Strength and Deformation Behaviors of Methane Hydrate-Bearing Marine Sediments in the South China Sea during Depressurization

Abstract

Investigating the mechanical behaviors of hydrate-bearing marine sediments can provide the basis for predicting the risks of silty reservoirs due to large-scale hydrate production, which are located in the South China Sea and contain huge hydrate reserves. To this end, hydrate-bearing marine sediments were remolded by the hydrate cores drilled from the South China Sea using the excess water method, and a series of triaxial tests with or without depressurization were conducted on these materials. The initial study shows that the hydrate obviously strengthens the strength and modulus of marine sediments, while the cement strength of the hydrate may depend on its cementing morphology content. No collapse occurs on this material during the depressurization even when the axial load reaches 90% peak strength of hydrate-bearing marine sediments, while serious deformations are observed and dominated mainly by the effective stress. The initial hydrate saturation plays a vital role in the deformation proportion of marine sediments induced by dissociation, while the stress also shows a certain impact during hydrate production. These results support the basic theory for hydrate production of silty reservoirs.

Influencing Factors of Hydration Number of Synthesized Methane Hydrates in Sediment

“…Natural gas hydrates, denoted by G · N h H 2 O, are non-stoichiometric inclusion compounds in which “guest” molecules G (mainly methane) are trapped within hydrogen-bonded cages formed by “host” water molecules H 2 O . Gas hydrates have been attracting more and more attention from both energy sectors and industry circles, due to their great potential to relieve the energy crisis and optimize the natural gas storage and transportation. − The hydration number N h , defined as the number of water molecules occupied by each guest molecule in the cavities, is an important structural parameter, which is usually used to characterize the gas storage capacity of the hydrate , and estimate the state parameters (say hydrate content) of hydrate-bearing sediments (HBSs) . However, how the hydration number evolves during hydrate formation or dissociation remains elusive experimentally due to the complex physicochemical interactions at molecular scales.…”

Phase Equilibrium of Methane Hydrate in Sediments: Experimental and Theoretical Characterization

“…However, hydrate dissociation can lead to the change in the stress state and the deterioration of the mechanical behavior of hydrate reservoirs. Inadequate design or improper disposal may give rise to serious engineering/geological disasters, such as platform failure, subsea landslides, seafloor subsidence, earthquake, and tsunami. , Therefore, proper characterization of the phase equilibrium condition of pore hydrate is crucial for determining the hydrate reservoirs, estimating the possible hydrate reserves, and designing the cost-effective exploitation of hydrate.…”

Microscopic Insights and Optimization of the CH4–CO2 Replacement in Natural Gas Hydrates

“…More seriously, it can set off a series of chain reactions. For instance, undiscovered hydrate reservoirs can be destroyed and release much CH 4 , causing greenhouse effects, sea level rise, climate deterioration, and ecological unbalance. , Second, the uncontrollable decomposition of NGH produces much gas and water. When they flow out, a lot of sand can be carried out, resulting in the breakdown of facilities and even the shutdown of the whole hydrate exploitation project. , In consideration of the risks mentioned above, more and more researchers have begun to realize the significance of the gas exchange method. − Different gases can be utilized to replace the CH 4 in hydrate, among which CO 2 is the most widely used gas. , For one thing, due to the higher stability of CO 2 hydrate than CH 4 hydrate, the newly formed CO 2 hydrate reduces the risk of uncontrollable decomposition of hydrates, sand production, and other secondary disasters mentioned above. , For another, the CO 2 replacement method can help not only extract CH 4 from NGH but also realize CO 2 storage and reduce the greenhouse effects.…”

“…For instance, undiscovered hydrate reservoirs can be destroyed and release much CH 4 , causing greenhouse effects, sea level rise, climate deterioration, and ecological unbalance. 19 , 20 Second, the uncontrollable decomposition of NGH produces much gas and water. When they flow out, a lot of sand can be carried out, resulting in the breakdown of facilities and even the shutdown of the whole hydrate exploitation project.…”