Numerical modelling of CO2 leakage through fractured caprock using an extended numerical manifold method

Abstract

In this study, the cohesive element-based numerical manifold method (Co-NMM) and unified pipe network method (UPM) are further developed and integrated to analyze the CO2 leakage through fractured caprock considering CO2-water two-phase flow, CO2 adsorption and deformation of both caprock matrix and fractures. First, the Co-NMM is modified to better treat complex discrete fracture networks (DFNs) problems, while the CO2 adsorption effect is introduced into the UPM governing equation of two-phase seepage. And then, a two-phase seepage-stress coupling model and an implicit sequential solution are successively adopted to combine the Co-NMM and UPM tightly to capture the interplay between the two-phase flow and fractured caprock accurately and flexibly. The extended Co-NMM formulation is verified by reproducing the CO2 plume distribution evolution process in deep brine aquifers for geological sequestration. Finally, we conduct a series of simulations where CO2 displaces water through caprock formation with different DFNs that serve as a dominant conduit for potential CO2 leakage from storage reservoirs. The effects of DFN connectivity and hydraulic characteristics on the CO2 leakage through caprock are discussed. The research results can offer a reference for the evaluation of caprock sealing efficiency and also indicate that the modified hybrid NMM-UPM method is a potentially effective tool for analyzing CO2 leakage through fractured caprock under complex DFN conditions.