CO₂ Migration and Leakage Risk in Dyke-Dominated Basaltic Reservoirs: A Multiphase Flow Modelling Study

CO₂ storage in basalt is considered one of the safest geological sequestration methods, as injected CO₂ reacts with basaltic minerals to form stable carbonates. Flood basalt provinces offer additional advantages, particularly their very low matrix permeability and their three-tier structure, where a vesicular or fractured zone lies between two low permeable massive units. The vesicular zone is often regarded as a suitable storage interval because of its high lateral permeability. These basalt flows are often intersected by dykes, which are commonly dominated with cooling joints. Similar dyke swarms are a characteristic feature of many basaltic terrains around the world, including the Columbia River Basalt Group, the Deccan Traps, and the Spanish Peaks. In India, such fractured dykes frequently serve as pathways for groundwater recharge during the monsoon. As the Deccan basalts in India, are now being examined as a potential large-scale CO₂ storage reservoir, the presence of tens of thousands of dykes presents a serious challenge. These dykes may act as conduits for groundwater contamination or possible leakage routes for injected CO₂. In this study, we numerically examined the effect of a fractured dyke with high vertical permeability intersecting a storage layer at 1.5 km depth using a multiphase flow model. Supercritical CO₂ was injected into a 50 m thick storage interval fully saturated with brine. The permeability of both the dyke and the host layer was derived from discrete fracture network modelling of representative field exposures. The results show that the dyke allows upward migration of CO₂, indicating a clear leakage risk that questions the practical feasibility of large-scale storage in such settings. Because sealing individual dykes is not realistic, and many serve as natural groundwater pathways, the hydrodynamics of dyke systems must be carefully evaluated before any CO₂ injection activity. The results also indicate that sills may offer a more secure storage option.