Introduction
Wide azimuth P-wave anisotropic fracture detectionMethod
For current gas and oil exploration worldwide, there are more and more fractured reservoirs. Especially in carbonate rocks, igneous rocks and tight sandstones, fractures are the major storage space for reservoir and therefore fractured reservoir prediction is of great importance. Many studies showed that in azimuthally anisotropic medium, seismic properties such as amplitude and velocity vary ellipsoidally with respect to the azimuth. Thus the AVO analysis of azimuthal P-wave seismic data can be used to predict fractures.Fractures are a kind of spatial surface on which rocks lost forces to stick together. The presence of fractures is required for forming fractured reservoirs of buried hill, or igneous rock type. Fractures can be mainly divided into two types: the structural fracture, mainly caused by structure distortion and faulting, and non-structural fracture such as sedimentary layers and diagenetic fracture. Two issues should be resolved in exploring fractured reservoirs: the orientation and the density or intensity of the fracture. Fractures cause the physical properties change along with the azimuth, known as azimuthal anisotropy in seismic exploration. By measuring these physical properties, we can detect fractures using pre-stack seismic data. In recent years fracture detection using wide-azimuth pre-stack P-wave seismic data has achieved many successes.As shown in Figure 1, when there are vertical or nearly vertical fracture zones of a certain scale which are aligned in parallel, the subsurface medium can be regarded as an anisotropic medium. When seismic wave travels in the direction perpendicular or parallel to the fracture orientation, the amplitude, frequency, velocity and other attributes will be affected by the fracture. Studies showed that amplitude, frequency and velocity are mostly influenced by fractures when the P wave propagates in the direction perpendicular to the fracture orientation, while the influence is the least when it propagates in the direction parallel to the fracture orientation. The P-wave AVO gradient in an anisotropic medium differs greatly in the directions parallel or perpendicular to the fracture strike(Perez and Gibson, 1996; Ramos and Davis, 1997; Mallick et al., 1998; Shen et al., 1999).
Application
General information of the survey
P-wave anisotropic fracture detection can be achieved by extracting P-wave seismic attributes at different azimuths and then fitting these attributes to an ellipse (Figure 2). The ellipticity corresponds to the density of the fracture, and the major axis and minor axis delineate the orientation of the fracture.In this survey, the general structure shows a nose feature inclined to northeast. The lithology of the main target zone is the sandstone which is tight and of low porosity. The reservoir space is mainly due to fracturing. The fold of seismic data in this area is relatively high, and the offsets between shots and receivers are relatively uniform. The data acquisition is of wide azimuth with relatively even fold at each azimuth (Figure 3). This established a good data basis for fracture detection. Finally we got six sets of azimuthal stack data for fracture
