Dilation behavior and acoustic emission response to water/CO2 injection-induced shear fracturing in coal seams

Abstract

Abstract The low permeability of coal seams significantly restrains the efficient extraction of coal-seam methane. Hydroshearing treatment is suitable for enhancing the permeability of coal seams because of their low mechanical strength and unique fracture–matrix structure. The shear dilation behavior associated with the increase in the void volume in porous media is critical for the evaluation of the permeability enhancement during hydroshearing. In this study, we performed conventional triaxial shear experiments to induce purely fluid injection-induced shear failure (hydroshearing) in coal specimens. The axial loading and mean stress unloading experiments were compared to investigate the effects of confinement and fluid injection on the dilation behavior. The experimental results and quantitative inelastic deformation analysis indicated the following: 1) The significant stress drop in the stress–strain curves and shear localization in computed-tomography images indicated the brittle failure of the coal specimens under the hydroshearing condition. 2) The dilation behavior of the coal specimens was sensitive to effective confinement. Mean stress unloading and hydroshearing improved the dilatancy factor from a negative or low value under axial loading to 0.3–1.0. With a suitable injection fluid (CO2) and rate (slow), the dilatancy factor could be further enhanced to 1.7–3.0.3) Owing to the significantly heterogeneous fluid-pressure distribution in coal, the acoustic emission (AE) system detected massive randomly localized micro-breakages, which could not be revealed by global deformation prior to the formation of the shear band in the coal specimens. Therefore, AE monitoring is essential for the hydroshearing experiments on coal to verify the reliability of global deformation measurements. 4) With slow CO2 injection, the fluid-pressure distribution became less heterogeneous. Moreover, the microfractures induced by CO2 sorption can enhance the brittleness and dilation behavior of coal specimens.