Abstract
Proper understanding of the physical properties of hydrate-bearing sediments is required for interpretation of borehole logs and exploration geophysical data, the analysis of borehole and submarine slope stability, and the formulation of reservoir simulation and production models. Yet current knowledge of geophysical and geotechnical properties of hydrate-bearing sediments is still largely derived from laboratory experiments conducted on disparate soils at different confining pressures, degrees of water saturation, and hydrate concentrations and with hydrates formed by methods unlike those that predominate in nature. We conducted a comprehensive laboratory program using sand, silts, and clay subjected to various confining effective stress levels in standardized geotechnical laboratory devices and containing carefully controlled saturations of tetrahydrofuran (THF) hydrate formed from the dissolved phase. Here, we undertake complete analysis of the trends in the measured geophysical and geotechnical properties (e.g., seismic velocities, strength, electrical conductivity and permittivity, and thermal conductivity) as a function of hydrate saturation, soil characteristics, and effective stress. Results reveal that the electrical properties of hydrate-bearing sediments are not very sensitive to the laboratory method used to form hydrate, which controls the pore-scale arrangement of hydrate and sediment grains, but are sensitive to hydrate saturation. Mechanical properties are strongly influenced by both soil properties and the hydrate formation method. Thermal conductivity depends on the complex interplay of a variety of factors, including formation history, and cannot be easily predicted by volume average formulations but will remain within physical upper and lower bounds. When hydrate forms from dissolved phase guest molecules, the resulting mathematical trends for all physical properties require that the hydrate saturation S-h in pore space, which is a quantity between 0 <= S-h <= 1.0, 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).