Natural gas hydrates (NGHs) are a new, clean, and effective energy source with great potential for exploitation. The efficient exploitation of NGHs has been a focus of research worldwide. Water migration in hydrate sediments is an important parameter influencing NGH exploitation. However, there is still little research in terms of visualization studies on the variation of hydrate distribution during the water flow process in hydrate-bearing sediment. Such variation of hydrate distribution and the influence of water migration on methane hydrate (MH) dissociation with different backpressures and water flow rates were systematically and visually analyzed in this study, where the influence of temperature and pressure variation on MH dissociation was completely eliminated. The results showed that the chemical potential difference between the hydrate phase and the aqueous phase caused MH dissociation during the water flow process and that the rate of MH dissociation increased with decreasing backpressure and increasing water flow rate. When the rate of MH dissociation is low, there will be a longer time for the flow channel to appear, vary, and disappear. Based on this conclusion, a new method of water flow erosion to improve NGH exploitation is proposed in this study.

Introduction

With the development of human society, the global energy demand is predicted to increase rapidly in the coming decades [1,2]. Nowadays, traditional fossil fuels such as oil and coal comprise approximately 85.9% of the global energy supply [3]. It is essential to find a sustainable alternate energy source to meet the continuously increasing energy demand with the decrease of traditional fossil fuels [2,4]. Natural gas hydrates, which are widely distributed in continental permafrost or marine sediment, are regarded as an alternative energy resource in consideration of their high energy density and purity [3,5,6]. In past decades, natural gas hydrates (NGHs) have attracted worldwide attention for investigation. NGHs are crystalline solids composed of methane molecules and water molecules under low-temperature and high-pressure conditions [7–۹]. The hydrate thermodynamic equilibrium is affected by temperature, pressure, chemical potential differences, and chemical potential correction caused by the dissolution of guest molecules in pure water [10]. All known hydration decomposition methods are based on shifting the thermodynamic equilibrium of the system [11]. There are mainly four methods of NGH exploitation, as follows: (1) depressurization [12–۱۴], (۲) thermal stimulation [15], (3) CO2 replacement [16–۱۸], and (4) inhibitor injection [19–۲۱]. These four methods are mainly based on the temperature and pressure variations. In past years, some measurements of NGH exploitation have been performed. The depressurization method has been widely used to liberate natural gas from methane hydrate (MH) reservoirs as a cost-effective solution because it does not require external energy [22]. However, the depressurization method has a low methane production rate, and the endothermic hydrate dissociation reaction may result in secondary hydrate formation [23]. In order to study changes during the hydrate dissociation process induced by depressurization, Yousif et al. [24,25] studied the process of hydrate dissociation in Berea sandstone by depressurization, and they derived a three-phase boundary model to describe the dissociation process. Kono et al. [26] studied the kinetic dissociation rate of hydrates using the depressurization method and found that sediment properties had a significant effect on the dissociation rate. Sloan [27] and Konno et al. [28] studied the characteristics of gas production from oceanic MH reservoirs by the depressurization method and both found that the effective permeability and initial temperature of the reservoirs were important factors of gas production. Regarding the other three NGH exploitation methods, Kawamure [29] studied the hydrate dissociation kinetics by dissociating pellet-shaped samples, which mimic naturally occurring hydrates in ocean sediments, with a viscous fluid or pure water at different temperatures. Tang et al.