Characterization of fractured seservoir using offset VSPs: case study from the Varandei Field, Northern Russia

摘要

Traditional seismic techniques of examining fracture-induced seismic anisotropy in a borehole utilize either walkaround observations of direct compressional waves (Home 2003; Home et al., 2002; Thompson et al., 2002), observations of vertical shear-waves with different polarizations (Brodov et al., 1991, Liu, Crampin and Queen, 1991; Winterstein, Ge, and Meadows 2001), or converted PS waves in offset VSP data (Home et al., 2002). The walkaround VSP data allows the construction of the P-wave velocities (Vp) indicatrix. The observations of vertical shear-waves are based on shear-wave splitting or birefringence measurements. The elastic moduli are usually calculated from the kinematic parameters of seismic waves assuming the model of transversally anisotropic media with a horizontal symmetry axis. For more complicated models (for example, thin-layer section with vertical fractures leading to orthorhombic model) the 3C multi-azimuth, multi-offset VSP is used (Dewangan and Grechka, 2002). Our goal is to extract information about seismic anisotropy due to the vertical fracturing from routinely recorded offset VSP data obtained with non-directional or explosive sources. We attempt to answer the following two questions: (1) Is it possible to control the polarization of converted PS waves in order to obtain pure SI and S2 modes? and (2), How to remove the influence of offset PS ray paths in order to estimate time delay between SI and S2 waves?