The effect of CO₂ injection on caprock permeability in deep saline aquifers

Department of Civil Engineering, Monash University, Clayton, Australia

^* Corresponding author: dinesha.wanigarathnajayasekara@monash.edu

Abstract

During CO₂ injection into deep saline aquifers, the overlying caprock may be subjected to geochemical reactions which can alter the leakage pathways for injected CO₂. Thus, it is crucial to identify the supercritical CO₂ (scCO₂) flow behaviour via fractures in caprock and its permeability to estimate the permanence of injected CO₂. The objective of this study is to find the effect of scCO₂ flow on fractured caprock permeability. A fractured siltstone sample was saturated in deionized water and conducted scCO₂ permeability tests using a high-precision advanced core flooding apparatus under different injection pressures and confinements. Next, the siltstone sample was saturated in 10% w/w NaCl brine and conduced scCO₂ permeability tests as described earlier. The results show that the brine-saturated sample has low permeability compared to water-saturated siltstone sample. The reason would be the deposition of evaporites during scCO₂ flow through the fractured sample. This is known as CO₂ dry-out phenomenon or absorbing moisture into the scCO₂, making the remaining brine saturated with salts. Thus, the CO₂ back-migration through the caprock discontinuities becomes minimized due to CO₂ dry-out phenomenon, which is an advantage for the caprock integrity in deep saline aquifers. In addition, aquifers with high salinity contents show significant dry-out phenomenon because pore fluid easily becomes supersaturated with salts due to evaporation of moisture into the scCO₂.