The drilling of a borehole leads to a change in the local in-situ rock stress state in the vicinity of the well bore. For a vertical hole drilled parallel with a principal stress axis a stress reduction occurs in the area of the borehole in the direction of maximum horizontal stress (SHmax). Tensional failure can occur where tangential stresses are lowest, in the direction of SHmax, with the formation of drilling induced tension fractures (DITF’s). They are usually recognized on image logs as two twinned vertical, or near vertical, irregular and discontinuous fractures set 180º apart that often terminate at lithological boundaries. The use of DITF’s is a recognised method for determining in-situ stress direction within boreholes.
Natural fractures that cut a borehole can also come under the influence of in-situ stress and result in “propped” or drilling-enhanced fractures. It is important to discriminate between true DITF’s and propped fractures.