Acquisition of wireline crossed-dipole data for analysis of shear wave azimuthal anisotropy is common practice nowadays, even in highly deviated wells. One issue that is rarely brought up is the selection of the correct tool orientation measurement that should be used to produce meaningful results: tool azimuth or relative bearing. This paper addresses this issue for all well deviations, sets guidelines for choosing the right tool orientation data, and provides illustrations from field data. This problem would also remain in the LWD (Logging While Drilling) setting when shear anisotropy logs become routinely available in this environment.
Wireline crossed-dipole shear-wave logs are commonly run for reservoir characterization1,2,3, geomechanical4,5 and seismic interpretation. Such logging tools contain a pair of dipole shear-wave sources which are normal to each other (called crossed-dipoles) and an array of crossed-dipole receivers6,7. This arrangement of sources and receivers records two shear-waves which travel along the well axis and whose particle motions are in the plane normal to the well axis. In this paper we call this plane the 'well cross-section plane'. If the rock is azimuthally isotropic along the well axis and the well has a circular cross-section then the two shear-waves are identical and their particle motions are along the orientation of the dipole source which generated them. If the rock is azimuthally anisotropic along the well axis, the two shear-waves, called fast and slow waves, travel with different speeds and the orientation of their particle motions on the well cross-section plane are determined by the symmetry of the rock's anisotropy8.9.10,11. Analysis of crossed-dipole waveforms generates three rock properties at each depth frame in the logged interval: a fast shear-wave polarization direction and the velocities of the fast and the slow shear-waves traveling along the well axis12. The fast shear-wave polarization direction is interpreted to be along the in-situ maximum compressive stress or along the strike of natural or induced fractures.
Knowledge of the fast shear-wave polarization direction helps in designing stable trajectories for deviated wells, sanding prediction, completion strategy, predicting permeability anisotropy for fractured reservoirs, in-fill drilling, etc. The two shear-wave velocities along the well axis can be used for evaluating the magnitude of azimuthal stress anisotropy13, improving seismic imaging etc.
Depending on the well deviation, the fast shear-wave polarization direction should be either relative to geographic North on the horizontal 'earth plane', or relative to the high side of the well on the well cross-section plane. This paper focuses on the criterion for choosing the appropriate reference plane for reporting the fast shear-wave polarization direction. This choice is made in the data analysis step.
Crossed-dipole logging tools have a fixed reference mark on the tool. A separate navigational package in the tool string continuously measures its orientation during logging to keep track of the tool orientation in the well. In addition to the tool reference azimuth (tool azimuth), the navigational package records other orientation data such as tool reference direction relative to high side of well (relative bearing), well deviation, and well azimuth.