ApplicAtions ■ Locate sidetrack wells for better

reservoir drainage ■ Target a fractured reservoir with

a directional well ■ Develop velocity models for surface

seismic processing and survey evaluation and design campaigns

ADVAntAGEs ■ Three-dimensional (3D) definition of the

salt flank for location of sidetrack wells ■ Selection of preferred drilling direction

with accurate minimum and maximum stress directions

■ Calibration of converted wave source seismic acquisitions to determine sensitivity to reservoir fracturing

■ Interpretation of subtle faults, pinchouts, and other stratigraphic features for enhanced survey evaluation

FEAturEs ■ Measurement of apparent fast and

slow shear wave directions ■ Identification of conversion points

for datasets rich in converted waves, both transmitted and reflected

■ Fracture orientation measurement ■ High-resolution converted wave

seismic images ■ Separation and individual analysis

of the four wave models

Traditionally, companies acquire offset vertical seismic profile (offset VSP) datasets to provide very high-resolution two-dimensional (2D) seismic sections in the source receiver plane. The advent of multiple-level, three-component geophone arrays and 3D advanced processing techniques enabled offset VSP datasets to be acquired for a much wider variety of applications.

Minimized drilling risk with 3D salt proximity surveysSalt proximity surveys determine how raypaths travel from a seismic source located above a salt dome to the geophones positioned in a wellbore drilled nearby (Fig. 1). This information generates an exact 3D definition of the salt flank, especially if salt proximity data from nearby wells can also be incorporated. Success requires a good understanding of the overburden and the use of a gyro tool to orient the geophones. The results minimize drilling problems and precisely locate sidetrack wells required for optimal reservoir drainage.

Fracture orientation determination with walkaround VspShear waves are sensitive to anisotropy because of a phenomenon known as shear-wave splitting or shear-wave birefringence. As the shear wave, or converted shear wave, enters the anisotropic region, it can split into two waves—a fast shear wave and a slow shear wave. As these two waves travel through the anisotropic medium, they separate because of the difference in their velocities so that, on emerging from the anisotropic medium, there is a time delay between the arrivals of the fast and slow shear waves.

Walkaround VSP datasets, typically acquired on land, are used to evaluate whether converted wave surface seismic is sensitive to reservoir fracturing. The datasets also aid in planning directional wells targeting a fractured reservoir. The offset surface sources are positioned around a wellbore with three-component geophones. Locally converted seismic waves are analyzed to determine their directions, from which the fracture orientation is inferred (Fig. 2).

The hodograms (blue) in Fig. 2 indicate downgoing S-waves generated at the target interface by the downgoing P-wave. For directions parallel to the symmetry planes (in the fracture normal and fracture strike directions), the particle motion is linear and polarized in the radial

Offset Borehole Seismic SurveysSalt flank positioning, fracture orientation, seismic imaging, and velocity control

Figure 1. Salt proximity results reduce drilling risks and enable well placement precision.

direction. The incoming P-wave excites only the S-wave that is polarized in that plane; the second S-wave, polarized orthogonal to the symmetry plane, is not excited. For other directions, the incoming P-wave can excite both S-waves. These S-waves then propagate downwards with different velocities and polarizations, leading to elliptical particle motion. Analysis of this shear-wave behavior as a function of azimuth identifies the symmetry plane.

Drilling direction identificationStress orientation can be extracted from horizontal transverse anisotropy computed from the azimuthal source positions. The information promotes wide-azimuth model building and processing—and drilling, because it establishes the preferred drilling direction. Walkaround VSP–computed orientation can be directly compared to the stress measurements from the Sonic Scanner* acoustic scanning platform and core analysis, thus providing a calibration point at surface seismic scale.

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Offset Borehole Seismic Surveys

*Mark of SchlumbergerOther company, product, and service names are the properties of their respective owners.Copyright © 2011 Schlumberger. All rights reserved. 10-DC-0106

The analysis can measure transverse-to-radial (T/R) energies, the sense of rotation, the linearity, and the polarity as a function of azimuth. Figure 2 shows how T/R ratio varies as a function of azimuth. The four-leaf clover pattern (red line) shows zeros coinciding

Figure 2. Wavefield propagation and hodogram analysis enable determination of the symmetry plane.

FractureStrike

HodogramT/R ratio

Figure 4. Comparison of surface seismic section with offset-VSP images from P-to-P reflections (a) and from P-to-S reflections (b). The image derived from S-wave reflections has higher vertical resolution. Yellow indicates productive sands intersecting the borehole, and red indicates a regional fault. The VSP images give clear indications of smaller-scale faults (blue) and broken reflections that are only impleied in the surface seismic section (insert).

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with the fracture strike and fracture normal directions, where there is no energy on the transverse component.

optimized surface seismic processing with offset Vsp dataOffset VSP datasets are rich in converted waves, both transmitted and reflected. If datasets are displayed with the wireline logs (Fig. 3), it is not only possible to identify where the conversion points occur, but with modern processing algorithms, but also its is possible to separate the four wave modes so that they can be analyzed individually. When displayed in compressional time, the converted (PS) wave seismic image (Fig. 4) has very high resolution, enabling easier interpretation of subtle faults, pinchouts, and other stratigraphic features.

A by-product of the separation process is the converted wave velocity for both the transmitted and reflected wavefields. These velocities can be used to calibrate velocity models in surface seismic processing and in survey evaluation and design campaigns.

Figure 3. Horizontal wavefield determines stress orientation.