The South China Sea (SCS) stands as one of the world's key regions boasting substantial reserves of natural gas hydrates. The mechanical characteristics and stability of hydrate reservoirs during exploitation in the SCS have always been frontier topics in the domain of marine geological engineering and energy development. As a typical geological carrier of hydrate reservoir in the SCS, clayey silts possess characteristics of non-diagenesis, low permeability and weak cementation. The hydrate exploitation process is easy to trigger reservoir settlement and deformation, and even cause wellbore collapse, submarine landslide and other disasters. Therefore, according to the characteristics of clayey silts in hydrate reservoirs of the SCS, exploring transformation law of deformation and strength during exploitation is crucial for realizing the safe and efficient development of hydrate resources. Based on this, this review methodically summarizes physical, mechanical properties and constitutive relationships associated with hydrate reservoirs in the SCS. Firstly, physical properties of host sediments and hydrate accumulation habits in them are introduced; Secondly, the influences of effective confining pressure, hydrate saturation and stress history on stress-strain and creep properties of reservoirs in the SCS are described; In addition, deformation characteristics of hydrate reservoirs in the process of exploitation by heat injection, depressurization and CO2 replacement methods are demonstrated; Finally, mechanical constitutive models applicable to hydrate reservoirs in the SCS are introduced. This review will deliver theoretical guidance and data for the stability of reservoirs and safe exploitation of hydrate resources in the SCS.

Long-term extraction of natural gas hydrates can induce reservoir creep, leading to geological hazards such as submarine landslides and wellbore instability. The presence of widespread natural fractures can intensify this creep deformation by altering stress distributions, thereby significantly increasing engineering geological risks. However, the influence of key parameters on the creep behaviour of fractured hydrate-bearing sediments remains insufficiently understood. This study presents a systematic investigation into the creep properties of fractured and fracture-free hydrate-bearing sediments under various conditions, including effective confining pressures and hydrate saturations. It provides a reference for assessing the stability of fractured hydrate-bearing reservoirs. A comparative analysis of total creep strain, attenuation-creep strain, and rheological rate was conducted to elucidate the distinct mechanical responses of the two types of sediments. The results reveal that fractures fundamentally alter the sediment’s creep behaviour. In fractured sediments, key creep parameters exhibit a non-monotonic dependence on hydrate saturation, with total creep strain peaking at a critical saturation of approximately 20%. In contrast, the same parameters in fracture-free sediments show a consistent positive correlation with hydrate saturation. For both sediment types, the rheological initiation time increases with both effective confining pressures and hydrate saturation, indicating this relationship is independent of fracturing. Crucially, the total creep strain is predominantly governed by the attenuation-creep stage, regardless of fracture presence. This work improves the understanding of creep mechanics in fractured hydrate reservoirs and provides a critical theoretical basis for engineering geological safety assessments and the optimisation of hydrate extraction strategies.

With the growing trend of seeking resources, the significance of infrastructure construction in the upper part of soils has become increasingly prominent. Studying the geotechnical characteristics of soils is crucial for the safe operation of geotechnical projects and the efficient deep-resource development. The cone penetration test (CPT) indirectly measures the geotechnical properties of soils. However, Parkin and Lunne pointed out that the CPT data obtained solely on site cannot be reliably converted into soil mechanical parameters. It is difficult to obtain undisturbed samples of soils for laboratory tests (such as triaxial and pedometric tests), thus making it impossible to accurately obtain their mechanical parameters, resulting in a lack of comparison in the interpretation of in situ CPT data. Furthermore, due to the differences in testing procedures, experimental equipment, and soil types among various regions, the data and findings from CPT cannot be generalized into universal principles to guide engineering practices. CPT calibration chamber (CCC) testing, as an effective method to establish the correlation between laboratory measurements of soil and its in situ mechanical properties, holds significant research value for soils where undisturbed samples cannot be retrieved (such as cohesion-less sands and silty soils without plasticity). This paper first reviews the characteristics of large and small CCC equipment for different types of soil. It then discusses the boundary effect encountered in the interpretation of CCC test data and general solutions and summarizes the proposed empirical relationships based on CCC test data for geotechnical parameters. Finally, it outlines future directions for CCC equipment and data interpretation methods.

During natural gas hydrate (NGH) extraction, the decomposition of hydrates is always accompanied by gas-water migration. Existing studies have confirmed the significant promoting effect of seawater flow on the decomposition of methane hydrates in porous media sediments. However, the molecular-level structural evolution during hydrate decomposition driven by fluid flow has not yet received broad attention. In this study, molecular dynamics simulation is used to investigate the microscopic effect mechanism of fluid flow on methane hydrate decomposition. Through two different simulation approaches, it is clarified that the accelerated mass transfer of methane molecules from the hydrate surface due to solution flow is the fundamental reason for promoting hydrate decomposition. Additionally, simulations of hydrate decomposition caused by heating and driven by fluid flow above phase equilibrium were performed to trace the microstructural evolution of methane hydrate in these two conditions. Two different modes of hydrate cage rupture were identified, which explain the differences in decomposition rates under different driving factors. The results of this study reveal the decomposition mechanisms of methane hydrates under the fluid flow environment, and may provide theoretical support for the technological solution of water erosion promoting hydrate exploitation.

Hydrate-based CO2 storage in subsea sediments presents a promising solution for safe carbon sequestration, as CO2 hydrate caps effectively reduce CO2 leakage risk. However, the effectiveness of using large-scale hydrate caps to achieve substantial CO2 sequestration is still uncertain. This study developed a numerical model for CO2 sequestration in sediment environments. The distribution patterns of multi-state CO2 (i.e., free, dissolved, and hydrate states) and the effectiveness of hydrate caps were investigated using single-horizontal-well and dual- horizontal-well systems. The findings indicated that a higher injection rate expedited the formation rate of CO2 hydrate caps but reduced the dissolved CO2 sequestration efficiency within the hydrate formation zone and the free phase zone. At the same CO2 sequestration amount, a low-flow-rate prolonged injection strategy could mitigate the pressure accumulation near the well and broaden the distribution range of the hydrate cap. Smaller well spacing facilitated the formation of a larger hydrate cap during the dual-well CO2 sequestration, with the thickness of the hydrate cap increasing by approximately 12 m over 50 years after CO2 injection cessation. Furthermore, a low-permeability mud cap interfered with the processes of CO2 plume migration and heat transfer, exacerbating the stratum instability near the injection well within the hydrate formation zone. This study provided new insights into forming large-scale CO2 hydrate caps and contributed to developing the CO2 storage technology in subsea sediments.

Leakage prevention of CO2 is crucial for the safety of submarine geological carbon sequestration, with hydrate caps identified as effective barriers against leakage. The stability and efficiency of these caps are influenced by water saturation, initial hydrate saturation, and H2O-CO2 seepage behaviors, but mechanisms governing rapid cap formation and efficient sequestration remain unclear. This study investigates hydrate cap formation under H2O-CO2 co-injection and CO2 pre-injection modes, analyzing the effects of water and hydrate saturation on sealing caps stability and CO2 sequestration efficiency. Results show that CO2 pre-injection promotes uniform hydrate distribution in sealing caps, reducing localized accumulation and enhancing sequestration efficiency. Moreover, the cap formation time negatively correlates with hydrate saturation and positively with water saturation in the pre-injection mode, while no linear relationship is observed in the co-injection mode. Optimal water saturation for rapid cap formation in co-injection mode is 40%-80 %. Compared to co-injection, CO2 pre- injection reduces cap formation time by 61.43 %, CO2 leakage by 32.56 %, and forms a pressure drop buffer region during cap failure, preventing continuous leakage and improving long-term sequestration safety. The present study provides technical support for the marine geological CO2 sequestration using the hydrate cap method.

CO2 capture and sequestration via the hydrate method has attracted much attention because of its high sequestration density and energy density. However, the influencing mechanism of the promoters on the formation of CO2 hydrate was not yet clarified. Also, there was no comprehensive review of the effect of promoters on CO2 hydrate formation from laboratory-scale studies. Therefore, this paper reviewed the effect of promoters on CO2 capture and sequestration via the hydrate method from the laboratory research scale. Typical CO2 hydrate formation processes were summarized in detail. And the characteristic parameters of CO2 hydrate were analyzed. Then, the different types of promoters were carefully overviewed. In particular, the influencing mechanisms of promoters on CO2 hydrates were highlighted and compared. Thermodynamic promoters mainly affected the conditions of hydrate formation, while kinetic promoters mainly acted at the gas-liquid contact interface. However, the promotional mechanism of CO2 hydrate promoters was controversial and the solution formulation units were not consistent. In addition, the effects of different promoters on CO2 capture, separation, and sequestration were also summarized. The existing studies ignored the CO2 hydrate formation experiments under in-situ conditions in the presence of promoters. The influence of different promoters on the sequestration effect was critically evaluated through parameters such as phase equilibrium curves, induction time, gas consumption, and sequestration density. A uniform evaluation standard for CO2 hydrate promoters was lacking. Finally, future development recommendations should focus on large-scale, low-energy, commercial, in-situ, environmentally friendly, and intelligent research directions. The review may provide some theoretical guidance for the commercial application of CO2 hydrate.

Methane hydrate, a form of clean energy also called flammable ice, has drawn global interest as an alternative energy resource of traditional fossil energy. The effective permeability of formation is a key factor to determine the gas production rate, which is controlled by not only hydrate saturation but also the porosity changes of the host sediment. A decrease in pore pressure leads to an increase in the effective stress and the collapse of the bonded structure made by the hydrate resulting in the volume contraction and permeability reduction. On the other hand, the pore pressure drop induces hydrate dissociation, which increases the porosity and permeability. In this study, we conducted a series of experiments to measure the effective permeability of hydrate-bearing sediments with different hydrate saturation. The experiment results show that the effective permeability of specimens is 30 Darcy without hydrate under 1 MPa effective stress. It decreases with the increasing of hydrate saturation from 0 to 0.396 or effective stress from 1 to 9 MPa. The relationship between void ratio, hydrate saturation and effective permeability is derived. Combining the confined compaction analysis, we proposed a simple formula to estimate the change in effective permeability of hydrate-bearing sediments during hydrate dissociation by depressurization. The formula is embedded the thermal-hydraulic model to predict the gas production under the effective stress. The amount of gas production reaches the maximum value of 1.45 x 10-3 m3 at 50 min and 4.82 x 10-3 m3 at 95 min with soil compaction from experiment, the fitting degree of nu-merical simulation are 0.997 and 0.998 when hydrate saturation equals 10% and 30%, respectively. This study could evaluate the compression-induced dynamic change in effective permeability and predict the gas/water production under the effect of effective stress during hydrate dissociation by depressurization.

Multi-phase (gas, saturated water, and unsaturated water) seepage is bound to exist in natural gas hydrates (NGHs) production process. The effect of water flow, especially for unsaturated water flow, on the permeability variation of hydrate reservoir and gas production behavior do not appear well understood. In this study, the unsaturated water flow in hydrate-bearing sediment and hydrate-free sediment is simulated by controlling water flow velocity. The hydrate phase distribution was monitored using visualization magnetic resonance imaging system. The variation of temperature, pressure, and gas production rate during the unsaturated water flow were analyzed. The results show that the changing trend of the pressure difference presented the three stages for the hydrate-free sediment. In contrast, the pressure difference followed a five-stage change for the hydrate-bearing sediment in the unsaturated water flow process due to the hydrate dissolution. The hydrate dissolution caused an increase in permeability (maximum of log10(Kr) was 0.5) and formed the obvious unsaturated water flow channel. Moreover, the higher water flow velocity, which increased the chemical potential difference between hydrate phase and water phase, accelerated the MH dissolution, and further induced the faster increase rate of permeability and gas production. Surprisingly, when the unsaturated water flow velocity was improved to 15 mL/min from 0.5 mL/min, the gas production rate increased by 35 times. Furthermore, the average gas pro-duction rate was mainly determined by the unsaturated water flow velocity, it was changing in a linear fashion with the increasing water flow velocity. The findings could provide new knowledge on the strategy design on NGHs production with high efficiency.

Geological sequestration of carbon dioxide (CO2 ) has been considered one of the most effective strategies against global warming. The greatest concern on the stored CO(2 )in sub-seabed sediments is leakage risk and can be solved by the plugging effect of CO(2 )hydrate cap, which is derived from the capillary force change by hydrate crystal formation inside pores. This study experimentally simulated CO(2 )upward leakage process in water -containing sediments and investigated the plugging characteristics of formed hydrate cap via magnetic reso-nance imaging (MRI) and flow characteristic analysis. Different CO(2 )flow rates (0.3-4.0 ml/min) and initial pressures (1.8-3.0 MPa) were employed for experimental conditions, and the hydrate cap appeared with no CO(2 )efflux any longer after hydrate formation for several minutes. It is found that both slow flow of CO(2 )and high pressure are beneficial for the formation of hydrate cap, and the strength of hydrate caps formed in all cases is confirmed by 10.0 MPa pressure test without any CO(2 )leakage. In addition, the spatial water distribution and the hydrate cap location inside the sediments are analyzed by multi-level MRI images and pressure evolution calculation, respectively. Ultimately, this study conducted a CO2 -water flow case and found that the strength of hydrate cap increases with the continuous formation of hydrates. Approximately 27.8% of hydrate saturation is a watershed of the plugging strength of CO(2 )hydrate cap. This study provides experimental evidences for the plugging effect of hydrate cap on terminating CO(2 )leakage and is of great significance for the scheme design and risk assessment of CO(2 )geological sequestration.

Natural gas hydrate, a potential energy resource, is attracting worldwide attention. In this study, we propose a new method of hydrate dissociation which uses seawater and electrostatic fields (SE method) cooperatively. The hydrate molecular dissociation mechanism of gas hydrate is a key issue in studying the kinetic properties of gas hydrate using the SE method. Therefore, molecular dynamics simulations were used to investigate the thermodynamic properties and structural changes of methane hydrate (MH) in multiple kinds of salt solutions under an electrostatic field. The results show that the electric field can drive cations into the MH phase to form a series of random semiopen cages, which are essentially temporary and metastable. The variation in free energy indicates that it is more difficult for divalent cations to enter the hydrate phase than monovalent cations, meaning that the hydrate structures formed with divalent cations are more unstable. Then, the ion current occurred in the hydrate phase (called ion migration in this study), which greatly accelerated hydrate dissociation. In contrast, the promotion effect of cations with the same charge on MH dissociation is as follows: Sr2+ > K+ ≈ Na+ > Ca2+ ≈ Mg2+. In general, the presence of common marine cations enhanced the promotion effect of the electric field on gas hydrate dissociation.

Natural gas hydrates, mainly existing in permafrost and on the seabed, are expected to be a new energy source with great potential. The exploitation technology of natural gas hydrates is one of the main focuses of hydrate-related studies. In this study, a large-size liquid aqueous solution wrapping a methane hydrate system was established and molecular dynamics simulations were used to investigate the phase equilibrium conditions of methane hydrate at different methane concentrations and interfacial geometries. It is found that the methane concentration of a solution significantly affects the phase equilibrium of methane hydrates. Different methane concentrations at the same temperature and pressure can lead to hydrate formation or decomposition. At the same temperature and pressure, in a system reaching equilibrium, the size of spherical hydrate clusters is coupled to the solution concentration, which is proportional to the Laplace pressure at the solid-liquid interface. Lower solution concentrations reduce the phase equilibrium temperature of methane hydrates at the same pressure; as the concentration increases, the phase equilibrium temperature gradually approaches the actual phase equilibrium temperature. In addition, the interfacial geometry of hydrates affects the thermodynamic stability of hydrates. The spherical hydrate particles have the highest stability for the same volume. Through this study, we provide a stronger foundation to understand the principles driving hydrate formation/dissociation relevant to the exploitation of methane hydrates.

Hydrate re-formation increases blockage risk and further reduces gas production efficiency. Considering the huge water production and gas migration, it is essential to determine the key parameters that control hydrate re-formation and blockage in the two-phase flow process. However, little research reveals the mechanism of hydrate re-formation in the water-dominated two-phase flow system. In this study, two-phase flow in hydrate sediment is simulated by controlling the water-gas flow rate, and the effect of effective sectional velocity on hydrate re-formation characteristics is analyzed. The experimental results showed that temperature and pressure followed a three-stage change trend in the water-dominated two-phase flow process: including hydrate re-formation induction stage I, mass hydrate re-formation and agglomeration stage II, and pore gas consumption stage III. Moreover, a lower effective sectional velocity of water (WESV) would reduce the gas concentration gradient between water and hydrate to enhance the hydrate re-formation process. Meanwhile, the gas phase impeded the mass transfer on the water-hydrate interface and acted as the nucleation site to promote hydrate re-formation. Furthermore, it was noticed that the relationship between the onset time of flow blockage and WESV was linearly positive, however, the amount of hydrate re-formation reduced with increasing WESV.

In this work, a new approach of seawater flooding was proposed for offshore natural gas hydrate (NGH) exploitation, whose feasibility was numerically evaluated based on a large-scale NGH reservoir situated in the Shenhu Area of the South China Sea. Then, two artificial means of well location rearrangement and hydraulic fracturing, as well as their combination, were proposed to be incorporated with seawater flooding to promote gas production from offshore NGH deposits. Simulation results indicated that seawater flooding had great advantages over depressurization and could substantially promote hydrate dissociation, effectively prevent secondary hydrate formation, and greatly facilitate gas production. Furthermore, a stable gas production period could be achieved for a long duration, which made the most significant contribution to gas production. The two artificial means of well location rearrangement and hydraulic fracturing could both promote hydrate dissociation and enhance gas recovery, but the promotion mechanisms were different. When these two methods were combined, hydrate dissociation in the whole hydrate-bearing layer could be further facilitated, and a favorable gas production rate could be obtained with a low water production and a high gas-water ratio, which demonstrated great superiority of the combined method over these two methods when used alone. Therefore, it is expected that the newly proposed approach of seawater flooding combined with well location rearrangement and hydraulic fracturing can be applied in the future commercial offshore NGH development.

Sub-seabed sequestration of carbon dioxide (CO2) is the most promising and feasible disposal of large-scale carbon storage, and the capacity and safety of CO2 sequestration are vital considerations. However, the leakage prevention of injected CO2 lacks necessary solutions and experimental investigations. In this study, the upward leakage process of sequestrated CO2 through water-saturated sediments was simulated at constant 277.15 K and 3.0 MPa, and the dynamic water distributions were detected by magnetic resonance imaging (MRI) technique. Both the flux difference of CO2 inflow and outflow and the MRI images reveal the formation of hydrates inside the sediments with seven water saturation (21.6 %similar to 67.8 %) that represent different stages of sequestrated CO2 leakage process. When the initial water saturation exceeds 26.2 %, the CO2 leakage will be discontinued after several minutes of hydrates formation, indicating the formation of hydrate cap in the CO2-displaced sediment aquifer. Then, the pressure of sequestrated CO2 increases due to the plugging effect of hydrate cap, and the overpressure can reach at least 7.0 MPa. In hydrate cap zone, the overpressure of CO2 greatly improves the storage capacity by at least 13.1 times, which may induce the increase of CO2 sequestration capacity beneath the hydrate cap as well. These results contribute significantly toward safe CO2 sequestration and provide fundamental data for sequestration site selection under sub-seabed areas.

Natural gas hydrates are considered a possible sustainable energy source. Various methods have been proposed and developed in the past few decades to exploit hydrate. The purpose of this paper is to review the research advances, application potential, and limitations of hydrate production methods, including four traditional methods (depressurization, thermal stimulation, inhibitor injection, and CO2 replacement), and novel water flow erosion. For the four traditional methods, there still exist many bottlenecks to achieve commercial hydrate production owing to a series of problems such as vast sand production, ice generation, hydrate reformation, lower energy efficiency, and huge water production. For the water flow erosion method, it can be a novel strategy to promote hydrate decomposition by introducing the chemical potential difference and accelerating mass transfer. Significantly, if the traditional methods assist with water flow erosion, the problems such as insufficient decomposition driving force and ice generation are prevented. However, there is always a gap between the experimental results and the practical application of the water flow erosion method. Therefore, future work should be undertaken to investigate the selection criteria of suitable water flow rates for different reservoirs and the ecological security of the hydrate reservoir during the water flow erosion process.

Natural gas hydrates (NGHs) are a form of natural gas widespread in the seafloor, and the realization of commercial exploitation is of great significance to solve the energy problem. There must be large amount of seawater and gas migration during the hydrate exploitation process. Studies have shown that seawater flow at a constant flow rate will enhance hydrate exploitation, especially at high flow rates. However, the seawater flow rate varies as a result of different seawater saturation in different areas of hydrate reservoir. There is little research on the effects of variational flow rate on hydrate decomposition characteristics. In this study, the effects of step-up flow rate and step-down flow rate modes on hydrate decomposition were investigated using the magnetic resonance imaging (MRI) system. The experimental results showed that the flowing seawater filled the pores that were occupied by free gas or generated by hydrate decomposition, which is beneficial to maintain the hydrate reservoir stability during the exploitation process. Through comprehensive comparison, the energy consumption of the variational flow rate mode was more than 60% less than that of the constant flow rate mode. For low hydrate saturation (22.5%), large seawater flow rate maintained a large driving force (fugacity difference) for hydrate decomposition, the efficiency of step-down flow rate mode was 131.1% greater than the efficiency of step-up mode; for high hydrate saturation (36.5%), low seawater flow rate made the mass transfer between seawater and hydrate more sufficient, the efficiency of step-up mode was 40.6% greater than that of step-down mode.

Understanding the critical velocity at which hydrate particles can be removed from a solid surface is crucial for flow assurance issues. An improved model is proposed to predict the critical velocity in hydrate particle removal process by considering a deformed liquid bridge. In situ experiments were performed for validation, and it was observed that hydrate particle decomposition occurred during the removal process. The particle diameter was determined to be a more dominant factor influencing the critical velocity than the initial liquid bridge volume. The resulting confidence level was 95% within a 30% error, showing its robust capability for engineering applications.