地应力各向异性致密砂岩地层声反射测井正演模拟

为研究地应力各向异性对偶极子声反射测井中井外地质构造处形成的反射波的影响,以致密砂岩为例,采用时域有限差分方法,结合混合完全匹配层(Hybrid-PML)吸收边界,利用地应力各向异性模型建立不同方位角反射界面的声反射测井模型,正演模拟研究各向同性和地应力各向异性致密砂岩地层中的正交偶极子声反射测井,通过对比反射波分析地应力各向异性对偶极子声反射测井的影响。结果表明:对于偶极反射波,地应力各向异性导致横波速度增大以及界面处反射系数减小,因此SV-SV反射波到时和幅度减小;在中短源距范围内,地应力各向异性也会导致SH-SH反射波到时和幅度的减小,但在长源距条件下,地应力各向异性地层存在的非均质性使得各向异性模型中SH-SH反射波的幅度与各向同性模型基本一致。可见,在地层剖面的偏移成像中,需要考虑地应力各向异性对偶极反射波的影响。

Study on Acoustic Reflection Logging Simulation in Stress-Induced Anisotropic Tight Sandstone Formation

In order to study the influence of stress-induced anisotropy on dipole reflection signals, isotropic and stress-induced anisotropic tight sandstone models with a reflector with different azimuth angles away from the borehole were built, then finite difference method combined with Hybrid-PML absorbing boundary condition was used to simulate dipole acoustic reflection logging in the models. Then the influence of stress-induced anisotropy on reflection signals is analyzed through the comparison of reflection signals between isotropic and stress-induced anisotropic acoustic reflection models. As the synthetic results shows, for the dipole reflection signals, since stress-induced anisotropy results in larger shear wave velocity and smaller reflection coefficient at the reflection interface, the arrival time and amplitude of SV-SV reflected wave get smaller; at short and medium spacings, stress-induced anisotropy will also result in smaller arrival time and amplitude of SH-SH reflected wave, however, at long spacing, due to the formation heterogeneity resulted from stress-induced anisotropy, the amplitude of SH-SH reflected wave is basically the same as that in the isotropic tight sandstone model. It is concluded that the influence of stress-induced anisotropy on dipole reflection signals should be taken account of in the immigration and imaging process of formation profile.