Geothermal reservoir stimulation by shear slip on pre-existing fractures has been proposed as the main permeability enhancement mechanism. In recent years, this has been interpreted by some to exclude the possibility of natural fracture propagation in tensile mode. However, fracture mechanics studies have shown that propagation of mechanically-closed cracks under compressive stresses often involves both mode I and mode II propagation. The former is known as a wing crack, an out-of-plane growth of the pre-existing closed cracks triggered by the shear deformation of crack surfaces. Once initiated, the wing cracks tend to deviate and extend in the direction of the maximum compressive stress. However, crack extension along the initial shear surface also occurs. The relative dominance of each of these propagation modes is a function of the in-situ stress state, and pore pressure, and rock properties. A displacement discontinuity method with Mohr-Coulomb elements is used in this paper to study the response of natural fractures to injection. Modeling results indicate that the onset of fracture slip occurs when the initial shear stress exceeds the shear strength of the Mohr-Coulomb contact elements. Wing cracks form and propagate with an angle from the pre-existing crack and turn toward the maximum compressive stress afterward. “Shear” cracks may start to form, along with the tensile wing cracks, in a plane approximately parallel to the pre-existing natural fracture. These types of propagation can lead to coalescence of multiple fractures and formation of a fracture network.

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