Search Results (54)

The efficient extraction of natural gas from marine natural gas hydrate (NGH) reservoirs is challenging, due to their low permeability, high hydrate saturation, and fine-grained sediments. Hydraulic fracturing has been proven to be a promising technique for improving the permeability of these unconventional reservoirs. This study presents a comprehensive triaxial experimental investigation of the fracturing behavior and fracture initiation mechanisms of NGH-bearing sediments, using large-scale ice-saturated synthetic cubic models. The experiments systematically explore the effects of key parameters, including the injection rate, fluid viscosity, ice saturation, perforation patterns, and in situ stress, on fracture propagation and morphology. The results demonstrate that at low fluid viscosities and saturation levels, transverse and torsional fractures dominate, while longitudinal fractures are more prominent at higher viscosities. Increased injection rates enhance fracture propagation, generating more complex fracture patterns, including transverse, torsional, and secondary fractures. A detailed analysis reveals that the perforation design significantly influences the fracture direction, with 90° helical perforations inducing vertical fractures and fixed-plane perforations resulting in transverse fractures. Additionally, a plastic fracture model more accurately predicts fracture initiation pressures compared to traditional elastic models, highlighting a shift from shear to tensile failure modes as hydrate saturation increases. This research provides new insights into the fracture mechanisms of NGH-bearing sediments and offers valuable guidance for optimizing hydraulic fracturing strategies to enhance resource extraction in hydrate reservoirs. Full article

In the marine natural gas hydrate solid-fluidization mining process, current separation devices are insufficient in de-bonding the hydrate cementation between sand particles, affecting the hydrate collection efficiency. To address this issue, a composite separator was designed in this study that is used to restrict the axial movement of cemented particles, thereby achieving the goal of enhanced de-bonding efficiency. A combined computational fluid dynamics–discrete element method simulation method was used to verify the de-bonding performance of this composite separator on weakly cemented hydrate particles with different sizes and to study the influence of the spiral flow channel structural parameters and inlet types on the de-bonding performance. Full article

Methane gas hydrate-bearing sediments hold substantial natural gas reserves, and to understand their potential roles in the energy sector as the next generation of energy resources, considerable research is being conducted in industry and academia. Consequently, safe and economically feasible extraction methods are being vigorously researched, as are methods designed to estimate site-specific reserves. In addition, the presence of methane gas hydrates and their dissociation have been known to impact the geotechnical properties of submarine foundation soils and slopes. In this paper, we advance research on gas hydrate-bearing sediments by theoretically studying the effect of the hydromechanical coupling process related to ocean wave hydrodynamics. In this regard, we have studied two geotechnically and theoretically relevant situations related to the oscillatory wave-induced hydromechanical coupling process. Our results show that the presence of initial methane gas pressure leads to excessively high oscillatory pore pressure, which confirms the instability of submarine slopes with methane gas hydrate accumulation originally reported in the geotechnical literature. In addition, our results show that neglecting the presence of initial methane gas pressure in gas hydrate-bearing sediments in the theoretical description of the oscillatory excess pore pressure can lead to improper geotechnical planning. Moreover, the theoretical evolution of oscillatory excess pore water pressure with depth indicates a damping trend in magnitude, leading to a stable value with depth. Full article

Natural gas hydrates represent a promising clean energy source with vast reserves. Their efficient development is crucial for ensuring the sustainable advancement of human society. However, wellhead instability occurred in the long-term development, which poses a significant challenge that impacts its commercial development. In the present work, the properties of hydrate-bearing sediments were experimentally investigated. It was found that the elastic modulus, cohesion, and internal friction angle of hydrate-bearing sediments exhibit an increase with the effective stress. As an example, when the effective stress increases from 0 MPa to 25 MPa, the normalized elastic modulus exhibits a rise from 1.00 to 1.36. Conversely, the Poisson’s ratio, permeability, and porosity demonstrate a decline in accordance with this trend. As an example, both normalized porosity and permeability decrease to values below 0.40 as the effective stress increases to 25 MPa. Based on the experimental results and previous work, a comprehensive model for describing the effect of both hydrate saturation and effective stress on physical parameters was obtained. Subsequently, a multi-field coupled investigation methodology was developed to evaluate wellhead stability during the long-term development of hydrate-bearing sediments, and the evolution characteristics and mechanisms of wellhead instability were numerically explored. It reveals that development operation using the vertical wellbore decomposes hydrates in the surrounding sediments only within a radius of 19.52 m, which significantly undermines the wellhead stability. Moreover, the wellhead system not only sinks with sediment subsidence but also experiences additional sinking due to the failure of bonding between the wellhead system and sediments. Furthermore, the latter accounts for a significant portion, amounting to approximately 68.15% of the total sinking under the research conditions. This study can provide methodological prerequisites for exploring the impact of various factors on wellhead stability during the long-term hydrate development process. Full article

Natural gas hydrates, a promising clean energy resource, hold substantial potential. Porosity plays a crucial role in hydrate systems by influencing formation processes and physical properties. To clarify the effects of porosity on hydrate elasticity, we examined methane hydrate formation and its acoustic characteristics. Experiments were conducted on sediment samples with porosities of 23%, 32%, and 37%. P- and S-wave velocities were measured to assess acoustic responses. Results show that as hydrate saturation increases, sample acoustic velocity also rises. However, high-porosity samples consistently exhibit lower acoustic velocities compared to low-porosity samples and reach a lower maximum hydrate saturation. This behavior is attributed to rapid pore filling in high-porosity samples, which blocks flow pathways and limits further hydrate formation. In contrast, hydrate formation in low-porosity sediments progresses more gradually, maintaining clearer pore channels and resulting in relatively higher hydrate saturation. Higher porosity also accelerates the shift of hydrates from cementing to load-bearing morphologies. These findings underscore porosity’s significant influence on hydrate formation and provide insights into observed variations in hydrate saturation and acoustic velocity across different experimental conditions. Full article

The marine gas hydrates within seabed sediments and their subsequent extraction may cause landslides. Predicting landslides in hydrate-bearing sediments is particularly challenging due to the intricate nature of the marine environment. To address this issue, we have developed a Lagrangian gradient smoothing method (L-GSM) based on gradient smoothing techniques. This approach effectively eliminates the tensile instability inherent in the original Smoothed Particle Hydrodynamics (SPH) method used for modeling solid flow. Then, we applied the L-GSM to investigate the mechanics of hydrate-bearing sediments by integrating a constitutive equation specific to these sediments, which were modeled based on the artificial methane-hydrate-bearing sediment. The robustness and precision of the L-GSM were verified through various numerical examples. Furthermore, we modeled the landslides associated with hydrate-bearing sediments under varying hydrate saturation levels. The numerical findings revealed that hydrate saturation significantly affects the dynamics of landslide movement. These satisfactory results suggest that the L-GSM has the potential to be applied to geotechnical problems associated with hydrate-bearing sediment. Full article

Natural gas hydrates have great strategic potential as an energy source and have become a global energy research hotspot because of their large reserves and clean and pollution-free characteristics. Hydrate saturation affecting the electrical and acoustic properties of sediments significantly is one of the important parameters for the quantitative evaluation of natural gas hydrate reservoirs. The accurate calculation of hydrate saturation has guiding significance for hydrate exploration and development. In this paper, experiments regarding methane hydrate formation and dissociation in clay-bearing sediments were carried out based on the Ultrasound Combined with Electrical Impedance (UCEI) system, and the measurements of the joint electrical and acoustic parameters were collected. A machine learning (ML)-based model for evaluating hydrate saturation was established based on electrical–acoustic properties and a neural network ensemble. It was demonstrated that the average relative error of hydrate saturation calculated by the ML-based model is 0.48%, the average absolute error is 0.0005, and the root mean square error is 0.76%. The three errors of the ensemble network are lower than those of the Archie formula and Lee weight equation. The ML-based modeling method presented in this paper provides insights into developing new models for estimating the hydrate saturation of reservoirs. Full article

Natural gas hydrate reservoirs, with shallow burial, poor cementation, and low strength, are prone to submarine landslides triggered by hydrate decomposition during extraction. Prior studies have inadequately considered factors such as the dynamic decomposition of hydrates during depressurization, and its impacts on the reservoir’s geomechanical properties. In this paper, a coupled thermal–hydraulic–mechanical–chemical mathematical model of hydrate decomposition is proposed, and the dynamic geomechanical response and the effect of hydrate decomposition on seafloor settlement and slope destabilization during the process of depressurization mining are analyzed by combining the strength discount method with the example of a hydrate-bearing seafloor slope in the Shenhu area. Furthermore, the study employs an orthogonal experimental design along with range and variance analysis to gauge the impact of critical factors (degree of hydrate decomposition, seawater depth, hydrate reservoir burial depth, hydrate reservoir thickness, and slope angle) on slope stability. The findings suggest that hydrate decomposition is non-uniform and is influenced by stratigraphic temperature gradients and gravity. In the region where hydrate decomposition occurs, the decrease of pore pressure leads to the increase of effective stress. Additionally, the decomposition of hydrates decreases the shear modulus of sediments, leading to deformation and reduced permeability in the affected area. Over a three-year period of depressurization mining, the significantly reduced safety factor increases the risk of landslides. Various factors play a role in the control of submarine slope stability, with slope inclination being the primary factor, followed by the degree of hydrate decomposition, reservoir thickness, burial depth, and seawater depth. Among these factors, hydrate burial depth and seawater depth have a positive correlation with submarine slope stability, while increases in other factors generally decrease stability. These research findings have important implications for the safe exploitation of slopes that contain hydrates. Full article

Clarifying the mechanical characteristics of gas hydrate-bearing sediments (GHBS) from a mechanical perspective is crucial for ensuring the long-term, safe, and efficient extraction of natural gas hydrates. In this study, seabed soft clay from the northern South China Sea was utilized to prepare clayey silt samples, aligning with gradation curves related to hydrate extraction projects in the Shenhu area of the South China Sea. Utilizing the high-pressure low-temperature hydrate triaxial testing system (ETAS), twelve sets of triaxial shear tests were conducted. The results highlight that increases in hydrate saturation and confining pressure significantly enhance GHBS’ strength and stiffness, with more pronounced volume expansion observed during shearing. These tests have elucidated the mechanical responses of GHBS. Subsequently, empirical formulas were developed to characterize their properties under varying conditions. Additionally, based on the experimental data, the micro-mechanisms of GHBS were analyzed, suggesting that hydrates notably contribute to the filling and cementing effects in GHBS, with these effects varying with changes in hydrate saturation and confining pressure. This study contributes to a deeper understanding of the fundamental mechanical properties of GHBS. Full article

More than 90% of the natural gas hydrate resources are reserved as marine clayey silt sediments. It is of great significance to efficiently develop a clayey silt hydrate. At present, there are problems of low single well production and small depressurization range in its production test, which is still a long way from commercial exploitation. The combination of hydraulic fracturing technology and other methods such as depressurization method is regarded as one of the potential technical means to achieve the commercial exploitation of the hydrate. However, compared with shale gas reservoirs and coalbed methane reservoirs, clayey silt hydrate reservoirs have special mechanical properties, resulting in unique hydraulic fracturing processes. Therefore, it is necessary to study the fracture initiation and propagation laws of clayey silt hydrate reservoirs. To this end, we carried out large-scale (30 × 30 × 30 cm) true triaxial hydraulic fracturing experiments using a simulated material with similar mechanics, porosity, and permeability to clayey silt hydrate-bearing sediments. The effects of completion method, fracturing method, and fracturing fluid displacement on hydraulic fracture propagation of clayey silt hydrate-bearing sediments were studied. The results showed that a perforated completion can significantly increase the fracture reconstruction area and decrease the fracture initiation pressure compared to an open hole completion. Due to the small horizontal stress difference, it is feasible to carry out temporary plugging fracturing in clayey silt hydrate reservoirs. Temporary plugging fracturing can form steering fractures and significantly improve fracture complexity and fracture area. Increasing the fracturing fluid displacement can significantly increase the fracture area as well. When conducting fracturing in clayey silt hydrate-bearing sediments, the fracturing fluid filtration area is obviously larger than the fracture propagation area. Therefore, it is recommended to use a high-viscosity fracturing fluid to reduce the filtration of the fracturing fluid and improve the fracturing fluid efficiency. This study preliminarily explores the feasibility of temporary plugging fracturing in clayey silt hydrate reservoirs and analyzes the effect of completion methods on the propagation of fracturing fractures, which can provide a reference for the research conducted on the fracturing stimulation of clayey silt hydrate reservoirs. Full article

Large amounts of natural gas hydrates have been discovered in the Qiongdongnan Basin (QDNB), South China Sea. The chemical and stable carbon isotopic composition shows that the hydrate-bound gas was a mixture of thermogenic and microbial gases. It is estimated that microbial gas accounts for 40.96% to 60.58%, showing a trend of decrease with the increase in burial depth. A significant amount of gas hydrates is thought to be stored in the mass transport deposits (MTDs), exhibiting vertical superposition characteristics. The stable carbon isotopic values of methane (δ¹³C₁) in the MTD1, located near the seabed, are less than −55‰, while those of the methane below the bottom boundary of MTD3 are all higher than −55‰. The pure structure I (sI) and structure II (sII) gas hydrates were discovered at the depths of 8 mbsf and 145.65 mbsf, respectively, with mixed sI and sII gas hydrates occurring in the depth range 58–144 mbsf. In addition, a series of indigenous organic matters and allochthonous hydrocarbons were extracted from the hydrate-bearing sediments, which were characterized by the origin of immature terrigenous organic matter and low-moderate mature marine algal/bacterial materials, respectively. More allochthonous (migrated) hydrocarbons were also discovered in the sediments below the bottom boundary of MTD3. The gas hydrated is “wet gas” characterized by a low C₁/(C₂ + C₃) ratio, from 2.55 to 43.33, which was mainly derived from a deeply buried source kitchen at a mature stage. There is change in the heterogeneity between the compositions of gas and biomarkers at the site GMGS5-W08 along the depth and there is generally a higher proportion of thermogenic hydrocarbons at the bottom boundary of each MTDs, which indicates a varying contribution of deeply buried thermogenic hydrocarbons. Our results indicate that the MTDs played a blocking role in regulating the vertical transportation of hydrate-related gases and affect the distribution of gas hydrate accumulation in the QDNB. Full article

Studying the mechanical characteristics of hydrate-bearing sediments (HBS) contributes to the comprehensive understanding of the mechanical behavior in environments with natural gas hydrate (NGH) occurrences. Simultaneously, the distribution patterns of hydrates significantly influence the strength, deformation, and stability of HBS. Therefore, this paper employs particle flow code (PFC) to conduct biaxial discrete element simulations on specimens of HBS with different hydrate distribution patterns, revealing the macroscale–mesoscale mechanical properties, evolution patterns, and destructive mechanisms. The results indicate that the strain-softening behavior of HBS specimens strengthens with the increase in hydrate layer thickness, leading to higher peak strength and E₅₀ values. During the gradual movement of the hydrate layer position (A_y) from both ends to the center of the specimen (A_y = 0.40 mm → A_y = 20 mm), the strain-softening behavior weakens. However, when A_y = 20 mm, the specimen exhibits evident strain-softening behavior again. Moreover, with an increase in the angle between the hydrate layer and the horizontal direction (α) greater than 20°, the peak strength of the specimen increases, while E₅₀ shows an overall decreasing trend. The influence of axial loads on the hydrate layer in specimens varies with α, with larger contact forces and fewer cracks observed for higher α values. Full article

Hydraulic fracturing is widely recognized as a potential stimulation technology for the development of challenging natural gas hydrate. However, the fracturing behavior of non-diagenetic hydrate reservoirs has peculiar characteristics that are different from those of conventional oil and gas reservoirs. Herein, a fully coupled fluid-mechanical model for simulating hydraulic fracturing in hydrate-bearing sediments (HBS) was established based on the discrete element method, and the influence of hydrate saturation, in situ stress, and injection rate on the meso-fracture evolution was investigated. The results indicate that with the increase in hydrate saturation, the fracture morphology transitions from bi-wing to multi-branch, thereby enhancing fracture complexity. Both tensile and shear failure modes exist, and the tensile failure between the weakly cemented sediment particles is dominant. The tensile strength of HBS is an exponential function of hydrate saturation, with the breakdown pressure being governed by hydrate saturation and in situ stress, with the form being consistent with the classical Kirsch equation. Additionally, lower in situ stress and higher injection rates are conducive to the generation of microcracks, whereas an excessive injection rate reduces the fracture length. These findings contribute to understanding the meso-evolution mechanism of hydraulic fractures and guide the design of on-site hydraulic fracturing plans of natural gas hydrate reservoirs. Full article

High-latitude permafrost, including hydrate-bearing frozen ground, changes its properties in response to natural climate change and to impacts from petroleum production. Of special interest is the behavior of thermal conductivity, one of the key parameters that control the thermal processes in permafrost containing gas hydrate accumulations. Thermal conductivity variations under pressure and temperature changes were studied in the laboratory through physical modeling using sand sampled from gas-bearing permafrost of the Yamal Peninsula (northern West Siberia, Russia). When gas pressure drops to below equilibrium at a constant negative temperature (about −6 °C), the thermal conductivity of the samples first becomes a few percent to 10% lower as a result of cracking and then increases as pore gas hydrate dissociates and converts to water and then to ice. The range of thermal conductivity variations has several controls: pore gas pressure, hydrate saturation, rate of hydrate dissociation, and amount of additionally formed pore ice. In general, hydrate dissociation can cause up to 20% thermal conductivity decrease in frozen hydrate-bearing sand. As the samples are heated to positive temperatures, their thermal conductivity decreases by a magnitude depending on residual contents of pore gas hydrate and ice: the decrease reaches ~30% at 20–40% hydrate saturation. The thermal conductivity decrease in hydrate-free saline frozen sand is proportional to the salinity and can become ~40% lower at a salinity of 0.14%. The behavior of thermal conductivity in frozen hydrate-bearing sediments under a pressure drop below the equilibrium and a temperature increase to above 0 °C is explained in a model of pore space changes based on the experimental results. Full article

Changes in undrained shear strength are important to the stability analysis of hydrate reservoirs during natural gas hydrate production. This study proposes a prediction model of undrained shear strength of hydrate-bearing fine-grained sediments based on the critical state theory. Several consolidated undrained triaxial shear tests are conducted on hydrate-bearing fine-grained samples from the Shenhu area of the South China Sea. The effects of effective consolidation stresses and hydrate saturations on the undrained shear strength are investigated. The results show that the undrained shear strength increases linearly with increasing effective consolidation stress. When the hydrate saturation is greater than the effective hydrate saturation, the undrained shear strength significantly increases with increasing hydrate saturation. The undrained shear strength of hydrate-bearing fine-grained sediments is a two-parameter function of effective hydrate saturation and a void ratio. The instability risk of the hydrate reservoir under undrained conditions is greater than that of under-drained or partially drained conditions. Furthermore, low-porosity reservoirs face more shear strength loss from hydrate decomposition yet lower risk than high-porosity ones. These results can improve the understanding of mechanical properties of hydrate-bearing fine-grained sediments under undrained conditions. This study also has implications for the design of marine structures in areas with hydrate-bearing sediment. Full article