Simulation of multiphase CO2 migration in storage reservoir using discrete fracture network model

Carbon capture and storage has been suggested as a component of a multi-pronged approach to reduce anthropogenic carbon dioxide emissions. Geologic permeable formations with high pore volume overlain by an impermeable or low-permeability caprock are considered as ideal storage options. The caprocks can contain natural fractures, which can result in potential leakage of CO₂ out of primary storage reservoir. In order to assess effectiveness of a potential storage reservoir, it is critical to quantify potential leakage through the natural fractures in the caprock and to develop strategies to minimize it. Most of the earlier studies on quantifying caprock leakage through natural fractures have focused solely on caprock without taking into consideration the underlying primary storage reservoir. The migration of fluids through a caprock overlying a storage reservoir is not only a function of natural fracture characteristics but also of the pressure and saturation conditions that develop at the caprock-reservoir interface due to injection in the reservoir. We have developed a novel meshing approach that simulates multi-phase fluid flow through a permeable storage reservoir and fractured caprock. Two different numerical techniques are combined: traditional continuum volume approach for flow through a non-fractured storage reservoir and discrete fracture network (DFN) approach for flow through fractured caprock. The DFN approach is an advanced model for simulation and prediction of subsurface transport through fractured media, while the traditional continuum model is the most effective in modeling multi-phase fluid flow in permeable rock. We apply a novel meshing capability, where a topological 2D mesh in 3D space of a DFN is seamlessly connected with a continuum 3D volume mesh. We apply a novel computational approach that simulates multi-phase fluid flow through the resulting numerical mesh. We have applied this computational approach to characterize leakage through caprock using a model with realistic storage reservoir and fractured caprock characteristics and CO₂ injection conditions. Our results demonstrate that caprock leakage is influenced by both geologic and operational (injection) parameters. These results will be critical in effectively managing potential leakage through fractured caprocks.