ANALYSIS OF SHEAR-WAVE POLARIZATION IN VSP DATA - A TOOL FOR RESERVOIR DEVELOPMENT

This paper presents results of a new method of using vertical seismic profile (VSP) data to deduce information on certain rock properties th at are important for reservoir development. These properties are the d irection of maximum horizontal stress or the orientation of aligned fr actures and the variation of these quantities with depth. Such informa tion 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 recor d first-arrival S waves along predominantly vertical ray paths. The da ta are analyzed to determine particle motion directions (polarizations ) and relative travel time of the two S waves. In simple cases, the po larization of the faster S wave is oriented parallel to the direction of maximum horizontal compressive stress or the predominant fracture a lignment. The travel-time differences between the S waves contain info rmation on the effects of unequal horizontal stresses. We recorded a n umber 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 variati on of the S-wave polarization azimuths with depth. Polarization azimut hs, although typically consistent over depth intervals of several hund red feet, tended to change abruptly at various depths. A simple layer- stripping technique made it possible to follow the polarization change s and determine travel-time differences over successive depth interval s. The variations with depth are particularly interesting because they indicate changes of principal stress direction or fracture orientatio n.