套管井应力集中下正交偶极声测井的声弹理论、数值分析与实例

Acoustoelasticity theory, numerical analysis and practical example of the cased hole with stress concentration by cross-dipole acoustic logging

油田受异常地应力作用而损坏的油井绝大部分都是套管井,针对这一实际情况,通过求解套管井应力集中情况下各层介质中应力场的分布,应用摄动积分方法,将裸眼井声弹性方程发展为适用于套管井井孔体系的情况.数值考察了井外无限远单轴水平压应力作用下,声弹方程中各层介质敏感系数和速度-应力系数随频率的变化,及二相互垂直偏振的偶极弯曲波相速度的频散曲线.结果表明,套管井声弹方程中的速度-应力系数主要由地层参数决定,内层介质的贡献较小,其中与三阶弹性模量相对应的项又起决定作用;井孔周围应力集中将引起平行和垂直单轴压应力方向偏振的二弯曲波发生速度分裂,同时随频率的增大两条频散曲线产生交叉,说明这种交叉仍然可用作判断套管井情况下地层各向异性是本征的还是由异常应力诱导的判据,用正交偶极声波测井技术在套管井中无损检测异常地应力仍然可行;但低速地层交叉点将明显向高频区移动,这又是与裸眼井情况重要的不同之处.本文还给出现场实测数据的处理实例,理论结果得到验证.

In view of actual cases that most of damaged oil wells under the action of abnormal formation stress are all for cased hole, through solving distributions of stress fields around the well in each layers caused by stress concentration of borehole and utilizing perturbation integral method, acoustoelastical equations only for the open hole are developed into the cases of being suitable to cased hole. Under the action of biaxial stress, sensitivity coefficients and velocity-stress coefficient in acoustoelastieal equations and dispersion curves of two dipole flexural modes polarized parallel and perpendicular to the applied stress direction are numerically investigated, respectively. Results show that velocity-stress coefficients primarily depend on the formation velocities of flexural modes wi characteristics of the formation moved to the higher frequency th different polarization and crossover of their dispersion curves are still main anisotropy induced by stress for cased hole, but usually dispersion curves will be region, especially for lower velocity formation, which is a difference for cased hole from the case in open hole. This put forward new requirements to carry out forward and inverse operations in the cased hole case. A practical example of field data processing for a cased hole is also presented, which is in basic agreement with theoretical predict.