Horizontal Stress Orientation Analysis Using Tile Cross-Dipole Acoustic Log In The Ekofisk Field

Horizontal stress orientation is an important aspect of formation evaluation and a governing factor in the optimization of the development and drainage of a reservoir. This is especially true if the formation is hydraulically stimulated or is naturally fractured and stress governs the directional aspect of permeability and hence production. The standard methods for determining stress include the evaluation of borehole breakout from image logs, anelastic stress relaxation from whole core, and the monitoring of surface displacement through the use of surface tiltmeters, Increasingly, the use of cross-dipole acoustics is demonstrating its value in assessing the horizontal stress orientation. The ability of the cross-dipole acoustic log to identify large-scale fractures and fracture networks has been well documented. On a smaller scale, a naturally fractured reservoir will respond to azimuthal stress in like fashion. This response will induce a shear-wave anisotropy, which can be measured and evaluated by cross-dipole analysis. This paper presents a case history where cross-dipole measurements were made to determine the horizontal stress orientation of the Eldfisk Field in the North Sea. The study was conducted in conjunction with an extensive stress analysis done prior to a planned waterflood. The results are compared with borehole image logs that were run before and after micro-frac tests to record the direction of the induced fractures. In addition, whole core was taken across the section and anelastic strain recovery measurements were made. These differing methodologies yield the same results and demonstrate the value of cross-dipole acoustic measurements in determining the stress orientation in the Eldfisk Field.