Rocks at depth are subject to natural stresses called in-situ stresses (Amadei and Stephansson 1997). These are the stresses that exist within a rock mass prior to disturbance and reflect its geological development. They include gravitational, tectonic, tidal, residual and terrestrial stresses. Active tectonic stresses include ridge push, slab pull, local bending and isostatic compensation. The drilling of a borehole can modify native in-situ stresses and result in well bore damage or even failure. Knowledge of in-situ stresses is important in helping to minimise well bore damage through instability.
The analysis of in-situ stress indicators from well bores has helped in the mapping of global stress patterns within the lithosphere.
AMADEI, B. AND STEPHANSSON, O. 1997. Rock Stress and its Measurement. Chapman & Hall, London, p490.
See also borehole breakout, borehole ovalisation and drilling induced tension fractures.