The natural gas hydrate (NGH) reservoir in China is mainly distributed in the continental shelf with water depths ranging from 600–1500 m, about 90% of which is stored in the shallow area of the deep sea, with weak cementation and non-diagenetic characteristics. In order to test and study this type of NGH, samples must be prepared in situ, in large quantities, and at fast speed. At present, there are problems with the common stirring, spraying, and bubbling preparation techniques available, such as slow generation rate, low gas storage density, and lack of rapid preparation. Therefore, the rapid preparation of large samples of non-diagenetic natural gas hydrate has received extensive attention at home and abroad. In view of this technical bottleneck, Southwest Petroleum University innovatively established a rapid preparation kettle of 1062 L. In this paper, the preparation experiment of natural gas hydrate in the South China Sea (the pressure of the preparation kettle was reduced from 7 MPa to 3.3 MPa) was carried out in the preparation method of the ‘three-in-one’ (stirring method, spraying method, bubbling method) and experimental test method. In the process of preparation of non-diagenetic gas hydrate, the data of dynamic image, temperature, pressure, electrical resistivity, and reaction time are tested. During the preparation of natural gas hydrate, temperature, pressure, and electrical resistivity curves in four preparation methods were made, respectively. Through the experimental data analysis of different preparation methods of natural gas hydrate, it has been found that the preparation time of natural gas hydrate using the stirring method, the spraying method, and the bubbling method alone require a longer preparation time. However, when the three-in-one method is used to prepare natural gas hydrate, the preparation cycle of natural gas hydrate is obviously shortened. The preparation time of the single method of stirring method, spraying method, and bubbling method is respectively about 5.13, 3.59, and 3.37 times as long as that of three-in-one method. The three-in-one method for preparing natural gas hydrate greatly improves the preparation efficiency, which has a great significance to the scientific and technological progress of experimental research and evaluation methods of natural gas hydrate.

Horizontal well drilling is a highly effective way to develop marine gas hydrate. During the drilling of horizontal wells in the marine gas hydrate layer, hydrate particles and cutting particles will migrate with the drilling fluid in the horizontal annulus. The gravity of cuttings is easy to deposit in the horizontal section, leading to the accumulation of cuttings. Then, a cuttings bed will be formed, which is not beneficial to bring up cuttings and results in the decrease of wellbore purification ability. Then the extended capability of the horizontal well will be restricted and the friction torque of the drilling tool will increase, which may cause blockage of the wellbore in severe cases. Therefore, this paper establishes geometric models of different hole enlargement ways: right-angle expansion, 45-degree angle expansion, and arc expanding. The critical velocity of carrying rock plates are obtained by EDEM and FLUENT coupling simulation in different hydrate abundance, different hydrate-cuttings particle sizes and different drilling fluid density. Then, the effects of hole enlargement way, particle size, hydrate abundance and drilling fluid density on rock carrying capacity are analyzed by utilizing an orthogonal test method. Simulation results show that: the critical flow velocity required for carrying cuttings increases with the increase of the particle size of the hydrate-cuttings particle when the hydrate abundance is constant. The critical flow velocity decreases with the increase of drilling fluid density, the critical flow velocity carrying cuttings decreases with the increase of hydrate abundance when the density of the drilling fluid is constant. Orthogonal test method was used to evaluate the influence of various factors on rock carrying capacity: hydrate-cuttings particle size > hole enlargement way > hydrate abundance > drilling fluid density. This study provides an early technical support for the construction parameter optimization and well safety control of horizontal well exploitation models in a marine natural gas hydrate reservoir.