric ratio line. The presence of this line in the area of combustion was thought to result in high flame spread speed. The use of the Particle Tracking Velocimetry method and the experimental values of the methane injection rate have led to an unexpected result. High air velocities (maximum of 3–5 m/s) due to free convection have been found in the combustion region. At that in the combustion region, there is a significant excess of oxidant (oxygen) and the mole fraction of CH4 is 10–30 times lower than necessary for the maximum reaction rate. It is shown that high combustion rates (high flame spread speed) may be associated with extremely inhomogeneous dissociation inside the powder and inhomogeneous rate of methane injection over the layer surface. At combustion, “mushroom-like" pulsating flame and methane “bubbles” are formed. The frequency of pulsations increases with the growth of external flow rate. The influence of the initial concentration of methane in natural hydrate on its combustion has been considered. The studies prove that the description of the combustion process must take into account the free convection and heterogeneity of dissociation of methane hydrate in the powder volume.

Numerical analysis of gas production from layered methane hydrate reservoirs by depressurization

Many natural methane hydrate (MH) reservoirs are heterogeneous and are characterized by a layered structure. In this study, we numerically investigate gas production from a multi-layered hydrate reservoir by depressurization through a single vertical and a single horizontal well. This layered MH reservoir is constructed based on field test data at the AT1 site of the Eastern Nankai Trough in Japan, and involves three hydrate-bearing layers (HBLs): Upper HBL-1, middle HBL-2, and lower HBL-3. The simulation results indicate that the horizontal well shows a better gas production performance in comparison to the vertical well. Over a production duration of two years, the average gas production rate by using the horizontal well reached 7.3 × 104 ST m3/d, which is 5.7 times higher than that by using the vertical well. However, the gas-to-water ratio for both the vertical and horizontal well cases is low in absolute terms. Sensitivity analysis of gas production by the horizontal well indicates that both the very higher and lower levels of permeability in HBL-2 and hydrate saturation in HBL-3 are unfavorable for long-term gas production. In addition, decrease of vertical permeability in HBL-1 and HBL-3 can lead to lower gas production efficiency.

Surface area controls gas hydrate dissociation kinetics in porous media

This study integrates X-ray radiography and 3D computed tomography (CT) to measure xenon hydrate dissociation kinetics in a porous medium (sand) through depressurization at the quasi-isothermal condition. Before dissociation, we use X-ray CT to measure hydrate volume and surface area in the sand and find [hydrate volume]2/3 is a linear approximation of hydrate surface area regardless of differences in hydrate specific area. During dissociation, we use X-ray radiography to quantify the evolutions of hydrate volume and dissociation rate. We find [hydrate volume]2/3 (i.e., hydrate surface area) is a linear metric of hydrate dissociation rate. Additionally we observe two circumstances when dissociate rate is not linear with [hydrate volume]2/3. First, above a critical hydrate saturation of 35–45%, hydrate dissociation rate initially stays constant and then decreases with [volume]2/3. Second, the dissociation rates of patchy hydrates in large pores initially stay constant and are significantly slower than hydrates in small pores with the same sizes. Since methane hydrates in nature are patchy and heterogeneously distributed, the new knowledge will facilitate modeling of hydrates production with a simple metric of dissociation rate (i.e., [volume]2/3) and constraints of hydrate saturation and pore size.

Pore‐Scale Visualization of Methane Hydrate‐Bearing Sediments With Micro‐CT

AbstractX‐ray computed tomography (CT) has become a critical technique in the study of porous media. It has attracted growing attention for analyzing hydrate‐bearing sediment, but this has been done using surrogates (Xe/Kr) only due to difficulties in distinguishing methane hydrate from water. This study presents the successful imaging of methane hydrate coexisting with pore liquid, gas, and sediments. We used potassium iodide (KI) solutions and in‐line propagation‐based phase‐contrast CT analysis of X‐ray attenuation and diffraction to distinguish the four materials. Thus, consideration for CT‐related X‐ray physics was necessary to optimize KI concentrations, improve material separation with X‐ray propagation, and properly interpret artifacts within the images. The images clearly show methane hydrate in the pore space of sand (~250 μm) coexisting with KI solution and gas. Following this, X‐ray CT can now be used to visualize pore habits of natural methane hydrate in sediment cores.