Abstract
The ratio between the horizontal and the vertical effective stresses is defined as the coefficient of earth pressure at rest Ko. Ko in hydrate-bearing sediments is critical in understanding the stress states in hydrate-bearing sediments, yet has not been previously understood. An oedometer cell equipped with vertical and horizontal stress measurement sensors is used to measure the evolution of Ko in tetrahydrofuran hydrate- bearing sands during hydrate formation and dissociation and vertical stress changes. The results show that the response of Ko in hydrate-bearing specimens reflects the combined effects of hydrate cementation, the viscous nature of hydrate crystals, and the stress levels. These results can enhance the understanding of stress anisotropy and geomechanical behaviors of hydrate reservoirs during gas production.
Highlights
The coefficient of earth pressure at rest Ko indicates the in situ stress state in deposits, and can be defined as the ratio of vertical and horizontal effective stress, as expressed below, K = σ σ (1)where σv and σh are the vertical and horizontal effective stresses
The vertical stress is controlled the hydraulic pump in a reaction frame, and a diaphragm pressure transducer installed at the middle height of the specimen monitors the lateral stress response
The initiation of hydrate crystallization is indicted by the thermal spike
Summary
The value of Ko in soils can be influenced by many factors, including the effective stress, stress history, the over-consolidation ratio, cementation bonding and debonding, packing density, and particle shape and size [1,2,3,4,5]. For non-cemented soils, the values of Ko does not change significantly during loading, i.e., the lateral stress increases linearly with the increase in vertical stress [2, 3]. The Ko is mainly governed by the cementation among particles during loading, leading to lower Ko values at low strain conditions [2, 6]. The increased effective stress caused by the depressurization method for gas production in deep-sea hydrate deposits can induce significant changes in the Ko of hydrate-bearing sediments. It is essential to understand the affecting factors of the values of Ko in hydrate-bearing sediments
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