A Fully Coupled THMC Model Predicting the Mechanical Behavior During Hydrate Dissociation
Authors: Zhang Li , Li Qingping , Wu Bisheng , Zhang Haitao , Yan Chuanliang 
Published: September 2023

ABSTRACT
An in-depth understanding of the mechanical behaviors of sediments is important to maintain reservoir stability and sustainable mining during natural gas hydrate (NGH) dissociation. In this work, a fully coupled thermal-hydro-mechanical–chemical (THMC) simulator, i.e. DEHydrate, is developed for studying the multiphysics processes and predicting the mechanical behavior during NGH dissociation involving solid-liquid-gas flow, heat transfer, NGH phase change and solid deformation. The numerical solution is obtained by using the fully implicit finite element method, where the higher order elements are used for solid deformation while lower order elements are used for gas-liquid flow and thermal diffusion. The results obtained from the present model are compared with those from other numerical simulators proposed in the Second International Gas Hydrate Code Comparison Study by conducting a benchmark problem. The evolution trends of the mechanical responses, such as NGH saturation, pressure, temperature, radial displacement and gas and water production rates, are roughly the same. The permeability coefficient is also directly related to the hydrate saturation, resulting in a "hump" in the distribution of pressure and temperature near the dissociation front.

INTRODUCTION
Natural gas hydrate (NGH), widely distributed in shallow seabed or permanent frozen region with a global reserve of around 7.5×1018 m3 (Kvenvolden and Lorenson), has a high energy density and high combustion efficiency (Yan et al. 2020). Safely and sustainably extracting NGH commercially can effectively relieve the global energy pressure and contribute to achieving carbon reduction goals. However, the current production technology can only achieve 1/17 of the commercial production requirement (Wu et al. 2021) due to some chanllenges such as marine slope collapse, seabed subsidence and sand production, and thus needs more research on hydrate dissociation behavior.
As a relatively common and straightforward way, numerical simulation provides a useful tool for predicting short- and long-term behavior of hydrate exploitation. Hydrate dissociation involves solid-liquid-gas flow, heat transfer, NGH phase transition and mechanical deformation. Many mathematical models have been developed to simulate the mechanical behavior of reservoir during hydrate dissociation. Based on the thermodynamic constitutive law, Sun et al. (2018, 2015) developed a thermal-hydro-mechanical–chemical (THMC) model to describe dissociation process by using the commercial finite element software COMSOL and show that decoupling of deformation-seepage process would overestimate the NGH dissociation rate. Kimoto et al. (2007, 2010) established a two-dimensional THMC dissociation model, and pointed out that the reservoir deformation is affected mainly by water-gas production and dissipation and decrease in soil strength. However, application of this model is limited by the number of elements and simulation time, even if the influence of heat convective is neglected. Therefore, most of the existing models have been simplified to some extent, not fully coupling the multiphysics process of hydrate dissociation. Therefore, it is very necessary to develop a fully coupled THMC model for simulating the hydrate dissociation and studying its mechanical behavior.