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
The effects of initial soil fabric on the shear behaviour of granular materials are investigated by employing distinct element method (DEM) numerical analysis. Soil specimens are represented by an ...assembly of non-uniform sized spherical particles. DEM specimens that have different initial contact normal distributions were prepared at two different densities (loose and dense) and then isotropically consolidated triaxial tests were simulated with the following conditions: (a) different drainage conditions (undrained/drained), (b) different modes of shearing (compression/extension), and (c) different directions of shearing (vertical/horizontal). The numerical analysis results are compared qualitatively with the observed experimental data and the effects of initial soil fabric on resulting soil behaviours are discussed. The discussions include the effects of specimen reconstitution methods, effects of large preshearing, and anisotropic characteristics in drained and undrained conditions. The effects of initial soil fabric on the quasi-steady state are also investigated. The numerical analysis results can systematically explain that the observed experimental behaviours of sand are due principally to the effects of initial soil fabric. The outcome provides insights into the observed phenomena in microscopic view.
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.
Energy piles are an effective and economic means of using geothermal energy resources for heating and cooling buildings, contributing to legislative requirements for renewable energy in new ...construction. While such piles have been used for around 25 years with no apparent detrimental effect, there is limited understanding of their thermo-mechanical behaviour. This paper synthesises the results from three published field studies and illustrates some of the engineering behaviour of such piles during heating and cooling. Simplified load transfer mechanisms for a single pile subjected to pure thermal loadings (i.e. without mechanical load) and combined thermo-mechanical loadings have been developed and are used to interpret the field data with regard to change in axial stress and shaft friction during heating and cooling. The effect of end restraint and ground conditions on the thermo-mechanical response of energy piles is discussed. Values of change in axial stress and mobilised shaft friction due to thermal effects that may be useful in the design of energy piles are presented.
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
An experimental study of the cross-anisotropic elastic parameters was performed on two heavily overconsolidated clays: London Clay and Gault Clay. The study utilised a special triaxial testing ...apparatus that incorporates local strain measuring system and elastic wave velocity measurement system. Isotropically consolidated drained compression tests and shear wave velocity measurements were undertaken on both vertically and horizontally cut specimens. The experiments were performed under in situ isotropic stresses to study the inherent anisotropic characteristics of the clays in terms of the elastic stiffness. The deformation characteristics during triaxial compression were carefully investigated to obtain various elastic moduli and Poisson ratios.
Methane hydrate is estimated to be present in substantial amounts below deep sea floors. Particular scientific and engineering interests that encourage studies of mechanical behaviour of methane ...hydrate soils include submarine geohazards, such as the initiation of marine landslides through hydrate dissociation, wellbore stability and estimation of future gas production from wells. To study these problems, a formulation of a multi-physics model of methane hydrate flow coupled to soil deformation is developed. By assuming deformable porous media (soil matrix) that accommodate non-movable but dissociable hydrate, a two-phase flow formulation of water and methane gas is suggested according to Darcy's law and capillary pressure law. A single-phase elastic–perfectly plastic constitutive model for hydrate soil sediments, based on the concept of effective stress, is developed to account for the effect of hydrate saturation on mechanical strength and stiffness. The formulation is incorporated into the explicit scheme of finite-difference code FLAC by solving three boundary value problems in parallel. The code is used to simulate the behaviour of horizontal unsupported and supported wells in hydrate-bearing sediments under different in situ stress conditions during methane hydrate extraction. Axial force, bending moment and well displacements were compared for supported and unsupported wells.
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