Effects of pressure and sea water flow on natural gas hydrate production characteristics in marine sediment
Graphical abstract
Introduction
The worldwide energy demand will continuously increase in the coming years to meet the needs of national economies and social development [1], [2]. Currently, 85% of the global energy needs are reliant on the conventional fossil fuels, including coal, petroleum and natural gas [3]. However, the environmental pollution problem from combustion and global CO2 emissions impede the development of human society in consideration of the consumption of traditional fossil fuels. Finding a sustainable alternative energy source to satisfy the energy demand is essential [3], [4]. Natural gas hydrates are considered as a potential alternative form of energy due to their high energy density, non-polluting nature and wide existence in permafrost or ocean seabeds [5], [6]. NGHs have aroused worldwide interest for research since a large number of hydrate reservoirs have been discovered in past decades [7], [8].
NGHs are ice-like crystalline solids composed of methane molecules and water molecules created under low-temperature and high-pressure conditions, in which the methane molecules are trapped in a lattice structure formed by water molecules [9], [10], [11]. The influencing factors of hydrate thermodynamic equilibrium include pressure, temperature, chemical potential differences, and a correction of chemical potential caused by guest molecules dissolved in pure water [12]. The hydration dissociation can be achieved by shifting the thermodynamic equilibrium in a three-phase system (gas-liquid-hydrate) [13]. To extract the methane gas from NGH, the following methods have been proposed: (1) depressurization [14], [15], [16], (2) thermal stimulation [17], [18], [19], (3) inhibitor injection [20], [21], and (4) CO2 replacement [22], [23], [24]. These four methods mainly promote hydrate dissociation by changing the temperature and pressure in the system.
Numerous NGH exploitations, experimental research projects and numerical simulations were reported over the past few decades. Among the four methods, it has been found that the depressurization technique is the most cost-effective method and has been widely used to liberate natural gas from MH reservoirs [25]. The best advantage of depressurization is that it does not require external energy; the decomposition heat is provided by the ambient environment [26]. However, among these four methods, the depressurization method has the lowest production rate due to slow reaction for hydrate dissociation, which results in the longest dissociation time. Moreover, it may lead to a secondary formation of hydrates in the process of dissociation because of the endothermic hydrate dissociation reaction [27]. Many researchers have carried out experiments on the depressurization method to study the variation during the hydrate dissociation process. Holder [14] and Chong [28] reported on the depressurization process and heat and mass transfer in a reservoir containing gas hydrate and free natural gas and heat by means of a numerical simulation and additional experiments. Aoki [29] studied the compaction behavior of Toyoura sand containing methane hydrate with dissociation induced by depressurization, and the relationships among pore temperature and pressure and vertical displacement of reservoir sand were obtained. Yousif [30] developed a three-phase 1D model to study the process of gas production from Berea sandstone containing MH by the depressurization method. Lee [31] designed an experimental apparatus to analyze the characteristics of hydrate dissociation and gas productivity by using the depressurization scheme. They found that the degree of depressurization is a significant influencing factor on the gas production rate. Tang [32] performed experimental works on dissociation from the hydrate-bearing core by depressurization and deduced the intrinsic hydration dissociation constant by fitting the numerical simulation results with experimental data. Kono [33] studied the dissociation rate of methane hydrates in porous sediments by using the depressurization method and found that the dissociation rate can be adjusted by control of sediment properties. Regarding the other three NGH exploitation methods, Tang [34] measured the flowing characteristics of the water and gas from hydrate dissociation and temperature distribution in porous sediments using thermal stimulation. The results of their experiment showed that the hydrate content has a promoting influence on the energy ratio, while the injection temperature and rate have negative effects on it. Kawamure [35] investigated the dissociation kinetics of methane and methane-ethane hydrate by dissociating pellet-shaped samples, and the dissociation rates of gas hydrate were measured in a viscous fluid or pure water at different temperatures. Li [36] experimentally investigated the gas production efficiency from methane hydrate by injection of an ethylene glycol solution with different injection rates and different concentrations. Both the injection rates and concentrations were found to be capable of affecting the production efficiency. Ota [37] designed an experiment to study the replacement process between liquid CO2 and CH4 in hydrate and used Raman spectroscopy for analysis. Their experiments demonstrated that small cavities can be preoccupied by the CH4 molecules released from hydrate. Kvamme [38] conducted a visualization study on the conversion of CH4 hydrate into the CO2 hydrate in Bentheim sandstone matrix using magnetic resonance imaging (MRI) and utilizing the Phase Field Theory approach to simulate the corresponding model system.
No matter what the NGH exploitation method is, it is nearly always accompanied by gas-water migration during the hydrate dissociation process. Much research regarding the NGH dissociation behaviors have been reported. However, the visualization study on the variation of hydrate distribution and the influence of seawater migration on MH dissociation in hydrate-bearing sediment during the seawater flow process has not been previously reported. Furthermore, few investigations are found regarding the effect of the chemical potential difference on MH dissociation. The main focus of this study is to conduct a visualization experiment to investigate the MH dissociation behaviors under seawater flow processes. Specifically, this experiment completely eliminates the effect of temperature and pressure variations on MH dissociation by maintaining a higher backpressure than the phase equilibrium pressure and a lower system temperature than the phase equilibrium temperature. These experimental results can provide a theoretical basis for the exploitation of submarine NGH.
Section snippets
Apparatus and materials
Fig. 1 shows the schematic design of the experimental setup used in this study for the research on methane hydrate (MH) dissociation during the seawater flow process. Li [39] have found that hydrates will dissociate during water injection and that the hydrate dissociation rate is very slow under low water injection rates. The core component of the experimental system is an MRI system (Varian, Inc., Palo Alto, CA, USA), which operated at 400 MHz for measuring hydrogen to image the MH
Results and discussion
In this study, the method of water flow erosion was used for NGH exploitation by enhanced driving force of MH dissociation. Twelve experimental cases on the seawater flow process was carried out in this investigation. The experimental conditions and results are shown in Table 1. The hydrate saturation was controlled to approximately 18.5% for each case, and the seawater flow velocities were set at 10, 8, 6, 4, 2, and 1 mL/min in this investigation. The backpressure was set to 3200 and 3300 kPa,
Conclusion
The promotion effect of water phase migration process on MH dissociation was investigated to contribute to the exploitation of hydrates. The results are summarized as follows:
- (1)
A visualization study on the distribution of hydrates in hydrate-bearing sediment during the seawater flow process was conducted. The influence of temperature and pressure variations on MH dissociation was eliminated completely in the experiments. The method of water flow erosion for natural gas hydrate (NGH) exploitation
Acknowledgements
This study was financially supported by grants from the National Natural Science Foundation of China (51436003, 51822603 and 51576025), the National Key Research and Development Plan of China (2017YFC0307303 and 2016YFC0304001), the Fok Ying-Tong Education Foundation for Young Teachers in the Higher Education Institutions of China (161050) and the Fundamental Research Funds for the Central Universities of China (DUT18ZD403).
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