基于横波分裂思想的横波速度方位各向异性反演

Inversion of shear-wave velocity azimuthal anisotropy based on shear-wave splitting

地层中平行排列的近垂直裂隙导致介质呈现明显的方位各向异性,尤其是横波速度的方位各向异性特征更为显著,因此横波速度各向异性反演结果可用于裂隙发育区的预测。基于横波分裂思想的横波速度各向异性反演方法因扇区太少影响反演精度,扇区太多影响覆盖次数,故首先将OVT域道集或者CMP道集按照一定的规则划分为5~6个不同方位的扇区,对每一个扇区的道集数据进行横波速度无井反演,即三参数AVO反演,得到横波速度反射率数据;然后针对目的层段补充一个常数的横波速度低频分量,得到这些扇区的横波速度绝对值数据体;最后利用这些不同扇区的横波速度数据开展椭圆拟合处理以确定椭圆长轴方向、长轴和短轴大小以及长轴、短轴比值,进而得到裂隙方向和裂隙密度。将该方法应用于塔里木盆地TH-A区和TH-B区碳酸盐岩裂隙及裂隙溶洞体预测,结果如下:W2井证实了TH-A区为不发育裂隙地区的预测结果,TH-B区预测结果(裂隙或小规模缝洞)与16口生产井裂隙数据高度吻合。

Near-vertical parallel fractures in a formation result in azimuthal anisotropy,particularly with respect to the shear-wave(S-wave)velocity.By performing the inversion of the S-wave velocity accounting for anisotropy,the fracture zones within a formation can be identified.In this study,the offset vector tile domain gathers or common midpoint gathers were divided into 5~6 sectors having different orientations.The number of sectors was chosen to be large enough to ensure sufficient inversion accuracy of the anisotropy without compromising the identification of the folds.Elastic inversion of the S-wave velocity(in absence of a well log constraint)was carried out for each sector as follows.First,the reflectivity data of the S-wave velocity were obtained using a three-parameter amplitude-versus-offset inversion.Then,the constant low-frequency component of the S-wave velocity was attributed to the target formation to obtain the absolute S-wave velocity values.Datasets of S-wave velocity from different sectors were utilized for elliptic fitting.The direction of the ellipse's major axis,the length of the major and minor axes,and their ratio were determined to identify the direction and density of the fractures.The proposed method was applied to the prediction of fractures and dissolution caves in the carbonate formation in the TH A and B areas of the Tarim Basin.Data from well W2 confirmed that no fractures had developed in A,whereas the predicted patterns in B(fractures or small-scale fracture-caves)were consistent with the cracks identified in 16 production wells.