Gas hydrate-bearing sediments (GHBS) have been considered as a potential energy source for the future due to their global abundance. While hydrate significantly strengthens the host sediment, dissociation would inevitably alter hydro-mechanical properties of it. Consequently, this could trigger instabilities in the gas production casing. It is, therefore, essential to devise safe and economical strategies for energy harvesting from GHBS at commercial scale. The understanding of these inter-related causes and mechanisms is still very limited.

In this study, a state-dependent critical state model is developed for methane hydrate-bearing sediments (MHBS) within the theoretical framework of bounding surface plasticity. A phase parameter is newly introduced into the constitutive model to acc...[ Read more ]

Gas hydrate-bearing sediments (GHBS) have been considered as a potential energy source for the future due to their global abundance. While hydrate significantly strengthens the host sediment, dissociation would inevitably alter hydro-mechanical properties of it. Consequently, this could trigger instabilities in the gas production casing. It is, therefore, essential to devise safe and economical strategies for energy harvesting from GHBS at commercial scale. The understanding of these inter-related causes and mechanisms is still very limited.

In this study, a state-dependent critical state model is developed for methane hydrate-bearing sediments (MHBS) within the theoretical framework of bounding surface plasticity. A phase parameter is newly introduced into the constitutive model to account for the coupled effects of temperature and pore pressure on the mechanical behaviour of MHBS. This unique feature of the proposed model enables it to capture the behaviour of MHBS inside the methane hydrate stability region. A non-associated flow rule is adopted and a modified dilatancy expression is proposed considering the degree of hydrate saturation, morphology, phase parameter and stress state of MHBS. The comparison between the computed results and measured results of drained triaxial tests on MHBS reveals that the model is capable of capturing the key features such as the evident strain softening behaviour due to hydrate degradation and the change in stress-strain and volumetric behaviour of MHBS at different initial conditions inside the stability region.

A key contribution of this study is a newly developed centrifuge energy harvesting chamber (CEHC). This is the first chamber that can operate at elevated gravities with the capability of sustaining the thermodynamically favourable conditions for gas hydrate formation, sustaining a continuous inflow of high-pressure water at the boundaries during dissociation, and an in-flight control of wellbore pressure and surcharge loading. Centrifuge modelling can recreate the in-situ stress gradient in a relatively small model and expedite conduction and convection processes involved during dissociation. Consequently, long-term in-situ mechanisms can be evaluated with a small model and short time. The performance of CEHC and dissociation effects on vertical casing-sediment interaction and GHBS response are evaluated through two depressurization tests on GHBS. The results suggests that the temperature-pressure profiles continuously evolve as hydrate is dissociated, causing the extension of dissociation front and inducing changes in effective stress. The addition of 15% clay content significantly alters the temperature-pressure response of GHBS.

The gas flow rate is governed by the initial available latent heat as well as the conduction and convection heat through the surrounding sediments. However, as the depressurization progresses, the gas production rate is governed by the competing effects of hydrate dissociation and re-formation which both evolves the permeability of the sediment. The hydrate dissociation is triggered by the decrease in the pressure, while hydrate reformation is induced due to the gas pressure build-up in confined smaller pores in the low permeability sediment.

Dissociation within perforated interval leads to stress transfer within the formation, resulting in vertical arching. As hydrate dissociation continues, downward movements of soil mobilize negative skin friction. The progressive hydrate dissociation could induce large axial compressive and tensile loads within and above the perforated interval, respectively. During depressurization, the compressive load in the casing exceeds the yield strength of the modelled prototype casing. While the induced tensile loads are smaller than the yielding strength of the casing, the cement above perforated interval might crack in tension and jeopardize the well integrity. Hence, both induced compressive and tensile axial loads during long-terms hydrate dissociation should be carefully considered for safe gas production in GHBS.