Investigation of fracture properties of rocks under drilling fluid saturation

The invasion created by drilling fluids around borehole walls creates considerable variations on mechanical and physical properties of both rock materials and discontinuities. However, up to date there is no comprehensive study has been performed on the variation of fracture toughness values of rocks under drilling fluid saturation with different compositions. By considering this deficiency, the variation of fracture toughness values of five different rocks interacting with drilling fluids with different compositions was investigated in this study. The experiments showed that the negative effect of drilling fluids on fracture toughness values of rocks has a broad range and the addition of polymers to the drilling fluid prevents a dramatic decrease in the fracture toughness values of rocks. Statistical evaluations showed that the fracture toughness values of rocks can be predicted from tensile strength with moderate error by simple regression analyses. However, the multivariate regression analyses produced statistically significant prediction equations with stronger coefficients of determination. On the other hand, the prediction capability of Adaptive Neuro Fuzzy Inference System (ANFIS) model was higher than the prediction performance of simple and multivariate regression analyses.