Natural Fracture Characterization Using P-Wave Reflection Seismic Data, VSP, Borehole Imaging Logs, and the In-Situ Stress Field Determination

Abstract

Abstract Seismic data in which anisotropy is observed have been used to detect and characterize the natural fractures, in the Bluebell-Altamont, Utah, field. We show an (azimuth-dependent) amplitude variation with offset (AVO) anomaly in the P-wave reflection seismic, attributable to the gas-filled vertical oriented fractures, and substantiated by the 9-component VSP. Introduction The detection of gas-filled natural fractures ahead of the bit is desirable for making a producing well in many low permeability formations in the Rocky Mountain basins. The Dept. of Energy has funded a study to demonstrate the use of multi-component seismic to detect high fracture density zones and predict fracture orientation ahead of the bit. Multi-component means that both P wave and shear-wave seismic data are examined. We have acquired two crossing lines of 9-component reflection seismic and two 9-C VSPs (vertical seismic profiles) to characterize the seismic anisotropy in the Bluebell-Altamont field study area (Figs. 1 and 2). By anisotropy, we mean that the value of the property measured depends upon the azimuth (direction) in which the measurement is made. Nine-component means that three sources (P, and two shear wave) and three receiver types (vertical and two horizontal) were used in acquisition. We include four pieces of information as necessary in this study:the seismic anisotropy;the geology, fractures, stratigraphy, and structural setting;the in-situ stress field orientation (today);the preferred flow direction within the reservoir (the maximum horizontal permeability direction). Core and wireline data are used to describe the lithology and the natural fractures. The in-situ stress field orientation comes from the orientation of the nearby gilsonite dikes and from borehole ellipticity studies. The preferred flow-direction within the reservoir is under current study, The seismic an isotropy will be discussed in this paper. GENERAL GEOLOGY The Uinta Basin of northeast Utah is an asymmetric east-west trending basin with a steep north flank bounded by a thrust, and a gently sloping (1-2 degree) south flank. Bluebell-Altamont field is located along the basin axis and north-central portion of the basin in Duchesne and Uintah counties (Fig. 1) Production is mainly from numerous sandstone and carbonates in the Tertiary Wasatch, lower Green River and upper Green River formations. P. 493