THE IMPACT OF PERFORATION ON THE FRACTURE PROPAGATION OF HORIZONTAL WELL MULTISTAGE FRACTURING IN THE ROOF OF BROKEN SOFT COAL SEAMS

Horizontal well multistage fracturing in the roof of low-permeability and broken soft coal seams has proven to be an effective technique to extract coalbed methane. However, fracture propagation behaviors under this technology vary significantly with different perforation patterns. Therefore, based on the 5# coal seam in Hancheng block of Ordos Basin in China, large-scale true triaxial hydraulic fracturing physical simulation experiments were performed using the "roof-coalfloor" similar material pouring model to optimize the perforation pattern. The fracture propagation behaviors and pressure curves characteristics of horizontal well multistage fracturing in the roof of coal seams with spiral perforation, double lateral downward perforation with a phase angle of 120 degrees and vertically downward oriented perforation were investigated respectively. The experimental results show that a relatively simple fracture network covering half of the area of the coal seam was formed by spiral perforation fracturing. A 3D complex fracture network with good connectivity in the coal seam was generated by fracturing with double lateral downward perforation. A horizontal fracture with a small zone in the coal seam was induced by vertically downward oriented perforation fracturing. Finally, double lateral downward perforation with a phase angle of 120 degrees is considered the optimal perforation pattern due to the desired fracturing effect and relatively low cost. The presented findings can provide references for the fracturing design of horizontal well multistage fracturing in the roof of lowpermeability and broken soft coal seams.