Acoustic emission characteristics of the brittle-ductile transition for unsaturated rock during drilling process with confining pressures.

The enhancement of drilling efficiency under complex geological environments plays a vital role in modern drilling engineering. Drilling is a highly concealed process. Traditional methods lack the ability to monitor energy release during the drilling process. Acoustic emission monitoring technology provides a solution. This study aims to enhance drilling efficiency by analyzing the brittle-ductile transition mechanism of water-bearing rocks during drilling through the integration of acoustic emission parameters with drilling parameters. Drilling experiments were conducted on red sandstone under four confining pressures and three saturation levels. The results indicate that the number of brittle failure events at the friction stage is greater than that at the cutting stage. The number of brittle failure events gradually decreases as confining pressure and saturation increase. The enhancement of ductile failure characteristics in drilling induced by water and confining pressure tends to be mutually inhibited. Specifically, the number of brittle failure events increases when the confining pressure reaches 20 MPa or the saturation reaches 100%. The drilling critical depth of the brittle-ductile transition increases with rising saturation. The delay in the brittle-ductile transition during drilling is caused by water-induced rock deterioration. The findings of this study provide valuable guidance for improving drilling efficiency in water-rich formations under stress field conditions.