| Geophysical prospecting and logging are important methods for exploring and evaluating the physical properties of the natural gas hydrate sediments.The relationship between the hydrate sediments acoustic information sediments the hydrate saturation can be used as interpretation data for geophysical prospecting and logging.The acoustic characteristic change provides an explanation for the growth mechanism of hydrate.Therefore,the thesis conducted an experimental study on the acoustic characteristics of natural gas hydrate sediments.An acoustic test device was developed,simulating the growth conditions of marine gas hydrates sediments,which included the temperature,formation pressure,and pore pressure.Based on the device,the quantitative relationship between longitudinal wave velocity and hydrate saturation was therefore studied..The methane was pumped from bottom to top to realize the rapid generation experiment of hydrate in the vertical gas diffusion transport mode,and the acoustic wave probe was used to measure the longitudinal wave velocity and amplitude of the hydrate sediments.A simple and effective method using PVT equation for calculating the hydrate saturation in the different formulation stages of the hydrate sediments was proposed,which provided a reference for the actual hydrate sediments exploration.The data about the temperature-pressure characteristic,hydrate saturation and longitudinal wave velocity characteristic of the three groups with final saturation respectively at 57.9%,44.4% and 22.3% were obtained.After comprehensively analyzing the relationship between temperature,pressure,acoustic characteristics and saturation during the formation of hydrates,the three stages theory of hydrate formation mechanism in loose sediments were verified.By analyzing the Biot Gassmann Theory(BGT)equation of loose gas-containing hydrate sediments,it was found that the model was more suitable for the hydrates sediments with a final saturation of 40% to 45%. |
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