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Abstract Laboratory experiments and field fracturing practices have shown that complex radial fracture-network extended system is easily formed in the area around borehole due to the effect of natural fractures during fracturing of fractured reservoirs，which greatly differs from the single，bi-wing，symmetric planar fracture generated in the homogeneous reservoirs. As the forming of radial fracture-network is mainly controlled by the initiation and propagation of hydraulic fracture at the beginning stage of hydraulic fracturing，studying the initiation mechanism of hydraulic fracture for fractured formations is the prerequisite for understanding the forming of radial fracture-network. Based on the theories of elastic mechanics and rock mechanics，taking account of the spatial relation of natural fractures and perforations under intersection，and integrating with the criterion of tensile initiation，the calculating model of initiation pressure in which the hydraulic fracture is tensile initiation along natural fractures is established. The calculation results show that the deeper position of natural fractures intersecting with perforation is，the lower the tensile initiation pressure is. Tensile initiation pressure in the position where natural fractures intersect the top of perforation is the minimum value. The difference of tensile initiation pressure may sharply decreases from perforations in different azimuths around borehole due to the effect of natural fractures and geostress azimuth，which leads to the extending of hydraulic fracture simultaneously from perforations in different azimuths and forms radial fracture-network. The example calculation results indicate that this calculating model can obtain high calculating accuracy and reliable calculating results，and can be used to calculate initiation pressure of fractured formations and analyze the probability of radial fracture-network propagation during fracture-network fracturing.
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Received: 17 November 2011
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2013, Vol. 32 
