Hydraulic fracture initiation and propagation in the presence of multiple layers with different mechanical and flow properties are investigated experimentally utilizing a novel fracturing cell. Mixtures of plaster, clay and other materials are used to cast sheet-like test specimens in layers with different mechanical properties. The specimen is placed between two transparent plates that allow us to take videos of the speckled rock sample surfaces to facilitate digital image processing of rock displacement. In order to control fracture orientation, a far-field differential stress is applied to the specimen via pneumatic jacks. Hydraulic fracture growth during the experiment is recorded using a high resolution digital camera. Key frames are subsequently analyzed using Digital Image Correlation (DIC) to reveal micro-cracks, strains and other features that are difficult to detect with the naked eye. Experiments are conducted for fracture propagation across layered porous media in different stress states.

It is clearly shown that a fracture initiating in the vicinity of a layer interface may exhibit a range of behaviors including propagation straight through the layer interface, forming secondary fractures, fracture kinking, and fracture propagation along or parallel to the interface. Straight/plane fracture propagation is observed across an interface when going from a hard to a soft layer. Conversely, crossing from a soft to a hard layer, a fracture tends to kink at a slight angle as it approaches the interface. The angle at which the fracture turns increases as the contrast in Young's modulus and fracture toughness between the layers increases and/or the differential stress decreases. Tensile stresses on the intact (hard) side of the interface can also develop a secondary fracture at an offset to the parent fracture with localized interface opening resulting in a fracture step-over. Given a choice of multiple layers, fracture propagation into a harder layer is favored over a softer layer. The effect of hard inclusions that can act as fracture barriers is also investigated. Fractures tend to initiate and propagate parallel to the bounding interfaces when the fracture initiation layer is relatively thin or in the case of an isotropic stress state and propagate along the interfaces when the bonding between the layers is weak. A fracture approaching a dipping, harder layer tends to kink towards the low side of the interface. The experiments, for the first time, clearly demonstrate and quantify experimentally the factors that control fracture propagation across geological layers. The observed turning and branching of the fractures can have a profound effect on fluid leakoff and proppant transport.

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