Combined finite-discrete element modelling of hydraulic fracturing in reservoirs with filled joints

Hydraulic fracturing (HF) is known to be the most effective stimulation to enhance fractured reservoir perme-ability. The efficiency of an HF fluid injection largely depends on the pre-existing discontinuities or sources of heterogeneities. These features need to be considered in an HF operation design. Amongst these features, rock masses with filled joints are largely ignored despite their presence in deep rocks. This study focuses on HF development in such ground conditions and simulates their hydro-mechanical behavior by using the combined finite-discrete element method (FDEM), which is fully capable of modelling HF propagation in heterogeneous rocks. First, the interaction mechanism between an HF and a closed joint with approaching angles of 30, 60, and 90 degrees is studied. The influences of flow rate, fluid kinematic viscosity, and confining in-situ stresses are evaluated. Then models with a single filled joint with approaching angles of 30, 60, and 90 degrees are studied, interaction types explored, and failure types revealed by varying flow rate, fluid kinematic viscosity, and in-situ stresses. Having explored the interaction mechanism between an HF and a filled joint, three models with complex joint sets are considered; Voronoi filled joint, orthogonal filled joints, and randomly distributed discrete fracture networks (DFNs). The results show a complex interaction mechanism between the HFs and the filled joints. It is revealed that increasing approaching angle, flow rate, or fluid kinematic viscosity causes the development of more shear failure than tensile in models with Voronoi or orthogonal filled joints. However, it is shown that HF's interaction with the DFN is minimal and tensile failure governs considerably.