Abstract
This study investigates the geomechanical, hydraulic and thermal characteristics of natural sandy sediments collected during the Ulleung Basin gas hydrate expedition 2, East Sea, offshore Korea. The studied sediment formation is considered as a potential target reservoir for natural gas production. The sediments contained silt, clay and sand fractions of 21%, 1.3% and 77.7%, respectively, as well as diatomaceous minerals with internal pores. The peak friction angle and critical state (or residual state) friction angle under drained conditions were ~26° and ~22°, respectively. There was minimal or no apparent cohesion intercept. Stress- and strain-dependent elastic moduli, such as tangential modulus and secant modulus, were identified. The sediment stiffness increased with increasing confining stress, but degraded with increasing strain regime. Variations in water permeability with water saturation were obtained by fitting experimental matric suction-water saturation data to the Maulem-van Genuchen model. A significant reduction in thermal conductivity (from ~1.4–1.6 to ~0.5–0.7 W·m−1·K−1) was observed when water saturation decreased from 100% to ~10%–20%. In addition, the electrical resistance increased quasi-linearly with decreasing water saturation. The geomechanical, hydraulic and thermal properties of the hydrate-free sediments reported herein can be used as the baseline when predicting properties and behavior of the sediments containing hydrates, and when the hydrates dissociate during gas production. The variations in thermal and hydraulic properties with changing water and gas saturation can be used to assess gas production rates from hydrate-bearing deposits. In addition, while depressurization of hydrate-bearing sediments inevitably causes deformation of sediments under drained conditions, the obtained strength and stiffness properties and stress-strain responses of the sedimentary formation under drained loading conditions can be effectively used to assess sediment responses to depressurization to ensure safe gas production operations in this potential target reservoir.
Highlights
Gas hydrates are ice-like solid compounds that encapsulate gas molecules in hydrogen-bonded water cages
Natural gas hydrates were identified in the Ulleung Basin, East Sea, during the Ulleung Basin gas hydrate expeditions 1 and 2 (UBGH1 and UBGH2) in 2007 [1,2] and 2010 [3,4]
A majority of the studies on the characterization of Ulleung Basin sediments have focused on silty or clayey sediments [5,6,7,8]; few studies have characterized the properties of sandy sediments, except for Reference [9]
Summary
Gas hydrates are ice-like solid compounds that encapsulate gas molecules in hydrogen-bonded water cages. Natural gas hydrates are found in both permafrost and oceanic sediments, where. Energies 2016, 9, 775 temperature and pressure satisfy the thermodynamic conditions required for gas hydrates, and where an abundant gas flux is supplied. The abundance of natural gas hydrates has garnered interest as a potential energy resource. The characterization of the geomechanical, transport, and thermal properties of the sediments hosting the gas hydrates is essential to our fundamental understanding of natural gas hydrate formation, as well as for the safe and effective production of gas from hydrate deposits. The physical, geotechnical, and thermal properties of the sediment cores recovered were obtained via various on-board and on-shore laboratory experiments. Given that sandy sediment layers provide feasible conditions for safe and productive gas production (e.g., low compressibility and high permeability), their characterization is critical
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