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

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.