Fractures on Images
Learning objectives
- Distinguish an open (conductive, dark) fracture from a sealed (resistive, bright) one
- Recognize drilling-induced tensile fractures as thin vertical pairs along SHmax
- Recognize borehole breakouts as broad spalled zones along the minimum stress
- Use induced features to read the in-situ stress azimuth
Four Features, Two Stories
An image log is the only tool that sees a fracture directly, and learning to read them is the climax of the chapter. Four features turn up, and they tell two different stories. The first story is the natural fractures in the rock; the second is the stress acting on the borehole itself.
Open and Sealed
A natural fracture is a sinusoid, like a bed, but what matters is its fill. An open fracture is full of conductive mud and reads dark; it is a real flow path, the feature that makes a tight rock produce. A sealed fracture is filled with cement, calcite or quartz, reads bright, and is a barrier, not a conduit. The two look identical in shape, so the dark-versus-bright distinction is everything.
The Borehole Tells Its Stress
Two more features are made by the drilling itself. Drilling-induced tensile fractures are thin vertical pairs 180 degrees apart, formed where the wall is pulled apart along the maximum horizontal stress, so they point straight at the SHmax azimuth. Breakouts are broad spalled zones, also 180 degrees apart but 90 degrees from the tensile fractures, where the wall fails in compression along the minimum stress. Because they track the borehole stress and not the rock, rotating SHmax moves them while the natural fractures stay put, and that is exactly how you tell the two apart.
References
- Zoback, M. D. (2007). Reservoir Geomechanics. Cambridge University Press.
- Rider, M. and Kennedy, M. (2011). The Geological Interpretation of Well Logs, 3rd ed. Rider-French.