Hole stability of Red Wildmoor sandstone under anisotropic stresses and sand production criterion

The borehole stability of Red Wildmoor sandstone, a weak and porous outcrop which is often used as a reservoir analogue, was investigated experimentally under various degrees of axial and lateral stress anisotropies. A total of 14 different stress combinations were examined in hollow cylinder tests and polyaxial tests on prismatic specimens. The hole and external specimen deformations were monitored during testing to obtain the material response and determine borehole failure. Acoustic emission source location was used to follow the progression of internal damage and borehole failure localization. CT scan images of the tested specimens revealed two borehole failure modes, the lateral and the axial borehole failure mode depending on the stress combination. The results show that stress anisotropy has an impact on hole deformation, failure stress and failure mode. Borehole failure diagrams were constructed for the various degrees of stress anisotropy and a model was developed for predicting the effect of stress anisotropy on borehole failure in sandstones. The model is extended to a borehole or perforation in the field and a suitable expression for the critical drawdown for sand production is derived.