Study on the law of multi-fracture through-layer propagation of crushed soft coal seam roof by hydraulic fracturing

Abstract

The effects of different geological parameters, construction parameters, fracturing sequence and other factors on the propagation patterns of multiple cracks and stress interference between cracks in indirect fracturing were analyzed by the extended finite element method (FEM) in order to study the law of multi-fracture through-layer propagation during hydraulic fracturing of crushed soft coal seam roof. The results show that the larger the minimum horizontal principal stress of the roof layer is, the better the spreading effect of the crack in the coal seam is. The greater the thickness of coal layer, the less the possibility of fracture propagation. As the injection rate increases, the length of crack in coal seam increases, but the interference range of stress between cracks increases continuously. The larger the fracture initiation angle, the larger the fracture turning distance, the fracture mainly spreads in the roof. The larger fracture spacing can ensure the uniform expansion of multiple fractures into the coal seam after fracturing. The length and area of hydraulic fractures formed in coal seam after fracturing are twice as long as those in middle, left and right fractures in different fracture initiation sequences. When the fracture initiation sequence is left side, middle side and right side, the stress interference range will be increased, the fracture initiation pressure will be higher, and the area formed in the coal seam will be smaller.

Keywords

• crushed soft coal seam
• stress interference
• multi-fracture propagation
• through-layer extension
• hydraulic fracturing