Fracture Identification from Well Logs

ABSTRACT The Silo oil field in southeastern Wyoming produces oil from the fractured Niobrara chalk. Chalk has very little matrix permeability and produces best from areas where, the chalk contains pervasive open fractures. This fractured reservoir is examined with five physically independent fracture characterization techniques; oriented micro-resistivity logs, borehole elongations, sonic logs, nuclear logs, and vertical seismic profiles. In this reservoir, at this location, the oriented micro-resistivity logs give the best overall identification of fractured intervals, probably because the most productive fractures are open and fluid filled. Borehole elongations, commonly called breakouts, also correlate with fractured intervals. Fractures are found both parallel and perpendicular to borehole breakouts. Sonic waveform attenuation also correlates with fractured intervals, although no orientation information is provided by the sonic log or the other nuclear porosity logs. The M-N nuclear-sonic logging technique is shown to work only for one well in this area, yet the response is due entirely to density log anomalies. A second M-N plot from Silo shows no indication of fractures while the well produced oil from fractured chalk. The final fracture identification technique shown in this paper is the vertical seismic profile. Shear waves are polarized into fast and slow directions due to anisotropy of the fractured chalk.