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Experimental Study of Fracture Interaction between Natural Fractures and Hydraulic Fractures in Shale Gas Reservoir

Hou, Bing; Chen, Mian; Li, Qinghui

Shale gas reservoir is a typical naturally fractured reservoir, which has been widely concerned and explored during last few years. Hydraulic fracturing has become a valuable technique for the stimulation of oil and gas industry and has achieved notable results in mature shale plays around North America.

The mechanism of interaction between natural fractures and hydraulic fractures was observed in multiple fractured outcrops of black shale by a series of physical experiments. The effect of natural fractures, in-situ stress, basic mechanical properties and the brittle index on hydraulic fracture geometry was revealed. Three types of fracture geometry were observed.

The results of this study revealed that fracture network complexity can be influenced dramatically by natural fractures and brittle deformability of the samples. Shear-slip deformation could be more easily caused by hydraulic fracturing than filtrate-open soon after treatment beginning. The shear-slip failure of the rock can be caused by three mechanisms that are to decrease the effective stress, to bring down the interface cohesion stress and to reduce the internal friction angle. Permeability of the natural fractures could be largely enhanced by the way mentioned above. Brittleness is another critical factor affecting the geometry of hydraulic fractures, which can be more complex with higher brittle deformation characteristics. Hydraulic fractures tended to propagate parallel to the maximum stress and get a temporary turn along the pre-fractures and then reinitiate in random weak points back to the previous direction. Based on the laboratory results, we proposed an index for depicting the fracability of the reservoir, which could be much more practical useful for understanding the formation and selecting engineering sweet point.

The experimental and analyzing results revealed the real geometry of hydraulic fractures in shale gas reservoir. The interaction mechanisms between natural and hydraulic factures can be guide line for treatment design. Brittleness index calculating approach could be helpful in understanding the mechanical properties of the shale and fracability evaluation could be practical useful for stimulating target selection and post-fracture feedback analysis.


AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013