Fracture toughness tests of shale outcrops: Effects of confining pressure

ANTINAO FUENTEALBA, FABIÁN J.; GONZALO BLANCO; LEANDRO NOE BIANCHI; JOSÉ LUIS OTEGUI; GUSTAVO LUIS BIANCHI

Geoenergy Science and Engineering

Elsevier B.V.

Lugar: amsterdam; Año: 2023

2949-8910

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 conditionsremains insufficient. This article aims to analyze experimental results for the critical stressintensity factor (KIC) of outcrops from Vaca Muerta carbonate-rich shale rocks, tested under arange of crack depths and confining pressures (0 to 70 MPa). Fracture toughness (KIC) isdetermined in multi-notched 1.5’’ plugs using a novel experimental set up, in which the crackdriving-force KI and the confinement pressure are both applied by hydraulic systems. Finitefracture-mechanics-based models are used to calculate KI. Our experimental results show thattests carried out at well-bottom pressures lead to apparent rock toughness doubling those fortests at atmospheric pressure.Stress analysis demonstrates that the size of the tensile stressed zone ahead of the crack tiptends to decrease as confining pressure increases. Additionally, compressive deviatoricstresses are developed ahead of the tensile zone, with their magnitude being dependent onthe level of confinement. Moreover, triaxial stress states induced by confinement could lead tomicrocracking ahead of the crack tip. The mechanisms of crack closure and deviatoric stressinducedmicrocrack initiation are combined to assess a plausible mechanism for rocktoughness enhancement under confined conditions. It is concluded that increasing triaxialpressure confinement allows to accurately model the mechanical response of shale rocksunder reservoir conditions.