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22 pages, 11083 KiB

Open AccessArticle

A Probabilistic Assessment Framework for Submarine Landslide Susceptibility in Continental Slopes with Rich Gas Hydrates

by Lin Tan, Mingliang Zhou and Feiyang Wang

J. Mar. Sci. Eng. 2023, 11(7), 1366; https://doi.org/10.3390/jmse11071366 - 5 Jul 2023

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Abstract

Submarine landslides in regions enriched with gas hydrates pose a significant threat to submarine pipelines, cables, and offshore platforms. Conducting a comprehensive regional-scale susceptibility assessment is crucial for mitigating the potential risks associated with submarine landslides in gas hydrate enrichment regions. This study [...] Read more.

Submarine landslides in regions enriched with gas hydrates pose a significant threat to submarine pipelines, cables, and offshore platforms. Conducting a comprehensive regional-scale susceptibility assessment is crucial for mitigating the potential risks associated with submarine landslides in gas hydrate enrichment regions. This study conducted a preliminary exploration by presenting a probabilistic assessment framework that integrated database construction, rapid prediction model training, and landslide susceptibility assessment in hydrate enrichment regions. The database was a virtual repository constructed using numerical simulations of hydrate dissociation under various combinations of factors, including water depth, geothermal gradients, seafloor slope gradients, the seafloor temperature’s rate of increase, gas hydrate saturation, and the strength and permeability of sediments. The rapid prediction model was trained using machine learning techniques, relying on the virtual database. A probabilistic assessment was performed using Monte Carlo simulations, with the landslide susceptibility determined by the rapid prediction model. The probability of landslide susceptibility exceeding a certain threshold served as an indicator for classifying the susceptibility of the study area. The proposed framework was implemented in the Shenhu area of the South China Sea, which is a representative region known for its substantial hydrate enrichment and well-developed landslides. The trained rapid prediction model for landslide susceptibility exhibited a speed advantage of over 60,000 times compared to traditional numerical calculation methods. The statistical analysis of the results in Monte Carlo simulations suggested that the landslide susceptibility was subjected to a high level of uncertainty due to limited survey data availability. Based on the probability of landslide susceptibility exceeding 0.4 in Monte Carlo simulations, the study area was classified into three zones of susceptibility: low, moderate, and high levels. Full article

(This article belongs to the Special Issue Advances in Gas Hydrate Development: Experimental and Numerical Simulation)

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13 pages, 2452 KiB

Open AccessArticle

The Effect of Salinity on the Strength Behavior of Hydrate-Bearing Sands

by Shi Shen, Lei Wang, Yang Ge, Jiawei Chu and Huiyong Liang

J. Mar. Sci. Eng. 2023, 11(7), 1350; https://doi.org/10.3390/jmse11071350 - 2 Jul 2023

Cited by 5 | Viewed by 992

Abstract

The first prerequisite for the efficient and safe exploitation of gas hydrate resources is to accurately analyze the primary mechanical performance of hydrate-bearing sediments (HBSs). The mechanical performance of HBSs is complex and affected by many factors, including the reservoir environment in situ [...] Read more.

The first prerequisite for the efficient and safe exploitation of gas hydrate resources is to accurately analyze the primary mechanical performance of hydrate-bearing sediments (HBSs). The mechanical performance of HBSs is complex and affected by many factors, including the reservoir environment in situ (temperature, pore pressure, salinity). Several published studies have demonstrated a correlation of the mechanical behavior of hydrates with temperature and pressure (T-P_P). However, the research on the effect of salinity on the mechanical properties of hydrates or HBSs is still a relatively blank field. This study found that the strength of HBSs decreased with increasing salinity. This phenomenon can be attributed to the influence of salinity on the phase equilibrium state of hydrates. NaCl changed the relationship between the phase equilibrium curve of the hydrate and the T-P_P conditions. The distance between the T-P_P conditions and equilibrium curve was reduced with increasing salinity, which in turn led to a decline in sample strength. Moreover, the effect of the phase equilibrium of hydrates on the mechanical performance of HBSs was further explored. NaCl was added to HBSs to regulate the phase equilibrium state of the hydrate. When the T-P_P conditions were on the phase equilibrium curve, the strength behaviors of HBSs showed a high degree of consistency. Full article

(This article belongs to the Special Issue Advances in Gas Hydrate Development: Experimental and Numerical Simulation)

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14 pages, 4732 KiB

Open AccessArticle

Strain Rate-Dependent Mechanical Response of Hydrate-Bearing Sediments under Plane Strain Condition

by Qi Wu, Yingjie Zhao, Norimasa Yoshimoto, Jinan Guan, Yukio Nakata, Shintaro Kajiyama and Masayuki Hyodo

J. Mar. Sci. Eng. 2023, 11(6), 1161; https://doi.org/10.3390/jmse11061161 - 1 Jun 2023

Cited by 1 | Viewed by 1062

Abstract

Natural gas hydrate has gained significant attention in recent years. To safely and sustainably exploit the natural gas from gas hydrate-bearing sediments, it is crucial to understand the long-term mechanical characteristics of the hydrate reservoir. In this study, the influence of hydrate and [...] Read more.

Natural gas hydrate has gained significant attention in recent years. To safely and sustainably exploit the natural gas from gas hydrate-bearing sediments, it is crucial to understand the long-term mechanical characteristics of the hydrate reservoir. In this study, the influence of hydrate and fine particles on the strain rate dependence of hydrate-bearing sediments under plane strain conditions has been studied. The experimental results show that the strain rate dependency of the mechanical properties of hydrate-bearing sediments is positively correlated with hydrate saturation instead of the morphology of hydrate in sediments. The residual strength of hydrate-bearing sediments is primarily controlled by the hydrate saturation and is independent of the strain rate. Changes in hydrate saturation and fines content can affect the relationship between the strain rate and shear band angle. Finally, the local volumetric expansion effect of hydrate-bearing sediments without fines content is more significant and shows a strong strain rate dependence characteristic. Overall, this study provides valuable insights into the long-term mechanical characteristics of hydrate reservoirs. These insights can contribute to the development of a constitutive model of hydrate-bearing sediments with time dependence in the future, which is meaningful to the exploitation of natural gas hydrate. Full article

(This article belongs to the Special Issue Advances in Gas Hydrate Development: Experimental and Numerical Simulation)

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16 pages, 3376 KiB

Open AccessArticle

A Fully Coupled Thermo-Hydro-Mechanical-Chemical Model for Methane Hydrate Bearing Sediments Considering the Effect of Ice

by Fanbao Cheng, Xiang Sun, Peng Wu, Zhixiang Chen, Tao Yu, Weiguo Liu, Xin Ju and Yanghui Li

J. Mar. Sci. Eng. 2023, 11(4), 766; https://doi.org/10.3390/jmse11040766 - 31 Mar 2023

Cited by 8 | Viewed by 1983

Abstract

The ice generation is one of the challenges facing the methane hydrate depressurization, which, however, has not been fully addressed by existing numerical models for hydrate-bearing sediments (HBS). In this study, we develop a high-fidelity, fully coupled thermo-hydro-mechanical-chemical numerical model that incorporates the [...] Read more.

The ice generation is one of the challenges facing the methane hydrate depressurization, which, however, has not been fully addressed by existing numerical models for hydrate-bearing sediments (HBS). In this study, we develop a high-fidelity, fully coupled thermo-hydro-mechanical-chemical numerical model that incorporates the effect of ice. The model, developed using COMSOL, takes into account water–ice phase change, thermally induced cryogenic suction and constitutive relation in HBS. It is verified well against the temperature, pressure and cumulative gas production of Masuda’s experiment. The model is then employed to investigate multiphysical responses and gas/water production when ice generation is induced by setting a low outlet pressure. The results reveal that ice forms near the outlet boundary of the specimen center, leading to a reduction in intrinsic permeability and fluid velocity and an increase in the bulk modulus of ice-HBS. This enhanced bulk modulus results in higher porosity under axial load. Although the exothermic effect of ice generation promotes the hydrate dissociation, the effect on cumulative gas production is negligible after the ice melts. A negative correlation between ice saturation and water production rate is observed, indicating that a higher gas–water ratio can be achieved by adjusting the ice duration during hydrate production. The developed coupled model proves to be crucial for understanding the effect of ice on hydrate exploitation. Full article

(This article belongs to the Special Issue Advances in Gas Hydrate Development: Experimental and Numerical Simulation)

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15 pages, 3855 KiB

Open AccessArticle

Experimental Study on CH₄ Hydrate Dissociation by the Injection of Hot Water, Brine, and Ionic Liquids

by Siting Wu, Xuebing Zhou, Jingsheng Lu, Deqing Liang and Dongliang Li

J. Mar. Sci. Eng. 2023, 11(4), 713; https://doi.org/10.3390/jmse11040713 - 26 Mar 2023

Cited by 1 | Viewed by 1093

Abstract

Thermal stimulation is an important method to promote gas production and to avoid secondary hydrate formation during hydrate exploitation, but low thermal efficiency hinders its application. In this work, hydrate dissociation was carried out in synthesized hydrate-bearing sediments with 30% hydrate saturation at [...] Read more.

Thermal stimulation is an important method to promote gas production and to avoid secondary hydrate formation during hydrate exploitation, but low thermal efficiency hinders its application. In this work, hydrate dissociation was carried out in synthesized hydrate-bearing sediments with 30% hydrate saturation at 6.9 MPa and 9 °C. Ionic liquids, such as 1-butyl-3-methylimidazolium chloride (BMIM-Cl) and tetramethylammonium chloride (TMACl), were injected as heat carriers, and the promotion effects were compared with the injection of hot water and brine. The results showed that the injection of brine and ionic liquids can produce higher thermal efficiencies compared to hot water. Thermodynamic hydrate inhibitors, such as NaCl, BMIM-Cl, and TMACl, were found to impair the stability of CH₄ hydrate, which was conducive to hydrate dissociation. By increasing the NaCl concentration from 3.5 to 20 wt%, the thermal efficiency increased from 37.6 to 44.0%, but the thermal efficiencies experienced a fall as the concentration of either BMIM-Cl or TMACl grew from 10 to 20 wt%. In addition, increasing the injection temperature from 30 to 50 °C was found to bring a sharp decrease in thermal efficiency, which was unfavorable for the economics of gas production. Suitable running conditions for ionic liquids injection should control the concentration of ionic liquids under 10 wt% and the injection temperature should be around 10 °C, which is conducive to exerting the weakening effect of ionic liquids on hydrate stability. Full article

(This article belongs to the Special Issue Advances in Gas Hydrate Development: Experimental and Numerical Simulation)

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17 pages, 7541 KiB

Open AccessArticle

Strength Behaviors and Constitutive Model of Gas-Saturated Methane Hydrate-Bearing Sediment in Gas-Rich Phase Environment

by Yuqi Sun, Yiqun Zhang, Li Liu, Jingsheng Lu, Shouceng Tian and Gensheng Li

J. Mar. Sci. Eng. 2023, 11(1), 142; https://doi.org/10.3390/jmse11010142 - 7 Jan 2023

Cited by 3 | Viewed by 1511

Abstract

Natural gas hydrates occupy an important position in the development of clean energy around the world in the 21st century. It is of great significance to research the mechanical properties of methane hydrate-bearing sediment (MHBS). In this paper, gas-saturated MHBS were synthesized based [...] Read more.

Natural gas hydrates occupy an important position in the development of clean energy around the world in the 21st century. It is of great significance to research the mechanical properties of methane hydrate-bearing sediment (MHBS). In this paper, gas-saturated MHBS were synthesized based on the self-developed triaxial compressor apparatus. The triaxial shear tests were performed at temperatures of 2 °C, 3 °C, and 5 °C and confining pressures of 7.5 MPa, 10 MPa, and 15 MPa. Results indicate that the axial strain process can be divided into three stages: initial elastic deformation, initial yield deformation, and strain softening. When confining pressure is increased, the shear strength of MHBS increases at a lower confining pressure. In contrast, shear strength appears to decrease with increasing confining pressure at a higher confining pressure. There is a negative correlation between temperature and shear strength of MHBS. The initial yield strain of MHBS increases in condition due to the increase in confining pressure and the decrease in temperature. The change in strength degradation is kept within 2 MPa. Using test data, the Duncan-Chang model was modified to describe the strength behaviors of gas-saturated MHBS. The accuracy of the model was verified by comparing calculated values with test data. Full article

(This article belongs to the Special Issue Advances in Gas Hydrate Development: Experimental and Numerical Simulation)

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15 pages, 5248 KiB

Open AccessArticle

Numerical Simulation on Sand Production Based on Laboratory Gas Hydrate Production Experiment

by Jingsheng Lu, Guangrong Jin, Dongliang Li, Deqing Liang, Yong He, Lingli Shi, Yiqun Zhang and Youming Xiong

J. Mar. Sci. Eng. 2023, 11(1), 110; https://doi.org/10.3390/jmse11010110 - 5 Jan 2023

Cited by 3 | Viewed by 1444

Abstract

Gas from natural gas hydrate (NGH) is priced competitively with gas prices. Most marine NGH is stored in low cementing strata, which easily cause sand production problems, restricting the commercial production and environmental safety of NGH’s development. This study applied a numerical simulation [...] Read more.

Gas from natural gas hydrate (NGH) is priced competitively with gas prices. Most marine NGH is stored in low cementing strata, which easily cause sand production problems, restricting the commercial production and environmental safety of NGH’s development. This study applied a numerical simulation on sand production in hydrate-bearing sediments’ (HBS) exploitation. The numerical simulation on sand production was carried out for different productions of laboratory NGH exploitation. The results show radial strain appeared to be deformed away from the wellbore and show radial displacement close to the wellbore during mining. Due to the overburden stress condition, the boundary condition wall was a displace less rigid body. The radial displacement was greatly affected by depressurization, which showed the displacement to the wellbore and sanding. The radial strain was dominant by the shear shrinkage phenomenon in the mechanical model, while the reservoir’s radial displacement was away from the wellbore instead. The balance between the fluid driving force of production rates towards the wellbore and radial displacement drawing away from the wellbore is significant to sand production in HBS. The dominant forces of sanding were different mechanical and hydraulic combinations in three periods of GH production. Full article

(This article belongs to the Special Issue Advances in Gas Hydrate Development: Experimental and Numerical Simulation)

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13 pages, 5600 KiB

Open AccessArticle

Study on the Mechanical Properties of Silty Clay Sediments with Nodular Hydrate Occurrence

by Cheng Lu, Pengfei Xie, Hui Li, Xuhui Zhang, Xiaobing Lu, Bin Zhang, Ziqin Zhang, Xuwen Qin, Shuai Zhang and Hang Bian

J. Mar. Sci. Eng. 2022, 10(8), 1059; https://doi.org/10.3390/jmse10081059 - 1 Aug 2022

Cited by 4 | Viewed by 1606

Abstract

Natural gas hydrates are a strategic energy resource in China. The China Geological Survey has discovered segregated hydrate mass formations under the seepage mechanism in the South China Sea through exploration, and gas hydrates occur in nodular, massive, and vein formations in silty [...] Read more.

Natural gas hydrates are a strategic energy resource in China. The China Geological Survey has discovered segregated hydrate mass formations under the seepage mechanism in the South China Sea through exploration, and gas hydrates occur in nodular, massive, and vein formations in silty clay sediment. Previous work has focused on the analysis of sediment mechanical properties with respect to the uniform distribution of natural gas hydrates in pore spaces, but the mechanical properties of hydrate-bearing sediments containing segregated hydrate masses are not well understood. Spherical hydrates are used to characterize nodular hydrates, a method is proposed for the preparation of sediment samples containing segregated hydrates masses, and a series of triaxial compression tests are carried out on the samples containing spherical hydrates with two kinds of particle sizes at a certain volume fraction. The paper presents triaxial stress–strain curves for the samples containing spherical hydrates. A model for predicting elastic modulus is established. The results present two distinct stages in the triaxial compression tests of silty clay sediments containing spherical hydrates; they also show that the elastic moduli predicted by the model are in good agreement with the experimental results when the model parameters are set at α = 0.5 and β = −0.21. These results provide fundamental mechanical parameters for the safety evaluation of strata containing segregated gas hydrates. Full article

(This article belongs to the Special Issue Advances in Gas Hydrate Development: Experimental and Numerical Simulation)

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19 pages, 3001 KiB

Open AccessArticle

Modeling the Mechanical Behavior of Methane Hydrate-Bearing Sand Using the Equivalent Granular Void Ratio

by Jie Shen, Abraham C. F. Chiu and Charles Wang Wai Ng

J. Mar. Sci. Eng. 2022, 10(8), 1040; https://doi.org/10.3390/jmse10081040 - 28 Jul 2022

Cited by 4 | Viewed by 1477

Abstract

For the safe extraction of methane from hydrate reservoirs, modeling the mechanical behavior of the methane hydrate-bearing soil properly is crucial in order to enable designers to analysis hydrate-dissociation-induced geotechnical failures. Hydrate morphology is one of major factors affecting the mechanical behavior of [...] Read more.

For the safe extraction of methane from hydrate reservoirs, modeling the mechanical behavior of the methane hydrate-bearing soil properly is crucial in order to enable designers to analysis hydrate-dissociation-induced geotechnical failures. Hydrate morphology is one of major factors affecting the mechanical behavior of soil containing hydrate. This paper presents a new constitutive model for methane hydrate-bearing sand (MHBS) using the equivalent granular void ratio as a state variable, which can quantify the effects of the pore-filling and load-bearing hydrate morphology under a unifying framework. The proposed model is a combination of generalized plasticity and an elastic damage model so as to take into account the observed frictional and bonding aspects of MHBS, respectively. By using the concept of state-dependent dilatancy, the equivalent granular void ratio is formulated and adopted in the generalized plasticity model. In addition, a nonlinear damage function is implemented to elucidate the degradation of hydrate bonds with respect to shearing. Compared with the basic generalized plasticity model for host sand, only three additional parameters are required to capture key mechanical behaviors of MHBS. By comparing the triaxial test results of MHBS synthesized from a range of host sands with a predicted behavior by the proposed model, it is demonstrated that the new model can satisfactorily capture the stress–strain and volumetric behavior of MHBS under different hydrate saturations, confining pressures, and void ratios. Full article

(This article belongs to the Special Issue Advances in Gas Hydrate Development: Experimental and Numerical Simulation)

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18 pages, 5309 KiB

Open AccessArticle

Analysis of the Characteristics of Pore Pressure Coefficient for Two Different Hydrate-Bearing Sediments under Triaxial Shear

by Ruchun Wei, Chao Jia, Lele Liu and Nengyou Wu

J. Mar. Sci. Eng. 2022, 10(4), 509; https://doi.org/10.3390/jmse10040509 - 6 Apr 2022

Cited by 7 | Viewed by 2770

Abstract

It is important to determine the volumetric change properties of hydrate reservoirs in the process of exploitation. The Skempton pore pressure coefficient

A

can characterize the process of volume change of hydrate-bearing sediments under undrained conditions during shearing. However, the interrelationship between

A

[...] Read more.

It is important to determine the volumetric change properties of hydrate reservoirs in the process of exploitation. The Skempton pore pressure coefficient

A

can characterize the process of volume change of hydrate-bearing sediments under undrained conditions during shearing. However, the interrelationship between

A

value responses and deformation behaviors remain elusive. In this study, effects of hydrate saturation and effective confining pressure on the characteristics of pore pressure coefficient

A

are explored systematically based on published triaxial undrained compression test data of hydrate-bearing sand and clay-silt sediments. Results show that there is a higher value of the coefficient

A

with increasing hydrate saturation at small strain stage during shearing. This effect becomes more obvious when the effective confining pressure increases for hydrate-bearing sand sediments rather than hydrate-bearing clayey-silt sediments. An increasing hydrate saturation leads to a reduction in A values at failure. Although A values at failure of sand sediments increase with increasing effective confining pressure, there are no same monotonic effects on clayey-silt specimens. A values of hydrate-bearing sand sediments firstly go beyond 1/3 and then become lower than 1/3 at failure even lower than 0, while that of hydrate-bearing clayey-silt sediments is always larger than 1/3 when the effective confining pressure is high (e.g., >1 MPa). However, when the effective confining pressure is small (e.g., 100 kPa), that behaves similar to hydrate-bearing sand sediments but always bigger than 0. How the A value changes with hydrate saturation and effective confining pressure is inherently controlled by the alternation of effective mean stress. Full article

(This article belongs to the Special Issue Advances in Gas Hydrate Development: Experimental and Numerical Simulation)

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13 pages, 4375 KiB

Open AccessEssay

A Study on the Law of Ring-Shear Band Evolution

by Jiyan Qiao, Xiaobing Lu and Xuhui Zhang

J. Mar. Sci. Eng. 2023, 11(1), 70; https://doi.org/10.3390/jmse11010070 - 2 Jan 2023

Viewed by 897

Abstract

A separate-bottom Couette cell is used to carry out ring-shear tests on sand and hydrate, and the evolution of shear bands is recognized using the tracer method. Based on experimental verification, a numerical simulation method is applied to study the shear band variation [...] Read more.

A separate-bottom Couette cell is used to carry out ring-shear tests on sand and hydrate, and the evolution of shear bands is recognized using the tracer method. Based on experimental verification, a numerical simulation method is applied to study the shear band variation law with the height and strength of the sample. Analyzing the distribution of stress and strain gives a dimensionless number

(c + ρ g H \cdot t g φ) H / α G L

that affects the characteristics of the shear band, which indicates that the evolution direction and form of the shear band are controlled by the stiffness ratio of strength to load. Furthermore, the dimensionless law of the height and width of the shear band is given quantitatively. Full article

(This article belongs to the Special Issue Advances in Gas Hydrate Development: Experimental and Numerical Simulation)

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16 pages, 3662 KiB

Open AccessTechnical Note

Shear Modulus of a Carbonate Sand–Silt Mixture with THF Hydrate

by Yuzhe Ren, C. F. Chiu, Lu Ma, Y. P. Cheng, Litong Ji and Chao Jiang

J. Mar. Sci. Eng. 2022, 10(10), 1519; https://doi.org/10.3390/jmse10101519 - 18 Oct 2022

Cited by 1 | Viewed by 1301

Abstract

The maximum shear modulus (G_max) is an important factor determining soil deformation, and it is closely related to engineering safety and seafloor stability. In this study, a series of bender element tests was carried out to investigate the G_max [...] Read more.

The maximum shear modulus (G_max) is an important factor determining soil deformation, and it is closely related to engineering safety and seafloor stability. In this study, a series of bender element tests was carried out to investigate the G_max of a hydrate-bearing carbonate sand (CS)–silt mixture. The soil mixture adopted a CS:silt ratio of 1:4 by weight to mimic the fine-grained deposit of the South China Sea (SCS). Tetrahydrofuran (THF) was used to form the hydrate. Special specimen preparation procedures were adopted to form THF hydrate inside the intraparticle voids of the CS. The test results indicate that hydrate contributed to a significant part of the skeletal stiffness of the hydrate-bearing CS–silt mixture, and its G_max at 5% hydrate saturation (S_h) was 4–6 times that of the host soil mixture. Such stiffness enhancement at a low S_h may be related to the cementation hydrate morphology. However, the G_max of the hydrate-bearing CS–silt mixture was also sensitive to the effective stress for an S_h ranging between 5% and 31%, implying that the frame-supporting hydrate morphology also plays a key role in the skeletal stiffness of the soil mixture. Neither the existing cementation models nor the theoretical frame-supporting (i.e., Biot–Gassmann theory by Lee (BGTL)), could alone provide a satisfactory prediction of the test results. Thus, further theoretical study involving a combination of cementation and frame-supporting models is essential to understand the effects of complicated hydrate morphologies on the stiffness of soil with a substantial amount of intraparticle voids. Full article

(This article belongs to the Special Issue Advances in Gas Hydrate Development: Experimental and Numerical Simulation)

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