Determination Of The Orientation Of Open Fractures From Hydrophone VSP

Abstract

Open fractures are of interest in many areas such as ground water contamination, hazardous waste disposal, oil and gas recovery, and geothermal energy extraction. In borehole geophysics and engineering, fractures are usually located by acoustic borehole televiewer logging, however, not all of the observed fractures are permeable. The caliper log, on the other hand, provides the information about the change of borehole diameter, but increasing diameter does not prove the existence of open fractures. Nor can the combination of these two methods-televiewer logs and caliper logs-provide direct information about open fractures. However, tube waves, generated by P- and/or S-waves in hydrophone vertical seismic profiling (VSP) or cross-well seismic profiling section can detect open fractures intersecting the borehole. A new technique is developed to determine the orientation of open fractures using the normalized ratios of an S-wave-generated tube wave to a P-wave-generated tube wave. The fracture orientations determined by this method represent the average over the fracture planes for large radii, generally on the order of a meter. Numerical tests show that, given a good experiment design, a set of two independent measurements of these ratios with polarization information, or a set of three independent measurements without polarization information, provides a unique solution. The developed technique is stable in the presence of noise. This technique is applied to hydrophone VSP data from the Kent Cliffs test well in southeastern New York state. The orientations of the three major fractures which generate primary tube waves in the seismic profiling sections are obtained. The results agree well with the orientations measured from the borehole televiewer images in general. Any discrepancy may be attributed to the difference between the sampling size of this method and the borehole televiewer, to the deviation of rays from the straight lines due to inhomogeneity, and/or to possible BH-wave motion due to anisotropy and lateral inhomogeneity.