The ultralow permeability of the unconventional and geothermal reservoirs can be increased for economic production by hydraulic fracturing. Natural fractures and other discontinuities are inseparable elements of unconventional reservoir rock masses. During stimulation, hydraulic fractures often interact with the natural fractures to form a fracture network which communicates with the rock matrix. This study is an effort to develop numerical models for simulation of these interactions and cast light on the mechanisms involved in the stimulation of naturally-fractured reservoir.
State-of-the-art simulators are developed to investigate the different aspects of stimulation in naturally-fractured rocks. The models include a 2D elastic model that couples rock deformation and fluid flow, a 2D fully-coupled poroelastic model, and an integrated 3D HF-NF model with pressure dependent leak-off. Rock deformation and stresses are modeled using two- and three-dimensional displacement discontinuity (DD) method. Contact elements are used to represent the closed natural fractures along with the Mohr-Coulomb criterion to determine the contact status of the fractures. Fracture propagation is modeled using a mixed-mode propagation scheme. A novel fracture coalescence scheme is integrated in the 3D HF-NF model to investigate intersection problems for a wide range of NF dip angles and strikes. The simulation results indicate that propagation from critically-stressed and favorably-oriented natural fractures significantly contributes to the stimulation of enhanced geothermal systems (EGS). Wing-crack propagation which starts at injection pressures below the minimum horizontal stress and continues at pressures slightly above the minimum stress may lead to the generation of NF networks in en echelon pattern. The analyses regarding the stress conditions revealed that wing-cracks are likely to form when the confining stress is not significantly high. Conditions that lead to higher leakoff and development of back-stress such as high rock permeability, low reservoir pressure, and low injection rates were found to limit the propagation of wing-cracks. The simulation results indicate that hydraulic fractures experience pressure drop upon intersection with permeable natural fractures. The pressure drop is followed by an increase in the injection pressure as the hydraulic fracture pressurizes the natural fracture. Moreover, the results show that the HF may propagate in other directions away from the NF when it is partially arrested by the natural fractures. Simultaneous interaction with multiple NFs and/or stress barriers was found to result in complex HF geometries with non-uniform fracture aperture distributions that could, in turn, affect proppant placement. The simulation results indicate that the increase in the injection pressure that follows a period of pressure drop in the EGS field experiments is likely caused by fracture containment near natural fractures and stress barriers.
The DFIT results revealed that the interaction between the hydraulic and natural fractures impact the pressure transient behavior. Our results show that the closure of natural fractures which often precedes that of the HF could result in a signature similar to that of the system stiffness/compliance. The simulation result indicate that the multiple closure humps that is observed in some filed data such as one in the FORGE EGS site can be explained by the closure of a HF-NF system.