Three-dimensional Numerical Models of Drilling Induced Core Fractures

Abstract

Drilling-induced fractures in borehole cores have distinct morphologies (e.g., petal, petal-centreline, saddle, and disk) and are produced under pure tensional stress, although most in-situ stresses are compressive. 3D numerical models show that tensile stress concentrations occur near the bottom of a vertical borehole. A new algorithm is developed to trace 3D tensile fractures for a range of crustal stress conditions. In a normal fault regime, fractures change from petal/petal-centreline to saddle to disk with increasing minimum horizontal stress (Sh). In a strike-slip regime, saddle fractures occur, except where Sh is much less than the vertical stress and petal centre-line fractures are found. In a thrust fault regime, saddle and disk fractures occur at low and high Sh, respectively. The results demonstrate that in-situ stress is the dominant control on fracture morphology; variations in Poisson’s ratio and core stub length primarily affect the magnitude of tensile stress.