Borehole Seismic Application for the Qusaiba Shale Play in Saudi Arabia

Abstract

Understanding seismic anisotropy is important for the exploration and development of unconventional shale gas reservoirs. Most shales are intrinsically anisotropic and the presence of thick layers of shale can cause defocusing of the seismic image and significant positioning errors of seismic reflectors. Moreover, anisotropy can have a major impact on the elastic inversion results and geomechanical analysis. The seismic anisotropy of shales results from a partial alignment of anisotropic plate-like clay minerals and can be approximated as vertically transverse isotropic (VTI) (Sayers, 2005). At the same time, shales may also exhibit azimuthal anisotropy associated with fractures and / or stresses. Borehole seismic data allows analysis of both VTI using walkaway VSP and azimuthal anisotropy using walkaround VSP. The Thomsen’s parameters (Thomsen, 1986) reliably estimated from VSP data can be used at different stages of exploration and development of unconventional reservoirs. The VSP results can also play an important role in the assistance of horizontal well placement by providing more accurate imaging of the vicinity of the borehole especially in anisotropic media (Berman et al, 2013). This is especially true in areas lacking 3D seismic data and well control. Another application of borehole seismic is providing critical information for microseismic monitoring of hydraulic fracturing. At the initial stage of downhole microseismic monitoring, VSP data can be used to assist in the optimal positioning of receivers in the lateral borehole and in the absence of perforation shots due to completion design, provide orientation of its horizontal components. During the processing and analysis of recorded microseismic data, VSP results help to build the anisotropic velocity model for a more accurate location of the microseismic events.