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

Studying fracture toughness behavior at elevated temperatures and cofining pressures is valuable for a number of practical situations such as hydrauli c fracturing used to enhance oil and gas recovery from a reservoir, and the disposal or safe storage of radioactive waste in underground cavities. Mix ed-mode (I-II) fracture toughness under simulated reservoir conditions of h igh temperature and confining pressure was studied using straight notched B razilian disk (SNBD) specimens under diametrical compression. Rock samples were collected from a limestone formation outcropping in the Central Provin ce of Saudi Arabia. Tests were conducted under an effective confining press ure (sigma(3)) Of UP to 28 MPa (4000 psi), and a temperature of up to 116 d egrees C. The results show a substantial increase in fracture toughness und er confining pressure. The pure mode-I fracture toughness (K-IC) increased by a factor of about 3.7 under a sigma(3) of 28 MPa compared to that under ambient conditions. The variation of K-IC was found to be linearly proporti onal to sigma(3) The pure mode-II fracture toughness (K-IIC) increased by a factor of 2.4 upon increasing sigma(3) to 28 MPa. On the other hand, K-IC at 116 degrees C was only 25% more than that at ambient conditions. Some du ctile behavior was displayed by the rock samples at a high temperature and confining pressure. (C) 2000 Elsevier Science Ltd. All rights reserved.