Quantifying anisotropic rock properties from sonic data is a well-known approach that has many applications. For instance, the percentage sonic anisotropy in the formation provides vital knowledge when constructing Geomechanics mechanical models and when determining optimum well trajectory for horizontal wells. One of the main outputs from sonic anisotropy processing is the fast shear-wave polarization azimuth (FSA) derived from auxiliary tool orientation measurements included in the tool string with the sonic tools. This FSA is polarized along the direction of fracture strike, in case of fracture-induced anisotropic zones, and /or along maximum horizontal stress, in case of stress-induced anisotropic zones.

When the hole shape is oval in one axis by breakouts and in gauge in the other axis, the sonic tool remains oriented in one direction throughout logging intervals. As a result, the fast shear azimuth will be stable in one direction. This locked direction inherently has a 90° ambiguity if interpreted wrongly particularly if no borehole image data were available to validate this direction from the strike fractures or breakout directions.

The main objective of the study in this paper is to validate the direction of FSA, and then horizontal stress orientations, from sonic data in vertical wells when the sonic tool keeps a singular orientation throughout the logging and no borehole image data are available to support the direction of horizontal stress orientations. This validation is represented through a graphical interpretation of the FSA, both qualitatively and quantitatively, at intervals that exhibit stress-induced anisotropy and without assist from borehole image interpretations.

Three examples are presented in this paper. The first one shows a determining of the maximum horizontal stress from the graphical interpretation of FSA after processing the sonic data. This interpretation is validated with image data. The other two examples are from wells where only sonic data is used to determine the horizontal stress orientations using the FSA graphical technique

Keywords:
Reservoir Characterization, Upstream Oil & Gas, well logging, reservoir geomechanics, orientation, breakout direction, fsa direction, image data, maximum horizontal stress, azimuthal shear anisotropy processing
Subjects:
Reservoir Characterization, Formation Evaluation & Management, Reservoir geomechanics, Open hole/cased hole log analysis
Copyright 2017, Society of Petroleum Engineers
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