Higher Hydrate Saturation Research Articles

Effect of Hydrate Saturation on Triaxial Shear Mechanical Properties of Methane Hydrate-Bearing Sediment

Abstract As a clean energy source, natural gas hydrate resources are essential to the development of energy in the twenty-first century. However, the degree of consolidation of the hydrate reservoir is low, and complex accidents such as wellbore instability, reservoir collapse, and gas production leakage will be caused if improperly exploited. The laboratory employs a self-developed triaxial shear test equipment that operates under high pressure and low temperature conditions. This equipment is specifically designed to determine the mechanical parameters and strength characteristics of methane hydrate bearing sediment (MHBS) samples. It is used to create water-saturated hydrate deposits with varying levels of saturation. The synthesis of MHBS samples with varying saturations is achieved by precisely regulating the amount of water. The saturation of MHBS is then determined and confirmed by the utilization of the “water content method” and the “gas collection method.” Triaxial shear tests are conducted on MHBS to examine the impact of hydrate saturation on the mechanical parameters and strength behaviours of MHBS. The experimental findings demonstrate that the presence of hydrate crystals enhances the robustness of the depositing medium. The stress-strain curves of MHBS have a hyperbolic shape, indicating strain hardening features. The stress-strain curves may be categorized into three distinct stages: the initial elastic deformation, followed by a slow shift to the initial yield deformation, and ultimately leading to strain hardening. As axial strain rises, the behaviour of the MHBS transitions progressively from elastic deformation to plastic deformation. The shear strength and initial yield strength of MHBS enhance with higher hydrate saturation. The reason for this is that as the hydrate saturation rises, the influence of hydrate crystals on the enhancement of sediment strength becomes more pronounced. However, the initial yield strain of each MHBS sample is between 0.5% and 1.5%, and the difference is not significant. When the hydrate saturation rises gradually, the cohesion of MHBS enhances. Nevertheless, the hydrate saturation has a very minor impact on the internal friction angle.

Theoretical investigation of threshold pressure gradient in hydrate-bearing clayey-silty sediments under combined stress and local thermal stimulation conditions

Due to the characteristics of smaller grain size and higher clay mineral content, the threshold pressure gradient (TPG) exists in multi-phase flow within hydrate-bearing clayey-silty sediments (HBCSS), which significantly affects the gas production from hydrate reservoirs. However, multi-phase flow through HBCSS relates to thermal-hydrological-mechanical coupling, leading to the understanding of TPG in HBCSS with complex pore structures and hydrate distribution is unclear. In this study, a theoretical TPG model of HBCSS is developed by considering the combined influences of effective stress, temperature increase, pore structures, hydrate saturation and its growth patterns. The proposed TPG model has been thoroughly validated using available experimental data. Moreover, the parameter sensitivity analysis is conducted based on this derived model, revealing a positive correlation between TPG and both effective stress and temperature increase. While TPG generally increases with higher hydrate saturation when other parameters are held constant, the relationship between TPG and hydrate saturation is non-monotonic. This observation suggests that TPG is impacted not only by hydrate saturation but also by other factors, including hydrate growth patterns and pore structures. The findings of this study establish a theoretical foundation for characterizing the nonlinear flow behavior during hydrate exploitation.

Analysis of permeability anisotropy of marine hydrate-bearing sediments using fractal theory combined with X-CT

Permeability anisotropy plays a crucial role in accurately predicting fluid productions during hydrate exploitation. However, the mechanisms governing permeability anisotropy have yet to be fully elucidated. This study conducted experiments on methane hydrate formation/dissociation in both unsaturated and saturated quartz sands, followed by X-ray computed tomography scanning and pore structure characterizations. Fractal parameters, encompassing fractal dimension, tortuosity fractal dimension, and maximum pore diameter, were employed to characterize the evolution of pore structure and changes in permeability anisotropy. Results show that the fractal dimension and maximum pore diameter decreased, while the tortuosity fractal dimension increased with the increase of hydrate saturation. Larger maximum pore diameter and smaller tortuosity fractal dimension in the horizontal direction become more pronounced with increasing hydrate saturation, resulting in heightened permeability anisotropy (kh/kv). The permeability anisotropy increases from 1.77 to 3.20 with hydrate saturation increasing from 0 to 0.38 in unsaturated sands, and from 1.47 to 4.70 with hydrate saturation increasing from 0 to 0.49 in saturated sands. Gas and water relative permeability anisotropy also amplifies with increasing hydrate saturation. Extrapolating to field tests, the findings suggest that horizontal wells present a more favorable option for gas production, especially in reservoirs with high hydrate saturation.

Evolution Characteristics of the Strength and Dilation Parameters of Gas Hydrate-Bearing Sediments and a Modeling Study

Gas hydrate has gradually become a new potential energy resource. However, some engineering and environmental problems related to the mechanical properties of gas hydrate-bearing sediments (GHBS) during gas recovery may occur. Many studies have been carried out on the basic mechanical properties of GHBS samples based on laboratory tests, but their evolution characteristics and suitable models require further research. Based on a series of data analyses of published laboratory experimental results on GHBS samples with different hydrate saturations under various confining pressures, the evolution characteristics of strength and dilation parameters were investigated. It was found that cohesion (c) increases quickly to a peak value and then decreases gradually to a residual value with an increasing plastic shear strain, and the samples with higher hydrate saturations have higher initial values, peak values, and residual values of cohesion (c). The internal friction angle (φ) increases quickly with increasing plastic shear strain and then becomes stable at a residual value for all the samples with different hydrate saturations. The dilation angle (ψ) increases from negative to positive values with increasing plastic shear strain and then becomes stable at a residual value. These characteristics are likely to be related to the compaction occurring at the early stage of compression before expansion due to dilation. In this paper, a non-linearly fitted model is proposed considering the evolution of the mechanical parameters, and the verification tests show that the proposed model can simulate the stress–strain behaviors of the GHBS samples well. This model is also adopted in the stability analysis of submarine slopes containing hydrate reservoirs. The analytical approach is developed, accompanied by the strength reduction method.

Characterization of the Structural–Stratigraphic and Reservoir Controls on the Occurrence of Gas Hydrates in the Eileen Gas Hydrate Trend, Alaska North Slope

One of the most studied permafrost-associated gas hydrate accumulations in Arctic Alaska is the Eileen Gas Hydrate Trend. This study provides a detailed re-examination of the Eileen Gas Hydrate Trend with a focus on the gas hydrate accumulation in the western part of the Prudhoe Bay Unit. This integrated analysis of downhole well log data and published geophysical data has provided new insight on structural, stratigraphic, and reservoir controls on the occurrence of gas hydrates in the Eileen Gas Hydrate Trend. This study revealed the relatively complex nature of the gas hydrate occurrences in the Eileen Gas Hydrate Trend, with gas hydrates present in a series of coarsening upward, laterally pervasive, mostly fine-grained sand beds exhibiting high gas hydrate saturations. Most of the gas hydrate-bearing reservoirs in the Eileen Gas Hydrate Trend are laterally segmented into distinct northwest- to southeast-trending fault blocks, occur in a combination of structural–stratigraphic traps, and are only partially hydrate filled with distinct down-dip water contacts. These findings suggest that the traditional parts of a petroleum system (i.e., reservoir, gas source, gas migration, and geologic timing of the system formation) also control the occurrence of gas hydrates in the Eileen Gas Hydrate Trend.

Modeling of synthesized hydrate-bearing sands: Effect of hydrate morphology on absolute and relative permeabilities

Absolute and relative permeability of hydrate-bearing sediments are crucial flow parameters to hydrate exploitation processes. However, up to now, grain-coating hydrate-bearing sediments (GC-HBS) with high hydrate saturation (Sh) and pore-filling hydrate-bearing sediments (PF-HBS) are still prohibitive to be prepared in laboratory. In this work, three-dimensional pore structures of GC-HBS with high Sh and PF-HBS were numerically synthesized, based on the CT scanning of porous samples. Then, a state-of-the-art quasi-static pore-network model (PNM) was used to study the absolute and relative permeability of these HBS. Results show that the absolute permeability of both of the GC-HBS and PF-HBS declined rapidly with increasing of Sh, they can be well fitted by the Tokyo model with exponential parameter of 3.5 and 5.3, respectively. And the absolute permeability of PF-HBS is smaller than that of GC-HBS with same Sh for a bigger specific surface area of PF-HBS which increases the flow resistance for flows. It shows that the wetting and non-wetting relative permeability of GC-HBS decreases with the increase of Sh. But the gas-water two-phase relative permeability of PF-HBS has nonmonotonic changes with the increase of Sh.

Numerical investigation of gas and sand production from hydrate-bearing sediments by incorporating sand migration based on IMPES method

Accurately predicting gas and sand production from oceanic gas hydrate reservoirs considering multi-field coupling effects is crucial. To address this, we first systematically summarized the multi-field coupling effect of gas hydrate-bearing sediments (GHBS) during sand production and gas production. After that, we developed a thermo-hydro-mechanical coupled model that incorporates both sand detachment and sand migration (THMS) to describe the complex behaviors of fluid and solid flow. We then solved this model using the implicit pressure-explicit saturation (IMPES) method and incremental scheme for a cylindrical core and presented a detailed algorithm derivation process for reproducibility. The newly developed model was validated through comparison with available experimental data. Subsequently, several parameter effects, including outlet pressure, initial hydrate saturation, load stress, and sand detachable ratio, on the behaviors of fluid and solid flow were studied. The computation results indicate that high sand production can be induced by low outlet pressure, high initial hydrate saturation, high load stress, and high sand detachable ratio. Meanwhile, high cumulative gas production can be caused by low outlet pressure, low initial hydrate saturation, high load stress, and high sand detachable ratio.

A pore-scale study on microstructure and permeability evolution of hydrate-bearing sediment during dissociation by depressurization

Research data and visual evidence to understand the interaction between hydrate dissociation and micro-seepage in sediment are currently lacking. In this study, X-ray computed tomography (CT) is used to reveal the dynamic interaction process between sediment pore-scale structural changes and permeability during hydrate dissociation. After Xe hydrate formation in spherical quartz and then decomposed the hydrate by depressurization to obtain the real-time permeability and 3D structure of hydrate-bearing sediments (HBSs) with CT scans. The results show that the heterogeneity of hydrate spatial distribution does not change the power function relationship between permeability and hydrate saturation. The ball-and-stick model shows that the number and size of flow channels dominate the flow resistance of the fluid at high hydrate saturation, while the size of flow channels dominates it at low hydrate saturation. The saturation cut-off point is between 16.35% and 25.37%. The CT scan images show that the hydrate morphology changes from patchy to load-bearing, and then to cementing. The transition process from load-bearing to cementing has the greatest impact on sediment permeability. Further PNM simulations show that the size and number of flow channels and capillary forces are the main factors affecting the ability of gas–water two-phase flow in the sediment. Most importantly, a relative permeability model related to hydrate saturation parameters is proposed based on image processing technology. The good agreement between the model and available laboratory/field measurements proves its practicability. The method of proposing this model provides a new idea to establish new relative permeability models.

Numerical Analysis on Gas Production and Geomechanical Responses of Natural Gas Hydrate Reservoirs.

Natural gas hydrate (NGH) has attracted considerable global attention as a promising energy resource in recent years. To acquire valuable insights into regarding the interplay between mechanical properties and production outcomes during the production, in this study, a fully coupled thermo-hydro-mechanical-chemical (THMC) model based on the geological features of reservoirs in the Shenhu area of the South China Sea (SCS) was developed to analyze the response characteristics of various physical fields within the reservoir during the exploitation. Furthermore, the study examined the influence of mechanical behavior on hydrate exploitation and investigated the effects of varying initial hydrate saturation and seawater depth on production efficiency and reservoir deformation. The simulation results indicated that neglecting the impact of solid mechanics in the analysis of hydrate productivity can result in overestimated results, particularly during the initial production stage. Reservoirs with higher hydrate saturation experience lower initial production rates due to the influence of permeability and capillary force. Moreover, reservoirs with high hydrate saturation exhibit greater compression but lower wellhead subsidence during the long-term development. The impact of seawater depth on production capacity primarily arises from the pressure's influence on the gas-water ratio, where greater seawater depth corresponds to increased reservoir compression and wellhead subsidence.

Impact Assessment of Hydrate Cuttings Migration and Decomposition on Annular Temperature and Pressure in Deep Water Gas Hydrate Formation Riserless Drilling

The accurate prediction of wellbore temperature and pressure is important for safe drilling. However, annulus temperature and pressure changes are more complicated due to phase transition. To study this problem, a prediction model of temperature and pressure in deep water riserless drilling is established by considering hydrate cuttings decomposition, interphase mass transfer, and phase transition heat. Based on this model, the effects of hydrate cuttings decomposition on the temperature and pressure of drilling in a hydrate reservoir are explored. The results show that the influence of hydrate cuttings decomposition increases significantly with an increase in the inlet temperature. The influence of hydrate cuttings decomposition on temperature and pressure decreases with an increase in displacement. A small range in the variation of density and penetration rates has little impact on the annulus pressure but mainly affects the temperature. The influence of hydrate cuttings decomposition increases with an increase in the penetration rate. In normal drilling conditions, hydrate cuttings decomposition has little impact on annulus temperature and pressure, but under the conditions of a high inlet temperature, high hydrate saturation, low displacement, and high penetration rate, it is necessary to consider the impact of hydrate cuttings decomposition. This study can provide reference for the prediction of temperature and pressure in deep water hydrate reservoir riserless drilling.

Study of Multibranch Wells for Productivity Increase in Hydrate Reservoirs Based on a 3D Heterogeneous Geological Model: A Case in the Shenhu Area, South China Sea

Summary So far, a total of 11 hydrate trial production projects have been carried out all over the world, all of which used a single vertical well or horizontal well to carry out hydrate production by the depressurization method or depressurization combined with other methods. These traditional production methods have some limitations: The single vertical well has a small contact area with the reservoir, and the transmission range of the temperature and pressure is limited; therefore, the productivity is low. The horizontal well can improve hydrate productivity from magnitude order; however, there is a long distance from the standard of commercial production of marine hydrate. Therefore, it is an inevitable trend to find a highly efficient and advanced drilling technology for heterogeneous hydrate reservoirs. The multibranch wells based on horizontal wells can not only increase the contact area between the hydrate reservoir and well by branch structure to improve the conductivity of the reservoir temperature and pressure but also improve the hydrate productivity by laying the branch at high hydrate saturation for areas with extremely uneven distribution. Therefore, for this paper, we chose the Shenhu area as the research area to establish an approximate realistic 3D heterogeneous geological model contained with hydrate, then we laid the multibranch wells based on horizontal wells in high hydrate saturation area and optimized the branch direction, location, and spacing, and the production increasing effect was assessed. Finally, an optimal multibranch well scheme was obtained under the conditions of this paper setting, which is as follows: The vertical multibranch well was set at the root end of the horizontal main well with a branch spacing of 10 m, and the productivity after optimization was 31.64% higher than that before optimization.

Numerical simulation of failure properties of interbedded hydrate-bearing sediment and their implications on field exploitation

Marine natural gas hydrate is widely accepted as a promising substitute energy resource, whereas geotechnical responses of hydrate-bearing sediments (HBS) are crucial for the safe development of natural gas hydrate. The hydrate spatial distribution in sediments is multitype and present different coalescence degrees, its distribution pattern significantly influences the failure behavior of HBS, but the mechanics behind this influence is unclear. Numerical simulations are of great importance in revealing such mechanics. In this paper, a series of numerical simulations, using a modified Mohr–Coulomb (MC) model, were performed to analyze the deformation mechanisms of HBS when the hydrate was laminarly distributed in its host sediment. Considering the vertical heterogeneity of the hydrate distribution, the vertical-laminar hydrate distribution samples were simplified as interbedded samples. The interbedded HBS specimens consisted of three layers: the middle layer with relatively lower hydrate saturation; and the upper and lower layers with relatively high hydrate saturation. Interbedded samples with different hydrate saturations, depressurization ratios, and thickness ratios were designed to analyze their mechanical behaviors. The results indicated that in both the hydrate saturation and thickness ratio of the interbedded samples, the plastic yield area first occurred in the lower hydrate saturation layer; the failure mechanism was mainly determined by the strength of the lower hydrate saturation layer. In addition, the shear failure behavior of the HBS was controlled by the hydrate saturation difference between the middle and upper layers. With a smaller hydrate saturation difference between the middle and upper layers, the samples were more likely to experience shear failure. In the thickness ratio samples, the HBS changed from brittle to ductile failure with an increase in the thickness of the lower hydrate saturation layer. Under different depressurization amplitudes, the larger compression and shear yield zones were mainly confined in the upper layer. This study provides a theoretical framework for controlling the strength-weakening mechanism during natural gas development, which can be used to guide reservoirs characterization, the selection of depressurization schemes, and decomposition zones monitoring.

Experimental Investigation into the Erosion Performance of Water Jets on Marine Hydrate-Bearing Sediment

Fine-grained natural gas hydrate (NGH) reservoirs are widely distributed across the world and bear more than 90% of global NGH. However, it is difficult to exploit this kind of NGH reservoir economically and environmentally using conventional methods. Water-jet cutting is an efficient and environmentally friendly technique for mining such hydrate reservoirs, as the production process does not depend on mass and heat transfer within the formations. In this work, a series of physical experiments were conducted to clarify the erosion performance of marine hydrate-bearing sediment (HBS) impacted by water jets. The results show that the accumulation of sediment particles and hydrate particles at the bottom of erosion hole severely inhibits the vertical erosion of HBS by water jet. For a particular jet flow rate, the jet distance has an optimal value, which is between 4 mm and 28 mm. Moreover, the upwelling flow containing solid particles has a significant impact on the erosion of the hole top. In reservoirs with a low hydrate saturation (20–40%) and reservoirs with a high hydrate saturation (60–80%), the erosion holes exhibit a gourd shape and a bamboo shape, respectively. In addition, the volume erosion efficiency and the depth erosion efficiency are more sensitive to the variation in jet flow rate than jet distance and hydrate saturation. This study can provide theoretical and technical support for the application of water-jet cutting in the exploitation of marine HBS.

Geomechanical properties of artificial methane hydrate-bearing fine-grained sediments

A series of isotropic and triaxial compression tests was conducted on artificial methane hydrate-bearing fine-grained sediment (HBFS) specimens prepared via the excess-gas method. It is shown that the swelling index decreases and the yield stress increases linearly with the increase of the hydrate saturation, whereas the compression index is practically independent of hydrate saturation. The HBFS generally exhibits strain-hardening behavior, though strain-softening behavior may occur at low confining net stress and high hydrate saturation. The presence of hydrate can increase the stiffness and shear strength of the HBFS to various degrees, depending upon the applied confining net stress. It is revealed that the effect of hydrate cementation on the shear strength is weakened under high confining net stress conditions, owing to the deterioration of hydrate cementation. The hydrate cementation can influence the shear dilatancy of the HBFS, an effect that becomes less pronounced as the applied confining net stress increases. Based on the experimental results, empirical formulations are developed for the yield stress, strength, and shear dilatancy of the HBFS. The experimental results provide a comprehensive dataset for validating the constitutive models of HBFS.

Geological Reservoir Characterization of a Gas Hydrate Prospect Associated with the Hydrate-01 Stratigraphic Test Well, Alaska North Slope

Geological reservoir characterization is essential for accurate evaluation of gas production performance from gas hydrate reservoirs. Particularly, the understanding of reservoir architecture and heterogeneity is of great importance since these are considered as major controls on fluid hydrodynamic and thermodynamic conditions. This study deals with well log and three-dimensional (3-D) vertical seismic profile (VSP) data acquired from the Hydrate-01 Stratigraphic Test Well within the 7-11-12 prospect, Prudhoe Bay Unit, Alaska North Slope and reports on the results of geological/geophysical evaluation related to the geological structure and reservoir properties of the 7-11-12 prospect. The structural trends of the target reservoirs, based on well correlations, are mostly consistent with the predrill prediction using the surface seismic data, and infer the existence of subseismic faults cutting through the Hydrate-01 well. The 3-D VSP data confirm a down-to-the-east normal fault that offsets the reservoir units across the Hydrate-01 well, which is concordant with the well identification of the same fault, and indicate a northeast-dipping relay structure associated with the overstepping normal faults. The edge enhancement attribute associated with discontinuity generated from the 3-D VSP data shows small faults/fractures, possibly as part of a complex fault network within the imaged normal fault system. These results reveal that the 3-D VSP data provide detailed structural information that is not present from the surface seismic data. The Hydrate-01 well log data confirm the occurrence of gas hydrate at high saturation in the two targeted sand units (B1 and D1 sands), and the comparison to a nearby pre-existing well (7-11-12 well) shows the same general trend in gas hydrate saturation as a map of seismic impedance generated from surface seismic data. The well log data also suggest that the base of gas hydrate occurrence in the Hydrate-01 and 7-11-12 wells is almost aligned at the same depth in both of the targeted B1 and D1 sand reservoirs. Especially for the D1 sand in the Hydrate-01 well, the resistivity logs show a sharp transition from high gas hydrate saturation to fully water-saturated within the D1 sand, suggesting a common gas hydrate/water contact. The results of this study will be used to construct the geological models needed for reservoir simulation studies and they can provide important insights into the geological factors that control the occurrence of gas hydrate on the Alaska North Slope.

Coupled flow and geomechanical analysis for gas production from marine heterogeneous hydrate-bearing sediments

For natural gas hydrate exploitation, revealing the interaction between flow and geomechanics is the key to accurate prediction of gas production and formation stability. In this work, based on the site GMGS3-W19 of the South China Sea, a numerical model with multilayer hydrate deposits was established to analyze the evolution of geological parameters during depressurization. The results demonstrated that the reservoir underwent sequential process of expansion and compression, which resulted in the corresponding changes of porosity and permeability. The different initial geological parameters of the three hydrate layers showed respective coupling characteristics between flow and geomechanics. Additionally, we also analyzed the geological stability of reservoir. The calculation results of horizontal and vertical effective stresses were within the elastic region away from the Mohr-Coulomb yield function until 1800 d. In contrast, the layer with high hydrate saturation was more likely to produce shear failure. Finally, the gas-water production was determined after the coupled flow and geomechanical analysis. The result showed that gas productions of the two horizontal wells were on par, while water production of upper well was >7 times that of lower well. The numerical model and analysis could provide useful insight into the marine heterogeneous hydrate exploitation.

Numerical Simulation of Hydrate Decomposition during the Drilling Process of the Hydrate Reservoir in the Northern South China Sea

The process of drilling in natural gas hydrate reservoirs in sea areas involves problems such as hydrate decomposition and wellbore instability. To study the response behaviors of a reservoir during the drilling process, a two-dimensional numerical model of drilling fluid invading a hydrate reservoir in a cylindrical coordinate system was established to simulate the processes of heat and mass transfer, gas–liquid two-phase flow, and hydrate formation and decomposition in the hydrate reservoir during the drilling process. Based on the hydrate reservoir at station W17, Shenhu area of the South China Sea, the physical property response of the hydrate reservoir under different drilling fluid temperatures and salinity values was studied. The simulation results showed that during the drilling process, the temperature and pressure of the reservoir respond rapidly in a large area, further promoting the hydrate decomposition in the reservoir around the wellbore and leading to secondary hydrate formation. Moreover, a high hydrate saturation zone appears near the decomposed hydrate area in the layer without free gas, which corresponds to the low water saturation and high salinity zone. The hydrate decomposition area in the layer with free gas is larger than that without free gas. The increase in the drilling fluid temperature significantly enhances the hydrate decomposition in both layers of the reservoir. The hydrate decomposition near the wellbore under the high drilling fluid temperature will cause a sharp increase in the pressure in the reservoir, leading to the flow of pore fluid into the wellbore. The increase in drilling fluid salinity has little effect on the range of the hydrate decomposition in the reservoir but significantly increases the salinity of the pore water in the layer with free gas. As the drilling fluid temperature increases, the possibility of the gas invasion from the reservoir into the wellbore will be greatly increased at the early stage.

Sedimentation-driven cyclic rebuilding of gas hydrates

Gas hydrate recycling is an important process in natural hydrate systems worldwide and frequently leads to the high gas hydrate saturations found close to the base of the gas hydrate stability zone (GHSZ). However, to date it remains enigmatic how, and under which conditions, free gas invades back into the GHSZ. Here we use a 1D compositional multi-phase flow model that accounts for sedimentation to investigate the dominant mechanisms that control free gas flow into the GHSZ using a wide-range of parameters i.e. hydrate formation kinetics, sediment permeability, and capillary pressure. In the first part of this study, we investigate free gas invasion into the GHSZ without any sedimentation, and analyse the dynamics of hydrate formation in the vicinity of the base of GHSZ. This helps establish plausible initial conditions for the main part of the study, namely, hydrate recycling due to rapid and continuous sedimentation. For the case study, we apply our numerical model to the Green Canyon Site 955 in the Gulf of Mexico, where the reported high hydrate saturations are likely a result of hydrate recycling driven by rapid sedimentation. In the model, an initial hydrate layer forms due to the invasion of a specified volume of rising free gas. This hydrate layer is consistent with the local pressure, temperature and salinity state. This hydrate layer is then thermally de-stabilised by sedimentation resulting in free gas formation and hydrate recycling. A key finding of our study is that gas hydrate recycling is a cyclic process which can be divided into three phases of 1) gas hydrate melting and free gas nozzling through the hydrate layer, 2) formation of a new gas hydrate layer as the old layer vanishes, and 3) fast uninhibited grow of a new hydrate layer. High hydrate saturations of about 80% can be attained purely through physical, burial-driven recycling of gas hydrates, without any additional gas input from other sources. Hydrate recycling is, therefore, highly dynamic with its own inherent cyclicity rather than a gradual process paced by the rate of sediment deposition.

Experimental study on the influence of brine concentration on the dissociation characteristics of methane hydrate

Brine stimulation is a common method for assisting depressurization to produce hydrate, which significantly improves the efficiency of hydrate production. In this work, to study the influence of brine concentration on the natural gas hydrate production process, a novel cylindrical hydrate production simulation experimental device was established, and investigations of hydrate dissociation experiments induced by depressurization in conjunction with brine injection were carried out. The effects of different brine concentrations on the gas production rate and heat transfer during production were studied. The distribution, flow, and concentration evolution of brine during hydrate production were analyzed. Brine injection can significantly enhance cumulative gas production during the depressurization stage, increase the sensible heat for hydrate dissociation, enhance heat transfer between the hydrate reservoir and the environment during the recovery process, and significantly improve the gas production rate of hydrate. The increase in the gas production rate caused by brine injection mainly occurs at the end of the depressurization stage and the beginning of the constant pressure stage. The injected brine is mainly located in the middle and upper areas of the reactor. In the bottom area of the reactor, the hydrate has higher saturation and longer dissociation time, which is slightly affected by the injected brine. A higher hydrate saturation is not beneficial to brine injection and does not promote hydrate dissociation. Additionally, in the experiments with brine injection, the resistance decreases more obviously during the depressurization stage due to more hydrate dissociation. The resistance also shows that in the bottom area of the reactor, there is a higher hydrate saturation and a lower brine effect on hydrate.

Mechanical properties of remolded hydrate-bearing clayey-silty sediments

The vast majority of global gas hydrate resources exist in clay and silty fine-grained reservoirs, so fully grasping the mechanical properties of fine-grained hydrate is the key to realize the commercialization of hydrate resources. To date, the research on mechanical properties of hydrates has focused on sandy sediments, while research on the fine-grained hydrate has been relatively less common. In this paper, clay collected from the South China Sea and quartz sand were used to remold hydrate-bearing clayey-silty sediments (HBCSS). A series of triaxial shear tests were then carried out to explore the effects of hydrate saturation, effective confining pressure and clay content on the mechanical properties of HBCSS. This study not only reveals the influence of different factors on the mechanical properties of HBCSS, but also explains the action mechanism of clay. The results show that a higher hydrate saturation could cause strain softening due to spalling and crushing of hydrate particles, and this strain softening effect will also be affected by confining pressure. The failure strength of HBCSS increases with the increase of hydrate saturation and effective confining pressure, but decreases with the increase of clay content. The action mechanism of clay is the effect of lubrication, which can change the particle relationship from crushing to rotating or sliding. This research could be valuable for the safe and efficient exploitation of clay-bearing fine-grained hydrate.