Experimental study on critical index optimization of drilling cuttings method of water-bearing coal based on acoustic emission features

Due to the influence of the Luohe Formation sandstone aquifer in some rock-burst coal mines of Shaanxi Province and Inner Mongolia Region, the moisture content of coal seam is too high and the amount of drilling cuttings cannot reach the standard. The results of drilling cuttings method have errors or failures, which affects the monitoring and pre-warning of rock burst risk. Based on this, the coal mass with different water contents is taken as the research object for carrying out the triaxial loading and unloading drilling tests at different stress loading levels, exploring the acoustic emission (AE) and damage spatial characteristics of coal with 0, 1.7%, 3.7% and 5.6% water content during drilling. And the method of calculating theoretical drilling cuttings is obtained. The cuttings critical index of coal mass under different water conditions is determined by the cuttings mass ratio of theory and test. The results show that ① with the increase of stress level and water content, the AE events show that the borehole center diffuses to the surrounding area, and the failure trend changes from shear partial failure mode to shear integral failure mode. ② Load level and water content have great influence on the volume of damage ellipsoid (DE) established by the spatial distribution of AE events. The DE is consistent with the spatial distribution characteristics of AE events. The calculated mass of DE is the main damage space mass M of water-bearing coal subjected to stress and drilling pipe cutting. ③ Through the analysis of the overall change trend of cuttings quality in theory and test, it is found that the theoretical cuttings amount increases after the weakening effect of water on the water-bearing coal body. Compared to the weakening effect of water, water bonding plays a dominant role, which leads to a small amount of cuttings collected on site. ④ The cuttings correction coefficient H of water-bearing coal body is obtained according to the ratio of cuttings mass between the theory and test of coal specimens with different water contents. When the water content is 0−1.7%, 1.7%−3.7% and 3.7%−5.6%, H is 1.00, 2.32 and 4.05, respectively. If the water content is greater than 5.6%, the drilling cuttings method fails. The critical index of drilling cuttings for water-bearing coal based on the test can provide reference for drilling cuttings monitoring in the water-rich rock-burst coal mines.