Phase-field modelling of hydraulic fracturing

Hydraulic fracturing is an important technology for stimulating and enhancing the extraction of coal seam gas in Australia. The rapid implementation of hydraulic fracturing for coal seam gas is being driven by the economic benefits of a cleaner and cheaper energy source to satisfy increasing energy demands. Hydraulic fracturing is a complex hydro-mechanical coupling problem, and modelling of hydraulic fracturing remains challenging due to difficulties in dealing with the discontinuities caused by propagating fractures and the hydro-mechanical couplings. This thesis aims to develop computational algorithms for numerical modelling and analysis of the hydraulic fracturing process in porous media, considering both isotropic and transversely isotropic rock properties. To fulfil this aim, a phase-field model based on the variational approach is step-wisely developed and implemented for modelling the hydraulic fracturing process in saturated poroelastic rocks. The phase-field model replaces the discrete, discontinuous fractures by continuous diffused/smeared damage fields by introducing a scalar field parameter that identifies the degree of damage in solid media. Griffith’s theory is adopted in the phase-field model to provide the fracture criterion to predict quasi-static fracture initiation in brittle material for both isotropic and transversely isotropic elastic brittle rocks. To deal with the hydro-mechanical coupling process in the hydraulic fracturing, the Biot’s theory of poroelasticity is incorporated into the phase-field model. The developed phase-field model is implemented using the finite element method in 2D, which is step-wisely verified by available experiment data or analytical solutions. Typical computation examples are presented to investigate the fracture initiation and propagation patterns in both isotropic and transversely isotropic rocks for the Mode I fracture and the mixed Mode I and Mode II fractures. Using the developed phase-field model, the hydraulic fracture interactions with pre-existing natural fractures and fracture networks in both isotropic and transversely isotropic rocks are studied, with emphasis on the influence of the fracture distance, the rock permeability, the injection rate, the anisotropic degree, and the rotation angle of the weak plane in transversely isotropic rocks on the fracture propagation process. The results generally show that the phase-field method is promising for modelling the hydraulic fracturing process. Based on the variational approach and the Griffith fracture theory, the developed phase-field model can correctly simulate the fracture initiation and propagation process in brittle materials. It can be extended for modelling fracture initiation and propagation in transversely isotropic rocks by modifying the fracture surface energy function and the elastic energy function. Using the Biot’s theory of poroelasticity, the phase-field model can be extended to simulate the hydraulic fracturing process in saturated poroelastic rock. Investigations into the interaction between hydraulic fractures and natural fractures show that the distance between the hydraulic fracture and the near natural fracture, the rock permeability, the injection rate, the anisotropic degree, and the rotation angle of the weak plane in transversely isotropic rocks significantly affect the natural fracture propagation direction after connecting with the hydraulic fracture. Simulations of hydraulic fracture interaction with natural fracture networks show that the phase-field model of hydraulic fracturing can be applied to model the complex topology cases related to the realistic situations in hydraulic fracturing practice.