套管井井壁附近地层横波速度径向分布反演

套管外地层受异常地应力、油气开采的影响,在径向上表现出非均质性;采用声波测井可以对该非均质性进行探测,利用偶极子横波测井数据可以对横波速度的径向分布进行反演.本文建立了套管井外地层横波速度径向分层参考模型,采用修正的微扰法计算了该模型的偶极弯曲波频散曲线,建立了横波速度径向分布反演目标函数,采用高斯牛顿法和快速模拟退火法对目标函数进行了求解,得到了套管井外地层横波速度的径向分布.分析了偶极弯曲波频段、套管横波速度对反演结果的影响,对比了高斯牛顿法和快速模拟退火法对反演过程的影响.分析对比结果表明,采用偶极弯曲波激发强度较高的频段与采用全频段的反演结果相近;套管的横波速度准确度越高,反演结果越准确;高斯牛顿法和快速模拟退火法计算精度相同,都可以得到高精度的横波速度径向分布;快速模拟退火法的计算效率略低于高斯牛顿法,但其收敛性对初始值依赖更小,实际处理中应选择快速模拟退火法.

Inversion of the shear velocity radial profile in a cased borehole

Formation evaluation in cased boreholes is becoming more and more important to the exploitation of oil reservoirs. Formations outside the steel casing are often affected by the stress which leads to the radial heterogeneity. As one of the well logging methods that can measure the formation property outside the casing, the acoustic well logging tool can be used to acquire the velocity information of the formation. And the shear velocity radial profile of the formation can be inverted from dipole acoustic well logging data. A radial layered model of shear velocity in a cased borehole is established in this work. The modified perturbation method is used to calculate the dispersion curve of this model. Inversion cost function is constructed based on the dispersion curve. The Gauss-Newton (GN) algorithm and Very Fast Simulated Annealing (VFSA) algorithm are employed to solve the cost function. And the radial shear velocity profile is determined in the cased hole. The influences of frequency range and casing shear velocity on the inversion results are analyzed. The inversion results using the GN algorithm are compared with those using the VSFA algorithm. The analysis shows that the dipole flexural data of the dominant excitation frequency range can be used to do the inversion. The more accurate casing shear velocity the better inversion results can be obtained. The GN and VFSA algorithms have the same calculation accuracy while VFSA has less dependence on the initial inversion value. So VFSA algorithm should be used in real data processing.