用非强迫共振法研究饱和岩石的黏弹性响应

STUDY OF VISCOELASTIC BEHAVIOR OF SATURATED ROCKS BY USING NON-FORCED RESONANCE METHOD

通过Metravib热机械分析仪,用正弦振动对岩样进行加载,并以饱和岩石对正弦振动传播的黏弹响应,来模拟地震波在饱和岩石中传播时的黏弹性响应,以获取地震波在饱和岩石中传播时的黏弹行为。试验时固定静载为120 N,正弦波动载荷为80 N;在温度-40℃～100℃,频率5～400 Hz的范围内,对2种孔隙度的泵油饱和彭山砂岩样品进行单轴循环加载试验,以得到泵油饱和彭山砂岩的衰减、弹性模量、弹性波速度与温度的关系。由此研究饱和多孔岩石在弹性范围内的衰减、杨氏模量和弹性波波速随温度的变化规律。研究结果表明:随频率的增高,饱和多孔岩石的能量衰减峰的峰位向高温方向移动的热激活弛豫规律。利用阻尼衰减机制,解释了随振动频率提高,衰减峰的强度降低的试验结果。这些试验结果与低频共振的驻波试验得到的热激活弛豫规律一致,说明热激活弛豫机制在饱和多孔岩石中具有一定的普适性。与此同时,得到弹性模量和波速随温度升高而逐渐下降的一般规律,以及随频率的增高而增大的频散特性,但是样品的频散效应随温度升高有减弱的趋势。该频率和温度范围的研究结果对岩石理论模型研究具有指导意义,对地震资料的解释具有实用意义。饱和岩石的衰减研究对地震工程和勘探地球物理具有特殊的意义,尤其对勘探地热蒸汽库或天然气库有很好的实用价值。

Employing Metravib dynamic mechanical analyzer, saturated rock samples are loaded under sine wave loading model to simulate the viscoelastic behavior when seismic wave propagating in viscoelastic media. The static load is fixed to 120 N, and the dynamic load of the sine wave is 80 N; so that the total loading force is controlled under yield stress. Temperature is controlled between -40℃ and 100 ℃, and the frequency range is from 5 Hz to 400 Hz. Such two kinds of saturated rocks as pump-oil saturated Pengshan sandstones with two different porosities are tested under uniaxial cyclic loading. The variations of attenuation, elastic modulus, and elastic wave velocity with temperature or frequency are obtained. The energy attenuation peaks shift to higher temperatures when the frequency increases, which can be considered as thermal relaxation regularities. Damping attenuation mechanism is used to explain the experimental result that amplitude of attenuation peak decreases with the increasing frequency. The experimental results are in accord with the thermal relaxation regularities obtained by low-frequency resonant standing wave experiments, which show that the thermal relaxation regularities may widely exist in saturated rocks. The experimental results indicate that the elastic modulus and velocity increase with the increasing frequency and decrease when temperature increases. There is obvious frequency dispersion, and the dispersion weakens at high temperature area. The studies of ranges of frequency and temperature are very helpful to the study of theoretical model and seismic data interpretation. Study of attenuation of saturated rock is valuable for geophysical exploration, especially the exploration of geothermal steam or gas reservoirs.