Natural fractures in soft coal seams and their effect on hydraulic fracture propagation: A field study

摘要

Natural fractures provide direct flow channels for coalbed methane (CBM) production and gas outburst control. However, in soft coal seams, the characteristics of natural fractures and their effect on hydraulic fractures are still unclear. In this study, a spatial system of visible natural fractures was established through underground dissection and measurement of ground joints from a case study of the Xinyuan Coal Mine in the northern Qinshui Basin, China. Our results show that the orientations of the subsurface exogenetic fractures of the coal seams are consistent with the macrostructural joints on the ground surface in the same tectonic setting. The exogenetic fractures of the coal seams have dominant orientations with approximately equal spacing. Next, a field test of a large-scale hydraulic fracturing project with a volume of up to 2000 m(3) was conducted. The on-site observation of the underground coal seams revealed that the propagation orientation of the vertical propped fractures coincides with the predominant orientation of the natural fractures. Even at a high constant injection rate (12 m(3)/min), the corresponding injection pressure was not constant during the propagation of the hydraulic fractures, and this can be attributed to the effect of the depth of the coal seam, internal natural fracture network, and normal fault. Additionally, a two-dimensional (2D) particle flow simulation (PFC2D) showed that the propagation of the hydraulic fracture is controlled by the orientations of the natural fractures and the principal stresses. The orthogonal fracture network showed more sensitivity to joint spacing than the layered fracture network in controlling the hydraulic fractures. Finally, fluid detection of the coalbed demonstrated that the flow range of the fracturing fluid in the natural exogenetic fractures was up to a few hundred meters. The flow direction was consistent with that of the natural fractures. A comprehensive analysis of the natural fractures contributes to the identification of flow channels and helps in the design of hydraulic fracturing engineering in soft coal seams.