Supercritical fluid behavior at nanoscale interfaces: Implications for CO₂ sequestration in geologic formations

Abstract

Injection of CO₂ into subsurface geologic formations has been identified as a key strategy for mitigating the impact of anthropogenic emissions of CO₂. A key aspect of this process is the prevention of leakage from the host formation by an effective cap or seal rock which has low porosity and permeability characteristics. Shales comprise the majority of cap rocks encountered in subsurface injection sites with pore sizes typically less than 100 nm and whose surface chemistries are dominated by quartz (SiO₂) and clays. We report the behavior of pure CO₂ interacting with simple substrates, i.e. SiO₂ and muscovite, that act as proxies for more complex mineralogical systems. Modeling of small-angle neutron scattering (SANS) data taken from CO₂–silica aerogel (95% porosity; ∼7 nm pores) interactions indicates the presence of fluid depletion for conditions above the critical density. A theoretical framework, i.e. integral equation approximation (IEA), is presented that describes the fundamental behavior of near-critical adsorption onto a non-confining substrate that is consistent with SANS experimental results. Structural and dynamic behavior for supercritical CO₂ interaction with muscovite (KAl₂Si₃AlO₁₀(OH)₂) was assessed by classical molecular dynamics (CMD). These results indicate the development of distinct layers of CO₂ within slit pores, reduced mobility by one to two orders of magnitude compared to bulk CO₂ depending on pore size and formation of bonds between CO₂ oxygens and H from muscovite hydroxyls. Analysis of simple, well-characterized fluid-substrate systems can provide details on the thermodynamic, structural and dynamic properties of CO₂ at conditions relevant to sequestration.

Keywords:

• geologic sequestration
• nanostructures
• supercritical CO₂
• adsorption
• integral equation calculation
• molecular dynamics

Acknowledgements

Support for this work comes from the US Department of Energy through projects funded by the Office of Basic Energy Sciences: GRS (Section 3) from ‘Structure and Dynamics of Earth Materials, Interfaces and Reactions’ (FWP ERKCC72) and DRC (Section 1, 2, 3, 5), AAC (Section 4.1) and LV (Section 4.2) from ORNL's part of the LBNL ‘Center for Nanoscale Control of Geologic CO₂’ (FWP ERKCC67) under contract DE-AC05-00OR22725 to Oak Ridge National Laboratory, managed and operated by UT-Battelle, LLC. We thank two anonymous reviewers for their helpful comments and suggestions that improved this paper.