26. The Impact of Hydrate Saturation on the Mechanical, Electrical, and Thermal Properties of Hydrate-Bearing Sand, Silts, and Clay

Abstract

An understanding of the physical properties of hydrate-bearing sediments is important for interpretation of geophysical data collected in field settings, borehole and slope stability analyses, and reservoir simulation and production models. Yet current knowledge of geophysical and geotechnical properties of hydratebearing sediments is still largely derived from laboratory experiments conducted on disparate soils at different confining pressures, degrees of water saturation, and hydrate concentrations. Here we report on the key findings that have emerged from 5 years of laboratory experiments conducted on synthetic samples of sand, silts, or clays subjected to various confining pressures in standardized geotechnical laboratory devices and containing carefully controlled saturations of tetrahydrofuran hydrate formed from the dissolved phase. For the first time, we use this internally-consistent data set to conduct a comprehensive analysis of the trends in geophysical and geotechnical properties as a function of hydrate saturation, soil characteristics, and other parameters. Our experiments emphasize measurements of seismic velocities, electrical conductivity and permittivity, large strain deformation and strength, and thermal conductivity. We discuss the impact of hydrate formation technique on the resulting physical properties measurements and use our data set to identify systematic effects of sediment characteristics, hydrate concentration, and state of stress, extracting robust relationships (often based on micromechanical concepts) for the most relevant material parameters. The mathematical trends that emerge for the measured physical parameters always require that the hydrate saturation in pore space, which ranges from 0 to 1, be raised to a power greater than 1. This significantly reduces the impact of low hydrate saturations on the measured physical parameters, an effect that is particularly pronounced at the hydrate saturations characteristic of many natural systems (<0.2 of pore space). The results also reveal that the electrical properties of hydrate-bearing sediments are less sensitive to the method used to form hydrate in the lab (which controls the pore-scale arrangement of hydrate and sediment grains) than to hydrate saturation. Mechanical properties are strongly influenced by both soil properties and the hydrate loci. Thermal conductivity depends on the complex interplay of a variety of factors, including formation history, and cannot be easily predicted by volume average formulations.

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

“…2) suggests the core contains non-cementing gas hydrates. Using the Gassmann formulation, we can estimate V P from V S (Santamarina et al, 2001;Santamarina and Ruppel, 2008):…”

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

“…Our simulation results indicate that an exponent c = 3 is a reasonable approximation. [62] suggest that S h tends to be raised to a power larger than 1, which reduces the impact on stiffness at low gas hydrate saturations relative to that for high gas hydrate saturations. Since gas hydrates formed using the excess-gas-method are predominantly located in the pore throats rather than in the free pore space, the linear and relatively stronger dependence of E sh on S h during formation appears reasonable.…”

“…Similarly, we used fitting of the stiffness model parameters to match the experimental volumetric strain behavior. We chose a functional dependence of composite modulus E sh on hydrate saturation S h as proposed by [62] and other authors ( [60], [32,33]). Knowledge about mechanical stiffness and strength properties of gas hydrate-bearing sediments is still limited.…”

“…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. 28.…”

“…We, therefore, chose a uniquely invertible linear-elastic constitutive model in the geomechanics-block of our hydrate reservoir model. We account for the stiffening effect due to gas hydrates by parameterizing the Young's modulus as a function of hydrate saturation S h ( [62]). We also account for material compressibility with respect to hydrostatic pressure.…”