A fully coupled XFEM analysis of complex fracture network development during multi-cluster hydraulic fracturing in naturally fractured formations

Multi-cluster hydraulic fracturing in naturally fractured reservoirs is often limited by non-uniform fracture propagation, yet the controlling mechanisms of NFs remain insufficiently understood. This study develops a fully coupled XFEM-based framework to simulate multi-cluster hydraulic fracturing, incorporating rock deformation, fracture propagation, fluid flow, frictional contact, leak-off, fluid partitioning, and HF-NF interactions. A unified solution strategy is introduced by incorporating a virtual pressure node and perforation flow elements, enabling the coupled problem to be solved without external iteration. The model is validated against analytical solutions for the KGD problem and benchmark cases of multi-cluster fracture propagation. Parametric analyses demonstrate that NFs promote uneven fracture growth and require higher perforation friction to compensate for the additional resistance induced by fracture deflection following NF capture. When HFs cannot cross NFs, higher stress differences increase net pressure and improve fracture growth uniformity, while smaller intersection angles enhance uniform propagation but generate simpler fracture geometries. Increasing NF aperture initially promotes balanced fracture growth and subsequently favors the development of more complex fracture networks. NF friction has a limited effect when crossing is inhibited, but once crossing occurs, fracture evolution becomes strongly controlled by HF-NF interaction patterns. Random NF distributions introduce significant uncertainty, with near-wellbore NFs playing a key role in fracture competition. These findings provide novel insights for optimizing multi-cluster fracturing designs in naturally fractured reservoirs.