Experiment and evaluation on the in-situ rapid preparation of marine non-diagenetic natural gas hydrate

In China, marine natural gas hydrate (NGH) resources are mainly distributed in the deepwater areas of 300–3000 m deep in the coastal continental shelf, and they are characterized by weak cementation and non-diagenesis. In order to test and study this type of NGH, it is necessary to prepare samples quickly in situ in large quantities. At present, the commonly used preparation technologies (e.g. stirring method, spraying method and bubbling method) are defective with slow generation and low gas storage density. In view of this, the 1062 L rapid non-diagenetic NGH preparation kettle was designed and developed independently. Then, based on the physical properties of non-diagenetic NGH in the South China Sea, NGH preparation tests were carried out on stirring method, spraying method, bubbling method and “three-in-one” method (the combination of stirring method, spraying method and bubbling method). Temperature, pressure, resistivity and reaction time in the process of test were tested, and NGH generation situations and preparation efficiency of different preparation processes were analyzed and compared. And the following research results were obtained. First, in the process of NGH preparation by stirring method, spraying method and bubbling method, respectively, the generated NGH increases slowly and spreads the whole liquid level gradually. Second, inducing period is obvious in the preparation process by the stirring method, but not obvious by the spraying method, the bubbling method and the “three-in-one” method. Third, in the process of NGH preparation by stirring method, spraying method, bubbling method and “three-in-one” method, the resistivity increases as the reaction goes, and its change trend is basically accordant with the NGH preparation rate. Fourth, the preparation cycle of NGH by “three-in-one” method is greatly shorter than that by single preparation method. The preparation time by stirring method, spraying method and bubbling method is about 5.14 times, 3.59 times and 3.16 times that of “three-in-one” method respectively. In conclusion, the 1062 L rapid NGH preparation kettle developed in this paper can prepare marine non-diagenetic NGH samples quickly in situ. And compared with single preparation method, “three-in-one” method is much higher in NGH preparation efficiency.