Natural fractures are present in the Permian Wolfcamp shales and have the potential to impact well completion operations and hydrocarbon production. In this paper, natural fractures are characterized with the latest borehole image technology, including both acoustic and micro-resistivity measurements. Whole core CT-Scan images are also made available to fully calibrated borehole image interpretation for better understanding of the many fracture attributes, i.e. fracture density, filling minerals, opening types, and termination, etc. Orienting whole core using image logs from the same wellbore was also made possible in this study.
This allowed interpretation of all visible fractures of any size with available fracture dip, strike, length, type and density as output. Litho-bounded, calcite-healed or partial-healed fractures are the dominant type within the formation. Open fractures, even though far less common, are also observed, with most of the being partially open. The interpretation concluded that the predominant set of fractures strike NE-SW, with a secondary NW-SE set, in the studied field area. However, fracture density showed considerable variability vertically and spatially among the studied wells.
The fracture facies analysis is first introduced in this study using borehole images, conventional open-hole logs and whole core data. Three basic types of fracture facies are proposed including fractured shale, non-fractured shale and limy beds. The fracture facies data has potential applications in understanding relative rock fracability in the studied Wolfcamp reservoir. The naturally fractured shales are expected to be the easier rock to hydraulically fracture compared to non-fractured shale and limy beds, because, in a sense, they were already fractured in geological history. The existence of abundant planes of weakness (even secondary mineral-cemented fractures) within the facies, takes far less energy to reactivate (or re-open) than to create new fractures during the hydraulic fracturing process. The fracture facies results of relative rock fracability can be used as part of the input for “sweet-spot” definition and/or landing zone decisions, along with available petrophysical & geomechanical properties. The various fracture attribute outputs and the interpreted in-situ maximum horizontal stress (SHmax) helps to understand the hydraulic fracture growth orientation and overall fracture network complexity.