Drilling Induced Core Fractures and Crustal Stress

Drilling induced core fractures, usually ignored in standard core logging, contain substantial information on the directions of horizontal stresses and the relative magnitudes of the three principal stresses. These fractures have a variety of shapes, variously referred to as petal, petal-centreline, saddle, cup, and scallop. Surprisingly, the morphology of the fractures suggests they are produced under a pure tensional stress, despite the fact that in almost all in situ conditions the state of stress is entirely compressive. Earlier numerical modeling has demonstrated that such pure tensions result from complex stress concentrations around the cylindrical geometry of a core stub before it is broken from the rock mass. In addition to the fracture shapes, the points at which they initiate indicate the natural stress state in the Earth. Here, we describe some new finite element models of the stress distributions at the bottom of a borehole being cored. This builds on earlier work of Li and Schmitt (1997) but is focused on the development of an interpretive tool.