Predicting Natural or Induced Fracture Azimuths From Shear-Wave Anisotropy

Abstract

Summary 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 anisotropy and provide a relatively inexpensive alternative to tiltmeter surveys and nine-component vertical seismic profiles (9C VSP's) for such predictions. A multiwell study in Cymric oil field, California, was conducted to map the variations in the predicted azimuth of induced fractures in a shallow Miocene siliceous shale reservoir (>500 millions of barrels of oil in place). Results from anisotropy data showed that the azimuths predicted by anisotropy logs and VSP's agreed. The agreement is significant because these two technologies measure very different kinds of seismic waves and have scales of investigation differing by two orders of magnitude. Within the productive zone, the predicted fracture azimuths from anisotropy data differed by about 60° 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 depth across a regional Plio-Miocene (P-M) unconformity. Independent evidence, including observed steam breakthrough, natural fracture azimuths, and curvature analysis, supports the azimuths predicted from anisotropy data. Time-lapse temperature data from observation wells in a pilot steamflood showed anisotropy in horizontal permeability. Microresistivity borehole imaging logs allowed measurements of borehole breakouts and large, open natural fractures. The curvature analysis indicated a NW trending region of high curvature in the NW part of the Cymric structure. The large changes in azimuth with both depth and location in the development area show that regional stress maps are inadequate for planning oilfield development when stress patterns are complex, especially when deviated wells or induced fracturing are planned. Local stress configurations, therefore, must also be determined.