Storing CO₂ as solid hydrate in shallow aquifers: Electrical resistivity measurements in hydrate-bearing sandstone

Department of Physics and Technology, University of Bergen, 5007 Bergen, Norway

^* Corresponding author: Jarand.Gauteplass@uib.no

Abstract

A recent proposed carbon dioxide (CO₂) storage scheme suggests solid CO₂ hydrate formation at the base of the hydrate stability zone to facilitate safe, long-term storage of anthropogenic CO₂. These high-density hydrate structures consist of individual CO₂ molecules confined in cages of hydrogen-bonded water molecules. Solid-state storage of CO₂ in shallow aquifers can improve the storage capacity greatly compared to supercritical CO₂ stored at greater depths. Moreover, impermeable hydrate layers directly above a liquid CO₂ plume will significantly retain unwanted migration of CO₂ toward the seabed. Thus, a structural trap accompanied by hydrate layers in a zone of favorable kinetics are likely to mitigate the overall risk of CO₂ leakage from the storage site. Geophysical monitoring of the CO₂ storage site includes electrical resistivity measurements that relies on empirical data to obtain saturation values. We have estimated the saturation exponent in Archie’s equation, n ≈ 2.1 (harmonic mean) for CO₂ and brine saturated pore network, and for hydrate-bearing seal (SH < 0.4), during the process of storing liquid CO₂ in Bentheimer sandstone core samples. Our findings support efficient trapping of CO₂ by sedimentary hydrate formation and show a robust agreement between saturation values derived from PVT data and from modifying Archie’s equation.