Investigating the effect of confining pressure on fracture toughness of CO2-saturated coals

Abstract The present paper investigates the effects of CO2-saturation and confining pressures on the fracture toughness of bituminous coals. Three-point bending experiments were performed with semi-circular coal specimens (SCB) after they were immersed in different CO2 pressures. The experimental results show that the pure modes I and II fracture toughnesses of coal samples gradually decrease with the growth of CO2-saturation pressure. Additionally, the fracture toughness reduces significantly after the CO2-saturation pressure exceeds 7.38 MPa, the threshold value of its supercritical state. When the CO2-saturation pressure arrives at 10 MPa, the degradation degree of the fracture toughness of the coal reaches the maximum. Furthermore, we tested the fracture toughness of coal SCB specimens under N2-saturation and CO2-saturation confining pressure conditions, respectively. As the N2-saturation confining pressure rises, the mode I and II fracture toughnesses increase gradually. The relationship between confining pressure and fracture toughness can be characterized by linear and quadratic functions for the mode I and mode II loading, respectively. The fracture toughness of coals decreases more rapidly under supercritical CO2 (scCO2)-saturation confining pressure than that under N2-saturation confining pressure. Furthermore, the theoretical values based on modified maximum tangential stress (MMTS) criterion that takes into account the effects of the T-stress are found to comply better with experimental results than those predicted by traditional maximum tangential stress (MTS) criterion.