松散沉积物中天然气水合物生成、分解过程与声学特性的实验研究

为了解松散沉积物中天然气水合物的生成和分解规律以及水合物对沉积物声学特性的影响,在粒径为0.18~0.28mm天然沙中进行了甲烷水合物的生成和分解实验,并利用超声波探测技术和时域反射技术实时测量了反应体系的声学参数与含水量。结果表明:根据水合物的生成和分解速率,可将水合物的生成过程分为初始生长期、快速生长期和稳定期3个阶段,分解过程可分为初始分解期和样品表层水合物快速分解期以及样品内、外层水合物均快速分解期3个阶段;由温度和压力数据的分析,得出水合物先在沉积物表层生长,然后在沉积物内、外层迅速生成;由水合物分解过程3个阶段的平均分解率,得出水合物的分解是一个由慢到快的过程。对声学参数的研究表明:水合物在松散沉积物中先胶结骨架颗粒而生成,使纵波速度和声波衰减在饱和度0~1%之间陡然增大;随后水合物开始在沉积物孔隙中形成悬浮粒子,造成超声波信号在饱和度1%~90%间淬熄,声波速度无法获取。研究结果在揭示沉积物中水合物与颗粒间接触机制的同时,为海上地球物理勘探中地震信号的解释提供了新的思路。

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

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.