Two-Phase Flow Mechanisms Controlling CO2 Intrusion into Shaly Caprock

Geologic carbon storage in deep saline aquifers has emerged as a promising technique to mitigate climate change. CO2 is buoyant at the storage conditions and tends to float over the resident brine jeopardizing long-term containment goals. Therefore, the caprock sealing capacity is of great importance and requires detailed assessment. We perform supercritical CO2 injection experiments on shaly caprock samples (intact caprock and fault zone) under representative subsurface conditions. We numerically simulate the experiments, satisfactorily reproducing the observed evolution trends. Simulation results highlight the dynamics of CO2 flow through the specimens with implications to CO2 leakage risk assessment in field practices. The large injection-induced overpressure drives CO2 in free phase into the caprock specimens. However, the relative permeability increase following the drainage path is insufficient to provoke an effective advancement of the free-phase CO2. As a result, the bulk CO2 front becomes almost immobile. This implies that the caprock sealing capacity is unlikely to be compromised by a rapid capillary breakthrough and the injected CO2 does not penetrate deep into the caprock. In the long term, the intrinsically slow molecular diffusion appears to dominate the migration of CO2 dissolved into brine. Nonetheless, the inherently tortuous nature of shaly caprock further holds back the diffusive flow, favoring safe underground storage of CO2 over geological time scales.Open Access funding provided thanks to the CRUE-CSIC agreement with Springer Nature. The research presented in this article was funded by the European Research Council (ERC) under the European Union’s Horizon 2020 Research and Innovation Program through the Starting Grant GEoREST (www.georest.eu) (Grant agreement No. 801809). This work was partly supported by the Spanish Ministry of Science and Innovation through the grant CEX2018-000794-S, funded by MCIN/AEI/10.13039/501100011033. Additional support was provided by the grant IJC2020-043809-I funded by MCIN/AEI/10.13039/501100011033 and the European Union NextGenerationEU/PRTR. This study also received the support from US DOE through CarbonSAFE Macon County Project DE-FE0029381. Open access funding provided by the Spanish National Research Council (CSIC).Peer reviewe