ASSESSING LEAKAGE POTENTIAL OF FRACTURED CAPROCKS THROUGH DISCRETE FRACTURE NETWORK MODELING APPROACH

Understanding how fluids flow through networks of fractures and fractured porous media has multiple applications, including geologic CO₂ storage, hydrocarbon extraction, geothermal energy production, nuclear waste disposal, etc. Safe, long-term storage of CO₂ requires having a competent caprock above the primary storage reservoir. Evidence based on natural CO₂ reservoirs as well as oil and gas reservoirs demonstrates effectiveness of caprocks. In spite of the evidence it is critical that effectiveness of caprock to limit migration of CO₂ and brine at typical CO₂ storage sites should be assessed, including potential of flow through naturally present fractures and fracture networks. Over the years various approaches have been developed to simulate and predict fluid flow through fractured rocks, including those where fractures are either represented explicitly or as part of multiple, interacting continua. The Discrete Fracture Network (DFN) modeling approach allows to explicitly characterize effect of geometry and properties of fractures (and fracture network) on fluid flow. At Los Alamos National Laboratory we have developed a novel approach to generate high quality, conforming Delaunay triangulation of 3-dimensional discrete fracture network. The approach allows for computationally efficient representation of complex fracture networks that can be used in multi-phase fluid flow simulations. Additionally, we are have also developed an approach that explicitly connects DFNs to volumes such that the process of CO₂ injection into a storage reservoir and subsequent flow through an overlying caprock, which may be fractured, can be simulated. We are applying this approach to perform numerical simulations of multi-phase fluid flow through complex fracture networks in caprocks that overly CO₂ storage reservoirs. Results of our study demonstrate that not all of the fractures in a fracture network contribute to the flow. Additionally, we also observe that CO₂ flow through fractured caprock is affected by the nature of CO₂ movement in the storage reservoir and which parts of fracture networks are intercepted by CO₂ plume. These results have implications on not only assessing the effectiveness of caprocks but also on deploying technologies to effectively monitor leakage through caprocks.