Experimental research on progressive failure characteristics of water-immersed coal: Implications for hydraulic fracturing

Coal rock water immersion softening is common in coal geology, and it is of great significance to effectively control and utilize the process of water immersion softening and fracturing of coal. In this paper, the progressive failure and softening characteristics of coal samples under localized or full water immersion conditions were determined based on the stress-strain and acoustic emission (AE) response under uniaxial compression, and the obtained characteristics were compared to the properties of the as-received coal as a control. The uniaxial compressive strength, axial stiffness, and softening coefficient of the water-immersed coal samples were reduced compared to those of the control, and the softening degree increased with increasing degree of water immersion. Further, the stress threshold values for crack initiation and crack damage and their ratios to the peak stress decreased with increasing softening degree. The crack damage (CD) point marked the beginning of the unstable propagation of visible fractures and the intense evolution of tensile/shear cracks, accompanied by a rapid increase in the AE counts, regardless of whether the coal had been water-immersed or not. For quantitative characterization and identification of softening characteristics, a new comprehensive index and its softening coefficient are proposed considering the stress threshold and volumetric strain at the CD point, and it is applied to the step-by-step fracturing mechanism of softened coal by hydraulic fracturing. The effective utilization of coal softening and fracturing can avoid the harsh conditions imposed by high-pressure water injection in hydraulic fracturing, which alleviate the dynamic disaster of coal and rock to a certain extent. • Study of progressive failure and softening characteristics of water-treated coal. • Expanding the water immersion level significantly enhances the softening degree. • Crack initiation stress threshold value decreases as the softening degree increases. • New index and softening coefficient to quantitatively describe softening degree. • Investigation of softening/rupturing mechanisms of coal during hydraulic fracturing.