Summary Hydraulic-fracture initiation and propagation in the presence of multiple layers with different mechanical and flow properties are investigated experimentally using a novel fracturing cell. Mixtures of plaster, clay, and hydrostone are used to cast sheet-like and porous test specimens in layers with different configurations and properties. The layered specimens are hydraulically fractured under varying far-field differential stress. Fracture growth is recorded using a high-resolution digital camera. Key frames are subsequently analyzed using digital image correlation (DIC) to reveal microcracks, measure strains, and show other features such as shear-failure events that are difficult to detect with the naked eye. The problem of a hydraulic fracture induced in a soft layer bounded by harder layers is considered. We demonstrate numerous laboratory experiments that reveal a clear tendency for induced fractures to avoid harder bounding layers. This is seen as fracture deflection or kinking away from the harder layers, fracture curving between the harder bounding layers, and fracture tilt from the maximum far-field stress direction. These observations appear to be more pronounced as the contrast in Young's modulus and fracture toughness between the layers increases and/or the far-field differential stress decreases. Moreover, when a fracture is induced in a relatively thin layer, the fracture avoids the harder bounding layers by starting and propagating parallel to the bounding interfaces. Fracture propagation parallel to the bounding layers is also observed in relatively wide layers when the far-field stress is isotropic or very low. A fracture approaching a dipping, harder layer tends to curve away from the hard layer by kinking toward the high side of the interface. Nonplanar fracture trajectories are observed even in homogeneous materials when the far-field differential stress is relatively low. Furthermore, various other fracture behaviors in layered specimens are demonstrated and discussed, such as fracture offsetting at material interfaces, fracture branching and complex fracture trajectories, and shear failure of weakly bonded interfaces.
During hydraulic fracturing, the interaction of hydraulic fractures with natural fractures can result in the formation of complex fracture networks. In the past these interactions have been captured in hydraulic fracturing models using crossing criteria developed based on two-dimensional geometries. In this work, we investigate the interaction of hydraulic fractures and natural fractures in three-dimensions and demonstrate that there can be significant differences in the observed interactions. A hydraulic fracturing simulator is presented that solves the coupled fluid flow and geomechanics problem for three-dimensional fractures. The simulator captures the physics of fracture growth and the intersection of hydraulic fracture with pre-existing discrete fracture network. The model employs a robust algorithm to account for the stress relaxation due to the slippage of natural fractures. The displacement of failed natural fracture elements is calculated rigorously. The model allows the partial failure of three-dimensional natural fractures and accurately calculates the stresses acting on the plane of the natural fracture. It is shown that a natural fracture inclined at an angle to an approaching hydraulic fracture experiences compression in one region (due to the stress shadow of the growing hydraulic fracture) and tension in other regions (in front of the approaching hydraulic fracture tip). The generated stresses can fail the natural fracture partially. The failure of the natural fracture relaxes the stresses around it, which can modify the direction of propagation of the approaching hydraulic fracture. In addition, if the elliptical front of the hydraulic fracture crosses an intact planar natural fracture, the three-dimensional geometry results in a line of intersection (between natural fracture and hydraulic fracture). This can lead to failure of the natural fracture even after the elliptical front has partially crossed the natural fracture. Such an interaction can allow the hydraulic fracture to both cross the natural fracture and activate (or dilate) it. These effects cannot be captured by two-dimensional simulations. This work improves our understanding of the interaction between hydraulic fractures and natural fractures. The novel results provide new insights into the mechanisms responsible for the complexity that is often observed in hydraulic fractures.
Micro-seismic data suggest that complex fracture networks are formed frequently in unconventional reservoirs due to the interaction of hydraulic fractures (HF) with natural fractures (NF). Understanding this interaction is critical for optimizing fracturing design. It is generally accepted that under certain conditions, a propagating HF can cause remote shear failure of a NF before intersecting with it. This fact is not accounted for in the development of the existing fracture interaction criteria. The goal of this study is to account for these dynamic interactions and present new criteria that define the conditions under which a HF will cross, kink, branch, or turn along a NF. We have used our peridynamics-based poroelastic fracturing simulator in this study, which solves for rock displacements and fluid pressure in a fully coupled and implicit manner. Shear failure of the NF is modeled using a Mohr-Coulomb failure criterion. The frictional force on the NF surface is modeled implicitly. The stress distribution around the HF is monitored as the NF approaches it. Considering the effects of shear failure, different propagation behavior such as turning, and crossing are characterized as a function of in-situ stress ratio, angle of approach, NF characteristics, and matrix permeability. It should be noted that the peridynamics model used in this study does not require a crossing criterion as an input, rather it can predict the interaction behavior based on local poroelastic stresses. The model is validated against the analytical crossing criteria derived using Linear Elastic Fracture Mechanics (LEFM) by ignoring remote shear slippage prior to intersection and poroelasticity in our model. Recent experimental observations that show an increase in approach angle before intersection of a HF with a NF are also used to test the model. Remote shear failure of the NF before intersection results in relaxation of the stresses locally. This in turn leads to the HF bending towards the NF. Though these effects are found to be important in low permeability rocks (100 nD), they are more pronounced in high permeability rocks (10 mD). In high permeability rocks, poroelastic effects are much more significant, leading to greater stress relaxation and thus a near-orthogonal modified approach angle. When stress relaxation due to remote shear slippage of the NF is considered, the HF is more likely to turn along the NF. For low angles of approach and low stress ratios (1.0-1.1 for low permeability rocks and 1.0-1.2 for high permeability rocks), the poroelastic crossing criteria derived in this study are considerably different from those derived using LEFM. However, for near-orthogonal angles of approach or high stress ratios, the crossing criteria do not change much. The poroelastic crossing criteria derived here can serve as direct inputs for discrete fracture network models simulating the growth of complex fracture networks (Shrivastava and Sharma, 2018). The results and insights presented in this paper improve the understanding of the formation of such complex fracture networks in unconventional reservoirs.
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