Study of mixed mode fracture toughness and fracture characteristic in gypsum rock under brine saturation

Abstract

In the deep geological situation, the gypsum is almost always associated with high temperature and the other mineral substances such as sodium chloride. And, hot saline brine will greatly affect the strength and deformability of gypsum by influencing the petrographic characteristics such as microstructure, granular minerals, composition content and morphology. To quantify the effect of brine temperature and concentration on fracture toughness and fracture characteristics of gypsum, a serious of tests, including cracked straight through Brazilian disc (CSTBD) tests and scanning electron microscopy (SEM), were carried out. The results demonstrated that, when at a given crack inclination angle, the fracture peak of gypsum decreases with the increase of liquid temperature. When at a given liquid temperature, due to common-ion effect, the fracture peak first decrease then increase with the increase of brine concentration. In all these cases, due to the effect of friction coefficient, the fracture peak first decreases then increases with the increase of prefabricated crack angle. From SEM morphology results, the microcrack density increases with liquid temperature increasing, whereas first increase then decrease with brine concentration increasing. Subsequently, by adopting the approach of back-propagation neural network, the effective FPZ at different conditions were obtained. It was found that the size of effective FPZ is closely dependent on brine temperature and concentration.