Parametric study of the physical properties of hydrate‐bearing sand, silt, and clay sediments: 2. Small‐strain mechanical properties

Abstract

[1] The small-strain mechanical properties (e.g., seismic velocities) of hydrate-bearing sediments measured under laboratory conditions provide reference values for calibration of logging and seismic exploration results acquired in hydrate-bearing formations. Instrumented cells were designed for measuring the compressional (P) and shear (S) velocities of sand, silts, and clay with and without hydrate and subject to vertical effective stresses of 0.01 to 2 MPa. Tetrahydrofuran (THF), which is fully miscible in water, was used as the hydrate former to permit close control over the hydrate saturation S hyd and to produce hydrate from dissolved phase, as methane hydrate forms in most natural marine settings. The results demonstrate that laboratory hydrate formation technique controls the pattern of P and S velocity changes with increasing S hyd and that the small-strain properties of hydrate-bearing sediments are governed by effective stress, s′ v and sediment specific surface. The S velocity increases with hydrate saturation owing to an increase in skeletal shear stiffness, particularly when hydrate saturation exceeds S hyd ≈ 0.4. At very high hydrate saturations, the small strain shear stiffness is determined by the presence of hydrates and becomes insensitive to changes in effective stress. The P velocity increases with hydrate saturation due to the increases in both the shear modulus of the skeleton and the bulk modulus of pore-filling phases during fluid-to-hydrate conversion. Small-strain Poisson's ratio varies from 0.5 in soft sediments lacking hydrates to 0.25 in stiff sediments (i.e., subject to high vertical effective stress or having high S hyd ). At S hyd ≥ 0.5, hydrate hinders expansion and the loss of sediment stiffness during reduction of vertical effective stress, meaning that hydrate-rich natural sediments obtained through pressure coring should retain their in situ fabric for some time after core retrieval if the cores are maintained within the hydrate stability field.Citation: Lee, J. Y., F. M. Francisca, J. C. Santamarina, and C. Ruppel (2010), Parametric study of the physical properties of hydrate-bearing sand, silt, and clay sediments: 2. Small-strain mechanical properties,

Hydro-bio-geomechanical properties of hydrate-bearing sediments from Nankai Trough

“…Only very limited stress, if any, may get locked within the sediments due to friction between the sediment and core liner. The DSC subsection of core 10P has a low S h~0 .13, and V P should depend strongly on effective stress for S h <~50% (Lee et al, 2010a). This stress dependence and the additional impact of long-term core storage in the absence of effective stress are evident when comparing V P measurements made by the DSC, IPTC and the Pressure Core Analysis and Transfer System PCATS (Fig.…”

“…Choi et al, 2014;Lee et al, 2013;Priest et al, 2009), estimate the smallstrain bulk-, shear-and compressional-moduli as well as the Poisson's ratio (Helgerud et al, 2009;Lee et al, 2010a), and link seismic and well log measurements to in situ hydrate saturations (e.g. Fohrmann and Pecher, 2012;Hunter et al, 2011;Shankar and Riedel, 2014;Shelander et al, 2012).…”

Challenges, Uncertainties, and Issues Facing Gas Production From Gas-Hydrate Deposits

“…However, the sediment still suffers from other mechanical effects listed above associated to changes in effective stress and the ensuing shear and normal strains. Laboratory results show that the presence of distributed GH stabilizes the granular skeleton when S H is high (~0.5 or grater) and unloaded sediments experience minimal mechanical effects (Lee et al, 2010b). Furthermore, a second chamber to bring sediments in pressure cores back to the in-situ state of effective stress so that effective stress-dependent properties can be properly determined has been developed ( Figure 7b).…”

“…The current bottleneck in the state-of-practice of geophysical analysis is centered on the relationship between measured physical parameters and S H (Waite et al, 2010;Santamarina and Ruppel, 2010;Lee et al, 2010a;Lee et al, 2010b), in particular:  Electrical conductivity and Archie's Law: most applications use empirical and fitting parameters, leading to good fits to the data but uncertain predictions, particularly in the absence of accurate information on the electrical conductivity of the pore fluid in situ (Edwards, 1997;Riedel et al, 2006b;Scholl et al, 2008;Weitemeyer and Constable, 2009;U.S. DOE, 2010).…”

Testing a thermo‐chemo‐hydro‐geomechanical model for gas hydrate‐bearing sediments using triaxial compression laboratory experiments

“…Biot's constant is chosen from a range of typically expected values. The Poisson's ratio is assumed to be a constant independent of the hydrate saturation following the experimental studies by [45,38]. The Young's modulus is modelled using the parameterization proposed by [62], given by Eqn.…”