Experimental investigation into the fracture propagation behavior of horizontal well multi-stage and multi-cluster fracturing within the roof of crushed soft coal seams

The effectiveness of horizontal well multi-stage and multi-cluster fracturing in the fractured soft coal seam roof for coalbed methane (CBM) extraction has been demonstrated. This study focuses on the geological characteristics of the No. 5 and No. 11 coal seams in the Hancheng Block, Ordos Basin, China. A multi-functional, variable-size rock sample mold capable of securing the wellbore was developed to simulate layered formations comprising strata of varying lithology and thicknesses. A novel segmented fracturing simulation method based on an expandable pipe plugging technique is proposed. Large-scale true triaxial experiments were conducted to investigate the effects of horizontal wellbore location, perforation strategy, roof lithology, and vertical stress difference on fracture propagation, hydraulic energy variation, and the stimulated reservoir volume in horizontal wells targeting the soft coal seam roof. The results indicate that bilateral downward perforation with a phase angle of 120° optimizes hydraulic energy conservation, reduces operational costs, enhances fracture formation, and prevents fracturing failure caused by coal powder generation and migration. This perforation mode is thus considered optimal for coal seam roof fracturing. When the roof consists of sandstone, each perforation cluster tends to initiate a single dominant fracture with a regular geometry. In contrast, hydraulic fractures formed in mudstone roofs display diverse morphology. Due to its high strength, the sandstone roof requires significantly higher pressure for crack initiation and propagation, whereas the mudstone roof, with its strong water sensitivity, exhibits lower fracturing pressures. To mitigate inter-cluster interference, cluster spacing in mudstone roofs should be greater than that in sandstone roofs. Horizontal wellbore placement critically influences fracturing effectiveness. For indirect fracturing in sandstone roofs, an optimal position is 25 mm away from the lithological interface. In contrast, the optimal location for indirect fracturing in mudstone roofs is directly at the lithological interface with the coal seam. Higher vertical stress coefficients lead to increased fracturing pressures and promote vertical, layer-penetrating fractures. A coefficient of 0.5 is identified as optimal for achieving effective indirect fracturing. This study provides valuable insights for the design and optimization of staged fracturing in horizontal wells targeting crushed soft coal seam roofs.