Peak strength property of the pre-cracked similar material: Implications for the application of hydraulic slotting in ECBM

Chinese coal seams are characterized by high gas content and low permeability. The permeability of coal seams should be improved to achieve maximum extraction of coalbed methane. Hydraulic slotting has been extensively used in China for underground enhanced coalbed methane (ECBM) recovery. The coal strength after hydraulic slotting has a significant effect on the efficiency of this method. In this work, stress-strain curves characteristics of pre-cracked similar material were investigated. Three types of stress-strain curves are identified with the increase in slot inclination angle: bimodal curve with two obvious peaks, transitional curve with unobvious second peak, and unimodal curve with only one peak. In general, the first peak strength increases with the increase in slot inclination angle, whereas the second peak strength shows an opposite tendency. The former is mainly affected by initiation and propagation of wing cracks, and the latter is mainly influenced by frictional sliding between slot surfaces. The study results provide certain guidance for the selection of slot inclination angle in field tests.