Effect of Slot Inclination Angle and Borehole-Slot Ratio on Mechanical Property of Pre-cracked Coal: Implications for ECBM Recovery Using Hydraulic Slotting

Abstract

Low permeability is the main constraint on the high-efficiency coalbed methane recovery in deep coal seams. Hydraulic slotting has been proved to be a favorable method to stimulate low-permeability coal seams. In this paper, the coal samples with various slot inclination angles and borehole-slot ratios were used to investigate the weakening effect of slot inclination angle and borehole-slot ratio on the mechanical property of the pre-cracked coal. Besides, the crack patterns of the slotted coal specimens were identified to reveal the slot weakening mechanism. It is revealed that the variations in compression strength, elastic modulus and Poisson’s ratio with the slot inclination angle generally conform to a Boltzmann function, logistic function and quadratic function, respectively. With the increase in the borehole-slot ratio, the curve clusters of compression strength and elastic modulus show the horizontal “V” with left opening, and the curve clusters of Poisson’s ratio show the trend of rapid increase after slow increase. Compared with elastic modulus, the slot weakening degrees of compression strength and Poisson’s ratio are more significant. Moreover, the tensile and shear cracks mainly appear in the coal samples with small and large slot inclination angles, respectively, which verify the fact that the slot weakening effect on mechanical property of the slotted coal samples with small slot inclination angles is more significant. The research achievements are attributed to the improvement in the efficiency of the hydraulic slotting-based enhanced coalbed methane recovery.