Abstract
Carbon capture and storage (CCS) technology is routinely cited as a cost effective tool for climate change mitigation. CCS can directly reduce industrial CO2 emissions and is essential for the retention of CO2 extracted from the atmosphere. To be effective as a climate change mitigation tool, CO2 must be securely retained for 10,000 years (10 ka) with a leakage rate of below 0.01% per year of the total amount of CO2 injected. Migration of CO2 back to the atmosphere via leakage through geological faults is a potential high impact risk to CO2 storage integrity. Here, we calculate for the first time natural leakage rates from a 420 ka paleo-record of CO2 leakage above a naturally occurring, faulted, CO2 reservoir in Arizona, USA. Surface travertine (CaCO3) deposits provide evidence of vertical CO2 leakage linked to known faults. U-Th dating of travertine deposits shows leakage varies along a single fault and that individual seeps have lifespans of up to 200 ka. Whilst the total volumes of CO2 required to form the travertine deposits are high, time-averaged leakage equates to a linear rate of less than 0.01%/yr. Hence, even this natural geological storage site, which would be deemed to be of too high risk to be selected for engineered geologic storage, is adequate to store CO2 for climate mitigation purposes.
Highlights
The integrity of engineered subsurface geological carbon dioxide (CO2) reservoirs is governed by a range of geological, geochemical, and geotechnical factors
To study how a CO2 storage site may leak over many thousands of years, here we examine a very large (>4.7 × 1010 m3 of recoverable CO2) natural accumulation, within the St Johns Dome located on the border of northern Arizona and New Mexico
We examine the history of CO2 leakage along structural elements in the region above the natural CO2 reservoir
Summary
Travertine samples were obtained from 11 discrete travertine mounds along the Buttes Fault. Left panel: Mass of CO2 leaked based on precipitation ratios of 10% (blue circles) and 1% (red squares) for the three mounds with constrained minimum lifespans, the Buttes Fault and the whole of St. Johns Dome area, respectively. Four samples exhibit ages outside the dating-range of the U-Th method and two fall just within the dating range but have very large errors associated with them and have not been used for the calculation of leakage rates (Supplementary Fig. 1, Table 1). Travertine mound volumes along the Buttes Fault were calculated using surface areas, based on field mapping, a 1/3-arcsecond DEM, and mound thickness, measured during fieldwork (Supplementary Fig. 2, Table 2). Leakage rates are calculated based on the minimum lifespan given by the U-Th dating for each mound. For the CO2 volume calculations, a CO2 density of 185 kg/m3 (based on reservoir conditions) and a CO2 saturation of 80% in the gas cap (assuming 20% of the pore space occupied by connate and residual water) was used
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