Experimental investigation on fracture morphology in laminated shale formation by hydraulic fracturing

Abstract

Abstract A complex fracture network, which is the precondition of stimulated reservoir volume for shale formation, depends on the interaction between hydraulic fractures and discontinuities, such as bedding planes and natural fractures. However, the fracture propagation behavior in laminated shale formation still maintains ambiguous and the formation mechanism of complex fracture network has not been well understood. Therefore, hydraulic fracturing simulated experiments were conducted on shale outcrops to clarify above issues under the effects of multiple factors, such as horizontal stress difference, pump rate, and fluid viscosity. The results reveal that dilation and crossing are two main hydraulic fracture propagation behaviors because of discontinuities influencing, resulting in three categories of the ultimate fracture morphologies. Simple fracture is a single induced fracture. Complex fracture is consisted of two or more induced fractures which do not link up with each other. However, network fracture includes induced fractures which interweave with each other like a network. All the fracture geometries are generated under the mutual interaction of natural factors and engineering factors. A large horizontal stress difference and a high fracturing energy reduce the complexity of fracture geometry, whereas, a small horizontal stress difference and a low fracturing energy enhance the activated probability of discontinuities, resulting in a complex fracture network. Furthermore, weak cementing strength and large opening degree of discontinuities increase the complexity of the fracture. In addition, the fracturing curve reflects fracture morphology indirectly. Simple fracturing curve and steady extensive pressure indicate the simple fracture. The rising extensive pressure implies the complex fracture, whereas, the fluctuation of fracturing curve suggests the network fracture. In the meantime, each fracture morphology has its own dimensionless net pressure features, which could be applied to judge fracture morphology in shale formation.