Abstract:

The 7^th member of the Triassic Yanchang Formation in Ordos Basin is rich in shale oil and has been regarded as one of the key targets for reserve growth and production increase. However， the oil-bearing lacustrine shale with its broad lamination and bedding， high content of clay minerals， and heterogeneity in terms of structure and mechanism， responds poorly to hydraulic fracturing， calling for a better understanding to the evolution characteristics of hydraulic fracture network. In this paper， the effects of in-situ stress difference and fracturing fluid viscosity on hydraulic fracture propagation in lacustrine shale are determined through laboratory fracturing tests and numerical simulation analysis. The results show that the hydraulic fracture propagation is mainly controlled by weak bedding planes with fracture height being limited when the vertical stress difference is 15 MPa and gradually increasing when the vertical stress difference is larger than 20 MPa. Fluid with higher viscosity enhances the vertical propagation of hydraulic fractures across weak bedding planes. The rock mass of lacustrine shale reservoirs is the basis for the generation of complex hydraulic fracture networks of mostly activated natural fractures. The hydraulic fracture network is mainly composed by activated horizontal bedding-parallel fractures and nearly bedding-perpendicular fractures.

Key words: fracturing experiment, numerical simulation, hydraulic fracture, lacustrine shale, Yanchang Formation, Ordos Basin