Optimization and Field Evaluation of Multi-Cluster Hydraulic Fracturing Using Surface L-Shaped Well in Soft Low-Permeability Coal Seam

<title>Abstract</title> <p>This study investigated the effects of injection rate, sand-to-fluid ratio, and cluster spacing on multi-cluster hydraulic fracturing in a surface L-shaped horizontal well targeting the soft, low-permeability, outburst-prone No. 7 coal seam at Yongchun Coal Mine, Guizhou. Laboratory testing of coal and rock mechanical properties, three-dimensional discrete-element simulations in XSite, and comparative field monitoring were combined. The fracture area balance index (FBEI), fracture fluid-volume balance index (FVBI), fracture geometry balance index (FGEI), and comprehensive evaluation index (CEI) were formulated from the fracture area, fluid volume, and equivalent mean aperture of each cluster. The results showed that a higher injection rate increased the overall stimulation extent but intensified fluid competition among clusters. An excessively high sand-to-fluid ratio increased total fracture area and fluid volume while reducing inter-cluster uniformity. A cluster spacing of 5 m provided a favorable balance between stimulation extent and uniform fracture development. The parameter combination of 12 m³/min, 8%, and 5 m achieved the highest CEI (0.9233) among the seven independent combinations, with a total fracture area of 767.00 m² and a total fracture fluid volume of 11.845 m³. During 120 d of field monitoring, the fractured well yielded an average gas concentration of 65.68% and a cumulative methane drainage volume of 261,948.77 m³, both higher than those of the unfractured control well. These findings demonstrate the engineering feasibility and permeability-enhancement potential of the recommended parameter combination under the conditions investigated.</p>