Compression behavior of hydrate-bearing sediments

Comparative analysis of the consolidation and shear behaviors of CH4 and CO2 hydrate-bearing silty sediments

Most hydrate-bearing sediments worldwide exhibit mixed pore- and fracture-filling hydrates. Due to the high exploitation value, pore-filling hydrate production is the focus of current hydrate production research, and there is a lack of systematic research on the decomposition of fracture-filling hydrates and their effects on the evolution of temperature and pressure in hydrate-bearing sediments. If only the decomposition characteristics of pore-filling hydrates are studied while the fracture-filling hydrates decomposition and its effects on the hydrate-bearing sediments production process are ignored, the obtained research results would be inconsistent with the actual situation. Therefore, in this study, the effects of fracture-filling hydrates with different dipping angles on the hydrate production process were studied, and the necessity of considering the phenomenon of mixed pore- and fracture-filling hydrates in hydrate-bearing sediments was illustrated. On this basis, the simulation of a typical site (GMGS2-16) with mixed pore- and fracture-filling hydrates was constructed, and the production process was researched and optimized. The results indicated that: (a) fracture-filling hydrates formed in shallow fine-grained sediments and gradually approached the area of pore-filling hydrates, before a stable mixed zone was formed; (b) the occurrence of fracture-filling hydrates was conducive to the hydrate-bearing sediment depressurization production, and the promoting effect of the fracture-filling hydrate with smaller dipping angles was stronger; and (c) depressurization combined with heat injection could effectively compensate for the local low temperature and secondary hydrate caused by the mass decomposition of fracture-filled hydrates.

Natural gas hydrate reserves in the South China Sea are huge, and their commercial prospects have been proved by repeated trial production. It is essential to systematically clarify the mechanical properties of hydrate-bearing sediments for the safe drilling of hydrate reservoirs. To date, most research studies focus on the undrained mechanical behavior of hydrate-bearing sandy or clayed sediments under normal consolidation conditions, but there are few reports on underconsolidated-undrained triaxial shear experiments. In this study, the present research conducts undrained triaxial shear tests on hydrate-bearing clayed-silty sediments. The results show that underconsolidated hydrate-bearing sediments appear to show a strain-hardening phenomenon, which are different from hydrate-bearing sands in Nankai Trough. The excess pore water pressure of hydrate-bearing sediments remains positive during shear. The increasing degree of consolidation will increase the failure strength and the Secant Young's modulus E50. According to the Mohr–Coulomb criterion, the corresponding predicting formulas are established, and the cohesion increases with the increase in the degree of consolidation, while the internal friction angle is not sensitive to the degree of consolidation. In addition, the failure strength and the Secant Young's modulus E50 increase with the increase in initial effective confining stress before shearing.

The ever-increasing demand for the use of marginal lands for various human activities necessitates the need to understand the behavior of soft sediments under the action of external forces. As the segregated sediments are of common occurrence in a freshwater depositional environment, it is essential to understand the compressibility and permeability characteristics of segregated sediments, in addition to those of homogeneous sediments. The present paper discusses the compressibility and permeability behaviors of soft segregated and homogeneous sediments, making use of a simplified seepage consolidation testing procedure developed by the authors. The other variables included in the study are the clay mineralogy of the soil and the stress history of the sediments. The study clearly brings out the significant effect of clay mineralogy on the compressibility characteristics of segregated, normally consolidated sediments. The compressibility behavior of homogeneous soil sediments formed under a given depositional environment, having different clay mineralogy, appears to be similar. It is shown that the coefficient of permeability of any soil is a function of stress history in addition to void ratio and soil type.

A particle-scale investigation of mechanical behavior of cemented hydrate-bearing sediment using Discrete Element Method

Strength behaviors of CH4 hydrate-bearing silty sediments during thermal decomposition

A comprehensive study of the mechanical behaviors of hydrate-bearing sediments (HBSs) is the key to safely exploiting hydrate resources. The mechanical behaviors of HBSs are related to many variables, among which hydrate saturation (Sh) and pore pressure (PP) are vital factors. In addition, Sh and PP are related to the location of hydrates in the subsea layer, so it is of positive significance to investigate their comprehensive influence on the mechanical behavior of HBSs. In this work, a series of triaxial tests were conducted on the HBSs synthesized in the laboratory to explore the influence of Sh and PP on the mechanical properties of the HBSs. The results show that the strength of HBSs increases with increasing PP and Sh. With increased PP and Sh, the stress-strain behaviors will shift from strain-hardening to strain-softening. Moreover, under different Sh conditions, a critical PP point of strain-hardening and softening transition exists in numerical perspective. The critical PP point will develop towards low pressure with increased Sh.

Evaluating mechanical weakening behaviors of hydrate-bearing sediments (HBSs) induced by hydrate dissociation is crucial for safe gas production from hydrate reservoirs. The discrete element method (DEM) can be used to link macroscopic mechanical behaviors with mesoscopic mechanisms, thereby facilitating this evaluation. Yet, existing DEM-based hydrate dissociation simulations inadequately describe the dissociation process and capture its mechanical effects, limiting the comprehensive assessment of the mechanical properties of HBSs. Herein, we developed a generic DEM-based simulation method for characterizing the hydrate dissociation process, which incorporates hydrate volume reduction and cementation damage, termed the VRCD method. Combined with biaxial compression tests, the VRCD method was employed to evaluate the mechanical weakening of HBSs with multiple morphologies of hydrates in which dissociation occurs. The simulation results show that the VRCD method significantly enhances prediction accuracy in mechanical weakening behaviors of HBSs compared with the conventional particle random removal method. The mesoscopic mechanisms corresponding to macroscopic mechanical weakening, including increased bond breakage, reduced stress contribution from hydrate-related contacts, and weakened contact force chains, are clarified. Furthermore, a modified Mohr-Coulomb strength criterion, related to the hydrate saturation and dissociation ratio, is proposed. The VRCD method and strength criterion hold great potential for application in assessing geological and engineering risks during gas production from HBSs.

Gas hydrates are crystalline compounds made of gas and water molecules. Methane hydrates are found in marine sediments and permafrost regions; extensive amounts of methane are trapped in the form of hydrates. Methane hydrate can be an energy resource, contribute to global warming, or cause seafloor instability. This study placed emphasis on gas recovery from hydrate bearing sediments and related phenomena. The unique behavior of hydrate-bearing sediments required the development of special research tools, including new numerical algorithms (tube- and pore-network models) and experimental devices (high pressure chambers and micromodels). Therefore, the research methodology combined experimental studies, particle-scale numerical simulations, and macro-scale analyses of coupled processes. Research conducted as part of this project started with hydrate formation in sediment pores and extended to production methods and emergent phenomena. In particular, the scope of the work addressed: (1) hydrate formation and growth in pores, the assessment of formation rate, tensile/adhesive strength and their impact on sediment-scale properties, including volume change during hydrate formation and dissociation; (2) the effect of physical properties such as gas solubility, salinity, pore size, and mixed gas conditions on hydrate formation and dissociation, and it implications such as oscillatory transient hydrate formation, dissolution within the hydrate stability field, initial hydrate lens formation, and phase boundary changes in real field situations; (3) fluid conductivity in relation to pore size distribution and spatial correlation and the emergence of phenomena such as flow focusing; (4) mixed fluid flow, with special emphasis on differences between invading gas and nucleating gas, implications on relative gas conductivity for reservoir simulations, and gas recovery efficiency; (5) identification of advantages and limitations in different gas production strategies with emphasis; (6) detailed study of CH4-CO2 exchange as a unique alternative to recover CH4 gas while sequestering CO2; (7) the relevance of fines in otherwise clean sand sediments on gas recovery and related phenomena such as fines migration and clogging, vuggy structure formation, and gas-driven fracture formation during gas production by depressurization.

Natural methane hydrate, as a potential alternative energy source to fossil energy in the 21st century, is found in abundance in deep-water sediments and permafrost regions. During the production of methane gas from these sediments, the dissociation process may induce various changes to the geotechnical properties. Thus, it is important to study the geomechanical behavior of hydrate sediments and simulate the sediment deformation patterns. In this study, the effects of different sediment types on the mechanical properties of hydrate-bearing soils were investigated using multistage triaxial tests, and the methane hydrate critical state (MHCS) soil model was calibrated using an optimization-based technique. The results revealed that: (1) the strain-hardening phenomenon in methane hydrate-bearing sediments varies with the particle sizes of the host sediments; (2) the strengths of methane hydrate-bearing sediments are higher with the host sediments being pure sand, compared with those with clay-added sediments, and the strength is inversely proportional to the clay contents in these sediments; and (3) the multistage triaxial test data were fitted with the MHCS model, which illustrates the combined effects of the abovementioned factors on the model parameters and geomechanical behavior of methane hydrate-bearing sediments.

Gas hydrates are a potential future energy resource and are widely distributed in marine sediments and permafrost areas. The physical properties and mechanical behavior of gas hydrate-bearing sediments are of great significance to seafloor stability and platform safety. In 2013, a large number of pressure cores were recovered during China’s second gas hydrate drilling expedition in the South China Sea. In this study, we determined the gas hydrate distribution, saturation, physical properties, and mechanical behavior of the gas hydrate-bearing sediments by conducting Multi-Sensor Core Logger measurements and triaxial and permeability tests. Disseminated gas hydrates, gas hydrate veins, and gas hydrate slabs were observed in the sediments. The gas hydrate distribution and saturation are spatially heterogeneous, with gas hydrate saturations of 0%–55.3%. The peak deviatoric stress of the gas hydrate-bearing sediments is 0.14–1.62 MPa under a 0.15–2.3 MPa effective confining stress. The permeability is 0.006– 0.095 × 10 − 3 μ m 2 , and it decreases with increasing gas hydrate saturation and burial depth.

Mechanical properties of methane hydrate-bearing sandy sediments under various temperatures and pore pressures

SummaryUnlike the conceptual models, the natural hydrate spatial distribution in sediments is multitype and presents different coalescence degrees. In this study, we present pore-scale triaxial test results for hydrate-bearing sediments with different hydrate spatial distributions for the first time. It shows that the specimen with a more dispersed hydrate distribution yields later and exhibits larger peak strength. Correspondingly, the localized deformation develops more slowly, and the shear band is steeper and thinner. The cementation failure in the specimen with a more dispersed hydrate distribution develops more slowly. However, the changing rate of the pore space characteristic does not seem to be affected by the hydrate spatial distribution. Moreover, the specimen with a more dispersed hydrate distribution has a larger hydrate-sand interfacial area, and further axial loading would increase it rapidly firstly, and then the increasing rate would be slowed down since the cementation structure failure.

Analysis of shearing behavior in hydrate-bearing sediments from a micromechanical perspective: A DEM simulation study

The production of gas hydrate using the depressurization method can lead to subsidence due to decreased strength and stiffness of hydrate-bearing sediments. Since subsidence could damage sea-bed facilities and production wells, it is essential to study this problem. In this study, geomechanical behaviors of hydrate-bearing sediments were predicted considering the properties of a field production test site in the Ulleung basin, East sea, Korea. For this purpose, subsidence and displacement velocity were analyzed according to the depressurization conditions. As a result, geomechanical behaviors mostly occurred in the upper hydrate-bearing sediments when the depressurization method was applied. In addition, the effect of bottom hole pressure was greater than that of depressurization rate. This study can be used as a basic data for field production tests in the Ulleung basin.