含甲烷水合物松散沉积物超声波性质的实验研究

实验模拟海域含天然气水合物松散沉积物, 并测量其声学性质变化规律, 以期为天然气水合物地震勘探和资源评价提供科学依据。采用一套改进的水合物超声波速度测量装置, 根据反应釜内部气体压力的变化, 连续监测实验过程中甲烷水合物饱和度的变化, 获得被水完全饱和的松散沉积物中甲烷水合物饱和度与声波速度的关系。模拟南海神狐海域SH-7钻孔的HY-3样品, 进行多个轮次的甲烷水合物形成?分解实验, 实测纵、横波的速度均落在水合物非胶结曲线与胶结曲线之间, 表明甲烷水合物在沉积物孔隙中同时存在胶结和非胶结多种赋存形式, 在分解和合成过程中均以呈悬浮状或作为支撑颗粒的水合物优先, 但少量起胶结作用的水合物始终存在, 并对沉积物声学性质有明显的提升作用。

Experimental Study of The Ultrasonic Velocity of Methane Hydrate-Bearing Unconsolidated Sediments

It is the scientific basis for seismic exploration and resource evaluation of natural gas hydrate to study the acoustic properties of gas hydrate-bearing unconsolidated sediments in the laboratory. For this purpose, an instrument was designed to experimentally study the change in acoustic velocity of water saturated unconsolidated sediments during gas hydrate formation. The improved ultrasonic transducer was employed to measure the acoustic velocity. The saturation of methane hydrate was continuously monitored as referring to the gas pressure change in the reactor, which was then related to the corresponding acoustic measurement. With the artificial sample simulating the sediments of core HY-3 from SH-7 borehole in Shenhu area of South China Sea, several round experiments of methane hydrate formation and dissociation were conducted. The results showed that the measured compressional and shear wave velocities were located between the curves predicted as to the rock physics models with and without hydrate cementing sediment grains in pore space. Although methane hydrate preferred to suspend in pore space or precipitate on sediment grains in dissociation and formation processes, there was always a very small amount of hydrate cementing the grains, and it significantly increased the acoustic velocities of sediments.