Fracture toughness tests of shale outcrops: Effects of confining pressure

Abstract

Brittle rock fracture is a core concept in oil and gas and other rock mechanics projects. However, the understanding of fracture behavior under mechanical well-bottom conditions remains insufficient. This article aims to analyze experimental results for the critical stress intensity factor (KIC) of outcrops from Vaca Muerta carbonate-rich shale rocks, tested under a range of crack depths and confining pressures (0–70 MPa). Fracture toughness (KIC) is determined in multi-notched 1.5″ plugs using a novel experimental set up, in which the crack-driving-force KI and the confinement pressure are both applied by hydraulic systems. Finite fracture-mechanics-based models are used to calculate KI. Our experimental results show that tests carried out at well-bottom pressures lead to apparent rock toughness doubling those for tests at atmospheric pressure. Stress analysis demonstrates that the size of the tensile stressed zone ahead of the crack tip tends to decrease as confining pressure increases. Additionally, compressive deviatoric stresses are developed ahead of the tensile zone, with their magnitude being dependent on the level of confinement. Moreover, triaxial stress states induced by confinement could lead to microcracking ahead of the crack tip. The mechanisms of crack closure and deviatoric stress-induced microcrack initiation are combined to assess a plausible mechanism for rock toughness enhancement under confined conditions. It is concluded that increasing triaxial pressure confinement allows to accurately model the mechanical response of shale rocks under reservoir conditions.