This paper presents a new constitutive model that simulates the mechanical behavior of methane hydrate‐bearing soil based on the concept of critical state soil mechanics, referred to as the “Methane ...Hydrate Critical State (MHCS) model”. Methane hydrate‐bearing soil is, under certain geological conditions, known to exhibit greater stiffness, strength and dilatancy, which are often observed in dense soils and also in bonded soils such as cemented soil and unsaturated soil. Those soils tend to show greater resistance to compressive deformation but the tendency disappears when the soil is excessively compressed or the bonds are destroyed due to shearing. The proposed model represents these features by introducing five extra model parameters to the conventional critical state model. It is found that, for an accurate prediction of ground settlement, volumetric yielding plays an important role when hydrate soil undergoes a significant change in effective stresses and hydrate saturation, which are expected during depressurization for methane gas recovery.
Key Points
A new constitutive model for hydrate‐bearing soil was presented
The model incorporates volumetric yielding and degradation of hyrate effects
The model also considers stress relaxation due to hydrate dissociation
Gas hydrate is a crystalline solid found within marine and subpermafrost sediments. While the presence of hydrates can have a profound effect on sediment properties, the stress‐strain behavior of ...hydrate‐bearing sediments is poorly understood due to inherent limitations in laboratory testing. In this study, we use numerical simulations to improve our understanding of the mechanical behavior of hydrate‐bearing sands. The hydrate mass is simulated as either small randomly distributed bonded grains or as “ripened hydrate” forming patchy saturation, whereby sediment clusters with 100% pore‐filled hydrate saturation are distributed within a hydrate‐free sediment. Simulation results reveal that reduced sand porosity and higher hydrate saturation cause an increase in stiffness, strength, and dilative tendency, and the critical state line shifts toward higher void ratio and higher shear strength. In particular, the critical state friction angle increases in sands with patchy saturation, while the apparent cohesion is affected the most when the hydrate mass is distributed in pores. Sediments with patchy hydrate distribution exhibit a slightly lower strength than sediments with randomly distributed hydrate. Finally, hydrate dissociation under drained conditions leads to volume contraction and/or stress relaxation, and pronounced shear strains can develop if the hydrate‐bearing sand is subjected to deviatoric loading during dissociation.
Key Points
Stress‐strain response of hydrate‐bearing sands
Numerical study using DEM simulations
Hydrate dissociation depends on reservoir boundary conditions
Methane gas hydrates, crystalline inclusion compounds formed from methane and water, are found in marine continental margin and permafrost sediments worldwide. This article reviews the current ...understanding of phenomena involved in gas hydrate formation and the physical properties of hydrate‐bearing sediments. Formation phenomena include pore‐scale habit, solubility, spatial variability, and host sediment aggregate properties. Physical properties include thermal properties, permeability, electrical conductivity and permittivity, small‐strain elastic P and S wave velocities, shear strength, and volume changes resulting from hydrate dissociation. The magnitudes and interdependencies of these properties are critically important for predicting and quantifying macroscale responses of hydrate‐bearing sediments to changes in mechanical, thermal, or chemical boundary conditions. These predictions are vital for mitigating borehole, local, and regional slope stability hazards; optimizing recovery techniques for extracting methane from hydrate‐bearing sediments or sequestering carbon dioxide in gas hydrate; and evaluating the role of gas hydrate in the global carbon cycle.
Turbidite formation is a common feature of natural hydrate‐bearing sediments that has been observed and reported at several hydrate exploration sites. It is therefore important to incorporate this ...anisotropic geological feature into the constitutive modeling when evaluating the geomechanical risks involved during hydrate‐based gas production. To date, a number of constitutive models have been proposed to capture the isotropic geomechanical behavior of homogeneous hydrate‐bearing sediments. Since the turbidite formation contains soil layers at a scale much smaller than the size of the numerical element used for reservoir scale simulations, it is necessary to upscale the geomechanical behavior of a layered system to an equivalent anisotropic continuum model by adopting some homogenization techniques. In this study, an anisotropic methane hydrate critical state model is developed by modifying the original isotropic version of Uchida et al. (2012, https://doi.org/10.1029/2011JB008661). The calibration methodology of the anisotropic model parameters for a given set of hydrate heterogeneity and the turbidite formation at the Eastern Nankai Trough is proposed and demonstrated. The upscaled parameters are calibrated by curve fitting the numerically simulated stress‐strain curves of the layered system with the original isotropic constitutive model at the layered scale. Forty‐two sets of model parameters are calibrated from different site element models of this site. They are used to develop empirical correlations between the model parameters and the site input properties within the turbidite formation. This paper presents the details of the new anisotropic constitutive model and the performance of the proposed upscaling procedure for the Eastern Nankai Trough case.
Plain Language Summary
The prospects for gas production from methane hydrates have greatly improved since hydrate deposits were found to exist in sandy sediments. Heterogeneity is a very important feature of natural hydrate‐bearing sediments. In order to carry out accurate numerical simulations of the Eastern Nankai Trough gas production test, an advanced soil model is proposed in this study to better represent heterogeneous structure of the hydrate‐bearing sediments. The numerical simulations using the advanced model suggested more accurate results than the results determined from previous models. This study, as part of MH21 research program, Japan, offered important insights for offshore methane hydrate gas production in Eastern Nankai Trough, Japan.
Key Points
The heterogeneous structure of the hydrate‐bearing sediments influences both the gas production and the geomechanical behavior
An anisotropic hydrate critical state model was developed by modifying the isotropic version of the methane hydrate critical state model
Correlations between the model parameters of the anisotropic hydrate critical state model and the input variables were established
Fracturing Pressure in Clay Marchi, M; Gottardi, G; Soga, K
Journal of geotechnical and geoenvironmental engineering,
02/2014, Letnik:
140, Številka:
2
Journal Article
Recenzirano
AbstractHydraulic fracturing in clayey soils can be triggered by either tensile or shear failure. In this paper, the physical meanings of various equations to predict fracture initiation pressure ...proposed in the past are discussed using the cavity expansion theory. In particular, when fracturing pressure is plotted against initial confining pressure, published laboratory test results as well as analytical models show a linear relationship. When the slope is close to 2, fracture is initiated by tensile failure of the clay, whereas when the slope is close to 1, it is initiated by shear failure of the clay. In this study, the analytical models, validated only on laboratory test data to date, were applied to unique data from field grouting work in which extensive soil fracturing was carried out to improve the mechanical characteristics of the soft silty clay underlying a bell tower in Venice, Italy. By a careful assessment of initial confining pressure in the field, the variation in recorded injection pressures with confining pressure was examined. Results suggest that the fractures at this site were likely to be initiated by shear failure of the clay, and the values were similar to what was predicted by the model with the shear failure criterion.
Methane hydrate soil is a natural soil deposit that contains methane hydrate in its pores. The micro-scale processes of the geomechanical behaviour of methane hydrate-bearing soils are investigated ...by the Discrete Element method (DEM). A series of DEM simulations of triaxial compression tests were performed to study the influence of methane hydrate saturation (
S
h
) on the stress–strain relationship, the volumetric response and on the macroscopic geomechanical properties such as friction and dilation angle. Results of the numerical simulations are compared with laboratory triaxial test data performed on sandy methane hydrate samples. The simulations showed that for the pore-filling case, the hydrate contribution to the strength of the sediment is of a frictional nature, rather than of a cohesive nature.
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