Effect of cavity-shaped holes on the initiation and propagation of hydraulic fracturing cracks in coal seam: a numerical study

Abstract

Hydraulic fracturing is a pivotal technology for enhancing reservoir permeability and is extensively utilized in the production of coal seam gas. In soft coal seams, drilling operations can precipitate coal and gas blowouts, forming a cavity at the base of the drill hole. However, the effect of this cavity on the initiation and propagation of hydraulic cracks remains inadequately understood. This study employs the discrete element method in conjunction with the acoustic emission moment tensor algorithm to investigate how the cavity angle α impacts fracture propagation. The findings indicate that an increase in cavity angles correlates with a rise in the number of branch fractures observed, due to the increase in the projection area S_h of the cavity in the minimum horizontal principal stress direction. Additionally, as the cavity angle increased, there was a noted increase in initiation pressure and the statistical magnitude of acoustic emission events. Moreover, the spatial distribution fractal dimension D of acoustic emission events logarithmically increased with α. The b value of acoustic emissions escalated with α, reaching its maximum at α = 60°, where the stimulated influence area was maximized. These findings suggest that cavity-shaped holes can significantly enhance the complexity of hydraulic fractures, thereby facilitating a more extensive fracture network within coal seams, which is crucial for effective gas extraction.