A fully coupled thermal-hydro-mechanical-chemical model for simulating gas hydrate dissociation

Due to high energy density and huge reserves, many trial projects worldwide have been conducted to exploit natural gas hydrates (NGH). However, most of them did not meet the commercialization requirements because of submarine hazards, low gas production and sand production. Therefore, it is important to investigate the NGH dissociation process. In this paper, a fully coupled multiphase, strongly nonlinear three-dimensional (3D) thermal-hydro-mechanical-chemical model (DEHydrate) is developed to consider the multiphysics behaviours including solid-liquid-gas multiphase flow, heat transfer, NGH phase transition and solid deformation during hydrate dissociation. The model is solved using a fully implicit finite element method. The gas-liquid flow and heat transfer are simulated by low-order elements while the solid deformation is calculated by high- or low-order elements. The behaviours of the multiphase seepage and geomechanics are fully coupled and the physical properties of the reservoir are updated iteratively based on changes in pressure, temperature and displacement. The DEHydrate simulator provides an effective tool to predict the evolution of reservoir mechanical behavior during hydrate dissociation, including transient pressure, temperature, displacement and stress in the reservoir, as well as multiphase saturation, hydrate dissociation rate/mass and gas production rate/mass. During the NGH dissociation, the initial stage exhibits a rapid dissociation rate of up to 206.23 kg/(d & sdot;m) for NGH layer with a length of 600 m and a thickness of 22 m, which subsequently decreases to 11.64 kg/(d & sdot;m). Only a small portion of the gas generated during NGH dissociation is collected at the wellbore (approximately 35.6 % after 100 days), while the majority remains trapped in the reservoir. In addition, throughout NGH dissociation, the reservoir matrix continues to move toward the wellbore in the horizontal direction. While in the vertical direction, the reservoir matrix below the wellbore moves upwards, with decreasing displacement due to low-pressure diffusion, while the reservoir matrix above the wellbore moves downwards, with increasing displacement. The largest displacements occur near the wellbore during the early stage of NGH dissociation, while in the late stage, the largest displacements occur above the reservoir.