Fracture detection in oil-based drilling mud using a combination of borehole image and sonic logs

Abstract

The deeply buried (5.5–7.1 km) Lower Cretaceous Bashijiqike sandstones are important fractured reservoirs in the Kuqa depression. In these high temperature-high pressure drilling environments, oil-based muds are used to limit drilling risks and improve efficiency, making facture detection using resistivity-based image logs difficult. The maximum horizontal stress (SHmax) trends close to North-South(0–10°) based on orientations of the drilling-induced fractures and borehole breakouts. Core descriptions, thin sections and image logs used to describe the fractures in the Bashijiqike sandstone reservoirs show that opening-mode natural fractures are common, and the fractures range from fully open to sealed by calcite and clay minerals. The open fractures are dominantly steeply dipping or vertical and primarily strike north-south. Locally horizontal fractures or fractures with low angle dip are detected.We compare fracture visibility on image logs acquired in the same KS 2-2-12 key well in oil-based mud and water-based mud. FMI-HD logs successfully detect large (0.1 mm aperture) vertical fractures in oil-based mud better than OBMI image logs, and the two imaging tools have a similar ability in detecting the high dip angle fractures. Since the UBI tool is not affected by oil-based muds, most of the fractures can be picked out on UBI images. Minor negative deviations of bulk density could be observed in fractured zones, and the presence of fractures obviously causes an increase in sonic transit time. Full waveforms from DSI tools combined with electrical image logs detect fractured zones. Both the velocity and amplitude of the sonic waves are sensitive to fractures, and the fractured zones could be identified according to the attenuation of Stoneley waves. The combination of cores, image logs, and full-wave sonic logs offers comprehensive detection of fractures in oil-based mud environments.