Critical pressure and scaling in cavity expansion tests

The study of fracture initiation from a cavity in rock has been largely motivated by the extraction of petroleum and natural gas from a borehole and therefore, is often considered as one of the most challenging problems in rock mechanics. An unresolved issue is relating the fracture parameters from small scale laboratory test to a large scale engineering process. To investigate fracture initiation from a borehole, cavity expansion tests were conducted in the laboratory on specimens of Berea sandstone. To capture the failure process near the circular cavity, digital image correlation (DIC) was used. In addition, a two dimensional bonded particle model was developed and validated. For investigating the effect of scaling on fracture parameters (e.g. critical pressure), several larger specimens with different tension softening behavior were tested numerically. It was shown that size plays an important role. Furthermore, using the DIC measurements, it was found that fracture initiates at 80% of peak internal pressure for the rock tested.