Hydraulic fracture propagation and interaction with natural fractures by coupled hydro-mechanical modeling

Abstract: Understanding the propagation of hydraulic fractures (HF) and their interaction with pre-existing fractures is essential in conducting effective stimulation in naturally fractured reservoirs. We explore fracture propagation within an equivalent continuum represented by linear elastic fracture mechanics, while updating the equivalent composite modulus of fracture and matrix based on deformation. The incremental length of fracture propagation is quantified based on the energy release rate at the fracture tips. Potential scenarios of interaction with natural fractures (crossing, arrested, or reinitiating) are interpreted through non-linear stress correction constitutive models using continuum simulator TOUGHREACT coupled with FLAC3D. Comparison between simulations and experimental observations highlight the three principal factors controlling hydraulic fracture interaction in defining whether HFs either directly cross, are arrested by, or reinitiate from the natural fractures. Simulation results reveal that a larger approaching angle and elevated deviatoric stress both favor direct crossing due to the comingled normal stress effects on the natural fractures. Conversely, when the approach angle and deviatoric stress difference are smaller, the natural fractures hinder HF crossing and either promote arrest of the HF or enable remote reinitiation from the initial intersection where a reinitiation criterion is satisfied. Direct crossing is favored only for a threshold minimum friction coefficient representing the case where the fracture is strong, replicates intact rock strength and is mechanically “invisible”. The development and verification of the model indicate the applicability in predicting the evolution of hydro-mechanical properties and fracture patterns due to the perturbation introduced by fluid pressurization. Simulation results from modelling of Panoche sand injectites reveals that the development of hydraulic fractures cause significant drawdown of borehole pressure through dissipation. Permeability evolution of fractures is concurrently evaluated based on the stress state and updated fracture geometry through dual-way coupling. The hydraulic fracture can connect natural discontinuities in sand injectites to facilitate fluid flow in otherwise low-permeability matrix, with field-scale permeability of sand injectites can be significantly increased by ~ 2 orders of magnitude. The arrest of hydraulic fracture by a natural fracture leads to fluid pressure buildup, which yields higher permeability enhancement than direct crossing of the hydraulic fracture which dissipates more pressure. Thereby, an optimized strategy for selection of well locations and injection schedules is critical in determining pattern of complex fracture networks which is beneficial in improving reservoir conductivity. Article Highlights: HF propagation and interaction with NFs were developed and studied in a coupled continuum simulator.HF may directly cross, be arrested by, or reinitiate from interacted NFs, influenced by multiple physical factors.Development of fracture networks in sand injectites can enhance field-scale permeability by ~ 2 orders of magnitude.