PREDICTING NATURAL OR INDUCED FRACTURE AZIMUTHS FROM SHEAR-WAVE ANISOTROPY

Prediction of fracture orientation to determine depletion geometry is a key step in reservoir characterization and development optimization. Wireline crossed-dipole anisotropy logs measure shear-wave velocity a nisotropy and provide a relatively inexpensive alternative to tiltmete r surveys and nine-component vertical seismic profiles (9C VSP's) for such predictions. A multiwell study in Cymric oil field, California, w as conducted to map the variations in the predicted azimuth of induced fractures in a shallow Miocene siliceous shale reservoir (>500 millio ns of barrels of oil in place). Results from anisotropy data showed th at the azimuths predicted by anisotropy logs and VSP's agreed. The agr eement is significant because these two technologies measure very diff erent kinds of seismic waves and have scales of investigation differin g by two orders of magnitude. Within the productive zone, the predicte d fracture azimuths from anisotropy data differed by about 60 degrees from the regional stress in the NW portion of the Cymric development, while they were approximately aligned with the regional stress in the SE portion. Marked changes in azimuth also occurred as a function of d epth across a regional Plio-Miocene (P-M) unconformity. Independent ev idence, including observed steam breakthrough, natural fracture azimut hs, and curvature analysis, supports the azimuths predicted from aniso tropy data. Time-lapse temperature data from observation wells in a pi lot steamflood showed anisotropy in horizontal permeability. Microresi stivity borehole imaging logs allowed measurements of borehole breakou ts and large, open natural fractures. The curvature analysis indicated a NW trending region of high curvature in the NW part of the Cymric s tructure. The large changes in azimuth with both depth and location in the development area show that regional stress maps are inadequate fo r planning oilfield development when stress patterns are complex, espe cially when deviated wells or induced fracturing are planned. Local st ress configurations, therefore, must also be determined.