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Sand production is a crucial geotechnical issue for safe and efficient gas recovery from natural gas hydrate (NGH) reservoirs. Although the same gas production method of depressurization was chosen for the production trials in the Nankai Trough and the Northern South China Sea, their sand production behaviors differed greatly due to the skeleton diversity of sand in the Nankai Trough and clayey‐silt in the Northern South China Sea, which resulted in different sand control and sand production characteristics. In this study, we conducted experiments to compare sediment responses and sand production behaviors from these two typical gas hydrate‐bearing sediments (GHBSs) before, during, and after hydrate dissociation under depressurization. The results reveal the fluid and sand production rates in both GHBSs are relatively low before and after hydrate dissociation. However, the sand production rate in sandy GHBS keeps stable at a relatively high value during the whole hydrate dissociation period, accompanied by a high fluid production rate and continuous sample subsidence. In contrast, the fluid and sand production rates are proved to be high only at the early stages of hydrate dissociation in clayey‐silty GHBS with sand production decreasing obviously afterward, and the sample subsidence rate exponentially decay until the ultimate subsidence is reached. After experiments, two kinds of particle plugging phenomena, sand bridge and mud cake, are found to form outside sand screens in sandy and clayey‐silty GHBSs, respectively. Differences in permeability, particle migration, and plugging characteristics are thought to be the main inducement of the variability in sand production behaviors in different GHBS types. These findings indicate that sand‐production control strategies should obey geological engineering integration and differ according to the GHBS type. Specifically, we should concern sand production risk in sandy GHBS during and after hydrate dissociation, whereas more attention should be paid to the early stages of hydrate dissociation in clayey‐silty GHBS. [ABSTRACT FROM AUTHOR]

Dynamic mechanical properties of natural gas hydrate (NGH) reservoirs are extremely important owing to their critical role in drilling safety and reservoir stability during NGH development. However, the comprehensive influence of reservoir skeleton and hydrate status on the dynamic mechanical properties of hydrate‐bearing sediments (HBS) has rarely been reported. In this study, three types of sandy sediments with different particle size were simulated. A customized resonant column system was used to synthesize HBS and to test the dynamic mechanical properties of the HBS under the conditions of different confining pressures and shear strains. In addition, the dynamic mechanical responses of the hydrate‐free and hydrate‐dissociated specimens were comprehensively studied to further demonstrate the dominant role of hydrate status and skeleton‐hydrate interactions on the dynamic mechanical properties of HBS. The results show that the shear moduli of the HBS specimens increase exponentially with an increase in confining pressure and decrease in amplitude strain. However, the damping ratios decreases with an increase in confining pressure and decrease in amplitude strain. With an increase in the median particle size (D50) of the simulated sediments, the shear moduli tend to increase, whereas the damping ratios decreas. In addition, the hydrate occurrence decreases differences in moduli between fine‐, medium‐ and coarse‐skeleton specimens, and the enhancement effect of hydrate on the mechanical properties of the specimen is the maximum for the fine sediment specimens. Interestingly, we find that the modulus of the hydrate‐dissociated specimen is relatively smaller than that of the hydrate‐free specimen, which implies that the structure of the sediments could be rearranged and weakened due to the hydrate formation‐dissociation processes. [ABSTRACT FROM AUTHOR]

Atomic force microscopy (AFM) indentation is widely used to determine mechanical parameters of various materials. However, AFM tip may lead to phase transition of the cold sample in the region of contact area. It is a long-standing challenge that low-temperature phase-change materials (e.g., ice and hydrate) are hardly characterized by AFM, especially for clathrate hydrates. Here, with theoretical analysis and numerical simulation, we investigated the temperature influence of AFM tip on the tetrahydrofuran (THF) hydrate stability. At first, a steady-state model of heat conduction was established between a v-shaped probe and THF hydrate sample. The temperature of the tip was estimated at different laser spot positions and laser intensities. Through numerical simulation, the heat loss by air convection is less than 1% of the total laser heat, and the influence of ambient air on the AFM probe temperature can be neglected. Meanwhile, the local temperature in the region of contact area was also calculated at the THF hydrate temperature of 0°C, -10°C, -20°C, and -30°C. We found out that the AFM tip causes the cold THF hydrate to melt. The thermal melting thickness decreases with the reduction of laser intensity and THF hydrate temperature. On the contrary, it is positively correlated with the thickness of liquid-like layer on THF hydrate surface and is also linearly increased with the contact radius. This indicates that the thermal melting continues as the press-in depth of the tip into THF hydrate increases. The local temperature rises when the tip touches the THF hydrate. It is easier for THF hydrate to be melted by an external pressure. In addition, the proposed model may be useful for guiding force tests on low-temperature phase-change materials by the AFM indentation. [ABSTRACT FROM AUTHOR]

The hydrate-bearing sediments (HBS) mechanical properties during hydrate dissociation are important to the safe, efficient and sustainable exploitation of natural gas hydrates (NGH). However, the inhomogeneous hydrate dissociation patterns inherent to HBS may make it difficult to accurately obtain its mechanical properties. In this study, a discrete element method (DEM) model was adopted to simulate the HBS mechanical behavior under various hydrate dissociation patterns. By investigating the simulation results and the corresponding internal mechanism, it is found that the stratified and radial inhomogeneous hydrate dissociation in HBS significantly affects its stress‒strain characteristics, and HBS under radial hydrate dissociation has a larger deviation. Moreover, it is suggested that when designing experiments to study the hydrate dissociation effect on the HBS mechanical strength, if the hydrate dissociation homogeneity cannot be guaranteed, a preference for a stratified hydrate dissociation pattern can lead to more desirable results. Finally, a method to judge the HBS hydrate dissociation pattern was also proposed. The results have a guiding significance for the experimental study of HBS mechanisms under hydrate dissociation and ultimately contribute to the realization of accurately predicting reservoir mechanical behavior during NGH exploitation. • A method to judge the hydrate dissociation pattern in sediment was proposed. • The impact of hydrate dissociation patterns on sediment mechanics was investigated. • Tips for study the effects of hydrate dissociation on sediment mechanics were given. [ABSTRACT FROM AUTHOR]

The dissociation of natural gas hydrates (NGHs) during production weakens the mechanical properties of hydrate-bearing sediments (HBSs). Understanding and predicting the mechanical behaviors of HBSs is crucial for the safe development of NGH reservoirs. This study presents a novel approach based on the discrete element method (DEM) to describe the mechanical weakening of cementing-type gas hydrate-bearing sandy sediments (GHBSSs). Hydrate dissociation is simulated by reducing the parallel-bond radius based on a specific damage evolution law. The Weibull statistical distribution is incorporated into the model to characterize the heterogeneity of hydrates in GHBSSs. Subsequently, a series of numerical triaxial compression tests are conducted to investigate the mechanical weakening behavior of cementing-type GHBSSs and the mechanical effect of the heterogeneous distribution of hydrates. The results show that the DEM model can successfully capture the mechanical weakening behaviors of cementing-type GHBSSs during hydrate dissociation. The peak strength and secant modulus increase with increasing homogeneity index, but their sensitivities decrease as the hydrate dissociation ratio increases. Finally, a fitting equation describing the relationship among the peak strength, initial hydrate saturation, and hydrate dissociation ratio was obtained. This study is valuable for evaluating the stability of GHBSSs during hydrate production. [ABSTRACT FROM AUTHOR]

Understanding the mechanical behaviours of gas hydrate-bearing sediments (GHBS) is important for their associated applications in wellbore stability, stratum deformation during exploitation, geological disaster prevention, and the risk assessment of replacing CH 4 with CO 2 in hydrate reservoirs and CO 2 sequestration in oceans. However, triaxial tests on mechanical properties of GHBS are taxing and time-consuming. Here, we presented an easy and efficient way to evaluate these behaviours by using a self-developed direct shear apparatus. Then a series of direct shear tests on GHBS represented by CO 2 hydrate-bearing silt were performed to investigate their mechanical behaviours and strength indices by changing the axial pressure, CO 2 hydrate saturation, shear rate and hydrate synthesis temperature. Our results indicate that CO 2 hydrate significantly strengthens specimens by cementing silt grains. In addition, when hydrate saturation increases, the cohesions are enhanced from 0.09 to 2.39 MPa, and the internal friction angles increase and decrease at the range from 28.6° to 43.3° under the experimental conditions. These findings have direct implications for evaluating the stability and safety of natural gas hydrate reservoirs, CO 2 replacement to extract CH 4 and CO 2 sequestration. [ABSTRACT FROM AUTHOR]

Knowledge of the dynamic responses of hydrate-bearing sediments under small strain conditions is a fundamental issue in terms of evaluating the stability of gas hydrate-bearing layers under natural forces, such as eustatic sea level and earthquakes as well as artificial activities for example gas production from hydrate reservoirs. However, this issue has not yet been fully understood. In this study, a customized hydrate resonance column was utilized to measure the dynamic response of tetrahydrofuran hydrate-bearing sediments with different conditions. The results show that the shear modulus increases exponentially with increases in the confining pressure or decreases in the shear strain, while the damping ratios decrease exponentially under identical conditions. Under the same confining pressure, the shear modulus and damping ratio increases as hydrate saturation rises. The stress sensitivity index (b) of shear modulus decreases monotonically with an increase in hydrate saturation. The shear modulus is negatively correlated with porosity when the tetrahydrofuran (THF) hydrate is predominantly pore-filling type, and the damping ratio and sensitivity index b are positively correlated with porosity. However, when the hydrate is mainly load-bearing type, the shear modulus and damping are both related positively with porosity. This is because the total amount of hydrate in specimens with identical hydrate saturation increases with higher porosity, while the sensitivity index b is negatively related to porosity. The hydrate formed in pores leads to a normal increase in the shear modulus and an abnormal increase in the damping, due to trace amounts of residual water between the surface of the hydrate and the particles. This study provides a basis for the comprehensive understanding of the dynamic responses of hydrate-bearing sediments under small strains. It is of great significance for seismic or sonic well logging evaluations on gas hydrate saturation and its dissociation degree in hydrate reservoirs. Image 1 • Effect of porosity on the dynamic responses of hydrate-bearing sediments under small strain is firstly investigated. • The abnormality of dynamic responses of hydratebearing sediments origin from the change of hydrate distribution pattern. • An empirical formula for predicting hydrate saturation of silty reservoir is established. [ABSTRACT FROM AUTHOR]

The heterogeneous distribution of hydrates in natural or synthetic hydrate-bearing sediments (HBSs) has definitively proven its influences on hydrate dissociation and related mechanical responses, which can alter hydrate production strategies. However, due to the limitations of experimental conditions, applicable hydrate heterogeneity characterization and corresponding evaluation of the mechanical properties of HBSs remain lacking. In this study, discrete element modelling of numerical HBSs with different hydrate heterogeneous distributions considering the coalescence phenomenon is performed. The hydrate heterogeneity degree is quantified through image recognition, and the constructed HBSs are subjected to biaxial compression tests. The results show that for two HBSs with the same hydrate saturation above 10 % but highly different hydrate heterogeneity degrees exceeding 0.1, the peak strength, secant modulus and maximum dilation angle of the strongly heterogenous HBSs are lower than those of the weakly heterogenous HBSs. In addition, the weakly heterogeneous HBSs exhibit a more uniform contact force chain structure, more hydrates participate in loading support, and more particle rearrangements, which are the meso - mechanisms for the above macro - mechanical behaviors. Combined with the consideration of the hydrate morphology and cementing ratio, the mechanical properties of HBSs fluctuate due to the randomness of hydrate coalescence but generally show a decreasing trend with increasing hydrate heterogeneity degree. These findings provide an important supplement for understanding the macro - and meso - mechanical properties of HBSs with different hydrate heterogeneity degrees, which indicates that we should precisely describe hydrate coalescence in laboratory experiments and numerical simulations and consider its representativeness in field applications. [ABSTRACT FROM AUTHOR]

Quasi–liquid layer (QLL) on the hydrate surface is discovered to possess multilayered structures. The regions of QLL near gaseous interface exhibit fluidity, while the majority at QLL–hydrate interface behaves similarly to bound water. [Display omitted] • Evaluating surface structure characteristics of nano-thick hydrate QLL by AFM measurement and MD simulation. • QLL fluidity increases with a steeper hydrate surface and higher temperature. • QLL distribution is related to the surface morphology of polycrystal hydrate. • QLL exhibits interface–dependent behavior, with outermost layer demonstrating highest instability. The surface structural characteristics of clathrate hydrates play a significant role in mass transfer, interfacial properties, and mechanical stability during the hydrate growth process. However, the microstructure of hydrate surfaces remains unclear and controversial. In this study, a modified Atomic Force Microscopy (AFM) system was employed to confirm the presence of a quasi–liquid layer (QLL) on the solid tetrahydrofuran (THF) hydrate surface. Subsequently, sensitivity analyses were conducted on factors affecting the instability (fluidity) of QLL, including relative position, inclination angle, environmental temperature, and surface morphology. The results indicate that higher temperatures enhance the instability of the QLL surface, and altering the slope of the hydrate surface leads to an increase in the average thickness of the QLL due to the driving forces of gravity and surface tension, resulting in increased fluidity. Additionally, the surface morphological structure formed by Oswald ripening in polycrystalline THF hydrates affects the thickness of QLL. Furthermore, molecular dynamics simulations reveal that water molecules on the hydrate surface exhibit a multilayered structure, with the QLL region near the gas phase showing greater instability, while the region closer to the hydrate exhibits solid–phase characteristics, consistent with experimental findings. [ABSTRACT FROM AUTHOR]

[Display omitted] • Nanobubble and temperature effects on hydrate dissociation were revealed at the molecular level. • Hydrate dissociation rate per unit of surface area was calculated for each system. • Formation of a nanobubble increases the hydrate dissociation rate by enhancing the directional methane transfer. • The nanobubble can increase the intrinsic hydrate dissociation rate. • The hydrate dissociation rate is linearly correlated with the mass flow rate toward the nanobubble. Hydrate dissociation is often accompanied by the formation of nanobubbles. Knowledge of the effects of nanobubbles on hydrate dissociation is essential for understanding the dynamic behavior of the hydrate phase change and improving the gas production efficiency. Here, molecular dynamics simulations were performed to study the methane hydrate dissociation kinetics with and without a pre-existing methane nanobubble. The results show that the hydrate cluster in the liquid phase dissociates layer-by-layer. This process is shown to be independent of the temperature and nanobubble presence at the simulation conditions. Hydrate dissociation does not always lead to nanobubble formation because the supersaturated methane solution can be stable for a long time. A steep methane concentration gradient was observed between the hydrate cluster surface and the methane nanobubble, which can enhance the directional migration of methane and effectively minimize the methane concentration in the liquid phase, thereby increasing the driving force for the hydrate dissociation. Our findings indicate that the presence of a nanobubble near the hydrate surface does not decrease the activation energy of hydrate dissociation, but it can increase the intrinsic decomposition rate. The average hydrate dissociation rate is linearly correlated with the mass flow rate towards the nanobubble. The mass flow rate is determined by the nanobubble size and hydrate-nanobubble distance. Our findings contribute to the fundamental understanding of the dissociation mechanism of gas hydrates in the liquid phase, which is crucial for the design and optimization of efficient gas hydrate production techniques. [ABSTRACT FROM AUTHOR]

• Tangential force between hydrate particles under different normal force, contact time and temperature are tested for the first time. • The results of tangential forces at macro and micro scales are compared. • Cohesion and friction Angle between hydrate particles are obtained. • The microscopic process of tangential failure between particles is observed and the failure mechanism is proposed. To reveal the mechanism of shear failure between gas hydrate particles can provide an important theoretical basis for controlling hydrate aggregation in oil and gas pipelines and for controlling sand production during gas production from natural gas hydrate reservoirs. Here we used a self-built micromechanical force test device to investigate the effects of normal forces, contact time, and temperature on tangential forces exerted between hydrate particles. The main controlling factors were analyzed according to the Mohr-Coulomb strength criterion. The test results show that the tangential force is proportional to the normal force and contact time, and that the temperature has a significant impact on the friction angle. Also, at a given temperature and for a short contact time (<30 s), the friction angle at the particle scale is very close to the friction angle measured by geotechnical tests at the macroscopic scale. Due to hydrate dissociation, the friction angle and cohesive force display the greatest variations around the phase-equilibrium temperature. In addition, possible tangential failure modes under various conditions are proposed to deeply reveal the mechanism of micro force between hydrate particles. [ABSTRACT FROM AUTHOR]

The Shenhu area is the key target area for marine gas hydrate industrialization in China. Significant progress has been made with respect to gas hydrate exploration and production in this area in recent years. However, a comprehensive understanding of the reservoir characteristics and hydrate accumulation mechanism are still lacking; in particular, the hydrate enrichment factors are not, as yet, completely clear. In this study, we systemically characterize the geological controls on gas hydrate accumulation in the Shenhu area, including the sedimentary environment, gas sources, fluid and gas pathways, and reservoir-caprock system. We subsequently provide a quantitative evaluation of some critical factors influencing hydrate enrichment. The depositional environment in the study area was characterized by low energy (sedimentary hydrodynamic conditions), and anoxic environment, and the gas hydrate accumulations were closely associated with turbidites. The methane captured in the gas hydrates in the GMGS1 area and at the SHSC-4 and SHSC2-6 production test sites was mainly derived from the microbial reduction of CO 2 , while thermogenic gas was a more important source of hydrocarbon gases in the GMGS3 and GMGS4 areas. In addition, we used a combination of 8-slice spiral X-ray computed tomography (CT) and micro-CT techniques for the first time for the in-situ investigations of the microfractures and foraminifera in the hydrate cores. The results indicate that microfractures and foraminiferal shells are crucial to the occurrence and accumulation of gas hydrates in fine-grained sediments of the Shenhu area. Interestingly, microfractures were found to be more developed in the hydrate layers where most sediment particles were relatively small in size. Hydrate saturation increased with i) a decrease in the specific surface area of sediment grain, ii) an increase in the median pore size, and iii) increased foraminifera abundance in the reservoir. Our findings could help locate high-saturation gas hydrate layers ('sweet spot') in fine-grained marine sediments of the Shenhu area, South China Sea. • Microscopic factors influencing the hydrate enrichment are comprehensively discussed. • A conceptional model for hydrate accumulation was proposed in the Shenhu area. • Microfractures prefer to develop in the hydrate layers where the sediment particles have relatively small grain size. • Thermogenic gas makes an important contribution to hydrate accumulation in turbidites connecting deep faults. [ABSTRACT FROM AUTHOR]

Gas hydrates that occur in offshore sediments and onshore permafrost represent a vast source of natural gas. However, current gas production trials indicate that sand production, which refers to the migration of sand into boreholes is an obstacle for long-term safe and efficient gas production from gas hydrate reservoirs. Therefore, understanding sand production behaviors during hydrate mining is especially important. In this study, we conducted laboratory-scale tests to investigate the effect of hydrate saturation and overlying stress on the sand production behaviors of hydrate-bearing sediments with a sand screen before, during, and after hydrate dissociation by depressurization. The synthesized methane hydrate saturations and simulated overlying stresses were ~20%, ~30%, ~40%, ~50%, and ~60% and ~11, ~12, ~13, ~14, and ~15 MPa, respectively. The experimental results indicate that sand production before hydrate dissociation is relatively minor and primarily silty. The amount of sand produced increases and silt, fine sand, and medium sand were observed during and after hydrate dissociation. The mass of sand production increases as hydrate saturation increases under constant overlying stress; however, under constant hydrate saturation, it increases then decreases as overlying stress increases. Friction among particles may become the primary control factor once the overlying stress exceeds a critical value. We considered hydrate saturation and overlying stress to establish a sand production prediction model. Our findings help understand sand production behavior using sand screens during gas production from hydrate reservoirs and provide a reference for sand production control design in future hydrate mining. • The effects of hydrate saturation and overlying stress on sand production are studied. • The reservoir coupling responses and sand production behaviors in different stages of hydrate dissociation are compared. • A sand production prediction model considering hydrate saturation and overlying stress is provided. [ABSTRACT FROM AUTHOR]