Improved technique for toughness testing of shale rocks

•New hydraulic testing device allows KIC toughness testing with low cost and unertainty.•Multi-notched 1.5” plugs were tested with polar and non-polar fluids and saturation methods.•200 mode I fracture tests of mostly calcareous Vaca Muerta shale rocks were performed.•Toughness results are discussed in terms of KIC, T-stress, and damage mechanisms. Mode I fracture mechanics tests of mostly calcareous Vaca Muerta shale rocks were performed in a new experimental device. A hydraulic system allows the injection of pressurized fluids inside of cracks to generate applied KI and measure fracture toughness in room pressure and temperature conditions. Multi-notched 1.5″ plugs were tested with different polar and non-polar fluids (water-based fluids and organic solvents). Some samples were subject to previous saturation in order to activate multiple micro crack initiation via various embrittlement mechanisms. The methodology allows to test a large number of plug samples with low cost and times, and with a comparatively low uncertatinty and scatter of results with respect to other techniques. Results are discussed in terms of the two stress terms at the crack tip: Stress Intensity Factor (KI) and T-stress, and according to the Extended Maximum Tangencial Strain criterion (EMTSN). The variation of toughness in dry and saturated samples are discussed in terms of damage mechanisms, as well as in terms of the stochastic nature of toughness results in these rocks. The convenience of repeating these tests at reservoir pressure and temperature conditions is recognized.