The content and distribution of pore water affects hydrate dissociation, gas migration and gas deliverability in the process of natural gas hydrate exploitation. Understanding the evolution characteristics of pore water during the hydrate dissociation is important for extracting natural gas from hydrate-bearing sediments (HBSs). In this work, methane hydrate formation and dissociation experiments were completed in unconsolidated sand with different initial water contents by using a self-designed low-field nuclear magnetic resonance monitoring apparatus. The evolution laws of water content were revealed during the process of methane hydrate dissociation by depressurization. The distribution characteristics of water were analyzed in the water-rich and the gas-rich sediments. Results show that the unclathrated water content is affected by mineral surface in gas-rich HBSs. It is affected by hydrate barrier effects in water-rich HBSs. The hydrate distribution is controlled by the initial water distribution in gas-rich HBSs. It is homogeneous in water-rich HBSs. The pore water content first decreases and then increases during the hydrate dissociation by depressurization in water-rich HBSs, while the pore water content keeps increasing in gas-rich HBSs. Compared with gas-rich HBSs, the migration efficiency of substances is lower,and the hydrate dissociation rate is slower in water-rich HBSs. The increase of water relative permeability and the decrease of pore temperature cause the reduction in growth rate of pore water content in the middle stage of hydrate dissociation by depressurization. For relatively closed HBSs, the pore water distribution after the hydrate exploitation by depressurization is similar to that before the hydrate formation.

The microscopic formation mechanism of methane hydrate is crucial for the sustainable exploitation and carbonneutral storage of natural gas hydrates. In our study, a novel Raman spectroscopy in situ method was developed to monitor dissolved CH4 concentration during hydrate formation process. Real-time Raman spectral information of the gas-hydrate-liquid three-phase system at different longitudinal positions was obtained under constant pressure. The formation mechanisms of methane hydrate were revealed at microscale. Results demonstrated that before hydrate formation, methane concentration in the liquid phase adjacent to the gas-liquid interface was 6.22 % higher than the theoretical solubility. After the hydrate film formation, the dissolved methane concentration decreased sharply to the critical level (0.07713 +/- 0.00209 mol/kg, 38.49 % below theoretical solubility within the scope of the research). In addition, small cages of hydrate were formed preferentially, but large cages exhibited higher thermodynamic stability. The dissolved methane molecules in the deeper liquid phase mainly contributed to the early hydrate formation, while those involved in the later hydrate formation mainly came from the methane gas above the hydrate film. These findings provide a new perspective and experimental evidence for understanding and verifying the mechanism of hydrate nucleation and growth at the interface, and provide engineering insights for hydrate-based methane sequestration and supports the development of integrated energy systems in marine environments.

The ocean represents a vast carbon reservoir, where methane concentrations reflect a balance between production and consumption processes. In this study, marine sediments harboring abundant aerobic methanotrophs were incubated under varying methane concentrations. We investigated changes in methane oxidation rates and the community structure of these methanotrophs during incubation. Results demonstrate that methane concentration regulates aerobic methane oxidation. The oxidation rate increased with rising methane concentration up to 50%. However, beyond this threshold, at 80% methane, the oxidation rate declined. Following incubation, the abundance of aerobic methanotrophs increased significantly, with Methylobacterium becoming the dominant genus. Relative abundances of other bacteria potentially associated with alkane degradation, including Planomicrobium, Bacillus, Methylophaga, Erythrobacter, Gillisia and Pontibacter, were also significantly enriched. These findings provide a scientific basis for predicting marine methane emission dynamics, which is crucial for assessing and mitigating the greenhouse gas impact of methane.

Hydrate-based carbon dioxide (CO2) sequestration is a promising pathway for long-term carbon storage, but its efficiency is governed by pore-scale processes that remain poorly understood. In this study, a self-developed low-field nuclear magnetic resonance (NMR) system was used to monitor transverse relaxation time (T2) during CO2 hydrate formation under both gaseous and liquid CO2 conditions in sediments with different grain size distributions, initial water saturations, and clay contents. A fractal dimension-based method was developed to characterize the spatiotemporal evolution of pore water and to quantify its relationship with hydrate conversion rate. Results show that CO2 hydrate preferentially formed in large pores, progressively decreasing the water signal in macropores and increasing the relative contribution of smaller pores. Hydrate conversion rate decreased with increasing grain size and initial water saturation, while clay minerals markedly inhibited hydrate formation by reducing free water availability and restricting CO2 transport. Liquid CO2 showed faster hydrate formation and higher conversion rate than gaseous CO2, especially in clay silty sediments, where hydrate formation under gaseous CO2 conditions was negligible. The fractal dimension increased continuously during hydrate growth and exhibited a clear power-law relationship with hydrate conversion rate, with rapid growth at the early stage followed by stabilization at a conversion of approximately 40–70%. These findings provide new insights into the pore-scale dynamics of CO2 hydrate formation and offer a quantitative basis for reservoir evaluation, site selection, and efficiency optimization in hydrate-based CO2 sequestration.

Understanding water-to-hydrate conversion behavior in marine sediments is critical for evaluating natural gas hydrate resources and optimizing CO2 sequestration strategies. This study enabled real-time in situ monitoring of CH4 and CO2 hydrate formation in sandy sediments using a self-developed low-field nuclear magnetic resonance system, enriching knowledge in sediment-hosted hydrate kinetics. Effects of guest molecule type, initial water saturation (28.96-79.24 %), and sediment particle size (20-600 mu m) on water-to-hydrate conversion behavior were quantitatively analyzed. Results show that CO2 hydrates occupy both macro- (>100 mu m) and micropores (<10 mu m) due to the high aqueous solubility of CO2 under experimental conditions (4.0 MPa, 274.65 K). In contrast, CH4 hydrates predominantly form in macropores, constrained by lower gas concentration. CO2 achieves 1.75-78.25 h faster conversion rates (t(90)) and 1.56-4.47 % higher final conversion percentages than CH4, driven by greater supercooling (7.5 degrees C vs. 6.0 degrees C), higher aqueous solubility, and enhanced mass transfer through partial liquefaction. Particle size <= 150 mu m accelerates conversion rates by 2.57-27.88 times through increased specific surface area (similar to 36 times higher in 20-40 mu m vs. 400-600 mu m sediments) and capillary-driven water migration. High initial water saturation (79.24 % and 77.69 %) triggers abrupt percentage losses (CH4: similar to 95 % to 65.75 %, CO2: similar to 98 % to 91.97 %) due to hydrate film termination and pore blockage effects. MRI data demonstrate asymmetric water redistribution during gas injection alters local conversion rates, explaining anomalous kinetics in mid-saturation CH4 systems. This work provides reliable data support and mechanism insights for gas hydrate resource assessment, CO2-CH4 exchange technology optimization, and subsea CO2 storage in heterogeneous marine reservoirs.

The synergistic production of gas hydrates and deep gas reservoirs assisted by the geothermal energy transport via circulating water is significantly influenced by the temporal coordination of reservoir production schedules. However, the correlation between production timing and system performance remains unexplored. This work has for the first time systematically demonstrated the decisive role of choosing the production timing in determining the production efficiency. Four combined extraction methods for the co-operation of hydrate reservoirs and deep gas reservoirs have been designed. Experimental results demonstrate that deep gas reservoirs maintain high productivity independent of hydrate decomposition dynamics. This finding highlights the critical role of deep gas production timing in modulating geothermal energy transport to accelerate hydrate dissociation. Strategic delay of deep gas production until after initial hydrate reservoir depressurization was found to elevate circulating water temperatures, thereby expanding the heat distribution within the hydrate reservoir. This approach increased hydrate production efficiency by 6.79% compared to simultaneous production of both reservoirs. To maximize this thermal enhancement effect, deep gas production was further delayed to the early and middle stages of the hydrate reservoir’s constant-pressure phase. Results revealed that sustained high-temperature water circulation until the mid-constant-pressure stage could boost cumulative geothermal energy release, achieving a 10.49% further improvement in hydrate production efficiency. These findings establish a positive correlation between deep gas production timing and hydrate recovery performance. The study provides actionable insights for optimizing field-scale co-production strategies, where deliberate sequencing of reservoir production can significantly enhance geothermal-driven hydrate dissociation rates.

Low-frequency electrical heating (LFEH) generates Joule heat by leveraging reservoir resistance, enabling in-situ heat generation independent of heat carrier transport; however, its heating effectiveness is strictly governed by the spatiotemporal distribution of electrical resistivity. During thermal dissociation, the hydrate occurrence habit exhibits significant heterogeneity, driving the simultaneous evolution of pore throat spaces and conductive channels. This results in complex, dynamic resistivity responses, for which traditional single-parameter Archie power-law models often yield fitting deviations or even non-physical extrapolations. To address this challenge, this study establishes a research framework comprising "in-situ CT observation-pore-scale electric field simulation-resistivity model construction-model verification". Based on a real core sample with a porosity of 34.2%, continuous in-situ CT scanning is conducted at a resolution of 3.25 mu m to extract typical hydrate occurrence patterns and geometric features; a comparison of hydrate morphology and saturation is achieved within an 3D homogeneous granular porous media model, employing the resistivity ratio to mitigate the influence of external factors such as salinity. Furthermore, a critical water saturation (Swc) is introduced to construct a piecewise resistivity ratio model that satisfies continuity and physical boundary constraints, explicitly characterizing the mechanistic differences arising from the stagewise evolution of pore throat geometry. The results indicate that under ideal unit geometric conditions, the critical water saturations for the three typical occurrence modes-grain-coating, pore-filling-round, and pore-filling-square-are 76%, 79.9%, and 61.6%, respectively. For the high water saturation range (60%-100%), the proposed model significantly reduces errors compared to the traditional single Archie fitting, drastically decreasing the predicted standard errors of the aforementioned three modes from 0.21, 0.12, and 0.04 to 0.03, 0.01, and 0.01, respectively. Finally, electric field simulations and verifications based on CT-reconstructed real pore structures demonstrate that the electrical response of real cores aligns closely with the grain-coating mechanism. The proposed model exhibits robust applicability in heterogeneous hydrate-bearing sediments, providing reliable physical property inputs for coupled electrical-thermal simulations of LFEH.

The mechanical stability of hydrate-bearing sediments (HBS) is critical for the safe extraction of natural gas hydrates and the prevention of geohazards. While existing research has focused largely on the macroscopic response of HBS, the underlying micromechanical mechanisms remain insufficiently understood. This study develops a coupled PFC3D-FLAC3D numerical framework to simulate triaxial shear behavior in HBS, incorporating flexible boundaries to more realistically replicate experimental conditions. By systematically varying hydrate saturation and confining pressure, we analyze the evolution of macroscopic strength and deformation characteristics in relation to microscopic factors including cementation bond breakage, coordination number, porosity, and fabric anisotropy. Results show that increased hydrate saturation significantly enhances cohesion, while high confining pressure suppresses strain softening by promoting shear-dominated bond failure. Increasing the confining pressure from 1.0 to 3.0 MPa leads to a more compact granular structure, boosting the coordination number by up to 40% at high hydrate saturations and significantly reducing porosity. Moreover, hydrate cementation amplifies mechanical anisotropy, influencing localized deformation modes. This study provides new insights into the micromechanical origins of strength and instability in HBS, offering a scientific basis for engineering design and risk mitigation in hydrate resource development.

Hydraulic fracturing technology has played an important role in the exploitation of unconventional oil and gas resources, however, its application to gas hydrate reservoirs has been rarely studied. Currently, there is still limited understanding of the propagation and extension of fractures around the wellbore during the fracturing process of horizontal wells in hydrate reservoirs, as well as the stress interference patterns between fractures. This study simulates hydraulic fracturing processes in hydrate reservoirs using a fluid-solid coupling discrete element method (DEM), and analyzes the impacts of hydrate saturation and geological and engineering factors on fracture extension and stress disturbance. The results show that hydraulic fracturing is more effective when hydrate saturation exceeds 30% and that fracture pressure increases with saturation. The increase in horizontal stress differential enhances the directionality of fracture propagation and reduces stress disturbance. The distribution uniformity index (DUI) reveals that injection pressure is directly proportional to the number of main fractures and inversely proportional to fracturing time, with fracturing efficiency depending on the spacing between injection points and the distance between wells. This work may provide reference for the commercial exploitation of natural gas hydrates.

We provide a comprehensive overview and summarizes the recent advances in the mechanical properties of hydrate-bearing sediments (HBS) from three aspects: experimental investigation, numerical simulation, and constitutive model. Mechanical properties and microscopic mechanisms under the influence of multiple factors are expounded in depth. The results show that hydrate saturation and confining pressure are the most significant factors affecting the mechanical behavior of HBS, and the effects of various factors are different and coupled. The essence of macroscopic mechanical properties is the evolution of microscopic particle relationships. Numerical simulation is an important means to study cross-scale mechanical behavior. The constitutive model is typified by the elastoplastic model based on critical state theory. Further, the challenges facing current research are discussed, and the solutions and development directions are clarified. In the future, the focus should be on the cross-scale unification of micro (atomic and crystal scales), meso (particle scale) to macro (sample and reservoir scales), also including the study of non-homogeneous fracture-filling hydrate. Meanwhile, there is an urgent need to establish unified industry standards and a global database to accelerate academic exchanges and sharing of achievements.

Analytical and Experimental Technology is an essential and important means for basic theoretical research on marine natural gas hydrates [...]

Natural gas hydrates are widely distributed in marine and permafrost environments. As a novel energy resource, accurately describing reservoir characteristics and assessing energy potential is crucial for its commercial development. Resistivity logging serves as a valuable approach for achieving these goals. Nevertheless, due to inadequate comprehension of the electrical conductivity mechanism in hydrate-bearing sediments, existing data processing models still encounter certain challenges. This study conducts both core-scale and pore-scale simulation experiments to examine the relationship between resistivity variations and the distribution of gas hydrate porosity. The results indicate that the characteristics of resistivity variation is associated with the gas hydrate formation process, and the gas hydrate saturation index, denoted as ‘n’, varies between 0 and 3 depending on different gas hydrate distribution patterns. As the saturation increases, gas hydrate distribution in pore spaces transitions from floating to contacting and cementing patterns. It is proposed that the aqueous pore tortuosity can be utilized to correct the saturation index ‘n’ in Archie’s equation. Based on the analysis of experimental data, a correction method for Archie’s equation is suggested, and its effectiveness in controlling relative error has been validated.

A significant portion of global hydrate reserves is located within clayey-silt marine sediments, primarily in the form of fracture-filling hydrates. The dynamic evolution of these hydrate occurrence states induces considerable alterations in the mechanical properties of the surrounding sediments, which in turn play a crucial role in submarine slope stability, environmental impacts, and engineering safety during resource extraction. Currently, the majority of research has concentrated on pore-filling hydrate reservoirs, while studies on fracture-filling hydrates remain relatively scarce due to experimental limitations. This study utilizes the discrete element method to predict the macroscopic mechanical behavior of fracture-filling hydrate-bearing sediments under varying hydrate occurrence characteristics and to explore the underlying microscopic mechanisms. The results reveal that an increase in the fracture-filling number leads to a higher proportion of tensile breakage in cementation, whereas a larger fracture-filling width enhances the occurrence of shear breakage in cementation. Both increases in width and number result in an increase in the proportion of strong force chains. Furthermore, increased tortuosity causes displacement and velocity changes in sediments near the fractures, resulting in shear slipping within the sediments. This study provides valuable insights into the exploitation of hydrate resources, submarine disaster mitigation, and the reduction of greenhouse gas emissions from hydrate dissociation.

High hydrostatic pressure in deep-sea environments potentially impacts microbial community diversity, the structure of cellular components and functions. The specific characteristics of aerobic methanotrophs originating from deep-sea environments and their responses to local pressure fluctuations in terms of community diversity and methane oxidation potential remain unexplored. This study investigates subsurface sediments rich in aerobic methanotrophs from the natural gas hydrate-bearing region in the Shenhu area, Northern South China Sea. By conducting aerobic oxidation of methane (AeOM) incubation experiments under various environmental pressures up to 10 MPa, the study aims to elucidate differences in microbial community diversity and AeOM rates. The results show a profound impact of pressure on both the taxonomic composition of bacterial and methanotrophic communities and their capacity for methane consumption. The key aerobic methanotrophs, that is, Methylococcales, exhibit a gradual decrease in composition as pressure rises. Accordingly, their AeOM rates also show a significant negative correlation with pressure (r = 0.986, P < 0.01). The composition of three dominant methanotrophic genera, that is, unclassified_Methylococcaceae, Methylobacter, and Methylocaldum, exhibited irregular fluctuations under varying pressure conditions, with the lowest abundance observed at 2 MPa. Our study also shows that unclassified_Methylococcaceae is the primary methanotroph that exhibits the main response to pressure changes in marine environments.

Fracture-filling hydrate reservoirs that have been extensively discovered commonly show anisotropic elastic properties. Comprehensive understanding of anisotropic elastic responses is an important premise to accurately assess the fracture-filling hydrate reservoirs. Rock physics experiment is one of the best means to understand the elastic responses of hydrate-bearing reservoirs. However, a synchronous measurement system for understanding the anisotropic elastic responses in fracture-filling hydrate reservoirs is still missing. In this paper, a novel experimental system that can synchronously measure five anisotropic acoustic velocities of fractured rocks during hydrate evolution to fully characterize their anisotropic elastic responses is reported. A synchronous experiment using a fractured rock with evolving hydrate was conducted to verify the validity of the synchronous measurement system and understand the anisotropic elastic responses in fracture-filling hydrate reservoirs. We find that hydrate formation and dissociation, respectively, increase and decrease the measured velocities, and the increasing gradients among the anisotropic velocities with forming hydrate are different when hydrate saturation is higher. We also find that the elastic anisotropic parameters calculated based on the measured velocities are dependent on hydrate saturation, and the compressional wave anisotropy ε sharply reduces with increasing hydrate as saturation exceeds about 10% and quickly enhances at the initial stage of hydrate dissociation. The results are reasonably analyzed and interpreted by the shaping and decomposition of bridging hydrate morphology in the aligned fractures. The results provide new insights into the anisotropic elastic responses and their influencing mechanism in fracture-filling hydrate reservoirs.

Dielectric properties with high sensitivity to the types and saturations of fluids are important means to assess the quality and leakage risk of carbon dioxide (CO2) sequestration in reservoirs. However, the understanding of frequency-dependent and anisotropic dielectric responses in fractured rocks during CO2-replacing brine, which is the key to accurately inverse the dielectric survey data from fractured saline aquifers during CO2 sequestration, is still missing. In this study, we investigate, via dedicated numerical simulations, for the first time, the frequency-dependent dielectric properties (electrical conductivity, relative permittivity, and dielectric loss factor) of a fractured rock during CO2-replacing brine in two directions: vertical and parallel to CO2 migration. The results show that, with increasing frequency , conductivities and relative permittivities increase and decrease, respectively, while the variations in loss factors present a bell-shaped trend. Enhancing the CO2 saturation is shown to reduce these dielectric properties and their dispersion amplitudes. More importantly, the reducing trend of the three properties with enhancing CO2 saturation occurs the maximum gradient at various frequencies. Analyses and discussion of the simulation results suggest that the frequency-dependent dielectric properties in the direction orthogonal to CO2 migration are more stable to characterize CO2 sequestration, and the three properties should be respectively chosen as the most reliable detecting parameters according to the operating frequency of dielectric tools with single frequency or narrow band. The results not only provide new insights into the frequency-dependent and anisotropic dielectric behaviors in fractured rocks during CO2 migration but also are helpful in assessing CO2 sequestration in fractured saline layers.

Mechanical properties of hydrate-bearing fine-grained sediments are crucial to effectively mitigate environmental risks caused by artificial and natural decomposition of natural gas hydrates, and the decomposition can induce laterally confined deformation. To explore the effect of natural gas hydrates on laterally confined compression properties, consolidation tests are conducted on remolded hydrate-free and hydrate-bearing samples by using natural fine-grained sediments collected from the northern South China Sea as the host sediments, and empirical equations are developed based on the analyses of consolidation characteristics. The results show that vertical loading induces a reduction in void ratio, and the reduction increases with decreasing hydrate saturation when samples are subjected to the same vertical stress change. The compression index of samples is about 0.53 whether there is hydrate or not, but the yield stress of samples increases sharply with increasing hydrate saturation once beyond the critical value. The coefficient of volume compression and the coefficient of consolidation of hydrate-bearing samples both increase firstly and then decrease to a relative stable level with increasing vertical stress, and the transition occurs at 200 kPa. The average consolidation degree with elapsed time increases rapidly under low vertical stresses, slowly under median vertical stresses, and under high vertical stresses, the consolidation increases a little faster but still slower than those under low vertical stresses.

Many marine hydrate reservoirs exhibit significant foraminifera abundance. These foraminifera alter the original properties of the sediments, potentially affecting the formation and accumulation of hydrates. This present study analyzed the pore-structure characteristics of foraminifera-containing sediments from the South China Sea and their influences on the hydrate formation process by using microscopic imaging techniques. Scanning electron microscopy (SEM) and X-ray computed tomography (X-CT) reveal the unique dual-porosity characteristics of these sediments, characterized by matrix pores and foraminiferal pores. These two types of pores diverge significantly in their size distribution, morphological attributes, connectivity, and other aspects. Further experiments show the formation and occurrences of tetrahydrofuran (THF) hydrate under different sedimentary conditions and solution concentrations. Foraminiferal shells are found to promote the formation of hydrate by increasing the volume fraction of hydrate-favoring pores, which is more pronounced under conditions of low hydrate saturation. These findings obtained may contribute to global hydrate resource assessments and enhance understanding of hydrate reservoirs.

Natural gas hydrates are not only substantial energy sources but also have significant applications in the chemical industry and other fields. Although investigating hydrate formation in sediment minerals is crucial for their development and utilization, the underlying hydrate formation mechanism remains unclear. Here, molecular simulations were conducted in systems incorporating hydrophobic and hydrophilic pores of different sizes to investigate methane hydrate formation processes. The findings suggest that, as the hydrophobic slit size increases, there is a larger number of dissolved methane after the system reaches a metastable equilibrium state. The probability of cage formation indicates that hydrate cages readily form on hydrophobic surfaces or in the solution phase near the solution/gas interface. The larger slits are preferred for hydrate nucleation, regardless of whether the surface is hydrophobic, with most initial nuclei located near the liquid/methane interface. However, the interface perturbation can lead to the movement and growth of hydrate nuclei near the solution/methane interface into the bulk solution phase. Additionally, hydrate can nucleate and grow on the hydrophobic surface, facilitated by the adsorbed methane molecules and nonstandard cages. Pores hinder methane storage capacity in the hydrate phase due to the confinement effect and the amorphous nature of the hydrate formed. These molecular-level findings enhance our understanding of hydrate formation in sedimentary environments and porous materials, benefiting the development of natural gas hydrates and the use of porous materials for gas storage and transportation.