Natural Gas Hydrate Production Research Articles

Monitoring the developmental trend and competitive landscape of natural gas hydrate through patent analysis.

Natural gas hydrate (NGH) is a significant alternative energy resource in achieving carbon neutrality. The developmental trend and competitive landscape of NGH exploitation and production are crucial for policymakers in government, managers of enterprises, and researchers. This study introduces a novel framework for conducting an in-depth analysis of NGH, integrating patentometrics, technology evolution, and correlation relationships to monitor developmental trends and competitive landscape through patent analysis. The results indicate that China, the USA, and Japan have distinct technology advantages. Current technological developments in the NGH field focus primarily on extraction technologies, equipment, and processing systems. The co-opetition analysis among countries reveals that the most extensive international cooperation network is primarily in Europe and the USA, with national partnerships in Asia concentrated in China and Japan. Institutional cooperation remains limited, primarily within universities in China, while both the USA and Japan foster collaboration between enterprises. The competitive landscapes of key NGH-related technologies among countries and institutions are also examined. This study contributes not only to monitoring the developmental trend and competitive landscape in NGH but also to providing policy recommendations for government and enterprises regarding strategic management and collaborative innovation.

Monitoring the developmental trend and competitive landscape of natural gas hydrate through patent analysis.

Natural gas hydrate (NGH) is a significant alternative energy resource in achieving carbon neutrality. The developmental trend and competitive landscape of NGH exploitation and production are crucial for policymakers in government, managers of enterprises, and researchers. This study introduces a novel framework for conducting an in-depth analysis of NGH, integrating patentometrics, technology evolution, and correlation relationships to monitor developmental trends and competitive landscape through patent analysis. The results indicate that China, the USA, and Japan have distinct technology advantages. Current technological developments in the NGH field focus primarily on extraction technologies, equipment, and processing systems. The co-opetition analysis among countries reveals that the most extensive international cooperation network is primarily in Europe and the USA, with national partnerships in Asia concentrated in China and Japan. Institutional cooperation remains limited, primarily within universities in China, while both the USA and Japan foster collaboration between enterprises. The competitive landscapes of key NGH-related technologies among countries and institutions are also examined. This study contributes not only to monitoring the developmental trend and competitive landscape in NGH but also to providing policy recommendations for government and enterprises regarding strategic management and collaborative innovation.

Time-dependent deformation of marine gas hydrate-bearing strata conditioned to a wellbore: Experiments and implications

Natural gas hydrate (NGH) is a promising alternative energy resource, whereas marine NGH exploitation suffers from various potential geo-environmental concerns. The existence of a production wellbore not only provides channel for gas extraction, but also causes specific boundary conditions for the occurrence of geo-environmental risks. Almost all kinds of geo-environmental risks during NGH production initiate from time-dependent deformation of the near-wellbore strata. In this paper, the pressuremeter test (PMT) method was firstly introduced to investigate the lateral creep behaviors of the NGH-bearing sediment. The PMT probe was used to apply outwards lateral stress onto the hydrate-bearing sediment to simulate the horizontal stress state of the near-wellbore reservoir during drilling, completion, and/or hydraulic fracturing, during which the pressure inside the wellbore is higher than the reservoirs static pressure. The results indicated that the decelerated creep accounts for more than 70% of the lateral strain before the effective stress reaches the long-term strength. The hardening modulus of the sediment was proven to be increased from the magnitude of ∼102 MPa–∼103 MPa, when the hydrate saturation increases from 2.2% to 50.0%. The PMT creep test results can be used for interpretation of the in-situ horizontal stress and geomechanical parameters such as pro-plastic pressure and ultimate pressure. Based on the fact that PMT test results complies with the previous results from other techniques, a field-applicable PMT technique for NGH reservoir test is suggested to be developed in future study, and a systematic hypothesis and workflow for NGH reservoir modelling is also proposed in this study.

A bibliometric analysis of two-decade research efforts in turning natural gas hydrates into energy

As a promising energy source, natural gas hydrate production has ignited scientific research in multidisciplinary fields globally, generating a vast amount of literature. A review in multidisciplinary and international perspectives will contribute to a comprehensive understanding about this field. This paper maps the two-decade spatiotemporal landscapes of scientific research on natural gas hydrate (NGH) production with a bibliometric analysis. The landscapes suggested that the study on gas hydrate production covers multiple research fields and international collaboration. Research topics on NGH production experienced three stages involving resource exploration, productivity enhancement, and environmental issues. An analysis of the research with the most activity showed that Energy & Fuel, Chemical Engineering, and Multidisciplinary Geoscience were the most active, with Thermodynamics, Geological Engineering, Ocean Engineering, and Civil Engineering experiencing fast growth. In terms of the landscape of international collaboration, the United States and European countries were the main contributors in the early stage, followed by China after 2008. The breakthrough of the commercial production threshold relies on the international collaboration and the multidisciplinary research among Thermodynamics, Geological Engineering, Ocean Engineering and Civil Engineering to characterize the energy consumption, gas production rate, and reservoir stability.

Experimental investigation on dynamic mechanical characteristics of casing during hydrate decomposition using depressurization

During marine hydrate production, some potentially undesirable consequences of casing strength failure are prone to occur. However, conventional methods used in casing mechanical analysis is unable to handle casing failure accurately, as the macro constitutive model employed in current simulation could not reflect the real stress-strain relationship of hydrate reservoir. This paper aimed at proposing an experimental method to investigate dynamic mechanical responses of casing during hydrate decomposition based on the self-developed experiment system, in which overlying stress was simulated to construct a similar mechanical environment of casing. By conducting laboratory-scale tests, the effect of overlying stress on casing mechanical behaviors could be revealed, and comparative analysis of casing axial stress obtained from experiment and simulation was carried out. Experimental results indicated that axial stress and extrusion pressure of casing increased 20.91 % and 7.18 % with the increase of overlying stress, respectively. Casing axial stress increased first and then gradually reached stable in different depressurization stage, which agreed well with the results calculated by numerical simulation. The maximum error between experimental data and simulation results was 11.7 %, 11.1 % and 10.2 % in different depressurization stage, respectively. The proposed approach could provide reference for casing safety design of natural gas hydrate production.

Pore-scale study of the dynamic evolution of multi-phase seepage parameters during hydrate dissociation in clayey silt hydrate-bearing sediments

Unlike conventional oil and gas sources, natural gas hydrate (NGH) is stored in solid form within the pores of loose submarine sediments. Production by any method poses a complex problem involving phase transition and multi-phase flow of gas and water. The evolution of hydrate dissociation from the solid phase to gas and liquid phases in a hydrate reservoir is a dynamic process incorporating pore structure and multi-phase seepage. The solid–gas-liquid phase dissociation of NGH affects reservoir seepage properties by altering the micropore structure of the hydrate-bearing sediments. This study, based on the lattice Boltzmann method of modeling fluids flow, clearly demonstrates the micropore-scale changes in pressure and heat transfer that take place during hydrate dissociation. Pore-scale heterogeneity is found to influence the complex development of hydrates in a porous clayey silt medium during dissociation. The effect of hydrate, gas and water saturation on the evolution of relative permeability is identified, and the relationship between relative permeability and hydrate saturation during hydrate dissociation is quantified. These findings provide theoretical support for the implementation of marine natural gas hydrate production testing and future commercial production in China.

Investigation of creep and transport mechanisms of CO2 fracturing within natural gas hydrates

Natural gas hydrates (NGH) are considered as a potential energy resource in the future because of its dramatic energy reserves. NGH formation owns the characteristic of viscoelasticity, to some extent behaves both like liquid and solid. Ignoring the viscoelastic properties of NGH can lead to inaccurate prediction of related characteristics. In 2020, horizontal-well drilling technology was adopted to improve the stimulation performance of NGH reservoirs in the Shenhu area of China. This indicates that it is possible to apply hydraulic fracturing or CO2 fracturing in the reservoir reconstruction of NGH. CO2 fracturing can generate more fracture networks and connect micro fractures. Additionally, CO2 can be stored geologically and integrated with injection, production, and storage. This study provides a relatively detailed demonstration of the formation and basic mechanical properties of NGH. Then, a comprehensive overview of potential NGH production enhancement methods is conducted. Finally, an evaluation method for dual conductivity considering creep effect is proposed, and the results show that the creep effect of NGH can reduce the conductivity of fractures and matrices. Thus, it is of great importance to investigate the viscoelastic properties of NGH formation, for the long-term prediction of the geo-mechanical response to gas extraction.

Exploration of production capacity-geomechanical evaluation and CO2 reinjection repair strategy in natural gas hydrate production by multilateral horizontal wells

Multilateral horizontal wells (MHW) can enhance gas production by expanding the natural gas hydrate (NGH) decomposition area, but inevitably leading to larger changes in reservoir pressure and rock stresses, causing subsidence or disasters. Therefore, this study established a THMC model that evaluated the production effect and geomechanical response of MHW in Class-I reservoirs with different permeability and proposed a CO2 reinjection repair strategy to achieve subsidence recovery and carbon storage. Results indicate that MHW with vertical branch arrangements performs best in short-term production for low-permeability reservoirs, while the temperature and pressure evolution in high-permeability reservoirs is more drastic than in low-permeability reservoirs, producing many unfavorable factors. Higher capacity also results in larger strata subsidence and stress changes, affecting the reservoir stability and wellbore safety: (i) subsidence occurs mainly in the NGH sediment and the upper zone of the wellbore; (ii) normal stresses around the wellbore increase most significantly in the vertical direction, while shear stresses generally decrease. The repair indicators of subsidence recovery rate, energy recovery rate, and hydrate conversion rate suggest that reinjecting CO2 into depleted NGH reservoirs can improve the mechanical properties of the strata and achieve carbon storage via the hydrate method as well.

Numerical Simulation of Hydrate Dissociation Behaviors in Hydrate Reservoir with Different Properties during Horizontal Well Drilling

The effectiveness of horizontal well drilling in improving the gas recovery efficiency of hydrate production makes it a promising technology for commercial exploitation. However, during horizontal well drilling in hydrate reservoirs, it is crucial to control hydrate dissociation to ensure the reservoir stability and drilling safety. In this work, a two-dimensional model using polar coordinates was built to study the influences of hydrate reservoir characteristics and drilling fluid salinity on gas production. The simulation applies to the hydrate reservoir of the second natural gas hydrate (NGH) production test in the Shenhu area of the South China Sea. The characteristics of hydrate dissociation and secondary formation and the drilling invasion behavior in the NGH layer and the mixing layer (free gas + hydrate) during horizontal well drilling were analyzed and compared. The simulation results indicated that the pressure and temperature transmission rates in the mixing layer (free gas + hydrate) are higher than those in the NGH layer. The invasion amount of drilling fluid in the mixing layer is 18.8 times more than that in the NGH layer. Under the high invasion of the drilling fluid, the hydrate dissociation amount in the mixing layer is similar to that of the NGH layer even though the initial hydrate saturation of the NGH layer was 2.65 times that of the mixing layer. The area of the hydrate dissociation in the mixing layer is much larger than that in the NGH layer, which may lead to the increase in risk of wellbore instability. The secondary hydrate formation is only observed in the NGH layer, which inhibits the drilling fluid invasion. The salinity of the drilling fluid has a more significant impact on the hydrate dissociation near the wellbore in the mixing layer compared to the NGH layer. With the increase in salinity from 3.05 wt% to 20 wt%, the hydrate dissociation range in the mixing layer increases from 0.16 m to 0.23 m, while the hydrate dissociation range in the NGH layer does not significantly change.

Pore-scale visualization of hydrate dissociation and mass transfer during depressurization using microfluidic experiments

Natural gas hydrate is a potential low-carbon energy resource. Extracting this resource efficiently remains a challenge, due to the intricacies involved in multiphase reactive transport during hydrate dissociation. This study employs microfluidic experimental techniques to observe pore-scale interactions during depressurization-induced hydrate dissociation, utilizing high-resolution spatial and temporal imagery. Supervised machine learning algorithms segmented these microscopic images, enabling the analysis of phase saturation changes and hydrate dissociation rates. The study identifies two distinct hydrate forms in microfluidic chips: hydrate films and crystals, each exhibiting unique dissociation characteristics. Hydrate films decomposed rapidly in response to pressure reductions below equilibrium with the dissociation rate of O(10−1 %/s) under stationary gas–water conditions. When considering gas–water migration, the gas encapsulated in the hydrate film will be displaced by water, leading to a reduced dissociation rate. The hydrate crystal dissociation is much slower with the dissociation rate of O(10−3 %/s) under stationary gas–water conditions compared to the hydrate film, hindered by mass transfer limitations. Gas-water migration can enhance crystal dissociation. The formation and expansion of gas microbubbles under depressurization accelerated hydrate dissociation within a confined temporal and spatial range, which was known as the self-promotion mechanisms. Significantly, depressurization-induced gas slug flow increases the dissociation rate by more than one order of magnitude, compared to that in stationary gas–water conditions. The depressurization process played a significant role, not only by facilitating thermodynamic feasibility for hydrate dissociation but also by inducing the critical gas slug flow. These results of the present study provide theoretical guidance for improving natural gas hydrate production efficiency.

An environmental-benign method to enhance hydrate production in clayey silty reservoir via splitting grouting

During the pivotal period of energy transition between carbon neutrality and carbon peaking, it is crucial to develop the natural gas hydrate (NGH) due to the advantages of high energy density and non-pollution. However, the NGH production also faces several significant environmental challenges that need to be addressed such as seawater intrusion, methane leakage, and submarine landslides. Therefore, an environmental-benign method to enhance hydrate production in clayey silty reservoir via splitting grouting was introduced. As a novel method for achieving the two goals of both reservoir skeleton reinforcement and permeability enhancement, is proposed to stimulate NGH reservoir. Herein, the formulation based on foamed ultrafine cement-based grout was established. The results showed that the grout has good fluidity with initial viscosity ∼500 mPa·s and consolidation time in the range of 28–50 min. Meanwhile, the consolidation performance evaluation was studied, and it showed that the consolidation is with porosity in the range of 9.41%–46.59%, permeability in the range of 51.23 mD-789.07 mD, and compressive strength in the range of 3.31 MPa–3.88 MPa. The internal pore channels are uniformly distributed with a large number of stomatal strings, which can be used as a multistage seepage channel for gas and water production from hydrate decomposition. To further examine the stimulating behavior, the effect of hydrate saturation, grouting well diameter, grouting flow rate, and in-situ stress on splitting pressure and grouting volume were analyzed, and it was found that the grout has good ability to split fracturing and grout diffusion under the conditions of axial pressure of 5 MPa, confining pressure of 4 MPa, and hydrate saturation not more than 0.30. In addition, an environmental-benign high-efficient reservoir stimulation method on a laboratory scale was proposed. The findings could provide fundamental insights and recommendations for the implementation of splitting grouting in NGH reservoirs.

Cementation breaking and grit separation characteristics of weakly cemented natural gas hydrate by a new structure hydrocyclone

Solid fluidization exploitation technology is one of the most promising methods for the safe exploitation of shallow natural gas hydrates (NGH) in the deep sea. However, after primary jet breaking in the solid fluidization exploitation, the hydrate and argillaceous silt particles are in a state of weak cementation and agglomeration. When the argillaceous silt particles enter the offshore drilling platform with the drilling fluid, it causes substantial sand production, high energy consumption, and pipeline blockage, considerably affecting the development process of the NGH test. Therefore, in-situ desanding has become one of the key technologies of solid fluidization exploitation. In this study, a coupling method of hydrocyclone cementation breaking and hydrocyclone desanding was proposed, and a new structure of cementation breaking and grit separation hydrocyclone was designed. In a lab-designed cementation breaking and grit separation hydrocyclone system, the cementation breaking and grit separation characteristics of weakly cemented NGH similar materials in the swirl flow field were studied. The experimental results under the operating conditions studied show that the swirl flow field exhibits good cementation breaking and grit separation performance for weakly cemented NGH similar materials. For weakly cemented NGH similar materials with a compressive strength of 0.06–0.42 MPa, the cementation breaking efficiency of the cementation breaking and grit separation hydrocyclone is in the range of 27.17–99.61%. After the cementation breaking, the mixture of hydrate similar materials (hydrophilic polyethylene powder) and quartz sand was separated in the hydrocyclone. The separation efficiency of hydrate similar materials is 27.59–99.64%, and the desanding efficiency is 87.66–99.98%. The cementation breaking and grit separation of weakly cemented NGH similar materials were successfully realized. Based on the experimental data, a multiple regression model for the cementation strength of similar materials and the swirl flow field strength was established using the least squares method. This method is expected to provide new technical support for in-situ desanding and safe exploitation of the deep-sea shallow NGH production process.

Numerical analysis of the production behaviors and geomechanical responses during natural gas hydrate production by vertical wells fracturing

Trial extraction of natural gas hydrates has made great progress, but the available capacity is still orders of magnitude away from commercial development. Reservoir modification measures are a potential approach to achieve commercial recovery, and previous studies have demonstrated the feasibility of fracturing modification in hydrate reservoirs. The laboratory experimental research on hydrate production by vertical well fracturing combined with depressurization method has been carried out to prove the effectiveness of improving capacity. The presence of fractures enhances conductivity near the wellbore, thereby facilitating the propagation of pressure drops and augmenting thermal energy transfer. Whereas hydrate decomposition causes a reduction in reservoir strength and stiffness, potentially resulting in formation subsidence, wellbore failure, and other catastrophic events. Therefore, it is important to understand the geomechanical response for designing fracturing modifications in hydrate reservoirs. As a continuation of previous research, we constructed a coupled model of thermal-hydro-mechanical to investigate the production characteristics and geomechanical response patterns during vertical well fracturing combined with depressurization method to extract hydrates. The results show that fracturing modification helps to shorten the duration of “self-locking effect” and improve the gas-water ratio of gas hydrate extraction. Fractures induces the stress concentration near the wellbore, exacerbating both the subsidence and uplift of caprocks. Under the condition of controlled geological risk, priority should be given to extracting reservoirs with high saturation and high original permeability.

On the Influence of Fine Particle Migration and Deposition on Gas Hydrate Production: Insights from the First Trial Production in the South China Sea

Summary As a promising alternative energy source with vast reserves, the primary challenge in exploiting natural gas hydrates (NGHs) lies in achieving long-term safety and efficiency. Several production tests conducted globally have demonstrated that the production of solid particles is one of the main factors contributing to this issue. However, most studies on assessing NGH production capacity have overlooked this phenomenon and its impact on production performance, potentially compromising the accuracy of simulation results. Therefore, this study aims to quantitatively analyze the impact of fine particle migration and deposition on gas production by numerically examining the first NGH production test conducted in the Shenhu area of the South China Sea (SCS). Through comparison of simulation results with the reconstructed gas production curve, the model was calibrated and enabled an explanation of unique gas production performance during field testing. It is found that the deposition of solid particles reduced the gas production rate by nearly an order of magnitude during the initial stage of exploitation. Long-term simulation results indicate that only the three-phase layer (TPL) remains partially unobstructed and serves as the primary source of gas production in later stages. Sensitivity analysis reveals that both depressurization and particle control strategies exert significant effects on particle migration, which in turn affects gas production. Specifically, the increase in depressurization amplitude does not necessarily lead to improved gas production behavior beyond a certain threshold (approximately 6 MPa). This study illustrates the inherent trade-off between preventing particle production and increasing gas production, highlighting the need for a safe and efficient production scheme that strikes a balance between these two objectives. Based on the current study, it is preliminarily recommended to maintain a depressurization amplitude of approximately 6 MPa with a maximum depressurization rate of 3 MPa/d and ensure a fine particle filtration ratio of no less than 95%.

Influence of depressurization mode on natural gas hydrate production characteristics: One-dimensional experimental study

The latest research suggests that using depressurization methods and their enhancements could be the most effective way to produce natural gas hydrates efficiently in marine areas. Therefore, it's crucial to investigate the gas production behavior and the changes in temperature and pressure within natural gas hydrate reservoirs under different depressurization methods. In this paper, the self-developed one-dimensional natural gas hydrate production simulation test system was utilized to conduct one-dimensional depressurization production tests during two depressurization modes of significant depressurization and slow depressurization. The findings indicated that the minimum temperature increment-depressurization rate curves in significant depressurization tests exhibited irregular patterns, primarily due to the frequency and intensity of hydrate reformation, while the smaller the depressurization rate, the larger the minimum temperature increment and the smaller the maximum dissociation rate under slow depressurization. In addition, when maintaining a fixed hydrate saturation level, the peak gas production rate showed a decline with decreasing depressurization rate. However, the exact correlation between the peak gas production rate and the depressurization rate remained uncertain due to the unpredictable nature of the “armoring” effect.

Fine Particle Migration in a Gas Hydrate Sand: Single- and Two-Phase Fluid Using a Device for Observation at the Pore Scale

The production of natural gas hydrates will change the cementation strength, porosity, and effective stress in the stratum, which may lead to engineering and geological disasters. Sand production is a phenomenon where sand particles are carried out of the reservoir along with fluids during gas extraction, posing challenges to safe and sustainable production. This study explored the mechanism of fine particle migration in multiphase flow by a microscopic visualization test device. The device can inject a gas–liquid–solid phase at the same time and allow real-time observation. Experimental tests on fine particle migration of single- and two-phase fluid flow were carried out considering different conditions, i.e., fine particle concentration, fine particle size, fluid flow rate, and gas–liquid ratio. The results show that in single-phase fluid flow, the original gas will gradually dissolve in the liquid phase, and finally stay in the test device as bubbles, which can change the pore structures, resulting in the accumulation of fine particles at the gas–liquid interface. In two-phase fluid flow with mixed gas–water fluids, there are two flow modes of gas–liquid flow: mixed flow and separated flow. The interfacial tension at the gas–liquid interface can effectively migrate fine particles when the gas–liquid flows alternately and the sand production rate further increases as the gas–liquid ratio increases. In addition, changes in the concentration of fine particles, particle size, fluid flow rate, and the gas–liquid ratio will affect the migration of fine particles, leading to differences in the final sand production.

Study of gas-liquid two-phase flow characteristics in hydrate-bearing sediments

The characteristics of gas-liquid two-phase flow in hydrate-bearing sediments (HBS) are critical for natural gas hydrate production. This study experimentally investigates the flow patterns in the presence of hydrates and employs the phase field method for accurate simulations. Additionally, flow pressure differences in a capillary model were analyzed, and the impacts of occurrence patterns, saturation, and wettability of hydrates were investigated by numerical simulation. The results show that flow patterns are predominantly influenced by flow velocity. At high flow rates (Reynolds number >10), different gas-liquid ratios result in a variety of flow patterns, including annular, Taylor, and bubble flow. However, at lower flow rates (Reynolds number <10), the flow pattern consistently manifests as Taylor flow, regardless of gas-liquid ratio variations. Notably, the Jamin effect (a significant pressure differential caused by hydrates within the flow) was observed, which intensifies with increasing hydrate saturation and decreasing contact angle. Pore-filling hydrate patterns, in contrast to grain-coating, exhibit a greater number of gas-liquid interfaces, leading to larger pressure differences and a more evident Jamin effect. These findings aid in understanding the fluid migration in HBS and help estimate the natural gas production capacity.

Synergistic performance of mixed agro-waste-based amino acids as kinetic hydrate inhibitors for natural gas hydrate control

The formation of plugs and clogs in pipelines caused by low temperatures and high pressures during drilling and production of natural gas hydrates causes difficulties for the oil and gas sector. Different commercially available kinetic hydrate inhibitors (KHIs) are used to manage this problem; however, they have disadvantages such as toxicity, low biodegradability, and high cost. Through the use of microbiological techniques, this study attempted to synthesize mixed amino acids from agricultural waste to prevent the formation of natural gas hydrates. The performance of mixed agro-waste amino acids (MBL4) was compared to single amino acids (glycine, histidine, proline, and alanine that are major components of MBL4) and PVCap as a control group at dosage of 0.1 and 0.5 wt%. The onset temperature and subcooling were used to assess the performance of the inhibitors in this study, which used the constant cooling method in a stirred high-pressure autoclave at 100 bar. The addition of MBL4 at 0.5 wt% retarded the onset of hydrate formation and increased the subcooling significantly when compared to PVCap and the individual single amino acids, with onset temperature and subcooling of 7.20 °C and 11.67 °C, respectively. The performance of MBL4 is observed to increase from 0.1 to 0.5 wt% dosage concentration, and MBL4 outperformed individual amino acids and PVCap. Mixed agro-waste amino acids showed better performance when used as KHI due to synergism, and this finding can offer a long-lasting, cheaper, and more efficient way of controlling gas hydrate formation in the oil and gas industry.

Investigation on hydrate exploitation in submarine slope: Insights from discontinuous pillar exploitation

The decomposition of natural gas hydrate (NGH) will cause a decrease in reservoir strength, which can easily induce submarine landslides when hydrate reservoirs are located on a submarine slope. Although there have been several studies on the stability of hydrate-bearing submarine slopes (HBSS), little attention has been paid to minimizing the risk of landslides due to NGH exploitation. The “multistage reduction method” based on strength reduction was described in this paper. Subsequently, “discontinuous pillar exploitation” was proposed to achieve safe extraction of NGH and avoid submarine landslides. Finally, the “discontinuous pillar exploitation” was analyzed using the “multistage reduction method”. The results show that NGH decomposition has a far-reaching impact on the HBSS; it reduces the stability of HBSS and can trigger submarine landslides. The “discontinuous pillar exploitation” can divide the continuous plastic failure zone into short discontinuous plastic zones through the setting up of pillars, which significantly improves the stability of HBSS. With the decrease in the exploitation ratio, the safety factor of the submarine slope increases continuously. When the “discontinuous pillar exploitation” is adopted for NGH production on a submarine slope, there are two positions that require special attention: the junctions between the pillars and the exploitation area (peak area of shear stress); the side of exploitation area near the top of the slope (peak area of tensile stress).

Application of the monitoring and early warning system for internal solitary waves: Take the second natural gas hydrates production test in the South China Sea as an example

Internal solitary waves (ISWs) contain great energy and have the characteristics of emergency and concealment. To avoid their damage to offshore engineering, a new generation of monitoring and early warning system for ISWs was developed using technologies of double buoys monitoring, intelligent real-time data transmission, and automatic software identification. The system was applied to the second natural gas hydrates (NGHs) production test in the Shenhu Area, South China Sea (SCS) and successfully provided the early warning of ISWs for 173 days (from October 2019 to April 2020). The abrupt changes in the thrust force of the drilling platform under the attack of ISWs were consistent with the early warning information, proving the reliability of this system. A total of 93 ISWs were detected around the drilling platform. Most of them occurred during the spring tides in October–December 2019 and April 2020, while few of them occurred in winter. As suggested by the theoretical model, the full-depth structure of ISWs was a typical current profile of mode-1, and the velocities of wave-induced currents can reach 80 cm/s and 30 cm/s, respectively, in the upper ocean and near the seabed. The ISWs may be primarily generated from the interactions between the topography and semidiurnal tides in the Luzon Strait, and then propagate westward to the drilling platform. This study could serve as an important reference for the early warning of ISWs for offshore engineering construction in the future.©2023 China Geology Editorial Office.