Study on Gas Hydrate Formation-dissociation and Its Acoustic Responses in Unconsolidated Sands

Abstract

Abstract:

To improve our understanding of the evolvement of gas hydrate in sediments and its acoustic impact on unconsolidated sediments, methane gas hydrate was formed and then dissociated in 0.18-0.28 mm sands, and subsequent testing was conducted to enable acoustic properties and water contents to be measured with ultrasonic detection and time domain reflectometry(TDR). The results of testing on several runs of experiments show that both of gas hydrate formation and dissociation could be divided into three periods according to formation speed and dissociation speed, respectively. Based on the analysis of temperature and pressure data, it comes to the conclusion that hydrate forms at the surface of sediments first and dissociates gradually fast during the three periods of dissociation. Moreover, acoustic properties shows that methane gas hydrates have the ability to cement sand grains at hydrate contents around 0-1% leading to a sharp increase in compression velocity and wave attenuation in specimens with no hydrate in the pore space. Whilst hydrates form as a part of the pore fluid and cause the blanking of ultrasonic signal appearing at the period of saturation 1%-90% during gas hydrate formation and dissociation. Consequently, this study reveals the contact mechanism between gas hydrate and sediment grains and provides a new idea to explain seismic signals in oceanic geophysical exploration. 

Key words: gas hydrate, loose sendiment, saturation, acoustic velocity, attenuation

CLC Number:

P631.5

HU Gao-wei, YE Yu-guang,ZHANG Jian. Study on Gas Hydrate Formation-dissociation and Its Acoustic Responses in Unconsolidated Sands[J]. Geoscience, 2008, 22(3): 465-474.

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