Coupled effects of stress damage and drilling fluid on strength of hard brittle shale

During well drilling process, original stress state of hard brittle shale will be changed due to stress redistribution and concentration, which leads to stress damage phenomenon around the borehole. Consequently, drilling fluid will invade into formation along the tiny cracks induced by stress damage, and then weaken the strength of hard brittle shale. Based on this problem, a theoretical model was set up to discuss damage level of shale under uniaxial compression tests using acoustic velocity data. And specifically, considering the coupled effect of stress damage and drilling fluid, the relationship between hard brittle shale strength and elapsed time was analyzed.