Modelling the thermo-hydro-mechanical behaviour of marine methane hydrate-bearing sediments

Abstract

This three-paper format thesis is divided into the following scientific topics; geomechanical investigation of methane hydrate-bearing sediments (MHBS), fully coupled thermohydro-mechanical (THM) modelling of their behaviour during hydrate dissociation and prediction of hydrate saturation in pores inhibited by capillary pressure.
The occurrence of hydrates in pores increases the sediment strength, stiffness and dilatancy and favours its mechanical stability. However, hydrate dissociation may compromise its structural integrity and cause failure. To assess the risk of occurrence of hydrate-related geohazards or geotechnical issues it is imperative to ascertain the factors controlling the mechanical behaviour of gas hydrate reservoirs. The first part of this thesis develops a new mechanical model for MHBS. The Hydrate-CASM model attributes the stress-strain changes observed in MHBS to variations in the host sediment available void ratio, isotropic yield stress and swelling line slope due to hydrate formation and dissociation. In particular, the model assumes that the decrease of the available void ratio of the host sediment during hydrate formation stiffens its structure and has a similar mechanical effect that the increase of the sediment density. The Hydrate-CASM model is successfully applied to published experimental tests that cover a wide range of hydrate saturation, several hydrate morphologies and confinement stress. The results capture the main features observed in the mechanical behaviour of synthetic MHBS and provide novel insights into understanding the role of hydrate saturation at governing the mechanical properties of the sediment.
MHBS are characterized by highly interdependent physical processes, including mechanical deformation, fluid flow, thermal flow and phase change reactions. Reliable simulations of their behaviour require efficient mathematical models capable of capturing the aforementioned interdependencies in a coupled manner. The second part of this thesis develops a fully coupled THM formulation for hydrate-bearing geological environments. The formulation extends the governing equations of energy and mass conservation of the 3D finite element simulator Code Bright and incorporates the Hydrate-CASM model, as
well as additional constitutive equations and equilibrium constrains required to describe the fundamental physical phenomena governing the behaviour of MHBS. The extended version of Code Bright offers a new open-source numerical tool to better understand the response of gas hydrate reservoirs to mechanical and thermal loading. The model capabilities are validated against the numerical solution of the second benchmark problem described in the first NETL-USGS international gas hydrate code comparison study and published experimental data from hydrate dissociation tests performed under triaxial shear. The results show that the formulation captures the dominant mass and heat transfer phenomena and the main mechanical features observed on synthetic MHBS during hydrate dissociation.
Accurate estimations of hydrate saturation in sediments are crucial in the simulation of gas hydrate reservoirs. Hydrate saturation controls the mechanic and hydraulic properties of the porous medium and determines the resource potential of the reservoir. The third part of this thesis explores the effects of the capillary pressure at the hydrate-liquid interface in inhibiting hydrate thermodynamic stability and controlling its saturation. To do so, a new equilibrium model for hydrate formation in pores is developed. The model combines a new expression of the Clausius-Clapeyron equation for the methanewater system with the van Genuchten’s capillary pressure to relate the thermodynamic properties of the system with the host sediment pore-size distribution and the hydrate saturation. The model is applied to simulate hydrate formation in sand, silt and clays under equilibrium conditions and without mass transfer limitations, so that capillary pressure at the hydrate-liquid interface is the only factor controlling the resulting hydrate saturation. The simulations show that for typical thermodynamic conditions found in the seabed, capillary effects in fine-grained sediments may limit the maximum hydrate saturation to about 50%.

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ORCID for Hector Marín-moreno: orcid.org/0000-0002-3412-1359

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