Abstract
An apparatus for the analysis of pressure cores containing gas hydrates at in situ pressures was designed, and a series of experiments to determine the compressional wave response of hydrate-bearing sands were performed systematically in the laboratory. Considering the difficulties encountered in performing valid laboratory tests and in recovering intact hydrate bearing sediment samples, the laboratory approach enabled closer study than the marine environment due to sample recovery problems. The apparatus was designed to achieve in situ hydrate formation in bearing sediments and synchronous ultrasonic detection. The P-wave velocity measurements enabled quick and successive ultrasonic analysis of pressure cores. The factors influencing P-wave velocity (Vp), including hydrate saturation and formation methodology, were investigated. By controlling the initial water saturation and gas pressure, we conducted separate experiments for different hydrate saturation values ranging from 2% to 60%. The measured P-wave velocity varied from less than 1700 m/s to more than 3100 m/s in this saturation range. The hydrate saturation can be successfully predicted by a linear fitting of the attenuation (Q−1) to the hydrate saturation. This approach provided a new method for acoustic measurement of the hydrate saturation when the arrival time of the first wave cannot be directly distinguished. Our results demonstrated that the specially designed non-embedded ultrasonic detection apparatus could determine the hydrate saturation and occurrence patterns in pressure cores, which could assist further hydrate resource exploration and detailed core analyses.
Highlights
Natural gas hydrate has become a potential alternative energy source with the characteristics of being clean and having large reserves [1,2,3]
The corresponding ultrasonic signal experienced a mutation, mainly reflected in the relative amplitude for the received signal increasing to 0.31 V with an increment of 210% compared to the hydrate-free state. This phenomenon indicated that the presence of hydrate could significantly change the elastic properties of the sediments, which might indicate that the change of P-wave velocity and amplitude mainly occurred at the late stages of hydrate formation
A method was developed to obtain the ultrasonic characteristics of pressure cores at in situ pressures
Summary
Natural gas hydrate has become a potential alternative energy source with the characteristics of being clean and having large reserves [1,2,3]. The exploration and development of natural gas hydrates in various countries in the world have promoted considerable research and numerous techniques [4,5,6,7]. With the development of natural gas hydrate sampling techniques, the detection and analysis of gas hydrate pressure cores become the bridge that connects hydrate exploration and exploitation [8,9,10]. Natural gas hydrate pressure cores reflect the reservoir characteristics and hydrate. Energies 2019, 12, 1997 accumulation [11,12,13] This information is important for the study of hydrate exploitation. The total amount of methane captured in the form of hydrate in nature is estimated to be on the order of
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