Simulation Study on Nanoparticles-Assisted CO2 Geological Storage for Enhancement of Safety

Abstract

Carbon dioxide (CO2) geological storage (CGS) is one of the promising measures for reducing greenhouse gas emissions and mitigating the impact of climate change. Deep saline aquifers have the largest potential as subsurface storage sites, which attract tremendous attention. CO2 injected into the deep saline aquifers migrates upward until it reaches a sealing formation due to its buoyancy effect. The concern is that CO2 might leak through the sealing rock while it is sitting on the top of the brine under the sealing layer. The risk of leakage will be higher if the sealing capacity of the sealing rock is not enough. CO2-dissolved water storage is one of the effective methods if the ability of the sealing layer is not sufficient. However, in-situ dissolution progresses slowly and takes many years to decades or centuries. This study proposes a novel concept for enhancing the safety of CO2 geological storage with nanoparticles in comparison with the conventional in-situ dissolution. The addition of nanoparticles to super-critical CO2 (scCO2) could increase its density and mitigate the gravity segregation. This effect can suppress the migration of CO2 to the upper part of the aquifer and promote the storage of CO2 in the lower part of the aquifer. As a result, the solubility trapping is promoted compared to injecting CO2 alone because injected CO2 contacts the formation brine in the lower part of the aquifer. Furthermore, the density contrast between CO2 dissolution formation brine and non-dissolved formation brine is enhanced, which can shorten the onset time of mixing and enhance convective mixing. This is expected to improve the safety of CO2 geological storage in the long term in addition to the enhancement of the CO2 storage efficiency. This paper presents the results of investigations considering the influence of nanoparticle types and concentrations on CO2 storage safety by using a commercial simulator. Nanoparticle types assumed in this study are common metal oxide nanoparticles which are widely used in the study of nanoparticle-based enhanced oil recovery. In particular, the effects of density and viscosity alteration were considered in this study. These results showed the leakage amount to the upper sealing layer decreased as the density and concentration of dispersed nanoparticles increased. Furthermore, the sensitivity studies for the sealing layer permeability and capillary pressure showed that the concept of nanoparticle-dispersed scCO2 injection has the effect of delaying the migration rate in the sealing layer when the capacity of the sealing layer is guaranteed but not enough. This paper discusses these effects on the safety in terms of long-term CO2 geological storage. This study is a preliminary simulation study on nanoparticles-assisted CO2 geological storage based on some important theories. This means further verification through some additional experiments is necessary in the future.

Keywords: CO2 geological storage; nanoparticle; CO2 storage safety; numerical simulation; fluid model