Analysis of Shear-Wave Polarization in VSP Data: A Tool for Reservoir Development

Abstract

Summary This paper presents results of a new method of using vertical seismic profile (VSP) data to deduce information on certain rock properties that are important for reservoir development. These properties are the direction of maximum horizontal stress or the orientation of aligned fractures and the variation of these quantities with depth. Such information can be important for secondary recovery, horizontal drilling, and infill drilling. The VSP method involves a shear-wave (S-wave) source at the surface and a downhole three-component (3-C) geophone to record first-arrival S waves along predominantly vertical ray paths. The data are analyzed to determine particle motion directions (polarizations) and relative travel time of the two S waves. In simple cases, the polarization of the faster S wave is oriented parallel to the direction of maximum horizontal compressive stress orthe predominantfracture alignment.The travel-time differences between the S waves contain information on the effects of unequal horizontal stresses. We recorded a number of VSP's for S-wave analysis in California and Texas oil fields. Results from three VSP's are presented here. In nearly every case, we found measurable travel-time differences and also significant variation of the S-wave polarization azimuths with depth. Polarization azimuths, although typically consistent over depth intervals of several hundred feet, tended to change abruptly at various depths. A simple layer-stripping technique made it possible to follow the polarization changes and determine travel-time differences over successive depth intervals. The variations with depth are particularly interesting because they indicate changes of principal stress direction or fracture orientation.