DRILLING-INDUCED CORE FRACTURES AND IN-SITU STRESS

The relationship between the shapes of drilling-induced core fractures and the in situ State of stress is developed. The stress concentratio ns at the well bore bottom are first determined using a complete three -dimensional finite element analysis. Existing in situ compressional s tresses generate large tensions in the immediate vicinity of the botto m hole which are sufficient to rupture the rock. Tensile fracture traj ectories within these concentrated stress fields are predicted using a simple model of fracture propagation. These modeled fracture trajecto ries resemble well the observed shapes of drilling-induced core diskin g, petal, and petal-centerline fractures. Further, this agreement sugg ests that both the shape of the drilling-induced fracture and the loca tion at which it initiates depends on the in situ stress state existin g in the rock mass prior to drilling; the core fractures contain subst antial information on in situ stress conditions. In all faulting regim es the coring-induced fractures initiate near the bit cut except for m ost cases under thrust faulting regime where the fracture initiates on the well bore axis. Further, under thrust faulting conditions only di sk fractures appear possible. Both petal and disking fractures can be produced in strike-slip and normal faulting regimes depending upon the relative magnitudes between the least compressive horizontal principa l stress and the vertical overburden stress. The predicted fracture sh apes are in good qualitative agreement with observations of drilling-i nduced fractures described in the literature from laboratory experimen ts and field programs in which in situ stresses are measured by other means. The relationship of the morphology of coring induced fractures and in situ stresses suggests that the fractures can be used as indepe ndent complementary indicators in identifying stress regimes.