Predicting Natural or Induced Fracture Azimuths From Shear-Wave Anisotropy
Authors: De G.S. , Winterstein D.F. , Johnson S.J. , Higgs W.G. , Xiao H. 
Published: December 2007

Predicting Natural or Induced Fracture Azimuths From Shear-Wave Anisotropy G.S. De, D.F. Winterstein, SPE, Chevron Petroleum Technology Co., S.J. Johnson, Chevron U.S.A. Production Co., W.G. Higgs and H. Xiao, Chevron Petroleum Technology Co.

Prediction of fracture orientation to determine depletion geometries is a key step in reservoir characterization and development optimization. Wireline crossed-dipole anisotropy logs measure shear-wave velocity anisotropy and provide a relatively inexpensive alternative to tiltmeter surveys and nine-component vertical seismic profiles (9C VSPs) for such predictions. A multi-well study in Cymric oil field, California, was conducted to map the variations in the predicted azimuth of induced fractures in a shallow Miocene siliceous shale reservoir &gt;500 MMBOOIP). Results from anisotropy data showed that the azimuths predicted by anisotropy logs and VSPs agreed. Furthermore, these two very different technologies showed that within the productive zone these azimuths differed by about 50 from the regional stress in the NW portion of the Cymric development area, while they roughly aligned with the regional stress in the SE portion. Marked changes in the azimuth also occurred across a regional Plio-Miocene unconformity. Observed natural fracture azimuths and Gaussian curvature analysis support the azimuths predicted from anisotropy data. Microresistivity borehole imaging logs allowed measurements of borehole breakouts and large, open natural fractures. The Gaussian curvature analysis indicated a NW trending region of high curvature in the NW part of the Cymric structure. The large changes in azimuth with both depth and location in the development area show that regional stress maps are incomplete for planning oil field development when stress patterns are complex, especially when deviated wells or induced fracturing are to be employed. Local stress configurations must, therefore, be determined. In a separate field study in consolidated, low porosity sands of the Hoback Basin, Wyoming, the predicted azimuths of natural or induced fractures from a crossed-dipole anisotropy log and a 9C VSP agreed qualitatively but not in detail. Due to its finer depth resolution compared to the VSP, the log revealed details that remain uncorroborated by other data.

Introduction

Anisotropy logging, or crossed-dipole logging, is an emerging wireline technology that provides a relatively inexpensive way to predict the direction of fluid flow when flow is controlled by natural or induced fractures. Reservoir rocks may be anisotropic due to oriented fractures, unequal horizontal stresses, oriented pores, structural layering as observed in shales, etc. If such anisotropy is present, then the rock fabric has a well-defined rigid direction and a weak direction orthogonal to it. Both crossed-dipole logs and VSPs provide velocities of the fast and the slow shear-waves traveling along the borehole axis and the polarization directions of the shear-waves. The fast shear-wave polarization azimuth is aligned with the rigid direction of the rock. Since the rigid direction lies in the plane of natural or induced fractures, anisotropy logs can predict fracture azimuths. Previous case studies have related fracture azimuth to fast shear-wave polarization azimuth predicted from anisotropy logs.

A reliable prediction of induced fracture azimuth can have large economic benefits, since "approximately 20,000 fracture treatments are performed annually at an estimated cost of about $860 million". Knowledge of fracture azimuths can be exploited to optimize in-fill drilling in secondary recovery processes and to enhance the effectiveness of fracture stimulation. Anisotropy logs can also be used to design high angle well paths, since the in-situ anisotropic stress field determines the most stable direction in which to drill a well. Furthermore, they can be used to orient horizontal wells to optimally intersect natural fractures. Also, since accurate velocities are key to seismic imaging, crossed-dipole logs can provide valuable input for producing sharper images in areas where the reservoir to be imaged lies below anisotropic formations.

Strictly speaking, anisotropy logs do not measure anisotropy but shear-wave splitting, one of the effects of anisotropy Shear waves "split" because the fast and the slow shear waves travel at different speeds.

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