document

Abstract
One of the ways to optimize the production of tight sand reservoirs is to take advantage of the geometry and density of the existing natural fractures in order to optimally design and locate the production wells. The detailed mapping of the reservoir fracture network is therefore essential.

To achieve such a detailed fracture mapping, a wide azimuth 3D surface seismic acquisition has been acquired and is being processed for azimuthal anisotropy estimation. The azimuthal variations of P-wave AVO and velocity are analyzed in terms of TIH anisotropy. In order to interpret the resulting anomaly maps in terms of fracture orientation and density, it is necessary to tie the measurements of anisotropy derived from the 3D surface seismic analysis to existing well data, such as well bore electric imaging and dipolar sonic logging. The multi-azimuth VSP technique is used to link these measurements with the surface seismic data.

Initial logging measurements carried out over the field show a very clear correlation between open fracture azimuths (determined by borehole electric imaging), the direction of fast shear azimuths (determined by dipolar sonic analysis) and the direction of tectonic maximum stress. Furthermore, the depth intervals with significant anisotropy correspond to the producing reservoir units.

Multi-azimuth VSP surveys are used to tie the interpretations of azimuthal anisotropy, carried out at the seismic scale, to the anisotropy and fracture measurements, which have been carried out inside the wells. Such well-seismic surveys should help to guide the surface seismic anisotropy processing by identifying the most suitable anisotropy attributes. It should also help to tie the 3D fracture interpretations to the well anisotropy and fracture observations.

Optimized multi-azimuth VSP surveys have been designed to link the ongoing surface seismic azimuthal anisotropy analysis to available well measurements.

The survey design is based on both 3D anisotropic modeling and synthetic multi-azimuth VSP data analysis.
This paper presents preliminary results on the multi-azimuth seismic anisotropy to well tie.

Introduction
Fracturing and fracture network identification are often parts of development projects of tight sand reservoirs.
As illustrated in Figure 1, the first step in any tight sand development project, which seeks to take advantage of the natural or induced fracturing, is to find a clear relationship between the production profiles of the well and the open fracture characteristics. The appreciation/development wells can then be designed and located in accordance with the orientation and density of the natural and induced fractures to optimize their productivity.

On the other hand, open fractures produce changes in the mechanical behavior of rocks. These changes impact seismic wave propagation and may cause azimuthal variations in seismic wave properties such as velocity and amplitude. Multi-azimuth seismic techniques infer the fracture characteristics indirectly through the analysis of the azimuthal changes in seismic properties, or azimuthal anisotropy.

Multi-azimuth seismic techniques have been used since the early 1990s to map fault and fracture patterns at the field level, in addition to field geology and regional tectonic studies.