Drilling-Induced Fractures: Their Identification and Analysis

Byron R. Kulander

Fractures formed during drilling provide useful information during core analysis and commonly occur with greater frequency than natural joints. Drilling-induced fractures originate at the bit and scribe knives or anywhere within the core barrel. In addition, some induced fractures propagate before the bit and, after coring, are present in the core as well as the borehole wall. These drilling-induced fractures are commonly mistaken for natural joints and assumed to be regionally pervasive. Such misidentification leads to errors in fractured reservoir characterization and flow models.

Drilling-induced fractures may show superficial resemblance to natural joints but can be identified by proper interpretation of fracture surface structures, specifically hackle plume components. These components, including fracture plane morphology and frequency of closely spaced induced fractures, show a unique geometrical relationship to core boundaries not developed with natural fractures. In every case, hackle plume geometry and fracture orientation reflect principal stress directions existing during propagation at any point on the fracture face. Furthermore, fractures induced before the bit commonly show unique orientations that reflect rock anisotropies as well as drilling and in-situ stresses. Similarly, bedding-parallel disc fractures, developed at the bit in vertical core thr ugh horizontal strata, commonly possess hackle plume axes with preferred orientations. Such orientations may reflect horizontal in-situ stresses and maximum sonic velocity directions. Furthermore, induced fracture frequency is commonly influenced by mechanical properties of contrasting lithologies. Properly logged drilling-induced fractures may predict preferred fracture stimulation directions and rock behavior to diverse stimulation procedures.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.