2D Phase-Field Modelling of Hydraulic Fracturing Affected by Cemented Natural Fractures Embedded in Saturated Poroelastic Rocks.

The interaction between a propagating hydraulic fracture (HF) and a pre-existing natural fracture (NF) embedded in saturated poroelastic rock formations is studied numerically in 2D plane–strain configurations. In this study, the phase-field method is further developed to be employed for modelling the HF propagation and the evolution of tensile and shear failure in geo-materials as gradient-type diffusive damaged zones. The shear slippage and dilation mechanisms inside the cemented NF are modelled using a Mohr–Coulomb–Griffith failure criterion that fitted in the framework of the phase-field fracture using appropriate energy functionals. The most important factors controlling the HF–NF interaction outcome are the approaching angle, differential in-situ stress, and hydro-mechanical characteristics of the NF. It is found out that higher tensile and shear strengths of the cemented NF are in favour of the crossing outcome when the differential stress is high enough to mobilise the resisting shear stresses against the slippage. Small hydraulic aperture (low hydraulic conductivity) for the NF is also in favour of the crossing outcome which helps to restrict the pressurised region local to the HF tip, lowering the possibility of shear slippage in the NF and the HF's diversion. It is also concluded that the injection rate and the viscosity of fracturing fluid are operative factors to be adjusted for increasing the chance of crossing, a critical element for successful operation of hydraulic fracturing for effective use of subsurface energy resources. Highlights: The phase-field method is developed for 2D modelling of mixed-mode tensile and shear failure as gradient-type diffusive damaged zones using Mohr–Coulomb–Griffith failure model. An extensive parametric study is conducted to identify the influential controlling factors in the interaction between a propagating hydraulic fracture (HF) and a pre-existing cemented natural fracture (NF) in plane–strain configurations. Hydro-mechanical characteristics of cemented NFs become important for intermediate approaching angles; the differential in-situ stresses and approaching angle are the most important controlling factors. Increasing the fluid injection rate can cause the HF to cross NFs with low permeability, whereas it can cause the HF to divert along the cemented NF that has a permeability much higher than the bulk rock. [ABSTRACT FROM AUTHOR]