Impact of Mechanical Anisotropy on Design of Hydraulic Fracturing in Shales

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Abstract Shale formations have laminated structures that result in directionally dependent mechanical properties. Conventional completion design approaches do not consider the material anisotropy or the laminated nature of shales. This can result in an underestimation of stresses, and lead to incorrect conclusions about the lateral landing points and the perforation intervals. In this paper, the authors demonstrate the importance of considering the anisotropy in the completion design using a case study from the Horn River Basin (HRB), the largest shale gas play in Canada. Shale formations in the HRB are strongly anisotropic with horizontal to vertical Young's modulus ratios varying from 1.2 to 3.5. Field data from the HRB is examined to evaluate the impact of mechanical anisotropy on break down pressure, fracture initiation and fracture containment. Numerical simulation of the completion design was conducted using a planar 3D fracture model. Results of the numerical simulation indicate that the mechanical anisotropy greatly influences the minimum horizontal stress which in turn impacts the fracture containment and fracture geometry. Strong mechanical anisotropy results in lower fracture initiation pressures and lower tortuosity at the wellbore face. Consequently, selecting the landing point in sections with high anisotropy will minimize the fracture initiation problems. The authors conclude that the heterogeneous and anisotropic nature of shales needs to be properly characterized and taken into account when making decisions on lateral landing points and completion design.

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