横波速度径向层析成像方法及应用研究

Research on radial shear-wave velocity tomography

近来三维声波成像技术的一个重要进展是利用偶极子弯曲波反演地层径向横波速度剖面。确定地层径向横波速度变化对评价井壁稳定性、估算地应力和优化油藏生产等有着重要的意义。偶极横波测井激发的弯曲波低频探测较深,高频探测较浅,本文根据偶极弯曲波的频散特征,采用约束反演的方法求取地层径向横波速度变化。在实际测井中,经常会遇到偶极子弯曲波的频散曲线在高频时缺失或者不准确的情况,这会使约束反演方法产生误差。针对这一现象,本文提出利用理论频散数据进行高频约束,并通过数值模拟验证了改进后约束反演方法的正确性。本文给出了一个致密气层的横波速度径向剖面综合应用实例,发现地层径向横波速度变化程度与岩石脆性有很好的对应关系,最终的压裂效果由压前、压后正交偶极横波各向异性的变化得到了印证,压裂后油气产量提高。

An important development for 3D acoustic imaging technology is inverting a radial shear-wave velocity profile away from wellbore using dipole acoustic logging data. Determining the radial property changes has been proven important for assessing borehole stability, estimating formation stress, and optimizing reservoir production, etc. We described a constrained inversion method using the dispersion data for estimating a radial shear-wave velocity changes. The constraint condition was based on a solid physical principle： a low-frequency dipole wave has a deep penetration depth and a high-frequency dipole wave has a shallow penetration depth. To improve the accuracy of constrained inversion method, we constrained high-frequency data using theory dispersion and verified the effectiveness using numerical simulation. We then showed an example as the application of radial shear-wave velocity tomography. We found that the radial shear-wave velocity changes consist of the brittleness of rock. The fracturing has been verified by anisotropy changes of pre-fracturing and post-fracturing and gas production was improved after fracturing.