各向异性岩石三阶弹性模量及其孔周声速的实验探究

发布时间：2018-07-26 11:54

【摘要】：声弹性现象是指弹性介质在受外力作用下,声波速度发生改变的现象。若干声弹实验研究表明岩石具有明显的非线性效应。通过实验测量不同应力作用下岩石声速及孔周声速分布,然后可由声速求出岩石的三阶弹性模量及判断主应力方向。在实际应用中,若已知地层岩石介质的三阶弹性模量,可以通过测量地层声速进而求出地层所受地应力的大小,同时已知孔周声速分布可判断出地层所受地应力的方向。因此,开展岩石三阶弹性模量及孔周速度分布的实验测量,对研究反演地应力理论与方法都具有十分重要的意义。本文通过以实验探究为主,以声弹理论为基础,分别对三种无孔岩石材料和钻孔岩石介质进行声弹性实验。通过实验测量单轴应力作用下岩石纵波速度和两个横波速度来求解岩石的三阶弹性模量。首先测量了参考态下(介质未受应力作用时)声波通过介质每个对称面的纵波和横波速度,通过三个对称面的(即三个坐标方向)纵波和横波速度测量值,发现岩石样品有弱各向异性,求出了参考态下样品的速度各向异性参数,对比Thomson参数结果看出本文实验采用的岩石样品近似为横向各向同性介质。其次改变横波偏振方向,测量同一平面内不同方位角处沿垂直该平面方向传播的横波速度,结果显示参考态下同一平面内沿不同方向偏振的横波速度发生变化,进一步确定了岩石样品有弱各向异性。最后在理论求解三阶弹性模量过程中考虑了岩石样品参考态下的本征各向异性,并计算出不同应力状态下的速度理论值,然后将速度测量值与理论值进行对比,发现通过岩石中心不同应力状态下的速度实验值跟理论值基本吻合,说明关于岩石三阶弹性模量的实验测量需要顾及参考态的各向异性。有关岩石样品孔周声速的实验探究主要集中在实验测量孔周纵波速度分布,关于孔周横波速度分布的实验探究很少。本文对钻孔的654岩样施加垂直孔轴方向的单轴力,然后分别测量不同应力状态下,在孔周不同方位角处沿孔轴方向传播的纵波速度、平行单轴力方向偏振沿孔轴方向传播的横波速度、垂直单轴力方向偏振沿井轴方向传播的横波速度。通过实验测量了不同应力状态下孔周纵波和两个横波速度,发现沿孔周一圈的最大波速方向与最小主应力方向一致,最小波速方向与最大主应力方向一致,因此通过实验测量孔周声速可以确定水平主应力的方向。
[Abstract]:Acoustoelastic phenomenon refers to the change of acoustic velocity in elastic medium under external force. A number of acoustoelastic experiments show that rocks have obvious nonlinear effects. The distribution of rock sound velocity and the sound velocity around a hole under different stresses are measured experimentally. The third-order elastic modulus of rock and the direction of principal stress can be obtained from the sound velocity. In practical application, if the third-order elastic modulus of formation rock media is known, the magnitude of in-situ stress can be obtained by measuring formation sound velocity, and the direction of in-situ stress can be determined by known distribution of acoustic velocity around holes. Therefore, the experimental measurement of the third-order elastic modulus of rock and the velocity distribution around the pore is of great significance to the study of the theory and method of inversion of in-situ stress. In this paper, three kinds of porous rock materials and borehole rock media are tested on the basis of acoustoelastic theory. The third order elastic modulus of rock is calculated by measuring the longitudinal wave velocity and two shear wave velocities under uniaxial stress. First, the velocities of sound waves passing through each symmetric plane of the medium under the reference state (when the medium is not subjected to stress) are measured, and the velocities of the longitudinal waves and the shear waves of the three symmetric surfaces (that is, the directions of the three coordinates) are measured. It is found that the rock sample has weak anisotropy, and the velocity anisotropy parameters of the sample under the reference state are obtained. The comparison of the Thomson parameters shows that the rock sample used in this paper is approximately transversely isotropic medium. Secondly, by changing the polarization direction of the shear wave, the velocity of the transverse wave propagating perpendicular to the plane at different azimuths in the same plane is measured. The results show that the velocity of the transverse wave polarized in the same plane varies in different directions under the reference state. It is further confirmed that the rock samples have weak anisotropy. Finally, the intrinsic anisotropy of the reference state of rock samples is considered in the theoretical solution of the third-order elastic modulus, and the theoretical values of velocity under different stress states are calculated, and then the velocity measurements are compared with the theoretical values. It is found that the experimental values of velocity at different stress states in the center of rock are in good agreement with the theoretical values, which indicates that the anisotropy of the reference state should be taken into account in the experimental measurement of the third-order elastic modulus of rock. The experimental investigation on the velocity of sound around the pore of rock samples is mainly focused on the measurement of the velocity distribution of the longitudinal wave around the hole, but little on the velocity distribution of the shear wave around the hole. In this paper, 654 borehole samples are subjected to uniaxial forces perpendicular to the hole axis, and the longitudinal wave velocities propagating along the hole axis at different azimuth angles around the hole are measured respectively under different stress states. The velocity of the transverse wave propagating along the hole axis is polarized in the direction of parallel uniaxial force and the velocity of the shear wave propagating along the axis of the well in the direction of the vertical uniaxial force. The longitudinal wave and two shear wave velocities around the hole under different stress states are measured experimentally. It is found that the direction of the maximum wave velocity along the circle of the hole is the same as the direction of the minimum principal stress, and the direction of the minimum wave velocity is the same as the direction of the maximum principal stress. Therefore, the direction of horizontal principal stress can be determined by measuring the velocity of sound around the hole experimentally.
【学位授予单位】：吉林大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：O42

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