In this study, we conduct two-dimensional hydraulic fracture (HF) simulations using Finite-Discrete Element Method (FDEM) in naturally fractured media with different matrix permeability and natural fracture density. Natural fractures (NFs) and fluid flow through the porous matrix and fractures are explicitly modeled in this framework through a fully coupled hydromechanical formulation. The stress redistribution due to the presence of discrete fracture network (DFN) and the complex pattern of HF propagation path due to HF/NF interactions are captured in these numerical simulations. For validation, the results of two-dimensional and hydromechanical FDEM simulations are compared to laboratory experiments and analytical solutions for hydraulic fracture initiation and propagation from a notch on a pressurized cavity in an impermeable and homogeneous medium under an anisotropic stress condition. Results of simulations reveal the significant role of NF pattern and permeability of the rock matrix on its response to HF stimulation. Hydraulic treatment of a medium with denser DFN activates more NFs and will more likely create flow channeling through some of the surrounding NFs. Size of the wet stimulated reservoir area depends on the permeability of the rock matrix, but the size of dry stimulated reservoir area is independent of the permeability.
Hydraulic fracturing technology has brought us a lot of economic and societal benefits because it makes the extraction of oil, gas, and heat from low permeability rocks possible. However, the accurate design of efficient well treatment operations to create sustainable stimulated reservoir volume (SRV) is not possible yet. Commonly used simplified models (linear elastic fracture mechanics integrated with lubrication theory) cannot predict the behavior of natural reservoirs because those models are developed for linear, elastic, homogeneous, isotropic intact rocks filled with Newtonian fluids. Natural rocks are Discontinuous, Inhomogeneous, Anisotropic, and Non-Elastic (DIANE) materials (Harrison and Hudson, 2000). In addition, they are porous and permeable, thus a complex set of poro-mechanical properties influence their behavior. Hydraulic fracturing, therefore, involves multiple interacting phases (rock blocks, granular materials, and fluids), and the behavior vary drastically depending on the involved scales, in-situ state of stress, host fluid properties, treatment parameters (e.g., viscosity and flow rate), poromechanical properties of the rock matrix, morphology, size, spacing, pattern, mineralization of the natural fractures (NF), and their relative orientation with respect to the wellbore and present-day principal stresses (Blanton, 1982; Warpinski and Teufel, 1987; Gale, et al., 2014; Raterman, et al., 2018; Daneshy, 2019).