Abstract
Summary Drilling-induced tensile fractures (DITFs) form due to stress concentrations around a wellbore and are in vertical wells typically parallel to the largest horizontal far-field stress and normal to the least horizontal far-field stress. The peak pressure in the wellbore exerted by the drilling mud that the wall rock can sustain is given by the so-called Hubbert-Willis (H-W) criterion, which predicts that wall rock failure takes place when the circumferential effective stress at the borehole wall reaches the tensile strength of the rock. However, even though the H-W criterion is a valuable fracture-initiation criterion, it cannot predict if and how an initiated fracture propagates. Linear elastic fracture mechanics (LEFM) can provide a solution to these questions. Results from three-dimensional numerical Rigid Body Spring Network (RBSN) lattice modelling are in good agreement with both the H-W initiation criterion and fracture lengths predicted by LEFM. The models can be adapted to situations where more complex fracture geometries are expected, for example due to mechanical layering or wellbores that are not aligned with one principal stress direction. The models are calibrated using log and core data from wells drilled by NAGRA for investigating possible site areas for nuclear waste repositories.
Published Version
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