Shear-Wave Polarizations and Subsurface Stress Directions at Lost Hills Field
WINTERSTEIN, D. F., and M. A. MEADOWS, Chevron Oil Field Research Company, La Habra, CA
Two closely spaced nine-component VSPs recorded in the Lost Hills oil field in the southern San Joaquin Valley of California show that the polarization of the fast shear (S) wave is aligned with the direction of maximum horizontal compressive stress as determined from analysis of tiltmeter data. Tiltmeters monitored fractures that were hydraulically induced in or near the VSP wells. When fractures were induced in one of the VSP wells, fracture azimuths as determined from tiltmeter data were N53 degrees E and N56 degrees E, respectively, in two different depth zones; in the same zones the fast S-wave polarization directions were N58 degrees E and N59 degrees E. When fractures were induced in a well 537 ft from the second VSP well, tiltmeter data indicated a mean fracture strike of N59 egrees E, while the fast S-wave polarization direction in that case was N40 degrees E. S-wave polarization directions were determined by minimizing energy on off-diagonal components of the 2 x 2 S-wave data matrix, accomplished by computationally rotating sources and receivers by the same angle.
S-wave polarizations from two concentric rings of offset VSPs were consistent in azimuth with one another and with polarizations of the near offset VSP. This consistency argues strongly for the robustness of the S-wave polarization technique as applied in this area. The S-wave polarization pattern in offset data fits a model of vertical cracks at N55 degrees E in a weakly transversely isotropic matrix with infinite-fold symmetry axis tilted at 10 degrees from the vertical toward N70 degrees E.
AAPG Search and Discovery Article #91009©1991 AAPG-SEPM-SEG-SPWLA Pacific Section Annual Meeting, Bakersfield, California, March 6-8, 1991 (2009)