Experimental Study on Fracture Propagation and Multi-stage Fracturing of Deep Shale:A Case of Longmaxi Formation in Western Chongqing
The deep shale in western Chongqing has the geological characteristics of high closure pressure and high horizontal stress difference,which leads to the difficulty and unclear law of fracture propagation in the process of shale gas exploitation.Based on the core mechanical parameters and mineral content of deep shale res-ervoir in western Chongqing,this paper selects the shale outcrop of Longmaxi Formation in southern Sichuan with similar parameters to carry out experiments.By simulating high vertical stress and normal fault stress,combined with acoustic emission monitoring data and pumping pressure data,the initiation and propagation laws of hydraulic fractures in rock sheet under different natural fracture development conditions are studied.The results show that un-der the condition of normal fault stress and undeveloped natural fracture,the fracture form is single.The develop-ment orientation of natural fractures significantly affects the morphology of the fracture network,and the high angle natural fractures guide the artificial fractures to turn and form step-like fractures.Horizontal natural cracks guide arti-ficial cracks to expand horizontally to form"T"or"work"cracks;Acoustic emission shows that under experimen-tal conditions,the main failure mechanism of hydraulic fracture is tensile failure,while that of horizontal natural fracture is shear failure.Multi-stage fracturing can effectively improve the local stress between fractures and further increase the fracture complexity.To sum up,natural fractures are the basic conditions for the formation of complex fracture network in Longmaxi deep shale reservoir.The experimental results of this paper can provide a reference for the understanding of experimental rules and measures to improve fracture complexity in deep shale gas fracturing.
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