Modeling Borehole and Perforation Collapse with the Capability of Predicting the Scale Effect

In this paper the writers examine a borehole failure model that is based on fracture mechanics and layer buckling theories and compare its predictions with experimental data. The model assumes that the main failure mechanism of borehole collapse takes place in the form of (preexisting or formed) layers buckling. The model introduces a combination of fracture mechanics parameters with length dimension that scales the size of the holes, thus allowing for size effect predictions. The writers review the model and compare its predictions with available experimental data of hollow cylinder tests on weak rocks. The writers found for the model predictions and for the experimental data a strong correlation between hollow cylinder strength normalized by the rock strength and hole size normalized by the square of the ratio of fracture toughness over tensile strength. These findings can be used for constructing efficient mechanical models for predicting or interpreting failure of boreholes and perforations in mining and petroleum engineering. (C) 2011 American Society of Civil Engineers.