Assessment of Diversion Fracture Paths during Large Hydraulic Fracturing in the Coal Seam

Natural fractures play an important role in fluid diversion and reservoir in the process of large hydraulic fracturing in coal seam. Under the action of tectonic stress, the complex fracture system in coal seam and wall rock and different types of coal body structure have been formed. There are few studies on the characteristics of fracture development and the spatial distribution of driving fluid in the soft coal seam. Taking the No.3 coal seam of Xinyuan coal mine in Shouyang, the development characteristics of coal seam fractures and coal body structure are found out through the detailed dissection in the pit, and the three-dimensional space system of fracture paths is established. Based on the large scale mapping of surficial joints in the research area, the development degree of fractures in corresponding coal seam is predicted. The results show that exogenetic fractures are the main diversion path, and their dominant strike is mainly NNE, followed by NW. There are significant differences in mechanical property about coal seam, roof wall rock, gangue belt and tectonic coal layer, which are not conducive to propagation for exogenetic fractures and hydraulic fractures. The non-planar fractures in the tectonic soft coal seam have poor diversion conductivity, which is easy to form an accumulation zone with the high content of gas. The horizontal propped fractures are easily formed along the weak combination surfaces between the coal seam and its roof, the bedding plane of the coal seam, and the soft coal layer of the coal seam. The vertical propped fractures are generally cut through the coal seam, which is in accordance with the natural exogentic fractures. The flow distance of driving fluid in fractured channel is much larger than that of proppant and hydraulic fractures, which can reach hundreds of meters. This study can predict the spatial distribution of driving fluid and assess the outburst control effect during large hydraulic fracturing.