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

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.