Dynamic responses of THF hydrate-bearing sediments under small strain: Resonance column test.

Knowledge of the dynamic responses of hydrate-bearing sediments under small strain conditions is a fundamental issue in terms of evaluating the stability of gas hydrate-bearing layers under natural forces, such as eustatic sea level and earthquakes as well as artificial activities for example gas production from hydrate reservoirs. However, this issue has not yet been fully understood. In this study, a customized hydrate resonance column was utilized to measure the dynamic response of tetrahydrofuran hydrate-bearing sediments with different conditions. The results show that the shear modulus increases exponentially with increases in the confining pressure or decreases in the shear strain, while the damping ratios decrease exponentially under identical conditions. Under the same confining pressure, the shear modulus and damping ratio increases as hydrate saturation rises. The stress sensitivity index (b) of shear modulus decreases monotonically with an increase in hydrate saturation. The shear modulus is negatively correlated with porosity when the tetrahydrofuran (THF) hydrate is predominantly pore-filling type, and the damping ratio and sensitivity index b are positively correlated with porosity. However, when the hydrate is mainly load-bearing type, the shear modulus and damping are both related positively with porosity. This is because the total amount of hydrate in specimens with identical hydrate saturation increases with higher porosity, while the sensitivity index b is negatively related to porosity. The hydrate formed in pores leads to a normal increase in the shear modulus and an abnormal increase in the damping, due to trace amounts of residual water between the surface of the hydrate and the particles. This study provides a basis for the comprehensive understanding of the dynamic responses of hydrate-bearing sediments under small strains. It is of great significance for seismic or sonic well logging evaluations on gas hydrate saturation and its dissociation degree in hydrate reservoirs. Image 1 • Effect of porosity on the dynamic responses of hydrate-bearing sediments under small strain is firstly investigated. • The abnormality of dynamic responses of hydratebearing sediments origin from the change of hydrate distribution pattern. • An empirical formula for predicting hydrate saturation of silty reservoir is established. [ABSTRACT FROM AUTHOR]