Effects of confining pressure and temperature on mixed-mode (I–II) fracture toughness of a limestone rock

Studying fracture toughness behavior at elevated temperatures and confining pressures is valuable for a number of practical situations such as hydraulic fracturing used to enhance oil and gas recovery from a reservoir, and the disposal or safe storage of radioactive waste in underground cavities. Mixed-mode (I–II) fracture toughness under simulated reservoir conditions of high temperature and confining pressure was studied using straight notched Brazilian disk (SNBD) specimens under diametrical compression. Rock samples were collected from a limestone formation outcropping in the Central Province of Saudi Arabia. Tests were conducted under an effective confining pressure (σ3) of up to 28 MPa (4000 psi), and a temperature of up to 116°C. The results show a substantial increase in fracture toughness under confining pressure. The pure mode-I fracture toughness (KIC) increased by a factor of about 3.7 under a σ3 of 28 MPa compared to that under ambient conditions. The variation of KIC was found to be linearly proportional to σ3. The pure mode-II fracture toughness (KIIC) increased by a factor of 2.4 upon increasing σ3 to 28 MPa. On the other hand, KIC at 116°C was only 25% more than that at ambient conditions. Some ductile behavior was displayed by the rock samples at a high temperature and confining pressure.

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