Multiple caprock layers offer confidence in permanent geologic CO2 storage at the gigatonne scale

Abstract

Widespread deployment of Geologic Carbon Storage (GCS) at the gigatonne scale is projected to play a vital role in reaching carbon neutrality and mitigating the climate crisis. One major concern with GCS scale-up is the ability of geologic formations to retain CO2 deep underground, at least over several thousand years. Of particular importance to this issue is the sealing capacity of the ideally low-permeability, high-gas-entry pressure caprock(s). Existing simulation studies to address the long-term fate of the injected CO2 could barely exceed multi-century time scales due to high computational costs. This work aims to provide an improved understanding of the extent to which the potentially leaked CO2 from basin-wide GCS may rise through a multi-layered system of laterally uniform aquifers and shale caprocks over geological time scales (million years). To this end, we develop a one-dimensional CO2 flow and transport model, which is arguably capable of capturing the dynamics of basin-scale upward CO2 migration. We consider two sets of caprock properties: (1) low intrinsic permeability (10-20 m2) and high capillary entry pressure (2.5 MPa), obtained from laboratory measurements on intact clay-rich shales, and (2) high permeability (10-16 m2) and low entry pressure (0.1 MPa), representative of pervasively fractured shales at regional scales. On the one hand, we find that the free-phase CO2 can hardly penetrate more than a few centimeters into the intact caprock directly overlying the storage reservoir. CO2 leakage in this scenario is exclusively governed by molecular diffusion with an estimated migration rate of 1 meter over thousands of years. On the other hand, the high permeability and low entry pressure of fractured caprocks enable CO2 to break through the whole primary caprock during the injection and through the secondary one(s) in the post-injection period. However, following the gradual CO2 pressure decline, brine imbibition back into caprocks suppresses CO2 leakage and the percolating path is cut by an overlying caprock. Once the pore fluid of upper aquifers becomes CO2-saturated, secondary CO2 accumulations form and may host a significant portion of the injected CO2. The extreme leakage scenario, which allows for further CO2 rise of nearly one hundred meters becomes eventually diffusion-dominated and hence relatively safe. Our model results suggest that the presence of multiple shaly caprock layers, even if pervasively fractured, provides secure CO2 containment in the subsurface over millions of years. ReferenceKivi, I. R., Makhnenko, R. Y., Oldenburg, C. M., Rutqvist, J., & Vilarrasa, V. (2022). Multi-layered systems for permanent geologic storage of CO2 at the gigatonne scale. Geophysical Research Letters, 49, e2022GL100443. https://doi.org/10.1029/2022GL100443